Improved Acetabular Cementing Techniques

Improved Acetabular

Cementing Techniques

Masters of Applied Science (Orthopaedics)

Queensland University of Technology

Faculty of Built Environment and Engineering

BN71

Author: Dr Bjorn N Smith BAppSc (PHTY), MBBS.

Principal Supervisor: Prof Ross Crawford.

Associate Supervisors: Dr Clive Lee (External),

Mr John Timperley (External).

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Abstract: The most common cause for revision total hip replacement surgey is aseptic loosening of

the acetabular component. This thesis explores the effect of three techniques to improve

the depth and quality of cemented acetabular component fixation in primary total hip

replacement. This may have beneficial effects on the longevity of cemented acetabular

components and reduce the rate of revision surgery for aseptic loosening.

Aims: 1. Determine the effect of the rim cutter on cement pressure during cup insertion.

2. Examine the effect of the rim cutter on cement penetration distance. 3. Evaluate the

effect of bone grafting of the acetabular notch. 4. Determine the effect of iliac suction

during cement pressurisation. 5. Compare the behaviour of bone cement with Play

Dough®.

Materials and Methods: 1. Sawbones hemi pelvis models were fitted with pressure

transducers at the rim and apex of the acetabulum. Peak pressure was measured upon

insertion of cups with different flange sizes and when the acetabulum was prepared with

the rim cutter. 2. Foam cavities were used to measure the depth of cement penetration

when the same cups and rim cutter were used. 3. Hemi pelvis models were modified to

simulate bone grafting of the acetabular notch. Again, pressure sensors were mounted at

the apex and rim of the acetabulum. Intra-acetabular cement pressure was compared with

native acetabulae. 4. A back bleeding model of the acetabulum was fitted with a suction

catheter. The effect on cement penetration into cancellous bone was measured compared

with no suction. 5. Play Dough® pressurisation and penetration into hemi pelvises and

foam was compared to bone cement.

Results: 1. Significant increase in peak apex and rim pressures when flanged cup inserted

into an acetabulum prepared with the rim cutter compared with both flanged and

unflanged cups alone. 2. Significant increase in cement penetration at the rim of the

acetabulum when rim cutter used and flanged cup inserted when compared with flanged

and unflanged cups alone. 3. Significant increase in intra-acetabular pressure when

cement pressurised in presence of simulated acetabular notch bone grafting compared

with normal acetabulae. 4. Significant increase in cement penetration distance when

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suction used compared with no suction. 5. Significant differences in the flow

characteristics between bone cement and Play Dough®.

Conclusion: The authors recommend preparation of the acetabular rim with the rim cutter

and bone grafting of the acetabular notch to improve the depth and uniformity of the

cement mantle in cemented primary THA. Play Dough® at room temperature is not a

suitable substitute for bone cement in in-vitro cementing studies.

Key words: hip, acetabulum, cement, flanged, cup, rim cutter, suction, pressurisation,

bone graft, acetabular notch, arthroplasty, penetration, Play Dough®.

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Contents:

Abstract………………………………………………………………..Page 1-2.

List of Diagrams and Photographs…………………………………….-5.

Statement of Authorship……………………………………………….Page 6.

Acknowledgements…………………………………………………....Page 7.

Prologue……………………………………………………………….Page 8.

Chapter One: Background……………………………………………..Page 9-19.

Chapter Two: Materials and Methods…………………………………Page 20-33.

Chapter Three: Results…………………………………………………Page 34-60.

Chapter Four: Discussion………………………………………………Page 61-83.

Chapter Five: Conclusion……………………………………………...Page 84-85.

Disclosure……………………………………………………………...Page 86.

Appendix 1……………………………………………………………..Page 87-88.

Bibliography……………………………………………………………Page 89-93.

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List of Diagrams and Photographs:

Photo 2.1: Sawbones hemipelvis model. Photo 2.2: Reamed hemipelvis model mounted in vice. Photo 2.3: Holes drilled in apex and rim of acetabulum. Photo 2.4: Holes drilled at apex and rim of acetabulum. Photo 2.5: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.6: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.7: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.8: Vinyl glove placed in acetabulum prior to cement insertion. Photo 2.9: Simplex bone cement. Photo 2.10: Load sensor mounted in shaft of acetabular pressuriser. Photo 2.11: Laptop computer and data acquisition unit. Photo 2.12: Patch of foam adhered to acetabular notch. Photo 2.13: Exeter Contemporary Cup prior to trimming. Photo 2.14: Exeter Contemporary Cup prior to trimming. Photo 2.15: Unflanged Exeter contemporary cup used. Photo 2.16: Flanged Exeter contemporary cup used. Photo 2.17: The Rim Cutter. Photo 2.18: The Rim Cutter assembled. Photo 2.19: Hemipelvis model reamed and prepared by using the rim cutter. Photo 2.20: Closer view of the acetabulum prepared with the rim cutter. Photo 2.21: Exeter contemporary cup with the flange trimmed to the second groove. Photo 2.22: Acetabular foam cavity used for cement penetration testing. Photo 2.23: Computer generated image of foam cavities. Photo 2.24: Schematic drawing of the foam cavities used to simulate acetabulae. Photo 2.25: Foam cavity mounted in jig. Photo 2.26: Pressurisation of the foam acetabulum. Photo 2.27: Pressurisation of the cup in the foam cavity. Photo 2.28: Closer view of cup pressurisation. Photo 2.29: Cavity prepared by rim cutter. Photo 2.30: Side view of cavity prepared by rim cutter. Photo 2.31: ‘Open cell’ foam used in the third part of the study. Photo 2.32: Closer view of ‘open cell’ foam. Photo 2.33: Foam pieces placed in metal box. Photo 2.34: Side view of foam in metal box. Photo 2.35: Pressurised cement in foam, with the various points labelled A to D. Photo 2.36: Side view of foam piece after cement pressurisation. Photo 2.37: Cement penetration measured by drawing a line between opposite corners of

the foam. Photo 2.38: The Eschmann TJ 240H portable suction machine. Photo 2.39: Motor oil fills the inverted 60mL syringe, suspended 77 cm above the metal

box.

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Photo 2.40: Motor oil fills the metal box and the suction catheter can be seen at point C. Photo 2.41: Pressurisation of the cement whilst the foam bleeds oil and suction catheter

aspirates fluid from the metal box. Photo 2.42: Manual pressurisation of the cement in the metal box. Photo 3.1: Pressurisation of cement in the native acetabulum. Photo 3.2: Unflanged cup has been inserted into the hemipelvis model. Photo 3.3: Holes seen in extracted cement mantle. Photo 3.4: No holes seen in extracted cement mantle of Rim Cutter group. Photo 3.5: Anterior view of Rim Cutter cement mantle after cup removed. Photo 3.6: Cementing of the unflanged cup into the acetabular foam cavity. Photo 3.7: Cement extrudes out of the base of the cavity in the rim cutter tests. Photo 3.8: Closer view of cement extruding from holes. Table 3.1: Pressures recorded in the normal acetabulum. Table 3.2: Pressures recorded using Play Dough® in normal acetabulum. Table 3.3: Pressures recorded using cement in a ‘grafted’ acetabulum. Table 3.4: Pressure recordings in unflanged cup group. Table 3.5: Pressures recorded in the flanged cup group. Table 3.6: Pressures recorded in Rim Cutter group. Table 3.7: Penetration of pressurised bone cement (in millimetres). Table 3.8: Measurements of Play Dough penetration in each trial (millimetres). Table 3.9: Mean force exerted on pressuriser (N). Table 3.10: Penetration of bone cement with insertion of unflanged cup (millimetres). Table 3.11: Penetration of bone cement into cavities with a flanged cup inserted (mm). Table 3.12: Penetration of bone cement in cavities prepared by rim cutter (millimetres). Table 3.13: Force exerted on pressuriser in different phases of penetration testing. Table 3.14: Maximal force required to seat each cup type. Table 3.15: Cement penetration distance at rim of dry foam (millimetres). Table 3.16: Cement penetration distance in dry foam with suction at point ‘C’

(millimetres). Table 3.17: Cement penetration distance into wet foam with no suction (millimetres). Table 3.18: Cement penetration distance with back bleeding and suction at point ‘C’

(millimetres). Table 3.19: Forces exerted on cement in foam under various experimental conditions. Table 3.20: Mean cement penetration at various points (in mm).

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Statement of Authorship:

The work contained in this thesis has not been previously submitted to meet requirements

for an award at this or any other higher education institution. To the best of my

knowledge and belief, the thesis contains no material previously published or written by

another person except where due reference is made.

Signature

Date

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Acknowledgements: The author would like to acknowledge the principal supervisors of this project: Mr John

Timperley, Professor Ross Crawford and Dr Clive Lee for their invaluable assistance,

guidance and support in this project. Without their input this study would not have been

possible. Thanks are also extended to the Engineering Department of the University of

Exeter whose generous assistance with the design and loan of custom built testing

equipment is much appreciated. Gratitude is also expressed to the management of the

Princess Elizabeth Orthopaedic Centre for the use of their laboratory facilities. Assistance

with statistical methods was generously provided by Ms Sarah Whitehouse from the

Queensland University of Technology to whom I am most grateful. The suction testing in

part three required an assistant to manage the flow of oil and this role was carried out by

Ms Fiona Graham. The author thanks her for her assistance and support throughout the

whole study. A special mention must go to Emeritus Professor Robin Ling who is a

fantastic role model and an inspiration to the author. Finally thanks must go to the Stryker

Corporation without whose financial contribution towards the testing equipment and

materials this study would not have been possible.

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“If a certain cup can, at the time of cup insertion, produce higher cement intrusion

pressure and depth than other cups, and if a cup, when completely inserted, is positioned

concentrically within the reamed acetabulum with a uniform cement thickness without

‘bottoming out’, then this feature would be highly desirable for lasting fixation of the

acetabular component.”

Oh, Sander and Trahearne (1985)

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Chapter One: Background: The aims of the studies in this thesis are fivefold: (1) To determine the effect of the rim

cutter on cement pressure during cup insertion, (2) to examine the effect of the rim cutter

on cement penetration distance, (3) to evaluate the effect of bone grafting of the

acetabular notch, (4) To determine the effect of iliac suction during cement

pressurisation, (5) To compare the behaviour of bone cement with Play Dough®. The

hypotheses being tested is that each technique will increase the cement pressure or

penetration distance in vitro. Play Dough® is hypothesized to behave similarly to bone

cement. A graphical representation of the thesis plan is shown on page 19.

The history of cemented acetabular components is an interesting one. This chapter will

endeavour to explain the history of cemented total hip replacement, the advances that

have been made in cementing technique, the current problems with acetabular

pressurisation and discuss the various experimental techniques, which aim to improve the

long term outcome of cemented sockets.

History:

The first total hip replacement was performed in 1961 by Sir John Charnley in

Wrightington, UK. A number of different materials for the cup were trialled by Sir

Charnley including Teflon®. After much consideration and experimentation the

components he used consisted of a stainless steel femoral stem, and femoral head, which

articulated with a high molecular weight polyethylene cup (Charnley, 1995). Sir Charnley

used both press-fit and cemented cups in his initial series of 582 patients. The cemented

components were held in place with polymethylmethacrylate bone cement, otherwise

known as PMMA. The techniques for cementing of the components used by Sir Charnley

became known as first generation cementing techniques. On the femoral side this

involved broaching of the femoral canal, bowl mixing of the cement, finger packing of

the cement into the femur and inserting of one of a limited selection of different sized and

shaped components. On the acetabular side the subchondral bone was reamed, cement

finger packed into the cavity and the cup inserted. Since that time, the technique of

inserting and fixing the femoral and acetabular components have advanced significantly.

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These changes have evolved to combat a variety of problems, which inevitably arise

when a completely new procedure is devised. Initially the results were very good, with a

high proportion of patients (>90%) experiencing relief of their debilitating arthritic

symptoms (Charnley, 1995). As the practice of cemented total hip replacement spread

over the years however, a number of the components appeared on radiographic review to

be loosening, and a (smaller) proportion of patients who developed symptoms related to

this required revision surgery. As the period of time since implantation of total hip

replacements increased, the problem of loosening of either of the components became

more pronounced. While the incidence of cemented femoral component loosening has

reduced significantly over the last twenty years, the rate of acetabular component

loosening has not shown such encouraging progress (Mulroy and Harris 1990). In fact,

countless authors over the last twenty years have stated that the most common reason for

revision total hip replacement is loosening of the acetabular component (Chandler et. al.

1981, Ranawat et. al. 1984, Wroblewski 1986, Kavanagh et. al. 1989, Schulte et. al.

1993, Ranawat et. al. 1995, Mulroy et. al. 1995, Sochart and Porter 1997, Wroblewski et.

al. 1999 and Callaghan et. al. 2000).

Each year there are more than one million hip replacement operations performed across

the globe (Flivik, 2005). According to the Australian Orthopaedic Association National

Joint Replacement Registry Annual Report of 2006, there were 20,683 total hip

replacements for the financial year 2004 – 2005, a number that is increasing each year.

Of these, only 9.5% (1965) used cemented acetabular fixation, a number that is gradually

decreasing each year. This indicates a trend in Australia away from cemented acetabular

fixation in favour of newer technologies such as cementless and hybrid fixation systems

and hip resurfacing. In other countries such as Sweden greater than 90% of the 10,000

plus hip replacements performed each year utilise cemented acetabular fixation (Flivik,

2005). Furthermore, because of its excellent long term results, cemented fixation systems

provide a level of reliability that at least matches if not exceeds that of cementless and

hybrid fixation. The Australian Joint Replacement Registry reports that in the 75 years

plus age group cemented total hip replacement has the lowest rate of revision at three

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years of all fixation systems (1.52%). Of the 1965 cemented cups implanted in 2004-05

in Australia, just under half (46%) were Exeter Contemporary cups.

Comparing the last five years of data from the Australian Joint Replacement Registry

shows that revision surgery accounts for 12% of all total hip replacement operations. In

total 9194 cases out of 19257 revision operations (47.7%) were performed because of

loosening of a prosthesis. Of all major hip revision operations performed acetabular

revisions far outnumber femoral revisions (37.2% vs. 21.2%). This further highlights the

need for improved acetabular fixation methods. Countless authors agree that the key

determinant in ensuring adequate, long lasting cup fixation in cemented total hip

arthroplasty is the quality of the cementing technique (Flivik et. al. 2004).

Strength of the Bone-Cement Interface:

The work of Krause et. al. in 1982 showed that bone cement fixation is dependent on the

penetration of bone cement into the cancellous interstices of the bone. They also

demonstrated that cleaning of the bone with high-intensity lavage allows improved

cement penetration, which thereby increased tensile and shear strength of the bone-

cement interface. These effects of improved cement penetration have been supported by a

number of other authors (Halawa et. al. 1978, Panjabi et. al. 1986, Macdonald et. al.

1993, Juliusson et. al. 1994). However, Majkowski et. al. in 1994 suggested that

penetration beyond three millimetres does not enhance the strength of the bone-cement

interface. A later study of the effects of bone-porosity and cement penetration on the

bone-cement interface was conducted by Graham et. al. in 2003. That study concluded

that the strength of the bone cement interface is increased significantly by increased bone

porosity and consequently by increased cement penetration. Kuivila et. al. (1989) also

showed that increased cement penetration leads to increased tensile strength at the bone-

cement interface, a finding that was also reported by Eftekhar and Nercessian (1988).

Causes of Prosthesis Loosening:

Anderson et. al. postulated in 1972 that the causes of prosthesis loosening in hip

replacements could be due to a combination of factors including micro motion at the

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bone-cement interface, thermal bone damage at the time of cement polymerisation or

chemical factors. In 1978 and 1979 a number of studies (Beckenbaugh and Ilstrup 1978,

MacBeath and Foltz 1979 and Gruen et. al. 1979) quoted the rates of prosthesis loosening

seen on radiological examination to be from 15 to 21 per cent after between one and

seven years of followup. Later, Mjoberg (1991 and 1994) suggested that loosening

resulted from early prosthetic instability and that the ‘late’ loosening being reported was

perhaps really just late detection of previously loose components. It has also been

suggested by DeLee and Charnley in 1976 that part of the loosening process may stem

from a layer of blood forming at the bone-cement interface, which could later go on to

form a fibrous tissue membrane. This concept has been supported by a number of other

authors (Lee and Ling 1981, Eftekhar and Narcessian 1988 and Bannister et. al. 1990).

Charnley suggested in 1975 that the accumulation of blood at the bone-cement interface

may be the cause of the radiolucent lines seen on post operative radiographs. Later,

DeLee and Charnley (1976) first classified the radiological appearance of those lucent

lines at the bone cement interface in their famous paper. Freeman et. al. 1982 first

suggested that radiolucency at the bone-cement interface may not be caused by micro-

motion but in fact be related to a biological cause. Later, Schmalzreid et al (1992)

claimed that the cause of aseptic loosening of the cemented acetabular component was in

fact different to that causing the femoral component to loosen. While the femoral

component is thought to loosen secondary to mechanical forces, they proposed that

acetabular loosening was indeed secondary to biologic phenomena. New et. al. (1999)

suggested that perhaps the better results seen recently are secondary to reduced fibrous

tissue between cement mantle and bone, which leads to reduced wear debris behind the

prosthesis.

Advances in Cementing Technique:

PMMA is by nature viscoelastic, which is important when considering the topic of

pressurisation of the acetabulum. The aim of pressurisation is to achieve an adequate

cement mantle with extensive cement interlock in cancellous bone interstices (Ranawat

et. al. 1997). This manifests stability at the bone-cement interface and prevents micro-

motion and subsequent loosening of the prosthesis. Pressurisation will therefore ideally

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force PMMA far enough into cancellous bone to achieve a strong interlock and then

prevent movement of the cement until it has polymerised. Pressurisation must be capable

of overcoming all additional forces that act either on or within the cement. The work of

Markolf and Amstutz in 1976 investigating the penetration and flow of bone cement was

able to highlight a number of important factors. Firstly, they showed that the penetration

of bone cement into cancellous bone is proportional to the pressure applied to the cement

bolus. They also demonstrated that the majority of the depth of cement penetration

occurred in the first two seconds of pressurisation (76%) and that the depth of penetration

was three times greater when cement was pressurised at four minutes from mixing rather

than the standard six and a half minutes. Another of their findings was that vigorous

finger packing of cement produced very high pressures for a short period of time and

caused greater cement penetration than low pressure sustained for a longer period of time.

Bayne et. al. (1975) showed that higher pressures applied to the cement mantle during

polymerisation may lead to a reduction in cement porosity. The relationship between the

pressure and penetration of bone cement was also studied by Panjabi et. al. in 1983 and

1986. Their research suggested a positive logarithmic relationship between the pressure

applied to the cement and the depth of cement penetration. They also showed that there

was a linear relationship between cement intrusion penetration and bending and axial

stiffness of the bone-cement interface. A similar link between pressure and penetration

has been reported by a number of authors (Convery and Malcolm 1980, Bannister et. al.

1988, Graham et. al. 2003, Flivik et. al. 2004). Oh and Harris in 1982 proposed a cement

fixation system by pressurising cement in the acetabulum for five to ten seconds. This

was followed up by Oh et. al. in 1983 in their study of the effect of pressurising keying

holes in a dry cadaveric acetabulum with multiple short bursts of high pressure, which

yielded a cement penetration depth of 19.2 to 25.5 millimetres. A landmark study was

performed by Benjamin et. al. in 1987 who studied the effects of bleeding on cement

penetration. Their findings showed that water was able to displace low viscosity PMMA

at a pressure of ten centimetres of water. They also found that water was able to displace

PMMA up to seven minutes after mixing of cement. This research suggested a change in

practice to apply pressure onto the cement mantle until such time as its viscosity would

be sufficient to resist the back bleeding pressure of blood. Otherwise, the cement mantle

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would be displaced both in the time between removing the pressuriser and inserting the

prosthesis and after insertion of the prosthesis if it was not itself pressurised, resulting in

a less adequate bone cement interlock. Majkowski et. al. in 1994 published results

suggesting that pressurisation for thirty seconds is adequate and that additional

pressurisation of cement in the dough stage after that is of no additional value at

physiological bleeding pressures. The simulated bleeding flow rates used in their study

however, were somewhat lower than those used in previous studies. These proposed

techniques were developed further by New et. al. (1999) who suggested that short

duration, high intensity pulses of pressure would be more effective when combined with

a sustained background pressure to resist back bleeding pressure and elastic recoil of the

cement.

Other aspects of cementing technique have been added and refined over the last thirty

years. In 1978, Oh et. al. found that when cementing femoral components, distal plugging

of the femoral canal with PMMA caused increased cement intrusion pressure, deeper

cement penetration and consequently increased tensile strength of the bone-cement

interface. Similar findings were also reported later by Macdonald et. al. (1993). In current

practice plastic or bioabsorbable femoral canal plugs are used. Cleaning of the cancellous

bone bed is known to remove debris and facilitate flow of cement and hence increased

cement penetration. This notion was supported by Krause et. al. (1982). Majkowski et. al.

(1993) examined cement penetration into bovine bone and reported no difference

between continuous lavage and pressurised pulsed lavage. However, jet lavage was

shown to be more effective at facilitating cement penetration than syringe lavage by

Breusch et. al. (2000) in their study of cadaveric femora, and also in the tibia by Dorr et.

al. (1984). Microcrystalline collagen was used as a topical haemostatic agent, but it was

later shown to significantly reduce the shear strength of the cement-bone interface by

Lange in 1979. Later, hydrogen peroxide was shown to reduce bleeding from cancellous

bone by Hankin et. al. (1984). This practice has since become common in cementing

techniques for all components. Halawa et. al. (1978) showed that in fact the greatest shear

strength at the cement-bone interface in the femur was achieved with pressurised low

viscosity cement. Noble and Swarts (1983) suggested that low viscosity cement would

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allow increased cement penetration into cancellous bone. These findings were confirmed

by Macdonald et. al. (1993) who showed that the use of low viscosity cement did indeed

improve cement penetration. However, a study by Mjoberg et. al. (1987) found no

difference in cup loosening rates between low viscosity and high viscosity cement use.

Current practice reflects a compromise between cement of sufficiently low viscosity to

allow adequate cement penetration and cement that is of such a viscosity that it is too

difficult for the surgeon to handle (Dorr et. al. 1984). Hypotensive epidural anaesthesia

has been frequently applied in cases of cemented total hip arthroplasty for over ten years.

Ranawat et. al. (1991) demonstrated that hypotensive epidural anaesthesia allowed deeper

penetration of cement into cancellous bone of the acetabulum on radiographic evidence.

Problems with Pressurisation:

Much attention has been focussed on improving the acetabular cement mantle. The

majority of authors claim that the optimal depth of cement penetration is three to five

millimetres (Huiskes and Sloof 1981, Walker et. al. 1984, Askew et. al. 1984, Eyerer and

Jin 1986, Miller 1990). A greater depth of cement penetration imparts minimal increased

interfacial strength and avoids some of the major problems with a thicker cement mantle

(Dorr et. al. 1984). Cement mantles thicker than ten millimetres may cause increased heat

generation when the cement is polymerising, which can cause bone necrosis, bone loss

and reduction in the strength of the bone cement (Homsy et. al. 1972, Huiskes and Sloof

1981, Sew Hoy et. al. 1983). This concept is contested by Jefferiss et. al. in their 1975

paper in which they claim that bone necrosis is not a consequence of thermal damage

caused by polymerisation of PMMA. The problem with adequate generation of pressure

in the acetabulum stems from two main features. The acetabulum itself is an open

hemisphere, which compared with the relatively closed, cylindrical shape of the femoral

canal makes containment of cement more difficult. This makes it more difficult to

generate the pressures required for adequate cement penetration in the acetabulum. The

second feature is the acetabular notch under the transverse ligament. This results in

inconsistency in the rim of the acetabulum, and hence makes achieving a closed cavity

for pressurisation much more difficult (Flivik et. al. 2004). A number of different shaped

and styled pressurisers have been designed to combat this problem and to date, there is no

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accepted gold standard design. The main drawback with current techniques in acetabular

cement pressurisation is extrusion of cement underneath the transverse ligament out

through the acetabular notch (Bernoski et. al. 1998). Martin et. al. (2003) support the

notion that the majority of cement extrusion occurs in this region with their report of a

series of post operative radiographs in which the main location of cement extrusion was

anteroinferiorly. Some authors (Oh et. al. 1983, Flivik et. al. 2004) have recommended

sequential pressurisation of keying holes in the acetabulum in order to overcome the high

pressures needed for adequate cement pressurisation. However, Gruen et. al. (1976) and

Saha and Pal (1984) have reported the formation of laminations in the cement mantle

with such pressurisation, which may cause weakening of the tensile and shear strength of

the cement mantle. Flivik et. al. (2004) claim that if the laminations form before 3.5

minutes from mixing of cement, that the polymerisation process is able to bridge the

laminations and hence prevent any reduction in the strength of the cement mantle.

Flanged and unflanged cups:

The problem of containing bone cement in the region of the acetabular notch has been

addressed since 1976 when the flanged socket was first introduced (Shelley and

Wroblewski 1988). The use of this socket was however, complicated by a tendency for it

to be placed in anteversion. Sir Charnley designed the Ogee flanged cup in the early

eighties when cross-linked polyethylene became available, which could be effectively

cast into an asymmetrical shape (Shelley and Wroblewski 1988). Oh et. al. (1985)

showed that cement pressures during cup intrusion are significantly elevated by the

employment of a cup with a continuous flange. This was supported by Shelley and

Wroblewski (1988) who showed that Ogee flanged cups generate three times more

pressure than standard unflanged cups. They also claimed that it is difficult to insert an

unflanged cup concentrically without it ‘bottoming out’. Bernoski et. al. (1998) also

agreed with this notion, by claiming that the use of a cup to generate pressure in the

acetabulum is not possible unless the cup is fitted with a suitably designed flange.

Beverland et. al. (1993) showed that inadequately sized and prepared flanges can actually

impair the pressurisation of cement in the acetabulum. In 2004, Parsch et. al. reported

that the inserting of Ogee flanged cups produced a higher intra-acetabular pressure than

17

unflanged cups, but did not result in significantly increased cement penetration. Similar

findings were also reported by Flivik et. al. (2004).

Bone grafting of the acetabular notch:

Bone grafting has had a role in cemented total hip arthroplasty for many years

(McCollum et. al. 1980). It has been utilised in patients suffering from developmental

dysplasia of the hip, where grafting allows a deeper and more stable socket to be formed

into which the cup can be cemented. Bone grafting in the acetabulum has also been used

in revision hip arthroplasty in cases of extensive bone loss in the form of impaction

grafting and primary hip arthroplasty in the presence of acetabular protrusio, where it is

essential to stabilise the medial wall of the acetabular cavity. McCollum et. al. reported in

1980 that in the case of acetabular protrusio bone grafts placed in the medial wall of the

acetabulum under cemented sockets had incorporated into the bone within three months

on radiographic evidence. Similar work into medial wall bone grafting by Heywood

(1980) and Mendes et. al. (1982), (1983) and (1984) suggested that the graft had united

within six to ten months. It has also been demonstrated by Xenakis et. al. in 1997 that

bone graft used with uncemented cups in the acetabulum will reliably consolidate and

incorporate in patients with developmental dysplasia of the hip. To the authors’

knowledge there have been no published studies specifically examining the effect of bone

grafting the acetabular notch in primary cemented total hip arthroplasty. However, an

unpublished study conducted in Exeter examining the effects of bone grafting of the

acetabular notch showed a reduction in cement extrusion clinically and an increase in

cement penetration radiographically.

Use of Iliac Sucker:

The use of a suction catheter or retractor has been proposed by Berend and Ritter (2002)

to maintain a dry acetabular cavity prior to and during cementing of sockets. Their

anecdotal evidence suggests that intra-pelvic suction may improve the cement mantle and

reduce the incidence of early radiolucent lines. This could be applied particularly to

DeLee-Charnley zone 1, where the presence of early radiolucency is currently the best

predictor of long-term stability of the acetabular component. Use of a suction catheter to

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improve cancellous bone drying and cement penetration has also been reported as being

effective in total knee arthroplasty by Banwart et. al. (2000). Their study used cadaveric

tibiae and although it did not show an increase in cement penetration, it did show better

bone-cement interlock, with fewer voids.

Play Dough

Experimental studies of acetabular cementing techniques tend to place a very high

demand on materials. For each experiment, all components need to be brand new and

once cemented cannot obviously be re-used. This means that unless non-stick coatings

are used on the various components, they are all single use. It is not known what effect

coating components has on their physical and mechanical characteristics. The authors

have postulated that Play Dough® used at a particular temperature would be a reasonable

substitute for bone cement in terms of physical and mechanical behaviour. This would

allow re-use of all materials in further studies involving bone cement.

Proposed rationale behind the study:

A new device has been designed to combat the problem of cement leakage, the rim cutter.

The rim cutter is designed to attach to conventional acetabular reamers. It allows a

standardised, uniform rim to be cut in the acetabulum. A flanged cup that has been

trimmed to fit the rim precisely can then be inserted into the acetabulum, which will

hopefully reduce the volume of cement extrusion, thereby increasing the cement pressure

and increasing the depth of cement penetration. It is hoped that the improvement in

cement penetration will lead to less radiolucency in zone 1 and reduce the incidence of

cup loosening. The purpose of this study is also to compare the pressure generated by two

different acetabular conditions: a native acetabulum and one in which the acetabular

notch has been bone grafted. The study will also investigate the pressures generated when

inserting flanged and unflanged cups using conventional reaming techniques and when

using the rim cutter. The depth of penetration of the cement mantle in each case will also

be compared. The study will also compare the effect of the iliac aspirator on cement

penetration in the presence and absence of back bleeding and the physical properties of

play dough compared with PMMA.

20

Chapter Two: Materials and Methods: The study was broken into three trials. The first trial was designed to measure the cement

pressure generated in the acetabulum under various conditions. The second trial aimed to

measure the depth of penetration of cement into simulated acetabulae under a variety of

conditions. The third trial attempted to assess the effectiveness of the iliac suction

retractor used in cemented total hip arthroplasty.

Part One

The first part of the trial was divided into six experiments, each of which was performed

six times. For each experiment the same basic set up was used. The variation lay in the

preparation of the acetabulum. For the testing of pressure sawbones (Sawbones Inc.,

Sweden) hemi pelvis foam models were used (Item #1307), shown in photo 2.1. Each

model hemi pelvis was mounted in a standard vice (City 60mm swivel table vice stock

no. 3335), such that the acetabular concavity was directed upwards. Next, the hemi pelvis

model was reamed in the conventional manner using Stryker acetabular reamers (Stryker,

Howmedica) up to a 56mm diameter as shown in photo 2.2.

Two drill holes were made using a 9mm diameter drill at the apex (90°) of the

acetabulum and at the rim (10° angle to the plane of the acetabular lip) (see photo 2.3 and

2.4). Next, the drill holes were manually tapped with a T-handled tap. A finely calibrated

pressure transducer [RDP Group model A105, 0 to 200 psi (0 to 13.5 bar), (Grove Street,

Heath Town, Wolverhampton WV10 0PY, UK)] was then screwed into each of the two

drill holes such that their upper edges were flush with the wall of the acetabular cavity as

shown in photos 2.5, 2.6 and 2.7. The pressure transducers were then coated with silicone

grease to prevent the cement from adhering to them. These pressure sensors were then

connected up to a laptop computer [Toshiba Tecra A2 CE] via an analog to digital

converter. Finally the acetabular cavity was lined with a vinyl glove, which had been cut

open so that a layer of vinyl separated the cement from the pressure transducers as shown

in photo 2.8. This also prevented cement from adhering to the model, allowing the

cement to be removed after each test and the same model to be used for all testing.

21

The control group consisted of a model prepared in exactly the way described above. One

mix of PMMA (Simplex, Stryker Howmedica, photo 2.9) was hand mixed at 2-3Hz and

the bone cement introduced to the acetabular cavity at exactly six minutes from the

commencement of mixing, when the cement was at dough stage. The ambient

temperature in the laboratory at the time of testing ranged from 11.2°C to 13.5°C. For

this reason, the cement took longer to reach dough phase. The cement was then

pressurised at 275N using a Stryker pressuriser (Stryker, Howmedica) for one and a half

to two minutes, depending on the quality of the seal around the lip of the acetabulum. A

constant force was manually applied to the pressuriser by an examiner for the duration of

the test. The force being applied to the pressuriser was measured by a load sensor (RDP

model MLC, 0 to 500 N), which was mounted in the middle of the pressuriser (see photo

2.10). The measured force was then displayed graphically and simultaneously on the

display screen of the laptop computer. This provided the examiner real time feedback

about their force on the pressuriser and ensured that a known, constant force was applied

to the cement. During pressurisation, continuous measurements were made of the

pressure generated in the cement mantle at the apex and the rim using a custom designed

computer program on a laptop computer mentioned earlier as shown in photo 2.11. The

cement mantle was then removed from the hemi pelvis model using the glove lining the

socket and the pressure and force trace saved on the computer. The whole process was

then repeated five more times.

The next step was to compare the pressures generated in a standard acetabulum with

those in which the acetabular notch had been bone grafted. Special hemi pelvis models

(Sawbones, Sweden, item #1307-4) were requested of and constructed by Sawbones Ltd.

in which a piece of foam had been adhered to the model, which covered the acetabular

notch so that the rim of the acetabulum took the form of a continuous circle (see photo

2.12). The model was prepared in exactly the same manner as previously i.e. reamed to

56 mm diameter, two holes drilled at apex and rim and pressure transducers screwed into

the holes. Hand mixed PMMA was again introduced to the acetabular cavity at exactly

22

six minutes after mixing began (again due to the low temperature in the laboratory), when

the cement had reached dough phase. The cement was then pressurised using the same

apparatus, by the same examiner using the same force (275N). Again pressure

measurements were recorded on the computer for the duration of the test. The cement

mantle was then removed and the process was again repeated five more times.

To determine whether the physical properties of Play Dough® matched those of bone

cement, another round of pressurisation tests were conducted. In these tests, the standard

hemi pelvis models were used. There was no simulated grafting of the acetabular notch in

these models. The hemi pelvis that was used in the initial cement pressurisation test was

re used in these tests. Instead of introducing low viscosity PMMA, the same volume of

Play Dough® was placed in the acetabular cavity and pressurised in the same manner as

the previous twelve times. The duration of pressurisation was chosen to be ninety seconds

as this was the approximate length of time of all of the pressure testing in the previous

experiments. The temperature of the play dough was maintained in the same range as the

bone cement 11.3° to 13.5° Celsius. At that temperature the play dough was quite

viscous, it was not warmed because the degree of warming and rate of cooling could not

be adequately controlled. There were also no previous reports of comparisons between

bone cement and play dough at the same temperature, which would be the obvious

starting point for experimentation. The pressure generated in the acetabulum was again

measured continuously and recorded on the computer. The process was repeated five

times, with the play dough being removed from the acetabulum and re-moulded into a

bolus prior to each test. Once the apparatus had been dismantled at the end of testing, the

hemi pelvis was labelled.

These three different experiments allowed comparison of pressure generated in normal

versus grafted acetabulae and also the pressure generated in normal acetabulae using

PMMA and using Play Dough®. The results are described in chapter 3.

It was also necessary to measure the pressure generated in normal acetabulae when either

flanged or unflanged cups are inserted, and also when the acetabular rim is prepared for a

23

flanged cup using the rim cutter. For this part of the study only two standard Sawbones

hemi pelvis models (Sawbones, Sweden, item #1307) were used. None of the hemi pelvis

models used had simulated acetabular notch grafting. For the first two trials the

acetabulum was prepared in exactly the same manner as in the previous trials. The model

hemi pelvis was mounted, reamed, drilled and two pressure transducers fitted. The

temperature in the testing room was controlled such that it ranged from 20.0°C to 21.1°C.

Simplex bone cement was again hand mixed at 2-3 Hz for the first one and a half minutes

and introduced into the acetabular cavity at exactly six minutes (i.e. the total standard

time before cement insertion into the acetabulum [4 minutes] and pressurisation [2

minutes] in vitro). It was not necessary to pressurise the cement for these tests as the

foam models used featured closed pores, which would not allow interdigitation of

cement. At this point an Exeter contemporary cup (Stryker, Howmedica) seen in photo

2.13 and 2.14 with either the flange completely removed flush with the edge of the cup

itself (i.e. unflanged, see photo 2.15) or the flange trimmed at the first groove (i.e. flange

intact, see photo 2.16) was inserted into the acetabular cavity using the modified Stryker

acetabular pressuriser (Stryker, Howmedica). At the six minute mark, the cup was

manually inserted into the acetabulum. The duration of cup insertion lasted twenty to

thirty seconds in an attempt to reflect in vivo practice. This aspect of the experiment was

tightly controlled as pressure measurements of this nature rely on reproducible insertion

forces on the cups. The force applied to each cup was defined as the force required to seat

the cup in twenty to thirty seconds. That force varied depending on the type of cup as a

greater force was required to seat a cup with a larger flange. The cup pusher being used in

these experiments was the Stryker pressuriser mentioned earlier, but with the silicone cap

removed from the head. The load sensor mounted in the centre allowed the same

measurements of force on the cup as in the earlier pressurisation tests. The forces applied

to the cups were recorded and displayed in real time on the computer screen. The

pressure generated in the acetabulum at the rim and the apex was again constantly

measured and recorded on the computer. After the cup had been seated, a constant force

of 100N was applied to the cup via the pressuriser for the duration of the recording trace,

which, in the case of our computer program was three and a half minutes (ten minutes

total trace on computer). The experiment was repeated six times for the flanged cup and

24

six times for the unflanged cup. The same model and cups were used for all of the

experiments.

In the final experiment in Part One, the acetabulum underwent extra preparation. Again a

hemi pelvis model was mounted and the two holes were drilled for the pressure

transducers. The acetabulum was reamed to 56 mm as it had been in all of the prior

experiments, however the rim of the acetabulum was also modified in this case. The rim

cutter attachment to the standard Stryker reamer was used (see photo 2.17 and 2.18). The

acetabular cavity was still reamed to 56 mm, but an additional 3 mm wide ledge was cut

into the acetabular rim, 3 mm deep (see photo 2.19 and 2.20). It is then easily to calculate

that the diameter of the rim of the acetabular cavity is exactly 56 + 6 mm. The flange of

the Exeter contemporary cup was then trimmed along the second groove, to fit the

acetabular cavity precisely as shown in photo 2.21. After the two pressure transducers

were fitted and one mix of hand mixed PMMA was inserted into the acetabular cavity,

the specifically flanged cup was inserted. It was pressurised in the same manner as in the

two previous experiments, with the load sensor in the middle of the pressuriser providing

continuous feedback to the examiner as to how much force they were applying to the cup.

Both the force being applied and the pressure generated in the acetabulum were recorded

and displayed on the computer.

25

A flow chart showing the timing of each part of the experiment is shown below:

These data allowed comparison of cup intrusion pressures between a flanged cup versus

an unflanged cup and a flanged cup versus a flanged cup with the rim of the acetabulum

cut. The results are again described in chapter three.

Mixing Starts Trace Begins

Mixing ceases

Cement In Hand

Cement In Cavity

Cement Not Pressurised

Cup Inserted

Cup Is Seated Pressurisation at 100N

Cup pressurisation Ceases

Time: 0 secs 1min30 3min30 4min 6 min 6 min30 10 min 3min50

26

Part Two

Part Two of the study was aimed at investigating cement penetration under various

conditions. Firstly a comparison was made between bone cement and Play Dough®, then

tests were conducted to compare the cement penetration when using flanged and

unflanged cups and also a flanged cup with the acetabular rim cut. A simulated

acetabular cavity was used for all experiments. These cavities were made from Sawbones

closed cell foam blocks (Sawbones Ltd., Sweden). Each rectangular block was machined

into a hemispherical cavity with an outer diameter of 75 millimetres and an inner

diameter of 60 millimetres1 (see photo 2.22). This was specially designed to fit a 56

millimetre cup with a 4 millimetre cement mantle. Each cavity was further prepared by

drilling 1 millimetre diameter holes through it at predetermined angles. Holes were

drilled at 5° from the vertical axis of the cavity, 45° and 85°. At each angle, four holes

were drilled in each quadrant of the cavity at each of the three aforementioned angles (as

shown in figure 2.23 and 2.24). Therefore, each cavity had a total of twelve x 1

millimetre holes drilled in it. This allowed calculation of an average cement penetration

at each of the three angles: 5°, 45° and 85°. The cavities were manufactured by Stryker

Europe at their plant in Northern France.

These cavities were then mounted in a specially designed jig, which consisted of a metal

box attached to the vice mentioned earlier. The metal box was fitted with a concave

rubber base on the inside and a screw down clear plastic face plate (see photo 2.25). The

cavity was placed in the metal box and the face plate screwed onto the top to hold the

cavity steadily in the box. Conveniently, the metal box was mounted such that the

concavity of the simulated acetabulum faced up obliquely towards the examiner at an

angle of 45°. All experiments were carried out in the same room, which was air

conditioned to a temperature range between 20.0°C and 21.1°C.

1 As the thickness of the blocks was a maximum of 40 mm, the cavity had only a thickness of 12.1 mm at the apex.

27

The first experiment involved setting up the apparatus as described above. PMMA was

hand mixed at 2-3 Hz and inserted into the cavity at exactly four minutes from the

commencement of mixing. The cement was then manually pressurised at a constant force

of 200 N using the same modified Stryker® pressuriser with a load sensor mounted in it

as had been used previously in Part One. The examiner was again able to watch the

computer screen to monitor the force of pressurisation to maintain a constant

predetermined level.

After two minutes, the pressurisation was reduced to 100N to simulate the forces on a cup

after it was inserted. Pressurisation of the foam acetabular cavity is shown in photo 2.26.

This force was maintained for a further one and a half minutes and removed at the eight

minute mark. When the cement had polymerised, the cavity was removed from the jig

and the depth of cement penetration was measured. This was done by using a fine one

millimetre gauge, which was inserted into each of the twelve holes drilled in the cavity.

The thickness of the cavity was pre measured and therefore known at each of the three

angles. By measuring the remaining space left in the hole, it is possible to determine the

depth of cement penetration. Using the four holes an average depth was then calculated

for each angle. This process was then repeated five more times.

In order to compare bone cement with Play Dough®, the bone cement in the previous

experiment was substituted for Play Dough®. The foam cavities were prepared and set

up in exactly the same manner as described above. However, instead of inserting bone

cement, an equal volume of Play Dough® was inserted into the cavity. It was pressurised

using the same modified pressuriser at a constant force for the same time as in the

previous experiment. The foam cavity was then removed from the jig and the holes

measured. The average depth of cement penetration was again calculated. This process

was repeated a further five times using the Play Dough®. The results of the experiments

are discussed in chapter three.

Cement penetration measurements were also required for assessment of the flanged and

unflanged cups and the rim cutter. In these experiments, the flanged and unflanged cups

28

were prepared in the same way as in part one. An Exeter Contemporary cup (Stryker,

Howmedica) was trimmed at either the first groove or flush with the edge of the cup to

create a flanged or unflanged cup respectively. The same foam cavities were used to

simulate the acetabulum. All apparatus was set up in exactly the same manner as

previously. Hand mixed PMMA bone cement at between 20.0°C and 21.1°C was

introduced at exactly four minutes after mixing commenced. The cement was then

pressurised for two minutes at the same known force (200N) and confirmed graphically

by the computer. After pressurisation, either a flanged or an unflanged cup was inserted

into the cavity. The cup was pushed in by the modified pressuriser with the load cell

fitted in it (as used in previous experiments in part one). Once again as in the pressure

tests of part one, the cup was aimed to be seated in 25 to 30 seconds to mimic in vivo

practice. Once seated, the cup was placed under a constant force of 100N by the

pressuriser for another one and a half minutes as shown in photo 2.27 and 2.28. This

again is identical to the technique used in part one. At the ten minute mark, the pressure

trace automatically stopped and the foam cavity with the cup cemented into it was

removed from the jig. The depth of the cement penetration into each of the twelve 1mm

holes was then measured and recorded. The cavity was then labelled and stored. The

process was repeated five times for each of the flanged and unflanged cups.

The final experiment in part two once again utilised the rim cutter. The rim cutter was

attached to the standard Stryker acetabular reamer and as was done in part one and a

ledge of foam three millimetres deep and three millimetres wide was cut in the rim of the

foam cavity as shown in photo 2.29 and 2.30. Again the flange of the Exeter

contemporary cup was trimmed to perfectly fit the rim, by cutting along the second

groove of the cup. This modified cavity was then mounted in the jig as had been done for

the previous twelve tests. Hand mixed bone cement was inserted and pressurised in the

same manner as before. The flanged cup was then inserted, again over a thirty second

period, and pressurised at 100N for one and a half minutes. Finally the foam cavity was

removed from the jig and the depth of cement penetration measured at each angle and

recorded. For each of the thirty experiments in part two, a new foam cavity and a new

cup was used. None of the materials were recycled. These data allowed comparison of

29

cement penetration depth using a variety of acetabular conditions and the results are

discussed in chapter three.

Above is a flow chart showing the timing of each part of the experiment.

Mixing Starts Trace Begins

Mixing ceases

Cement In Hand

Cement In Cavity

Pressurisation Begins 200N

Pressurisation Ceases Cup Inserted

Cup Is Seated Pressurisation at 100N

Cup pressurisation Ceases

Time: 0 secs 1min30 3min30 4min 6 min 6 min30 8min 3min50

30

Part Three

The final part of the study was to examine the effect of the suction retractor (a.k.a. iliac

aspirator) on the depth of cement penetration. Four different experiments were

performed, which examined cement penetration with pressurised PMMA in dry foam,

with and without the use of a suction retractor and pressurised bone cement in ‘bleeding’

foam with and without the use of a suction retractor. To simulate cancellous bone, an

open cell foam block (Sawbones, Sweden, item number 1521–59) was used (shown in

photo 2.31 and 2.32). A metal box of dimensions 7cm x 7cm x 6cm was constructed by

staff at the University of Exeter Engineering Department into which the foam would be

placed. It consisted of five closed sides and one open frame, facing upwards, which was

screwed onto the top (see photo 2.33 and 2.34). A corner of the box was engraved with

the letter ‘A’ so that all other corners could be labelled. This allowed the foam placed in

the box to be labelled after cementing to correspond with its position in the box. A clear

piece of plastic was used to cover the exposed foam in the centre of the metallic frame,

which formed the roof of the box. A hole 47 millimetres in diameter was cut in the

middle of the plastic so that the foam below could be reamed and cemented. Finally,

silicone was used to seal all joins on the inside of the box so that it was both water and

airtight on all sides. This set up was constructed in an attempt to mimic, on a small scale,

a pelvis with the circular hole in the plastic representing the acetabular rim. The box also

had a fluid inlet port installed on one side near the base, and a small four millimetre

diameter hole drilled in one corner of the metallic roof of the box, through which air or

fluid could be aspirated. The foam pieces, originally 15cm x 7.5cm x 2cm were manually

cut into 7cm x 7cm x 2cm squares by a bandsaw. Three such pieces needed to be placed

on top of one another in order to fill the metal box. The clear plastic and the metal frame

were then screwed onto the top of the box. Play dough was used around all joins in the

roof of the box to ensure that it was both air and watertight. This step was required for

practicality reasons as the continual removing and reinserting of the foam pieces from the

box displaced the original silicone (which took 24 hours to cure once reapplied). The

foam was then reamed to a diameter of 47 mm through the hole in the plastic. Once this

31

standard preparation of the foam had been carried out, a variety of experiments were

carried out.

Firstly, one mix of Simplex PMMA bone cement (Stryker, Howmedica) was hand mixed

at 2-3 Hz as in all previous experiments. The ambient temperature in the testing room

remained controlled such that it ranged between 20.0°C and 21.5°C. The pressure trace

on the computer began at the commencement of cement mixing (T=0 mins). The cement

was then manually pressurised at a constant force of 200N by the aforementioned

modified Stryker acetabular pressuriser in the same manner as described in parts one and

two. Visual feedback was provided to the examiner via the display on the computer

screen. Pressurisation at 200N lasted for two minutes as in previous tests, at which point

the force on the pressuriser was reduced to 100N for the final four minutes of the pressure

trace. After pressurisation each region of the cement mantle was labelled A to D

according to the position of each quadrant in relation to the labelled corners of the metal

box. When the cement had polymerised, the foam with its embedded cement mantle was

removed from the jig. The uppermost two pieces of foam became solidly bound together

by the cement interdigitation and had effectively become one piece (shown in photos 2.35

and 2.36). Lines were drawn on the piece of foam from corner to corner, which

highlighted the thickness of the cement mantle at each corner of the foam block as shown

in photo 2.37. The depth of cement penetration from the edge of the simulated acetabular

cavity to the outer limit of the cement in the line marked was measured and recorded.

Finally, the foam and attached cement mantle from each test was placed in a labelled

clear plastic bag and stored. This process was repeated another five times.

The next step was to determine whether the depth of the cement penetration was different

when a suction catheter was used. A small hole of four millimetres in diameter was

drilled in a corner of the frame plate of the box. Into this hole was placed a (spinal)

suction catheter to a depth of twenty millimetres. The same process as earlier was

repeated, with hand mixed bone cement introduced at four minutes and pressurised with

the modified pressuriser at 200N for two minutes, then pressurised at 100N for a further

four minutes. In these tests however, the suction catheter was switched on for the

32

duration of the cementing. The suction used in these experiments was provided by a

portable suction machine (Eschmann TJ 240H, shown in photo 2.38) as used in some

operating rooms. It provides a variable suction pressure depending on the resistance to

flow of material. It was set to provide 200mmHg of suction when the three millimetre

calibre spinal sucker is fitted (in air). This pressure increased automatically as the

viscosity of the material being aspirated increased, or the resistance to aspiration

increased. The maximum suction possible with this machine was 700mmHg, although

this suction pressure was far greater than any achieved during testing. The quadrants of

the foam block were then labelled A to D and then the foam block was removed from the

box. The cement mantle thickness was again measured in the same way as previously in

each of the four lines drawn in the four quadrants of the foam.

The third part of the trial aimed to simulate back bleeding of the cancellous bone. The

foam used in all of the experiments in part three is open cell. This means that fluid and air

is able to move easily throughout the foam. Unfortunately this foam, even though it is the

most dense foam that is possible to manufacture, is more similar to osteoporotic bone

than healthy cancellous bone. Due to this increased porosity of the cancellous bone foam,

a liquid more viscous than saline was required in order to closer mimic the flow

characteristics of blood in native acetabular cancellous bone. Motor oil (Havoline®

10w30) was chosen as a suitable fluid as it was shown on preliminary testing to flow

through the pores of the foam block in a similar pattern and at approximately the same

rate at which blood flows through the cancellous interstices of the acetabulum at

operation. The cancellous bone foam was cut to size as it had been done previously and

placed in the metal box with its plastic cover and metal frame. It was then reamed to a

diameter of 47 millimetres as had been done in all previous tests. Play dough was used as

mentioned earlier to seal all edges of the roof of the box from the inside, and a piece of

play dough was used to plug the hole in the metal face plate since the four millimetre

hole for the spinal sucker would not be used in this part of the testing. The metal box was

filled with motor oil up to a level at the apex of the reamed hemispherical cavity. A

reservoir for the oil in the form of an inverted 60mL syringe was set up, the top of which

was suspended at a point 77 centimetres above the box as shown in photo 2.39. This was

33

shown on preliminary tests to provide a (back bleeding) pressure of forty centimetres of

water (which is roughly equal to 30mmHg). The cement bolus at dough stage was

inserted at three minutes and fifty seconds from mixing and the oil clamp was released,

allowing the remainder of the box to fill with oil around the cement bolus. As in previous

tests at four minutes from mixing the cement was pressurised at 200N for two minutes

and then at 100N for a further four minutes. The oil was seen to have completely filled

the box just prior to the commencement of pressurisation. After the cement had

polymerised, the foam was dried, labelled A to D based on its position in the box and

then each quadrant marked and measured. The piece of foam was then labelled wet foam

without suction and stored in a plastic bag. This process was repeated five more times in

an identical fashion.

The final step in the experiments of part three was to examine the effect of suction on the

simulated ‘bleeding’ acetabulum. An identical set up to the previous three steps was used.

The foam pieces were placed in the metal box, the lid sealed with Play Dough® and

screwed down. The spinal suction catheter was again inserted and set to the same suction

pressure as previously used (200mmHg). The metal box was filled with motor oil, again

just to the apex of the reamed foam cavity (see photo 2.40) and filled completely at three

minutes and fifty seconds after the cement bolus had been placed in the ‘bleeding’

acetabular cavity. The cement was pressurised in the same manner as had been done for

all of the study with the load cell positioned in the middle of the pressuriser, providing

real time graphical feedback to the examiner on the force applied. Pressurisation is shown

in photo 2.41 and 2.42. When the cement had polymerised, the foam was dried, the four

quadrants labelled A to D and removed from the metal box. The depth of the cement

mantle thickness in each quadrant was recorded. The same process was then repeated five

more times. These data allowed comparison of the cement penetration in dry bone with

and without a suction catheter and bleeding bone with and without a suction catheter with

respect to its position relative to the suction catheter. The results are discussed in chapter

three.

34

Chapter 3: Results: All measurements taken in these experiments were converted from analog to digital by

the data acquisition unit and then recorded and displayed on the computer. The data was

recorded in channels such that channel one represented the pressure in the acetabulum at

the apex, channel two represented the pressure in the acetabulum at the rim and channel

seven represented the force being applied to the cement mantle via the pressuriser. The

data was displayed on the computer screen as a graph with time on the X axis and both

pressure in Bar and force in Newton on the Y axis. However, the data acquisition unit

was not capable of simultaneously converting two different units of measurement (Bar

and Newton) on two different scales. As a result, all data was recorded as a value on an

arbitrary scale and needed to be converted manually to the appropriate units after all data

had been recorded. In this study, pressure was expressed as millimetres of mercury

(mmHg) and force was expressed as Newton (N). In the following report of the results of

the experiments, in all tables and graphs, measurements will be expressed in terms of the

absolute value assigned by the computer and then converted to the appropriate units at the

end of each section. To convert the units of pressure measured to mmHg, each value was

multiplied by 2.34. Similarly for values of force each value was multiplied by 0.64. Data

was acquired and recorded in all three channels every 0.5 seconds whilst the pressure

recording computer program was running. The pressure recording program was able to

run for a maximum of nine minutes and fifty five seconds. It could be stopped earlier, but

once it had been stopped it could not be restarted again. This ensured that the pressure

recording program ran continuously for the complete duration of all experiments reported

in this paper. The only exception to this was in the Play Dough® tests (which did not

require mixing) and in the initial pressure tests, where the laboratory temperature was

very low. In these tests, the cement needed to be inserted into the acetabulum at dough

stage at six minutes from the commencement of mixing (rather than four). The pressure

recording trace was only started twenty to thirty seconds prior to the commencement of

pressurisation as it was initially uncertain how long the cement would take to reach

dough stage.

35

Statistical analyses in this paper were carried out using Microsoft® Excel for

Macintosh® using paired and unpaired t-tests and ANOVA. The t-score could then be

used to generate the p-value for each comparison between groups. This is based on 10

degrees of freedom (dF) for each comparison as each group consisted of six trials [dF =

(n1 + n2) – 2].

36

Part One: Pressure Testing.

Pressurisation of Native Acetabulum:

As mentioned earlier in chapter two, cement was pressurised in a model of a human

acetabulum, which had undergone no modification of any kind. The cement was

pressurised at 275N at a time six minutes from mixing of cement for one and a half

minutes. The pressure was recorded at the apex and rim of the acetabulum and also the

force placed on the acetabular pressuriser. The process was repeated five more times. The

pressure trace recorded for each of these six tests are shown in Appendix 1. The peak

pressure generated in the acetabulum at the rim and apex was recorded for each trial as

was the total pressure generated at the apex and rim (represented by units [in mmHg] x

seconds). The mean force applied manually to the pressuriser by the examiner was also

calculated. These results are shown in table 3.1 below. A photograph of the cement

pressurisation is shown in photo 3.1, note the extrusion of cement from around the

pressuriser.

Normal Acetabulum

Trial Peak apex pressure

Peak rim pressure

Total pressure generated at apex

Total pressure generated at rim

Mean force applied to pressuriser

1 214 194 23762 25977 405 2 156 151 24712 26171 410 3 177 165 28267 29554 413 4 153 148 16067 16738 411 5 149 142 22875 23187 419 6 150 146 24936 26207 418

Mean values

167 units = 390mmHg

158 units= 370mmHg

23437 units= 54843mmHg

24639 units = 57655mmHg

413 units= 264 N

Table 3.1: Pressures recorded in the normal acetabulum.

Pressurisation of Play Dough in Native Acetabulum:

In exactly the same way, play dough was pressurised in a model pelvis with no structural

modifications to it. The same force was applied to the pressuriser and the same

measurements of apex and rim pressure and force on the pressuriser were recorded. The

pressure trace from each of the six tests are shown in Appendix 1. The peak pressure

generated in the acetabulum at the rim and apex was recorded for each trial as was the

total pressure generated at the apex and rim (represented by the area under the curve).

37

The mean force applied to the pressuriser was also calculated. These results are shown in

table 3.2 below.

Play Dough

Trial Peak pressure at apex

Peak pressure at rim




1 384 388 53428 52254 417 2 402 381 50808 51714 417 3 347 349 50681 53104 427 4 339 348 56768 58023 405 5 291 302 50016 53764 417 6 297 326 54979 55732 394

Mean values

345 units = 807 mmHg

349 units= 817 mmHg

52780 units= 123505mmHg

54099 units = 126592mmHg

413 units= 264 N

Table 3.2: Pressures recorded using Play Dough® in normal acetabulum.

Pressurisation of Cement in ‘Grafted’ Acetabulum:

The third series of tests were conducted using bone cement in a model hemi pelvis.

However, this model hemi pelvis had undergone modification in the form of simulated

bone grafting of the acetabular notch. The bone cement was introduced into the

acetabular cavity at the same time as in the initial tests (i.e. six minutes from the

commencement of mixing of the cement), and pressurised at the same force, for the same

duration. Again there were measurements taken at the rim and apex of the acetabulum as

well as the force applied to the pressuriser. Six tests were conducted and the results of the

pressure trace recorded on the computer are shown in Appendix 1. The results are

summarised in table 3.3, which shows the measured peak pressure generated at the apex

and at the rim as well as the calculated total pressure generated at both the apex and the

rim. It also shows the mean force applied to the pressuriser for each test and for all six

tests.

38

Grafted Acetabulum






1 307 291 46944 48529 406 2 303 296 41253 41966 371 3 261 255 44565 46408 411 4 320 303 52438 54982 440 5 355 315 51948 53669 416 6 302 302 48689 49477 421

Mean values

308 units = 721 mmHg

295 units= 690 mmHg

47640 units= 111478mmHg

49172 units = 115062mmHg

411 units= 263 N

Table 3.3: Pressures recorded using cement in a ‘grafted’ acetabulum.

Comparisons:

The initial tests in part one were designed to allow comparison between the cement

pressure generated in the normal human acetabulum and an acetabulum in which the

acetabular notch had been bone grafted. It was also designed to allow comparison

between the behaviour of bone cement and Play Dough® in the normal human

acetabulum. Firstly, the peak pressure in the normal acetabulum will be compared with

the acetabulum with the simulated bone grafting of the acetabular notch. The mean peak

pressure at the apex of the acetabulum was 390mmHg for the native acetabulum

compared with 721mmHg for the ‘grafted’ acetabulum. When the peak pressure at the

rim of the normal acetabulum is compared with the ‘grafted’ acetabulum a similar trend

is seen. The mean peak pressure at the rim is 370mmHg in the normal acetabulum and

690mmHg in the ‘grafted’ acetabulum. This represents a significant increase in mean

peak pressure in the acetabulum at the apex (p-value < 0.001) and at the rim (p-value <

0.001) when the acetabular notch is bone grafted. Chart 3.1 in the appendix shows the

differences in peak pressure at the apex and rim of the acetabulum under the three

different conditions.

Next it is necessary to compare the total pressure generated at the apex and rim of the

normal acetabulum with the ‘grafted acetabulum’. As mentioned earlier in this chapter,

the total pressure generated at each point was calculated by measuring the total area

under the curve in each trial. The mean total pressure generated at the apex of the normal

39

acetabulum was 54.8 x 103 mmHg compared with 11.1 x 104 mmHg in the ‘grafted’

acetabulum. At the rim of the acetabulum, the mean total pressure generated was 57.7 x

103 mmHg in the normal acetabulum compared with 11.5 x 104 mmHg in the ‘grafted’

acetabulum. This again represents a significant increase in the mean total pressure

generated in the acetabulum at the apex (p-value < 0.001) and at the rim (p-value <

0.001) when the acetabular notch is ‘grafted’. The mean force applied to the cement

mantle via the pressuriser was 264N for the normal acetabulum and 263N for the

‘grafted’ acetabulum. There was no significant difference between these two groups in

terms of the forces applied to the cement (p value = 0.88).

The other important comparison in this section is between bone cement and Play

Dough®. The mean peak pressure generated by cement at the apex of the normal

acetabulum is 390mmHg compared with the mean peak pressure of 807mmHg generated

by Play Dough® in the same acetabulum. At the rim of the same acetabulum a mean

peak pressure of 370mmHg is generated by cement compared with 817mmHg when Play

Dough® is used. This represents a statistically significant increase in the peak pressures

of Play dough over bone cement at the apex (p-value < 0.001) and rim (p-value < 0.001)

of the acetabulum. When the total pressures generated at the apex and the rim are

compared, a similar association is seen. The mean total pressure at the apex of the

acetabulum is 54.8 x 103 mmHg when cement is pressurised compared with 12.4 x 104

mmHg when Play Dough® is pressurised. A similarly large difference exists at the rim

with the mean total pressure generated in the cemented acetabulum being 57.7 x 103

mmHg compared with 12.7 x 104 mmHg in the Play Dough filled acetabulum. This again

represents a statistically significant difference between the two groups at both the apex

(p-value < 0.001) and at the rim (p-value < 0.001). The mean force exerted on the

pressuriser in the cement group was 265N, which is exactly the same as the Play Dough®

group. There was no statistically significant difference between the force exerted on the

pressuriser between the groups (p-value = 0.99).

Insertion of Unflanged Cup:

40

Similar pressure measurements were taken in the tests involving the insertion of the

unflanged Exeter Contemporary cup into the acetabular model. Again peak pressure was

measured and total pressure generated was calculated (based on the area under the curve)

at the rim and the apex of the acetabulum. The force required to seat each cup in the

designated time was also measured via the load sensor in the pressuriser for each trial cup

insertion. The data for each of the six trials is again displayed in the form of the pressure

trace recorded on the computer. These graphs are shown in Appendix 1. The peak and

total pressures generated in the acetabular model at the apex and the rim are shown in

table 3.4 below. This table also shows the peak force required to seat each cup and the

mean pressures and forces for the group. Photo 3.2 shows the insertion of an unflanged

cup into the acetabulum, note the significant volume of cement that has extruded around

the cup during insertion.

Unflanged cups





Peak force required to seat the cup

1 196 67 14205 3821 163 2 239 100 8850 3589 225 3 284 94 10901 3585 234 4 269 71 10974 2637 268 5 289 132 6283 2212 282 6 216 109 7107 2834 231

Mean values

249 units = 583mmHg

96 units= 225mmHg


3113 units = 7284mmHg

234 units= 150N

Table 3.4: Pressure recordings in unflanged cup group.

Insertion of Flanged Cup:

Pressures and cup insertion forces were again measured in the six trials conducted using

flanged Exeter Contemporary cups. Peak pressure at the apex and rim of the acetabulum,

total pressure generated at the apex and the rim and the force required to seat the cup

were all recorded for each trial. The results recorded by the pressure trace on the

computer of the six trials are again shown in Appendix 1. The data and mean values for

all six trials is summarised in table 3.5. Again the total pressure generated is calculated

based on the area under each curve.

41

Flanged cups





Peak force required to seat the cup

1 283 198 9848 7601 3262 339 259 23060 13454 362 3 272 155 12226 4831 332 4 312 211 13547 8550 362 5 293 245 9611 5229 314 6 286 225 20867 12319 340

Mean values

298 units = 697mmHg

216 units= 505mmHg



339 units= 217N

Table 3.5: Pressures recorded in the flanged cup group.

Insertion of Flanged cup into Acetabulum prepared with Rim Cutter:

The final tests in part one consisted of insertion of a specially flanged cup into a hemi

pelvis model in which the acetabular rim had been prepared with the rim cutter. The same

measurements were recorded as had been done in all prior tests in part one. The pressure

traces recorded on the computer for each trial are shown in Appendix 1. Again the total

pressure generated at the apex and rim of each trial are calculated based on the area under

the respective curves. These results are summarised in table 3.6 below.

Rim Cutter





Peak force required to seat cup

1 306 266 11789 7803 484 2 446 306 13981 8581 560 3 394 348 9585 7302 567 4 278 233 7673 5925 419 5 365 352 13382 10734 565 6 378 325 14770 10053 550

Mean values

321 units = 751mmHg

305 units= 714mmHg



524 units= 335N

Table 3.6: Pressures recorded in Rim Cutter group.

Comparisons:

42

The three tests described above were designed to demonstrate any differences between

the pressure generated in the acetabulum when cups with different sized flanges were

inserted. Firstly, the pressures generated when an unflanged cup is inserted into a normal

acetabulum will be compared with the pressures generated when a flanged cup is

inserted. The mean peak pressure generated at the apex when an unflanged cup is inserted

is 583mmHg compared with 697mmHg when a flanged cup is inserted. The mean peak

pressure at the rim of the acetabulum is 225mmHg for the unflanged cup group and

505mmHg for the flanged cup group. Similarly, the mean total pressure generated at the

apex was 22.7 x 103 mmHg for the unflanged cup group and 34.8 x 103 mmHg for the

flanged cup group. At the rim the mean total pressure generated was 7.3 x 103 mmHg for

the unflanged cup group and 20.3 x 103 mmHg for the flanged cup group. These data

represent a statistically significant increase in the peak intra-acetabular pressure

generated on cup insertion at the apex (p-value = 0.03) and rim (p-value < 0.001) when a

flanged cup is inserted compared with an unflanged cup. The total pressure generated at

the apex and the rim is also statistically significantly higher when a flanged cup is

inserted compared with an unflanged cup (p-value = 0.091 and 0.013, for apex and rim

respectively).

It was also necessary to compare the pressures generated by insertion of a flanged cup

alone with the larger flanged cup inserted into the acetabulum prepared with the rim

cutter. The mean peak pressure generated at the apex of the acetabulum was 697mmHg

for the flanged cup group compared with 751mmHg for the rim cutter group. A similar

difference was seen at the rim with the mean peak pressure generated being 505mmHg

for the flanged cup group compared with 714mmHg for the rim cutter group. These data

again represent a statistically significant increase in the peak pressure generated at the

apex (p-value = 0.049) and the rim (p-value = 0.005) of the acetabulum when the rim is

prepared with the rim cutter and a flanged cup is inserted. When the total pressures

generated at each point are considered, the differences are less marked. The mean total

pressure generated at the apex is 34.8 x 103 mmHg for the flanged cup group compared

with 27.8 x 103 mmHg for the rim cutter group. A small difference exists at the rim with

43

the mean total pressure generated being 20.3 x 103 mmHg for the flanged cup group

compared with 19.7 x 103 mmHg for the rim cutter group. This represents no significant

difference between the total pressure generated at the apex and the rim when comparing

the rim cutter to a standard flanged cup at the apex and the rim (p-value = 0.29 and 0.86

respectively) .

The mean peak force required to seat each cup was 150N for the unflanged cup group,

217N for the flanged cup group and 335N for the rim cutter group.

As mentioned in chapter two, the cups used to test pressure were removed from the hemi

pelvis after each test, along with the cement mantle. At that time, each cement mantle

was inspected and it was this examination that revealed an interesting discovery. The

cement mantles removed from the normal hemi pelvises i.e. all unflanged cup trials and

all flanged cup trials had four small circular holes. Examples of the cement mantles with

holes is shown in photo 3.3. However, when the cement mantles in the rim cutter group

were examined, there were no such holes seen (see photo 3.4 and 3.5).

44

Part Two: Penetration Testing.

Penetration of pressurised bone cement:

As mentioned in chapter two, the initial testing of penetration began with bone cement

alone, pressurised with no cups inserted. The results of each trial are shown in table 3.7

below, with the distance of cement penetration listed at each of the four sets of holes at

the apex, middle and rim of the cavity.

Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration Distance in mm. 1

Rim 9 5 5 5 Middle 6 5 5 6

Apex 9 11 9 10 2

Rim 8 7 7 9 Middle 12 9 12 11

Apex 10 9 9 10 3

Rim 10 9 9 9 Middle 9 8 9 10

Apex 8 10 11 10 4

Rim 4 5 5 5 Middle 8 5 4 5

Apex 9 11 10 8 5

Rim 10 9 10 9 Middle 9 10 9 10

Apex 11 10 12 11 6

Rim 9 10 10 10 Middle 7 7 9 9

Apex 9 10 10 10 Table 3.7: Penetration of pressurised bone cement (in millimetres).

45

Penetration of pressurised Play Dough:

Play Dough was also pressurised in the same manner and under the same conditions as

the bone cement. It also underwent six trials. The results are shown in table 3.8 below.

The play dough penetration described in millimetres.

Position 1 Position 2 Position 3 Position 4 Trial Number Play Dough Penetration in mm. 1

Rim 4 4 4 3 Middle 4 4 4 3

Apex 3 4 3 3 2

Rim 3 5 4 3 Middle 4 5 5 2

Apex 3 4 3 4 3

Rim 4 4 5 3 Middle 4 5 5 3

Apex 4 4 4 4 4

Rim 4 4 4 3 Middle 4 4 4 4

Apex 4 3 3 4 5

Rim 4 5 4 2 Middle 4 4 4 5

Apex 5 4 3 2 6

Rim 5 3 3 3 Middle 4 3 3 4

Apex 4 3 3 2 Table 3.8: Measurements of Play Dough penetration in each trial (millimetres).

Comparisons:

The purpose of the first two penetration tests was to determine whether Play Dough

behaved in a similar manner to bone cement. To do this the mean cement penetration was

calculated at the apex, middle and rim of each cavity for the bone cement group and for

the Play Dough group. Considering the apex, the mean cement penetration was 10mm for

the bone cement group compared with 3.5 mm for the Play Dough group. A similar result

is seen at both the middle and rim of the cavity with the mean bone cement penetration

46

being 8mm at both and the Play Dough penetrating 3.5mm at both. These results show a

statistically significant difference between the penetration of the play dough at the apex,

the middle and the rim (p-value < 0.001 for all).

The force applied to the bone cement and the Play Dough was recorded on the computer

in each trial for its duration. The mean force applied during each trial is displayed in table

3.9 below.

Material Bone Cement Play Dough

Trial 1 308 304

2 304 302

3 304 300

4 295 306

5 296 305

6 301 299

Mean values 301 units= 193N

303 units= 194N

Table 3.9: Mean force exerted on pressuriser (N).

The mean force applied to the bone cement for all six trials was 193N while the mean

force applied to the play dough was 194N. There is no statistically significant difference

between the forces applied in the two groups (p-value = 0.58).

47

Penetration of bone cement with pressurisation and insertion of unflanged cup:

The next step was to build on the data from the initial tests described above, and assess

the effect on cement penetration of inserting a cup into the cavity after cement

pressurisation. The Unflanged Exeter Contemporary cups were inserted into the cavity

after cement pressurisation and then pressurised at 100N for two minutes. Six trials were

carried out. The distance of cement penetration was again measured in millimetres at the

apex, middle and rim in each of the four sets of holes in the cavities. The results of all

trials for the unflanged cup group are shown in table 3.10 below.

Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration in mm. 1

Rim 9 8 10 9 Middle 9 7 8 9

Apex 9 9 7 10 2

Rim 9 10 10 11 Middle 11 11 10 11

Apex 12+ 12+ 12+ 12+ 3

Rim 8 9 9 9 Middle 8 9 8 8

Apex 9 10 11 8 4

Rim 10 10 11 11 Middle 11 11 11 11

Apex 12 12 12 12 5

Rim 10 10 10 10 Middle 10 9 10 10

Apex 11 9 12 10 6

Rim 11 9 10 10 Middle 9 9 8 9

Apex 10 9 9 9 Table 3.10: Penetration of bone cement with insertion of unflanged cup (millimetres).

Photo 3.6 shows the extrusion of cement from the acetabular cavity on insertion of an

unflanged cup.

48

Penetration of bone cement with pressurisation and insertion of flanged cup:

The same procedure was carried out for the flanged cup group, with the cement

pressurised in the cavity, a flanged cup inserted and then pressurised for a further two

minutes at 100N. Again the cement penetration was measured at the apex, middle and rim

of the acetabulum in each of the four sets of holes. These measurements of cement

penetration (in millimetres) are shown in table 3.11 below.

Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration in mm. 1

Rim 11 10 10 11 Middle 11 11 10 11

Apex 9 10 12 12 2

Rim 10 10 10 10 Middle 12 12 12 13

Apex 10 10 12 11 3

Rim 11 11 11 11 Middle 12 11 12 11

Apex 12 12 12 12 4

Rim 9 10 10 10 Middle 11 12 11 13

Apex 10 10 12 12 5

Rim 11 12 12 11 Middle 11 12 11 12

Apex 12 12 12 12 6

Rim 11 11 12 11 Middle 10 12 9 12

Apex 12 12 12 11 Table 3.11: Penetration of bone cement into cavities with a flanged cup inserted (mm).

Penetration of pressurised bone cement in cavity with rim cut and insertion of specially

flanged cup:

In this final test of part two, all six cavities were prepared by the rim cutter. A three

millimetre wide and deep ledge was cut in the rim of each cavity prior to cement

insertion. The rest of the procedure remained unchanged from the previous cup

49

insertions. The cement was pressurised, a cup with the flange specially trimmed for the

rim of the cavity was inserted and then pressurised at 100N for a further two minutes.

The same measurements were taken as in all previous penetration tests. Of note in the rim

cutter trials, the cement at the apex in all but one trial penetrated so far that it extruded

out from the holes as shown in photo 3.7 and 3.8. The cavity was thinner at this point due

to the design limitations described earlier in chapter two. As a result, measurement in the

case of cement extrusion from a hole was recorded as 12+mm, as it was not possible to

accurately measure the distance of cement extrusion. The results are shown in table 3.12

below.

50

Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration Distance in mm. 1

Rim 12 14 13 15 Middle 13 14 13 14

Apex 12+ 12+ 12+ 12+ 2

Rim 15 15 14 15 Middle 13 14 13 14

Apex 12+ 12+ 12+ 12+ 3

Rim 13 8 10 11 Middle 13 9 8 15

Apex 12+ 12+ 12+ 12+ 4

Rim 14 14 14 14 Middle 15 14 14 15

Apex 12+ 12+ 12+ 12+ 5

Rim 8 10 10 14 Middle 10 6 9 6

Apex 12+ 12+ 12+ 12+ 6

Rim 10 10 9 12 Middle 9 9 10 13

Apex 8 12 12 11 Table 3.12: Penetration of bone cement in cavities prepared by rim cutter (millimetres).

51

Forces applied to the cavities:

Again the force applied to the cement mantle during pressurisation and during cup

insertion was measured and recorded for the duration of each trial. The mean force

applied during pressurisation was calculated for each trial, as was the mean force of

pressurisation after each cup had been seated. The peak force required to seat each cup

was also noted for each trial. These data are listed for each of the three groups in table

3.13 below.

Table 3.13: Force exerted on pressuriser in different phases of penetration testing.

The mean force of pressurisation was 191N for the unflanged cup group, 192N for the

flanged cup group and 195N for the rim cutter group. There was no statistically

significant difference in the mean force used to pressurise the cement in each group (p-

value = 0.30). Similarly, the mean force used to pressurise the cup after it was seated

remained reasonably constant. The mean force applied to pressurise the cup was 98N for

the unflanged cup group, 93N for the flanged cup group and 103N for the rim cutter

group. Although the rim cutter group was pressurised with slightly more force than the

other two groups, this was not significant (p-value = 0.06).

Cup Type Unflanged Flanged Rim cutter Pressurisation Cement Cup Cement Cup Cement Cup

Trial 1 303 140 296 159 307 1632 302 158 294 145 304 1683 296 141 304 130 295 1624 292 160 299 158 302 1705 294 160 307 149 303 1516 309 158 306 131 312 151

Mean Value 299 units= 191N

153 units= 98N

301units = 192N

145 units = 93N

304 units = 195N

161 units = 103N

52

Table 3.14 Shows the maximal force required to seat the cup in each group.

Maximal Insertion Force (units) Trial Unflanged Flanged Rim Cutter

1 331 422 396 2 357 497 390 3 346 591 417 4 357 693 337 5 338 725 396 6 344 656 407

Mean Values 345 units = 221N 597 units = 382 N 390 units = 250 N Table 3.14: Maximal force required to seat each cup type.

The mean force required to seat each cup was 221N for the unflanged cup group, 382N

for the flanged cup group and 250N for the rim cutter group.

53

Comparisons:

When the cement penetration with pressurisation alone is compared with cement

pressurisation and unflanged cup insertion an increase in penetration is seen. The mean

cement penetration distance at the apex is 10mm with pressurisation alone compared with

10.5mm with pressurisation and cup insertion. The mean cement penetration in the

middle was 8mm with pressurisation alone and 9.5mm with pressurisation and unflanged

cup insertion. At the rim a similar picture is seen with 8mm mean penetration with

pressurisation alone and 9.5mm mean penetration with both pressurisation and cup

insertion. These results indicate a statistically significant increase in the cement

penetration at the middle (p-value = 0.015) and the rim (p-value < 0.001) when an

unflanged cup is inserted after pressurisation. However, the difference at the apex was

not significant (p-value = 0.23).

Cement penetration distance with unflanged cup insertion was also compared with

flanged cup insertion. The mean cement penetration distance at the apex was 10.5mm in

the unflanged cup group compared with 11.5mm in the flanged cup group. Similar results

were seen in the middle and at the rim with mean cement penetration in the unflanged

cup group being 9.5mm and 9.5mm respectively compared with 10.5mm and 11.5mm

respectively in the flanged cup group. These results demonstrate a statistically significant

increase at the apex (p-value = 0.008), middle (p-value < 0.001) and rim (p-value <

0.001) of the cavity when a flanged cup is inserted instead of an unflanged cup.

The final comparison is between the cement penetration in the flanged cup group

compared with the rim cutter group. At the apex the mean cement penetration was

11.5mm for the flanged cup group compared with 12+mm in the rim cutter group. In the

middle, the difference between the two groups was again relatively small, the mean

cement penetration being 11.5mm in the flanged cup group compared with 12mm in the

rim cutter group. At the rim there was a slightly larger difference in the cement

penetration. The mean cement penetration here was 10.5mm for the flanged cup group

and 12mm for the rim cutter group. These data show no statistically significant difference

54

between cement penetration at the apex (p-value = 0.12) or middle (p-value = 0.37) with

insertion of a flanged cup alone or a flanged cup with the rim cutter.

However, there was a statistically significant increase in cement penetration at the rim

with the rim cutter used compared with the flanged cup alone (p-value = 0.003).

Charts 3.2, 3.3 and 3.4 in Appendix 1 show the cement penetration differences between

all of the groups at the apex, middle and rim respectively.

55

Part Three: Suction Testing.

The cement penetration distance was measured at the rim of the piece of foam in each

corner of the foam block. The measurement was taken on a line drawn diagonally across

the foam block between opposite corners of the block. Measurements were recorded

based on their position on the foam block with a letter corresponding to its position in one

corner of the box. This is shown in photo 2.37.

Cement pressurised in dry foam with no suction:

The first tests used to examine the effect of suction in acetabular cementing consisted of a

control group in which there was no suction. Cement was pressurised at 200N into dry

foam with no suction for two minutes followed by pressurisation at 100N for four

minutes. The force exerted on the cement for the duration of each trial was recorded on

the computer. The measured distance of cement penetration (in mm) at each point in each

of the six trials is shown in table 3.15. below.

Position A B C D Trial 1 8 9 9 9

2 7 7 7 8 3 4 5 6 6 4 5 4 6 7 5 7 6 6 7 6 5 8 6 6

Mean Value 6 6.5 6.5 7 Table 3.15: Cement penetration distance at rim of dry foam (millimetres).

Penetration of bone cement into dry foam with suction:

The next series of trials measured the distance of cement penetration in dry foam with

supplemental suction. Measurements were taken in the same way as in the previous tests

and recorded based on their position in the metal box. Note that the suction catheter was

positioned in the corner of the box labelled point ‘C’. These measurements are shown in

table 3.16 below.

56


2 4 7 9 7 3 7 7 10 10 4 7 8 10 8 5 7 9 7 6 6 1 7 10 7

Mean value 5.5 8 9.5 8 Table 3.16: Cement penetration distance in dry foam with suction at point ‘C’

(millimetres).

Cement penetration in foam with simulated back bleeding:

Cement was also pressurised in wet foam with simulated back bleeding. Measurements

were again taken in the same manner as previously and the force applied to the

pressuriser was again recorded for the duration of each of the six trials. There was no

suction applied in these trials and the hole in the ceiling of the box for the suction

catheter was blocked with Play Dough. The results are shown in table 3.17 below.


2 5 6 5 4 3 5 6 5 7 4 7 5 5 6 5 5 6 5 5 6 7 10 10 6

Mean value 6 6.5 7 6 Table 3.17: Cement penetration distance into wet foam with no suction (millimetres).

Cement penetration into foam with simulated back bleeding and suction:

In the final six trials, cement was pressurised in wet foam with the effect of back bleeding

and active suction at point ‘C’. Measurements of cement penetration were again taken in

all corners of the piece of foam as well as continuous measurement of the force applied to

the cement mantle via the pressuriser. The results are shown in table 3.18 below.

57


2 7 6 11 11 3 10 9 13 11 4 7 9 9 5 5 5 5 12 10 6 4 11 11 5

Mean value 6.5 8 11 8.5 Table 3.18: Cement penetration distance with back bleeding and suction at point ‘C’

(millimetres).

Forces exerted on cement mantle:

Once again the mean force exerted on the cement mantle during pressurisation of the

cement was calculated for each trial. The pressurisation in part three consisted of an

initial pressurisation of around 200N for two minutes (i.e. from 4 minutes to 6 minutes),

followed by a second pressurisation of 100N for four minutes (i.e. from 6 minutes to 10

minutes). The mean force applied to the pressuriser in each of these two bouts was

calculated for each trial. The results are listed in table 3.19 below.

58

Table 3.19: Forces exerted on cement in foam under various experimental conditions.

The mean force applied to the cement mantle in the dry foam group was 192N for the

first two minutes and 93N for the four minute pressurisation. The mean force applied in

the dry foam with suction group was 193N for the first pressurisation and 93N for the

second pressurisation. Similar mean forces were recorded for the wet foam group and the

wet foam with suction group with 186N and 188N respectively for the first pressurisation

(i.e. lasting two mins) and 99N and 95N respectively for the second pressurisation (i.e.

lasting four minutes). There is no statistically significant difference between the mean

force applied to the cement mantle in any of the four groups (p-value = 0.41 for the initial

pressurisation and p-value = 0.054 for the second pressurisation).

Comparisons:

The mean cement penetration at point ‘A’ (i.e. opposite the point of suction) was

relatively unchanged depending on the conditions in the foam. In dry foam the mean

cement penetration was 6mm, in dry foam with suction 5.5mm, in wet foam 6mm and in

wet foam with suction 6.5mm. However, points ‘B’ and ‘D’, which were located mid way

between point ‘A’ and the suction catheter at point ‘C’ (when it was switched on) showed

differences in mean cement penetration depth. The mean cement penetration at point B

was 6.5mm in dry foam and wet foam, and 8mm in dry foam and wet foam with suction.

Foam

Conditi

ons Dry Foam Dry Foam + Suction Wet Foam Wet Foam + Suction

Dry Foam Dry Foam + Suction Wet Foam Wet Foam + Suction Trial

1st Pressurisation

2nd Pressurisation

1st Pressurisation

2nd Pressurisation

1st Pressurisation

2nd Pressurisation

1st Pressurisation

2nd Pressurisation

1 300 139 307 151 285 148 307 150 2 299 142 313 150 273 152 313 150 3 291 149 278 143 300 168 270 144 4 302 145 303 135 292 164 287 158 5 302 153 310 146 296 151 280 150 6 305 146 293 146 298 148 304 141

Mean Values

300 units = 192 N

146 units = 93 N

301 units= 193 N

145 units= 93 N

290 units= 186 N

155 units = 99 N

293 units= 188 N

149 units= 95 N

59

At point D the mean cement penetration was 7mm in dry foam and 6mm in wet foam

compared with 8mm in dry foam with suction and 8.5mm in wet foam with suction.

The suction catheter was located at point C in the box and it did increase the penetration

of cement in its immediate vicinity. In dry foam cement penetration was 6.5mm, however

in dry foam with suction it was 9mm. Similarly, in wet foam the cement penetration was

7mm, compared with 11mm in wet foam with suction.

Comparing points in each group:

In the dry foam and wet foam groups there were no significant differences between

cement penetration at any of the points. ANOVA of all points in dry foam p-value = 0.6

and in wet foam ANOVA of all points had a p-value = 0.8.

In the dry foam with suction group point A was used as the reference point against which

the other points could be compared. Point B and D both exhibited a statistically

significant increase in the mean cement penetration over point A (p-values = 0.039 and =

0.042 respectively). An even larger increase in cement penetration was demonstrated at

point C compared with point A in the dry foam with suction group (p-value = 0.006).

In the wet foam with suction group point A was again used as the reference point. A

significant difference was not see when comparing point B and D to A (p-value = 0.13

and 0.10 respectively). However, an even larger increase was seen at point C (p-value <

0.001). Chart 3.5 in appendix 1 shows the differences in cement penetration at each point

in the four groups.

Comparing between groups:

As stated previously, there was no significant difference between the mean cement

penetration at all points in the dry foam group and the wet foam group. Comparing the

mean cement penetration distance at point A in all four different groups, there was also

no significant difference (p-value = 0.90). Point A has essentially acted as an

experimental control point across all groups. The increase in mean cement penetration

over the control (point A) observed at point C and to a lesser extent points B and D in the

60

two groups involving suction remain valid and applicable to the two groups in which

suction was not used.

The mean total cement penetration across all points was also calculated for each group.

The results are shown in table 3.20 below.

Position A B C D Total Mean Cement Penetration

Dry Foam 6 7 6 7 26 Dry Foam + Suction 6 8 9 8 31 Wet Foam 6 7 7 6 26 Wet Foam + Suction 7 8 11 8 34

Table 3.20: Mean cement penetration at various points (in mm).

It is seen from these data that the mean total cement penetration is higher in the groups

where suction was used. Analysis of variance reveals a significant difference in the wet

foam with suction group (p-value = 0.04). An observable increase in mean total cement

penetration is also seen in the dry foam with suction group, however, this difference was

not significant (p-value = 0.07).

61

Chapter Four: Discussion: This discussion will begin with some general comments regarding the equipment and

techniques common to all parts of the study. The specific details and results of the

experiments in parts one two and three will then be discussed in order.

Many previous studies into cement pressurisation in vitro have stressed the need to

consider the effect of back bleeding on cement penetration. Cadaveric bone had

previously been used for testing of pressure and penetration in the presence of back

bleeding (Breusch et. al. 2004). In our institution the authors were faced with a paucity of

cadaveric pelvises for laboratory use. The other issue of using cadaveric pelvises is the

relative inconsistency of the bone quality. The use of animal pelvises was considered,

but it was decided that the pelvic bone structures of other animals did not sufficiently

match that of the human. An ideal model for in vitro testing would have the mechanical

properties of bone, a correct anatomical shape and have open pores to simulate cancellous

bone, which would allow liquid to flow through it. Finally, large numbers of almost

identical models would be readily available. Unfortunately such a model does not exist.

For these experiments the authors chose Sawbones® Hemi pelvis models. This decision

was based on both their fidelity to the shape of the human acetabulum, which is of

particular importance in part one of the study and on their consistency of character and

mechanical behaviour. They do not, however, allow liquid to flow through them being of

“closed cell foam” construction. Therefore there was a necessity to separate the study into

three distinct parts, each of which investigated a particular aspect of the experimental

variables. The authors feel that in the absence of an ideal model for testing that

individual, controlled experiments would provide more reproducible and valid results.

Another important variable, which has been mentioned sporadically in the literature is the

force used to pressurise the acetabulum. The average force that an orthopaedic surgeon

places on a pressuriser intra-operatively has not been clearly defined, although it has been

suggested that 35 to 50 kPa pressure sustained for one minute would generate an

acceptable cement mantle (Noble and Swarts 1983). New et. al. (1999) measured an

62

average pressure of 48 kPa by two surgeons in their study. This is in stark contrast to the

paper by Juliusson et. al. in 1994 which claims that a force of 0.3 MPa is ideal to generate

a cement penetration distance of three to five millimetres. This level of force (300kPa) is

far in excess of any force able to be generated in our laboratory during the experiments in

this study. It is clear that for in vitro studies of pressurisation, the force applied to the

pressuriser must be both relatively constant and at a level which is clinically relevant.

The average surgeon is not capable of maintaining pressurisation at an exact force at

exactly the same angle for the whole duration of pressurisation. Therefore the study was

designed to simulate the effects of manual pressurisation of cement within a relevant

force range as one could expect to find in vivo. This study used a load cell mounted in the

shaft of a Stryker® acetabular pressuriser, which measured the force in Newton applied

through it. The diameter of the pressuriser head was five centimetres, so the area of the

head was 78 square centimetres (= 0.0078 m2). To convert this to one square meter we

calculate 1/0.0078 (=128). Since one Pascal (Pa) is equal to one N/m2, then the force on

the pressuriser needs to be multiplied by 128 to convert to Pascals. In this study,

pressurisation of the cement mantle alone was chosen to be 275N [35,200 N/m2 = 35.2

kPa], pressurisation of the cement mantle prior to cup insertion 200N [25,600 N/m2 =

25.6 kPa], while pressurisation of the cup was chosen to be 100N [12,800 N/m2 = 12.8

kPa].

Simplex bone cement was used for all experiments in the study. It was necessary to

specifically control the environment in which the cement was prepared to avoid

variations in cement polymerisation time and rate. Therefore, the most temperature stable

room in the hospital was chosen for testing. Unfortunately the initial three tests involving

pressurisation of a native acetabulum, pressurisation of play dough and pressurisation of

a hemi pelvis with simulated bone grafting were carried out in a laboratory whose

temperature was kept far too low 11.2° to 13.5°C. As a result the bone cement took six

minutes to reach dough phase rather than the planned four minutes. After this was noted,

a warmer room that was equally temperature stable was chosen. The temperature ranged

from 20.0° to 21.1°C, which not only allowed the cement to reach dough phase in four

minutes, but also replicates fairly well the temperature in the operating theatre during

63

surgery. The ambient temperature is of pivotal importance as the viscoelastic properties

of bone cement are very sensitive to temperature. Subtle reductions in temperature can

result in greatly increased time to polymerisation. As the cement polymerises, it becomes

rapidly more viscous. All of the experiments in this thesis rely on the cement to

polymerise at the same rate, so that the rate of change of the viscosity is constant, which

in turn ensures that the flow characteristics of the cement remain constant. Another

property of PMMA is also worthy of mention namely its mechanosensitivity. Bone

cement polymerisation rate is influenced by the degree of agitation of the cement. This

means that unless the cement is mixed at the same frequency in the same way and for the

same period of time it risks polymerising at an inconsistent rate. In order to control these

variables hand mixing was chosen over vacuum mixing, to allow the examiner more

control over the frequency of mixing. The frequency was chosen to be one to two cycles

per second as it was both effective and easily reproducible. The duration of mixing was

also kept constant at ninety seconds as it has shown on preliminary testing to provide the

most reliable cement polymerisation times. Finally, the cement was taken into the hand at

a consistent time and once in the hand not manipulated any more than was necessary to

keep it there until it was inserted as a bolus into the acetabulum, simulated acetabular

cavity or piece of foam.

In order to control the force applied to the cement and to the cups in these experiments, it

was necessary to incorporate a mechanism by which the force could be measured and

displayed. A load sensor was specially adapted and mounted in an acetabular pressuriser

by the staff in the Engineering Department of Exeter University. This sensor conveyed

force readings via the data acquisition unit mentioned in chapter two. It displayed the

force applied graphically in real time at half-second intervals. The examiner was able to

then watch the computer screen and adjust the force applied to ensure a relatively

constant pressurisation force. The laptop computer had installed on it specially designed

software to record and display the data from the force transducer and the two pressure

transducers. Since the computer program ran for the duration of each test it was able to

record all forces applied to the cement mantle. Statistical analysis of the force readings

64

across all of the tests proved that there was no significant difference between the

treatment of each group.

When considering the analysis of results in this thesis it is important to note that the

absolute values measured are not meant to be applied to in vivo practice. The absolute

values measured are important when compared relative to each other. For example it is

of no matter how far bone cement penetrates through a foam cavity when merely

pressurised. It is however important to note that if under the same conditions a flanged

cup is then inserted after pressurisation, then the cement will penetrate much further

through the foam. The materials used are not a valid representation of normal human

anatomy, but the biomechanical principles demonstrated in this thesis and their effects on

the experimental models are applicable to normal human anatomy.

65

Discussion of Part One:

The choice of materials for pressure testing has been discussed above. As discussed an

ideal model for the testing of cement pressurisation of the acetabulum would have the

same shape and strength as a human pelvis as well as the ability to adequately adhere

bone graft to it in the laboratory. A variety of different options were considered including

cadaveric pelvises, bovine pelvises and cast metal models. Previous studies of acetabular

cementing and pressurisation have used cadaveric pelvises (Parsch et. al. 2004 ), (Parsch

et. al. 2004 ) and (Oh et. al. 1983). Two problems were reported with regard to the use of

cadaveric pelvises, which were the difficulty in re-using the same specimen and the

inconsistency in acetabular size between the different cadavers. Shelley and Wroblewski

(1988) used a synthetic socket and rightly pointed out that in a comparative study a

“simple but consistent model” is valid and acceptable. Oh et. al. (1985) used polyethylene

blocks as a simulated acetabulum while Flivik et. al. (2004) and New et. al. (1999)

examined acetabular cementing pressure by using pelvises intraoperatively. Finally,

Bernoski et. al. (1998) used Sawbones® hemi pelvises in their study of acetabular

pressurisers. The authors in our study chose Sawbones® hemi pelvis models. This

ensured that the models were identical and also that the simulated bone grafting of the

acetabular notch, in this case foam, was carried out by the manufacturer prior to their

arrival in the laboratory. The foam also allowed ease of drilling holes for the pressure

transducers and ease of mounting in a vice.

Initially the examiners planned to use a separate hemi pelvis for each individual test.

However, the variability in the reaming, drilling of holes for pressure transducers and

tapping of a thread into the holes was thought to introduce confounding factors to testing

that ultimately requires very precise measurements. In order to reduce the possibility of

confounding variables, a single hemi pelvis was used for the pressurisation tests of bone

cement and play dough in a native acetabulum. The same hemi pelvis was used to

measure the pressure during unflanged cup and flanged cup insertion tests. In order to

allow the repeated use of the hemi pelvis, the reamed acetabular cavity was covered with

a latex glove to allow the cement to be removed from the model after the test. Silicone

grease was applied to the tip of the two pressure transducers to reduce the effect of any

66

shear forces created by the latex as the cement was pressurised by the cup as it was

inserted.

Pressurisation of bone cement, Play Dough® and ‘grafted’ acetabular notch:

The force of pressurisation for cement in native and grafted acetabulae and play dough

was 275N. This force was chosen as it was the maximum reliable force the examiner

could place on the pressuriser for a duration of ninety seconds. As higher forces are

generated, the reliability of the pressurisation is more difficult to maintain. The force

used in this part of the study compares with the 210N force used by Bernoski et. al.

(1998), but is far more than the 60N force used by Parsch et. al. (2004) in their

comparison of two acetabular pressurisers. Shelley and Wroblewski (1988) used an eight-

kilogram weight in their study of flanged and unflanged cup insertion pressures.

In all trials, the initial pressure generated in the acetabulum was higher than the final

pressure at the end of the ninety-second pressurisation time. This was due to cement

extruding from the acetabulum. As cement extrudes, the pressure is seen to fall until the

pressuriser has sunk far enough into the cement mantle to cover all of the inconsistencies

between the rim of the acetabulum and the pressuriser head. At this point, as there are no

pores in the foam models (in contrast to human cancellous bone, into which the cement

would be forced) the pressure trace levels out as a steady state is reached where there is

no net movement of cement. Had the pressurisation been continued for a further five

minutes, the cement would eventually polymerise and the pressure trace would fall to

zero. The acetabulum with the grafted notch has much smaller gaps and as a result allows

less extrusion of cement. Hence the volume of cement retained within the acetabulum is

greater and the pressuriser cannot be forced far into the acetabulum. This is postulated to

be the cause for the higher intra acetabular pressure measurements recorded with

simulated grafting of the acetabular notch. The results show a significantly higher

pressure generated in the cement mantle when the acetabular notch is covered. The total

cement pressurisation pressure listed is a function of the area under the pressurisation

curve and reflects the improved retention of cement in the acetabulum achieved when the

acetabular notch is covered. Since the pressure trace plateaued at a higher pressure with

67

the acetabular notch grafted than in the native acetabulum, then the total cement

pressurisation will consequently be significantly higher.

The peak pressure generated in the normal acetabulum was 390mmHg at the apex and

370mmHg at the rim. These figures fall far short of the results reported by Bernoski et.

al. (1998) of 900mmHg (120kPa) at the apex with a standard pressuriser and 1350mmHg

(180kPa) at the apex using a pressuriser with an additional flap to cover the acetabular

notch. Shelley and Wroblewski (1988) reported a peak pressure of 193mmHg when

testing with an acetabular pressuriser, however, the difference may be accounted for by

their use of Palacos® cement (rather than Simplex®) and the fact that the pressurisation

in their study was done at two minutes from the commencement of mixing. A greater

peak pressure was also measured by Parsch et. al. (2004) who reported 510mmHg

(68kPa) at the rim and 607mmHg (81kPa) at the apex with the Bernoski pressuriser and

585mmHg (78kPa) and 645mmHg (86kPa) at the rim and apex respectively with the

Exeter pressuriser. Their study used simplex bone cement inserted at 5.5 minutes from

the commencement of mixing. New et. al. (1999) measured a peak pressure of 570mmHg

(76kPa) and 698mmHg (93kPa) by the two surgeons in their study. Pressurisation in that

study began at one to two minutes from the commencement of mixing the Palacos® bone

cement. They also measured negative pressurisation on the cement mantle when the

pressuriser was removed to insert more cement into the acetabulum in the case of cement

leakage during pressurisation. Flivik et. al. (2004) measured a mean peak (Palacos®)

cement pressurisation of 647mmHg with conventional pressurisation at 2.5 minutes from

mixing in their in vivo study using two surgeons. The higher pressure mentioned in these

studies relate more closely to the pressures recorded in the acetabulum with simulated

bone grafting of the acetabular notch, which was 721mmHg at the apex and 690mmHg at

the rim. To the author’s knowledge there are no published studies reporting the pressure

generated when cementing an acetabulum with bone grafting of the acetabular notch.

Play Dough® and bone cement are not dissimilar in consistency and flow at particular

temperatures. When bone cement reaches dough stage it behaves rather like warm play

dough. As the cement continues to polymerise, its consistency mimics more that of Play

68

Dough® at room temperature and finally just prior to polymerisation it is much like cold

Play Dough®. Bone cement is a dynamic substance whose viscosity level once mixed,

follows a roughly predictable path. Play Dough® on the other hand must be manually

warmed to experience changes in its viscosity, and in fact undergoes the opposite change

to that of bone cement. Where bone cement viscosity increases with warming, Play

Dough® viscosity deceases with warming. In this study using Play Dough® at room

temperature it did not behave at all like that of bone cement. Its relatively higher viscosity

at room temperature resisted extrusion from the acetabulum and hence the intra-

acetabular pressure was much higher than that of bone cement alone. Perhaps if the Play

Dough® had been warmed, it may have performed more like bone cement. However, the

degree of warming, cooling rate and volume could not be adequately controlled for use in

the study unless extensive testing in its own right was carried out to create a valid

standard. The authors opted not to use warmed Play Dough® for this reason. It will be

seen in a subsequent section that the penetration of Play Dough® into the foam model is

also hindered by its high viscosity at room temperature.

Insertion of cups into model hemi pelvis:

Previous pressurisation tests were carried out by Bernoski et. al. in 1998 to test their new

acetabular pressuriser. Their study utilised Sawbones® hemi pelvises and also placed

pressure transducers at the apex and rim of the acetabulum. Tests of the pressuriser were

done using Boneloc® cement at four minutes from mixing, as was the cement used for

the insertion of a flanged cup. Shelley and Wroblewski (1998) inserted cups in their study

at two minutes from the commencement of mixing, whilst Oh et. al. (1985) inserted 48

millimetre cups at 3.5 minutes from mixing. Parsch et. al. (2004) used CMW 2000®

cement and inserted cups (of different sizes) at three minutes from mixing. Flivik et. al.

(2004) inserted cups at 4.5 minutes from mixing. We inserted our cups at six minutes

from mixing to be more in line with in vivo cementing technique in Exeter. At this time,

the viscosity of the cement is much higher and hence the propensity for cement extrusion

through the acetabular notch was less.

69

In this study the peak pressure generated by the insertion of an unflanged cup was

583mmHg at the apex and 225mmHg at the rim and for a flanged cup 697mmHg at the

apex and 505mmHg at the rim. These pressures are slightly higher than those measured

by Parsch et. al. (2004) which were 293mmHg (39kPa) and 150mmHg (20kPa) with an

unflanged cup at the apex and the rim respectively. In their study a flanged cup generated

pressures of 593mmHg (79kPa) at the apex and 368mmHg (49mmHg) at the rim. These

lower pressures may be accounted for by the reduced viscosity of the cement as the cups

were inserted (at three minutes from mixing) in their study. Our results are also much

higher than the pressures noted by Shelley and Wroblewski (1988) who recorded a peak

insertion pressure of 46mmHg with CMW1® cement and an unflanged cup and

185mmHg with Palacos® and a flanged cup. Cups were inserted in their study at two

minutes from mixing, which again may account for the reduced pressure if the cement

viscosity is lower. However, three studies reported much higher cement pressures when

cups were inserted. Oh et. al. (1985) recorded pressures of 848mmHg (11.3N/cm2) at the

apex and 548mmHg (7.3N/cm2) at the rim with an unflanged cup and 1080mmHg

(144N/cm2) at the apex and 788mmHg (105N/cm2) at the rim with a flanged cup. As

mentioned previously, cups in that study were inserted at 3.5 minutes from mixing. Flivik

et. al. (2004) measured a peak pressure on cup insertion (at 4.5 minutes from mixing) of

1115mmHg. Finally, Bernoski et. al. (1998) reported mean peak cup insertion pressures

of 900mmHg (120kPa) at the apex and 412mmHg (55kPa) at the rim.

After each cup was inserted it was pressurised at 100N for three and a half minutes. This

step was designed to simulate in vivo practice where pressurisation of the cup after it is

seated is necessary to resist the effect of back–bleeding which displaces bone cement

from cancellous bone interstices as discussed in an earlier section. Even though there

were no pores or back bleeding in the foam hemi-pelvis models used in the experiment,

the pressurisation of the cup was retained to better reflect in vivo practice. As we will see

later, this step highlighted some distinct differences in the behaviour of the cups and hints

at future improvements in cementing technique. Another reason for keeping this step was

to allow the experimental technique to remain the same throughout all parts of the study.

This means that in the experiments of cement pressurisation, the experimental technique

70

was the same as in the experiments involving cement penetration and those investigating

the effect of the suction catheter.

It was necessary in this study to also control the time in which the cups are inserted into

the acetabulum of the foam model. We aimed to have all cups seated within thirty

seconds. Only the study by Bernoski et. al. (1998) mentions the time in which cups are

seated. In their study cups were seated over a twenty-second time period. In our study the

volume of the acetabular cavity was held constant for all tests (i.e. reamed to 56mm).

Also, each bolus of bone cement inserted is a constant volume (i.e. one mix). For a cup to

be properly seated the edge (or flange) of the cup must be positioned just inside the rim

of the foam acetabulum. For this to occur the cup must sink a set distance into the

acetabulum. This distance is therefore also constant. The movement of the cup as it is

seated will displace bone cement from the acetabulum. It follows that a specific volume

of cement will be displaced in order for the cup to be seated. The net force required to

seat each cup must be sufficient to create intra-acetabular cement mantle pressure

sufficient to displace that volume of cement out of the acetabulum. Consider an

unflanged cup. It is possible to apply a low level force to the cup so that it displaces

cement slowly from the acetabulum. It will create low intra-acetabular cement mantle

pressure and the cup will be seated over a period of minutes. Conversely, if an extremely

high pressure was applied to the cup, it would create high intra-acetabular cement mantle

pressures and the cup would be seated swiftly over a period of seconds. So it is possible

to alter the intra-acetabular cement mantle pressure by inserting the cup faster. It was

clear that this variable also needed to be controlled in order to compare the intra

acetabular cement mantle pressure between different cup types. As can be seen from the

results of the initial tests where an unflanged cup was inserted into the acetabulum, if the

time of cup insertion is held (relatively) constant, the intra-acetabular cement mantle

pressure generated is also within a limited range.

When tests were carried out using a flanged cup, the intra-acetabular cement mantle

pressure was seen to be much higher at both the apex and the rim when compared to the

unflanged cup. The only change to the experimental conditions was the size of the flange

71

on the cup. This increase in flange had the effect of decreasing the area through which

cement could extrude from the acetabulum. As discussed above, a set volume of cement

must extrude from the acetabulum in order for the cup to be seated correctly. Since the

time of cup insertion was kept constant as is the volume of cement extruded, then the rate

of cement extrusion must also be constant. Hence the following relationships apply:

Volume of cement extruded = rate of cement extruded x time (all constant)

Rate of cement extrusion ∝ area through which cement can extrude x intra-acetabular cement mantle pressure

The rate of cement extrusion remains constant and is proportional to the intra-acetabular

cement mantle pressure multiplied by the area through which cement can extrude. It

follows that as the area through which the cement can extrude decreases, then the intra-

acetabular cement mantle pressure must increase in order to maintain the same rate of

cement extrusion. The flanged cup reduces the area through which cement can extrude by

virtue of its flange being closer to the rim of the acetabulum and covering the defects

present when the unflanged cup was used. These physical principles are postulated as the

reason for the increases in peak and rim pressures seen in the results of our study.

This phenomenon can be extrapolated to also explain why the use of the rim cutter and a

cup with an even larger flange showed even higher pressure measurements. When the rim

cutter is used to prepare an acetabulum it essentially smooths out the rim of the

acetabulum. The flange on the cup is cut such that it fits precisely into the ledge cut in the

acetabular rim by the rim cutter. This ensures that when the cup is seated, there are no

inconsistencies between the rim of the acetabulum and the flange of the cup.

It was expected that as the flanged cup engaged with the cut rim of the acetabulum, that

the pressure recorded on the pressure trace would rapidly increase at both the apex and

the rim. In reality something different was observed. For the last half of the duration of

cup insertion in the rim cutter tests, the intra-acetabular cement mantle pressure was

much higher at both the apex and the rim than the flanged cup alone. This effect was

72

most marked at the rim, suggesting that the extra size of the flange had the effect of

retaining more cement at the rim of the acetabulum by resisting its extrusion from the

acetabulum. The results from the cup insertion tests in part one showed that there is a

significantly increased intra-acetabular cement mantle pressure generated when a rim

cutter is used in conjunction with a flanged cup when compared to a flanged cup or an

unflanged cup alone. These findings are similar to those of Shelley and Wroblewski

(1988) who reported a significant increase in cement pressurisation with a flanged cup

over an unflanged cup. A significant increase in cement pressure with insertion of a

flanged cup was also reported by Oh et. al. (1985) and Parsch et. al. (2004) . However, if

that increase in pressure does not cause an increase in cement penetration distance (which

is considered to be a major benefit of using the rim cutter), then the relevance of these

findings is questionable.

An interesting finding was seen when the cement mantles were removed from the hemi-

pelvis models. Holes were seen in the cement mantles removed from the hemi-pelvis in

the flanged cup and unflanged cup groups (see photo 3.3 previously). These holes

corresponded with the size, orientation and location of the pods on the back of the Exeter

Contemporary cup. The holes appeared to have been formed by the pods impacting on

the floor of the acetabulum as the cup continued to be displaced into the acetabulum after

it had been seated. This phenomenon is also termed ‘bottoming out’ of the cup and is

generally associated with a poor position of the cup in the acetabulum. This ‘bottoming

out’ of the cup is believed to be caused by the 100N force applied to the cup after it was

seated in each trial cup insertion. However, these holes were not present in the cement

mantles removed from the hemi pelvis whose acetabulum had been prepared with the rim

cutter, as shown in photo 3.4. This is believed to be due to the flange impacting on the

ledge cut into the acetabular rim by the rim cutter. This implied that flanged cups

inserted into the acetabulum that had been prepared by the rim cutter do not ‘bottom out’,

since there is a mechanical block to the cup displacing further posteriorly. When a cup

‘bottoms out’, the pods on the back of the cup are in direct contact with the floor of the

acetabulum. Hence any further force applied to the cup will be transmitted via the pods

directly into the acetabulum, bypassing the cement mantle. Therefore, no additional

73

pressure is generated in the cement mantle after the cup has ‘bottomed out’. If the cup is

physically unable to ‘bottom out’, then any additional forces applied to the cup would be

transmitted into the rim of the acetabulum. Parsch et. al. (2004) “avoided” bottoming out

in their study of cup insertion whereas all cups bottomed out in the study by Bernoski et.

al. (1998).

It is important to consider the mechanical properties of the region of the cup that

transmits force into the acetabulum after it has been seated. The pods on the posterior

surface of the cup are solid pieces of UHMWPE and it is through these very rigid

hemispherical structures that forces in a ‘bottomed out’ cup are transmitted. In the case of

cups seated in a ledge created by the rim cutter, forces are transmitted through a thin

flange of UHMWPE, which is directed obliquely posterior. The flange of the Exeter

Contemporary cup has been designed to be deformable in order to allow the cement

mantle beneath to be pressurised if the surgical situation allows. Therefore, it is

reasonable to suggest that it may be possible for the surgeon to apply effective pressure to

the cement mantle after a flanged cup has been seated on the ledge created by the rim

cutter. The results of the rim cutter trials described in chapter three do not however

support this. The pressurisation force after cup insertion was only 100N, which was not

sufficient to generate detectable pressure within the acetabulum. However, the maximum

force that could be generated by the investigators was in the order of 280N. A final test

was carried out to determine whether there would be any effective pressurisation of the

cement mantle after cup insertion if a surgeon’s maximum force was applied to the cup.

The result of this trial is displayed in the form of the super pressure trace shown in

appendix 1. In this trial it is clear that pressure can be generated in the cement mantle

after a cup has been seated if the force applied is of sufficient magnitude. Future studies

will be required to further investigate this phenomenon and determine whether it is

unique to cups inserted in conjunction with the rim cutter.

In our study we measured a mean peak cup insertion force of 150N for the unflanged cup,

217N for the flanged cup and 335N for the rim cutter group. The result of the unflanged

cup group is similar to those obtained by Oh et. al. (1985) who reported a mean cup

74

insertion force of 113N – 171N for the unflanged cup group. However, their results with

regard to the flanged cup group was much higher at 2167N – 2912N. Cup insertion forces

recorded by Parsch et. al. (2004) were between 60N and 100N.

75

Discussion of Part Two:

Part two of the study used closed cell foam blocks, which had been machined into the

shape of an acetabular cavity. This was unfortunately the most troublesome feature of the

experimental design. The foam blocks made by Sawbones® are made in a uniform

thickness of forty millimetres. The foam blocks were available in a range of sizes, but

only one thickness. As a result it was necessary for the thickness of the cavity at its apex

to be restricted to twelve millimetres. In hindsight it would have been better to use a foam

that was thick enough to allow the cavity to be a uniform fifteen millimetre thickness.

The interior diameter of the cavity was made 56 millimetres to match the size of the

hemi-pelvis cavities cemented in part one. This would allow a degree of correlation

between the pressure and penetration of the cement under similar pressurisation forces.

The holes drilled, through which penetration would be measured, were one millimetre in

diameter. This size was chosen to best mimic the calibre of the cancellous bone in the

floor of the acetabulum and yet be large enough to be easily measured by inserting a

small probe. Holes were drilled at 5°, 45° and 85° to the axis of the acetabulum to

roughly correlate with the penetration at the sites of the pressure transducers in the

previous models in part one. All preparation of the foam cavities was carried out prior to

delivery to the testing laboratory (except preparing six for use in the rim cutter trials). To

the author’s knowledge, no prior studies using pre-made foam cavities have been

reported.

The shape of the rim of the acetabular cavities was perfectly flat having been machined

from a solid block of foam. In this respect it differs from the hemi-pelvis model used in

the first part of the study where the rim of the acetabulum mimicked the shape of the

normal human acetabulum with its myriad of subtle irregularities and the large acetabular

notch. Therefore the seal of the pressuriser onto the rim of the cavity was much better

than in part one and the fit of the various cups used was also much better. This fact must

be considered when evaluating the absolute values obtained from measuring the cement

penetration in part two. It again underscores that the individual values of the results in

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this paper are primarily designed to be compared relative to one another rather than as

absolute values.

77

Bone cement and play dough tests:

From the initial comparisons of the behaviour of bone cement and Play Dough® in part

one, it was clear that the difference in viscosity plays a major role in the respective flow

characteristics. The same cavity was re-used for each of the Play Dough® tests as it was

easy to remove the play dough from the cavity after each test. The results of the Play

Dough® penetration tests revealed significantly less penetration distance compared to the

bone cement tests. This is a valid result as the experimental conditions were kept the

same for each testing group and only the substance pressurised changed. These results are

consistent with those in part one, where much greater Play Dough® pressures were

recorded indicative of the relative resistance of Play Dough® to flow when at the same

temperature as bone cement. The results of this part of the study show that bone cement

and Play Dough® do not behave similarly in experimental conditions. When compared to

the results seen in part one of the study, it would appear that even though a greater intra-

acetabular Play Dough® pressure is generated, it in fact does not lead to an increase in

Play Dough® penetration.

Cup insertion tests:

In our study the mean cement penetration depth at the apex was 10.5 millimetres in the

unflanged cup group, 11.5 millimetres in the flanged cup group and 12 millimetres in the

rim cutter group. These results differ from those obtained by Oh et. al. (1985) who

recorded a cement penetration distance of 17 millimetres with an unflanged cup and 93.6

millimetres with a flanged cup. In their study cups were inserted at three minutes from

mixing, whereas in our study cups were inserted at six minutes, which may account for

the differences in results. However, our results are similar to those of Flivik et. al. (2004)

who measured a mean cement penetration radiologically of 10.3 millimetres. As

mentioned previously, their cup insertion time was 4.5 minutes from mixing.

Previous studies (Oh et. al. 1985, Shelley and Wroblewski 1988) have suggested that the

use of an unflanged cup to generate pressure in the acetabulum is not a viable possibility.

However, the results of this study have shown that even an unflanged cup is able to exert

a pressurisation force on the cement mantle at the middle and the rim. The results of this

78

study have also shown that that increase in cement pressure leads to an increase in

cement penetration distance. When the pressurised cement mantle alone is compared to

the pressurised cement mantle and an unflanged cup inserted, the cement penetration

increases significantly at the middle and the rim of the acetabulum. This finding implies

that the viscosity of the bone cement at six minutes is perhaps still low enough for

increases in cement mantle pressure to displace the cement. This finding is significant in

light of the other results that compare flanged cups with the rim cutter.

It was thought that six minutes after mixing, bone cement would simply be forced out of

the cavity and not into the small holes as a cup was inserted. In fact, the engineer behind

the design of the rim cutter stated that he felt no difference would be seen in cement

penetration distance with the rim cutter. It can be seen from the results that the cement

penetration is significantly increased at the apex, middle and the rim when a flanged cup

is compared to an unflanged cup. This increase in cement penetration distance is thought

to be due to the same physical properties of cement extrusion from the cavity through a

smaller space around the cup. The result is not surprising in light of what we know about

the flow characteristics of acrylic bone cement discussed in chapter one. It is also

supported by the work done by Shelley and Wroblewski (1988) who showed that an Ogee

flanged cup can be used to generate increases in cement penetration as it is inserted.

Increases in cement mantle pressure lead to increases in cement penetration and in these

experiments that is what we have seen. The cement penetration distance is globally

increased when a flanged cup is used compared with an unflanged cup, which suggests

that the addition of the flange is pivotal if not solely responsible for the improvement in

cement retention in the cavity. It follows that if more cement is retained in the cavity

then the pressure will increase and the cement will flow preferentially down the path of

least resistance i.e. into the twelve holes drilled in the cavity.

The same physical principles are postulated to be the cause for the improved cement

penetration seen when the rim cutter is used. Since the contact between the rim of the

cavity and the flange of the cup are so intimately matched, more of the cement is retained

in the cavity and hence more is forced into the measuring holes. The results show a

79

significant increase in cement penetration distance at the rim of the cavity. This region

corresponds with DeLee-Charnley zones 1 and 3, where lucent lines on initial post

operative radiographs are correlated with an increased rate of aseptic loosening (Breusch

and Malchau, 2005). Therefore any device or mechanism that improves cement

penetration in this area potentially reduces the incidence of radiolucent lines and also

may improve the long-term survival of the cup.

Previous studies are divided on whether cement penetration can be increased with cup

insertion. Oh et. al. (1985) reported a significant increase in cement penetration distance

when a flanged cup is inserted compared with an unflanged cup. However, even though

they reported a higher cement mantle pressure during cup insertion, Parsch et. al. (2004)

reported no significant increase in cement penetration distance with insertion of a flanged

cup compared to an unflanged cup.

Unfortunately the design characteristics of the foam cavity used may have limited the

power of the study to highlight other differences. The results of the cement penetration

measurements of the rim cutter group at the apex of the cavity show that in five out of the

six trials cement extruded right through the cavity. As mentioned previously, the cavities

could only be machined to a maximum thickness of twelve millimetres at the apex.

Ideally this would have been fifteen millimetres as was the case at the middle and rim

positions. As a result, the cement penetration distance could not reliably be measured

once the cement had exuded through the holes and the maximum distance was therefore

twelve millimetres. In reality the cement penetrated much further than that at the apex. It

is postulated that had the cavity been manufactured at a uniform thickness of fifteen

millimetres then the distance of cement penetration at the apex would have been

measurable. It may have also shown a significant increase in cement penetration depth at

the apex in the rim cutter group compared with the flanged cup group.

80

Discussion of Part Three:

The effect of back bleeding of cancellous bone on cementing technique was first

highlighted by Benjamin et. al. in 1987 as previously mentioned. It has been stated often

in world literature that in order to be valid, studies of cement penetration should take into

account the effect of back bleeding pressure (Breusch et. al. 2004). Majkowski et. al. in

1994 in their study of cement penetration utilised a bleeding model of cancellous bone

formed by cancellous bone discs of bovine femora enclosed in an aluminium chamber.

More recently, Parsch et. al. (2004) used human cadaver pelvises to assess the effect of

back-bleeding.. This study used Sawbones® “open cell” foam. This foam has a density of

0.12 g/cc and a strength of 0.28 MPa. It is in reality a closer match to osteoporotic

cancellous bone rather than normal human cancellous bone. It did, however, allow liquid

to flow through it although rather more readily than normal cancellous bone. This foam

was used as although its physical characteristics were not identical to normal cancellous

bone, it was thought that the relationship between the cement and the suction was the

relevant factor rather than the absolute value of the depth of cement penetration. Other

materials were proposed for use such as flower arranging foam, stainless steel foam,

animal cancellous bone and trabecular metal but the respective strengths, ease of

manipulation, lack of fidelity and cost made these materials unsuitable for the scale of the

testing involved.

To compensate for the increased porosity of the cancellous bone foam being used, a

substitute for blood was required. The author sought a liquid that, although thicker than

blood, would have similar flow characteristics to blood when allowed to flow through the

cancellous bone foam. Previous studies had used dyed saline but as mentioned

previously, while it flows well through cadaveric pelvises, it flows rather more readily

through foam. Motor oil (Havoline® 10w30) was found on preliminary testing to flow

through the foam at a rate and fashion similar to blood through cancellous bone in vivo.

As the temperature of the experimental work station was controlled, the changes in motor

oil viscosity due to temperature were not experienced. The spinal suction catheter was

inserted twenty millimetres into the cancellous bone foam in keeping with the reported

81

depth of suction retractor insertion by Berend and Ritter (2002) in their technical note on

the use of suction to aid cement penetration.

The height of the column of oil above the metal box was of pivotal importance. It must be

placed at a height such that it generates a back bleeding pressure equivalent to 30mmHg.

Parsch et. al. (2004) used dyed saline held at a point one metre above the pelvis to

provide a back-bleeding pressure of 25 to 30 mmHg. Their results yielded cement

penetration of two to 3.6 millimetres with cement introduced five and a half minutes from

mixing. In comparison, our experiments found a mean cement penetration distance of 6.5

millimetres in the wet foam group. Shelley and Wroblewski (1988) used a simulated back

bleeding pressure of 25mmHg in their study of simulated acetabular cementing. One

mmHg is equal to 1.33cm of water. Therefore the column of motor oil was set such that it

was in equilibrium with 40cm of water. This height was measured to be 77cm and

remained constant throughout all testing.

In this part of the study, pressurisation of the cement mantle was chosen to be 200N

initially for two minutes, followed by 100N for four minutes in keeping with the previous

parts of the study. These times were chosen in an attempt to recreate faithfully the in vivo

pressurisation of cement in the acetabulum for two minutes, followed by a period of

pressurisation at 100N for four minutes representing the more gentle pressurisation of a

prosthesis in the acetabulum to resist back bleeding until the cement had polymerised.

This pressurisation appears to have been effective in that no reduction in penetration of

cement was noted between the dry foam and foam with simulated back bleeding (p-value

= 0.33). These results further underscore the importance of correct cement pressurisation

technique to prevent the displacement of cement by back bleeding in the acetabulum.

Parsch et. al. (2004) claim that simulation of back bleeding is mandatory in experiments

involving acetabular cementing. However, our results show that adequate pressurisation

of the cement mantle and the cup after seating is able to resist the effects of back

bleeding. Our methodology was unchanged in each of the three parts of the study. Since

our pressurisation of cement and cup was able to nullify the effects of back bleeding (as

seen in the comparison between dry foam and wet foam) it is reasonable to assume that it

82

was not mandatory to include the effect of back bleeding in parts one and two of the

study.

Most importantly, a significant increase in cement penetration distance was noted when a

suction catheter was introduced at point C. This effect was seen both in dry foam and also

in foam with simulated back-bleeding. However, the effect is greater in foam with

simulated back-bleeding than in dry foam (p-value = 0.03). This difference is likely to be

due to the added pressure of back-bleeding on the cement mantle, which was the only

factor added to the experimental system. It remains unclear as to exactly how this effect

is brought about. The most likely explanation is that in the back-bleeding tests the cement

mantle opposite the suction catheter is subject to two forces, the force of the pressuriser

on one side and the force of the back-bleeding on the other. In the tests using dry foam

and suction there existed the force on the cement of the pressuriser alone. These findings

suggest that bone cement is propelled deeper into cancellous bone by two distinct

mechanisms. First is the direct effect of the suction on the cement (as seen in the dry

foam) and secondly by removing the opposing force of back-bleeding in the cancellous

bone (as seen in the foam with simulated back-bleeding) in the region of the suction

catheter. The effect of the suction catheter in improving cement penetration was not

apparent at all points in the metal box. The cement penetration at point A remained

unchanged during all experiments. This implies that the effect of the suction catheter is

confined to the region in which it is located and that its beneficial effects are reduced as

the distance from the suction catheter is increased.

Interestingly, both the dry foam group and the wet foam group had the same mean total

cement penetration, and in fact more cement penetration was noted in the wet foam with

suction group over the dry foam with suction group. The cause for this increase in cement

penetration is not known. It is therefore reasonable to suggest that if a suction catheter

was inserted under the iliac crest of a patient’s pelvis during cementing of an acetabular

prosthesis, it may serve to increase the cement penetration around the rim. The

importance of improved cement penetration at the rim of the acetabulum was mentioned

83

previously with regard to reducing the incidence of radiolucent lines on the initial post

operative radiograph.

84

Chapter Five: Conclusion:

The majority of the study was designed to explore the in vitro effect of three techniques

that may improve cement penetration into the acetabulum in total hip arthroplasty namely

1) bone grafting of the acetabular notch, 2) use of the rim cutter and 3) use of a suction

retractor in the ilium. This study aimed to help answer the question of whether these

techniques improve cement penetration and cement mantle thickness. The results

reported in this paper are valid and show that there is indeed an increase in cement

pressurisation and hence penetration when the various techniques are employed in vitro.

In the first part of the study the more uniform and constricting rim of the acetabulum in

the hemi pelvis with simulated bone grafting of the acetabular notch experiments

contained bone cement more effectively than the native acetabulum. This resulted in

higher pressures generated within the cement mantle. The pressure generated within the

acetabulum also increased with the size of the flange on the cup inserted and also when

the rim cutter was used. In the second part of the study the insertion of the flanged cup

resulted in a global increase in cement penetration distance when compared to an

unflanged cup. The effect of the rim cutter was to increase the cement penetration at the

rim only. In the third part of the study the suction catheter was used to show that suction

in simulated acetabular cementing results in an increase in cement penetration in the

immediate vicinity of the catheter with a gradually diminishing effect as distance from

the catheter increases.

What has been demonstrated by this study is that acetabular notch bone grafting, the use

of the rim cutter and suction catheters exhibit some effect on the cement penetration into

foam. This effect is supplemental to current acetabular cementing technique when used

on the same type of foam. Whether that effect applies to human cancellous bone in vivo

remains to be seen.

It is possible that in isolation or in concert these acetabular cementing techniques may

lead to a thicker and more uniform cement mantle, particularly around the rim of the

acetabulum. This may lead to a reduction in the rates of aseptic loosening of the

85

acetabular component and a lower revision rate for aseptic loosening. This study has

highlighted improvements in vitro relative to current cementing techniques and it is not

unreasonable to extrapolate this to in vivo practice. Further clinical studies are required to

properly assess the effectiveness of the new techniques in vivo.

The first element of this study compared the behavior of Play Dough® with Simplex®

bone cement in an attempt to find an alternative substance to use in future testing. It was

shown that at the same temperature, the two materials behave quite differently.

Therefore, this study does not recommend the use of room temperature Play Dough® as a

substitute for bone cement in experimental studies. Further studies may show more

similar biomechanical characteristics between the two materials when the Play Dough®

is warmed to a specific temperature.

86

Disclosure: This study was performed with the assistance of the Stryker group who provided the bone

cement, acetabular pressuriser, jig for mounting foam cavities and Exeter Contemporary

cups for testing. They also financed the purchase of all foam and foam models, the vice

and the pressure transducers. The Stryker plant in Cedex, France prepared the foam

blocks by machining them into cavities and drilling the measurement holes. They also

supplied the testing jig for the foam cavities. Exeter University generously loaned the

laptop computer, data acquisition unit and software. They also constructed the metal box

for the testing of the suction catheter. The patent on the Rim Cutter is jointly held by

Stryker Corporation, the French engineer who designed the Rim Cutter (also a Stryker

employee) and the hip surgeons at the Exeter Hip Centre (one of whom was a principal

supervisor of this project). No past or future financial or non-financial benefits have been

offered to or accepted by the author.

87

Appendix 1 1. Pressure Tracings:

Native Acetabulum Cement Trial 1 Native Acetabulum Cement Trial 2 Native Acetabulum Cement Trial 3 Native Acetabulum Cement Trial 4 Native Acetabulum Cement Trial 5 Native Acetabulum Cement Trial 6 Native Acetabulum Play Dough Trial 1 Native Acetabulum Play Dough Trial 2 Native Acetabulum Play Dough Trial 3 Native Acetabulum Play Dough Trial 4 Native Acetabulum Play Dough Trial 5 Native Acetabulum Play Dough Trial 6 ‘Grafted’ Acetabulum Cement Trial 1 ‘Grafted’ Acetabulum Cement Trial 2 ‘Grafted’ Acetabulum Cement Trial 3 ‘Grafted’ Acetabulum Cement Trial 4 ‘Grafted’ Acetabulum Cement Trial 5 ‘Grafted’ Acetabulum Cement Trial 6 Unflanged Cup Trial 1 Unflanged Cup Trial 2 Unflanged Cup Trial 3 Unflanged Cup Trial 4 Unflanged Cup Trial 5 Unflanged Cup Trial 6 Flanged Cup Trial 1 Flanged Cup Trial 2 Flanged Cup Trial 3 Flanged Cup Trial 4 Flanged Cup Trial 5 Flanged Cup Trial 6 Rim Cutter Trial 1 Rim Cutter Trial 2 Rim Cutter Trial 3 Rim Cutter Trial 4 Rim Cutter Trial 5 Rim Cutter Trial 6 Comparison of all three cup types Super Pressure Test

2. Charts: Mean Peak Pressure Generated: Cemented, Play Dough and Grafted Acetabulum Mean Peak Pressure Generated: Unflanged Cup, Flanged Cup, Rim Cutter Mean Cement Penetration at Apex

88

Mean Cement Penetration in Middle Mean cement Penetration at Rim Average Total Cement Penetration Per Cavity Comparison of Cement Mantle Thickness Under Various Conditions

89

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Improved Acetabular Cementing Techniques

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