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 degrees), and ASD with knee extension (ASD-KE) otherwise (knee flexion < 5 degrees). 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 degrees vs. 18 +/- 9 degrees and 11 +/- 7 degrees) and PI-LL mismatch (20 +/- 26 degrees vs - 1 +/- 18 degrees and - 13 +/- 10 degrees) when compared to ASD-KE and controls (all p < 0.05). ASD-KF also had a more tilted (34 +/- 11 degrees vs. 28 +/- 9 degrees and 26 +/- 7 degrees), anteverted (24 +/- 6 degrees vs. 20 +/- 5 degrees and 18 +/- 4 degrees) and abducted (59 +/- 6 degrees vs. 57 +/- 4 degrees and 56 +/- 4 degrees) acetabulum, with a higher posterior coverage (100 +/- 6 degrees vs. 97 +/- 7 degrees 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 R-2 [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.

Automated summary

The study found that among adult spinal deformity patients, those who compensated with knee flexion had altered hip orientation characterized by increased posterior coverage and decreased anterior coverage. This could potentially lead to posterior femoro-acetabular impingement and limit pelvic retroversion, which may be involved in the hip-spine syndrome.