THE IMPORTANCE OF ADEQUATE STEM ANTEVERSION FOR ROTATIONAL STABILITY IN THA. AN RSA-STUDY WITH 5 YEARS FOLLOW UP.

Abstract

Introduction: Progressive retroversion of the stem within the femur has been suggested to be an important initial mode of hip prosthesis failure. We have assessed the relationship between postoperative stem anteversion angle, measured with CT, and the rotational stability as measured with repeated radiostereometric analysis (RSA) with 5 years follow up.

Patients and methods: 57 patients were operated on with THA using a cemented, matt and collared stem. The achieved stem anteversion angles were measured postoperatively with 3-D CT-examinations. The patients were divided into three groups depending on their ante-version angle: ≤10°, 11°–25° and ≥25°. They were followed with repeated RSA examinations for 5 years to determine the stem migration pattern.

Results: The mean postoperative anteversion angle was 20.5° (range 1°–43°). At 5 years, all except one stem had rotated into retroversion. There was a strong correlation between the postoperative anteversion angle and later rotation into retroversion (p=0.007). The group with ≤10° of stem anteversion rotated significantly more into retroversion, seen as early as 3 months (p=0.02), but more obvious at 5 years (p=0.002) with a mean of 9.9° of retroversion compared to 3.8° in the 11°–25° group and 2.4 ° in the ≥25° group. The distal stem migration results were accordant with more migration at 5 years (p=0.008) for the ≤10° anteversion group (1.6 mm subsidence compared to 0.5 and 0.3 mm respectively). Two stems have been revised because of aseptic loosening, both with a low initial anteversion angle (7° and 1°) and large retroversion at 5 years (7 ° and 31° respectively).

Conclusion: Measured by RSA, rotation into retroversion of the femoral stem was a regular finding in this study using a conventional prosthesis design. We propose that such rotation is a common finding in hip arthroplasty, but the degree may be design sensitive. Our results strongly suggest that the initial rotational position of the femoral component during surgery is decisive for the degree of later retroversion and probably prosthetic longevity; the less anteverted position the more the stem will migrate into a more retroverted position after the operation. This rotational migratory pattern is correlated to subsidence and eventual loosening. Hence, meticulous attention should be paid to the rotational position of the femoral stem during surgery, with less than 10° of anteversion appearing deleterious. However, too much anteversion will risk impingement and possibly other unwanted biomechanical effects, and an upper limit still remains to be established.