Medical Student Washington University in St. Louis Washington University in St. Louis School of Medicine St Louis, MO, US
Disclosure(s):
Karan Joseph: No financial relationships to disclose
Introduction: Post-operative alignment errors, including L1 Pelvic Angle (L1PA) error and T4 Pelvic Angle (T4PA) -L1PA mismatch, are postulated to be significant contributors to mechanical failure following spinal deformity surgery. Identifying the variables that drive these errors can help inform surgical planning and patient management. This study investigates the patient- and procedure-specific factors influencing these alignment errors, with particular attention to differences in short and long fusions. Despite low explanatory power, significant findings may shed light on the complex factors contributing to post-operative deviations from ideal alignment.
Methods: We retrospectively reviewed 116 thoracolumbar fusion patients at an academic tertiary care center between 2015 and 2024 with at least 6 months of follow-up. Multivariable regressions were performed on potential predictors, including psoas index, BMI, age, sex, and fusion length (short vs. long fusions). Separate models were built for L1PA error, T4PA error, and their absolute values to assess their relationship with mechanical failure and other post-operative outcomes.
Results: The multivariable regression for L1PA error identified psoas index (p = 0.009) and sex (p = 0.028) as significant predictors, though the overall R² was low (0.035), suggesting that these factors explain only a small portion of the variability in L1PA error. In contrast, T4PA error was significantly influenced by fusion length (p = 0.016, R² = 0.026), particularly in short fusions. The model for absolute T4PA error similarly showed a strong association with fusion length (p = 0.014, R² = 0.055), intuitively demonstrating that short fusions yield more variability in T4PA alignment post-operatively.
Conclusion : This study highlights key factors associated with post-operative L1PA and T4PA alignment errors in spinal deformity surgery. Although the low R² values suggest that these are only a small subset of the variables that contribute these alignment deviations, significant predictors such as psoas index, sex, and fusion length provide important insights. The findings suggest that short fusions may leave more degrees of freedom for T4PA to deviate from ideal alignment, potentially leading to increased post-operative mismatch. Further research is needed to explore additional contributors, to fully understand the drivers of these alignment deviations.