Change in cup orientation from supine to standing posture: a prospective cohort study of 419 total hip arthroplasties

Background and purpose: Arthroplasty surgeons traditionally assess cup orientation after total hip arthroplasty (THA) on supine radiographs. Contemporary hip–spine analyses provide information on standing, functional cup orientation. This study aims to (i) characterize cup orientations when supine and standing; (ii) determine orientation differences between postures; and (iii) identify factors associated with magnitude of orientation differences.Methods: This is a 2-center, multi-surgeon, prospective, consecutive cohort study. 419 primary THAs were included (57% women; mean age: 64 years, standard deviation SD 11). All patients underwent supine and standing antero-posterior pelvic and lateral spinopelvic radiographs. Cup orientation and spinopelvic parameters were measured. Target cup orientation was defined as inclination/anteversion of 40°/20° ± 10°. A change in orientation (Δinclination/Δanteversion) between postures > 5° was defined as clinically significant. Variability was defined as 2 x SD.Results: Inclination increased from 40° (supine) to 42° (standing) corresponding to a Δinclination of 2° (95% confidence interval CI 2–3). Anteversion increased from 25° (supine) to 30° (standing) corresponding to a Δanteversion of 5° (CI 5–6). When supine, 69% (CI 65–74) of THAs were within target, but only 44% (CI 39–49) were within target when standing, resulting in a further 26% (CI 21–30) being out of target when standing. From supine to standing, a clinically significant change in anteversion (> 5°) was seen in 47% (CI 42–52) of cases. Δanteversion was higher in women than in men (6°, CI 5–7 vs 5°, CI 4–5) corresponding to a difference of 1° (CI 1–2), which was dependent on tilt change, standing cup anteversion, age, and standing pelvic tilt.Conclusion: Cup inclination and version increase upon standing but significant variability exists due to patient factors.