Risk Factors Predicting C- Versus S-shaped Sagittal Spine Profiles in Natural, Relaxed Sitting: An Important Aspect in Spinal Realignment Surgery

Study design A cross-sectional study on a randomly selected prospective cohort of patients presenting to a single tertiary spine center. Objective The aim of this study was to describe the clinical and radiographic parameters of patients with S- and C-shaped thoracolumbar sagittal spinal profiles, and to identify predictors of these profiles in a natural, relaxed sitting posture. Summary of background data Sagittal realignment in adult spinal deformity surgery has to consider the sitting profile to minimize the risks of junctional failure. Persistence of an S-shaped sagittal profile in the natural, relaxed sitting posture may reflect a lesser need to accommodate for this posture during surgical realignment. Methods Consecutive patients with low back pain underwent whole body anteroposterior and lateral radiographs in both standing and sitting. Baseline clinical data of patients and radiographic parameters of both standing and sitting sagittal profiles were compared using χ, unpaired t tests, and Wilcoxon rank-sum test. Subsequently, using stepwise multivariate logistic regression analysis, predictors of S-shaped curves were identified while adjusting for confounders. Results Of the 120 patients included, 54.2% had S-shaped curves when sitting. The most common diagnoses were lumbar spondylosis (26.7%) and degenerative spondylolisthesis (26.7%). When comparing between patients with S- and C-shaped spines in the sitting posture, only diagnoses of degenerative spondylolisthesis (odds ratio [OR], 5.44; P = 0.01) and degenerative scoliosis (OR, 2.00; P = 0.039), and pelvic incidence (PI) >52.5° (OR, 5.48; P = 0.008), were predictive of an S-shaped sitting sagittal spinal alignment on multivariate analysis. Conclusion Stiffer lumbar curves (e.g., patients with degenerative spondylolisthesis and degenerative scoliosis) or those who have a predilection for an S-shaped standing sagittal profile when sitting (e.g., high PI) may be more amenable to fusion in accordance with previously studied sagittal realignment targets. In contrast, more flexible curves may benefit from less aggressive lordotic realignment to prevent potential junctional failures. Level of evidence 3.