Coronal offset of C7 is associated with uneven joint degeneration between right and left hips after spinal fusion

The aim of the study was to investigate the effects of coronal offset of C7 after spinal fusion on the discrepancy in joint space narrowing between the right and left hips. We retrospectively reviewed data from patients who underwent lumbar spinal fusion from 2011 to 2018 at our institute. The rate of hip joint space narrowing after spinal fusion was measured in 190 patients (380 hips). We assessed the effects of the distance between the C7 plumb line and the central sacral vertical line (C7-CSVL) on the discrepancy in joint space narrowing between the right and left hips. Using multivariate regression models, we controlled for the effects of age, sex, body mass index, fusion length, and several spinopelvic alignment parameters (sacral slope, pelvic incidence, lumbar lordosis, pelvic incidence minus lumbar lordosis, and sagittal vertical axis) on the joint space narrowing rate. Multivariate regression showed that the C7-CSVL was associated with the discrepancy of the joint space narrowing rate between right and left, indicating that when C7 deviated to right, the joint space narrowing was larger in the right than left hip (standardized coefficient, 0.203; p = 0.0005). A larger C7-CSVL was associated with a larger right-left discrepancy in hip joint space narrowing after spinal fusion. These findings indicate that spinal coronal balance affects the distribution of joint degeneration in the right and left hips. Surgeons should understand the potential risk of uneven progression of degeneration between the right and left hips in patients with large coronal offset.