title: Evaluation of in vivo rotator cuff muscle function after acute and chronic detachment of the supraspinatus tendon: an experimental study in an animal model.
abstract: BACKGROUND
  Surgical repair of large chronic rotator cuff tears can be technically demanding because it requires manipulation of a muscle-tendon unit that is scarred, retracted, and stiffer than normal, all of which contribute to increased tension at the repair site. The purpose of the present study was to characterize the in vivo rotator cuff muscle-tendon unit function after acute and chronic injury at surgically relevant preload tensions.
  
  
  METHODS
  Sixty-two Sprague-Dawley rats were divided into a healthy, uninjured (control) group (n = 22), an acute injury group (n = 20), and a chronic injury group (n = 20) and underwent in vivo muscle force testing and electromyographic testing of the supraspinatus muscle-tendon unit at various preload tensions.
  
  
  RESULTS
  Preload tension affected the maximum supraspinatus muscle contractile force in all groups (p < 0.05). At the peak tension required to repair an acute tear, there was a 28% to 30% reduction in maximum tetanic contraction amplitude in all groups (p < 0.05). At the peak tension required to repair a chronic tear, there was a 40% to 53% reduction in maximal tetanic contraction amplitude in all groups (p < 0.05). The uninjured (control) group showed increased muscle endurance (p < 0.05) in comparison with the acute injury and chronic injury groups at all preload tensions. The chronic injury group showed reduced compound motor action potential amplitude (p < 0.05).
  
  
  CONCLUSIONS
  Both the acute and chronic injury groups demonstrated functional impairment related to increasing preload tensions. Higher repair tensions, associated with the chronic injury setting, resulted in greater functional impairment. The present study also demonstrates an association between increased time from rotator cuff tendon injury and impaired in vivo rotator cuff muscle electromyographic findings.
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1070, USA. smannava@wakehealth.edu
authors.forename: Sandeep
authors.initials: S
authors.lastname: Mannava
authors.forename: Johannes F
authors.initials: JF
authors.lastname: Plate
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Whitlock
authors.forename: Michael F
authors.initials: MF
authors.lastname: Callahan
authors.forename: Thorsten M
authors.initials: TM
authors.lastname: Seyler
authors.forename: L Andrew
authors.initials: LA
authors.lastname: Koman
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Smith
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Tuohy
mesh_terms: D000818:Animals; D004576:Electromyography; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017006:Rotator Cuff / Q000473:pathology / Q000502:physiology*; D000070636:Rotator Cuff Injuries; D013708:Tendon Injuries / Q000503:physiopathology*; D013710:Tendons / Q000473:pathology / Q000502:physiology*