title: The role of muscle imbalance in the pathogenesis of shoulder contracture after neonatal brachial plexus palsy: a study in a rat model.
abstract: 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 denervation 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.
  
  
  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 muscles. 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 contracture in patients with neonatal brachial plexus palsy.
authors.affiliation: Pediatric Hand Surgery and Microsurgery, Vall Hebron Research Institute, Universitat Autònoma de Barcelona, Barcelona, Spain. Electronic address: drsoldado@cirugiamanoinfantil.com.
authors.forename: Francisco
authors.initials: F
authors.lastname: Soldado
authors.affiliation: Pediatric Hand Surgery and Microsurgery, Vall Hebron Research Institute, Universitat Autònoma de Barcelona, Barcelona, Spain.
authors.forename: Cesar G
authors.initials: CG
authors.lastname: Fontecha
authors.affiliation: Pediatric Hand Surgery and Microsurgery, Vall Hebron Research Institute, Universitat Autònoma de Barcelona, Barcelona, Spain.
authors.forename: Mario
authors.initials: M
authors.lastname: Marotta
authors.affiliation: Orthopaedic Surgery Department, Fundació Hospital de l'Esperit Sant, Barcelona, Spain.
authors.forename: David
authors.initials: D
authors.lastname: Benito
authors.affiliation: Pediatric Hand Surgery and Microsurgery, Vall Hebron Research Institute, Universitat Autònoma de Barcelona, Barcelona, Spain.
authors.forename: Marcelo
authors.initials: M
authors.lastname: Casaccia
authors.affiliation: UIME, Centro de Imagiologia, Hospital da Luz Lisboa, Lisbon, Portugal.
authors.forename: Vasco V
authors.initials: VV
authors.lastname: Mascarenhas
authors.affiliation: Shriners Hospital for Children, Department of Orthopaedic Surgery, Temple University, Philadelphia, PA, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Zlotolow
authors.affiliation: Shriners Hospital for Children, Department of Orthopaedic Surgery, Temple University, Philadelphia, PA, USA.
authors.forename: Scott H
authors.initials: SH
authors.lastname: Kozin
mesh_terms: D000818:Animals; D001720:Birth Injuries / Q000150:complications / Q000473:pathology / Q000503:physiopathology*; D001917:Brachial Plexus / Q000293:injuries; D020516:Brachial Plexus Neuropathies / Q000150:complications* / Q000473:pathology; D002675:Child, Preschool; D003286:Contracture / Q000209:etiology* / Q000473:pathology / Q000601:surgery; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000009:adverse effects; D008297:Male; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000294:innervation / Q000473:pathology; D009133:Muscular Atrophy / Q000503:physiopathology; D016059:Range of Motion, Articular; D051381:Rats; D017207:Rats, Sprague-Dawley; D012782:Shoulder / Q000294:innervation / Q000473:pathology; D012785:Shoulder Joint / Q000294:innervation / Q000473:pathology* / Q000601:surgery; D014102:Torsion Abnormality / Q000209:etiology / Q000503:physiopathology*
keywords: Neonatal brachial plexus palsy; glenohumeral dysplasia; muscle growth; muscle imbalance; shoulder contracture; subscapularis muscle