102. THE EFFECT OF FEMORAL NECK CUT, CABLE TENSION AND MUSCLE FORCES ON THE STABILITY OF GREATER TROCHANTER REATTACHMENT

Abstract

Purpose: Greater trochanter reattachment is frequently accomplished using cable grip type systems. There is a relatively high failure rate for these systems, the mechanisms of which are unclear. One possible source of instability could be femoral neck cut location. Another concern is the effect of variability in cable tension. The objective is to create a femur implant model which allows for variation in cable tension, common muscle forces and the placement of the femoral neck cut in order to analyse trochanter fragment fixation.

Method: A finite element model (FEM) of a femur with simulated greater trochanter osteotomy (30°) was combined with the femoral component of a hip prosthesis and a greater trochanter reattachment system with 4 cables (Cable-Ready®, Zimmer). A total of 18 simulations were modeled in a full factorial design using three independent variables; cable tightening (178N, 356 N and 534 N), muscle forces (rest, walking and stair climbing) and femoral neck cut (10 mm and 15 mm above the lesser trochanter). Displacement of the fragment, in terms of both gap and shear components, as well, stress in the bone were investigated.

Results: The location of the femoral neck cut reduced contact surface area by 20% and had the largest influence on displacement (0.24 mm). Pivoting of the fragment was observed with a maximum gap (0.38 mm) and maximum total displacement (0.41 mm) at the bottom of the fragment. This was observed during stair climbing, while the cables were tightened to 177.9 N and with the femoral neck cut at 10 mm. Increased tightening of the cables provided no significant reduction in fragment displacement. However, higher cable tension significantly increased the stress in the bone (8 MPa and 26 MPa for cable tension of 178 N and 534 N respectively).

Conclusion: Placement of the femoral neck cut closer to the lesser trochanter significantly increased fragment displacement. Preservation of the contact surface area is recommended. Excessive cable tightening did not reduce fragment movement and only exacerbated bone stress. Caution must be used to not over tighten the cables. This model can be used to test and compare the performance of new implant designs.