Surgical Variables Affect the Mechanics of a Hip Resurfacing System

Abstract

Recent clinical studies have linked failure to surgical variables of stemmed hip resurfacing systems. We used finite element analysis to investigate the effects of implant position, stem orientation, and extent of fixation both on the local stresses and strains associated with implant loosening, neck fracture, and stem fracture, as well as on the load transfer distribution in the bone-implant system. The range of peak stress in the cement was reduced from 11 to 13 MPa for the varus stem to 3.2 to 4.2 MPa for the valgus stem. The range of peak strain in the bone was also reduced from -0.35% to -0.45% strain for the varus stem to -0.19% to -0.27% strain for the valgus stem, but only when reamed cancellous bone remained exposed. Peak stresses in the stem were low for all cases. Additionally, the implant's load transfer distribution was generally insensitive to all variables examined and the femoral head was substantially unloaded. Our data indicate the local stresses and strains associated with implant loosening and neck fracture were reduced by placing the implant in a valgus orientation and covering reamed cancellous bone, but unloading of the femoral head, found for all variables examined, may lead to adverse bone remodeling.

Hip resurfacing increases bone strains associated with short‐term femoral neck fracture

“…We scaled head load magnitudes to account for kinetic differences between resurfacing and traditional hip replacement patient populations. 5 Accordingly, head load magnitudes were increased by 26%, which was the difference between the mean, peak abduction moment during walking for resurfacing patients 18 compared to the instrumented patients. This is a reasonable scaling factor because the instrumented patients had temporal agreement between peak abduction moment and peak head load, and the abduction moment was the primary joint moment component at this point in the gait cycle.…”

“…The thickness of the cement mantle was 1 mm. 5 The stem diameter was 6 mm at its base and 65 mm long with the stem hole 2 mm longer than the stem to avoid load transfer at the tip. Two stem-hole geometries were analyzed: a 0.58 tapered stem in a 0.58 tapered hole (S 1 ), and a 0.58 tapered stem in a 6 mm diameter straight hole (S 2 ).…”

“…Implant shell diameters were 50 mm for Bone 1 and 46 mm for Bone 2, which were the smallest diameter implants that did not violate the neck geometry. For both bones, the implant rim was located at the head-neck junction, and the stem axis was aligned with the neck axis because a valgus orientation 1,5 would have required violation of the neck in these bones. The FE meshes for all components of the intact and resurfaced models were generated using Truegrid (v2.2; XYZ Scientific Applications, Livermore, CA).…”

“…3,4 In recent finite element (FE) analyses, it was observed that placing a stiff cobalt-alloy implant around the femoral head remnant unloaded this bone. 5,6 An additional consequence of hip resurfacing is increased bone strains near the rim of the shell, which is likely due to distal load transfer from the implant to the bone. 5 It is near the shell rim where fractures generally occur.…”

“…5,6 An additional consequence of hip resurfacing is increased bone strains near the rim of the shell, which is likely due to distal load transfer from the implant to the bone. 5 It is near the shell rim where fractures generally occur. 4 Even in viable bone, damage can accumulate if the strains are sufficiently large (approximately À0.35% strain in compression and 0.25% strain in tension) 7,8 and applied repeatedly over a short time.…”