WEDNESDAY, March 16 (HealthDay News) -- Impaired growth of biceps and brachialis muscles causes elbow flexion contractures, and impaired subscapularis muscle growth causes shoulder internal rotation contracture following brachial plexus injuries in neonatal mice, according to research published in the March 2 issue of The Journal of Bone & Joint Surgery.
Sia Nikolaou, Ph.D., from the Cincinnati Children's Hospital Medical Center, and colleagues excised the supraclavicular C5-C6 nerve root on neonatal mice models, causing unilateral brachial plexus injuries, to determine the role of denervated muscle in contracture pathogenesis. Over the next four weeks, the functional length, fibrosis, and cross-sectional area of the biceps, brachialis, and subscapularis muscles were measured. Myogenic capability was assessed by culturing muscle satellite cells from denervated and control biceps muscles. Shoulder and elbow motion range was evaluated after four weeks.
The investigators found that, within four weeks of the injury, elbow flexion and shoulder internal rotation contractures developed on the involved side. Elbow flexion contractures were relieved by excision of the biceps and brachialis. Involved biceps and brachialis showed reduced cross-sectional and longitudinal growth, but they still had myogenically capable satellite cells. Histologically, the biceps were fibrotic and fatty infiltration was seen in the brachialis and rotator cuff muscles. Reduced longitudinal growth of upper subscapularis resulting in internal rotation contracture was observed only after brachial plexus injury and not after excision of the external rotators.
"Impaired growth of neonatally denervated muscle may be at least in part responsible for loss of passive joint range of motion following neonatal brachial plexus injury," the authors write.