Impact of Lordosis Distribution Index on Proximal Junctional Kyphosis and Clinical Outcomes in Patients Undergoing Thoracolumbar Fusion for Degenerative Flatback Syndrome

Study Design. Retrospective study using prospectively collected data. Objective. To investigate the impact of the lordosis distribution index (LDI) on proximal junctional kyphosis (PJK) and clinical outcomes in patients with degenerative flatback syndrome (DFS). Summary of Background Data. Proper lumbar lordosis (LL) correction is essential for successful outcomes in corrective surgery for DFS. However, the clinical significance of LDI remains debatable, likely due to the lack of a clear definition regarding normal LDI. Methods. Patients with DFS who underwent fusion from the low thoracic spine (T9-T12) to the pelvis with ≥2 years of follow-up were included. LDI correction status was categorized as undercorrection, matched correction, and overcorrection based on pelvic incidence (PI)-adjusted normative LDI values. The impact of LDI correction on PJK development and clinical outcomes was evaluated using logistic regression analysis, both with and without age-adjusted PI-LL correction status. Results. A total of 222 patients were included in the study (female, 89.6%; mean age, 69.6 y; mean fusion length, 7.7). PJK developed in 69 patients (31.1%) during a mean follow-up of 45.1 months. Logistic regression analysis revealed that LDI alone did not affect PJK development, but when combined with overcorrection of age-adjusted PI-LL (vs. matched correction; odds ratio [OR]=2.255, P<0.001), LDI overcorrection (vs. matched correction; OR=1.784, P=0.030) significantly increased the risk of PJK. Clinical outcomes were not influenced by the LDI correction status but were significantly affected by age-adjusted PI-LL correction status. Conclusions. LDI correction alone did not affect PJK occurrence, but in conjunction with age-adjusted PI-LL overcorrection, LDI overcorrection significantly increased the risk of PJK. Clinical outcomes were affected by age-adjusted PI-LL correction with best outcomes in matched correction group but not by the LDI correction status. This lack of association between LDI and clinical outcomes may be attributable to the fact that LDI alone did not have a significant effect on determining spinopelvic alignment.