Is the Lower Lordotic Arc a Driver of Sacral Slope Among Spinal Deformity Patients?

Introduction: Lordotic and kyphotic curves maintain a geometrically balanced relationship defined by inflection points and apices along the spine. One proposed relationship suggests that the sacral slope (SS) should situate equal and opposite to the lower lordotic arc (LLA), defined as the cobb angle between the lordotic apex and L5. We sought to validate this relationship in adult spinal deformity and hypothesized that the lower lordotic arc would influence sacral slope such that changes in the LLA from surgery should induce a corresponding change in the sacral slope in a linear fashion.

Methods: We reviewed data of 81 thoracolumbar fusion patients over 65 years of age between 2015 and 2024. Radiographic measurements were performed for sagittal spinopelvic parameters from preoperative and 6-week postop scoliosis radiographs. The LLA was measured using KEOPS software to segment the sagittal spine and calculate the cobb angle between the superior endplate of the calculated lumbar apex and inferior endplate of the L5 vertebral body.

Results: Our cohort included 58 females (71.6%). Average age and BMI were 71.3 +/- 4.0 years and 28.2 +/- 5.1.
Pearson’s correlation between the SS and LLA was –0.661 (p < 0.001, 95CI: [-0.771, -0.514]) among preoperative measurements and –0.632 (p < 0.001, 95CI: [-0.750, -0.474]) among post-operative measurements. A simple linear regression yielded an estimate of –0.786 (intercept=-4.0, R^2=0.44) for preoperative measurements and –1.00 (intercept=0.79, R^2=0.40) for postoperative measurements.

When comparing the changes in sacral slope versus changes in lower lordotic arch caused by deformity correction, a Pearson coefficient of –0.27 (p=0.02, 95CI: [-0.466, -0.0458]) was found with a linear regression estimate of –0.46 (intercept=-3.43, R^2=0.07).

Conclusion : SS measurements were correlated with the LLA in both the pre- and post-operative setting. The agreement between LLA and SS was particularly strong post-op. This relationship was consistent when observing changes in SS versus changes in the LLA induced by deformity correction. LLA accounting for 40% of the SS post-operatively suggests that modulating LLA is a major driver of changing SS-and by extension pelvic tilt-in deformity correction.