Effects of botulinum toxin on paraspinal muscle imbalance during spinal motions in adolescents with idiopathic scoliosis: Assessment using high-density electromyography.

[BACKGROUND] The pathogenesis of adolescent idiopathic scoliosis (AIS) remains unclear, with muscle imbalance being a widely discussed hypothesis.

[OBJECTIVE] This study examined the impact of muscle imbalance on AIS by injecting botulinum toxin A (BTX) into patients' unilateral muscles and investigating altered back muscle synergies.

[METHODS] Three AIS patients received BTX injections in the concave-side paraspinal muscles. High-density electromyogram arrays placed from T8 to L4 recorded signals during five spinal motions at four timepoints (one pre-BTX and three post-BTX). Muscle synergies were extracted using non-negative matrix factorization and compared with data from ten healthy and ten AIS subjects from our previous studies.

[RESULTS] Post-BTX, muscle activity maps during flexion/extension, sitting, and standing exhibited reduced symmetry, with concave/convex ratios decreasing and being statistically lower than those of healthy subjects at post-2 and post-3 follow-ups (< 0.01). Muscles on the dominant side during lateral bending or axial rotation demonstrated decreased activation and differently distributed center of gravity positions on synergy maps compared to healthy subjects at all timepoints (< 0.05). Post-BTX changes were particularly notable for the patient with mild deformity.

[CONCLUSIONS] BTX affected the activation of paraspinal muscles, providing insights into the role of muscle imbalance in AIS and informing future therapeutic strategies.