Surgical Planning for Adult Spinal Deformity: Anticipated Sagittal Alignment Corrections According to the Surgical Level

Study Design: Retrospective cohort study. Objectives: Establish simultaneous focal and regional corrective guidelines accounting for reciprocal global and pelvic compensation. Methods: 433 ASD patients (mean age 62.9 yrs, 81.3% F) who underwent corrective realignment (minimum L1-pelvis) were included. Sagittal parameters, and segmental and regional Cobb angles were assessed pre and post-op. Virtual postoperative alignment was generated by combining post-op alignment of the fused spine with the pre-op alignment on the unfused thoracic kyphosis and the pre-op pelvic retroversion. Regression models were then generated to predict the relative impact of segmental (L4-L5) and regional (L1-L4) corrections on PT, SVA (virtual), and TPA. Results: Baseline analysis revealed distal (L4-S1) lordosis of 33 +/- 15 degrees, flat proximal (L1-L4) lordosis (1.7 +/- 17 degrees), and segmental kyphosis from L2-L3 to T10-T11. Post-op, there was no mean change in distal lordosis (L5-S1 decreased by 2 degrees, and L4-L5 increased by 2 degrees), while the more proximal lordosis increased by 18 +/- 16 degrees. Regression formulas revealed that Delta 10 degrees in distal lordosis resulted in Delta 10 degrees in TPA, associated with Delta 100 mm in SVA or Delta 3 degrees in PT; Delta 10 degrees in proximal lordosis yielded Delta 5 degrees in TPA associated with Delta 50 mm in SVA; and finally Delta 10 degrees in thoraco-lumbar junction yielded Delta 2.5 degrees in TPA associated with Delta 25 mm in SVA and no impact on PT correction. Conclusions: Overall impact of lumbar lordosis restoration is critically determined by location of correction. Distal correction leads to a greater impact on global alignment and pelvic retroversion. More specifically, it can be assumed that 1 degrees L4-S1 lordosis correction produces 1 degrees change in TPA / 10 mm change in SVA and 0.5 degrees in PT.