45. WHAT IS THE OPTIMAL FEMORAL OFFSET AFTER TOTAL HIP ARTHROPLASTY?

Abstract

Purpose of the study: Theoretically, in first-intention total hip arthroplasty (THA), restoration of femoral offset (distance between the femoral axis and the joint centre) enables optimal function. The purpose of this study was to determine acceptable limits for variation in femoral offset without loss of function.

Material and method: We studied 122 hips (108 patients) who had THA with a straight cemented stem and a modular cone which could be adapted to enable three dimensional adjustment of the offset (more than 100 configurations). Mean patient age was 64 years. Most had primary or secondary degenerative disease (n=80) of the hip joint or osteonecrosis of the femoral head (n=21). The preoperative PMA score was 11.9 and the Harris score 49. Clinical and radiographic assessment was noted at mean 4.5 years follow-up. The radiographic femoral offset was measured semiautomatically in comparison with the healthy hip using the method described by Steinberg and Harris.

Results: At last follow-up, the mean PMA score was 16.4 and the mean Harris score 89. These clinically scores were statistically different depending on the degree of variation of the femoral offset. Outcome was better for offset increased 0 to 5 mm (PMA 17 and Harris 93). They were less satisfactory for decreased offset (PMA 15.9 and Harris 83) (p=0.01). They were also less satisfactory for an offset increased more than 8 mm, but non significantly.

Discussion: It has been established that increasing the femoral offset decreases the rate of dislocation, reduces the incidence of limping, the use of crutches, and increases the force of the gluteus medius, as well as range of motion and abduction. However, there is no known limit value.

Conclusion: It is advisable to increase the femoral offset during total hip arthroplasty; the increase should be to the order of 0 to 5 mm, and never be too great.