Normal spino-pelvic alignment in healthy adolescents: defining the PI-SS relationship and laying the foundations for pre-operative sagittal planning

Purpose: Surgical treatment of adolescent spinal deformities requires accurate pre-operative planning. Few studies confirmed that sagittal alignment restoration is crucial to reduce the risk of early degeneration, yet sagittal alignment targets are lacking in adolescents. No predictive formula for ideal sacral slope (SSideal) based on pelvic incidence (PI) has been developed specifically for surgical planning in adolescents. The aim of the present study is to investigate the relationship between PI and SS in healthy adolescents aged 11–18 years to identify the most accurate model for SSideal prediction based on PI. Methods: Full-spine standing biplanar X-rays were analyzed. PI, SS and pelvic tilt (PT) were measured. Three regression models (linear, polynomial, and logarithmic) were generated to describe the PI–SS relationship. The adult-derived model by Yilgor et al. was also applied to the study cohort. Accuracy and precision of all models were assessed globally and within PI-based subgroups. External validation was performed using a previously published dataset. Results: A total of 122 patients (mean age 14 years) were included. Logarithmic and polynomial models achieved the best fit within the study cohort (R² = 0.63 and 0.63). The Yilgor et al. model showed substantially lower fit (R² = 0.39) and systematic underestimation of SS in intermediate and high PI subgroups. The linear model performed best within the external validation cohort showing the best goodness-of-fit, accuracy, precision, and robustness. The Yilgor model again showed systematic SS underestimation and higher errors. Conclusion: Our results suggest that the formula SSideal = 0.61 x PI + 11.4 can be reliably employed in the pre-operative planning of adolescent spinal deformities.