The Lower Lordotic Arch Is a Driver of Pelvic Positioning and Inflection Point Migration in Adult Spinal Deformity

Study Design. – Retrospective cohort. Objective. – To evaluate the relationship between the lower lordotic arch (LLA) and other parameters of spinopelvic alignment in adult spinal deformity (ASD). Summary of Background Data. – Lordotic and kyphotic curves maintain a geometrically balanced relationship defined by inflection points and apices along the spine. One proposed relationship suggests that pelvic positioning should be intimately related to the LLA, defined as the Cobb angle between the lordotic apex and L5. Methods. – We reviewed data of 304 adult thoracolumbar fusion patients between 2014 and 2024. KEOPS software was used to segment the sagittal spine and calculate relevant Cobb angles from preoperative and six-week postoperative scoliosis radiographs. Results. – Sacral slope (SS) and LLA lordosis were correlated among preoperative (ρ=0.64, P<0.001) and postoperative (ρ=0.64, P<0.001) measurements. Linear regression of changes in SS versus change in lordosis of LLA from surgery was significant (y=0.77x+3.6, R=0.47, P<0.001). Extending the caudal measurement of LLA to S1 (LLA-S1) increased strength of correlation preoperative (ρ=0.73, P<0.001) and postoperatively (ρ=0.86, P<0.001). Change in SS versus change in LLA-S1 lordosis was also more precise (R=0.62) than using LLA. Inflection point (IP) position was correlated to LLA lordosis preoperatively and postoperatively. Rostral migration of the IP was significantly associated with additional lordosis in the LLA after surgery (ρ=0.22, P<0.001). Conclusions. – SS measurements were correlated with LLA before and after surgery. This relationship was consistent when observing changes in SS versus changes in the LLA induced by deformity correction, and these relationships were further strengthened when comparing SS to LLA-S1 instead of LLA. LLA was also found to rostrally migrate the IP position. Increasing lordosis in the LLA is a major driver of increasing SS without affecting PI—and by extension decreasing pelvic tilt—in ASD correction.