Gluteal Muscle Degeneration Is Associated with Compensation Failure in Age-Related Spinopelvic Malalignment: A Cross-Sectional Study

Introduction: The role of pelvic stabilizing muscles, particularly the gluteus muscles, remains largely unexplored in the context of sagittal alignment. Understanding the relative vulnerability of each muscle group during the sequential breakdown of sagittal alignment is thus important. We hypothesize that gluteal muscle degeneration precedes or accompanies the failure of pelvic compensation in sagittal malalignment, making it a key indicator of early decompensation. Here, we examine which trunk or pelvic muscles demonstrate the greatest deterioration in morphology and quality across progressive stages of sagittal alignment decompensation, and aim to identify muscle-related predictors of spinopelvic malalignment. Methods: In this cross-sectional study, we analyzed 204 outpatients stratified into groups based on standing whole-spine radiographic parameters: normal (sagittal vertical axis [SVA] ≤50 mm and pelvic tilt [PT] ≤20°), compensated (SVA ≤50 mm and PT >20°), and decompensated (SVA >50 mm). We measured computed tomography-derived cross-sectional area (CSA) and Hounsfield unit (HU) values for the paravertebral (erector spinae and multifidus), psoas, gluteus medius, and gluteus maximus muscles. Group comparisons were performed using non-parametric tests, and logistic regression models were used to identify independent predictors of alignment deterioration. Results: Both gluteal and paraspinal muscles exhibited significant declines in CSA and HU values across worsening alignment stages, whereas the psoas muscle did not. In multivariate analyses, reduced HU values of the gluteus medius and maximus and decreased paraspinal CSA values were independently associated with abnormal and decompensated alignment. Notably, the gluteus medius HU value was positively associated with compensation maintenance, whereas the gluteus maximus HU value had a negative association. Conclusions: Degeneration of the gluteal muscles, especially in muscle quality, is strongly associated with the breakdown of sagittal alignment compensation. Our results suggest the gluteal musculature is a useful early biomarker and therapeutic target in preventing age-related spinal imbalance.