Abstract
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. Our study uses Finite Element Analysis (FEA) to examine the effects of the implant orientation on bone remodelling following hip resurfacing arthroplasty. A radiographic analysis of the proximal femur following hip resurfacing was conducted in order to draw a comparison to the FEA findings.
A 3D FEA model of the Birmingham Hip Resurfacing (BHR) was created based on the geometry and material properties of a 45 year-old female donor hip. Hip joint and muscle loads were applied. Bone remodelling stimuli was determined using changes in strain energy. A range of implant orientations were compared to study the affect on bone remodelling. A retrospective radiological analysis was undertaken on 100 hips with a minimum of 5 years follow up. Femoral neck diameter was measured at post-op, 2 and 5 years, as well as neck and stem shaft angles.
FEA showed that valgus orientation was associated with increased resorption underneath the shell. Varus orientation showed increased bone formation at the stem tip. The radiological analysis identified 2 distinct patterns of neck thinning. Slow thinners (76%) had < 5% reduction in neck diameter at 2 years and < 10% at 5 years. Rapid thinner (24%) had > 5% thinning at 2 years and > 10% at 5 years. The mean percentage reduction in neck diameter was significantly different between the two groups at the two time points (p< 0.01). The rapid group had a higher proportion of valgus aligned implants (88%) and a significant decrease in reconstructed offset (p=0.0023).
The FEA results have shown that stem alignment can affect bone resorption resurfacing. FEA results were consistent with the radiological findings. Additional retrieval studies are necessary to help understand aetiology of implant failures.
Correspondence should be addressed to Editorial Secretary Mr ML Costa or Assistant Editorial Secretary Mr B.J. Ollivere at BOA, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England; Email: mattcosta@hotmail.com or ben@ollivere.co.uk
