Neuromuscular Partitioning of Subscapularis Based on Intramuscular Nerve Distribution Patterns: Implications for Botulinum Toxin Injections

Abstract

Subscapularis muscle spasticity is commonly treated with botulinum toxin injections; however, there are challenges in determining optimal injection sites within the muscle. The purpose of this study was to document the intramuscular innervation patterns of the subscapularis (1) to determine how the muscle is neuromuscularly partitioned and (2) to identify a strategy for botulinum toxin injection based on neuromuscular partitioning. In 50 formalin-embalmed cadaveric specimens, the extramuscular and intramuscular innervation was (1) serially dissected, digitized, and reconstructed in 3 dimensions (n=7); or (2) serially dissected and photographed (n=43). Intramuscular innervation patterns were compared among specimens to identify neuromuscular partitions. Variation was observed in the number (2–5) and origin of extramuscular nerve branches to the subscapularis. Despite variation in extramuscular innervation, the intramuscular innervation was consistent. Based on intramuscular innervation patterns, the subscapularis had 3 neuromuscular partitions (superior, middle, inferior) in 78% of specimens, and 2 partitions (superior, inferior) in 22% of specimens. The superior and middle partitions were most commonly innervated by branch(es) from the posterior cord, and the inferior partition by branch(es) from the axillary nerve. Injection of botulinum toxin into each partition may help to optimize results in the treatment of shoulder spasticity, and may be achieved by a combination of medial and inferior approaches. Clinical studies are required to determine whether the combination approach is more effective than any single approach and whether the number of partitions injected correlates with clinical outcomes.