Supine Versus Standing Pelvic Radiographs Yield Significantly Different Radiographic Parameters in Femoroacetabular Impingement and Hip Dysplasia: A Systematic Review and Meta-analysis.

PURPOSE: To compare radiographic parameters obtained from supine versus standing pelvic radiographs in patients with femoroacetabular impingement (FAI) and acetabular dysplasia (AD). METHODS: This study was registered on the International Prospective Register of Systematic Reviews and performed in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. PubMed, Cochrane Central Register of Controlled Trials, and Scopus were queried in June 2025. Included studies compared supine versus standing radiographic parameters in patients with FAI or AD. Articles were excluded if they failed to provide extractable radiographic data for both positions. Demographics, study design, and radiographic parameters, including lateral center-edge angle (LCEA), pelvic tilt, Tönnis angle, femoral head translation, Sharp angle, joint space width, and femoroepiphyseal acetabular roof index, were analyzed. Meta-analyses with a random-effects model were performed to evaluate mean differences in supine versus standing measurements for LCEA, Tönnis angle, and pelvic tilt, stratified by FAI and AD cohorts. RESULTS: Twelve studies were analyzed, comprising 824 hips in 675 patients. Statistically significant differences in individual-study supine versus standing comparisons were identified for pelvic tilt (10/10 studies), Tönnis angle (7/8), and LCEA (6/10). Four studies reported sex-disaggregated data. Meta-analyses showed greater pelvic tilt in supine in FAI (+1.38 cm, 95% confidence interval [CI]: 0.92, 1.84; P < .001) and AD (+1.98 cm, 95% CI: 0.37, 3.59; P = .020) populations. Tönnis angle was smaller in supine in FAI (-1.14°, 95% CI: -2.19, -0.09; P = .030) and AD (-1.93°, 95% CI: -3.20, -0.65; P = .003). LCEA measurements were not significantly different in FAI (+0.97°, 95% CI: -0.80, 2.74; P = .28) and AD (+1.03°, 95% CI: -0.70, 2.76; P = .24). CONCLUSIONS: Supine pelvic radiographs resulted in greater pelvic tilt measurements, decreased Tönnis angles, and similar LCEA compared with standing radiographs. Supine and standing radiographic parameters should not be considered interchangeable. LEVEL OF EVIDENCE: Level IV, systematic review of Level II to IV studies.