How Strict Is the Balance Between the Lower Kyphotic Arc and Upper Lordotic Arc in Spinal Deformity?

Introduction: The thoracolumbar spine can be divided into four arcs delineated by a kyphotic apex, lordotic apex, and inflection point in-between. Theoretically, the lower kyphotic arc (LKA) and upper lordotic arc (ULA) are equal and opposite within a healthy spine. The extent that this balance is maintained in spine deformity and subsequent instrumented correction is unknown. Complications of the rostral spine may be potentially affected by management of the caudal, instrumented section. We sought to better understand this balance of arcs in spinal deformity and hypothesized that changing upper lordotic arc distance would be correlated to changing lower kyphotic arc distances after surgery.

Methods: We reviewed radiographic data of 81 thoracolumbar fusion patients over 65 years between 2015 and 2024. Measurements were performed for sagittal spinopelvic parameters from preoperative and 6-week postop scoliosis radiographs. The LKA distance was measured as levels between the kyphotic apex vertebrae and the superior vertebrae of the inflection point. The ULA distance was measured as levels between the inferior vertebrae of the inflection point and lordotic apex vertebrae.

Results: Our cohort included 58 females (71.6%). 44 patients underwent long fusions (UIV>T10) and 37 underwent short fusions (UIV<=T10).
The average distance of the LKA was 3.1 +/- 2.0 levels (preop) and 4.9 +/- 1.4 levels (postop). The average distance of the ULA was 1.9 +/- 1.0 levels (preop) and 2.5 +/- 0.8 levels (postop).
Deformity correction induced an average 1.2 +/- 2.6 level greater change in LKA (1.9 +/- 2.3 levels) than ULA (0.6 +/-1.1 levels).
The Spearman’s correlation coefficient between changes of the LKA and ULA distance induced by deformity correction was 0.003 (p=0.98). The correlation was –0.067 (p=0.71) among short fusions and 0.016 (p=0.92) among long fusions.

Conclusion : There is little-to-no correlation between the size changes induced to the ULA and LKA by surgery. These findings suggest that kyphotic imbalances in the rostral spine may have a complicated relationship to the caudal spine that requires further evaluation global alignment in the context of arc imbalance.