Predicting postoperative mechanical complications with the ethnicity-adjusted global alignment and proportion score in degenerative scoliosis: does paraspinal muscle degeneration matter?

Background: The global alignment and proportion (GAP) score was developed to predict mechanical complications (MCs) after adult spinal deformity surgery but showed limited sensitivity in the Asian population. Considering variations in sagittal parameters among different ethnic groups, our team developed the ethnicity-adjusted GAP score according to the spinopelvic parameters of 566 asymptomatic Chinese volunteers (C-GAP score). Notably, degenerative scoliosis (DS) patients with MCs following corrective surgery have more severe paraspinal muscle degeneration. For DS patients with various sagittal alignments, the unevenly distributed degeneration of paraspinal muscle may exert different influences on MC occurrence and largely affect the accuracy of the C-GAP score in clinical assessment. Therefore, incorporating paraspinal muscle degeneration indices within the C-GAP score may improve its accuracy in predicting MC occurrence.

Purpose: We aimed to clarify the influence of paraspinal muscle degeneration on the C-GAP score predicting MC occurrence following DS surgery and modify the C-GAP score with paraspinal muscle degeneration parameters.

Study design: A retrospective case-control study.

Sample size: A total of 107 adult degenerative scoliosis patients.

Outcome measures: Demographic information, postoperative sagittal spinopelvic parameters, the GAP score, the C-GAP score, and paraspinal muscle degeneration parameters.

Methods: A total of 107 DS patients undergoing posterior spinal fusion surgery (≥4 vertebrae) with a minimum of 2 years follow-up (or experiencing MCs within 2 years) were retrospectively reviewed. Their C-GAP score was calculated based on our previous study and patients were divided into 3 C-GAP categories, "proportioned" (P), "moderately disproportioned" (MD), and "severely disproportioned" (SD). Relative cross-sectional area (cross-sectional area of muscle-disc ratio×100, rCSA) and fat infiltration rate, FI% at L1/2, L2/3, L3/4, and L4/5 discs were quantitatively evaluated using magnetic resonance imaging (MRI). In each C-GAP category, patients were additionally divided into the MC group and the non-MC group to analyze their paraspinal muscle degeneration. A multivariable logistic regression model consisting of the CSA-weighted average FI% (total FI%) and the C-GAP score, C-GAPM was constructed. The area under the curve (AUC) of the receiver operating characteristic (ROC) curves was used to evaluate the predictability of the GAP score, the C-GAP score, FI%, and C-GAPM. This project was supported by the National Natural Science Foundation of China (No.82272545) and Special Fund of Science and Technology Plan of Jiangsu Province (No.BE2023658).

Results: For all 107 patients, FI% at L1/2, L2/3, L3/4, and L4/5 discs and the total FI% of the MC group (n=32) were significantly higher than those of the non-MC group (n=75). The MC rates of 3 original GAP categories, P, MD, and SD categories were 25.00% (6/24), 27.03%(10/37), and 34.78% (16/46) (χ2=0.944, p=.624). Based on the C-GAP score, the MC rates of the P, MD, and SD categories were 11.90% (5/42), 34.69% (17/49), and 62.50% (10/16), showing significant differences (χ2=15.137, p=.001). In the C-GAP MD category, compared with the non-MC group (n=32), the MC group (n=17) has a higher total FI% (26.16(22.95, 34.00) vs. 22.67(16.39, 27.37)), p=.029). A similar trend was identified in the C-GAP SD category (34.79±11.56 vs. 19.00±5.17, p=.007), but not in the C-GAP P category (25.09(22.82, 32.66) vs. 24.66(17.36, 28.63), p=.361). The AUC of the GAP score, the C-GAP score, the total FI%, and C-GAPM were respectively 0.601, 0.722, 0.716, and 0.772.

Conclusions: Paraspinal muscle degeneration exerts a significant effect on the occurrence of MC in the C-GAP MD, SD instead of P category. The integration of paraspinal muscle FI% with the C-GAP score (C-GAPM) enables a more accurate prediction of MCs following DS surgery. Surgeons should pay adequate attention to paraspinal muscle degeneration during surgical planning and postoperative management for patients in the C-GAP MD and SD categories.