Compensatory mechanisms in adult degenerative thoracolumbar spinal deformity – Radiographic patterns, their reversibility after corrective surgery, and the influence of pelvic morphology

OBJECTIVE: Loss of lumbar lordosis (LL) in degenerative deformity activates spinal compensatory mechanisms to maintain neutral C7 sagittal vertical axis (C7SVA), such as an increase in pelvic tilt (PT) and decreased thoracic kyphosis (TK). We study the extent to which PT increase and TK reduction contribute to the compensation of pelvic incidence (PI)-LL mismatch. METHODS: A cohort of 43 adult patients with adult degenerative thoracolumbar deformity were included in this retrospective study. Radiographic spinopelvic measurements were obtained before and after corrective surgery. Pearson correlations were calculated. RESULTS: Preoperative PI-LL mismatch significantly correlated with an increase in PT and a decrease in TK in the whole cohort r = +0.66 (95% confidence interval [CI] 0.44–0.8) and r = −0.67 (95% CI − 0.81–−0.47), respectively, at a relative rate of 0.37 (standard deviation [SD]: 0.07) and − 0.57 (SD: 0.09), respectively. In patients with low PI, only TK showed a significant correlation with PI-LL mismatch, r = −0.56 (95% CI − 0.8 to − 0.16), at a rate of − 0.57 (SD: 0.19). The high PI subgroup showed a significant correlation with PT, TK, and C7SVA, r = 0.62 (95% CI 0.26–0.82), r = −0.8 (95% CI − 0.9–−0.58), and r = 0.71 (95% CI 0.41–0.87) at rates of 0.48 (SD: 0.11), −0.72 (SD: 0.12), and 0.62 (SD: 1.27). CONCLUSIONS: Decreased TK represented a more consistent compensatory mechanism in patients with high and low PI when compared to an increase in PT. PI-LL mismatch induced more pronounced changes in TK than did PT in both subgroups. Patients with high PI relied more on increases in PT and a relative decrease in TK to compensate for PI-LL mismatch than patients with low PI.