The role of muscle imbalance in the pathogenesis of shoulder contracture after neonatal brachial plexus palsy: a study in a rat model Francisco Soldado, MD, PhD a, *, Cesar G. Fontecha, MD, PhD a , Mario Marotta, MD, PhD a , David Benito, MD b , Marcelo Casaccia, MD a , Vasco V. Mascarenhas, MD c , Dan Zlotolow, MD d , Scott H. Kozin, MD d a Pediatric Hand Surgery and Microsurgery, Vall Hebron Research Institute, Universitat Aut onoma de Barcelona, Barcelona, Spain b Orthopaedic Surgery Department, Fundaci o Hospital de l’Esperit Sant, Barcelona, Spain c UIME, Centro de Imagiologia, Hospital da Luz Lisboa, Lisbon, Portugal d Shriners Hospital for Children, Department of Orthopaedic Surgery, Temple University, Philadelphia, PA, USA Background: An internal rotation contracture of the shoulder is common after neonatal brachial plexus injuries due to subscapularis shortening and atrophy. It has been explained by 2 theories: muscle denerva- tion and muscle imbalance between the internal and external rotators of the shoulder. The goal of this study was to test the hypothesis that muscle imbalance alone could cause subscapularis changes and shoulder contracture. Materials and methods: We performed selective neurectomy of the suprascapular nerve in 15 newborn rats to denervate only the supraspinatus and the infraspinatus muscles, leaving the subscapularis muscle intact. After 4 weeks, passive shoulder external rotation was measured and a 7.2-T magnetic resonance imaging scan of the shoulders was used to determine changes in the infraspinatus and subscapularis mus- cles. The subscapularis muscle was weighed to determine the degree of mass loss. An additional group of 10 newborn rats was evaluated to determine the sectional muscle fiber size and muscle area of fibrosis by use of images from type I collagen immunostaining. Results: There was a significant decrease in passive shoulder external rotation, with a mean loss of 66  ; in the thickness of the denervated infraspinatus, with a mean loss of 40%; and in the thickness and weight of the non-denervated subscapularis, with mean losses of 28% and 25%, respectively. No differences were found in subscapularis muscle fiber size and area of fibrosis between shoulders after suprascapular nerve injury. Conclusions: Our study supports the theory that shoulder muscle imbalance is a cause of shoulder contrac- ture in patients with neonatal brachial plexus palsy. This study was approved by the Ethic Commission of Vall Hebron Research Institute (Act No. 24, Extraordinary Session, November 20, 2008). *Reprint requests: Francisco Soldado, MD, PhD, Vall Hebron Research Institute, Universitat Aut onoma de Barcelona, Passeig Vall Hebron 119- 129, 08035 Barcelona, Spain. E-mail address: drsoldado@cirugiamanoinfantil.com (F. Soldado). J Shoulder Elbow Surg (2014) 23, 1003-1009 www.elsevier.com/locate/ymse 1058-2746/$ - see front matter Ó 2014 Journal of Shoulder and Elbow Surgery Board of Trustees. http://dx.doi.org/10.1016/j.jse.2013.09.031