Rod angulation does not reflect sagittal curvature in adult spinal deformity surgery: comparison of lumbar lordosis and rod contouring

Study designA retrospective study.ObjectiveRelationship between rod and spinal shape in the sagittal plane in adult spinal deformity (ASD) surgery.BackgroundCorrective surgery for adult spinal deformity (ASD) involves the use of contoured rods to correct and modify the spinal curvatures. Adequate rod bending is crucial for achieving optimal correction. The correlation between rods and spinal shape in long constructs has not been reported previously.MethodsWe conducted a retrospective analysis of a prospective, multicenter database of patients who underwent surgery for ASD. The inclusion criteria were patients who underwent pelvic fixation and had an upper instrumented vertebra at or above T12. Pre- and post-operative standing radiographs were used to assess lumbar lordosis at the L4S1 and L1S1 levels. The angle between the tangents to the rod at the L1, L4, and S1 pedicles was calculated to determine the L4S1 and L1S1 rod lordosis. The difference between the lumbar lordosis (LL) and the rod lordosis (RL) was calculated as Delta L = LL-RL. The correlation between this difference (Delta L) and various characteristics was analyzed using descriptive and statistical methods.ResultsEighty-three patients were included in the study, resulting in 166 analyzed differences (Delta L) between the rod and spinal lordosis. The values for rod lordosis were found to be both greater and lesser than those of the spine but were mostly lower. The range for total Delta L was -24 degrees-30.9 degrees, with a mean absolute Delta L of 7.8 degrees for L1S1 (standard deviation (SD) = 6.0) and 9.1 degrees for L4S1 (SD = 6.8). In 46% of patients, both rods had a Delta L of over 5 degrees, and over 60% had at least one rod with a Delta L difference of over 5 degrees. Factors found to be related to a higher Delta L included postoperative higher lumbar lordosis, presence of osteotomies, higher corrected degrees, older age, and thinner rods. Multivariate analysis correlated only higher postoperative L1S1 lordosis with higher Delta L. No correlation was found between a higher Delta L and sagittal imbalance.ConclusionsVariations between spinal and rod curvatures were observed despite the linear regression correlation. The shape of the rod does not seem to be predictive of the shape of the spine in the sagittal plane in ASD long-construct surgeries. Several factors, other than rod contouring, are involved in explaining the postoperative shape of the spine. The observed variation calls into question the fundamentals of the ideal rod concept.

Automated summary

This study retrospectively analyzed the relationship between rod and spinal shape in adult spinal deformity surgery. The results showed variations between the curvatures of the rod and spine, suggesting that the shape of the rod does not seem to predict the shape of the spine in the sagittal plane.