This study examined the effect of a 10° valgus positioning of the femoral component, comparing it in a matched manner to neutral implantation. The authors used an experimental method, cyclically loading implanted cadaver femora to fracture. The results suggest that in femora with good quality bone a valgus position gives a 12% increase in the number of cycles to failure; however in femora with osteoporosis a valgus position resulted in a 40% reduction in the number of cycles to failure.
The authors are to be congratulated on completing a difficult experimental study; however, it is important to bear in mind the study’s limitations. The paper is not able to provide an explanation of the apparently contradictory consequences of valgus positioning; this may be related to the fact that the same body mass was assumed for all cadaver specimens. The fractures in this study are due to an increasing number of loading cycles applied to a non-living bone/implant construct. In vivo, there will be a biological response, leading to healing and remodelling, and the construct will not be loaded in a continuous cyclic manner.
Other studies have shown that the mechanical consequences of implantation with a hip resurfacing device may not be the most important risk factors for femoral neck fracture.1 Biological factors, such as bone necrosis,2,3 and patient factors4 have been shown to be associated with fracture; these factors are obviously not easy to investigate with an in vitro study.
This study raises further questions about the mechanisms of femoral neck fracture after hip resurfacing.
1. Little JP, Taddei F, Viceconti M, Murray DW, Gill HS. Changes in femur stress after hip resurfacing arthroplasty: response to physiological loads. Clin Biomech 2007;22:440-8.
2. Little CP, Ruiz AL, Harding IJ, McLardy-Smith P, Gundle R, Murray DW, Athanasou NA. Osteonecrosis in retrieved femoral heads after failed resurfacing arthroplasty of the hip. J Bone Joint Surg [Br] 2005;87-B:320-3.
3. Steffen RT, Foguet PR, Krikler SJ, Gundle R, Beard DJ, Murray DW. Femoral Neck Fractures After Hip Resurfacing. J Arthroplasty 2008 (e-pub).
4. Beaule PE, Dorey FJ, LeDuff M, Gruen T, Amstutz HC. Risk factors affecting outcome of metal-on-metal surface arthroplasty of the hip. Clin Orthop 2004;418:87-93.
Nuffield Department of Orthopaedic Surgery, Oxford, United Kingdom