Abstract
Adverse bone remodeling in the proximal femur may be detrimental to the long term survival of resurfacing prosthesis. Bone resorption beneath the femoral shell and thinning of the femoral neck have been observed. We present a radiological analysis of the incidence, rate, site of neck thinning and changes observed within the femoral neck, in 100 cases, with a minimum of five years follow-up.
Femoral neck diameter was measured at zero, two and five years post-operatively, at the head neck junction and five mm distally. Pre and post-operative head to neck ratios, natural and reconstructed offset, femoral neck-shaft and stem-shaft angles and cup inclination angle were measured.
Two distinct patterns of neck thinning were observed. In 76 cases (slow thinning group), we observed a reduction of < 5% of original neck diameter at two years and < 10 % at five years (mean 1.5%, sd+/− 1.5). In 24 cases (rapid thinning group), a reduction of > 5% of original neck diameter at two years and > 10% at five years (mean 10.4%, sd+/− 4.8) was observed. The difference in the percentage reduction in neck diameter was significantly different between the two groups at both time points (p< 0.01). Larger head-neck ratios were observed in the rapid thinning group, both pre and post operatively (p< 0.01).
The viability of bone underneath the femoral head may be compromised as a consequence of a non-physiological bone loading mechanism. FEA has predicted stress shielding underneath the femoral head and loading of the mini stem. Compromised blood supply of the retained epiphyseal remnant may play a part in femoral head resorption.
Femoral neck thinning is a phenomenon of unproven aetiology which affects almost 25% of our resurfacing cases.
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
