Special Poster Session 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

INTRODUCTION

Adult spine deformity is often associated with paraspinal muscle dysfunction, though the cause remains unclear [1]. It has been previously demonstrated that bilateral glycerol injections of the lumbar paraspinal muscles cause muscle degeneration and lead to hyper-kyphotic deformity [2]. Studies have shown that paraspinal muscle asymmetry is present in patients with adult degenerative scoliosis [3,4]. The purpose of this study was to evaluate the effect of asymmetrical glycerol-induced paraspinal muscle degeneration on spine deformity. It was hypothesized that an asymmetrical pattern of paraspinal muscle degeneration would create a muscle imbalance in the coronal plane and result in a scoliotic curvature.

METHODS

17 female C57BL/6 mice were divided into two groups (ipsilateral and contralateral), which determined the pattern of glycerol injections administered to induce paraspinal muscle degeneration. Glycerol (50% v/v) was injected in the multifidus (15µL) and erector spinae (20µL) muscles in the thoracic and lumbar regions (based on group) at 4 timepoints, each 2 weeks apart. For the ipsilateral group, glycerol was injected in the right lumbar and right thoracic regions of the multifidus and erector spinae muscles. The contralateral group received injections in the multifidus and erector spinae muscles of the right lumbar and left thoracic regions. The mice were sacrificed two weeks after the last glycerol injection. MicroCT imaging was conducted before the first injections (week 0), before the third injections (week 4) and after the last injections before sacrifice (week 8) to monitor spinal curvature progression. Spinal curvatures were evaluated in the coronal and sagittal planes by measuring the Cobb angles (superior endplate of T5 to the inferior endplate of L5). Statistical significance was determined using two-way repeated measures ANOVA (a=0.05), followed by Tukey’s multiple comparisons test.

RESULTS

Mean coronal plane Cobb angles were small (< 6 degrees) at all timepoints in both groups (Figure 1). There was a significant effect of group (injection pattern) in the coronal plane (p=0.0460), but there was no significant interaction or effect of time. In the sagittal plane, a significant effect of group (p=0.0061), time (p<0.0001), and interaction (p=0.0004) was found (Figure 1). Surprisingly, both groups demonstrated a significant decrease in sagittal plane Cobb angles over time, indicating a reduction in kyphosis. This effect was strongest in the contralateral injection group.

DISCUSSION

No clinically meaningful scoliotic curvatures were observed in the coronal plane, contrary to the hypothesis. However, glycerol-induced asymmetrical paraspinal muscle degeneration in the thoracic and lumbar regions did significantly alter the sagittal plane curvature, most significantly in the contralateral group. Surprisingly, this muscle degeneration resulted in lower sagittal plane Cobb angles (i.e. reduced kyphosis) compared to baseline. Our previous work demonstrated that isolated bilateral (i.e. symmetrical) lumbar muscle degeneration leads to hyper-kyphosis [2], while here we demonstrate that lumbar combined with thoracic asymmetric muscle degeneration instead leads to hypo-kyphosis. In this case, the thoracic muscle degeneration appears to dominate the effect on spinal curvature, and the effect is largest when the degeneration is contralateral in the thoracic and lumbar regions.