Erect weight-bearing pelvic radiographs are superior to supine radiographs for diagnosis of mixed type acetabular retroversion plus developmental dysplasia hip deformity.

Purpose: This study focused on the mixed-type deformities of acetabular retroversion (AR) and developmental dysplasia hip deformity (DDH) and aimed to ascertain the changes in pelvic tilt from a supine to a standing position in these cases and identify potential underlying mechanisms.

Methods: A retrospective study was conducted on cases with symptomatic DDH from January 1, 2019, to April 30, 2023. DDH was defined as LCEA < 20°. AR was diagnosed by using a crossover index threshold of 0.2 in standing pelvic X-ray. Two observers assessed the supine and weight-bearing pelvic radiographs, along with computed tomography (CT) scans. The evaluated parameters included pelvic tilt (sacrofemoral-pubic angle (SFP), symphysis to sacrococcygeal distance (PSSC), pubic symphysis to sacroiliac (PSSI)), acetabular retroversion (crossover index, posterior wall sign), acetabular coverage (lateral center-edge angle (LCEA), ischial spine sign (ISS)), and axial rotation of the hemipelvis (pelvic width index, obturator index, and ilio-ischial angle). Acetabular orientation and coverage was measured by CT through anterior sector angle (ASA), posterior sector angle (PSA) and acetabular anteversion (AA). Cases with acetabular retroversion plus DDH were defined as the mixed-type deformity. Comparative analyses between mixed-type deformities and DDH cases were performed along with subgroup and correlation analyses within mixed-type cases. Inter-observer and intra-observer reliabilities were assessed using intraclass correlation coefficients.

Results: A total of 85 were included. 26 cases (30.59%) had mixed-type deformity, where transition from the supine to standing position led to an increased posterior pelvic tilt (SFP (supine: 64.35±4.6°, standing: 74.75±4.16°, p<0.001), PSSC (supine: 6.37±2.47, standing: 2.08±1.32, p<0.001) and PSSI (supine: 9.47±1.66, standing: 6.33±1.08, p<0.001)). Compared to cases with DDH, CT examination revealed a significantly greater anterior acetabular coverage and less posterior superior coverage, with smaller posterior sector angle and greater anterior sector angle (ASA) (p<0.05) for cases with AR. The superior iliac wing angle (mixed type: 45.63±9.22°, isolated type: 50.70±8.77°, p=0.013), inferior iliac wing angle(mixed type: 60.77±8.24°, isolated type: 65.24±8.02°, p=0.013), and ischiopubic angle (IPA) (mixed type: 32.27±3.19°, isolated type: 36.71±5.38°, p<0.001) were significantly reduced in AR cases, suggesting external rotation of the hemipelvis. Subgroup analysis showed that cases with a higher crossover index had a significantly higher PSSC and a significantly lower IPA.

Conclusion: AR was observed in 31% of DDH cases and was associated with a notable posterior pelvic tilt during postural transitions. This tilt appeared to be a compensatory mechanism affecting the AR diagnosis. Key changes in the acetabular coverage, including increased anterior coverage and decreased superior posterior coverage, were also observed. Additionally, external rotation of the hemipelvis in mixed-type cases correlated strongly with the extent of AR and anterior acetabular coverage, suggesting that it may be a key contributor to the underlying mechanism of this mixed-type deformity.

Level of evidence: III, retrospective case series.