Although sagittal alignment is known to influence pelvic position, few studies accurately identify the relationship between sagittal alignment and acetabular orientation. We hypothesized that postoperative PT should be correlated with acetabular change in native hips after surgical correction of adult spinal deformity.

The objective of this study was therefore to describe the correlation between the change in pelvic tilt and the change in acetabular orientation two years after surgical correction of adult spinal deformity.

Based on a retrospective study of a prospective mono center database, 127 acetabuli out of sixty-nine patients were analyzed preoperatively and at two years postoperatively of surgical management of sagittal imbalance by posterior arthrodesis extended to the pelvis. The analysis was based on bi-planar EOS radiographs with 3D reconstructions of the pelvis and spine using SterEOS 3D software.

The following specific parameters were analyzed: sacral slope, pelvic tilt, lumbar lordosis, SVA, acetabulum tilt (AT), anteversion (AA) (orientation of the acetabulum in the axial plane), abduction (AAbd) (orientation of the acetabulum in the frontal plane), inclination (AI) (orientation of the acetabulum in the sagittal plane), and anterior acetabulum coverage (ACA).

A Pearson correlation was performed between the pre-and postoperative change in acetabular parameters (right and left) and pelvic parameters. Linear regressions were performed to identify the most relevant pelvic and spinal parameters. A subgroup analysis was performed to identify a difference between distal sacral and distal ilium fixations.

All measured acetabular parameters were significantly different two years after surgery. Changes in AT (p = 0.03), AI (p = 0.03) and ACA (p = 0.05) were significantly greater in the ilium fixation group. Postoperative PT reduction was strongly correlated with the decrease of AT and AA ( = 0.61 and = 0.57, p < 0.001), it was also correlated with the increase of AI and ACA and the decrease of AAbd. The entire cohort linear regression analysis revealed that a 1 ° decrease in PT resulted in a 0.4 ° decrease in AA and a 0.6 ° decrease in AT (R2 = 0.45 and = 0.38).

Our study highlights the significant influence of the change in sagittal alignment on acetabular orientation in standing position. This correlation explains the increased risk of anterior hip impingement, the change in acetabular load distribution that might lead to early hip osteoarthritis, and the overall change in the subjects' gait pattern.

IV.