1

Biomechanical Evaluation of the ODI Hip Screw System

Research Report for Compression and Torsion Biomechanical Tests

February 23, 2000

Research Sponsor: Orthopedic Designs, Inc.

PO Box 7778

St. Petersburg, FL 33734

Phone: (888) 635-8535

Fax: (727) 344-3244

Research Institution: University of Florida

Department of Orthopaedics

Division of Research

1600 SW Archer Rd.

Box 100246

Gainesville, FL 32610

Investigators: Donna Wheeler, PhD

Alex Rose, BS

Bryan Conrad, BS

Andy Petrella, MD

Phone: (352) 392-4251

Fax: (352) 392-8637

e-mail: wheelerd@ortho.ufl.edu

2

BIOMECHANICAL EVALUATION OF A NEW TYPE OF HIP COMPRESSION

SCREW WITH RETRACTABLE TANGS

Bramlet, Dale G. MD, Petrella, Andrew MD and Wheeler, Donna PhD

The advantages of treatment by open reduction and internal fixation for

intertrochanteric, subtrochanteric, and some femoral neck fractures of the proximal femur

has been well known to orthopedic surgeons for several decades25, 39, 49-51, 70, 84, 86, 87, 89, 100,

101. Rigid internal fixation with interfragmentary compression permits early mobilization

and reduces patient morbidity and mortality53, 66, 79, 99. Initial results with the Jewett

triflanged nails102 were dampened when long term reports revealed as high as a 26%

failure rate commonly associated with varus displacement and joint penetration with these

devices14, 57, 93. The introduction of a sliding hip compression screw lowered these

complications by allowing the fracture site to collapse upon itself as patients ambulated

rather that having the lag screw erode through the bone and penetrate into the hip joint1, 27,

31, 36, 37, 42-45, 69. Technical failure, defined as loss of fixation, loosening of the lag screw

and/or cut out of the screw with hip joint penetration has still been reported in from 4 -

19% of cases7, 26, 71, 95, 103.

Unstable fractures with a large posterior spike of bone and/or medial comminution are

particularly challenging and both medial displacement34, 41 and valgus osteotomies9, 17, 28,

88 have been proposed for restoring stability to this fracture pattern. Other authors have

advocated the use of methylmethacrylate as an adjunct to internal fixation with

intertrochanteric fractures in osteoporotic patients4, 12, 15, 16, 35, 80. Anatomic reduction and

rigid internal fixation with a compression screw – plate system is the preferred technique

by the majority of current authors13, 23, 29, 40, 93. Restoring a stable fracture pattern is

fundamental to obtaining a high union rate of these challenging fractures48, 67, 82 and

reoperation following nonunion of intertrochanteric fractures that fail to unite with

primary treatment is uniformly advocated62. In many series, from 30 to 65% of all

intertrochanteric fractures are classified as unstable52. With weak osteoporotic bone and

in unstable, comminuted 3 and 4 part intertrochanteric fractures of the proximal femur,

varus angulation of the fracture fragments and joint penetration rates of from 8 - 34%

have been reported58, 68. A high grade of osteoporosis or the presence of osteomyelitis

have been shown to be the important predisposing factors in the spontaneous

development of a subcapital femoral neck fracture after a healed intertrochanteric hip

fracture47, 54, 60, 63, 77.

While some authors have advocated the use of intramedullary nails5, 10, 18, 19, 46, 78, 96 instead of

hip compression screws for intertrochanteric fractures, the compression hip screw remains the

most popular method for treatment of these sometimes challenging fractures21, 24, 65, 74, 83, 106.

Intramedullary hip - screws have been shown to have significantly less sliding of the lag-screw,

effectively converting these into fixed angle devices with the attendant risks of cut-out, distal

cortical hypertrophy or fracture, and limb shortening, especially in unstable fractures33.

Furthermore, intramedullary devices are technically more demanding with higher intra-

operative and post-operative complications and inferior return of mobility38, 73, 90-92.

Several commercially available hip compression screw systems are “keyed” whereby

they have a design fit between the lag screw and side plate that minimizes torque or

3

rotation forces from occurring at the fracture site by limiting rotation between the lag

screw and the barrel of the side plate. These systems still allow sliding to occur between

the lag screw and side plate, enabling fracture fragments to settle into a stable pattern as

the patient walks. A compression screw at the time of internal fixation can be utilized to

apply compression forces across the fracture site. This minimizes the potential for

excessive lag screw slide and subsequent impingement of the threads of the lag screw

against the barrel, effectively converting a sliding screw into a fixed angle device8, 105.

Sliding occurs mainly during the first 2 weeks post-operatively and excessive sliding has

been associated with a delay in union of the fracture72 and lag screw cut-out105. In cases

of severe osteoporosis, excessive tightening of the compression screw can lead to a

stripping out of the thread “purchase” within the bone of the femoral head and a

subsequent loss of fixation. Increasing the strength of attachment of the lag screw within

the femoral head (or the “purchase” of the lag screw) can enable greater compressive

forces to be applied across the fracture site. With appropriate lag screw length choice,

initial compression at the fracture site should result in less settling of the lag screw, and

thus less chance of lag screw thread - barrel engagement61, 85.

We tested a new type of hip compression screw1 incorporating a reversibly deployable

series of four tangs (Figures 1 and 2) that protrude from the base of the threads of the lag

screw. The tangs are designed to engage into the cortical bone at the base of the femoral

head - neck junction in the inferior portion of the femoral head. We hypothesized that the

engagement of the tangs into the dense cortical bone serves three purposes. First, they

increase the “purchase strength” of the lag screw within the femoral head, enabling the

surgeon to tighten the compression screw with greater force without the lag screw

stripping out. Second, the tangs resist torque forces between the femoral head and the lag

screw by counteracting rotational forces occurring at this interface. Third, by increasing

the amount of bone engaged by the screw, particularly the dense cortical bone at the base

of the femoral neck, it theoretically should resist joint penetration by increasing the

column of bone necessary for a lag screw to penetrate through prior to perforating

through the femoral head and entering into the hip joint space. This paper reports the

results of testing the first two of our three hypotheses.

Because the final location of the lag screw relative to the femoral head geometry has been

shown to be significant in terms of failure2, 3, 20, 22, 32, 97, we chose to test the central versus

the inferior head position of the lag screw and its relationship to purchase within the

femoral head. Furthermore, we hypothesized that resistance to torque forces is a critical

factor leading to the failure of hip pins in osteoporotic bone. As the patient ambulates in

the early post-operative period, the repetitive torque and loading forces are paramount in

leading to a wedge effect of the lag screw in the minimally dense femoral head.

Therefore, we chose to test the torque or rotational strength of the talon hip pin.

Compression at the fracture site is critical to preventing subsequent excessive slide of the

lag screw as the patient ambulates in the post-operative period11. We therefore chose to

test compressive forces across the fracture site utilizing the Talon hip pin.

1 Talon Dynamic Compression Hip Pinning System (ODi: Orthopedic Designs Inc., P. O Box 7778 St. Petersburg, FL

33734)

4

Materials and Methods

A total of 25 matched pairs of embalmed adult cadaveric femurs were obtained

and the soft tissue removed. Radiographs were taken of each femur to assure symmetry

between right and left limbs, rule out bone pathology and assess femoral anatomy. Dual

energy x-ray absorptiometry (DEXA) (Lunar Corporation, Madison, WI) was used to

quantify the bone mineral density (BMD) of each femoral pair at the region of Ward’s

triangle. Femoral pairs were excluded from the study if there was: (1) assymetry between

right and left femurs; (2) a pathological condition; or (3) bone mineral density difference

greater than 15% between right and left femurs.

In order to maintain alignment and assist reduction, the femoral heads were

drilled and tapped for implantation of hip pins prior to creation of intertrochanteric

fractures. A pneumatic saw was used to score the femur in a circumferential manner

extending from the proximal superior aspect of the greater trochanter to the inferior

aspect of the femoral neck, just superior to the lesser trochanter. The fracture was then

completed with an osteotome.

In all studies, for one side of each pair of femurs, an ODI compression hip screw

system with a 4-hole side plate was surgically implanted using the procedure

recommended by the manufacturer. The tangs (Figure 2) of the ODI lag screw were

deployed in one femur from each pair (tang) while the tangs remained retracted in the

contralateral femur (screw). The femurs receiving the tang treatment were alternatively

placed in the higher BMD bone of a pair and then the lower BMD bone of the next pair.

All femurs were fixed with a standard barrel (38 mm)/standard barrel angle (135°) plate.

The lag screw length was determined by the femoral neck length and set as specified by

the manufacturer. Radiographs were taken after implantation to document tang

deployment and lag screw position. All surgical implantations were conducted by two

orthopaedic surgeons.

Torsional Stability Eight pairs of embalmed, adult cadaveric femora were used for this phase of the

investigation. All femoral pairs underwent cyclic torsional loading followed by a

torsional load to failure. Femoral heads were potted in dental stone (Snap Stone, Whip

Mix, Louisville, KY) in such a way that the fracture would be perpendicular to the axis of

rotation. Each femur was mounted on a custom made jig that held the shaft at

approximately 45° (Figure 3). A torsional load was applied to the femur using an Instron

materials testing system (Instron Corp, Canton, MA). The cyclic torsional load applied

to the femur was ramped between 1 and 11 N-m in a clockwise direction (tightening

action). Torsional loading was applied to the femurs at 1 Hz for 5000 cycles. Angular

displacement and torque were collected during cycling to determine rotational stability of

the construct. All femurs underwent cyclic testing prior to torque to failure.

After cycling, the compression screw was removed from the lag screw and the

femoral head fragment removed from the lag screw barrel of the side plate and the

femoral shaft. The lag screw was securely mounted to the materials testing equipment

using a 4-jaw Jacobs chuck. The femoral head, mounted in dental stone and reinforced

with k-wires, was mounted to the actuator of the materials testing equipment. The

construct was loaded to failure in torque at a loading rate of 1 degree/sec to a maximum

rotation of 90 degrees in a clockwise direction.

5

Statistical analyses involved an analysis of variance (ANOVA) with a complete

block design (paired samples) evaluating the effects of bone mineral density and

treatment (tang or screw). All statistical tests were run using SAS statistical software at a

significance level of α = 0.05.

Interfragment Compression

Seventeen pairs of embalmed, adult cadaveric femora were assigned for this phase

of the investigation. Intertrochanteric osteotomies were performed, instead of

ostectomies, to provide space between fracture fragments for the compression transducer.

Care was taken to assure parallel fragment surfaces for uniform load distribution to the

interposed ring load transducer. The load transducer placed between intertranchanteric

fragments is shown in Figure 4.

Four pairs of specimens were tested with the lag screw in the central aspect of the

femoral neck (center). The lag screw was placed in the inferior position (IP) for the

remaining femoral pair to provide better cortical purchase. Both center and IP positions

were tested to determine if lag screw position affected compressive characteristics.

A 4400 N (1000 lb) capacity ring load cell (Transducer Techniques, Inc,

Temacula, CA) was used to monitor the compression of intertrochanteric fragments. The

calibrated load cell was placed between the fracture fragments prior to insertion of the

device lag screw. For each quarter turn of the compression screw, the interfragment

compression and torque applied to the compression screw were recorded in a systematic

fashion. Compression force and torque were recorded for a total of 15 complete turns of

the compression screw. Failure was defined as a drop in compression of more than 50%

within one turn of the compression screw. Data was acquired 5 screw turns beyond peak

compression. Radiographs were acquired following testing to document the device

position at failure.

Statistical analyses involved an analysis of variance (ANOVA) with a complete

block design (paired samples) evaluating the effects of bone mineral density, lag screw

position (center or IP) and treatment (tang or screw). If a significant effect of position

was found, the effects of treatment by position were determined. Similarly, if a

significant effect of treatment was found, the effects of position by treatment were

determined. All statistical tests were run using SAS statistical software at a significance

level of α = 0.05.

6

Results

Torsional Stability

Table 1 summarizes the results of the cyclic torsional and torque to failure tests.

The relative rotational displacement during cyclic testing from preload to maximum load

was defined as amplitude. The absolute rotational displacement from beginning to end of

cycling (5000 cycles) was defined as migration. The amplitude during cyclic loading for

the tang and screw groups was not significantly different. However, the femurs fixed

with the tang treatment had significantly less rotational migration during 5000 torsional

cycles than the screw group (p = 0.023), with angular values of 1.59 ± 3.15 degrees and

2.11 ± 2.97 degrees, respectively. The peak torque for the tang device was significantly

greater than the torque for the screw device (p = 0.00016) with torque values of 6.48 ±

1.73 N-m and 1.92 ± 1.71 N-m, respectively. The average rotational displacement

occurring prior to peak torque was 27.13 ± 18.20 degrees for the tang group and 30.13 ±

19.38 degrees for the screw group. The rotational displacement at peak was not different

between the tang and screw groups. The torque at 15 degrees of rotation, a more

clinically relevant value, was also significantly greater for the tang compared to the

screw device (p=0.0028) with torque values of 4.185 ± 2.002 N-m and 1.21 ± 1.15 N-m,

respectively. Peak torque and torque at 15 degrees of rotation are presented graphically

in Figure 5.

Interfragment Compression Of the 17 paired specimens assigned to compression testing, 5 pairs of specimens

were deleted from the analysis. The specimens were eliminated due to testing errors

(n=2), implantation errors (n=1), and BMD differences between contralateral femurs

greater than 15% (n=2). The remaining 12 femur pairs were evaluated for interfragment

compression as a function of compression screw rotation. Table 2 and Figures 6 – 9

summarize the results from the compression test. The peak compressive forces generated

by the tang treatment group were significantly greater than the screw group in the inferior

position (p = 0.0046) with compression values of 1398.41 ± 466.84 N and 712.83 ±

322.77 N, respectively (Figures 7 & 9). No differences between tang and screw

treatments were noted when the device was implanted in the central position (Figures 8 &

9). The compression was significantly greater for the tang group when the lag screw was

placed in the inferior position compared to the central position (p=0.011) but no

difference in the screw group was noted (Figure 10). The tang group produced

significantly greater compression than the screw group in the inferior position (p=0.036)

but no difference in the central position (Figure 7). The BMD did not have a significant

effect on interfragment compression for the screw or tang group (p > 0.05).

7

Discussion

Numerous papers have addressed the ideal positioning of the lag screw within the

femoral head to maximize purchase of the screw and to minimize the risk of cut out59,

95, 104. Our data clearly demonstrates that an inferior placement of the lag screw within

the lower portions of the femoral head results in greater purchase of the lag screw

regardless of whether the tangs are deployed . However, with tang deployment, a 96%

greater peak compression strength is obtained prior to failure when compressive forces

are applied across the fracture site. These results were statistically significant [Figures

7 & 9; (p=0.0046)]

Our results show that one of the critical factors maximizing the purchase of the lag screw

within the femoral head is to engage the tangs within the dense cortical bone at the

junction between the femoral head and the neck. If a central location is chosen and the

bone is weak and osteoporotic, deployment of the tangs provides no further increase in

purchase resistance to compressive forces within the femoral head (Figure 8). If the

tangs are engaged within the dense cortical bone provided by an inferior lag screw axis,

then a statistically significant (p=0.036) increase in compressive force across the fracture

site can be produced (Figure 10) prior to lag screw failure.

Regardless of tang deployment, an inferior position is supported by this study. With both

the torque testing and the compression testing, at the extremes of testing, representing

non-physiologic loads, radiographs revealed gross tang deformity. In all instances, when

the actuator retrieval mechanism was utilized, the tangs fully retracted and no tangs broke

or failed to retract.

We believe that the mechanism of failure accounting for lag screw cut out and subsequent

pin penetration so commonly reported is multifactorial. Gill J. et al reported that high

streeses in the surrounding cancellous bone contribute to the failure of repairs30. Failure

to compress the fracture site fully has been noted clinically to be accompanied by

excessive slide of the lag screw, which has been shown to be associated with a poor

functional outcome6. The advantage of enhanced compression across the fracture site has

been advocated by numerous authors55, 56, 64, 75, 76, 98. Indeed, excessive slide, presumably

associated with poor initial compression has been shown to prolong time to union of

operatively treated intertrochanteric fractures72. Prior to the introduction of the talon hip

pinning system, few hip pinning systems were designed to prevent rotation forces

between the lag screw and the proximal femoral bone. While hip bolt procedures have

been advocated by some authors to increase the purchase of the device within the femoral

head, most authors have favored use of the hip compression screw81. Our results clearly

demonstrate an approximately three times greater peak rotational purchase of the Talon

lag screw within the femoral head compared to those lag screws wherein the tangs were

not deployed (Figure 5). There are no comparative studies to date testing different

commercially available hip pinning systems with the Talon hip pin. Thread length and

screw designs vary between manufacturers.

8

Most current hip pinning systems fail to adequately address rotational or torque forces

that occur between the femoral head and the lag screw as the patient ambulates and

arises from or resumes the sitting position. We postulate that with weakened

osteoporotic bone, the lag screw acts like a wedge, slowly working its way upward as

the patient ambulates and subjects their recently fractured hip to rotational forces. This

is particularly a problem in the non-compliant and slightly demented patients who

nevertheless remain ambulatory. Many authors have noted the extreme resorption of

bone in the femoral head and metaphyseal regions of the proximal femur and torsional

forces have been implicated in implant failure94. Enlarged threads patterns or expansile

devices such as molley-bolt designed hip lag screws fail to reach to the dense remaining

cortical bone that we believe holds the greatest promise for maximizing purchase of the

lag screw within the femoral head.

Our prior experience has shown great difficulty in experimental models for reproducing

“cut-out” of the femoral head. Investigators have difficulty obtaining only osteoporotic

femurs for testing, and many matched specimens vary in bone density even in the same

person. We alternated between most and least dense femurs for deployment of our tang –

deployed and control lag screws, but a better test design would be to have matched

controls that were tested in only osteoporotic femurs. Furthermore, our fracture design

was a stable model whereby a simple 2 part intertrochanteric fracture was created. This

insured a stable reconstruction of the fracture pattern simulating a reproducible construct

expected after open reduction internal fixation. Clinical experience dictates that unstable

and multipart fractures are those most at risk for cut-out, in addition to the extremely

osteoporotic. Furthermore, in vivo experience would suggest greater extremes of fracture

site bone interdigitation depending upon the extremes of fracture comminution and the

success of the fracture site reduction.

Our results clearly demonstrate that with a stable 2 part intertrochanteric fracture of the

proximal femur treated with a Talon lag screw with tangs fully deployed, the resistance to

torque forces (Table 1, p=0.0028) in our experimental model were greater with tangs

deployed that with comparable lag screws wherein the tangs were not deployed. Many

manufacturers and numerous studies have attempted to prevent or reduce the incidence of

cut out of the lag screw by enlarging the threads of the lag screw or by expanding a

molley-bolt type device within the head of the femoral head. An association with over-

reaming and weak osteoporotic bone have been shown to be significant risk factorsa.

We believe that the unique combination of deployable tangs within the femoral head has

been shown to be superior to resisting rotational forces than a comparable lag screw

without tangs deployed. Furthermore, the tangs deploy a full 1 inch at maximum

deployment. Most standard lag screws have an external thread diameter of

approximately ½ in. The column of bone that a lag screw must erode through to penetrate

through the superior cortex of the femoral head is thus doubled with the talon hip pin

with tangs deployed.

9

The peak compressive forces generated by the tang treatment group were significantly

greater than the screw group in the inferior position (p = 0.0046) with compression values

of 1398.41 ± 466.84 N and 712.83 ± 322.77 N, respectively (Figures 7 & 9). No

differences between tang and screw treatments were noted when the device was

implanted in the central position (Figures 8 & 9). The compression was significantly

greater for the tang group when the lag screw was placed in the inferior position

compared to the central position (p=0.011) but no difference in the screw group was

noted (Figure 10). The tang group produced significantly greater compression than the

screw group in the inferior position (p=0.036) but no difference in the central position

(Figure 7). The BMD did not have a significant effect on interfragment compression for

the screw or tang group (p > 0.05).

In summary, the talon hip pinning system in our matched cadaveric model provided a

two fold increase of purchase of the lag screw when tangs were deployed within the

femoral head when compressed to failure at the fracture site. Optimal location of any lag

screw with an axis inferior to the center of the femoral head as advocated by Wu, C. et

al104 in supported by our results in this study. This leads to greater purchase of the lag

screw within the femoral head in an area that presumably has more dense bone and thus

theoretically should resist cut-out. Deployment of the tangs into or through the cortical

endosteal surface increases the purchase strength of the lag screw in this inferior position.

Furthermore, tang deployment appears to be the critical factor in resisting rotational

torque forces about the femoral head in our test model. After cyclical testing of the

fracture construct, peak torque for the tang device was significantly greater than the

torque for the screw device (p = 0.00016) with torque values of 6.48 ± 1.73 N-m and

1.92 ± 1.71 N-m, respectively. Our results support our hypotheses that the Talon hip pin

system with tangs deployed provides significantly increased purchase of the lag screw

within the proximal femoral head and will allow greater compression forces to be applied

across the fracture site. The increased purchase afforded by the tangs within the femoral

head counteracts rotational forces and provides a greater column of bone that must be

eroded through prior to lag screw penetration in our model. While our results did not

address the degree of tang penetration into the dense cortical bone, we believe from our

experience with this device that engagement or penetration into or through the cortical

bone at the base of the femoral head-neck junction is the critical technical step to

maximize the tang purchase within the femoral head.

10

BMD

(g/cm3)

Amplitude (degrees)

Migration (degrees)

Peak Torque (N-m)

Torque at 15 degrees

(N-m)

Screw 0.516 ± 0.234 0.953 ± 0.409 2.11 ± 2.97 1.92 ± 1.71 1.21 ± 1.15

Tang 0.547 ± 0.243 1.07 ± 0.581 1.59 ± 3.15 6.476 ± 1.73 4.185 ± 2.002

Screw vs Tang

(p-value) p = 0.292 p = 0.723

Tang < Screw p = 0.023

Tang > Screw p = 0.00016

Tang > Screw p = 0.0028

Table 1. Results of Torsional Biomechanical Tests

Implantation Location

Measurement Screw Tang Screw vs Tang

(p-value)

Inferior

Peak Compression (N) (n = 8)

712.83 ± 322.77 1398.41 ± 466.84 *p = 0.0046

Peak + 1 turn (N) (n = 6)

380.88 ± 196.64 1033.47 ± 451.37 *p = 0.021

Peak + 3 turn (N) (n = 6)

343.90 ± 192.90 712.46 ± 384.01 *p = 0.024

Peak + 5 turn (N) (n = 6)

258.55 ± 132.22 472.99 ± 215.43 p = 0.078

Central

Peak Compression (N) (n = 4)

520.44 ± 346.22 562.70 ± 362.26 p = 0.853

Peak + 1 turn (N) (n = 4)

270.67 ± 204.61 286.91 ± 166.14 p = 0.905

Peak + 3 turn (N) (n = 4)

145.01 ± 108.25 205.73 ± 60.81 p = 0.396

Peak + 5 turn (N) (n = 4)

71.73 ± 55.42 180.15 ± 30.55 *p = 0.024

Table 2. Results of Interfragmentary Compression Tests

11

Figure 1. Talon Hip Compression Screw System

12

Figure 2. Talons fully deployed

13

Figure 3. Specimen in Test Fixture for Cyclic Torsional Loading

14

Figure 4. Specimen with Load Transducer Interposed between

Intertrochanteric Fragments to Measure Compression

15

Figure 5. Peak Torque and Torque at 15 Degress

0

1

2

3

4

5

6

7

8

9

Peak 15 degrees

To

rqu

e (

N-m

)

Screw

Tang

Tang > Screw

(p = 0.00016)

Tang > Screw

(p = 0.0028)

16

Figure 6. Interfragment Compression for Each Femur Pair

in the Inferior and Central Lag Screw Positions

0

500

1000

1500

2000

2500

#8952 #9126 #9210 #9273 #9337 #9363 #9391 #9134 #9130 #9240 #9271 #9159

Specimen ID

Fo

rce

(N

)

Screw

Tang

Inferior Screw Position Central Screw Position

17

Figure 7. Peak & Post-Peak Compressive Forces

Inferior Postion

0

500

1000

1500

2000

2500

Peak Peak + 1 turn Peak + 3 turn Peak + 5 turn

Fo

rce

(N

)

Screw

Tang

Screw < Tang

(p = 0.0046)

Screw < Tang

(p = 0.021)

Screw < Tang

(p = 0.024)

Screw = Tang

(p = 0.078)

18

Figure 8. Peak and Post-Peak Compressive Forces

Central Postiion

0

200

400

600

800

1000

1200

Peak Peak + 1 turn Peak + 3 turn Peak + 5 turn

Fo

rce (

N)

Screw

Tang

Screw = Tang

(p = 0.853)

Screw = Tang

(p = 0.905)Screw = Tang

(p = 0.396)

Screw < Tang

(p = 0.024)

19

Figure 9. Peak Interfragment Compression

for Inferior and Central Lag Screw Placement

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Inferior Central

Screw Position

Fo

rce

(N

)

Screw

Tang

Screw = Tang

(p=0.853)

Screw < Tang

(p = 0.0046)

20

Figure 10. Inferior versus Central Placement of Lag Screw

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Screw Tang

Treatment

Fo

rce

(N

)

Inferior

Central

Inferior > Central

(p = 0.0110)

Inferior = Central

(p = 0.3635)

21

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