Background: Injury to the brachial plexus during birth results in paralysis of the upper extremity in as many as one in 250 births and can lead to substantial functional deficits in the shoulder. The goal of this study was to characterize the development of bone and joint deformities in paralyzed neonatal shoulders and to assess the improvement of these deformities after muscle function recovery with use of an animal model.
Methods: Intramuscular injections of botulinum toxin were used to paralyze the supraspinatus, infraspinatus, and posterior deltoid of the left shoulders of mice at birth. Seventy mice were divided into three groups: Botox, recovery, and normal. The twenty-five mice in the Botox group received botulinum toxin injections until they were killed. The twenty mice in the recovery group received botulinum toxin injections for different durations and then were allowed injection-free recovery periods until they were killed. The twenty-five mice in the normal group received saline solution injections until they were killed. Radiographs were used to measure shoulder and elbow contractures. Microcomputed tomography was used to examine anatomical parameters of the supraspinatus muscle, humerus, and scapula.
Results: The Botox group showed bone and joint deformities including delayed mineralization and flattening of the humeral head, hypoplasia, and introversion (i.e., anteversion) of the humerus, contractures of the shoulder and elbow, hypoplasia of shoulder muscles, hypoplasia of the scapula, and hypoplasia and retroversion of the glenoid. In the recovery group, a significant trend toward normal properties was observed with longer recovery periods (p < 0.05). However, only soft-tissue contractures of the shoulder and elbow were resolved completely with the longest recovery period.
Conclusions: This mouse model successfully simulates human neonatal brachial plexus palsy, reproducing most of the bone and joint deformities found in the human condition. The deformities started to develop early in the postnatal period in the paralyzed shoulders and progressed with longer durations of paralysis. Early restoration of muscle function completely resolved the soft-tissue contractures of the shoulder and elbow. However, osseous deformities of the humerus and scapula were never resolved completely. These findings demonstrate the time-dependence of reversibility of musculoskeletal deformities in developing shoulders with neurological deficits.
Clinical Relevance: The information in this study provides the basis for further study of the mechanisms and factors influencing upper-extremity musculoskeletal development.