MODERN CONCEPTS IN PLATE OSTEOSYNTHESIS...MODERN CONCEPTS IN PLATE OSTEOSYNTHESIS Karl Kilian...

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MODERN CONCEPTS IN

PLATE OSTEOSYNTHESIS

Karl Kilian Stoffel MD

This thesis is presented for the degree of Doctor of

Philosophy of The University of Western Australia

School of Surgery and Pathology and

School of Mechanical Engineering

University of Western Australia

2007

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Abstract

Renewed interest in the fixation of fractures using plates has been stimulated by an

improved understanding of the biology of fracture healing and a drive towards

minimally invasive surgery. This has led to a change in the way we use plates

nowadays and the way in which we build the bone-plate construct, as well as the

development of new implants better suited to these techniques. As a result of this,

we have now the potential to safely expand the indications for plate fixation

especially in the management of fractures in osteopenic bone. This thesis provides

scientific evidence allowing for better formulation of the optimum way to use the

modern plating systems in the clinical setting.

Biological fracture repair with conventional plates, in terms of a less rigid construct

to enhance fracture healing, is becoming increasingly popular. By omitting screws

the construct becomes more flexible with a risk of fixation failure. It was the aim of

the first paper to investigate in an experimental model the construct strength of

different conventional plate lengths and number / position of the screws, and if an

oblique screw at the plate end could increase the fixation strength. Our data

suggest that the plate length is the most important factor in withstanding forces in

cantilever bending. Longer plates with an equal number of screws require greater

peak loads to failure than short plates with more screws. Furthermore, an oblique

screw at the plate end produces an increased strength of fixation in all different test

setups. However, the difference is more significant in shorter plates and in

constructs with no screw omission adjacent to the fracture site.

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The second paper investigates the mechanical properties of locking plates (LCP) in

torsion and axial compression in composite bone cylinders. The influence of

different variables of the implant on construct stiffness and strength are described.

The results show that axial stiffness and torsional rigidity of the construct is mainly

influenced by the working length of the plate. More than three screws per fragment

have little effect on axial stiffness, while more than four screws do not increase

torsional rigidity. The closer an additional screw is positioned towards the fracture

gap, the stiffer the construct becomes under compression. The rigidity under

torsional load is mainly determined by the number of screws, and is for a given

working length independent of their position. Finally, shorter plates compared to

long plates with an equal number of screws, cause a reduction in axial stiffness but

do not affect torsional rigidity.

In the third paper the behaviour of compression plate technology, the internal

fixator technology and the combination of both principles was biomechanical

investigated in simulated shaft fractures and comminuted femoral supracondylar

fractures (C2). The tests were conducted using composite bones and osteoporotic

cadaveric bones. Results indicate that in comminuted fractures, the construct

stiffness under axial compression using a locking plate compared to a compression

plate is not significantly different. Following cyclic loading, however, locking plates

can better retain fracture reduction compared to compression plates. On the other

hand, under torsional load the compression plate appears to be biomechanical

superior to the locking system. In supracondylar comminuted femur fractures,

combining the two principles results in less plastic deformation, and a higher load

to failure compared to their single application.

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The last two papers examine the behaviour of locking plates in osteopenic bone. In

cadaveric intra-articular calcaneal fractures, the locking plate showed a

significantly lower irreversible deformation during cyclic loading and a significantly

higher load to failure. In dorsal and volar fixed angle distal radius constructs in a

cadaveric model, all constructs showed adequate stability with minimal deformation

on fatigue testing under physiological conditions in good bone quality. In

osteoporotic bone, however, dorsal fixed angle constructs are stiffer and stronger

than volar constructs. The addition of a styloid plate to a volar plate does not

significantly improve stability.

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Table of Contents

Declaration for thesis containing published work and/or

work prepared for publication

10

Statement of Author Contribution (%)

12

Chapter 1 Introduction and Review of the Literature 14

Chapter 1.1 Biology of Bones

14

Chapter 1.2 Mechanics of Fracture Healing

17

Chapter 1.3 Bio-Mechanics of Fracture Healing

29

Chapter 1.4 Evolution of Plate osteosynthesis

22

Chapter 2 Aims and Hypotheses of the thesis 49

Chapter 3 Oblique screws at the plate ends increase the fixation

strength in synthetic bone test medium

52

Chapter 4 Biomechanical considerations in plate osteosynthesis:

the effect of plate to bone compression with and without

angular screw stability

74

6

Chapter 5 How can stability in internal fixators be controlled? A

mechanical and Finite Element Analysis

107

Chapter 6 Intraarticular calcaneus factures in human cadavers: A

biomechanical comparison of a conventional versus a

locking plate

133

Chapter 7 Volar versus dorsal locking plates with and without

radial styloid locking plates for the fixation of dorsally

comminuted distal radius fractures: A biomechanical

study in cadavers

152

Chapter 8 Summary and Conclusion

175

Chapter 9 Future Work

188

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List of Abbreviations

AO Arbeitsgemeinschaft für Osteosynthesefragen (Working Group for

Osteosynthesis Questions)

DCP Dynamic Compression Plate

FEA Finite Element Analysis

LCP Locking Compression Plate

LISS Less Invasive Stabilization System

MIPO Minimally Invasive Plate Osteosynthesis

NCB Non Contact Bridging Plate

PAP Periarticular Compression Plate

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Acknowledgements

I am deeply indebted to my supervisors, Professor Markus Kuster and Professor

Gwidon Stachowiak, for their guidance, stimulating suggestions and

encouragement during all the time of research for and writing of this thesis.

The project was conducted at the University of Western Australia and I would like

to acknowledge and thank both, the School of Surgery and Pathology and the

School of Mechanical Engineering for their comprehensive support.

I acknowledge the financial support from the Max-Biedermann Foundation, Berlin,

Germany (Paper 1, 2, 4), the Swiss Orthopaedic Association, Bern, Switzerland

(Paper 1), the Zimmer Inc., Warsaw, USA (Paper 2) and the South Metropolitan

Area Health Service Human Research Ethics Committee (Paper 5). Similarly I

wish to thank Synthes, Bettlach, Switzerland and Zimmer Inc., Warsaw, USA for

providing implants for testing (Paper 1-5). .

Thanks are to Miss Hannah Ozturk, Miss Claire Jones and Greg Scott from the

School of Mechanical Engineering for their technical support during mechanical

testing (Paper 1, 5), to Mr Gago Mario from Mathys Medical Ltd, Bettlach,

Switzerland for his assistance during finite element analysis (Paper 1), to Mr

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Geoff Roth from Nuclear Medicine, Royal Perth Hospital, for his assistance with the

bone densitometry (Paper 2,4,5), to Mrs Wendy Davies from the School of

Medicine and Pharmacology for her assistance with the statistics (Paper 2,4), to

Mrs Joy Buchanan for proof reading of Paper 3, to Dr. Brian Sweeney and Dr.

Nicholas Wambeek from the Radiology Department, Fremantle Hospital for kindly

classifying the calcaneal fractures (Paper 4). I also express my sincere thanks to

all members of the Biomechanical Laboratory at Fremantle Hospital and the

Engineering Laboratory at the University of Western Australia, past and present. A

special thank also to my colleague Mr Piers Yates, who looked closely at the final

version of the thesis for English style and grammar, correcting both and offering

suggestions for improvement.

The greatest thanks, however, goes to my family, particularly to my wife Nadine.

Without her continuous support this project would not have been possible. Her love

and patience enabled me to complete this work. I also want to thank my parents

who have done everything they possibly could to make sure all their five children

had a chance in this word.

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Declaration for thesis containing

published work and/or work prepared

for publication

This thesis contains published work and/or work prepared for publication, some of

which has been co-authored. The bibliographic details of the works and where they

appear in the thesis are set out below.

Stoffel K, Forster T, Stachowiak G.W., Gächter A, Kuster M.S. Oblique screws at

the plate ends increases the fixation strength in synthetic bone test medium.

Journal of Orthopaedic Trauma 2004, Vol 18/9, 611-17

This work appears on page 52 of the thesis.

Stoffel K, Ulrich D, Stachowiak G, Gächter A, Kuster M.S. How can

stability in internal fixators be controlled? A mechanical and Finite Element

Analysis.

Injury 2003; Suppl. 2: 11-19


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Stoffel K, Lorenz KU, Kuster M. Biomechanical considerations in plate

osteosynthesis: the effect of plate to bone compression with and without angular

screw stability.

Accepted for publication in the Journal of Orthopaedic Trauma


Stoffel K, Booth G, Roehl S, Kuster M. Intraarticular calcaneus factures in human

cadavers: A biomechanical comparison of a conventional versus a locking plate.

Clinical Biomechanics 2007, Jan;22(1):100-105


Blythe M, Stoffel K, Jarrett P, Kuster M. Volar versus dorsal locking plates with and

without radial styloid locking plates for the fixation of dorsally comminuted distal

radius fractures: A biomechanical study in cadavers.

J Hand Surg [Am]. 2006 Dec;31(10):1587-93


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Statement of Author Contribution (%)

Stoffel K (70%), Stachowiak G (5%), Forster T (5%), Gachter A (5%), Kuster M.

(15%). Oblique screws at the plate ends increases the fixation strength in synthetic

bone test medium. Journal of Orthopaedic Trauma 2004, Vol 18/9, 611-17

Signature Coordinating Supervisor Prof.Kuster M

Stoffel K(70%), Dieter U (5%), Stachowiak G (5%), Gachter A (5%), Kuster M

(15%). Biomechanical testing of the LCP--how can stability in locked internal

fixators be controlled? Injury 2003;34 Suppl 2:B11-9


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Stoffel K (70%), Lorenz KU (10%), Kuster M (20%). Biomechanical consideration in

plate osteosynthesis: the effect of plate to bone compression with and without

angular screw stability. Resubmitted to J Orthop Trauma.


Stoffel K (80%), Booth G (10%), Rohrl SM(5%), Kuster M (5%). A comparison of

conventional versus locking plates in intraarticular calcaneus fractures: A

biomechanical study in human cadavers. Clin Biomech 2007, Jan;22(1):100-105


Blythe M (50%), Stoffel K (40%), Jarrett P (5%), Kuster M (5%). Volar versus

dorsal locking plates with and without radial styloid locking pates for the fixation of

dorsally comminuted distal fractures: a biomechanical study in cadavers. J Hand

Surg [Am]. 2006 Dec;31(10): 1587-93


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Chapter 1 Introduction and

Review of the Literature

Chapter 1.1 Biology of Bones

Normal bone structure and remodelling

Bone is a living tissue made by deposition of minerals. It undergoes constant

turnover, with simultaneous bone formation and resorption [1]. There are two types

of bone: compact (lamellar) bone and cancellous bone.

Lamellar bone exhibits a circular arrangement of “Haversian” system [2].. This

system results from the formation of a tunnel, in a longitudinal direction in long

bone, and the filling of the tunnel by layers of collagen that are concentrically

organized. Cancellous bone is made of a network of trabeculae forming large

marrow spaces, which contain hematopoietic cells and fat. Such lamellar, non-

Haversian bone is made up of alternating bands of collagen oriented at slightly

different angles to each other. Trabeculae are believed to have an optimal

orientation and interconnection to resist compressive load [3].

The cellular components of lamellar and cancellous bone consist of lining cells

(inactive osteoblasts), osteoblasts, osteoclasts, and osteocytes. The collagen fibrils

of the osteoid become oriented into the orderly arrangement of lamellar bone

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except in areas where woven, non-lamellar bone is being laid down. Woven bone,

which is generally temporary, is the only type of bone that shows no lamellar

pattern.

Cortical bone remodelling occurs through so-called Haversian remodelling

(formation of secondary osteons). The process is initiated by osteoclastic

resorption to create longitudinally oriented tubular channels. Osteoblasts on the

surface of these channels then deposit successive layers of lamellar bone until the

diameter of the cavity is reduced to a small, singular vascular canal [4]. The close

coupling of bone resorption and bone formation is one of the basic phenomena of

bone remodelling, which seems to occur not only in lamellar bone but also in

cancellous bone. The mechanism of the coupling is poorly understood, but it is

generally believed to be mediated through bone load carrying optimization

functions [5]. Numerical optimization methods have often been employed to

determine the laws that govern the process of bone growth, maintenance and

remodelling. A local optimization goal was proposed as a relationship between

local density of the bone and “effective stress” to which it is subjected [6].

Successively this criterion was used for optimization of femur density distribution

[7,8]; an alternative criteria accounting for strain were also formulated. However, a

particular global mechanical criterion responsible for bone morphology optimization

is still missing. The correlation between mechanical stress history and tissue

differentiation in initial fracture healing has been investigated by Carter et al. [9].

His results suggest that intermittent stresses play an important role in determining

tissue differentiation and morphological patterns of fracture healing.

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Fracture mechanism of long bones

Fractures can be classified according to the factors characterizing the force

causing the fracture. Fractures caused by direct forces can be classified according

to the magnitude and area distribution of the force, as well as according to the rate

at which the force acts on the bone. Soft tissue injury and fracture comminution are

especially related to the loading rate and trauma energy [10].

Fractures caused by indirect forces are produced by a force acting at a distance

from the fracture site. When a long bone is loaded, each section of the bone will be

subjected to both normal and shear stress. When these stresses exceed the limit

of the bone, the bone will fracture. Different loads will generate different normal

and shear stresses along different orientation planes within the bone. From the

morphology of the fractures lines, it is possible to infer the type of indirect injury

mechanism [11].

Depending on the material strength, the three principal planes (maximum tensile

stress plane, maximum compressive stress plane, and maximum shear stress

plane) dictate the fracture plane and predict when and how the material will fail.

Cortical bone as a material is generally weak in tension and shear, particularly

along the longitudinal plane. These anisotropic properties influence the bone

fracture failure under external loads.

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The failure patterns of long bones follow basic rules. Under bending, the convex

side is under tension, and the concave side under compression. Because bone is

more susceptible to failure in tension then in compression, the tension side fails

first. Tension failure then occurs progressively across the bone; the resulting

comminution on the compression side often creates a single “butterfly” fragment or

multiple fragments. Under torsion injury, there is always a certain bending moment

that prevents the propagation of the endless spiral fracture line. The 45º fracture

line is a result of maximum tensile stress acting at a 45º plane.

The susceptibility of a bone to fracture with a single injury is related to its energy-

absorption capacity and modulus of elasticity. Bone undergoing rapid loading will

absorb more energy than when loaded at the slower rate [12]. The energy

absorbed by the bone during loading is realized when the bone fractures. This

phenomenon helps to explain why injuries with rapid loading involving higher

velocities dissipate greater energy and result in greater fracture comminution and

displacement.

Chapter 1.2 Mechanics of Fracture Healing

The amount and quality of callus formation is influenced by biological factors such

as blood supply, hormones and growth factors [13-16] and by biomechanical

conditions at the fracture site [17-19]. The process of fracture healing can

principally be divided into three stages: inflammation, reparation, and remodeling.

Following a fracture, haematoma occurs which develops a granulation tissue [20].

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Typically the new bone formation in this “soft callus” starts at the periosteal and

endosteal surface of the cortical bone at some distance from the fracture [21] and

proceeds in the direction of the fracture gap [22]. The next critical phase of fracture

healing involves the formation of an intact bone bridge between the fragments, and

because this involves the joining of hard tissue, it follows that the whole system

must become immobile at least temporarily. This section of the fracture healing is

divided into two healing patterns: primary bone healing and secondary

(spontaneous) fracture healing [23]. The primary bone healing mechanism occurs if

there is accurate cortical apposition and rigid fixation. In this case the bone fracture

ends unite directly by haversian modeling in the contact areas and by formation of

osteons bridging the tissue across the fracture. In primary bone healing the

production of the fibrocartilaginous callus is minimal. However, rigid fracture

fixation with a plate (e.g. the use of lag screws and interfragmentary compression)

has shown a high complication rate, including delayed- or non-union, infection,

hardware failure, and refracture after plate removal [24-30]. On the other hand,

spontaneous fracture healing (healing with periosteal and endosteal callus

formation) is considered “secondary” because, initially, an intermediate fibrous

tissue or fibrocartilage is formed between the fracture fragments and is only

subsequently replaced by new bone [31]. The biomechanical function of the callus,

formation of which characterizes secondary bone healing [32], is to reduce the

relative movement between the proximal and distal bony fragment to such an

extent that both fragments can be united by bony bridging [31,33]. The amount and

quality of callus formation is influenced by biological factors such as blood supply,

hormones and growth factors [14-17,34,35]] and by biomechanical conditions at

the fracture site [17-19,34]. The final phase of fracture healing is governed by

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Wolff’s law [36] where bone is being remodelled back to its original shape and

load-carrying strength. Weight bearing is important in order to allow the healing

bone to be subject to normal stresses.

Chapter 1.3 Bio-Mechanics of Fracture Healing

The mechanical situation at the fracture site can be described by the local strain

and is influenced by the strain magnitude, the strain rate, and the duration of the

strain [37-39]. Which specific parameters of the functional strain environments

govern the osteogenesis has not yet been clearly established. In vivo experiments

have begun to isolate and identify the nature of the osteoregulatory stimulus.

Lanyon et al. [40] have demonstrated strain rate and strain distribution to be of

prime importance in the osteoregulatory process.

It seems that local tissue strain and hence the fracture healing process are

dominated by the size of the fracture gap and the interfragmentary moment, which

is induced by the load and the stability of the fracture fixation [41]. A theory that

takes into account the gap size and the interfragmentary movement is the

interfragmentary strain hypothesis by Perren [42]. He predicted that fracture

healing will occur only if the interfragmentary strain (IFS=interfragmentary

movement divided by gap width) is less than the rupture strain of bone (2%) [43].

Several clinical and experimental studies, mainly performed using an external

fixator, showed good results in fracture healing even with higher IFS [24,37,44-46].

Claes et al. [44] indicated that intramembranous bone formation occurs when

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stresses and strains are below 0.15 Mpa and 5% strain. Enchondral bone

formation occurs when this stress is exceeded, but the strain remains below 15%.

Goodship and Kenwright [37] demonstrated in animal studies and in clinical trials a

significantly enhanced clinical and mechanical healing process in subjects with

tibial fracture, where the fractures were stabilized with an external fixator and

allowed axial micro movement of 1mm (resp.33% interfragmentary strain in a 3mm

gap). Other animal experiments have shown that an optimal axial inter-fragmentary

movement seems to be within the range of 0.2-1.0 mm [18,47]. Axial and shear

bone strains are believed to have distinct strategic roles in defining bone

architecture and tissue differentiation (Rubin et al, 1996) Similarly, inter-

fragmentary axial and shear strains seem to affect oppositely the process of

fracture healing. While axial strain positively stimulates the callus formation, shear

strain within the fracture site is considered harmful for the process of bone growth

[48]. Low-contact [49], point-contact [50] and non-contact plates [51,52] have been

used for biological plate osteosynthesis. Long conventional plates are also used for

subfascial non-contact fixation in order to not damage viable tissue. Every attempt

is made to maintain all the soft tissue attachments and blood supply of the

intervening comminuted fragments, since union will depend primarily on the

formation of the bridging callus rather than primary bone union [53-60].

Biological Plate Osteosynthesis

The aim for the surgeon is to achieve a beneficial mechanical environment for

fracture healing in terms of a less rigid construct. However, fracture stabilization is

a balancing act between flexible fixation, which can enhance callus formation and

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thus improve the healing process [25,61] and an unstable fixation, which can lead

to non-union and/or implant failure [28,62]. With the use of more dynamic

osteosynthesis techniques, such as external fixator and intramedullary nailing, the

complication rate could be reduced [63,64]. Increasing evidence of the importance

of biological factors and callus formation led to the development of the concept of

“biological plate osteosynthesis”, which has been promoted by several authors and

become an option for the treatment of metaphyseal and diaphyseal fractures [64-

67]. In this principle every attempt is made to maintain all the soft tissue

attachments and blood supply of the comminuted fragments, since union will

depend primarily on the formation of the bridging callus rather than primary bone

union [56,57,59,60]. The optimal parameters for influencing fracture healing by

altering the mechanical environment using an external fixator are more or less well

established [18,24,37,38,68-74].

When choosing plate osteosynthesis the outstanding problem is to determine how

mechanical stability, and thus interfragmentary movement, can be controlled. The

choice of plate length and the number of cortices of fixation is still made largely on

the basis of anecdotal reports and clinical experience with mechanical failure.

Most of the literature published to date report of the strength to failure or the

stiffness of the fixation construct in relation to the unfractured bone [62,75-83].

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Chapter 1.4 Evolution of Plate Osteosynthesis

Different principles of operative fracture fixation have been developed since the

end of the 18th century. The first attempt for open fracture fixation was the use of a

brass wire by Icart, published in a French manuscript in 1775 [84]. At the beginning

of the 19th century operative fixation became more popular with an increased

number of publications [85-89]. However, the rather unstable internal fixation

methods actually just combined the disadvantages of a conservative and an

operative fracture treatment. The fracture site had to be opened with the inherent

risk of an infection, which often ended in catastrophe for the patient. In order to

increase the stability of the construct the German surgeon Hansmann reported in

1885 a new fixation method which he used in 21 tibia fractures [90]. His concept of

fracture fixation was to bridge the 2 fracture ends with a narrow metal bar in which

he predrilled holes.

Figure 1. The first internal fixation by means of a plate and percutaneous placed

screws was described by the surgeon Hansmann in 1885 [90].

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Hansmann’s more stable technique also faced failure due to infection, and the real

breakthrough for surgery in general, and particularly for operative fracture

treatment, only came with the introduction of asepsis at the end of the 19th century.

This included a progressive understanding of bacterial contamination, the use of

early splintage, and the application of open wound treatment.

It was Lambotte who pioneered at this time the research of newer fixation

principles [91]. He was the founder of the concept of modern osteosynthesis by

using different plates and screws, as well as external fixators, with the aim of an

anatomical reconstruction of the limb, together with an early rehabilitation of the

patient.

Figure 2. Lambotte, a Belgian surgeon, developed new plate and screw designs.

His plate was thin, round and tapered at both ends [91].

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Plates were further refined by Sherman, Pauwels, Lane and Koenig, [92-97] who

reported their results with varying rates of success. In 1949 Danis described a new

plate technology, which allowed compressing the 2 main fragments together to

achieve increased fixation rigidity [92-97]. This allowed for the first time early

mobilization of the adjacent joints and helped to preserve the function of the limb in

the early postoperative period. He observed that fractures under rigid fixation

healed without external callus formation and described this phenomenon of

fracture healing as “primary bone healing”.

Figure 3. Danis described for the first time direct bone healing following

interfragmentary compression in his book Theorie et pratique de l’osteosynthese.

Interfragmentary compression is achieved by tightening the side screw on the left

side of the plate [123].

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Any signs of callus formation were interpreted as a sign of instability, with the

associated risk of non-union and implant failure.

Mueller read Danis book and enthusiastic about early functional care after rigid

fixation, he founded with Allgoewer and Willenegger in 1951, a study group for

internal fixation with a scientific basis. In 1958 a group of orthopaedic surgeons,

general surgeons and scientists formed an association known by the name AO

(Arbeitsgemeimschaft fuer Osteosynthefragen) or ASIF (Association for the study

of internal fixation) [124].

Figure 4. The first study group for internal fixation (1951) and later members of the

AO Foundation Group (1958) (left) and a standardized Instrumentation Set by the

AO 1958 (right) [124].

The philosophy of the AO group was aimed at the restoration of the function of the

injured extremity. To achieve this they focused their work on the biomechanical

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prerequisites and pitfalls of internal fixation [100]. The working hypothesis included

the following 4 basic principles:

1. Anatomical reposition

2. Absolute stability through interfragmentary compression

3. Preservation of the blood supply by an atraumatic surgical technique

4. Early active mobilization

In the beginning compression between the major fragments was achieved with the

use of interfragmentary compression screws and later the axial co-adaptor [101]. In

1969 the Dynamic Compression Plate (DCP) was designed and became the

implant of choice for almost 2 decades [102,103].

Figure 5. Interfragmentary compression was achieved with the axial co-adaptor

(left) [101] or later the dynamic compression plate (DCP) (right). Since the holes

have an inclined plane upon tightening, the screws produces plate translocation

and hence interfragmentary compression [102].

The principle of rigid fracture fixation allowed early mobilization, however, clinically

the disadvantages of such a fixation principle became more and more evident with

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a high complication rate including delayed- or non-union, infection, hardware failure

and re-fracture after plate removal [24-30,104]. These complications were mainly

caused by 3 factors. First, in order to achieve a rigid fixation, very strong and

voluminous plates had to be used. Since the plates were much stiffer than bone,

there was stress protection beneath the plate, with subsequent osteoporosis

induced by mechanical unloading of the bone [105]. Secondly, the vascular supply

to the bone was further impaired by large skin incisions with further dissection of

the soft tissue e.g. muscles and periosteum around the fracture site. Also, in order

to achieve primary bone healing without callus formation the smallest fragments

were aligned anatomically, with the consequence of further bone and soft tissue

damage. The blood supply to the fracture site was even further disturbed by

compressing the plate to the bone, obliterating the periosteum. Thirdly, primary

bone healing without callus formation is relatively weak and together with the 2

reasons mentioned above, increases the risk of a re-fracture after removal of the

implant.

These insights, together with the excellent results of diaphyseal fractures treated

with more flexible fixation systems, such as intramedullary nailing [106] and

external fixation [107,108], have changed attitudes toward the use of plates in

fracture treatment. The increasing evidence of the importance of biological factors

and that callus formation may be seen to be beneficial for some plate

osteosynthesis, has led to the development of the concept of “biological plate

osteosynthesis” [64-67]. This concept aims to integrate the 2 factors of biology and

mechanics. Hand-in-hand with this evolution of plate fixation has been the

development of new devices, which often utilize newer materials.

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The biology is respected by creating plate fixation without further impairment of the

vascular supply to the bone, soft tissue and periosteum. Isolated bone fragments

were left in place without the risk of further devascularisation. The fracture is

reduced indirectly and stabilized using a bridging technique [109].

The “Wave Plate” popularised by Weber [110] can be seen as the first attempt to

preserve the blood supply beneath the plate. The wave of the plate lifts the plate

away from the bone near the fracture. The elevated part increases the lever arm,

allows better blood supply and can be used to accommodate bone grafts.

Figure 6. The “Wave Plate” offers biological advantages in fracture fixation. The

periosteum beneath the plate is not compressed and bone graft can be packed

under the wave portion of the plate. [110]

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It has been shown that the disturbance of the periosteal blood supply correlated

with the contact area between the bone and the plate [111]. Perren et al. [112] and

Gautier et al. [113] showed that the remodelling beneath the plate was more the

result of internal remodelling of cortical bone induced by necrosis rather than by

unloading. In order to minimize the damage to the periosteal blood supply the

contact area between the plate and the bone was reduced. This led to the

development of the Limited Dynamic Contact Plate (LC-DCP). As a result of

undercutting the plate surface the contact could be reduced by more than 50%

compared to a conventional DCP [114]. The stiffness of the plate was uniform over

the whole plate length and allowed precontouring of the plate without the risk of

kinking the plate at the level of a hole. The compression unit of the DCP was

incorporated on both ends of the LC-DCP plate in case the surgeon wanted to

achieve interfragmentary compression at different levels in a segmental fracture.

Figure 7. With the introduction of the Limited Contact Dynamic Compression Plate

(LC-DCP) the contact between the plate and the bone could be reduced by 50%

[114]. Stainless steel was replaced by the more bone friendly titanium alloys. The

trapezoid form of the plate allows the formation of rounded bone lamellas at the

extremities of the plate, which will not be damaged at removal.

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A further reduction of the contact area between the plate and the bone was

achieved with the Point Contact Fixator (PC-Fix) [115]. The first generation PC-Fix

I was a combination between a conventional plate, which still relied on plate-to

bone compression, and an internal fixator. By tightening the screws the plate was

compressed against the bone and the conical screw head was “locked” in the

conical plate hole. The plate-screw connection, however, was not absolutely angle

stable. Furthermore, a slight angulation of the conical screw head in the plate

caused a cold welding and sometimes created problems when the plate had to be

removed.

Figure 8. The PC-Fix I (left) still relied on plate to bone compression to pretension

the screws with the conical head “locked” in the plate [116]. The later version (PC-

Fix II) is an internal fixation system in which the mixed mode of load transfer has

been eliminated in favour of screw-only transfer, made possible by locking of the

screw head into the plate.

Although the contact area in the PC-Fix I is reduced, the fixation principle is still

based on friction between the plate and the bone. This shortcoming can only be

avoided if the plate can be fixed in a relative distance to the bone with no contact

31

and therefore no friction between the plate and the bone. In order to achieve this

goal the screw and the plate need to be in a fixed angle relative to each other. The

construct looks like a plate, but the fixation principle is more a fixator, and if

positioned beneath the skin an “internal fixator”. The screws only need to be

anchored in one cortex, because the locking within the device replaces the

stabilizing effect of the second cortex. The first internal fixator for long bones was

developed by Zespol during the 70’s in Poland where square screw heads were

held in the plate with special bolts [117].

A B

Figure 9. A) The Zespol system consists of a plate (1), platform screws (2), and

nuts (3) that together form a small clamp fixator. The Zespol system enables a

surgeon to perform compression, neutralization (protecting) contact and bridging

osteosynthesis. A) The screws are inserted into the bone in a definitive distance to

each other. The platform screws have different distances to the undersurface of the

plate. B) By tightening the nuts, the screws and the plate bent with the

consequence of an interfragmentary compression [117]. B) The Zespol system is

still in use with newer materials and different plate and screw designs [118].

32

Another form of internal fixator was the Schuhli nut introduced by Mast et al. [119].

This system consists of Schuhli washer and a Schuhli nut, which lies directly under

the plate. The main body linking the locked screws consists of a standard internal

fixation plate. Their screws are held in a rigid position using a washer on the side of

the plate facing the bone. This has the effect that the screws are locked and that

the plate is elevated from the bone. After a screw is threaded through a Schuhli

nut, the screw is incorporated into the plate as a fixed-angle device.

Figure 10. The Schuhli nut. The nut engages the screw below the plate, elevating

the plate, and locking the screw at a 90 degree angle, thus preventing toggling

[119].

As a consequence of the cold welding between the conical head and the plate, the

AO group changed the connection mechanism between the screw and the plate.

The new design conical threaded screws lock in the also conical threaded plate,

and create an absolute angular stability with no need for plate to bone contact (PC-

Fix II). With this concept the screw geometry also changed: in conventional plate

osteosynthesis screws function by pressing the plate to the bone and create friction

33

between the two surfaces. Friction transfers load tangentially and screws are

therefore subjected to minimal bending. In an internal fixator the screws act like

bolts and transfer more bending load and their core is therefore thicker. The

shallow threads must resist only the pull out forces and do not produce or maintain

compression [120]. The concept of the PC-Fix II is incorporated in newer fixator

systems like the Less Invasive Stabilization System (LISS) [121,122].

Figure 11. Screws in a compression plate technique (left) are differently loaded

than screws of an internal fixator (right) [66].

The next step in the development of plates was the introduction of the Locking

Compression Plate (LCP). This system gives the surgeon intraoperative the

options of whether to use it with conventional screws, with locked screws, or with a

combination of both. This could be achieved by the development of the

combination hole of the LCP (Figure 12).

34

A B

Figure 12 A and B. With the Combihole technique of the LCP (A), the first half of

the hole comprises a Dynamic Compression Unit and is intended for a standard

cortical or cancellous screw. The threaded half of the hole is conical and permits

the locking of a special locking head screw. Another Locking plate technology

where conventional and locking screws can be combined in one plate is the

Locking Cup Technology of the NCB (B). The NCB houses round holes for 5mm

screws with a ball head. Notably without the locking nut the systems acts like a

conventional plate. Angular stability is achieved by inserting a locking cup

consisting of threads on the outside which engage with the plate, and a convex

inside which presses the screw into the plate hole and maintains screw position.

35

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49

Chapter 2 Aims and Hypotheses

of the Thesis

The overall aim of the thesis was to determine how the mechanical environment of

the fracture and implant failure can be controlled with the use of conventional

compression plates, locking plates alone or in combination with the compression

plate technique and in the treatment of fractures in different clinical situations. The

5 peer reviewed papers included in the thesis detail the biomechanical

investigations and including reviews of the relevant biomechanical and clinical

literature. Specifically, the aims of the individual results chapters of this thesis are

as follows:

- Chapter 3 investigates in vitro the construct strength of different

conventional plate lengths and number / position of the screws,

and examines if an oblique screw at the plate end can increase

fixation strength.

- Chapter 4 has sought to examine the mechanical properties of

locking plates in composite bone cylinders and how construct

stiffness and hence implant failure can be controlled using these

new implants.

50

- Chapter 5 has sought to evaluate the behaviour of compression

plate technology, the locking plate technology and the combination

of both principles for specific fractures in composite bones and

cadaveric bones.

- Chapter 6 of this thesis has analysed the use of locking plates in

cadaveric osteoporotic bone of the calcaneus to achieve more

reliable fixation than conventional plates

- Likewise, chapter 7 tested, in cadaveric osteoporotic distal radius,

the biomechanical effect of different plate positioning and numbers

on construct stiffness and load to failure.

The overall hypotheses are as follows:

- The surgeon has the ability to control the biomechanical behaviour

of implanted plating constructs during fracture fixation.

- An oblique screw at the end of a compression plate increases the

fixation strength thus allowing the omission of screws to create a

more flexible construct with enhanced callus formation.

51

- Compression plate and locking plate principles behave differently

when applied individually, but when combined, certain rules must

be followed to allow prediction of biomechanical properties.

- Locking plates in osteoporotic bone increase the fixation stability

compared to compression plates.

- In osteoporotic bone of extraarticular distal radius fractures, a

single volar locking plate is biomechanically similar to double volar

or double dorsal locking plates constructs, with less complications.

The overall findings will allow more specific clinical recommendations for the

aforementioned fractures, based on the resultant biomechanical conclusions.

52

Chapter 3

Oblique screws at the plate ends increase the fixation strength in synthetic

bone test medium.

Karl Stoffel, MD,*† Gwidon Stachowiak, PhD,† Thomas Forster, MD,‡ Andre

Gaechter, MD,‡ and Markus Kuster, MD, PhD*

* Department of Orthopaedic Surgery, Fremantle Hospital, Fremantle, Australia

; †Department of Mechanical Engineering, University of Western Australia,

Crawley, Australia

‡ Department of Orthopaedic Surgery, Kantonsspital, St. Gallen, Switzerland.

Journal of Orthopaedic Trauma, 2004, Vol 18/9, 611-617

The principles of biological plate osteosynthesis in conventional plating and the

technique of indirect reduction relate to long plates and emphasize a limited

number of screws. However, the omission of screws in conjunction with the

presence of thin cortical bone (as observed in osteoporosis) increases the risk of

screw pull out. In these cases, the fixation strength can be increased by the

application of screw reinforcement methods. Some authors found an increased

holding strength for a single screw with increased screw angulation. However, the

effect on construct fixation strength of the screw insertion angle at the plate end

has not been investigated. This study focuses on the extent at which an oblique

53

screw at the end of a plate was able to compensate for loss of construct strength

due to the omission of screws.

54

Abstract

To test the hypothesis that oblique screws at the ends of a plate provide increased

strength of fixation as compared to standard screw insertion.

This was a biomechanical laboratory study in synthetic bone test medium.

Narrow 4.5-mm stainless steel low-contoured dynamic compression plates were

anchored with cortical screws to blocks of polyurethane foam. The fixation strength

in cantilever bending (gap closing mode) and torsion was quantified using a

material testing system. Different constructs were tested to investigate the effect of

the screw orientation at the end of the plate (straight versus oblique at 30°), the

plate, and bridging length as well as the number of screws. Results: An oblique

screw at the plate end produced an increased strength of fixation in all tests;

however, the difference was more significant in shorter plates and in constructs

with no screw omission adjacent to the fracture site. Both longer plates and

increased bridging length produced a significantly stronger construct able to

withstand higher compression loads. Under torsional loading, the fixation strength

was mainly dependent on the number of screws. Conclusions: The current data

suggest that when using a conventional plating technique, plate length is the most

important factor in withstanding forces in cantilever bending. With regard to

resisting torsional load, the number of screws is the most important factor.

Furthermore, oblique screws at the ends of a plate increase fixation strength.

Key Words: oblique screw, fixation strength, plate length, bridging length, screw

number

55

Introduction

The principles of biologic plate osteosynthesis in conventional plating 1–3 and the

technique of indirect reduction 4,5 relate to long plates and emphasize a limited

number of screws. By omitting screws, the construct becomes more flexible 6,7

which enhances fracture motion and callus formation. However, in more flexible

constructs, the risk of screw pullout at the end of the plate is increased because

the greatest amount of motion between the plate and the bone occurs at the end of

the plate.8 If bone quality is good, the most efficient way to improve the fixation

strength is to increase the amount of torque applied to the screws 8,9 whereas in

the presence of thin cortical bone, as seen in osteoporosis, the risk of stripping the

threads in the bone is high 10–12. In these cases, the fixation strength can be

increased by screw reinforcement methods, such as the use of cement 13 by

inserting longer plates and/or more screws 14–18 or 2-plate constructs 19 and by the

use of fixed-angle implants, e.g., the Locking Compression Plate 20 or the Schuhli

nut 21. The holding power of screws can further be increased by the insertion of

self-drilling screws 22,23 and of screws with 4 full-length flutes. 24 Also, crystalline

hydroxyapatite-coated screws 25 and screws with an increased major-to-minor

diameter are reported to have a superior holding strength 26.Some authors 27–29

found an increase in holding strength for single screws with increased screw

angulation. However, the effect of the insertion angle of the screw at the plate end

on construct fixation strength has not been reported previously. The present study

focuses on the extent to which an oblique screw at the end of a plate was able to

compensate for loss of construct strength due to the omission of screws. Of

additional interest are the factors influencing construct failure, namely, plate length,

56

working length (distance of the first screw to the fracture site), and the number of

screws. It is hypothesized that fixation strength resulting from compression and

torsional loading increases with an angulated screw at the end of a plate. If true,

this technique could reduce the risk of construct failure and would permit the use of

shorter plates where anatomically warranted.

Materials and Methods

Homogeneous polyurethane foam (model 1522-01; Pacific Research Laboratories,

Vashon Island, WA) was selected as the synthetic cancellous bone test medium.

The density of the the polyurethane blocks was 10 pcf with the compressive and

tensile strength of 2.2 MPa and shear strength of 1.4 MPa. The blocks were

isotropic, thus eliminating variations in geometry and material properties such as

bone density and with the experimental advantage of lower variability. 30 Narrow

4.5mm stainless steel low-contoured dynamic compression plates (LC-DCP) were

anchored to uniform 40-mm blocks of foam material using 4.5mm cortical bone

screws inserted into 3.2-mm pilot holes in accordance with the surgical protocol.

The hole at the plate end was drilled at an open angle of 30° inclination in the

longitudinal axis of the plate using a specially designed drilling device to a depth of

44 mm. The remaining screws were inserted per-pendicular to the plate to a depth

of 38 mm. All screws were tightened to the same torque value of 2 Nm ± 2%,

measured with a torque-controlling device (model 320300; Warren & Brown Ltd,

Melbourne, Australia). The constructs were tested in cantilever bending in a

fracture gap-closing mode (Fig. 1) and in torsion. Eight independent repetitions

were conducted for each construct, which were defined by plate length, the

57

position of the first screw in relation to the fracture gap (bridging length), and the

number of screws, as shown in Table 1. The numbering of the plate holes started

with the hole adjacent to the plate center as hole number 1. For instance, construct

(2+4) stands for 2 screws in an 8-hole plate with 1 unoccupied plate hole adjacent

to the center of the plate, then an occupied hole, an unoccupied, and an occupied

hole at the plate end (Fig. 1). The cantilever bending tests were performed using a

servohydraulic materials testing machine (model 8501; Instron, Victoria, Australia).

For these tests, a right-angled steel plate with an identical block height was

clamped to the base to prevent the foam from collapsing under the applied load.

The foam-plate construct was clamped horizontally to the base in direct contact

with the right-angled plate. The actuator came in contact with the LC-DCP via a

ball fixed on a freely mobile horizontal adjustable rig over the first plate hole. The

specimens were then loaded in cantilever bending by displacement of the actuator

at a rate of 10 mm/second. For torsional tests, 2 blocks of foam with a gap of 1 mm

were bridged with the same constructions as for cantilever bending tests (Table 1)

and clamped horizontally in the torsional testing machine (model TOSM21;

Instron). Therefore, the center of rotation was aligned to the center of the block

with the plate–block interface at a distance of 2 cm. The actuator was rotated at a

rate of 6°/second until construct failure occurred. Both axial load/torque and

crosshead displacement history were continuously recorded at a sampling rate of

100 Hz. The maximal recorded values for bending load and torque were taken to

indicate the point of construct failure. Non parametric post hoc paired comparisons

(Wilcoxon signed rank tests) were used to test for differences in the biomechanical

properties of the constructs (Analyze-it Software, Ltd., Leeds, England). A

probability level of P < 0.05 was considered significant.

58

Results

Construct failure in cantilever bending and torsion resulted either in screw pullout

with or without prior plastic deformation of the plate or only in plastic deformation of

the plate without screw pullout. In all cases of screw pullout, failure occurred due to

shearing of a cylindrical area of the host material corresponding to the outer

diameter of the screw threads. In the case of plastic deformation, the plate bent

between the 2 innermost screw holes. Figure 2 shows typical load-displacement

curves for different plate lengths under cantilever bending loading. The foam

blocks were not broken in any of the tests.

Cantilever Bending

Plate Length, Number, and Position of the Screws

Figure 3 shows the pullout strength of the different constructs in percentage,

whereby the strength of construct (1+3) with a straight screw at the plate end was

chosen to be 100%. Longer plates, as in construct (1+6), provided significantly

more fixation strength than shorter plates (P < 0.003), even when all screw holes in

a shorter plate as in construct (1+2+3) were occupied (P < 0.028). Prior to screw

pullout, construct (1+6) and construct (1+2+6) showed plastic deformation of the

plate between the 2 innermost plate holes at a mean yield point of 268%, whereas

in the construct without screw pullout (1+5+6), an ultimate strength of 354%

without screw pullout was reached (P < 0.001). By increasing the bridging length,

the construct (2+4) became significantly stronger than construct (1+4) (P < 0.02).

59

There was no significant difference in strength between constructs (2+4) and

(1+2+4) and constructs (1+2+3) and (1+4), respectively.

Orientation of the Screw at the Plate End (Straight Versus Oblique).

An oblique screw at the plate end increased the strength in all constructs. In

general, the weaker the construct, the greater the effect of an oblique screw. The

difference was significant (p<0.05) for specimens/hole constellations (1+3),

(1+2+3), (1+4), and (1+6) but not for constructs (1+2+4; 1+2+6, 2+4).

Torsion

Longer plates provided higher fixation strengths in torsion. The difference was

significant between construct (1+6) versus construct (1+3) (P < 0.05), but not

between (1+6) and (1+4) or (1+4) versus (1+3), respectively. Three screws in

each block of foam caused significantly higher fixation strength than constructs

with only 2 screws (P < 0.026). Increasing the bridging length (1+4 versus 2+4)

led to a significant decrease in torsional strength of about 30%. Although an

oblique screw at the plate end caused an increased strength of fixation, the

difference was not significant.

Discussion

Biological fracture repair is becoming an increasingly popular means of fracture

fixation. This technique involves a reduction in the amount of surgical trauma,

thereby preserving vascular supply and soft-tissue integrity with the implantation of

less screws. The aim is to achieve a beneficial mechanical environment for fracture

60

healing in terms of a less rigid construct. However, fracture stabilization is a

balancing act between flexible fixation, which can enhance callus formation and

thus improve the healing process 31–36 and an unstable fixation, which can lead to a

nonunion and/or implant failure 37–39. The operative decision as to the length of the

plate and the number screws should be made on an individual basis along with a

decision as to the bridging length and screw angulation at the end of the plate.

Our data suggest that the plate length is the most important fact or in withstanding

forces in cantilever bending. Longer plates with an equal number of screws

required greater peak loads to failure than short plates with more screws. These

results are in agreement with other reports on synthetic models 15,18 or cadaver

bones 14,16,17. Also, by increasing the working length (e.g., the distance from the

fracture site to the first screw), the fixation strength increased significantly for both

a perpendicular and an oblique screw at the end of a plate. These findings are in

agreement with the report of Ellis et al 15, who found a decreased stress

concentration at the plate end after omitting screw holes near the fracture site. The

effect of placing a screw toward the plate end is comparable to the effect of

spacing the screws more widely to increase the strength of fixation 4,17,18. On the

other hand, omission of screws near the fracture site causes a decreased construct

stiffness both for the internal fixator and plate osteosynthesis 7,40 .Fixation strength

under torsional load increased significantly with more screws, plate length, and a

decreased working length. Field et al 7 reported that in an equine cadaver bone

model, a significant reduction in torsional strength arose from omitting screws near

the fracture site and from using shorter plates. The number of screws did not have

a significant effect in their study, although they only compared constructs with 3 or

more screws on each side of the fracture. It seems that more than 3 screws do little

61

to increase torsional strength. Toernkvist et al 18 on the other hand, reported an

increased torsional strength with increasing number of screws but no effect of plate

length. Because Toernkvist mainly tested shorter plates and reported his results as

a percentage of the fixation strength of a plate construct with screws in the holes

1,2 and 3, the results can not be compared.

This study has shown that an oblique screw at the end of the plate increased the

fixation strength in all cases in cantilever bending and torsion. This is probably due

to the increased number of threads engaged in the cortex, which leads to

progressive fixation strength as previously shown by Seebeck et al 28. Because the

plate length was the major factor in increasing fixation strength in cantilever

bending, the addition of an oblique screw in these constructs had a relatively low

additive effect. In particular, shorter plates benefitted from an oblique screw at the

end of the plate (up to 43%). The difference between 2 screws with 1 screw

oblique at the plate end and 3 screws with 1 screw perpendicular at the plate end

was not significant. Therefore, placing an oblique screw at the end of the plate is a

way of increasing fixation strength without insertion of an additional screw. Hence,

the construct becomes less stiff at the plate end, which causes a decrease in

stress shielding. This may reduce the refracture rate of the bone after plate

removal. The author’s clincical experience, which is enforced by biomechanical test

results 41 , is that the risk of screw breakage for an oblique screw (up to 40°) at the

time of insertion is low because the minimal moment that leads to screw breakage

is higher than the maximal insertion moment that was recorded during surgery.

Assessment of the pullout strength according to the formula from Seebeck et al 28

(ultimate strength = 391 N × number of threads + 6.6N/° × loading angle + 177 N)

was not practicable because the formula predicts the pullout strength of a single

62

screw and not of a construct system. We used synthetic bone materials for testing

because the absolute fixation strengths were not considered important. Only the

relative differences for each parameter tested under the same conditions were of

interest. The variability in mechanical properties of human bones has presented

obstacles to the experimental determination of factors affecting bone screw holding

strength, especially for osteoporotic bones 28. Hence, the use of synthetic materials

with more consistent properties 30,42 is an accepted method for mechanical testing

and might be the reason for the statistically significant differences in fixation

strength. Despite the synthetic testing material, these data cannot be directly

extrapolated to the clinical setting. Clearly, we only tested the effect of fixation

strength in simple transverse fractures with an increased bridging length. In

comminuted fractures where the plate between the inner-most screws is free and

unsupported, the effect of the plate length and an oblique screw is even more

obvious. We appreciate the limitations of this biomechanical study. However, in our

clinical experience using the minimally invasive plate osteosynthesis technique

with long plates, omitting 2 screws over the fracture gap in a simple transverse or

short oblique fracture and placing an oblique screw at the plate end has not led to

breakage of the screw at the plate end (Fig. 4). Screws near the fracture site

should not be inserted at an oblique angle because the shear stresses in this

region tend to be high and screw breakage has been observed (Fig. 4). In theory, it

would be possible to minimize surgical dissection further by inserting the last screw

obliquely, thus eliminating the need for full exposure of the last plate hole.

However, such surgical dissection remote from the fracture site probably has only

a very minimal biologic influence on fracture healing. one has to keep in mind that

an extensive arterial or venous damage during the surgical approach proximal to

63

the fracture site may well have an impact on fracture healing; a process also

dependend on anatomical location. We hypothesize that, for a small fracture gap,

a longer plate with increased working length and 2 to 3 screws in each main

fragment with the last screw inserted obliquely offers biomechanical advantages.

These advantages are increased plate fixation strength and construct flexibility

together with the induction of callus formation. We therefore recommend this

technique for patients with osteoporotic bone and in those cases where a shorter

plate must be used because of anatomic constraints.

Conclusions

Based on our findings in the synthetic bone medium, we suggest the insertion of

the screw at the end of the plate in 30º angulation to the longitudinal axis to

improve the fixation strength. Additionally, for bones such as the femur and tibia

that are exposed to large bending forces, long plates with a small number of

screws should be considered. Because torsional strength is mainly restricted by

the number of screws, fractures of the humerus and radius, which are exposed to

large torsional forces, should be stabilized with a plate with a high number of

screws on either side of the fracture line.

64

Figures

Figure 1. Cantilever bending for construct (2+4) in gapclosing modus. The foam-

plate construct (a) was clamped horizontally to the base with a right-angled steel

plate (b) and loaded in compression until failure.

Figure 2. Typical load-displacement curves for different plate lengths. All curves

have an initial linear portion starting at point A which ends for constructs (15) and

(16) at point B: the load at B is the yield load; y/x, the gradient of AB is the stiffness

65

value (N/mm). The curves continue and reach the ultimate tensile load only in

construct (16). Furthermore, it is shown that by increasing the plate length the load

to failure also increases. Note the lower stiffness values for shorter plates.

Figure 3. The mean load to failure in percentage of the fixation strength of

construct (1+3) in cantilever bending with the coefficient of variation for each

construct (n = 8); s *= p<0.05, ns = not significant for straight vs. oblique..

66

Figure 4. Simple distal transverse fractures of the femur (left) and the tibia (right)

were stabilized in a bridge-plating technique and both healed. The enlarged

working length over the fracture site led in both cases to a large fixation callus

without any instrumentation failure.

67

Figure 5. A comminuted diaphyseal femoral fracture with plate fixation showed

secondary fracture healing within 6 months. The 2 screws near the fracture site

were found to be broken at the radiologic control at 6 weeks after the operation.

This resulted in an unplanned increase in working length and a relative increase in

interfragmentary motion with increased callus formation. Although this fracture

healed, screws near the fracture site should not be inserted obliquely because the

shear stresses are expected to be very high.

68

Tables

Table 1. The Plate-Bone Constructs tested

Plate length Constructs Screws in Hole To investigate

the Effect of the

6-hole plate (1 + 3) 1 and 3



Plate length

8-hole plate (2 + 4) 2 and 4 Bridging length

6-hole plate (1 + 2 + 3) 1 and 2 and 3



(1 + 5 + 6) 1 and 5 and 6

Plate length

and position of screws

The screw at the plate end (bold type) was inserted either perpendicular to the

plate or at an open angle of 30° to the direction of the plate. Each Plate-Bone

construct was tested 6 times.

69

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19. Rubel IF, Kloen P, Campbell D, et al. Open reduction and internal fixation of

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74

Chapter 4

How can stability in Internal fixators be controlled? A mechanical and Finite

Element Analysis.

Karl Stoffel 1, 2, Ulrich Dieter 3, Gwidon Stachowiak 2, André Gächter 1, and Markus

S. Kuster 4

1 Department of Orthopaedic Surgery, Kantonsspital, 9007 St.Gallen, Switzerland

2 Department of Mechanical Engineering, University of Western Australia, Crawley

6009, Australia

3 Mathys Medical Ltd, 2544 Bettlach, Switzerland

4 Department of Orthopaedic Surgery, Fremantle Hospital, Fremantle 6160,

Australia

Injury 2003; Suppl. 2: 11-19

A popular option to eliminate the possibility for the screw to toggle, slide, or

dislodge is the use of internal fixators in which the screws are locked to the plate,

effectively forming a fixed angle device. The knowledge of fixation stability provided

by these new implants is limited and clarification is necessary to determine how the

mechanical stability and hence the risk of implant failure can be controlled. The aim

of this study was to investigate in vitro and by finite element analysis (FEA), the

extent to which a locked internal fixator could provide satisfactory stability for

75

clinical use on load bearing bones. Of special interest were the factors influencing

the mechanical conditions at the fracture site, the control of interfragmentary

movement and implant failure, namely: gap size, working length (distance

between the first two screws on each side of the fracture site), number and position

of the screws, plate length, distance between the plate and the bone and the

material properties.

76

Summary

New plating techniques, such as non-contact plates, have been introduced in

acknowledgement of the importance of biological factors in internal fixation.

Knowledge of the fixation stability provided by these new plates is very limited and

clarification is still necessary to determine how the mechanical stability, e.g.

fracture motion, and the risk of implant failure can best be controlled. The results of

a study based on in vitro experiments with composite bone cylinders and finite

element analysis using the Locking Compression Plate (LCP) for diaphyseal

fractures are presented and recommendations for clinical practice are given.

Several factors were shown to influence stability both in compression and torsion.

Axial stiffness and torsional rigidity was mainly influenced by the working length,

e.g. the distance of the first screw to the fracture site. By omitting one screw hole

on either side of the fracture, the construct became almost twice as flexible in both

compression and torsion. The number of screws also significantly affected the

stability, however, more than three screws per fragment did little to increase axial

stiffness; nor did four screws increase torsional rigidity. The position of the third

screw in the fragment significantly affected axial stiffness, but not torsional rigidity.

The closer an additional screw is positioned towards the fracture gap, the stiffer the

construct becomes under compression. The rigidity under torsional load was

determined by the number of screws only.

Another factor affecting construct stability was the distance of the plate to the bone.

Increasing this distance resulted in decreased construct stability. Finally, a shorter

plate with an equal number of screws caused a reduction in axial stiffness but not

in torsional rigidity.

77

Static compression tests showed that increasing the working length, e.g. omitting

the screws immediately adjacent to the fracture on both sides, significantly

diminished the load causing plastic deformation of the plate. If bone contact was

not present at the fracture site due to comminution, a greater working length also

led to earlier failure in dynamic loading tests. For simple fractures with a small

fracture gap and bone contact under dynamic load, the number of cycles until

failure was greater than one million for all tested constructs. Plate failures

invariably occurred through the DCP hole where the highest von Mises stresses

were found in the finite element analysis (FEA). This stress was reduced in

constructions with bone contact by increasing the bridging length. On the other

hand, additional screws increased the implant stress since higher loads were

needed to achieve bone contact.

Based on the present results, the following clinical recommendations can be made

for the locked internal fixator in bridging technique as part of a “minimally invasive

percutaneous osteosynthesis” (MIPO): for fractures of the lower extremity, two or

three screws on either side of the fracture should be sufficient. For fractures of the

humerus and forearm, three to four screws on either side should be used as

rotational forces predominate in these bones. In simple fractures with a small inter-

fragmentary gap, one or two holes should be omitted on each side of the fracture

to initiate spontaneous fracture healing, including the generation of callus

formations. In fractures with a large fracture gap such as comminuted fractures, we

advise placement of the innermost screws as close as practicable to the fracture.

Furthermore, the distance between the plate and the bone ought to be kept small

and long plates should be used to provide sufficient axial stiffness.

78

Keywords: Locking Compression Plate, biomechanical investigation, finite

element analysis, fracture fixation

Introduction

Plate osteosynthesis with rigid fixation (e.g. the use of interfragmentary

compression and lag screws) has shown a high complication rate including

delayed or non-union, infection, hardware failure and refracture after plate removal

21-28. With the use of more dynamic osteosynthesis techniques, such as external

fixation and intramedullary nailing, the complication rate has been reduced 35,106.

Hence, for fractures of the diaphyseal tibia, intramedullary nailing has become the

gold standard at many centres.

However, recent studies have shown that more than 75% of fractured tibiae heal in

malposition after intramedullary nailing 123. Therefore, biological plate

osteosynthesis [20,21,40] and new plate developments such as internal fixators

have become an interesting option for treating metaphyseal and diaphyseal

fractures. One of the early developments of an internal fixator can be seen in the

form of the wave bridging plate as popularised by B.G. Weber 38. The wave of the

plate lifts the plate away from the bone near the fracture and allows better blood

supply as well as the possibility of cancellous or cortical bone grafts.

Newer innovative forms of internal fixators were the so-called ’Shuhlis‘ from Mast

124, the non-contact internal fixator 95, the Zespol plate 94, the PC Fix 125,126, the

LISS 127 and the Locking Compression Plate (LCP, Mathys Inc, Bettlach,

Switzerland) 128. The optimal parameters required to influence fracture healing by

79

altering the mechanical environment are established for the external fixator

81,108,109,112. No recommendations exist in the current literature for the control of

construct stability or how the risk of implant failure can be reduced for internal

fixators such as the LCP.

The aim of the present study was to investigate in vitro and by finite element

analysis (FEA), the extent to which a non-contact plate such as the LCP, acting as

an internal fixator, could provide satisfactory stability for clinical use on load-

bearing bones. Of special interest were the factors influencing the mechanical

conditions at the fracture site, the control of interfragmentary movement and

implant failure, namely: gap size, working length (distance between the first two

screws on each side of the fracture site), number and position of the screws, plate

length, distance between the plate and the bone and the material properties. These

principles have not yet been established and clear definitions would allow surgeons

to choose a construct with ’sufficient‘ stability for a specific injury, while minimising

dissection of the surrounding soft tissue and possibly reducing the number of

complications.


Biomechanical tests

56 homogeneous composite cylinders representing bone and made of epoxy

reinforced glass fibres filled with rigid polyurethane foam (length 250 mm, outer

diameter 35 mm, cortical thickness 2.5 mm; model 3003-4; Pacific Research

80

Laboratories, Vashon Island, WA, USA; Figure 9) were used for mechanical

testing. The cylinders were uniform, which eliminated variation in geometry and

material properties such as bone density.

All tests were performed at room temperature. The cylinders were cut in half with a

band saw. A 1 and a 6 mm gap were bridged with eight or twelve-hole 4.5 mm

titanium LCPs, respectively, at a distance of either 2 or 6 mm from the bone. The

different positions and numbers of screws are shown in Figure 1. Standard AO

technique was used 129. Only monocortical, self-tapping and self-cutting, locking

head screws (18 mm) were used. The proximal and distal ends of the composite

cylinders were mounted perpendicular to the longitudinal axis in a metal fixture at a

depth of 0.8 cm. The torque applied to each screw was standardized to 4 Nm ± 2%

using a torque-controlling device (model 320300; Warren & Brown Ltd, Melbourne,

Australia).

Axial stiffness and torsional rigidity

These static tests were determined for each intact and stabilized bone cylinder.

Axial compression was achieved using a dynamic servo-hydraulic universal

material testing machine (model 8501; Instron, Victoria, Australia). The test

apparatus was constructed as a universal joint system to allow free rotation in all

six degrees of freedom. This was achieved by placing a hip joint implant between

the metal fixture and the actuator at both the proximal and distal ends. Silicon

grease was used as a lubricant for the hip implant. The compression load was

applied from 20 N to 200 N at 20 N/sec. Force and displacement histories were

recorded at a sampling rate of 100 Hz, and the mean slope (stiffness) was

obtained from the curve. Six data sets were recorded for each test set-up following

81

the achievement of steady-state hysteresis whereby the kind of composite bone

specific pre-conditioning was not recorded.

During torsion tests, the tibia was held horizontally using a custom-made jig as

previously described by Karnezis et al. 106 and the rigidity was calculated as shown

in Figure 2. The load was gradually applied in steps of 5 N until a torque of 5 Nm

was reached (load controlled test setup). Four data sets were taken for each

sample.

Fatigue test

Static and dynamic loading tests under axial compression were performed to

evaluate the fatigue strength in relation to the gap size (2 or 6 mm) and working

length (6-1, 6-2, 6-3) with the plate 2 mm from the bone. For these tests, the

specimens were positioned in the Instron as described above for axial

compression. For the static loading tests, load was applied under load control until

failure occurred by reaching the yield point, which was defined as the force

required to induce plastic deformation 106.

For dynamic tests, cyclic loads were applied in load control. The loading system

employed pneumatic drivers to apply a sinusoidal axial load ranging from 23 to 230

N at a rate of 5 Hz. The screw torque was measured every 100,000 cycles using

the same torque-controlling device as described above. Fatigue failure was defined

as a plate or screw breakage or a loosening of the screws in the plate or bone.

Tests were abandoned after 1 million cycles if the construct had not already failed.

Finite element analysis

82

Linear FEA was performed for the different screw configurations on a 3D model of

the LCP attached to a cylinder. For each screw configuration the solid models of

the internal fixator and cylinder were transformed into FE models using Scenario

Software integrated within Unigraphics v17 (Plano, Texas, USA) with solid

parabolic linear-elastic finite elements. The model represented the mechanical

laboratory that was set up to test the plate in bending by applying the same axial

forces and boundary conditions as in the mechanical tests. The threads of the

fixed-angle screws and LCP holes were not considered because it was assumed

they would have little influence on the results. This way, mesh-mating conditions

were established between the solid models of the screws without threads and the

drilled holes in the cylinders, thus, making coincident nodes at corticalis and

spongiosa, simulating a perfect anchorage of the screws in the bone. The implant

material was pure titanium represented in the FE model by its Young's modulus of

115 GPa and a Poisson's ratio of 0.34 and stainless steel with an E-modulus of

220 GPa and a Poisson’s ratio identical to titanium. The tibia was characterized by

distinguishing between cortical bone and trabecular bone. The cortical bone was

modelled with a Young’s modulus of 17 GPa and a Poisson’s ratio of 0.3 130,131,

whereas the cancellous bone was modelled with a Young’s modulus of 700 Mpa,

and a Poisson’s ratio of 0.2 132. The stiffness under axial compression as well as

the local stress in the plate was calculated for the same test configurations as in

the in vitro experiments. Since titanium is a ductile material, von Mises stresses

were calculated for comparison with the uniaxial yield strength of the material. The

fracture gap was also chosen to be 1 and 6 mm with a plate-bone distance of 2

mm. Since meta-diaphyseal tibial fractures are often the result of direct trauma to

83

the tibia, e.g. car bumper on the tibia of a pedestrian 104 and frequently associated

with a fibular fracture, the additional support of the fibula was neglected.

For each mechanical test performed, the mean and standard deviations were

calculated. The Shapiro-Wilk test for normality indicated the data did not follow a

normal distribution. Therefore, non-parametric statistical testing (Wilcoxon signed-

ranks test; Analyse-it Software, Ltd., Leeds, England) was used to test for

significance between construct stability of different working lengths, the number of

screws, the distance from the plate to the bone and the plate length. A probability

level of p<0.05 was considered significant.

Results

Axial stiffness and torsional rigidity

The mean axial stiffness and torsional rigidities of the intact composite bone

cylinders without the fixation device were 7.47 (SD, 0.53) kN/mm and 3.47 (SD,

0.38) Nm2/deg, respectively. The results obtained during axial and torsional loading

after osteotomy and osteosynthesis are shown in Figures 3-5. The mean axial

stiffness and torsional rigidity diminished by more than 90% of the intact bone

cylinder. Working length was shown to be the most important factor affecting axial

stiffness and torsional rigidity.

On omission of one screw hole near the fracture site, axial stiffness and torsional

rigidity decreased significantly by 64% and 36%, respectively. For both axial

stiffness and torsional rigidity, every further unoccupied screw hole decreased the

stability by about 10% (Fig. 3).

84

The number of screws had more influence for a short than a long working length.

Furthermore, more than three screws on either side of the fracture did little to

increase axial stiffness; nor did four screws increase torsional rigidity (Fig 3). As

can be seen in Figure 4, the position of a third screw on either side of the fracture

significantly influenced axial stiffness. The closer this screw was positioned

towards the fracture site, the stiffer the construct for axial compression. Torsional

rigidity, however, was unaffected by the position of the middle screw (Fig. 4).

Increasing the distance from the plate to the bone and a shorter plate also resulted

in a decreased axial stiffness and torsional rigidity as shown in Figure 5.

Furthermore, the influence of a larger plate-bone distance was less marked for

larger working lengths.

Fatigue tests

During the static tests, the yield point strongly depended on the working length.

Yield occurred at 520 N (SD ± 14) for the smallest working length (6-1), at 350 N

(SD ( 28) for one omitted screw hole near the fracture site (6-2) and at 300 N (SD

( 28) for two empty holes on either side of the fracture (6-3). Due to the use of

composite bone cylinders, the variation of the results remained low.

The results of the dynamic sinusoidal loading tests are presented in Figure 6. With

a 1 mm fracture gap no construct failed after one million cycles, independent of the

working length. If, however, the fracture gap was increased to 6 mm, as occurs in

comminuted fractures, only the shortest working length (e.g. 6-1) lasted one

million cycles. If one or even two screws on each side of the fracture were omitted,

the number of fatigue cycles to failure was significantly reduced to fewer than

85

500,000 cycles. All failures in axial loading resulted in plate failure rather than

screw loosening/breakage or pullout. Since the screws showed no macroscopic

damage they were not further investigated in a metallurgic sense or with electron

microscopy. Plate failure occurred in eleven out of twelve cases through the

dynamic compression unit hole, and in one case obliquely through the dynamic

compression unit hole and the locking screw hole (screws in positions 6-3). This

was also the case with the lowest cycles to failure. In one case, with the screws in

position 6-3, plate failure was observed in three of four unoccupied DCP holes.

Finite Element Analysis

In the FEA, values found for axial stiffness compared well to the mechanical tests.

The discrepancy in the results between the two methods was below 2% for the

different working lengths and increased to 8% with additional screws. Figures 7

and 8 show the maximal von Mises stress in the plate and screws for different

bridging lengths. The maximum stress concentrations were found next to the

compression unit of the combihole and in the innermost screws at the shaft-head

junction.

For an interfragmentary gap of 1 mm the von Misses stress in the plate and the two

innermost screws decreased when the bridging length was increased because

bone contact occurred under load conditions. Leaving one plate hole (6-2)

unoccupied on each side of the fracture, reduced the stress in the plate by 10%

and in the screw by 63% while two empty holes (6-3) caused a stress reduction in

the plate of 45% and in the screw of 78%. For an interfragmentary gap of 6 mm,

86

the von Misses stress in the plate increased by 133% if one hole on each side of

the fracture was left empty and remained unchanged if the bridging length was

further increased.

More than two screws in each fragment caused increased stress at the plate for

small bridging lengths and small fracture gap sizes. For larger gap sizes, an

additional screw placed closer to the fracture gap was able to reduce the peak

stress at the innermost screw. Furthermore, axial stiffness was approximately twice

as high for a stainless steel plate than for a titanium plate. However, the maximum

stress in the plate and screws remained the same for both the stainless steel and

the titanium plate if the interfragmentary gap size was 6 mm, but was twice as high

for the stainless steel plate if the gap size was only 1 mm.

Discussion

In the past, the appearance of callus in plate osteosynthesis was assumed to

indicate a lack of stable fixation. Today, indirect healing with callus formation is no

longer regarded as a disturbance to healing but as a goal in itself and a welcome

sign of a prompt and positive bone reaction. However, in the stabilisation of

fractures of long bones there is a fine line between flexible fixation, which

enhances callus formation and improves the healing process, and an unstable

fixation, which leads to non-union and/or implant failure.

When selecting an internal fixator for plate osteosynthesis, the main problem is to

determine how the mechanical environment of the fracture and implant failure can

87

be controlled. The first clinical results with internal fixators were promising

94,95,127,133,134, although determining the number and positions of screws and the

plate length was mainly based on clinical experience with conventional plates as

described in numerous studies 105,114,115,117-120,135. The present study primarily

focused on understanding the control mechanisms of stability and fatigue failure for

internal fixators such as the LCP for fractures with different gap sizes.

The working length had the most important effect on construct stability. By omitting

one screw hole on either side of the fracture, the construct became more flexible in

both compression (by about 60%) and torsion (by about 30%). These results are

in agreement with those of biomechanical investigations for conventional plating

techniques 136. As in conventional plating more than three screws on each side of

the fracture did little to increase the axial stiffness of the system 119,122,135. In

contrast to conventional plating where more widely spaced screws lead to an

increased strength of fixation 118,137,138, the stiffness under axial load decreased for

the LCP by placing additional screws towards the plate ends. Rigidity under

torsional load increased significantly with an increasing number of screws for up to

four screws per fragment and was independent of the position of these additional

screws.

The distance from the plate to the bone also affected construct stability. By

increasing this distance from 2 mm to 6 mm, both torsional rigidity and axial

stiffness decreased by as much as 10-15%. As shown by Kowalski et al. 139, the

longer, unsupported free part of the screw between the plate and the bone allowed

greater construct deformation under torque whereas in compression the lever arm,

and hence the bending moment, increased. In contrast to the tests in rotation,

where a longer plate with equal number of screws did not show a significant

88

advantage 118, axial stiffness decreased for shorter plates, as has already been

shown for conventional plating 115,117,118.

In the finite element calculations under pure axial compression, axial stiffness

values were found to be similar to those derived from the in vitro tests. These

differences were comparable to those reported by Duda et al. 140: Under axial

compression the plate is not straight but bent, especially when the bridging length

is increased. Because of this extensive deformation, the direction of the load

changes during the loading process. This phenomenon is a non-linear elastic

effect. For the FEA, a linear-elastic solver was used, thus, this non-linear effect

was not taken into account. It was concluded that the FEA of the bone-plate

construct resembled the in vitro behaviour under axial compression directly

postoperatively and could be used to calculate those stresses in the plate and the

screws that appear in the early stages of fracture healing.

In the FEA, we demonstrated the maximum von Mises stress in the plate when no

bone contact occurred and with screws placed away from the gap as also shown

for a broad dynamic compression plate 135. If the system was flexible enough

(longer bridging length) and bone contact occurred, the stress in the plate was

reduced by between 50% and 85% (Figure 7) since less force was needed to

achieve interfragmentary contact and this was reflected in lower peak stress

magnitudes.

On the other hand, Duda et al. 140 reported a considerably lower von Mises stress

in the LISS plate (Less Invasive Stabilization System) but with a large working

length. However, in this investigation not only was the working length increased but

also the plate length. Since we have been able to show that a larger plate caused a

significant stress reduction under axial load, it is more likely that the reported stress

89

reduction in the plate was due to the increased plate length rather than the

additional working length.

In contrast to fractures under interfragmentary compression where the greatest

concentration of applied force occurs for the screws at the end of the plate 135,141, in

bridging plating technique the FEA showed the highest stress concentrations for

the screws close to the fracture gap. The stress in the screw can be reduced if the

fragments can be adapted for contact between the fracture surfaces during

dynamic loading. In this case, increasing the bridging length can further reduce the

stress on the plate and the screws and hence improve fatigue failure. If the fracture

gap, however, is large as is the case in comminuted fractures, no bone contact and

load sharing can occur during dynamic loading and the stress within the screw and

the plate increases with increasing working length. In this case, additional screws

should help to decrease the stress on the screws near the fracture site.

The threads of the screws were not taken into account in the LCP 3D model

because of an assumed negligible effect on the bending stiffness of the system.

However, the threads could cause high stress concentrations due to their relatively

sharp angles and, under cyclic loading, the combination of these sharp angles and

high stress concentrations could facilitate the appearance and growth of fissures,

leading to system failure in these areas.

Hardware failure (plate failure, or screw breakage) is a complication that has been

reported to occur in as many as 7% of plate fixations 24, although clinical

experience with non-contact fixation has shown that loosening of the implant by

bone resorption in the area of the screw-bone interface is the most frequent

complication 136. During the fatigue tests we did not observe any screw loosening.

This might be due to the relatively strong epoxy material representing cortical

90

bone, which remained the same throughout all the test phases. In the clinic, bone

density reduction occurs that is caused by demineralisation under partial load

bearing 24 and due to stress shielding of the implant 24. Plate failure occurred in

eleven of twelve plates through the DCP hole and in one case obliquely through

the DCP / LCP-hole. In the latter case, the number of cycles was the lowest of all

for the tests performed. Plate failure through the DCP hole is explained by the fact

that the plate cross-section is smallest in this area 128 and thus the highest stress

concentrations occur. The risk of implant failure can be reduced if the

interfragmentary gap can be kept small and the fixation system is flexible enough

to allow bone contact to occur opposite the plate under load conditions.

There are a number of issues that were not addressed in the current study. The

results of this study are based on mechanical tests and FEA. Biomechanical

results cannot be directly extrapolated to the clinical setting as the composite

cylinders used in the current study cannot replace real bone and the in vivo

situation e.g. soft tissue attachment, local temperature is far more complex. The

tests conducted on homogeneous cylinders exclude the high variation in geometry

and quality of real bone, thus increasing the reproducibility of the results. In

addition, the applied load used in this model may not represent the multifaceted

manner of loading that occurs in humans. During the torsional testing a vertical

beam was meant to prevent any axial bending. However, possible compressive

force with this setup would have provided a more complex state of stress with

influence on the test results. During fatigue tests, a combined load regime (axial

compression and torsion) acting on the plate is likely to decrease the life of the

fixator. Lower limb biomechanics, however, reveal that forces at the fracture site

are predominantly compressive during two-legged stance, while transverse and

91

torsional contributions to load bearing are relatively low. This might be different if

the plate is not in line with the mechanical axis and if an additional torque is applied

to the plate.

The current investigation offers a parametric evaluation of a potential mechanism

underlying the mechanical properties of the internal fixator. Some of the constructs

remain to be tested on bone, in particular, investigations should be directed

towards the fixation of fractures in osteoporotic bone, where plate pullout is a

common mechanism of failure 94, partly caused by the poor holding power of the

screws 135 and bone resorption at the screw-bone interface.

For the clinical use of the LCP as a locked internal fixator in bridging plate

technique and MIPO, we recommend two or three screws on either side of the

fracture for femoral and tibial fractures, which are mainly loaded in compression.

The position of the first screw near the fracture and the additional screw depends

on the fracture gap size. In simple fractures with an interfragmentary gap smaller

than 2 mm, one or even two plate holes near the fracture gap should be omitted to

allow fracture motion and bone contact to occur. For comminuted fractures, we

recommend three screws on either side of the fragment with two screws as close

as practicable to the fracture site. In plate osteosynthesis of the humerus and the

forearm, where mainly torsional load predominates, three to four screws in each

main fragment are recommended, as torsional rigidity depends more on the

number of screws than axial stiffness. Three screws can be placed as described

above with the fourth screw in any position. If the plate must be placed at a

distance from the bone for anatomical reasons, the screws should be positioned

closer to the fracture site to improve construct stability.

92

Internal fixators provide a number of biological and many technical advantages in

comparison to existing fixation methods. Further laboratory investigations into the

biomechanical parameters, particularly of metaphyseal fractures with joint

participation, are needed and the effects of combining both the compression and

locked screw principles should be addressed.

93

Figures

Fig. 1. Schematic representation of the pattern bridging length and number of

screws shown for the 12 hole LC plate.

Bridging length

Number of screws

654321

6 21

6 1

6 43

6543

6 3

65

94

Fig. 2. To measure torsional rigidity, dhe distal end (de) of the bone was rigidly

fixed in rig, the proximal end was supported vertically (g) to prevent axial bending

and a ball bearing (*) allowed free rotation. The torsional rigidity (GJ, expressed in

Nm2/deg) is given by a formula: GJ=TּL / φ, where T is the applied torque (Nm)

and φ the angle of twist. The applied torque was calculated by T = F ּ e, where F is

the applied load (N) and e the horizontal deflection at a distance A from the axis of

rotation (m). The angle of twist φ (calculated in grad) was calculated as the

inverse function of sin (b/c), where b is the vertical deflection (as measured using

a dial gauge) at a distance c from the axis of rotation.

F

eb φc

l

A*

de

g

95

Axial Stiffness Torsional Rigidity

0

50

100

150

200

61 621

6321

6432

165

4321

62 632

63 64 65 61 621

6321

6432

165

4321

62 632

63 64 65

N/m

m

0,00

0,10

0,20

0,30

0,40

0,50

Nm

2 /deg

Fig. 3. The mean axial stiffness and torsional rigidity with standard deviation as a

function of bridging length and number of screws for a 12 hole titanium LCP with a

distance from the plate to the bone of 2 mm. p-values axial stiffness: 1 < 0.001; 2 <

0.001; 3 = 0.011; p values torsional rigidity: 4 = 0.008; 5 = 0.01; 6 < 0.001; 7 =

0.011; 8 = 0.008; 9 = 0.014.

1 2

3

4 6

7 8

9 5

96


0

50

100

150

200

621

631

641

651

61 621

631

641

651

61

N/m

m

0,00

0,10

0,20

0,30

0,40

0,50

Nm

2 /deg

Fig. 4. The mean axial stiffness and torsional rigidity with standard deviation as a

function of screw position for a 12 hole titanium LCP with a distance from the plate

to the bone of 2 mm; p-values: 1= 0.004; 2 < 0.001; 3 < 0.001.

1 2

3

97


0

40

80

120

6 1 1 6 4 1 1 4 6 1 1 6 4 1 1 4

N/m

m

0,00

0,08

0,16

0,24

Nm

2 /deg

2 mm 6 mmDistance plate-bone

Fig.5. The mean axial stiffness and torsional rigidity with standard deviation as a

function of plate length and of the distance of the plate to the bone. p-values axial

stiffness: 1= 0.015; 2 = 0.011; 3 = 0.008; 4 = 0.002; p values torsional rigidity: 5 =

not significant, 6 =0.038; 7 = 0.011; 8 = 0.008.

1 2

3 4

5 6

7 8

98

6116 6226 6336

Cyc

les

to fa

ilure

1 mm gap6 mm gapx 105

10

5

1

Fig. 6. The mean number of cycles with standard deviation under dynamic

sinusoidal loading tests as a function of bridging length and gap size for a 12 hole

plate with a distance of the plate to the bone of 2mm; p-values: 1 < 0.001; 2 =

0.043.

12

99

6116, 6mm gap

6116, 1mm gap

6226, 6mm gap

6226, 1mm gap

6336, 6mm gap

6336, 1mm gap

Fig. 7. The maximal von Mises stress in the plate as a function of the bridging

length and the interfragmentary gap size.

6116, 6mm gap

6116, 1mm gap

6226, 6mm gap

6226, 1mm gap

6336, 6mm gap

6336, 1mm gap

Fig. 8. The mean maximal von Mises stress in the screw as a function of the

bridging length and the interfragmentary gap size.

100

Figure 9. Composite cylinders made of epoxy reinforced glass fibres filled with rigid

polyurethane foam (length 250 mm, outer diameter 35 mm, cortical thickness 2.5

mm)

101

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47. Wagner M, Frigg R. Locking Compression Plate (LCP): Ein neuer AO-

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Chapter 5

Biomechanical considerations in plate osteosynthesis: the effect of plate to

bone compression with and without angular screw stability.

Karl Stoffel MD a, Kai Lorenz MD a,b, Markus S Kuster MD, PhD a,b

a Fremantle Orthopaedic Unit, University of Western Australia, Fremantle 6160,

Australia

b Orthopaedic Department, Kantonsspital St.Gallen, St.Gallen 9007, Switzerland

Submitted Revised Version to Journal of Orthopaedic Trauma

As a consequence a new implant has been designed which combines the features

of a conventional plate with plate to bone compression, of an internal fixator with

angular stability, and additionally the possibility of polyaxial screw placement.

There are no recommendations in the current literature on the issue when

compression or angular stability is indicated. The combination of the two methods

brings with it the risk of incorrect handling. Cases of secondary dislocation of a

load-bearing articular fragment in a lateral tibia plateau fracture treated by LCP

plate were reported. All proximal screws were inserted in the locking mode. Hence,

compression of the intra-articular lateral fragment was minimal and the fragment

dislocated secondarily. The effects of plate to bone compression on stability are

extremely important in osteoporotic bones and to be further investigated. It is the

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purpose of the proposed investigation to establish recommendations regarding

compression and angular stability for diaphyseal and metaphyseal fractures.

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Abstract

Objective: We compared the biomechanical stability of bone-plate constructs using

a compression plate (CP), an internal fixator (IF), and a plate with the combination

the two fixation principles (CP/IF).

Methods: Standardized simulated shaft fractures with a segmental defect in

composite bones (n=60), and intraarticular distal femur fractures with a

comminuted supracondylar zone in fresh frozen paired cadaveric femurs (n=36)

were stabilized by CP, IF, and CP/IF. Construct stiffness, plastic deformation, and

fixation failure strength were measured under axial compression and torsion using

a bi-axial testing machine.

Results: The experimental results indicate that IF can retain better reduction under

axial load compared to CP in both fracture models. Under torsion, the CP shows

significantly (P<0.05) increased stiffness (CP PAP 144% of CP/IF NCB in shaft

fracture model) compared to the IF. The combination of the two fixation principles

(CP/IF) results in a lower plastic deformation under axial compression and torsion,

together with a higher load to failure (CP/IF NCB 245% fail load of CP PAP).

Conclusions: The combination of CP and IF technology in one plate shows a

biomechanical advantage in long bone intraarticular fractures with multifragmentary

extension into the diaphysis, when exposed to high compressional and torsional

load. Anatomical reduction with interfragmentary compression of the articular

component, and plate to bone compression with secondary IF locking screws,

increases primary stability in terms of compression and torsion. Bridging of the

reconstructed joint block to the diaphysis using the plate as an IF helps to retain

reduction.

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Key words: Biomechanics, Compression Plate, Internal Fixator, NCB, PAP, LISS

Introduction

The Locking Compression Plate (LCP) and the Non-Contact Bridging (NCB) plate

are new technologies that give the surgeon the option of using the device as a

compression plate (CP), an internal fixator (IF), or combining both techniques

(CP/IF). In CP techniques, the stability of the bone-plate construct is a result of

friction generated between the plate and the bone, a factor mainly dependent upon

the torque a screw can be tightened without damaging the bony thread. When

using an IF, stability is determined by the angular conformity of the plate-screw

interface, and is partially independent of bone quality. Many surgeons are

uncertain as how to apply these variable biomechanical principles to the clinical

setting, with no recommendations existing in the current literature. As commented

by Frigg, there is “a need for further laboratory investigations … in order to address

the question of when each method (e.g. angular stability or compression) should

be applied?” 142. Hence, various recent biomechanical tests have been conducted

into the behavior of CP 41,42 , IF 143,144 and the comparison of CP versus IF for

different fracture scenarios 145-148.

In simple shaft fractures with good bone quality and no bony defect, no differences

in biomechanical testing 145,146 or clinical outcomes 148-150 exist between the CP and

the IF. However, when using a lateral CP for fixation of a distal comminuted femur

fracture the entire load is transferred by the implant from the proximal to the distal

fragment. In this case, the risk of a loss of reduction with a progressive collapse of

the distal fragment into a varus position is as high as 50% 151-155. Such studies

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highlight a lack of specific selection criteria for assessing the most indicative plate

system for differential fracture diagnoses; an issue heavily reliant on preclinical

comparative plate fixation research.

To the best of our knowledge, no study has addressed the issue of which

combination of fixation method is most suitable with differential fracture localization

(shaft vs. metaphyseal area of long bones). A pure CP, an IF, and a CP with the

option to lock the screws (CP/IF) were tested for construct stiffness, plastic

deformation following cyclic loading, and fixation strength under axial compression

and torsion. Specifically, the current study was designed to investigate the

comparative biomechanical effect of these three fracture fixation methods in

composite shaft fractures and cadaveric human distal intraarticular femur fractures

with supracondylar comminution. The knowledge gained through these tests may

prove useful in formulating recommendations regarding suitable combinations of

compression and/or angular stability plating systems to optimize stability in

diaphyseal and intra-articular fractures.


Biomechanical testing was conducted on composite and cadaveric bones in two

fracture models. All testing was conducted under strict regulations outlined by the

University of Western Australia Research Ethics Committee (C230406).

The following plates were tested to evaluate the three aforementioned fixation

principles: a CP - the Periarticular compression Plate (PAP, Stainless steel,

Zimmer, Warsaw, IN, USA) and the Non-Contact Bridging plate (NCB, Titanium,

Zimmer Inc, Warsaw, IN, USA); an IF - the NCB and the Less Invasive Stabilization

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System (LISS, Titanium Alloy, Synthes, Bettlach, Switzerland) and as the

combination CP/IF - the NCB (Figure 1). The NCB plate consists of an

anatomically contoured condylar buttress plate into which polyaxial, self-tapping

cortical or cancellous screws are inserted. The inserted screws can also be locked

in position using locking cups (Figure 1 insert) to allow the plate to act like an IF,

therefore combining the design of a CP with the features of an IF. The different

testing configurations are shown in Table 1.

The first test setup evaluated the fixation characteristics of the three different

biomechanical fracture fixation principles in a comminuted composite shaft fracture

model. Sixty homogeneous composite cylinders representing bone, and made of

epoxy reinforced glass fibers, were filled with polyurethane foam (length 250mm,

outer diameter 35 mm, cortical thickness 2.5 mm, model 3003-4, Pacific Research

Laboratories, Vashon Island, WA, USA). The cylinders were uniform, therefore

eliminating variation in geometry and material properties such as bone density. The

material properties of the cylinders are similar to third generation composite femurs

156. The cylinders were cut in half to produce an 8mm gap between the two tubes

(to simulate a loss of bone contact opposite the plate), which was bridged with

either a CP, an IF, or with the fixation principle of both plates (CP/IF). In the latter

mode all four screws were locked. For the shaft fracture test, all plates had their

distal component removed so that only the straight part of the plate with 6 screw

holes remained. The proximal and distal plate fixation was provided by two

bicortical screws in each fragment whereby one screw was inserted at the plate

end and the other screw close to the fracture gap. The construct was oriented in

line with the loading axis of the bi-axial testing machine (Zwick Z010, Zwick Inc.,

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Ulm, Germany), and fixed at both sides in a custom-made device using dental

cement (Glastone 3000, Die Stone, Type V, Densply, Perth, AU). The proximal rig

was connected to a universal joint. Load was then applied to the distal rig which

was fixed to the base plate of the material testing machine (Figure 3A), and

controlled by the manufacturer’s software (test-expert-Software; Zwick Inc., Ulm,

Germany). Each configuration in each loading mode was tested 6 times with each

time new implants and new test bones.

The second test simulated the clinical situation of a distal intraarticular femur

fracture with a comminuted fracture zone in the metaphyseal area. Eighteen pairs

of fresh-frozen cadaveric distal femurs were retrieved (median age 72±8.1;

10F:8M). Biplanar radiographs were taken to exclude any pre-existing osseous

disease or trauma. The specimens were wrapped in saline soaked cloth, and

stored at -30º C in sealed plastic bags. The bone mineral density (BMD) of the

cadaveric femoral necks was determined by dual-energy X-ray absorptiometry

scans using a Hologic QDR-4500A densitometer (Hologic Inc, Waltham, MA,

USA). The 18 pairs of femurs were divided into three series of six pairs to give

each series a nearly equal and evenly distributed BMD. The NCB was then paired

to the LISS plate, and randomization to the right side was done by drawing lots

from a randomization envelope. The PAP was tested in six pairs on the right and

on the left side of the same cadaver. The contra lateral femur within each pair for

each series was not significantly different in BMD. A simple T-shaped distal

intraarticular femur fracture with a comminuted fracture zone of 20 mm located 55

mm proximal to the most distal point of the lateral condyle (AO classification 33-

C13) was created (Figure 3B). Proximal fixation was achieved by using 4 bicortical

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screws for each plate. The distal bone fragment was fixed by the insertion of 5

screws. Unicortical partially threaded cancellous screws were used in the NCB and

PAP, and unicortical locking screws in the LISS plate. All inserted screws were 10

mm less in length than the corresponding drill hole. To achieve interfragmentary

compression using the LISS plate, two partially threaded cancellous screws were

first placed on either side of the plate. All screws were inserted according to the

manufacturer guidelines and were tightened with 4 Nm. The locking cups in the

NCB were tightened with 6 Nm. The load was applied to the top of the femoral

fragment through a lever arm attached to a universal joint to allow for movement,

simulating physiological loading conditions. The load axis was aligned through the

medial condyle such that the ratio of medial to lateral was 1:2. Only the medial

condyle was fixed in dental cement (Glastone 3000, Die Stone, Type V, Densply,

Perth, AU). This simulated the worst case scenario in a varus knee, where the load

is mainly transferred through the medial compartment.

To measure and simulate plastic deformation during the postoperative period of

fracture healing, cyclic tests were performed. Cyclic tests where conducted under

compression and torsion loading in both the shaft and the intra-articular fracture

model (for each configuration n=6). The loads where defined by a step-wise

function over a 40 cycle period. Specimens were preloaded with a 100 N axial load

and 0.5 Nm torque. Axial loading started from 1000 N, increasing 30 N with each

cycle increment, applied at a rate of 40 N/s (load control). Torsional loading started

from 1 Nm and was incremented by 0.1 Nm every cycle at a rate of 1.9 °/min

(torque control). The plastic deformation opposite the plate (at the level of the

osteotomy defect under compression load) was measured with a manual external

clip-on extensometer (Zwick Inc., Ulm, Germany), attached by two 2 mm diameter

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Kirschner wires positioned 2 mm above and below the osteotomy. The angle

formed under torsion was measured with the lineal variable differential transformer

on the bi-axial testing machine.

Failure tests where also conducted on the previously cyclical loaded constructs in

both torsion and axial compression (i.e. torsion and axial compression fatigued to

failure) for each fracture model and relative fixation system (n=6). The load to

failure under axial compression was recorded at an actuator speed rate of 200 N/s,

with a torque to failure rate of 2º/s. The tests were stopped after observable failure.

The point of failure was defined when the yield point on the load/torque-

displacement/angulation curve was reached, or after a sudden change of the curve

due to loss of fixation (fracture at points of fixation or breakage of the implant). A

one percent axial yield strength was calculated as the intersection of the one

percent strain offset line from the stiffness linear regression with the deformation

curve by using the linear interpolation between measured values 147. From the

linear section of the load/torque-displacement/angulation curve, the stiffness was

calculated for each configuration. In the case of plastic deformation, the origin of

deformation (plate, screw, cylinder, or fixation) was also documented. Both the

proximal and distal test setup and the testing procedure was similar to previous

biomechanical studies testing the implant stability of distal femoral fractures

143,147,157,158.

All data was collected via the testXpert software used by the Zwick testing

machine. All subsequent data analysis and force/torque-displacement/angulation

curve plotting was achieved with Microsoft Excel (Microsoft, Seattle, WA)

software. Data were analyzed using Analyse-it statistical software (Analyse-it

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Software, Ltd., Leeds, England). Univariant descriptive statistics were used to

determine means, and standard deviations. A multivariant linear model was used

to test for differences in age, bone mineral density, and gender across the three

aforementioned fixation constructs. Nonparametric paired comparisons (Wilcoxon

signed rank tests) were used to test for differences in the biomechanical properties

of the constructs. A rank correlation of the average paired BMD was examined to

determine whether differences between the groups were dependent on BMD. For

all analyses, statistical significance was determined as P<0.05.

Results

Firstly, we assessed the three different fixation principles in a composite bone

model of comminuted shaft fracture. Figure 3 shows a comparison of the stiffness,

plastic deformation, and load failure under axial compression and torsion. Stiffness

under axial compression (Figure 3A) was not significantly different between the 3

fixation principles, however, the total plastic deformation under axial compression

of a plate-bone construct with a cortical defect opposite the plate however was

significantly less (P<0.01) using an internal fixator with (NCB) or without (LISS)

plate to bone compression. Interestingly, plastic deformation under axial

compression was significantly higher (P<0.05) in the PAP CP than in the NCB CP,

although the stiffness was comparable between these two compression plates.

Under torsional load (Figure 3B), stiffness was significantly (P<0.05) higher in all

three compression plate constructs, whereby the combination of CP/LP showed

the lowest plastic deformation following cyclic loading, although not significantly

(P>0.05). Loading the constructs in axial compression until failure revealed no

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significant (P>0.05) difference between tested constructs (Figure 3C). However,

under torsion, the combination of compression plate and internal fixator (CP/IF)

showed the highest load to withstand torsion (P<0.001). No significant (P>0.05)

difference between the NCB as a compression plate and as an internal fixator

could be found however. The mode of failure for the compression plates (PAP and

NCB) under axial compression and torsion was angulation of the screw in the plate

hole, whilst the combination CP/IF system (NCB) failed four times due to bending

of the plate, and twice due to angulation of the screws in the proximal locking

mode. On the other hand, the internal fixator (LISS) failed twice due to bending of

the plate, and four times due to a break of the most distal screw. Under torsion, the

failure mode for the combination CP/IF system (NCB) was always a partial loss of

the locking mechanism with a slippage of the screw head beneath the locking cup,

whereas the internal fixator (LISS) failed in all six trials due screw breakage.

The cadaveric fracture setup simulated a distal intraarticular femur fracture with a

comminuted fracture zone in the metaphyseal area. All specimens had mild to

moderate osteopenia (mean bone mineral density 0.624 ± 0.043 g/cm2). Age,

gender, and bone mineral density were not significantly (P>0.05) different among

the three groups. The stiffness under axial compression (Figure 4A) was highest

for the combination CP/IF system, although the differences between the implants

were not significant (P>0.05). In the CP group, 2 specimens failed in axial

compression cyclic testing at loads of 1219 N and 1844 N. In those cases, the

plastic deformation was taken just before failure occurred. The CP group showed

significantly (P<0.05) higher plastic deformation than the internal fixators, with

(NCB) or without (LISS) plate to bone compression. In torsion (Figure 4B), the

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stiffness was significantly (P<0.05) higher and plastic deformation was significantly

(P<0.05) lower in the CP group with (NCB) or without (PAP) internal fixation when

compared to a pure internal fixator (LISS). The load to failure (Figure 4C) of the

CP group under axial compression was significantly (P<0.05) lower than that of

the CP/IF group, but not the pure internal fixator (LISS). The difference between

the two internal fixators, compressed to the bone (NCB) or not (LISS), was not

significant (P>0.05). Whilst, although not significant (P>0.05), the highest

torsional load to failure was found in the pure CP system (PAP). The mode of

failure for all constructs under compression and torsion was loss of distal screw

fixation, either by screw cut out (NCB, LISS) or the toggling of the screws around

the lateral plate hole (PAP).

Discussion

In this biomechanical study, we have comparatively analyzed the stability of three

different fracture fixation systems in plate osteosynthesis: a pure compression

plate (PAP), a pure internal fixator (LISS), and a combination plate which can be

used as a compression plate with the option to lock the screws (NCB). We firstly

investigated a model of shaft fracture in composite bones to assess the anchorage

of screws in the bone, and the effect of plate to bone compression with or without

locking the screws. Secondly, we simulated the clinical situation of a distal femoral

intra-articular fracture with a metaphyseal comminution in cadaveric bone. Our

data indicate that in comminuted shaft fractures loaded under axial compression,

only angle-stable IF implants provide sufficient stability, whereas under torsion, the

CP evidenced biomechanically superior results compared to IF. A combination of

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these two principles (CP/IF) produces superior fixation in supracondylar

comminuted fractures with intra-articular fracture extension.

In simple shaft fractures, the correlation between plate biomechanics and clinical

outcomes with the CP and IF systems remains unclear 145,146,148-150. We have

illustrated with our tests that a plate may have high stiffness at the beginning of the

test but is unable to maintain the reduction. This is even more obvious in the case

of a comminuted fracture zone opposite the plate. Although the CP plate system

has a similar initial stiffness under axial compression to the IF system, it cannot

maintain the reduction under a cyclic load. This is well known phenomenon,

previously described as the 'windshield wiper effect', 158-161 which can be reduced

using angle-stable implants like the modified angular-stable Condylar Buttress

Plate (CBP) 139,161 or the LISS 147. Noteworthy, IF are not only angle-stable at the

plate-screw interface, but also the larger core diameter of the screws provide a

better stress distribution in the supporting bone and is less likely to become loose

at the bone-screw interface than smaller compression screws 147. However, screw

loosening in the metaphyseal area in osteoporotic bone also remains a partially

unsolved problem with the use of angle-stable implants. In order to increase

construct stability other authors recommend additional medial plating 153,162, iliac-

crest grafting 152,153,155,163, cement augmentation of the screws 164, the use of a long

interfragmentary screw angulated towards the medial condyle in a diagonal

position 165, the insertion of a medial endosteal plate 166, or even double plating

with locked implants 162 for severely comminuted fractures. Although, the present

results suggest that a single lateral locking plate might suffice to retain reduction of

a comminuted fracture under cyclic axial loading.

120

In both fracture models, CP showed less plastic deformation and a higher load to

failure under torsion compared to IF. The reason for this finding is not absolutely

clear. It may be that under torsional load, the broad anatomical preshaped CP

plate gives additional stability through the enlarged geometric contact area when

compressed to the bone. Hence, some of the conflicting results reported may not

only be related to the different fixation principles, but also the type of plate material,

plate geometry, and the different type of screws implemented. When considering

material properties, the stainless steel CP (PAP) was nearly twice as stiff as the

Titanium Alloy IF (NCB and LISS). The fact that the straight part of the NCB plate

is slightly thicker than the PAP and the LISS, may explain the similar stiffness

under compression in the shaft fracture model of the two CP (stainless steel PAP

and Titanium Alloy NCB). On the other hand, the PAP showed an increased plastic

deformation compared to the NCB. This may be explained due to differential screw

hole geometry and screw diameter. The smaller screw in the oval hole of the PAP

can angulate more than the thicker screw in the round hole of the NCB, where the

shaft of the screw integrates earlier with the screw hole and plate, and so can gain

secondary stabilization. Differences between the constructs may also be

attributable to the use of different distal screws. Using the LISS, the distal screws

were cortical screws, whereas in the PAP and the NCB the distal screws were all

cancellous screws. Noteworthy, it has been shown that cancellous screws have a

significantly higher fixation strength than cortical screws in the distal metaphyseal

area 167.

The authors recognized that the use of a biomechanical model is a limitation of this

study. Soft tissue tension which contributes to the alignment and maintenance of

121

reduction is lost in a laboratory model. For both simulated fractures, the plastic

deformation during cyclic loading may be different in a clinical scenario since

interval fracture healing and bone remodeling around the bone-screw interface

occurs. The use of 2 instead of 3 bicortical screws on either side of the fracture

may have biased the results and favored the internal fixator over the compression

plate. This may be more relevant for the composite fracture model since the plastic

deformation in the cadaver model occurred in the metaphyseal region. Also, the

clip gauge and travel extensometer of the materials testing machine was used to

measure axial and angular displacement, although given fracture displacement is

three-dimensional, only one direction of displacement was detected with this

method at a time. The average age of the donor for cadaveric specimens used in

this study for the distal fracture setup was 72 years, which is substantially older

than the population of patients who typically sustain this type of comminuted

fracture. Since the mechanical properties of trabecular bone decrease with age 168,

and given the mild osteopenia with mild degenerative changes observed in our

cadavers, the response to loading in this group may have been influenced.

Biomechanical testing in this study demonstrated that (1) under axial compression,

both IF systems (NCB and LISS) provide similar fixation in fracture patterns with a

comminution and are superior to CP (PAP) in order to avoid a secondary varus

displacement of the fracture; (2) For torsional loading, compression of the plate to

the bone is more important for stiffness, plastic deformation, and load to failure

than using angle-stable IF implants; (3) The combination of both fixation methods

(CP/IF) seems to be clinically advisable in articular fractures of long bones with a

metaphyseal comminution. An anatomical reduction and compression of the plate

122

to the articular bone increases stability in torsion, and bridging the reconstructed

joint block to the diaphysis using the plate as an internal fixator avoids secondary

varus displacement of the fracture. This data may be utilized by surgeons to build a

more specific treatment plan in patients with diaphyseal and intra-articular distal

femur fractures with a metaphyseal comminution.

123

Tables and Figures

Table 1. Test configurations for the composite shaft and comminuted metaphyseal

fracture models.

124

Figure 1. The three tested bone-plate constructs. (left) Compression plate (CP;

Periarticular Plate PAP). (middle) Compression plate with the option to lock the

screws like an Internal fixator (CP/IF; Non- Contact Bridging Plate NCB). (right)

Internal fixator (IF; Less Invasive Stabilization System LISS). (insert) Locking

cups used in the NCB CP/IF plate system. Angular stability is achieved by inserting

a locking cup (consisting of thread to engage the plate and a convex inside to

press the screw) into the plate to hold screw head position.

Figure 2. Test rig setups of the composite shaft model with the LISS, and the

cadaveric distal intra-articular fracture model with the LISS. (A) Schematic

diagram (left) and photograph (right) of the test setup for the composite shaft

fracture model with affixed LISS. (B) Schematic diagram (left) and photograph

(right) of the test setup for the cadaveric distal intra-articular fracture model with a

metaphyseal comminution zone and affixed LISS.

126

Figure 3. Mean stiffness, plastic deformation, and failure loads of the shaft fracture

model under axial compression and torsion. (A) Mean stiffness and plastic

deformation under axial compression (** P<0.01 compared to the CP PAP and CP

NCB). (B) Mean stiffness and plastic deformation under torsional loading (*

P<0.05 compared to the IF LISS and IF NCB). (C) Mean axial compression and

torsional loads to failure (** P<0.01 compared to all other systems). Data

presented as mean ± standard deviation.

127

Figure 4. Mean stiffness, plastic deformation, and failure loads of the distal femur

fracture model under axial compression and torsion. (A) Mean stiffness and plastic

128

deformation under axial compression (* P<0.05 compared to the IF LISS and

CP/IF NCB). (B) Mean stiffness and plastic deformation under torsional loading (*

P<0.05 compared to the IF LISS). (C) Mean axial compression and torsional loads

to failure (** P<0.01 compared to the CP/IF NCB). Data presented as mean ±

standard deviation.

129

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133

Chapter 6

Intraarticular calcaneus factures in human cadavers: A biomechanical

comparison of a conventional versus a locking plate.

Karl Stoffel MD a, Grant Booth MD a, Stephan M. Rohrl MD, PhD a, Markus Kuster

MD, PhD a,b


Australia

b Orthopaedic Department, Kantonsspital St.Gallen, St.Gallen 9007, Switzerland

Clinical Biomechanics 2007, Jan;22(1):100-105

An area where locking plates might change the indication for operative treatment in

elderly patients is the displaced calcaneus fracture. The results of operative

treatment of these fractures in younger patients are favourable. In older patients,

however, there are still certain contraindications, including osteopenia of

osteoporosis which can result in loosening of the implants and, ultimately, failure of

fixation. In this experimental study, the stability of calcaneus plates with locking

screws and conventional screws were compared in a cadaver fracture model. Our

results showed that in older patients the use of locking plates should be considered

when treating displaced intraarticular calcaneal fractures.

134

Abstract

Internal fixation of displaced intraarticular calcaneal fractures in patients older than

fifty years remains controversial. This is, in many cases, due to fear of loss of

fixation and the risk of implant failure in osteoporotic bone. It is the objective of this

study to compare the fixation strength obtained using calcaneal plates with and

without locking screws, in the fixation of osteoporotic cadaveric intraarticular

calcaneal fractures.

In seven pairs of fresh frozen lower limbs cadavers, intraarticular calcaneal

fractures were created with a dynamic single impact loading device and stabilized

using either the low profile locking plate, or the conventional calcaneus plate.

Radiographs were obtained to assess reduction. The specimens were then

subjected to cyclic loading followed by loading to failure, using matched pairs of

cadaveric lower limbs. The Wilcoxon signed rank test was used to test for

differences in the results.

The locking plate showed a significant lower irreversible deformation during cyclic

loading and a significant higher load to failure. The difference between the ultimate

displacement, and work to failure was not significant. A low bone mineral content in

the area of the posterior facet correlated only in the conventional plate group

significant with an increased irreversible deformation.

This study, performed for the first time on human cadavers, supports the

mechanical viability of using locking calcaneal plates for the fixation of intraarticular

calcaneal fractures in elderly patients.

135

Key words: Calcaneal fracture; plate fixation; locked screws; conventional screws;

biomechanical testing

Introduction

Several large comparative clinical studies 169-174 have shown good to excellent

results in younger patients with surgical treatment of displaced intraarticular

calcaneal fractures. Controversy still persists for the treatment of calcaneal

fractures in the elderly 170,175,176. Besides lower physical demand, many elderly

patients have lower bone quality impairing implant fixation leading ultimately to

failure 170. In these cases locking plates might potentially improve the outcome 174.

Richter et al., 177 showed in a biomechanical study that locking plates provide a

greater stability during cyclic loading than conventional plates. No statistical

difference in failure tests between the two plates was found. However, artificial

bones with a single fracture pattern by defined osteotomies of the os calcis were

used. In the present study, different clinical fractures of the os calcis on

osteoporotic lower limb cadavers were produced through a direct impact to the

specimens. Superior fixation strength of the locking plate would encourage the use

of this plate in active elderly patients.

Material and Methods

Ten pairs of osteoporotic human cadaveric fresh frozen limb cadavers were

retrieved within 24 h post mortem (median age 67, standard deviation 5.1; 3 male

and 7 female). Radiographs of the foot were taken to exclude any osseous

136

pathology of pre-existing disease or trauma. The bone mineral density (BMD) of

the cadavers’ femoral neck and Ward’s triangle region was determined using dual-

energy X-ray absorptiometry scans using a Hologic QDR-4500A densitometer

(Hologic Inc, Waltham, MA, USA) with the running software v9.8D. Bone density

was evaluated according to the WHO guidelines for osteoporosis and osteopenia

(2.5 and 1 standard deviations respectively below the mean of a young healthy

reference population of the same gender). To evaluate bone quality of the

calcaneus bone density was also measured in the 3 locations intended for screw

fixation using different software (Rat Whole Body v8.26a) (Figure 1). The

specimens were then wrapped in saline soaked linen, and stored at -80 deg C in

sealed bags.

The intraarticular fractures of the calcaneus were created according to Carr et al.

178 by dropping a weight of 148N guided by a stainless steel rod which was

inserted through a reamed (11 mm) canal in the tibia 20 cm above the ankle joint.

The tibia was in an upright position and the foot in a 10 º dorsally extended position

supported by a frame to prevent it from sliding off the stand. A small notch was

made in the sinus tarsi to produce a standard fracture pattern. The dropping height

was lowered from 106 cm (maximal available energy 156Nm) to 80 cm (118 Nm)

after the first 2 specimens from different cadavers showed fractures of the distal

tibia as well as through the talar neck. In spite of this modification 1 specimen

sustained an unintended talus fracture and was also excluded from the study.

Hence 14 paired specimens from 7 cadavers remained in the study with isolated

fractures of the calcaneus.

Two independent musculoskeletal radiologists then classified the fractures

according to Sanders 173 using CT scans. The Böhler angles (BA) on lateral

137

radiographs were also measured before and after fixation of the fracture and again

after failure testing.

By means of an extended lateral approach the calcaneal fractures were reduced to

less than 2 mm of intraarticular displacement and stabilized with implants in a

standard manner as described by Sanders and Gregory 179. In both groups a total

of 9 bicortical screws were placed in the calcaneal bone: 3 beneath the posterior

facet, 3 into the tuberosity fragment far dorsally, and 3 into the fragment of the

anterior process close to the calcaneo-cuboidal joint (Figure 3.). All screws were

inserted through the plate holes. The implants were either a locking or a non-

locking stainless steel AO “Sanders plate” with self-tapping screws (Synthes,

Sydney, Australia). In the Calcaneal locking plate the conical threaded head

screws are firmly locked into the also conical threaded plate hole providing axial

and angular stability of the screw relative to the plate. In the Calcaneal

conventional plate the screws are tightened to compress the plate onto the bone.

The actual stability results from friction between the plate and the bone (Figure 2).

The type of plate was randomly assigned to the right foot in each pair by drawing

lots from the randomization envelope. The left foot was then stabilized with the

other type of plate.

To minimize the soft tissue contributions to irreversible deformation during testing

and to prevent the foot from slipping the heel pad was removed. The plantigrade

hind foot was placed into a box containing dental cement (Glastone 3000, Die

Stone, Type V, Densply, Perth, Australia). The box was then clamped to the

baseplate of the material testing machine (Zwick Z010, Zwick Inc., Ulm, Germany)

allowing free rotation. The tibia was oriented in line with the loading axis of the

testing machine and proximally rigidly fixed in a custom made device (Figure 3).

138

The testing machine was controlled by special software (Test-expert; Zwick Inc.,

Ulm, Germany) on a standard IBM compatible personal computer that also directly

stored the data.

Each specimen was cyclically loaded 1000 times from a preload of 20 N to a

compressive load of 200N at a rate of six cycles per minute. The irreversible

deformation was measured with the linear variable differential transformer on the

Zwick Machine. The load to failure was then recorded at an actuator speed rate of

45 millimeters per minute. Ultimate load, ultimate deformation, and work to failure

were recorded at the failure point, which was defined as the yield point on the load-

deformation curve. Because the specimens were tested as an osteo-cartilago-

ligamentous unit, the measured values reflect load, displacement and work to

failure of the construct as a whole. After failure we dissected each specimen to

identify the mode of failure.

Statistical Analysis

The Wilcoxon signed rank test was used to compare the results of the irreversible

deformation testing, the failure testing and, the BMD measurements of the two

groups. To compare the BMD measurements of the different areas of the

calcaneus the Kruskal-Wallis test was applied. The Spearman's correlation

coefficient was used to assess the degree of correlation between values obtained

by mechanical testing and BMD of different areas. Kappa statistics were used to

analyse the interobserver variability of the CT based fracture classification. The

hypothesis was that both plates would perform similarly in cyclic loading and in

failure testing. P-values of < 0.05 were considered significant.

139

Results

Imaging

All specimens were osteopenic. The bone density of the calcaneus was similar

between the two groups (for all regions P> 0.75). The bone below the posterior

facet (R2) showed the highest density, followed by the anterior process region

(R1) and the calcaneal tuberosity (R3). Both groups displayed significant (P

<0.05) differences between these three areas.

The distribution of fracture type for both groups was similar (Table 1). The

interobserver variability of the classification system was 0.57 (95% confidence

interval (CI) 0.46 to 0.68). The subclasses were then combined and assessed. An

agreement between the general classes gave a kappa value of 0.61 (95% CI 0.52

to 0.70).The BA was restored within the 95% CI in 5/7 cases using the locking

plate and in 6/7 cases using the conventional plate.

Mechanical Testing

After 1000 cycles, the mean irreversible deformation excluding the first cycle was

2.75 mm (SD 0.47) and 3.49 mm (SD 0.96) for the locking and conventional

plates, respectively (Figure 4 and 5) (P= 0.031).

The ultimate load to failure, the ultimate displacement, and work to failure for the

locking plates were 3818 N (SD 1712), 15.9 mm (SD 4.5), 23.1 Joules (SD 13.8)

and for the conventional plates 3176 N (SD 1424), 17.3 mm SD (5.6), and 29.3

Joule (SD 23.2), respectively (Figure 5). In all cases the locking plates showed a

higher load to failure. In 2 constructs a higher ultimate displacement and, in 1

140

construct a higher work to failure were recorded. A significant difference between

the two groups could be found for the load to failure (P = 0.031) but not for the

ultimate displacement and the work to failure.

A low bone mineral content in the area of the posterior facet correlated significantly

with an increased irreversible deformation for conventional plates (r=0.87,

P=0.024) but not for locking plates (r=0.36, P=0.29).

At the point of failure all specimens failed by loss of reduction. The force applied to

the tibia caused subsidence of the posterior facet in both groups. The locking plate

failed by bending of the plate itself while the screws maintained their plate-screw

angle, whereas in conventional plates the plate remained intact but the screws

angulated relative to the plate by pivoting in the plate hole.

Discussion

The optimal treatment of displaced intraarticular calcaneal fractures in the elderly

patient is challenging. The suitability for internal fixation in this population group

should be determined by the patient’s overall condition and functional demand

rather than their age 180. Herscovici et al. 181 reported good clinical results in elderly

patients following an operative treatment. He stated that careful patient selection is

necessary as individuals presenting with severe osteopenia have the risk of

implant loosening with loss of reduction. In patients with poor bone quality locking

plates have been shown to have an advantage over conventional plates 144,182-184.

141

The present study investigated the fixation strength of locking versus non locking

screws in the fixation of intraarticular fractures of the calcaneus in osteoporotic

human cadavers. The locking plates provided a significant increase in fixation

strength with a lower irreversible deformation during cyclic loading and a higher

load to failure. When comparing the ultimate deformation and the work to failure no

statistical difference between the two groups could be found.

The mean age of our osteoporotic cadaveric specimens was 67 years with a low

standard deviation (5.1), providing suitable material for demonstrating a difference

in constructs stability between the two types of plate fixation. Furthermore

randomizing within the specimen pairs for the alternate forms of fixation helped

reduce the effect of differences in bone quality and made the data more

comparable. We used the dynamic single impact fracture model described by Carr

et al. 178. In order to avoid additional fractures around the ankle joint we had to

lower the dropping height from 106 cm to 80 cm. With this slight modification we

produced clinically relevant fracture patterns with variation of the primary fracture

line and different patterns of comminution of the posterior facet 170,185,186. These

variations can not be seen by creating intraarticular “fractures” by defined

osteotomies 177,187,188. In spite of only fair agreement among the two radiologists in

classifying the fractures, we consider the fractures as being consistent with typical

fractures encountered clinically in impact trauma. We encountered low

interobserver reliability in classifying our fractures but consider this to be a

common problem of classifications in general and not unique to the Sanders

classification 189.

142

The rate (six cycles per minute) and the load (200N) used for cyclic compressive

loading mimics crutch walking as described in earlier biomechanical studies 190,191.

Others used compressive loads during cyclic testing of either 100N 178 or 800N 177

or tested only single load to failure 188.

During cyclic loading the resistance to irreversible deformation was significantly

higher in locking plates compared to conventional plates. This supports the findings

of Richter et al. 177. In our model the deformation was most likely due to a

subsidence of the posterior facet. We measured the displacement only in the

primary loading axis with the setup for both sides and plates being identical. Only

in the conventional plate group could a significant correlation between an

increased irreversible deformation and a low bone mineral density be found.

Accordingly it can be assumed that the irreversible deformation of a construct with

a locking plate is mineral content independent.

In all cases the load to failure was well above the average physiological force of

755 N acting on the calcaneus at heel strike during normal gait 192. The load to

failure in our series was relatively high in both groups, which might be related to

the adequate restoration of the Böhler’s angle (tripod effect) as described by

Letournel 193. However, the results of failure testing were significantly higher in the

locking plate group and did not correlate with the fracture type. From a failure point

of view the success of surgery is mainly dependent on the bone quality and hereby

indirectly, also the implant design (locking vs. conventional) and the restoration of

the Böhler’s angle. This is supported by the fact that the restoration of Böhler’s

angle has been found to be crucial for good long term results independent of the

treatment mode 194. The low density of the tuberosity (R3) was not reflected in the

mechanical strength of the different implants or the failure mode. All constructs

143

failed in the area of the main direct axial load impact, the posterior facet. No failure

was detected from the loss of fixation of the anterior or posterior process in any of

the implants. These findings, together with the findings from the cyclic loading

tests, suggest that a plate with locking holes beneath the posterior facet and

compression holes in the rest of the plate might provide as stable a construct as a

plate with all locking screws. The fact that Richter et al. 177 found no significant

difference in the load to failure might be related to the specimens tested. They

tested composite bones with the mechanical properties of a young patient. It has

been shown that in good bone quality locking plates demonstrate only a subtle

mechanical superiority compared to compression plates 145,195. In addition to the

bone density the failure mode must be attributed to the fixation principle (locking

vs. conventional plate) since the plate thickness between the screw holes is the

same for both. Also with the screws in place the construct thickness is the same

since the locking screws are countersunk into the plate. As a result of these facts

the locking and the conventional plate are of equally low profile and reduce the risk

of a possible wound breakdown due to stretching the overlying soft tissue over the

implant as requested by Carr et al. 178.

The absence of stabilizing forces of the muscles in our cadaveric limbs compared

to that in-vivo is a limitation of the study. Further limitations include the uniaxial

mode of loading and the measurement of the displacement of the entire construct

registered by the material testing machine. However, it has been shown that the

motion measurements of the mechanical testing machine are equivalent to those

recorded when measuring the motion between the plate and the bone 177. In our

experimental setting we used either conventional or locking screws. In a clinical

setting however, the surgeon may chose to use compression screws separate from

144

the plate to achieve interfragmentary compression followed by a locking plate to

stabilize the fracture. This might help to maintain the reduction as well as to

improve the fixation strength.

This is the first biomechanical study performed on fresh frozen human cadavers

that investigated the fixation strength of plates with and without locking screws in

intraarticular calcaneal fractures. We found significantly less irreversible

deformation regardless of bone quality and a significant higher load to failure in

locking plates in osteoporotic bone. We conclude that locking plates are beneficial

when treating displaced intraarticular calcaneal fractures in patients with

osteoporotic bone.

145

Figures

Figure 1. Three regions of interest were defined (R1-R3) and BMD was measured

in each.

Figure 2. Calcaneal locking plate (left) and Calcaneal conventional plate (right)

146

Figure 3. Material testing setup using the Zwick Machine.

Figure 4a and 4b. Irreversible deformation (y; excluding the first cycle) during the

cyclic loading sequence, for plates with (a) and without locking screws (b).

147

Figure 5. Ultimate load, ultimate deformation and, irreversible deformation for each

group.

148

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152

Chapter 7

Volar versus dorsal locking plates with and without radial styloid locking plates for

the fixation of dorsally comminuted distal radius fractures: A biomechanical study in

cadavers.

Murray Blythe MBBS a, Karl Stoffel MD a, Paul Jarrett MB ChB a , Markus Kuster

MD, PhD a,b


Australia.

b Orthopaedic Department, Kantonsspital St.Gallen, St.Gallen 9007, Switzerland.

J Hand Surg [Am]. 2006 Dec;31(10):1587-93.

A common fracture, where locking plates may find a useful application is in dorsally

comminuted and displaced distal radius fractures in elderly patients. Traditionally

these fractures are buttressed dorsally and the functional outcome can be

satisfactory . However this has been associated with complications of extensor

tendon irritation and rupture, which may necessitate implant removal. Also elderly

patients with osteoporotic bone have a higher risk of loss of reduction due to screw

loosening and due to the toggle effect of the screws within the distal part of the

plates. Recently, angle stable constructs have become available in both volar and

dorsal plates which may afford fixation sufficiently stable to allow early mobilisation.

153

The biomechanical model of the three columns of the distal radius led to the

development of a new low profile 2.4-mm Titanium Alloy Locking Distal Radius

System (LDRS) (Synthes Ltd, Paoli, PA). The pre-contoured locking plate system

offers either a single volar or a dorsal plate with the option of an additional styloid

plate in both cases for an angle stable double plate fixation.

In this study the stability of clinically relevant constructs under near physiologic

conditions will be tested in human cadaveric osteopenic distal radii.

154

Abstract

Purpose: To compare the stability and stiffness of dorsal and volar fixed angle

distal radius constructs in a cadaveric model.

Methods: The Synthes Titanium Alloy Locking Distal Radius System 2.4 was used

in a combination of a dorsal and styloid plate (group 1), a single volar plate (group

2) and a combination of a volar and styloid plate (group 3) configuration. In

addition a single volar 3.5mm stainless steel locking plate was used in group 4.

The constructs were tested on 6 fresh frozen radii each; with a simulated unstable

dorsally comminuted extra-articular distal radius fracture. Specimens were tested

on a Zwick testing machine with an extensometer and subjected to axial

compression fatigue and load to failure testing.

Results: No construct failed in fatigue testing of 250N for 5000 cycles. Two

specimens in each group were tested for 20 000 cycles without failure. The plastic

deformation in the double plating groups was lower compared to the single plating

groups although the difference was not statistically significant. Group 1 had the

highest and group 4 the lowest failure load and stiffness, respectively. The

difference between group 1 and the other groups, except failure load compared to

group 3, was statistically significant. Group 2 and 3 had a significantly higher load

to failure and group 3 had a significantly higher stiffness compared to group 4.

Conclusions: All constructs offer adequate stability with minimal deformation on

fatigue testing under physiological conditions. Dorsal fixed angle constructs are

stiffer and stronger than volar constructs. The addition of a styloid plate to a volar

155

plate did not significantly improve stability in this model of simulated extra-articular

dorsal comminution loaded in axial compression.

Introduction

Dorsally comminuted and displaced distal radius fractures in elderly patients with

osteoporosis are a difficult management problem. The treatment goal is a full

functional recovery of the wrist. A prerequisite is the maintenance of an anatomical

reduction which is sufficiently stable to allow early mobilisation of the wrist and

hand while maintaining its position until union. Traditionally these fractures are

buttressed dorsally and the functional outcome can be satisfactory196-201. However

this has been associated with complications of extensor tendon irritation and

rupture, which may necessitate implant removal198,200,202-205.

Volar plating of dorsally displaced fractures with conventional plates has been

described with good clinical results in younger age groups and a mix of fracture

complexity 206,207. However in dorsally comminuted and displaced fractures the

volar plate has no buttress effect. Elderly patients with osteoporotic bone may have

a higher risk of loss of reduction due to screw loosening and due to the toggle

effect of the screws within the distal part of the plate. Recently, angle stable

constructs have become available in both volar and dorsal plates which may afford

fixation sufficiently stable to allow early mobilisation.

Trease et al have shown in a simulated dorsal comminution model using 3.5 mm

steel implants that dorsal constructs are more rigid and stronger than volar

constructs in both locking and compression plates208. They found a biomechanical

156

advantage of the dorsal compression plate compared to the volar locking plate.

This is in contrast with other reports, where volar angle stable constructs showed

an improved biomechanical stability compared to dorsal conventional plates209-211.

The biomechanical model of the three columns of the distal radius led to the

development of a new low profile 2.4-mm titanium Locking Distal Radius System

(LDRS) (Synthes Ltd, Paoli, PA). The precontoured plate system offers volar,

dorsal and styloid fixation options with locking head screws forming an angle stable

construct. To date there has been no biomechanical comparison of dorsal versus

volar and single versus double plating configuration of a low profile locking system

under physiological loads. The purpose of this study was to compare the stability of

clinically relevant constructs under near physiologic conditions in a simulated extra-

articular dorsally comminuted distal radius fracture model.


Twelve pairs of forearms with intact soft tissues were harvested from cadavers

within 24 hours postmortem. The median age was 77 years with a range of 56 to

92 years, with four female and eight male. Radiographs were performed to exclude

osseous pathology of pre-existing disease or trauma. The specimens were stored

at -30 degrees Celsius in tightly sealed plastic bags and were defrosted in air at

room temperature. Bone mineral density (BMD) of the intact specimens was

measured in the ultra-distal lunate fossa portion of the radius by dual-energy x-ray

absorptiometry (Hologic QDR 4500A, Waltham, Massachusetts, USA, using

version 9.8 software).

157

The radius was transected 12cm from the radial styloid and all soft tissues except

periosteum were removed. Dorsal comminution was simulated by an incomplete

10mm wide dorsal wedge osteotomy. The osteotomy commenced 20mm from the

articular surface in groups 1 to 3 and 15mm from the articular surface for group 4

so as to place the defect centrally between the proximal and distal screws. The

defect overlay the elongated screw hole in all plates. Before the volar cortex was

osteotomised to the thickness of the saw blade (1mm), the osteotomy was

stabilized.

The specimens were divided into four groups of six radii. The first three groups

were plated with the LDRS without further contouring. Group 1 were fixed with one

dorsal intermediate and one styloid plate (Figure 1a), group 2 with a single volar

plate (Figure 1b), and group 3 with one volar plate and one styloid plate (Figure

1c). The styloid plates in groups 1 and 3 were placed in the same position. Group 4

were fixed with one volar 3.5mm steel LCP oblique T plate (Figure 1d) which was

shaped manually to a similar configuration as the LDRS plates. All plates were

implanted according to the manufactures guidelines and were of similar length.

The proximal end of the radius was potted in polymethylmethacrylate bone cement

in a custom built stainless steel cylinder which was attached to the baseplate of the

electromechanical material testing machine (Zwick Z010, Zwick Inc., Ulm,

Germany) (Figure 2). The radius shaft was aligned with the loading axis. A

negative mold of the articular surface was formed with the same cement, avoiding

158

encroachment on the plates, and articulated with a 32mm diameter steel ball to

form a universal joint allowing free rotation (Figure 2). The testing machine was

controlled by special software (test-expert-Software; Zwick inc., Ulm, Germany) on

a standard IBM compatible personal computer that also directly stored the data.

The plastic deformation was measured with the linear variable differential

transformer on the Zwick Machine.

Specimens were tested in axial compression. For fatigue testing a manual clip-on

extensometer (Zwick Inc., Ulm, Germany) was attached by two 2.0mm Kirschner

wires inserted in Listers tubercle and 5 cm proximally.

Each specimen was cyclically loaded at 100N/s from a preload of 100N to a total

compressive load of 250N representing the load across the wrist when the digits

are flexed based on in vitro measurements212. All specimens were subjected to

5000 cycles at a rate of 1 Hz as described in a previous study.211 Two specimens

in each group were subjected to 20 000 cycles to assess additional plastic

deformity. The plastic deformation at 5000 and 20000 cycles was measured with

an extensometer with a resolution of 0.0001mm and an accuracy of 0.016mm.

All specimens were then loaded to failure at 2mm/s as per previous

studies210,211,213. Axial displacement was measured using the linear variable

differential transformer on the Zwick Machine. Stiffness was calculated using the

linear part of the load/displacement curve. Load to failure was defined as a sudden

change in the load/displacement curve, construct breakage or closure of the

osteotomy gap.

159

The Mann-Whitney U Test was used to compare the results from the plastic

deformation, failure testing and the BMD measurements of the four groups.

Spearman's correlation coefficient was used to assess the degree of correlation

between measurements of the mechanical testing and BMD. The hypothesis is that

all four constructs would perform likewise in cyclic loading and failure tests.

Statistical analysis was performed using Microsoft Excel 2002 with Analyse-It

software (Microsoft Corporation, Redmond, USA). Statistical significance was

accepted at a p value of less than 0.05. Post hoc power calculations were based

on an alpha of 0.05 and beta of 0.8.

Results

There were no significant differences between the groups for age or BMD (Table

1). No construct failed during fatigue testing. There was no significant difference in

plastic deformation between the groups after 5000 cycles (Figure 3). The

additional plastic deformation between 5000 and 20000 cycles with all eight

constructs tested (two from each group) was less than 0.2 mm. Figure 4 presents

the cyclic test result of one specimen from group 3. There was no association

between bone mineral density and plastic deformation at 5000 cycles for all

constructs (Spearman rank correlation coefficient r = 0.18, p = 0.39) and for each

group. A post hoc power analysis based on group 1 and group 4 showed that to

detect a real difference of means of 0.2mm would require 40 samples in each

group.

160

The average stiffness is shown in Figure 5. Group 1 had significantly greater

stiffness than the other groups (group 2, p = 0.026; group 3, p = 0.026 and group

4, p = 0.002). Group 3 was also significantly stiffer than group 4 (p = 0.015). There

was no difference between group 2 and group 3 and no difference between group

2 and group 4 (p = 0.81). Stiffness did not correlate with bone mineral density.

As shown in Figure 6, the average failure loads (±SD) for groups 1 to 4 were

1660N (±460), 1047N (±268), 1089N (±412) and 710N (±215) respectively.

Group 1 had a higher failure load than all other groups which was significant

compared to group 2 (p = 0.026) and group 4 (p = 0.002). Group 2 and 3 had a

significantly higher failure load than group 4 (p = 0.041). There was no significant

difference between groups 2 and 3 (p = 0.94). Failure load did not correlate with

bone mineral density for pooled results (Spearman rank correlation coefficient r =

0.26, p = 0.22) or group 1 but did for the volar groups. This was statistically

significant for groups 3 and 4 and high but not statistically significant for group 2

(Figure 7). An investigation of potential outliers in group 2 was undertaken using a

boxplot. None of the constructs were outside the 1.5 interquartile range from the

median.

All specimens in group 1 failed with volar angulation and displacement of the distal

fragment after slippage or fracture of the volar cortex. In three specimens the

metaphyseal screws of the styloid plate cut out and in three specimens the

metaphyseal screws bent or fractured at the screw head-shaft junction, although

161

this occurred at loads higher than the construct failure load. All specimens in

groups 2, 3 and 4 failed with dorsal angulation of the distal fragment. All specimens

in groups 2 and 3 bent at the second (elongated) hole between the threaded and

dynamic compression junction. In two specimens in group 2 the distal shaft screw

fractured at the screw head-shaft junction, although this occurred at loads higher

than the construct failure load. In group 3 either the styloid plate metaphyseal

screws fractured or cut out. All plates in group 4 bent at the first (elongated) hole

between the threaded and dynamic compression junction. No screws bent or failed

in group 4.

Discussion

This study compared the stability of a dorsal fixed angle distal radius system to

three volar fixed angle constructs in a simulated dorsally comminuted extra-

articular distal radius fracture in osteoporotic cadaver bone. All constructs

withstood fatigue testing under physiologic loads. The dorsal plating system was

the strongest and stiffest.

Most previous biomechanical papers compared volar locking plates to dorsal

conventional plates208,210,211,213,214. Trease et al208 compared single dorsal to single

volar fixed angle constructs in a simulated dorsal comminution model in static

testing only. They found the dorsal locking plates stiffer and stronger. This study is

the first comparing volar and dorsal locking plates in cyclic testing as well as with

styloid plates.

162

During our testing no construct failed under cyclic fatigue loading of 250N. This is

in agreement with the findings of another biomechanical study performed by

Liporace et al211 . This suggests that any of these fixed angle constructs are stable

enough for early post-operative unloaded mobilization in osteoporotic patients with

extra-articular fractures. The fatigue load was based on measured wrist joint loads

in cadavers while simulating wrist and digit motion212 and is the highest force

estimated by others which exists in the wrist joint in unloaded conditions210.

Liporace et al211 compared the plastic deformation of a volar locking plate to a

dorsal conventional plate in a dorsal wedge model. They found that a locking plate

showed significantly less loss of initial reduction than the conventional plate. The

question arises whether a dorsal locking plate will show less irreversible

deformation than a volar locking plate in fatigue testing. In our study there was no

statistically significant difference in plastic deformation between the groups.

Locking implants seem to maintain the initial reduction after fatigue testing

satisfactorily independent of their position.

The dorsal and styloid plate group showed significantly higher stiffness values than

the other tested constructs. In a similar test setup Liporace et al211 found

comparable stiffness values for the Distal Volar Radius plate (Hand Innovations,

Miami, FL). However, Osada et al210 reported only half of the stiffness values when

testing a custom designed volar locking plate. Compared to their study not only the

implants were different but also the setup. They tested their implants in a radio-

163

ulna preparation with a defect in the volar cortex and used the carpi to load the

construct

The average failure load found in the three volar constructs tested is similar to the

values reported by Osada et al213 for their custom designed volar locking plate

tested in isolated radii. The failure load of all constructs exceeds the 250N which

has been calculated as the load across the radius for unloaded grasping210,212.

However none of the constructs would tolerate the calculated 2410N load across

the radius with the average male power grip205. We found in this study that the

dorsal construct had a significantly higher failure load compared to all volar

constructs, including the volar and styloid plate construct. This is an expected

finding in a dorsal wedge model where there is a buttress effect for a dorsally

positioned plate but not a volar plate. Interestingly the volar and styloid plate

construct was not significantly stronger or more rigid than a single volar plate.

There are no previous studies available to compare failure loads of dorsal versus

volar locking plates. Comparing conventional plates using a similar setup to ours

Peine et al 196 reported that dorsal double plating with 2mm conventional plates

showed a significantly higher strength than a single dorsal 3.5 mm locking plate.

While Gesensway et al209 showed a dorsal blade plate to be significantly stronger

than a conventional dorsal T plate Osada et al210 showed a volar blade plate to

have a higher failure load than conventional volar and dorsal plates.

We are not aware of any other studies of biomechanical testing on volar and styloid

plating of distal radius fractures. According to our tests volar and styloid plating has

no significant biomechanical advantage in extra-articular fractures with dorsal

164

comminution when tested in compression. The addition of a styloid plate may offer

greater stability to bending or torsion forces. Although there was a trend toward a

stiffer and stronger construct, the results did not reach significance. The styloid

plates are of use in the fixation of styloid column intra-articular fractures, which

were not modeled in this study. Musgrave and Idler215 reported a series of patients

treated with volar and styloid locking plates. The styloid plates were used if the

styloid fragment was difficult to control or if supplementary fixation was required.

They reported good radiographic and clinical results in all cases.

All of the volar plate groups failed by bending at the elongated hole between the

threaded and dynamic compression junction. The osteotomy gap was positioned

over this hole in all constructs, which did not have screws, so the bending moment

was concentrated in this area of the plate. The elongated hole may represent a

region of weakness due to the decreased cross-sectional area which may warrant

reinforcement. An unfilled hole at the level of the defect has been noted as a site of

weakness by Trease et al208

The dorsal and styloid constructs had a significantly higher load to failure

compared to the volar constructs. The dorsal plates buttressed and bridged the

defect on the dorsum of the radius while the volar cortex was in contact and failure

occurred due to slippage or fracture of the volar cortex. In this group the failure

load did not significantly correlate with bone mineral density. In comparison, the

volar plates were fixed opposite the defect and while loading the constructs in

compression the screws seemed to have first compressed the distal cancellous

bone and then the osteotomy gap closed subsequently. In our study the failure

165

load of the volar constructs correlated with the bone density. To minimize this

cancellous bone compression the screws of a volar locking plate should be

positioned as close as possible to the subchondral bone.

There are several limitations to this study. Our aim for the setup was to simulate

the most common clinical situation with an extra-articular distal radius fracture with

dorsal comminution and an accurately reduced volar cortex, which has been used

previously196,211,213. A single dorsal plate construct was not tested in this study. The

number of plate combinations which could be tested was limited by cadaver

availability. A single dorsal locking plate might be biomechanically superior to the

tested constructs. Although the styloid plate did not provide additional stability in

this extra-articular model, it might in other fracture patterns, like intra-articular

styloid column fractures, or in other loading conditions such as bending or torsion.

Our results can not be extrapolated to intra-articular fractures and difficulties in

reliably reproducing an intra-articular fracture model has been noted previously

210,213.

The osteotomy defect was positioned centrally between the proximal and distal

screws in all constructs and overlay the elongate screw hole. Due to the different

plate geometry in group 4 the resultant defect was 5mm distal compared to the

other groups. This did not change the relationship to the elongated screw hole

through which all the plates bent or the positioning of the distal screw row relative

to the articular surface. However, the bridging length was shorter in group 4. As

shown in previous biomechanical studies by our group testing different locking

constructs144 this would be expected to result in a higher failure load and stiffness,

166

which was not found in this study. Therefore the slightly different position of the

defect cannot explain the lower mean failure load and stiffness in group 4.

Furthermore it has been suggested that a wedge osteotomy to simulate dorsal

comminution would test the strength of the volar cortex only210. We disagree on

this point as we found a correlation between bone density and failure load for the

volar constructs but not with plastic deformation or rigidity nor for the dorsal

constructs.

In conclusion for extra-articular distal radius fractures with dorsal comminution, the

combination of a dorsal and styloid locking plate is significantly stiffer and stronger

than either single volar or the combination of a volar and styloid locking plate. The

combination of a volar and styloid plate showed no biomechanical advantage over

volar single plating. Bone mineral density correlated with load to failure in the volar

groups but not the dorsal and styloid plate group. In the case of severe

osteoporosis to maximise strength of fixation consideration could be given to the

use of a dorsal plate construct. In the case of good quality bone in a distal extra-

articular radius fracture with dorsal comminution we recommend the use of a single

volar locking plate.

167

Figures

A B C D

Figure 1. A, LDRS 2.4 intermediate and styloid plates. B, LDRS 2.4 volar plate. C,

LDRS 2.4 volar and styloid plates. D, 3.5mm stainless steel T plate.

Figure 2. Diagram of a lateral view of a specimen loaded in the Zwick testing

machine. This demonstrates the universal joint at the distal end and the custom

168

potting cylinder at the proximal end. The loaded extensometer is on the right,

attached by K wires.

Figure 3. Comparison of mean (95% confidence interval) plastic deformation after

5000 cycles.

169

Figure 4. Sample curve of fatigue testing of a specimen from group 3 showing

strain (mm) by cycle number. The curve flattens before 5000 cycles and there is

minimal additional plastic deformation after this point.

Figure 5. Comparison of mean (95% confidence interval) stiffness .

p<0.026

p<0.026

p<0.002

p<0.015

170

Figure 6. Comparison of mean (95% confidence interval) failure load.

p<0.026

p<0.002

p<0.041

171

Figure 7. Graph of failure load by bone mineral density with lines of best fit. There

was no correlation for group 1 (Spearman rank correlation coefficient r = -0.23).

There was good, but not statistically significant, correlation for group 2 (r = 0.71, p

= 0.11) and statistically significant correlation for group 3 (r = 0.89, p = 0.02) and

group 4 (r = 0.83, p = 0.04).

Tables

Group 1

(n = 6)

Group 2

(n = 6)

Group 3

(n = 6)

Group 4

(n = 6)

Age (years) 78.0 (9.5)

78.0

(9.5)

75.8

(11.6)

75.8

(11.6)

BMD (g/cm2)

0.28

(0.14)

0.35

(0.10)

0.32

(0.18)

0.35

(0.14)

Table 1. Mean (SD) age and BMD of cadavers by group. No significance between

age and BMD for all four groups was observed (p>0.05).

172

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6. Hove LM, Nilsen PT, Furnes O, Oulie HE, Solheim E, Molster AO: Open

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7. Jakob M, Rikli DA, Regazzoni P: Fractures of the distal radius treated by

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13. Osada D, Fujita S, Tamai K, Iwamoto A, Tomizawa K, Saotome K:

Biomechanics in uniaxial compression of three distal radius volar plates. J

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14. Osada D, Tamai K, Iwamoto A, Fujita S, Saotome K: Dorsal plating for

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15. Osada D, Viegas SF, Shah MA, Morris RP, Patterson RM: Comparison of

different distal radius dorsal and volar fracture fixation plates: a

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175

Chapter 8 Summary and Conclusions

Summary

Early clinical results obtained when using locked plates as “internal fixators” were

promising . However, determining the factors that influenced stiffness and strength

of the construct was based largely on clinical experience with conventional plating

(1, 2). The objective of this thesis was to determine, in the light of the works, how

the mechanical environment of the fracture and implant failure can be controlled

with the use of conventional compression plates, locking plates alone or in

combination with the compression plate technique, in the treatment of fractures in

different clinical situations.

Conventional compression plate technique and the oblique screw at the plate

end

The operative decision on the length of the plate and the number screws in

biological fixation of fractures should be made on an individual basis along with a

decision on the bridging length. This study has shown that in conventional

compression plate osteosynthesis, the fixation strength in cantilever bending and

torsion can be increased by insertion of an oblique screw at the end of a

conventional plate. Because the plate length was the major factor in increasing

fixation strength in cantilever bending, the addition of an oblique screw had a

relatively low additional effect. However, shorter plates benefited significantly from

an oblique screw at the end of the plate (up to 43%). Therefore, placing an oblique

176

screw at the end of the plate is a way of increasing fixation strength without

insertion of an additional screw.

Screws near the fracture site should not be inserted at an oblique angle because

the shear stresses acting in this region tend to be high and screw breakage has

been observed. In theory, it would be possible to minimize surgical dissection

further by inserting the last screw obliquely, thus eliminating the need for full

exposure of the last plate hole. However, such surgical dissection remote from the

fracture site probably has only a minimal biologic influence on fracture healing. It is

thus hypothesized that, for a small fracture gap, a longer plate with increased

working length and two to three screws in each main fragment with the last screw

inserted obliquely offers biomechanical advantages. The advantages are increased

plate fixation strength and construct flexibility, together with the induction of callus

formation. It is therefore recommend that this technique is used for patients with

osteoporotic bone and in those cases where a shorter plate must be applied

because of anatomic constraints. This principle can only be applied to a

compression plate and not to a locking plate which is fixed at a distance to the

bone.

Based on the findings of this work with synthetic bone, it is suggested that the

screws at the ends of the plates are inserted obliquely to improve the fixation

strength. Additionally, for bones such as the femur and tibia that are exposed to

large bending forces, long plates with a small number of screws should be

considered. Because torsional strength is mainly restricted by the number of

screws, fractures of the humerus and radius, which are exposed to large torsional

177

forces, should be stabilized with a plate with a high number of screws on either

side of the fracture line.

Locking plates

In the first stage of this study, the experiments were primarily focused on

understanding of the control mechanisms of stability and fatigue failure of internal

fixators such as the LCP for fractures with different gap sizes. In all constructs the

plate length had the most important effect on the construct’s stability. As in

conventional plating more than three screws on each side of the fracture did little to

increase the axial stiffness of the system. The position of the middle screw on each

side of the fracture influenced the construct stiffness significantly. Placing the

middle screw close to the fracture gap increased the stiffness, while it had almost

no effect on stiffness when placed close to the plate end. This is in contrast to

conventional plating, where the highest stiffness was found by placing the middle

screw half way between the two other screws . Two screws at the plate end,

however, increased the fixation strength. Rigidity of the construct under torsional

load increased significantly with an increasing number of screws, for up to four

screws per fragment, and was independent of the position of the screws.

The distance from the plate to the bone also affected the construct’s stability. By

increasing this distance from 2 mm to 6 mm, both torsional rigidity and axial

stiffness decreased by as much as 10-15%.

In contrast to fractures under inter-fragmentary compression, where the greatest

concentration of applied force occurs for the screws at the end of the plate,

bridging plating technique, the FEA, showed the highest stress concentrations for

178

the screws close to the fracture gap. It was found that the stress in the implant can

be reduced if the fracture ends can be adapted for contact between the fracture

surfaces during dynamic loading. In this case, increasing the bridging length can

further reduce the stress on the plate and the screws and hence decrease the risk

of fatigue failure. However, if the fracture gap is large as is the case in comminuted

fractures, no bone contact and load shearing can occur during dynamic loading

and the stress within the screw and the plate increases with increasing working

length of the plate. In this case, additional screws should help to decrease the

stress on the implant near the fracture site.

The risk of implant failure can be reduced if the inter-fragmentary gap can be kept

small and the fixation system is flexible enough to allow bone contact of the

fracture ends to occur opposite the plate under load conditions.

For the clinical use of the LCP, as a locked internal fixator in bridging plate

technique and MIPO, it is recommended that two or three screws are placed on

either side of the fracture for femoral and tibial fractures, which are mainly loaded

in compression. The position of the first screw near the fracture and the additional

screw depends on the fracture gap size. In simple fractures with an inter-

fragmentary gap smaller than 2 mm, one or even two screw holes on either side of

the fracture gap should be omitted to allow for fracture motion and contact of the

fracture ends to occur. For comminuted fractures, at least three screws are

recommended on either side of the fracture, whereby two screws should be placed

as close as practicable to the fracture site. In plate osteosynthesis of the humerus

and the forearm, where mainly torsional load predominates, three to four screws in

each main fragment are recommended, as torsional rigidity depends more on the

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number of screws than axial stiffness. Three screws can be placed as described

above with the fourth screw in any position. If the plate must be placed at some

distance from the bone for anatomical reasons, the screws should be positioned

closer to the fracture site to improve the construct’s stability.

The combination of the conventional compression plate- and locking plate

technique.

The new information gained about screw and plate construct patterns in internal

fixators was applied to investigate the stability of three different fracture fixation

systems in plate osteosynthesis using different bone quality. These were a

compression plate (PAP), a pure locking plate (LISS), and a plate which can be

used as a compression plate with the option of locking the screws in position

(NCB). Experimental results gained indicate that in comminuted fractures, the

stiffness under axial compression is similar for both tested internal fixators (NCB,

LISS) and significantly higher for the compression plate (PAP). The difference in

stiffness under axial compression between the internal fixators and the

compression plate can be explained due to the different implant materials

(Titanium alloy for the internal fixators and steel for the compression plate) and the

slightly unequal plate geometries. However, internal fixators can maintain the initial

reduction under cyclic axial loading significantly better compared to a compression

plate. Under torsion, the compression plate demonstrates biomechanical superior

results compared to the locking system.

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A combination of these two principles, compression and locking screw technology,

produces superior fixation in supracondylar comminuted fractures with intra-

articular fracture extension. We conclude from this study that:

(1) for axial loading, the use of an internal fixator is the most important factor in

avoiding secondary displacement of a comminuted fracture.

(2) for torsional forces, compression of the plate to the bone is more important for

both stiffness and plastic deformation than an angle-stable implant.

(3) the combination of both methods (compression and internal fixator) seems to

be clinically advisable in articular fractures of long bones with multi-fragmentary

extension into the diaphysis, with:

(1) Anatomical reduction and compression of the plate to the articular

bone showing increased stability in torsion.

(2) Bridging the reconstructed joint block to the diaphysis using the plate

as an internal fixator to avoid secondary varus displacement of the

fracture.

The use of locking plates in osteoporotic bone

In patients with poor bone quality locking plates have been shown to have an

advantage over conventional plates (5). The final two papers of this thesis focused

on the use of locking plates in order to achieve more reliable fixation in

osteoporotic bone of the calcaneum and the distal radius.

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The calcaneal study investigated the fixation strength of locking versus non-locking

screws in the fixation of intra-articular fractures of the calcaneus in osteoporotic

human cadavers.

The locking plates provided a significant increase in fixation strength with a lower

irreversible deformation during cyclic loading and a higher load to failure. In all

cases the load to failure was well above the average physiological force of 755 N

acting on the calcaneus at heel strike during normal gait. From a failure point of

view the success of surgery is strongly dependent on the bone quality and hereby

indirectly, also the implant design (locking vs. conventional) and the restoration of

the Böhler’s angle. This is supported by the fact that the restoration of Böhler’s

angle has been found to be crucial for good long term results independent of the

treatment mode. All constructs failed in the area of the main direct axial load

impact, the posterior facet. No failure was detected from the loss of fixation of the

anterior or posterior process in any of the implants tested. These findings, together

with the findings obtained from the cyclic loading tests, suggest that a plate with

locking holes beneath the posterior facet and compression holes in the rest of the

plate might provide a construct as stable as a plate with all locking screws.

It can be concluded that locking plates are beneficial when treating displaced

intraarticular calcaneal fractures in patients with osteoporotic bone.

In the distal radius, the stability of a dorsal or volar fixed angle distal radius system

was compared with or without the addition of a radial styloid plate in a simulated

dorsally comminuted extra-articular distal radius fracture in osteoporotic cadaver

bone.

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The combination of a dorsal and radial styloid locking plate was the strongest and

stiffest among all the tested constructs. The combination of a volar and styloid

plate showed no biomechanical advantage over volar single plating. However, no

construct failed under cyclic fatigue loading of 250N, suggesting that any of these

fixed angle constructs are stable enough for early post-operative unloaded

mobilization in osteoporotic patients with extra-articular fractures.

The failure load of the volar constructs showed significant correlation with the bone

density, which was not seen with the dorsal constructs. The dorsal plates

buttressed and bridged the defect on the dorsum of the wrist while the volar cortex

was in contact. Failure occurred due to slippage or fracture of the volar cortex. The

volar plates, however, were fixed opposite the defect and while loading the

construct in compression the screws seemed to have first compressed the distal

cancellous bone and then subsequently closed the osteotomy gap. This

compression was dependent on the bone density. In the case of good quality bone

in a distal extra-articular radius fracture with a dorsal comminution it is

recommended to use a single volar locking plate, and in osteoporotic bone of the

same fracture to use a dorsal locking plate.

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Conclusions

From the work conducted during this PhD project the following conclusions and

clinical recommendations can be drawn:

- To provide sufficient stability in compression and torsion long plates should be

used.

- The fixation strength of compression plates can be effectively increased by

inserting the screw at the plate end in an oblique way. This is even more

important if due to anatomical reasons a shorter plate must be used like in hand

and foot surgery.

- In a comminuted fracture which is under axial compression load, the use of an

internal fixator is the most important factor in avoiding secondary displacement.

- In fractures of the lower extremity when the locked internal fixator is used in a

bridging technique, three screws on either side of the fracture should be

sufficient.

- If the plate must be placed at a distance from the bone for anatomical reasons,

the screws should be positioned closer to the fracture site to improve construct’s

stability.

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- In simple fractures with a small interfragmentary gap, one or two holes should

be omitted on each side of the fracture to allow bone contact under loading

conditions and thereby to unload the plate. In fractures with a large fracture gap

such as in comminuted fractures, it is recommended to place the innermost

screws as close as practicable to the fracture.

- For torsional forces, compression of the plate to the bone is more important for

both stiffness and plastic deformation than an angle-stable implant.

- For fractures of the humerus and forearm, three to four screws on either side

should be used as rotational forces predominate in these bones.

- In metaphyseal fractures stabilized with a broad condylar shaped plate which is

under high torsional forces, compression plate technique should be considered

to increase primary stability. Different systems of locking plates (e.g. LISS,

NCB) give a biomechanical advantage in comminuted fractures to maintain the

initial reduction under axial compression. The combination of both methods

(compression and internal fixator) seems to be clinically advisable in a long bone

articular fracture with multifragmentary extension into the diaphysis. First the

preshaped plate should be compressed against the bone (compression plate

technique) to neutralize the torque and then the plate should be locked in this

position (locking plate technique) to reduce the deformation force under axial

load.

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- In osteoporotic bone locking plates improve construct viability, potentially

increasing the indications for operative fixation of distal radius and calcaneus

fractures in elderly patients.

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References

1. Aro HT, Kelly PJ, Lewallen DG, Chao EY: The effects of physiologic

dynamic compression on bone healing under external fixation. Clin

Orthop:260-73., 1990

2. Dennis J, Sanders R, Milne T: Minimal vs. maximal compression plating of


Trauma 7:152-153, 1993

3. ElMaraghy AW, ElMaraghy MW, Nousiainen M, Richards RR, Schemitsch

EH: Influence of the number of cortices on the stiffness of plate fixation of

diaphyseal fractures. J Orthop Trauma 15:186-91., 2001

4. Field JR, Tornkvist H, Hearn TC, Sumner-Smith G, Woodside TD: The

influence of screw omission on construction stiffness and bone surface

strain in the application of bone plates to cadaveric bone. Injury 30:591-8.,

1999

5. Gautier E, Perren SM: [Limited Contact Dynamic Compression Plate (LC-


21:11-23., 1992

6. Johnston SA, Lancaster RL, Hubbard RP, Probst CW: A biomechanical


compression plates. Vet Surg 20:235-9., 1991

7. Karnezis IA, Miles AW, Cunningham JL, Learmonth ID: "Biological" internal

fixation of long bone fractures: a biomechanical study of a "noncontact"

plate system. Injury 29:689-95., 1998

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8. Korvick DL, Monville JD, Pijanowski GJ, Phillips JW: The effects of screw

removal on bone strain in an idealized plated bone model. Vet Surg 17:111-

6., 1988

9. Miclau T, Remiger A, Tepic S, Lindsey R, McIff T: A mechanical comparison

of the dynamic compression plate, limited contact-dynamic compression

plate, and point contact fixator. J Orthop Trauma 9:17-22., 1995

10. Ramotowski W, Granowski R: Zespol. An original method of stable

osteosynthesis. Clin Orthop:67-75., 1991

11. Tornkvist H, Hearn TC, Schatzker J: The strength of plate fixation in relation

to the number and spacing of bone screws. J Orthop Trauma 10:204-8,

1996

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Chapter 9 Future work

This project produced several significant results and highlighted some of the

important biomechanical issues associated with modern plate osteosynthesis.

However, a great deal of details still remains unknown and thus further research is

required in the following areas:

• Clinical studies will be required to evaluate the proposed theoretical

biomechanical advantages of angle stable implants. It has to be confirmed

whether this surgical technique provides a safe and effective management

option.

• Animal studies need to be conducted to investigate the percutaneous

application of the angle stable implants with regard to the blood supply at the

fracture level, the adequacy of fracture reduction and the overall surgical

safety.

• So far the biomechanics and biology of fracture repair has been widely

studied. However, many issues regarding the interaction between the

mechanical milieu and the cell response still remain poorly understood. The

interaction between fracture motion and fracture healing patterns for the most

common fractures stabilized with plates are not known. Further studies are

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needed to determine whether Radiostereometric Analysis might accurately

detect and measure fracture motion and fracture stiffness, and whether the

construct’s stiffness can directly be correlated to the time of fracture healing.

• An increasing problem in fracture fixation for the future is the aging population

and the osteopenic/osteoporotic bone quality. The fixation strength of the

screws can be increased by cement augmentation. Other solutions might be

the use of local osteoinductive materials and/or systemic agents like

bisphosponates.

• Another problem with the aging population is stress shielding of the bone if

the plate remains in situ. Bioresorbable implants for fracture fixation of long

bones might be a possible solution for this problem. These implants might

even be loaded with osteoinductive agents. This would also resolve some of

the problems associated with implant removal like prominent hardware, screw

back out, and the attendant risk of a second operation in general and cost.

• The likelihood of a joint replacement increases with the age and therefore

also the risk of a periprosthetic fracture. The fixation of those fractures with or

without revision of the prosthesis is very demanding and requires special

implants. The effect of different periprothetic fracture fixation on the survival

rate of the prosthesis is still unknown and requires further biomechanical

investigations.

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• The history of fracture fixation is surrounded with dogma. Therefore the

algorithm of the stepwise introduction of new implants should follow the

progressional finite element and laboratory studies, animal studies, limited

pilot studies in humans in a controlled research setting followed by

randomized controlled trials (multicentre) and finally overseen by an implant

registry.

MODERN CONCEPTS IN PLATE OSTEOSYNTHESIS...MODERN CONCEPTS IN PLATE OSTEOSYNTHESIS Karl Kilian...

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