EFFECT OF HIP RESURFACING ORIENTATION ON FEMORAL NECK REMODELING–A FINITE ELEMENT STUDY

Abstract

Introduction: Cemented hip resurfacing component orientation may, in part, be associated with femoral neck fracture. Orientation offset may be introduced due to the cement setting prior to achieving a completely seated component. Varus/valgus orientation error may occur due to surgical error or poor instrumentation design. We modeled a number of different orientations and investigated bone mineral density change using the finite element method.

Methods: CT scans were used to reconstruct the femoral geometry and create a finite element model. The boundary conditions applied were hip muscle forces at the 45% position of the gait cycle. Two models were created, a preoperative (reference) and a postoperative (reconstructed) model. The post operative model was reconstructed using the Birmingham Hip Replacement (BHR). Implant offsets and varus/valgus orientations were analysed. The bone mineral density (BMD) changes at nine positions along the superior and inferior aspects of the alignment stem were analyzed.

Results: Results suggest bone loss decreases with increasing offset distances. Femoral offset distance is defined as the perpendicular distance from the center line of the femoral shaft to the center of the femoral head. Greater femoral stem offsets increases the abductor moment arm and this decreases the abductor force need for walking as well as the overall articulating reactive force at the articulating surface. As the BHR orientation deviates away from the an extreme valgus to a more varus position, the volume of bone that will decrease in BMD increases.

Discussion: There is minimal difference between the 1mm and 3mm offsets and their respective bone remodeling volumes. The 5mm offset has a larger bone volume where the BMD will increase; this is due to the larger moment applied to the proximal femur and is not an advisable surgical position as there may be a large density gradient at the mouth of the resurfacing component and could predispose the femoral neck to fracture. There is also not a lot of difference in bone remodeling volume between the extreme valgus, 5° and 10° cases. However, the extreme valgus case does present a “notching” risk. The objective of this study was to implement a consistent theoretical adaptive bone remodelling rule that may, in part, give an understanding as to how a femoral resurfacing component’s orientation would influence and simulate BMD changes in the proximal femur.