Prediction of mechanical complications after corrective surgeries in adult degenerative scoliosis: modification and validation of the global alignment and proportion (GAP) score

Abstract

Objective To modify the global alignment and proportion (GAP) score according to the spinopelvic alignment of healthy volunteers and to validate the reliability of the GAP score and the modified GAP (M-GAP) score in the prediction of mechanical failure after corrective surgeries in adult degenerative scoliosis (ADS). Methods Clinical and radiographical data of 66 ADS patients undergoing correction surgeries in our center between January 2014 and January 2017 were retrospectively reviewed. The average age of the cohort was 60.2±7.1 years, including 13 male patients and 53 female patients. The predictive ability of GAP score to the mechanical failure was validated with Fisher's exact test and Linear-by-linear association test. Sagittal spinopelvic parameters were measured on lateral full-spine X-rays of 67 healthy volunteers aged between 50 and 70. The average age of these subjects was 58.2±5.4 years. Values of spinal and pelvic parameters including pelvic incidence (PI), sacral slope (SS), lumbar lordosis (LL), global tilt (GT) and sagittal vertical axis (SVA) were measured. Simple linear regression analysis was conducted between PI and SS, LL and GT, respectively. The equations of regression analysis were used to calculate patients' ideal SS, LL and GT, and to establish the modified GAP (M-GAP) score. The predictive ability of M-GAP score to the mechanical failure was also validated. Results Sagittal spinal and pelvic parameters of healthy subjects were measured and illustrated. The correlations between SS, LL and PI were SS=0.40×PI+12, LL=0.46×PI+22 and GT=0.46×PI-5. According to the GAP score, the postoperative sagittal spinopelvic alignment was proportioned in 16 (24.2%) cases, moderately disproportioned in 32 (48.5%) cases and severely disproportioned in 18 (27.3%) cases according to the original GAP score, and the occurrence of mechanical complication was 18.8% (3/16), 12.5% (4/32) and 22.2%(4/18) respectively. The GAP categories were not significantly correlated with the prevalence of mechanical failure (P=0.633), and no significant linear correlation was found (χ2=5.022, P=0.822). After re-evaluation of M-GAP score, the postoperative sagittal spinopelvic alignment was proportioned in 32 (48.5%) cases, moderately disproportioned in 25 (37.9%) cases and severely disproportioned in 9 (13.6%) cases, and the occurrence of mechanical failure was 6.3% (2/32), 24.0% (6/25) and 30%(3/9), respectively. The prevalence of mechanical failure was statistically different between proportioned, moderately disproportioned and severely disproportioned spinopelvic alignment patients classified by M-GAP score (P=0.048), and there was a statistically significant linear association between the M-GAP categories and the occurrence of mechanical complications (χ2=0.093, P=0.034). Conclusion Poor predictive ability of the original GAP score to the mechanical failure was detected in ADS patients. However, the modified GAP score seemed to be a reliable predictor for mechanical failure in ADS patients. Key words: Adult; Lumbar vertebrae; Intervertebral disc degeneration; Scoliosis; Postoperative complications