Objective: Cup angles in total hip arthroplasty (THA) vary between standing and supine and with chronological changes in pelvic inclination. We studied these chronological changes in pelvic inclination and cup angles standing and supine after THA.
Methods: Subjects were 37 hips of 36 patients (mean age : 64 years) . Pelvic inclination angle (PTA) (Kitajima et al.) and cup angle (Lewinnek et al.) were measured in radiographs of the pelvis standing and supine and chronological changes in these parameters clarified. We also studied the correlation between PTAs and cup angles.
Results: PTA decreased with time after THA, and averaged 4.5 degrees greater supine than standing. Due to retroversion of the pelvis after THA, the cup anteversion angle tended to increase 1.6 degrees in standing position one year after THA. A significant correlation was seen between PTA before THA and postoperative change in cup angles (p<0.05) .
Conclusions: Pelvic position varies between standing and supine. Cup angles tend to be anteverted after THA because the pelvis tends to tilt with time, particularly in standing. Patients with a large PTA before THA tend to have a large postoperative change in cup angle. Special attention thus is required when positioning the acetabular component in patients with anterior pelvic tilt.