Modified global alignment and proportion scoring with body mass index and bone mineral density (GAPB) for improving predictions of mechanical complications after adult spinal deformity surgery

Abstract

Background context: The global alignment and proportion (GAP) score for predicting mechanical complications of adult spinal deformity (ASD) surgery has limitations due to its lack of bone quality and patient characteristics such as obesity, which has a significant impact on surgical outcome, especially in the elderly population with ASD.

Purpose: This study aimed to improve the predictability of GAP score after ASD surgery by adding body mass index (BMI) and bone mineral density (BMD).

Design: A retrospective comparative study.

Patient sample: Between January 2009 and December 2016, 203 consecutive patients with ASD underwent corrective fusion of more than 4 levels and were followed up for more than 2 years.

Outcome measures: The ability of the Scoliosis Research Society (SRS)-Schwab classification, age-adjusted alignment goals, GAP score, and modified global alignment and proportion scoring with BMI and BMD (GAPB) to predict mechanical failure was compared by calculating the area under the receiver operating characteristic curve (AUC).

Methods: The GAPB was developed and validated in patients randomly assigned to derivation (n=125, 61.6%) and validation (n=78, 38.4%) cohorts. Considering multicollinearity, a multivariable logistic regression model with BMD, BMI, and the GAP score was constructed.

Results: Patients' average age was 66.8±12.28 years, and they were followed for an average of 30.54±10.25 months. Fifty-five patients of the derivation cohort (44%) and 34 patients of the validation cohort (43%) had mechanical complications after ASD surgery. AUCs of the SRS-Schwab classification, GAP score, age-adjusted alignment goals, and GAPB were 0.532 (95% confidence interval [CI], 0.463-0.602), 0.798 (95% CI, 0.720-0.877), 0.568 (95% CI, 0.495-0.641), and 0.885 (95% CI 0.828-0.941), respectively.

Conclusions: The GAPB system, which includes BMI and BMD, showed improved predictability for predicting mechanical complications compared to the GAP score. Given these results, surgeons should keep in mind the importance of bone quality and BMI as well as proportional alignment.