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

Abstract

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.

The role of subscapularis muscle denervation in the pathogenesis of shoulder internal rotation contracture after neonatal brachial plexus palsy: A study in a rat model

“…Around 25% decrease in the thickness and weight of the non‐denervated subscapularis occurred without muscle fiber atrophy or fibrosis. These findings were similar to those found in subscapularis histology samples taken from children with GHD . The authors concluded that a muscle hypoplasia occurred as a result of rotator shoulder imbalance with lack of passive stretching in the absence of active shoulder external rotation …”

“…This could explain the differences with the subscapularis muscle histology findings in children with NBPP and glenohumeral dysplasia. The absence of muscle fiber atrophy in children could be rationalized by muscle reinnervation from C5‐C6 and preservation of innervation arising from C7 level …”

“…These findings were similar to those found in subscapularis histology samples taken from children with GHD. 7 The authors concluded that a muscle hypoplasia occurred as a result of rotator shoulder imbalance with lack of passive stretching in the absence of active shoulder external rotation. 5 In our study, a selective subscapularis muscle denervation produced a slower shoulder contracture as compared to the neural imbalance model 4 (Fig.…”

“…2,3,4 While the natural history of the changes in bone and articular alignment has been extensively studied, the pathogenesis of muscular changes preceding them remains unclear and is currently receiving more attention. [5][6][7]8,9,10 The mechanism leading to subscapularis muscle growth impairment with subsequent volume loss and contracture has been explained by two differing hypotheses: denervation 8,11,9,12 and muscle imbalance. 5,6,7,13,14,10 The imbalance theory infers that the subscapularis changes are secondary to the lack of passive stretching in the absence of active shoulder external rotation, which leads to progressive shortening and stiffness of subscapularis muscle due to an abnormal muscle growth or muscle hypoplasia.…”

“…[5][6][7]8,9,10 The mechanism leading to subscapularis muscle growth impairment with subsequent volume loss and contracture has been explained by two differing hypotheses: denervation 8,11,9,12 and muscle imbalance. 5,6,7,13,14,10 The imbalance theory infers that the subscapularis changes are secondary to the lack of passive stretching in the absence of active shoulder external rotation, which leads to progressive shortening and stiffness of subscapularis muscle due to an abnormal muscle growth or muscle hypoplasia. 5 The denervation hypothesis states that denervation of the subscapularis muscle causes progressive muscle fibrosis, shortening, and contracture.…”