Schwab alignment targets use fixed numerical cutoffs that fail at the extremes of pelvic incidence, yet mechanical complications persist even after ideal correction. This study develops and validates the GAP score, a pelvic-incidence-based proportional method of assessing postoperative sagittal alignment. The aim was to predict mechanical complications after adult spinal deformity surgery.
The core teaching point: stop reading sagittal alignment as fixed numbers and start reading it as proportion to pelvic incidence. Schwab's targets (PI-LL ≤10°, PT <20°, SVA <4 cm) were calibrated to quality-of-life scores, not to implant failure, and they break down at high or low pelvic incidence. A patient corrected to Schwab 0-0-0 can still fail.
The GAP score asks instead whether each parameter is proportioned to that individual's pelvic incidence. A proportioned construct (GAP 0-2) sits in a position needing minimal compensation, so loads on rods and junctional segments stay normalized. That is why the complication rate drops to 6% versus 95% in severely disproportioned spines.
Use it both ways: as a preoperative planning target and as a 6-week postoperative check. A high GAP score early flags a patient for closer surveillance, bracing, and osteoporosis treatment before the rod breaks. This remains a single-cohort derivation; external reliability and reproducibility were not yet established in this paper.
Schwab alignment targets use fixed numerical cutoffs that fail at the extremes of pelvic incidence, yet mechanical complications persist even after ideal correction. This study develops and validates the GAP score, a pelvic-incidence-based proportional method of assessing postoperative sagittal alignment. The aim was to predict mechanical complications after adult spinal deformity surgery.
The core teaching point: stop reading sagittal alignment as fixed numbers and start reading it as proportion to pelvic incidence. Schwab's targets (PI-LL ≤10°, PT <20°, SVA <4 cm) were calibrated to quality-of-life scores, not to implant failure, and they break down at high or low pelvic incidence. A patient corrected to Schwab 0-0-0 can still fail.
The GAP score asks instead whether each parameter is proportioned to that individual's pelvic incidence. A proportioned construct (GAP 0-2) sits in a position needing minimal compensation, so loads on rods and junctional segments stay normalized. That is why the complication rate drops to 6% versus 95% in severely disproportioned spines.
Use it both ways: as a preoperative planning target and as a 6-week postoperative check. A high GAP score early flags a patient for closer surveillance, bracing, and osteoporosis treatment before the rod breaks. This remains a single-cohort derivation; external reliability and reproducibility were not yet established in this paper.