Sagittal Alignment in Adolescent Idiopathic Scoliosis with Major Thoracic Curves: Pelvic Incidence and Its Influences on Lumbar Lordosis and Thoracic Kyphosis.

OBJECTIVES: To analyse the correlations between pelvic incidence (PI), segmental lumbar lordosis (LL), and segmental thoracic kyphosis (TK) in AIS patients with major thoracic curves. To evaluate the influence of PI, stratified into low, average and high PI groups, on segmental LL and spinopelvic alignment in AIS patients.

SUMMARY OF BACKGROUND DATA: PI is a key anatomic determinant of sagittal spinal alignment. While the relationship between PI and LL is well established, its influence on the distribution of proximal lumbar lordosis (PLL) versus distal lumbar lordosis (DLL) and its relationship with segmental TK in AIS patients remains poorly defined.

METHODS: This retrospective study included 506 patients with AIS (Lenke types 1-4). Preoperative sagittal radiographic parameters measured were LL, PLL, DLL, TK, and PI. Patients were stratified into low PI (<45°), average PI (45-60°), and high (>60°) PI groups.

RESULTS: PI demonstrated a moderate positive correlation with LL (r=0.395, P <0.001), primarily attributable to PLL (r=0.470, P <0.001), but not DLL (r=0.007, P =0.869). PI showed a weak negative correlation with TK (r=-0.199, P <0.001), predominantly due to DTK (r=-0.276, P <0.001). Higher PI groups had a significantly higher LL (low PI: 51.4±10.7°; average PI: 57.4±10.9°; high PI: 63.4±10.8°) ( P <0.001), driven by a corresponding increase in PLL (16.8±8.4° vs. 23.3±9.3° vs. 28.2±8.3° ( P <0.001). In contrast, DLL were comparable between PI groups ( P =0.507).

CONCLUSION: PI significantly influences PLL, but not DLL in AIS patients with major thoracic curves. Although DLL constitutes the majority of LL, it remains relatively constant across varying PI. Increasing PI is associated with reduced TK, primarily driven by a decrease in DTK. These findings highlight the importance of individual PI in planning sagittal rod contouring, as patients with higher PI may require greater proximal lumbar contouring to achieve their native sagittal profile.

STUDY DESIGN: Retrospective study.