The Effect of Component Alignment on Proximal Femoral Strength with the Birmingham Mid-Head Resection

The Birmingham Mid-Head Resection (BMHR) is a bone-conserving, short-stem femoral prosthesis designed for patients with compromised femoral head anatomy unsuitable for a traditional hip resurfacing arthroplasty. Similar to hip resurfacing, the BMHR preserves the femoral neck, however, it is not known if the femoral neck fracture risks associated with hip resurfacing pose the same risk to a mid-head resection arthroplasty. The current study examined the effect of coronal alignment on proximal femoral strength with the BMHR. Sixteen pairs of fresh-frozen cadaveric femurs were divided into 2 equal alignment groups. Individual pairs were divided into a control and a test specimen. A power analysis revealed 8 pairs each would be needed to detect an effect size of approximately 500 N (α = 0.05; β = 1-0.8; population SD 500 N). The first alignment group prepared test specimens in 10° of relative valgus alignment, while the second group prepared test femurs in 10° of relative varus. All control specimens were prepared with the implant aligned with the native neck-shaft angle of the femur. Femurs were tested in axial compression in single-leg stance. There was no significant difference between matched paired femurs prepared in varus alignment (mean 4324 N; SD 2207) compared to controls (mean 4114; SD 2153; P = .996). Likewise, there was no difference between femurs prepared in valgus alignment (mean 4623; SD 1608) and matched paired control specimens (mean 4761; SD 1290; P = .999). There was also no difference between control groups (P = .897). Femurs were well matched for BMD and anatomic parameters. The findings of the current study are in contrast to previous studies investigating implant alignment in hip resurfacing. A valgus aligned implant did not appear to strengthen, nor a varus implant weaken, proximal femoral strength compared to a neutrally aligned implant. Failure of the proximal femur implanted with a BMHR appears less sensitive to variations in implant alignment than a typical hip resurfacing.