Relationship between paraspinal muscle degeneration and the global alignment and proportion (GAP) score: cross-sectional study in middle-aged and older adults

Background The paraspinal muscle, a crucial component of the stable sagittal balance of the spine, is closely associated with the structural degeneration of ligaments. The global alignment and proportion (GAP) score can predict spinal pelvic ligament degeneration and examine the overall sagittal balance of the spine. This study aimed to determine the correlation between the paraspinal muscles and the GAP score using imaging evaluation and to investigate better methods for evaluating the sagittal balance of the spine compared to spinal pelvic parameters. Methods This study enrolled 61 patients aged over 45 years who suffered from chronic lower back pain or leg pain. The functional cross-sectional area (FCSA) and fatty infiltration (FI) of the paraspinal muscle were quantified using ImageJ software. The spinopelvic sagittal parameters were measured. Pearson correlation analysis was used to investigate the correlations between paraspinal muscle measurements and GAP score. The participants were stratified into a proportioned (P) group, a moderately disproportioned (MD) group, and a severely disproportioned (SD) group according to the GAP score. The paraspinal muscles were subsequently compared among the three groups. Results A significant negative correlation was observed between the multifidus (MF) FCSA, erector spinae (ES) FCSA, and GAP scores ( r = − 0.339, − 0.409, P < 0.01). Additionally, a significant positive correlation was observed between MF FI, ES FI, and GAP scores ( r = 0.379, 0.354, P < 0.01). The SD group showed significantly greater FI and significantly lower FCSA of the MF and ES than the MD and P groups( P < 0.05), with no significant differences between the MD and P groups ( P > 0.05). Conclusions The degree of paraspinal muscle degeneration is strongly associated with the GAP score; therefore, more severe paraspinal muscle degeneration corresponds to a higher GAP score. Moreover, the relative sagittal plane parameters (relative pelvic version, relative lumbar lordosis, and relative spinopelvic alignment) exhibited significant correlation with the paraspinal muscle parameters and were more strongly correlated with the paraspinal muscle parameters than with the actual spinal pelvic parameters. These findings suggest that the relative sagittal plane parameters serve as sensitive indicators for assessing sagittal spine balance.