JBJS | Forces Required to Initiate Sliding in Second-Generation Intramedullary Nails*

The Journal of Bone & Joint Surgery, Volume 80, Issue 11

Articles | November 01, 1998

Forces Required to Initiate Sliding in Second-Generation Intramedullary Nails*

DEB A. LOCH, M.S.†; RICHARD F. KYLE, M.D.†; JOAN E. BECHTOLD, PH.D.†; MICHAEL KANE, M.D.†; KIMBERLY ANDERSON, M.S.†; ROBERT E. SHERMAN, PH.D.†, MINNEAPOLIS, MINNESOTA

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Investigation performed at the Orthopaedic Biomechanics Laboratory, Midwest Orthopaedic Research Foundation at Hennepin County Medical Center, Minneapolis

The Journal of Bone & Joint Surgery. 1998; 80:1626-31

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Abstract

Second-generation intramedullary nails, which allow the fixation screw that is placed in the femoral head to slide distally and thus allow compression of the fracture of the femoral neck, have become a popular option for the treatment of ipsilateral fractures of the femoral neck and shaft. However, the sliding characteristics of the screw within the barrel of the nail or the side-plate have not been assessed biomechanically, to our knowledge. The goal of the current study was to investigate the forces required to initiate sliding of the proximal screw in intramedullary devices and to compare these forces with those required to initiate sliding of hip screws. The loading configuration simulated the typical angle of 135 degrees between the intramedullary nail and the proximal screw. The forces required to initiate sliding of the proximal screw, with the screw extended fifty-one, seventy-six, eighty-six, and 102 millimeters beyond the proximal end of the barrel, were measured for three different types of second-generation intramedullary nails (Recon, ZMS, and Gamma), a sliding compression hip screw, and an intramedullary hip screw, and these forces were then compared.With each amount of extension of the screw, the hip screws required lower forces to initiate sliding than did the second-generation intramedullary devices. Of the second-generation devices, the Gamma nail required the highest forces to initiate sliding; the Recon and ZMS nails required 20 to 40 percent lower forces compared with the Gamma nail. None of the devices jammed in any of the loading configurations that were tested. When the extension of the screw was increased, higher forces were required to initiate sliding.CLINICAL RELEVANCE: Since sliding allows continued compression of the fracture, surgeons should be aware that, compared with hip screws, the second-generation nails that we tested required higher loads to initiate sliding and to generate subsequent compression of the fracture.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Ipsilateral fractures of the femoral neck and shaft are uncommon injuries that are challenging to treat. Stabilization of both fracture sites with use of a second-generation femoral intramedullary nail, which can be distinguished from a first-generation intramedullary nail by the superomedial orientation of the channel for the proximal locking screw, is a suitable treatment for this combination of fractures. The superomedial orientation of the channel allows the screw, which is inserted into the femoral head, to slide distally, providing physiological compression at the site of the fracture of the neck or the intertrochanteric fracture as the screw slides distally and laterally through the barrel of the nail. The mechanism of the sliding of the locking screw through the nail is similar to that of the sliding compression hip screw, which has a long history of successful use in the treatment of intertrochanteric fractures.

The function of the compression hip screw depends on the sliding of the screw within the barrel as well as the proper placement of the screw within the femoral head^13,15-17,20. The capability of the screw to slide provides a controlled collapse of the fracture along the axis of the implant. Failure of the screw to begin sliding when compressed results in a rigid fixation device, which is likely to cut out or to penetrate into the joint space^5,9,11,16,22. Additional potential complications resulting from the inability of the screw to compress dynamically include nonunion and fatigue fracture of the hardware^12,19.

The ease of sliding of the screw in the barrel of the intramedullary device is determined mechanically by the angle between the screw and the nail as well as by the amount of the screw that is engaged in the barrel of the device (Fig. 1)^15,16. This finding has led to the increased use of larger-angle compression hip screws for the fixation of intertrochanteric fractures of the femur. Clinically, the placement of the hip screw in the center of the femoral head, to ensure adequate purchase within the proximal bone fragment, is the most important step during the procedure; thus, the largest-angle screw that allows such placement is used. With second-generation intramedullary nails, the channel for the proximal screw has a fixed orientation of 130 to 135 degrees to provide ease of insertion of the proximal screw.

The success of fixation of a femoral fracture with a sliding device depends on the ability of the device to slide. Thus, the objective of the current study was to determine the forces required to initiate sliding of the proximal screw in the barrels of second-generation intramedullary nails under physiological loading conditions, and to compare these data with those for sliding compression hip screws and intramedullary hip screws.

*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were Richards Medical Company (Smith and Nephew Richards, Incorporated), Memphis, Tennessee; Ace DePuy, Los Angeles, California; Howmedica, Incorporated, Rutherford, New Jersey; and Zimmer, Incorporated, Warsaw, Indiana. All devices were donated by the manufacturers.

†Orthopaedic Biomechanics Laboratory, Midwest Orthopaedic Research Foundation, 914 South Eighth Street, Mail Code 860-C, Minneapolis, Minnesota 55404. E-mail address for Dr. Bechtold: joan.bechtold@co.hennepin.mn.us. Please address requests for reprints to Dr. Bechtold.

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Fig. 1 Free-body diagram depicting the loads applied to the femur and the corresponding resultant loads. P = load vector, applied at an angle of 159 degrees from the vertical axis; ß = screw-nail angle; b1 and b2 = reaction forces; a1 and a2 = static frictional forces; A0 = axial (vertical) component of load vector P; B0 = perpendicular component of load vector P; LS = extension of the screw beyond the proximal end of the barrel; and LB = length of the barrel of the nail.

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Fig. 2 Diagram of the experimental setup, showing application of load with use of a static weight and measurement transducers for the monitoring of vertical displacement and generated load. IM = intramedullary, and LVDT = linear variable differential transformer.

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Fig. 3 Graph showing the force required to initiate sliding as a function of extension of the screw for a 135-degree nail-screw angle and a body weight of 690 newtons. IMHS = intramedullary hip screw, and CHS = compression hip screw.

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TABLE I PARAMETERS OF THE LOGARITHMIC STATISTICAL MODEL*

*The load required to initiate sliding (LOAD) was analyzed as a function of the bending moment (BMOM). A linear regression line was fit to the measured data with use of least-squares estimates. The regression coefficients c and d define the equation to predict the load on the basis of the bending moment. The standard errors of c and d are a gauge of the estimation of the coefficients. The coefficient of determination represents the degree to which the variability of the load is attributable to the variability in the bending moment.

Device Tested†	No. of Samples	Regression Coefficient		Standard Error		r² Value
c	d	Of Coefficient c	Of Coefficient d
Intramedullary nails
Recon	3	3.040	0.944	0.094	0.040	0.962
ZMS	4	2.915	1.072	0.091	0.041	0.974
Gamma	3	2.639	1.345	0.162	0.068	0.958
Hip screws
Sliding compression	5	2.940	0.840	0.130	0.110	0.918
Intramedullary	5	2.044	1.207	0.195	0.086	0.872

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TABLE I PARAMETERS OF THE LOGARITHMIC STATISTICAL MODEL*

Device Tested†	No. of Samples	Regression Coefficient		Standard Error		r² Value
c	d	Of Coefficient c	Of Coefficient d
Intramedullary nails
Recon	3	3.040	0.944	0.094	0.040	0.962
ZMS	4	2.915	1.072	0.091	0.041	0.974
Gamma	3	2.639	1.345	0.162	0.068	0.958
Hip screws
Sliding compression	5	2.940	0.840	0.130	0.110	0.918
Intramedullary	5	2.044	1.207	0.195	0.086	0.872

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

The sliding characteristics and jamming potentials of three types of second-generation intramedullary nails (Recon [Smith and Nephew Richards, Memphis, Tennessee], ZMS [Zimmer, Warsaw, Indiana], and Gamma [Howmedica, Rutherford, New Jersey]), a sliding compression hip screw (Ace DePuy, Los Angeles, California), and an intramedullary hip screw (Smith and Nephew Richards) were tested. The effect of extension of the screw beyond the proximal end of the barrel on the jamming potential was determined. The experimental design was based on earlier work, by the senior one of us (R. F. K.), on sliding compression hip screws¹⁵.

Experimental Design

Four ZMS, three Recon, and three Gamma second-generation femoral nails that can be locked, five sliding compression hip screws, and five intramedullary hip screws were tested to determine the force on the screw that is required to overcome the static frictional force and thereby initiate sliding. Four different levels of extension of the screw were tested for each nail (except for the ZMS nail, which was tested at only three levels), with a fresh surface of the screw contacting the nail each time. To provide a fresh surface for each test, the screw was rotated in the barrel, or bearing surface.

Each fourteen-millimeter nail was attached to an angle block that was cut so that the proximal screw extended vertically from the barrel (Fig. 2). The apparatus was placed on the actuator of a servohydraulic test frame (MTS, Minneapolis, Minnesota; controller: Interlaken, Eden Prairie, Minnesota). A static load was applied perpendicular to the tip of the screw through a cable connected to a weight by means of a pulley (Fig. 2). The screw was held in place by the frictional forces a₁ and a₂ resulting from the reaction forces b₁ and b₂ (Fig. 1). The surface of the load-cell platen contacted the tip of the screw (Fig. 2). The surface of the platen was hardened to a Rockwell C of fifty-five to sixty and then was polished to minimize frictional coupling between the screw and the platen.

During each test, the screw was fully engaged in the barrel of the nail. The tip of the screw was parallel to and concentric with the load-cell axis of the test-frame. The test was run in displacement control at a rate of 0.5 millimeter per second. The load that was applied to the tip of the screw and the displacement of the nail were recorded. The nail-displacement and load data were measured with the screw extended fifty-one, seventy-six, eighty-six, and 102 millimeters beyond the proximal end of the barrel. (Because of its length, the screw for the ZMS nail can be extended a maximum of ninety millimeters beyond the barrel; thus, it was tested only at extensions of fifty-one, seventy-six, and eighty-three millimeters.)

We simulated a 130-degree angle between the screw and the nail by applying a seventy-two-newton perpendicular load component, and we simulated a 150-degree angle by applying a 225-newton perpendicular load component. The performance of the nail with the screw at a 135-degree angle was calculated by interpolating the appropriate perpendicular load component (or bending moment). Only the results for the 135-degree angle are presented. The screw was rotated 90 degrees between the 150 and 130-degree tests to ensure that any effects due to galling along the surface of the screw were eliminated. A testing sequence was established so that the configurations that were least likely to cause damage to the nail (those with the smallest bending moments) were tested first.

Several improvements were made to the previously reported experimental design for the testing of the compression hip screws¹⁵. A linear variable differential transformer was attached parallel to the screw and the barrel of the nail, in order to isolate sliding of the screw from bending of the nail, as measured by the displacement transducer of the test-frame. For example, it was known that no bending had occurred if the measurement obtained with the linear variable differential transformer was equal to the displacement recorded by the test-frame transducer. To prevent bending of the intramedullary nails, a test run was terminated when the measurement obtained with the linear variable displacement transducer became less than the displacement recorded by the test-frame transducer or when the load exceeded 950 newtons. Thus, the screw was considered to have jammed within the barrel of the nail at an axial load component of 950 newtons. To ensure an adequate response time to terminate the test run if excessive bending or jamming suddenly occurred, the displacement rate was decreased to 0.5 millimeter per second compared with the rate of sixty millimeters per second that was used previously¹⁵.

A logarithmic statistical model was used to compare the data for the different devices. The load required to initiate sliding (LOAD) was analyzed as a function of the bending moment (BMOM, measured from b₂), compared across each nail, and combined to form a summary description. The data suggested that a logarithmic transformation should be used because the variability of the observations appeared to be proportional to the expected value as the amount of the extension of the screw increased. Thus, the following statistical model was used: ln(LOAD) = c + d * ln(BMOM), in which both c (the y-intercept) and d (the slope) are constants. The observations for each design of intramedullary nail or hip screw were fit separately to the model with use of standard least-squares regression methods. The mean forces required to initiate sliding of the compression hip screw¹² were used as single data points for the compression hip screw regression line (the logarithmic data were best represented by a linear relationship). The best-fit line for each device was transformed back to the algebraic scale with use of the antilog: LOAD = e^{(c + d * ln[BMOM])}, or, equivalently, LOAD = e^c * e^{ln(BMOM * d)}.

The force required to initiate sliding was calculated for each device, assuming a 690-newton body weight and a joint-reaction force of three times body weight.

Results

Introduction | Materials and Methods | Results | Discussion | References

The forces required to initiate sliding of all of the second-generation nails were significantly higher than those required for sliding of the compression hip screws and the intramedullary hip screws (which have a configuration similar to that of a compression hip screw) at each level of extension (p < 0.05) (Fig. 3; Table I). The forces required for sliding of the intramedullary hip screws and those required for the compression hip screws were similar. The lengths of the barrels of the intramedullary hip screws and the compression hip screws also are similar; both barrels are longer than those of the second-generation nails. Of the second-generation devices, the Gamma nail required the highest forces to initiate sliding; the forces required to initiate sliding of the Recon and ZMS nails were 20 to 40 percent lower. When the extension of the screw was increased, higher forces were required to initiate sliding. None of the intramedullary nails jammed in any of the loading configurations that were tested.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

There have been numerous studies comparing the sliding hip screw with alternative methods of fixation of intertrochanteric fractures^3,10,18,22. The sliding hip screw is twice as strong and five times more rigid than either Ender pins or the Harris nail¹⁰. Sernbo et al.²⁴ prospectively compared the results of fixation with Ender pins with those of fixation with a sliding hip screw in more than 200 patients. The patients who had an unstable fracture that had been treated with Ender pins needed more secondary operations because of loss of fixation compared with a similar group that had been managed with a compression hip screw. In a prospective, randomized study, Bannister et al.¹ compared the results of fixation with a Jewett nail with those of fixation with a sliding hip screw. The prevalence of failure of the Jewett nails (twenty-five [69 percent] of thirty-six) was much higher than that of the hip screws (eight [16 percent] of fifty). In a retrospective study of 173 intertrochanteric fractures, Jacobs et al.¹³ found that the rates of acetabular penetration and loss of fixation after use of Jewett nails were higher than those after use of hip screws. Simpson et al.²⁵ studied the mechanism of failure of thirty-five of 223 sliding hip screws that had been used in the treatment of intertrochanteric fractures. In two hips, the implant had separated. In the remaining thirty-three, the compressive action of the device had been lost as a result of jamming, an insufficient amount of sliding, or interference due to additional fixation. In prospective, randomized studies by Leung et al.¹⁸ and Radford et al.²², the outcomes of treatment with Gamma nails and compression hip screws were compared. With the numbers of patients available, no difference in the outcomes was found between the groups in either study. However, the patients who had been managed with a Gamma nail had more perioperative complications. The Gamma nail must be sized properly to avoid intraoperative fracture of the femur.

Loads on fixation devices during actual and simulated clinical use have been evaluated experimentally. In one study, the Jewett nail and the hip screw were compared by measuring strains on the implant laterally and at the fracture site medially¹³. Strains on the Jewett nail were higher than those on the bone. Conversely, the compressive strains on the hip screw were lower than those on the bone, suggesting that the bone was bearing a major part of the load. In another study, insertion of a nail-plate with instrumentation in the proximal part of the femur after an osteotomy allowed determination of the in vivo force on the screw⁷. The screw was found to transmit one-fourth of the total hip load. The use of this nail-plate construct demonstrated that, for a bedridden patient who has a fracture of the femoral neck, the forces on the femoral head during activities of daily living are similar to those experienced during walking with external supports⁸. Peak loads on the hip joint have been determined with gait analyses as well as with studies of hip prostheses with instrumentation^6,8,14,21. During the stance phase of gait, the peak force was found to be a mean of 2.7 times body weight, with the resultant force on the anterior-superior aspect of the femoral head applied at an angle of approximately 159 degrees from the vertical^6,8,14,21. Hence, for the loading protocol in the current study, we assumed a joint-reaction force of three times body weight, with one-fourth of the load transmitted to the device.

The biomechanical advantages of a sliding hip screw or a second-generation intramedullary nail depend on adequate purchase of the proximal screw within the femoral head, sufficient stability of the implant, and proper sliding. Sliding is influenced by the bending moment on the screw; with larger bending moments, a higher force is required to initiate sliding. The bending moment is increased in a heavier patient, by a smaller screw-nail angle, and by a longer extension of the screw. In practice, all intramedullary devices need to have a small angle between the nail and the screw because of operative and design-related constraints, and this increases the bending moment. The distance over which the screw is engaged in an intramedullary nail is shorter than the distance over which it is engaged in a compression hip screw, and this decreases the ability of the screw to slide through the intramedullary device. Thus, because of the smaller angle and the decreased amount of engagement in the barrel, the sliding properties of clinically available second-generation intramedullary devices would be expected to be lower than those of smaller-angle compression hip screws¹⁵.

In the present study, the force required to overcome the static frictional forces between the screw and the barrel of five different devices was measured. The effect of the extension of the screw on the sliding characteristics also was examined. This information may help surgeons to determine which devices promote bone impaction and therefore have the greatest potential to resist jamming and cut-out. Although this was an in vitro study and therefore not a direct representation of the clinical situation, it did simulate forces that have been reported in previous in vitro and in vivo investigations^6,8,14.

In general, the forces required to initiate sliding in devices with a longer barrel are lower than those required to initiate sliding in devices with a shorter barrel. Intramedullary devices have a shorter barrel, and hence the forces required for sliding were higher.

In most second-generation intramedullary devices, the barrel through which the screw slides is the proximal hole in the nail. Since these intramedullary nails are hollow, the sliding surface differs from the broad, continuous barrel of a sliding hip screw; rather, it consists of two arcs, one at either side of the wall of the intramedullary nail. The thickness of this supporting surface is the thickness of the wall of the nail, and the diameter of the arc is the diameter of the hole for the locking screw. Since these arcs are narrow, the available sliding surface is small. When the sliding surface is small, higher forces are required to initiate sliding¹⁵, as was demonstrated in the current study.

There was a considerable difference between the Gamma nail and the other intramedullary nails, with the former requiring substantially higher forces to initiate sliding. This could be a result of the proximal flare of the Gamma nail, which decreases the screw-nail angle. The Gamma nail also had the shortest barrel. Our findings with regard to the Gamma nail are in agreement with the clinical results of its use^2,4,18,22,23.

It should be noted that we did not address the potential for protrusion of the proximal portion of the intramedullary nail to prevent sliding of the proximal bone fragment in the current study. This protruding strut can act as a buttress that obstructs sliding of the proximal bone fragment. The screw in the proximal bone fragment would therefore also slide less, but only secondary to interference with the bone and not because of mechanical factors inherent in the design of the screw or the sliding surface. The effect of this osseous obstruction was expected to be the same for all of the intramedullary devices and was not examined in this study. Such an obstruction may provide the advantage of preventing mechanical collapse of an unstable fracture, but it also prevents impaction of bone fragments. Mechanical jamming of the screw did not occur in any device that was tested.

The data for the intramedullary hip screws were similar to those for the compression hip screws. The hip screws required lower forces to initiate sliding compared with the three designs of intramedullary nails that were tested, under both high and low-angle loading. This finding supports an earlier determination that the length of the barrel is important¹⁵. The intramedullary hip screw was designed for comminuted intertrochanteric, proximal subtrochanteric, and peritrochanteric fractures but not for fractures of the femoral shaft. It was included in this study because of its intramedullary nature. This device probably slides better than the screws of the other intramedullary devices because it has a longer barrel. Since the indications for use of an intramedullary hip screw are different than those for use of an intramedullary nail, the data should be interpreted with this limitation in mind. The difficulty of operative implantation was not addressed in this study.

In summary, devices with a shorter barrel required higher loads to initiate sliding than did devices with a longer barrel. Hence, all of the intramedullary nails, with their shorter bearing surface, or barrel, required higher forces to initiate sliding than did the hip screws. The Gamma nail required the highest forces to initiate sliding. High bending moments, which would be found in a heavier patient or be due to a long extension of the screw, necessitated higher loads to initiate sliding in a given device than did low bending moments, which would be found in a lighter patient or be due to a short extension of the screw.

References

Introduction | Materials and Methods | Results | Discussion | References

Bannister, G. C.; Gibson, A. G. F.; Ackroyd, C. E.; and Newman, J. H.: The fixation and prognosis of trochanteric fractures. A randomized prospective controlled trial. Clin. Orthop.,254: 242-246, 1990.254242 1990 [PubMed]

Boriani, S.; Bettelli, G.; Zmerly, H.; Specchia, L.; Bungaro, P.; Montanari, G.; Capelli, J.; Canella, P.; Regnoli, R.; Rispoli, R.; Trabucchi, L.; Simonini, D.; Bonicoli, F.; D'Imporzano, M.; Rava, M.; Ghera, S.; Giacomelli, M.; Dorigotti, P.; Ragazzi, E.; Montella, S.; Renier, F.; Parduzzi, S.; Torelli, L.; and Spagnolo, R.: Results of the multicentric Italian experience on the Gamma™ nail: a report on 648 cases. Orthopedics,14: 1307-1314, 1991.141307 1991 [PubMed]

Bridle, S. H.; Patel, A. D.; Bircher, M.; and Calvert, P. T.: Fixation of intertrochanteric fractures of the femur. A randomised prospective comparison of the Gamma nail and the dynamic hip screw. J. Bone and Joint Surg.,73-B(2): 330-334, 1991.73-B(2)330 1991

Davis, J.; Harris, M. B.; Duval, M.; and D'Ambrosia, R.: Pertrochanteric fractures treated with the Gamma™ nail: technique and report of early results. Orthopedics,14: 939-942, 1991.14939 1991 [PubMed]

Davis, T. R. C.; Sher, J. L.; Horsman, A.; Simpson, M.; Porter, B. B.; and Checketts, R. G.: Intertrochanteric femoral fractures. Mechanical failure after internal fixation. J. Bone and Joint Surg.,72-B(1): 26-31, 1990.72-B(1)26 1990

Davy, D. T.; Kotzar, G. M.; Brown, R. H.; Heiple, K. G.; Goldberg, V. M.; Heiple, K. G., Jr.; Berilla, J.; and Burstein, A. H.: Telemetric force measurements across the hip after total arthroplasty. J. Bone and Joint Surg.,70-A: 45-50, Jan. 1988.70-A45 1988

Frankel, V. H.: The Femoral Neck; Function, Fracture Mechanism, Internal Fixation; An Experimental Study, p. 7897. Springfield, Illinois, Charles C Thomas, 1960.

Frankel, V. H.; Burstein, A. H.; Lygre, L.; and Brown, R. H.: The telltale nail. In Proceedings of the American Academy of Orthopaedic Surgeons. J. Bone and Joint Surg.,53-A: 1232, Sept. 1971.53-A1232 1971

Griffiths, W. E. G.; Swanson, S. A. V.; and Freeman, M. A. R.: Experimental fatigue fracture of the human cadaveric femoral neck. J. Bone and Joint Surg.,53-B(1): 136-143, 1971.53-B(1)136 1971

Gurtler, R. A.; Jacobs, R. R.; and Jacobs, C. R.: Biomechanical evaluation of the Ender's pins, the Harris nail, and the dynamic hip screw for the unstable intertrochanteric fracture. Clin. Orthop.,206: 109-112, 1986.206109 1986 [PubMed]

Halder, S. C.: The Gamma nail for peritrochanteric fractures. J. Bone and Joint Surg.,74-B(3): 340-344, 1992.74-B(3)340 1992

Heyse-Moore, G. H.; MacEachern, A. G.; and Jameson Evans, D. C.: Treatment of intertrochanteric fractures of the femur. A comparison of the Richards screw-plate with the Jewett nail-plate. J. Bone and Joint Surg.,65-B(3): 262-267, 1983.65-B(3)262 1983

Jacobs, R. R.; McClain, O.; and Armstrong, H. J.: Internal fixation of intertrochanteric hip fractures: a clinical and biomechanical study. Clin. Orthop.,146: 62-70, 1980.14662 1980 [PubMed]

Kotzar, G. M.; Davy, D. T.; Goldberg, V. M.; Heiple, K. G.; Berilla, J.; Heiple, K. G., Jr.; Brown, R. H.; and Burstein, A. H.: Telemeterized in vivo hip joint force data: a report on two patients after total hip surgery. J. Orthop. Res.,9: 621-633, 1991.9621 1991 [PubMed]

Kyle, R. F.; Wright, T. M.; and Burstein, A. H.: Biomechanical analysis of the sliding characteristics of compression hip screws. J. Bone and Joint Surg.,62-A: 1308-1314, Dec. 1980.62-A1308 1980

Kyle, R. F.; Cabanela, M. E.; Russell, T. A.; Swiontkowski, M. F.; Winquist, R. A.; Zuckerman, J. D.; Schmidt, A. H.; and Koval, K. J.: Fractures of the proximal part of the femur. J. Bone and Joint Surg.,76-A: 924-950, June 1994.76-A924 1994

Larsson, S.; Friberg, S.; and Hansson, L.-I.: Trochanteric fractures. Influence of reduction and implant position on impaction and complications. Clin. Orthop.,259: 130-139, 1990.259130 1990 [PubMed]

Leung, K. S.; So, W. S.; Shen, W. Y.; and Hui, P. W.: Gamma nails and dynamic hip screws for peritrochanteric fractures. A randomised prospective study in elderly patients. J. Bone and Joint Surg.,74-B(3): 345-351, 1992.74-B(3)345 1992

Mainds, C. C., and Newman, R. J.: Implant failures in patients with proximal fractures of the femur treated with a sliding screw device. Injury,20: 98-100, 1989.2098 1989 [PubMed]

Massle, W. K.: Extracapsular fractures of the hip treated by impaction using a sliding nail-plate fixation. Clin. Orthop.,22: 180-202, 1962.22180 1962 [PubMed]

Pauwels, F.: Der Schenkelhalsbruck ein mechanisches Problem. Grundlagen des Heilungsvorganges Prognose und kausale Therapie. Stuttgart, Ferdinand Enke, 1935.

Radford, P. J.; Needoff, M.; and Webb, J. K.: A prospective randomised comparison of the dynamic hip screw and the Gamma locking nail. J. Bone and Joint Surg.,75-B(5): 789-793, 1993.75-B(5)789 1993

Sabharwal, S.; O'Brien, P. J.; Meek, R. N.; Blachut, P. A.; and Broekhuyse, H. M.: Intertrochanteric hip fracture fixation—Gamma nail vs dynamic hip screw. A randomized, prospective study. Orthop. Trans.,17: 92, 1993.1792 1993

Sernbo, I.; Johnell, O.; Gentz, C.-F.; and Nilsson, J.-Å.: Unstable intertrochanteric fractures of the hip. Treatment with Ender pins compared with a compression hip-screw. J. Bone and Joint Surg.,70-A: 1297-1303, Oct. 1988.70-A1297 1988

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TABLE I PARAMETERS OF THE LOGARITHMIC STATISTICAL MODEL*

Device Tested†	No. of Samples	Regression Coefficient		Standard Error		r² Value
c	d	Of Coefficient c	Of Coefficient d
Intramedullary nails
Recon	3	3.040	0.944	0.094	0.040	0.962
ZMS	4	2.915	1.072	0.091	0.041	0.974
Gamma	3	2.639	1.345	0.162	0.068	0.958
Hip screws
Sliding compression	5	2.940	0.840	0.130	0.110	0.918
Intramedullary	5	2.044	1.207	0.195	0.086	0.872

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TABLE I PARAMETERS OF THE LOGARITHMIC STATISTICAL MODEL*

Device Tested†	No. of Samples	Regression Coefficient		Standard Error		r² Value
c	d	Of Coefficient c	Of Coefficient d
Intramedullary nails
Recon	3	3.040	0.944	0.094	0.040	0.962
ZMS	4	2.915	1.072	0.091	0.041	0.974
Gamma	3	2.639	1.345	0.162	0.068	0.958
Hip screws
Sliding compression	5	2.940	0.840	0.130	0.110	0.918
Intramedullary	5	2.044	1.207	0.195	0.086	0.872