Structural characteristics of the subscapularis muscle in children with medial rotation contracture of the shoulder after obstetric brachial plexus injury.

The aim of this study was to obtain a better understanding of the cause of the medial rotation contracture of the shoulder after obstetric brachial plexus lesions by studying the morphology of the shortened subscapularis muscle. Muscle biopsy specimens were harvested from 13 children with obstetric brachial plexus palsy who underwent corrective surgery for the rotation contracture. The majority of the subscapularis muscle biopsy samples had an essentially normal morphology and showed a predominance of type I myosin heavy chain isoform, while one biopsy showed signs of marked fibrosis and a predominance of type II myosin heavy chain isoform. The findings support the assumption that shortening of the subscapularis is caused primarily by the nerve injury, which weakens the antagonistic lateral rotators, but that direct injury to the muscle might be a contributory factor.