Strain pattern following surface replacement of the hip.

The aim is to compare the strain pattern in intact and resurfaced femurs using validated third-generation composite femurs and rosette strain gauges. The rosette strain gauges were applied to an intact and a resurfaced third-generation composite femur at three sites: the narrowest part of the lateral surface of the neck, the narrowest part of the medial surface of the neck, and the medial surface at the level of the lesser trochanter. The maximum and minimum principal strains were calculated at axial loads of 600, 800, and 1000 N. Further tests were carried out with an additional abductor load. The maximum principal strains in the resurfaced femur were approximately 50 per cent higher in the lateral surface of the neck and about 25 per cent higher in the lesser trochanteric region than in the intact femur. Inclusion of the abductor force decreased the strains in both the intact and the resurfaced femurs, particularly at the lateral surface of the femoral neck. Increased strain at the lateral surface of the femoral neck following hip resurfacing could be a cause of neck fracture, particularly if there are other predisposing factors such as notching of the femoral neck and/or abductor dysfunction. Meticulous repair of the abductors is warranted if a lateral approach is used.