OTA 1997 Posters - Hip Fractures

*A Comparison of Fixation Devices for the Treatment of Unstable Subtrochanteric Femur Fractures

Michael A. Dube, MD, Andrew N. Pollak, MD, Neil Price, FRCS, Riyaz H. Jinnah, MD, Vincent P. Novak, MS, Stephen M. Belkoff, PhD

Orthopaedics, R Adams Cowley/Univ. of Maryland Medical Center, Shock Trauma Center, Baltimore, Maryland, USA

Purpose: Subtrochanteric femur fractures are usually unstable injuries. Because the subtrochanteric region of the femur is subject to high stresses, fixation devices used to treat fractures in this region are prone to mechanical failure. Fixation for these fractures needs to be sufficiently stable to withstand the stresses incurred during the portion of the patient's rehabilitation which occurs before fracture union. The purpose of this study was to compare the mechanical properties of two devices currently marketed for use in the treatment of unstable subtrochanteric femur fractures.

Methods: Eleven matched pairs of fresh cadaveric femurs were harvested. All specimens were x-rayed and templated. Two pairs were not used in comparative testing because of previous fracture of one of the proximal femurs. The contralateral specimen from each of these pairs was used for pilot testing and system calibration. Of the 9 remaining pairs, one proximal femur from each was instrumented with an Unreamed Femoral Nail with Spiral Blade Interlocking (URFNSB) (Synthes) and the contralateral specimen was instrumented with the Alta CFX (Howmedica) reconstruction nail. Treatment was assigned to right and left femurs randomly. All specimens were x-rayed again after instrumentation to document hardware position. A circumferential section of the subtrochanteric region of each femur from the distal tip of the lesser trochanter to a point 3 cm distally along the shaft of the proximal femur was then removed to simulate the most unstable situation, i.e. no cortical opposition medially or laterally after fracture fixation. Specimens were then mounted in a position simulating the single leg stance phase of gait and were loaded quasistatically using an Instron servo hydraulic testing system. Force-deflection data and mode of failure for each specimen were recorded. Each test was also videotaped to aid in documenting the mode of failure. Failure was defined as 2.5 cm of vertical displacement, and the maximum load to create this displacement was defined as the ultimate load. Catastrophic failure was defined as blade, screw, or rod cutout from the bone. Nail deformation without associated hardware cutout was considered noncatastrophic. Specimens were x-rayed again after mechanical testing to assist in assessing failure mode.

Results: Mean ultimate loads for the URFNSB (1708 ± 478 N) and Alta CFX (1709 ± 479 N) were not significantly different. Stiffness values were also similar for the two constructs (Alta CFX 154 ± 53 N/mm versus URFNSB 136 ± 48 N/mm). The mode of failure in the two groups differed: in the Alta CFX group, 4 (44%) specimens failed catastrophically (2 by fracture through the greater trochanter/piriformis fossa and 2 by screw cutout from the femoral head); in the URFNSB group, 8 (89%) specimens failed catastrophically (6 by fracture through the greater trochanter/piriformis fossa and 2 by screw cutout). However, these differences were not statistically significant (Chi square = 2.25, p > 0.05).

Discussion/Conclusion: The Alta CFX and URFNSB provided similar stability for the treatment of unstable subtrochanteric femur fractures in a cadaveric model. There was a trend toward increased frequency of catastrophic versus noncatastrophic failure in the URFNSB group.