Purpose: Amstuz introduced the notion of a lever arm ratio to describe the relationship between the abductors and weight in arthroplasty. He demonstrated that patients may limp if defective lateralisation produces a low lever arm ratio. We analysed a continuous series of arthroplasty patients with excellent outcome at one year to compare restoration of lateralisation with the preoperative status. We also performed the same analysis in a series of patients who limped after arthroplasty, excluding cases with classical causes of failure. We then compared these two series.

Material and methods: We had a series of 100 arthroplasties with excellent results at one year. These patients had undergone unilateral arthroplasty and had a healthy contralateral hip. The centre of the arthroplasty head was identified on preoperative and one-year AP x-rays of the pelvis (same magnification). We measured the lateralisation in relation to the femoral axis. The position of the cup was measured with a U ratio. Results were expressed in percent restitution of the preoperative status. Amstutz’s lever arm ratio was also measured. Finally, we measured the distance between the pubic symphysis and the outermost point of the femur on the prosthetic and healthy side.

We also had a second population of twelve patients presenting persistent limping at one year with no objective cause. The same parameters were measured for this population.

Results: We found that we had achieved only partial restitution of the initial lateralisation and had a tendency to medialise the acetabulum. The restitution of lateralisation was significantly different between the series with excellent outcome and the series with limping.

Discussion: Deficient lateralisation appears to be a factor involved in persistent limping. There is a threshold for restitution of lateralisation; limping is always observed under this threshold. Data in the literature reveal a very wide variability in lateralisation. It would thus appear important to restore the initial lateralisation to avoid limping; this has led us, like others, to use lateralised implants for certain patients.