Background and aims:
Restoration of the biomechanical forces around the hip with appropriate femoral offset (FO) and leg length is an important goal in total hip arthroplasty (THA). It is still controversial as to how much postoperative leg length discrepancy (LLD) and FO change are acceptable. The aim of this prospective study is to analyse whether the problem of postoperative leg lengthening and FO reduction is located in the stem or cup or both.
Material and methods:
Between September 2010 and April 2013, 174 patients with unilateral primary OA treated with THA were included. Postoperative LLD and global FO were measured using a standardized protocol of the antero-posterior hip radiograph. Patients whose operated leg became ≥ 10mm longer compared with the contralateral side (n=41) or reduction of the global FO > 5mm (n=58) were further studied to investigate the amount of leg lengthening and global FO reduction that took place in the stem and in the cup compared with the contralateral side. We compared the measurements made by an orthopaedic surgeon with the measurements made by a radiologist.
The leg lengthening was located in the stem while the FO reduction was located in both the stem and cup, for the two observers. For observer 1, the mean stem length of the operated side was 57.7 mm vs. 50.9 mm, p=0.003, while the mean cup length of the operated side was 19.0 mm vs. 18.9, p=0.95. For observer 2, the mean stem length of the operated side was 59.5 mm vs. 50.9 mm, p=0.001, while the mean cup length of the operated side was 18.2 mm vs. 18.8, p=0.90. The global FO reduction was located both in the stem and cup for the two observers. For observer 1, the mean stem offset of the operated side was 49.2 mm vs. 57.1 mm, p<0.001, while the mean cup offset of the operated side was 35.1 mm vs. 39.9, p<0.001. For observer 2, the mean stem offset of the operated side was 48.9 mm vs. 55.1mm, p<0.001, while the mean cup offset of the operated side was 35.5mm vs. 40.4mm, p<0.001. Both interobserver reliability and intraobserver reproducibility were substantial to excellent (ICC ≥ 0.79).
Post THA lengthening of the operated leg is mainly caused by improper femoral stem positioning while global FO reduction results from improper positioning of both acetabular and femoral components. Surgeons should be aware of these observations in order to avoid them.