Toward understanding the underlying mechanisms of pelvic tilt reserve in adult spinal deformity: the role of the 3D hip orientation

PURPOSE To explore 3D hip orientation in standing position in subjects with adult spinal deformity (ASD) presenting with different levels of compensatory mechanisms. METHODS Subjects with ASD (n = 159) and controls (n = 68) underwent full-body biplanar X-rays with the calculation of 3D spinopelvic, postural and hip parameters. ASD subjects were grouped as ASD with knee flexion (ASD-KF) if they compensated by flexing their knees (knee flexion ≥ 5°), and ASD with knee extension (ASD-KE) otherwise (knee flexion < 5°). Spinopelvic, postural and hip parameters were compared between the three groups. Univariate and multivariate analyses were then computed between spinopelvic and hip parameters. RESULTS ASD-KF had higher SVA (67 ± 66 mm vs. 2 ± 33 mm and 11 ± 21 mm), PT (27 ± 14° vs. 18 ± 9° and 11 ± 7°) and PI-LL mismatch (20 ± 26° vs - 1 ± 18° and - 13 ± 10°) when compared to ASD-KE and controls (all p < 0.05). ASD-KF also had a more tilted (34 ± 11° vs. 28 ± 9° and 26 ± 7°), anteverted (24 ± 6° vs. 20 ± 5° and 18 ± 4°) and abducted (59 ± 6° vs. 57 ± 4° and 56 ± 4°) acetabulum, with a higher posterior coverage (100 ± 6° vs. 97 ± 7° for ASD-KE) when compared to ASD-KE and controls (all p < 0.05). The main determinants of acetabular tilt, acetabular abduction and anterior acetabular coverage were PT, SVA and LL (adjusted R2 [0.12; 0.5]). CONCLUSIONS ASD subjects compensating with knee flexion have altered hip orientation, characterized by increased posterior coverage (acetabular anteversion, tilt and posterior coverage) and decreased anterior coverage which can together lead to posterior femoro-acetabular impingement, thus limiting pelvic retroversion. This underlying mechanism could be potentially involved in the hip-spine syndrome.