Acetabular Morphotype Is Associated with Change in Acetabular Orientation Between Supine and Standing.

Abstract

Debate exists on which functional position (supine or standing) should form the basis of radiographic assessment of the native acetabulum. Pelvic tilt (PT) differs between positions influencing radiographic acetabular assessment. This study aims to (1) quantify PT changes from supine to standing amongst volunteers and patients; (2) assess changes in acetabular parameters between the postures; and (3) determine whether changes in acetabular parameters differ according to acetabular morphotype. This is a prospective, consecutive, cohort study of 105 asymptomatic volunteers (53% male; mean age 36.5 ± 13.6 years; body mass index [BMI] 25.0 ± 2.0 kg/m²) and 437 patients presenting to a hip preservation clinic without osteoarthritis (Tonnis ≤1) (40% male; mean age 35.7 ± 8.6 years; BMI 25.0 ± 2.0 kg/m²). All underwent standardized supine and standing anteroposterior pelvic radiographs. PT change was quantified using the sacrofemoral-pubic (SFP) angle. Acetabular morphology was evaluated in detail to characterize morphotype according to the Ottawa classification (lateral dysplasia, anterior dysplasia, posterior dysplasia), pincer or "normal." Changes between supine and standing were compared between cohorts and morphotypes. The mean SFP decreased by -3.7° (IC95% 4.0-3.5) from supine to standing. Acetabular measurements demonstrated no differences in change between volunteers and symptomatic patients when transitioning between supine to standing. Measurements of lateral coverage changed minimally between supine (volunteers: 30.7° ± 6.1°; symptomatics: 27.3° ± 8.3°) and standing (volunteers: 30.3° ± 5.7°; symptomatics: 26.1° ± 8.1°), whereas measurements reflecting anteroposterior coverage changed significantly (ΔAWI volunteers: -0.10 ± 0.08 and symptomatics: -0.09 ± 0.07; p < 0.001 and ΔPWI volunteers: 0.09 ± 0.08 and symptomatics: 0.09 ± 0.07; p < 0.001). Patients with anterior dysplasia demonstrated smaller ΔSFP (-1.5° ± 2.5° vs. -3.7° ± 2.9°; p = 0.004) compared with the rest of the cohort. By contrast, posterior dysplasia showed greater ΔSFP (-4.5° ± 2.7° vs. -3.6° ± 2.9°; p = 0.128) than the rest. Groups demonstrated comparable changes in PT and acetabular parameters between positions. The magnitude and direction of change was associated with acetabular morphology uncoupling compensatory mechanisms. Standing radiographs may be preferred to supine, as they account for compensatory mechanisms. Level II, diagnostic. See Instructions for Authors for a complete description of levels of evidence.