A COMBINED FINITE ELEMENT AND RADIOLOGICAL ANALYSIS OF THE PROXIMAL FEMUR FOLLOWING HIP RESURFACING ARTHROPLASTY

Abstract

Introduction: Sectioned femoral components retrieved from failed hip resurfacing arthroplasties show resorption of proximal femoral bone or formation of a fibrous membrane at the bone cement interface, in a proportion of cases. We hypothesize that both scenarios create a functional discontinuity zone (FDZ), which exacerbates offloading the proximal bone and promoting resorption. Our study uses finite element modeling to examine the effects of the presence of an FDZ on bone remodeling following hip resurfacing arthroplasty. A radiographic analysis of the proximal femur following hip resurfacing was also conducted in order to draw a comparison to clinical findings.

Methods: The hip resurfacing FE models were oriented in 3 distinct stem-shaft angles: 136 ‘neutral’, 120 ‘varus’ and 150 ‘valgus’. A low-modulus (2 MPa) FDZ (approx. 2 mm thick) was simulated beneath the implant head. Femoral joint and muscle loads were applied to simulate peak joint loading during gait. Interface stress was compared for the normal and simulated FDZ resurfaced femurs. Bone remodeling stimuli was determined using changes in strain energy. A retrospective radiological analysis was undertaken on 98 hips (74 males and 24 females) with a minimum of 5 years follow up. Measurements of the prosthesis-shaft angle, pre–and post-operative femoral head offset and femoral neck diameter at 2 and 5 years were undertaken.

Results: The presence of the simulated FDZ in the FE analysis resulted in increased proximal-medial bone resorption and slightly greater bone formation surrounding the stem. Correspondingly, device-bone interface stresses were found to decrease proximally under the loading platform and increase at the stem, particularly adjacent to the stem-head junction. The valgus BHR femur led to increased resorption, especially around the periphery of the neck and on the medial side. The radiological analysis identified 2 groups; 22 hips (Group 1) had a mean 5.61mm (sd 2.07) reduction in neck diameter over 5 years and 76 hips (Group 2) demonstrated slow reduction in neck diameter, mean 1.13mm (sd 0.97). Neck thinning at 2 and 5 years was significantly greater for Group 1 (p< 0.0001). Group 1 hips had significantly greater reduction in femoral offset (p=0.041), with greater valgus angle oriented components (p=0.09). Reduction in femoral offset was significantly associated with greater valgus orientations (p< 0.0001). The Group 1 revision rate was 36.4% compared to 2.6% in Group 2 (p< 0.0001).

Discussion: The FE results support the hypothesis that the presence of a FDZ decreases load transfer to the proximal bone, resulting in increased medial stress shielding and resorption. These results are consistent with the Group 1 clinical findings. In order to better understand the cause of implant failures in hip resurfacing arthroplasty, additional retrieval studies are necessary.