title: Chronic Degeneration Leads to Poor Healing of Repaired Massive Rotator Cuff Tears in Rats.
abstract: BACKGROUND
  Chronic rotator cuff tears present a clinical challenge, often with poor outcomes after surgical repair. Degenerative changes to the muscle, tendon, and bone are thought to hinder healing after surgical repair; additionally, the ability to overcome degenerative changes after surgical repair remains unclear.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate healing outcomes of muscle, tendon, and bone after tendon repair in a model of chronic rotator cuff disease and to compare these outcomes to those of acute rotator cuff injuries and repair. The hypothesis was that degenerative rotator cuff changes associated with chronic multitendon tears and muscle unloading would lead to poor structural and mechanical outcomes after repair compared with acute injuries and repair.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  Chronic rotator cuff injuries, induced via detachment of the supraspinatus (SS) and infraspinatus (IS) tendons and injection of botulinum toxin A into the SS and IS muscle bellies, were created in the shoulders of rats. After 8 weeks of injury, tendons were surgically reattached to the humeral head, and an acute, dual-tendon injury and repair was performed on the contralateral side. After 8 weeks of healing, muscles were examined histologically, and tendon-to-bone samples were examined microscopically, histologically, and biomechanically and via micro-computed tomography.
  
  
  RESULTS
  All repairs were intact at the time of dissection, with no evidence of gapping or ruptures. Tendon-to-bone healing after repair in our chronic injury model led to reduced bone quality and morphological disorganization at the repair site compared with acute injuries and repair. SS and IS muscles were atrophic at 8 weeks after repair of chronic injuries, indicating incomplete recovery after repair, whereas SS and IS muscles exhibited less atrophy and degeneration in the acute injury group at 8 weeks after repair. After chronic injuries and repair, humeral heads had decreased total mineral density and an altered trabecular structure, and the repair had decreased strength, stiffness, and toughness, compared with the acute injury and repair group.
  
  
  CONCLUSIONS
  Chronic degenerative changes in rotator cuff muscles, tendons, and bone led to inferior healing characteristics after repair compared with acute injuries and repair. The changes were not reversible after repair in the time course studied, consistent with clinical impressions.
  
  
  CONCLUSIONS
  High retear rates after rotator cuff repair are associated with tear size and chronicity. Understanding the mechanisms behind this association may allow for targeted tissue therapy for tissue degeneration that occurs in the setting of chronic tears.
authors: Department of Orthopaedic Surgery, Washington University, St Louis, Missouri, USA.
authors: Megan L
authors: ML
authors: Killian
authors: Department of Orthopaedic Surgery, Washington University, St Louis, Missouri, USA Orthopaedics and Traumatology, University of São Paulo, São Paulo, Brazil.
authors: Leonardo M
authors: LM
authors: Cavinatto
authors: Department of Radiology, University of California, San Diego, La Jolla, California, USA Department of Orthopaedic Surgery, University of California, San Diego, La Jolla, California, USA Department of Bioengineering, University of California, San Diego, La Jolla, California, USA.
authors: Samuel R
authors: SR
authors: Ward
authors: John Cochran Division, St Louis Veterans Affairs Medical Center, St Louis, Missouri, USA.
authors: Necat
authors: N
authors: Havlioglu
authors: Department of Orthopaedic Surgery, Washington University, St Louis, Missouri, USA ThomopoulosS@wudosis.wustl.edu galatzl@wudosis.wustl.edu.
authors: Stavros
authors: S
authors: Thomopoulos
authors: Department of Orthopaedic Surgery, Washington University, St Louis, Missouri, USA ThomopoulosS@wudosis.wustl.edu galatzl@wudosis.wustl.edu.
authors: Leesa M
authors: LM
authors: Galatz
mesh_terms: D000818:Animals; D002908:Chronic Disease; D004195:Disease Models, Animal; D008297:Male; D009133:Muscular Atrophy / Q000150:complications / Q000175:diagnosis*; D019637:Orthopedic Procedures / Q000379:methods*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017006:Rotator Cuff / Q000601:surgery*; D000070636:Rotator Cuff Injuries; D012421:Rupture; D013708:Tendon Injuries / Q000150:complications / Q000175:diagnosis / Q000601:surgery*; D014945:Wound Healing; D055114:X-Ray Microtomography
keywords: fatty degeneration; rotator cuff; rotator cuff repair; shoulder