Compensation Characteristics of Spinal Sagittal Malalignment According to Pelvic Incidence in Elderly Patients with Degenerative Lumbar Spinal Stenosis

OBJECTIVE: Compensation characteristics of spinal sagittal malalignment according to pelvic incidence (PI) have yet to be elucidated. This study aimed to investigate the difference in compensatory segments according to PI in elderly patients with degenerative lumbar spinal stenosis (DLSS). METHODS: This retrospective study included 196 patients (143 females, 53 males) suffering from DLSS with an average age of 66.73 years in our department. Sagittal parameters were obtained on the whole spinal lateral radiograph, including T1‐T12 slope (T1S‐T12S), Cobb angle (CA) of the functional units of the thoracic spine, thoracic kyphosis (TK), lumbar lordosis (LL), sacral slope (SS), pelvic tilt (PT), PI, the ratio of PT to PI (PT/PI), PI minus LL mismatch (PI‐LL), sagittal vertical axis (SVA). Patients were divided into the low and high PI groups according to the median value of PI. With reference to the value of SVA and PI‐LL, each PI group was further grouped into the balance subgroup (SVA < 50 mm, PI‐LL ≤10°), hidden imbalance subgroup (SVA < 50 mm, PI‐LL > 10°), and imbalance subgroup (SVA ≥50 mm). Independent samples t‐test/Mann–Whitney U test, one‐way ANOVA/Kruskal‐Wallis test, and Person correlation analysis were utilized for statistical test. RESULTS: The median value of PI was 47.65°. 96 and 100 patients were assigned to low and high PI groups, respectively. Correlation analysis indicated that the T8‐T12 slope and T10‐T12 slope were associated with PI‐LL in the high and low PI groups, respectively (all, p < 0.01). For segmental lordosis, T8‐9 to T11‐12 CA and T10‐11 to T11‐12 CA were associated with PI‐LL in the high and low PI groups, respectively (all, p < 0.01). In the high PI cohort, T8‐12 CA and PT increased significantly from the balance to imbalance subgroups (both, p < 0.05). In the low PI cohort, T10‐12 CA and PT first increased and then decreased from the balance to imbalance subgroups (both, p < 0.05). CONCLUSION: T8‐12 was the main compensatory segment of the thoracic spine in patients with high PI, while it was T10‐12 in patients with low PI. In addition, the compensation potential of the lower thoracic spine and pelvis in patients with low PI was inferior to those with high PI.