Use of the Global Alignment and Proportion score to predict postoperative health-related quality of life in adult spinal deformity surgery

OBJECTIVE The purpose of this study was to validate the Global Alignment and Proportion (GAP) score as a predictor of health-related quality of life (HRQOL) outcomes for patients undergoing adult spinal deformity (ASD) surgery. METHODS This was a retrospective cohort study of patients with ASD undergoing long-segment spine fusions (≥ 5 vertebrae fused) at a single institution over a 2-year period (n = 85). Radiographic parameters were measured at preoperative, 6-week postoperative, 1-year postoperative, and 2-year postoperative visits. GAP scores were calculated using 4 sagittal parameters: relative pelvic version, relative lumbar lordosis, lordosis distribution index, and relative spinopelvic alignment. Patients were stratified into 3 GAP categories at each time point: proportioned (score 0–2), moderately disproportioned (score 3–6), and severely disproportioned (score ≥ 7). HRQOL outcomes were collected at preoperative, 1-year postoperative, and 2-year postoperative visits; these measures included patient self-reported outcome measures (i.e., PROMIS), Scoliosis Research Society-22 spinal deformity questionnaire (SRS-22), and Oswestry Disability Index (ODI) scores. RESULTS Overall, 42% of cases were revision surgeries and 96.5% of patients underwent fusion to the sacrum. The mean preoperative GAP score significantly improved from preoperative (7.84) to immediate postoperative (3.31) assessment (p < 0.001). Similarly, the percentage of patients categorized as proportioned improved from 9.4% at preoperative to 45.9% at immediate postoperative evaluation. The preoperative GAP score or category was not significantly associated with any preoperative HRQOL outcome metrics. The immediate postoperative GAP score was not correlated with any 1-year HRQOL outcomes. However, the immediate postoperative GAP score was significantly associated with 2-year SRS-22 outcomes, including SRS-22 function (r = −0.35, p < 0.01), self-image (r = −0.27, p = 0.044), and subtotal (r = −0.35, p < 0.01) scores. As compared to severely disproportioned patients, proportioned patients had better SRS-22 pain (4.08 vs 3.17, p = 0.04), satisfaction (4.40 vs 3.50, p = 0.02), and subtotal (4.01 vs 3.27, p = 0.036) scores. The immediate postoperative GAP score was also significantly associated with 2-year PROMIS outcomes, including PROMIS pain (r = 0.31, p = 0.023) and physical function (r = −0.35, p < 0.01) scores. As compared to severely disproportioned patients, proportioned patients had better PROMIS pain (53.18 vs 63.60, p = 0.025) and physical function (41.66 vs 34.18, p = 0.017) scores. Postoperative GAP score or category did not predict any ODI outcomes. CONCLUSIONS The postoperative GAP score is a predictor of long-term HRQOL outcomes following ASD surgery, and proportioned patients are more likely to have less pain and be satisfied with their surgery. However, the postoperative GAP score does not predict outcomes as measured by ODI.