Pelvic Incidence Affects Age-adjusted Alignment Outcomes in a Population of Adult Spinal Deformity

STUDY DESIGN: A single-center retrospective cohort study. OBJECTIVE: The objective of this study was to assess the effects of patient height and pelvic incidence (PI) on age-adjusted alignment outcomes of surgical adult spinal deformity (ASD) patients. SUMMARY OF BACKGROUND DATA: Patient height and PI have yet to be evaluated for their individual effects on achieving age-adjusted alignment targets. METHODS: Surgical ASD patients were grouped by percentile (low: <25th; normative: 25th-75th; high: >75th) for height and PI. Correction groups were generated at postoperative follow-up for actual alignment compared with age-adjusted ideal values for pelvic tilt (PT), pelvic incidence minus lumbar lordosis mismatch (PI-LL), and sagittal vertical axis, and PI-adjusted ideal alignment values for sacral slope (SS), as derived from clinically relevant formulas. Means comparison tests assessed differences in rates of matching ideal alignment (±10 y threshold for age-adjusted targets; -7 to 5 degrees measured minus ideal for SS) across height and PI groups. RESULTS: Breakdown of all included 198 patients by PI group: low (25%, 38±11 degrees), normative (50%, 57±5 degrees), high (25%, 75±7 degrees). Breakdown of patient height groups: low (25%, 1.52±0.04 m), normative (50% 1.64±0.05 m), and high (25%, 1.79±0.06 m). Overall, 29% of patients met postoperative age-adjusted alignment targets for PT, 23% for PI-LL, and 25% for sagittal vertical axis. Overall, 26% of patients met PI-adjusted SS alignment. There were no differences across patient height groups in rates of achieving adjusted alignment target (all P>0.05). Patients with high PI reached age-adjusted ideal alignment for PT at a lower rate (16%) than patients with normative (33%) or low PI (33%, P=0.056). Of patients that matched at least 1 ideal alignment target, those with high PI showed inferior preoperative to postoperative changes in EuroQol 5-dimension questionnaire as compared with normative and low PI patients (P=0.015). CONCLUSIONS: Patients with high PI reached ideal postoperative age-adjusted PT alignment at a lower rate than patients with normative and low PI. Height had no impact on postoperative age-adjusted alignment outcomes. Current postoperative ideal alignment targets may warrant an adjustment to account for PI.