title: Changes in rotator cuff muscle volume, fat content, and passive mechanics after chronic detachment in a canine model.
abstract: BACKGROUND
  Long-standing tears of the rotator cuff can lead to substantial and perhaps irreversible changes in the affected rotator cuff muscles. We developed a chronic rotator cuff tear in a canine model to investigate and quantify the time-related changes in passive mechanics, volume, and fat of the infraspinatus muscle. We hypothesized that infraspinatus muscle stiffness would increase, volume would decrease, and fat content would increase at twelve weeks following tendon detachment.
  
  
  METHODS
  The right infraspinatus tendon of eight adult mongrel dogs were surgically detached from the proximal part of the humerus. The uninvolved left shoulder served as a control. Muscle volume changes were quantified with use of magnetic resonance imaging. At twelve weeks, the passive mechanical properties of the chronically detached and control muscles were determined intraoperatively with use of a custom-designed device. Intramuscular fat was evaluated histologically at the time that the animals were killed.
  
  
  RESULTS
  After twelve weeks of detachment, the stiffness was significantly increased in the detached infraspinatus muscles relative to that in the controls (p < 0.0001). Magnetic resonance image analysis demonstrated that the detached muscle volumes decreased by an average of 32% in the first six weeks and remained constant thereafter. Intramuscular fat increased significantly in the detached muscles and to a greater extent in the lateral regions (p < 0.05).
  
  
  CONCLUSIONS
  The chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. The detached muscle becomes stiffer, and the passive loads required to repair it can become excessive. A significant reduction in muscle volume occurs within days to weeks following tendon detachment (p < 0.0001). The nonuniformity of changes in muscle fat suggests that fat content should be used cautiously as an indicator of muscle quality.
authors.affiliation: Department of Orthopaedic Surgery and the Orthopaedic Research Center (O.S., K.A.D., K.P., and J.P.I.) and the Department of Biomedical Engineering (K.A.D., K.P., and J.P.I.), The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195. iannotj@ccf.org.
authors.forename: Ori
authors.initials: O
authors.lastname: Safran
authors.forename: Kathleen A
authors.initials: KA
authors.lastname: Derwin
authors.forename: Kimerly
authors.initials: K
authors.lastname: Powell
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Iannotti
mesh_terms: D000273:Adipose Tissue / Q000473:pathology; D000818:Animals; D001696:Biomechanical Phenomena; D002908:Chronic Disease; D004285:Dogs; D005260:Female; D008297:Male; D018482:Muscle, Skeletal / Q000293:injuries* / Q000503:physiopathology; D009135:Muscular Diseases / Q000209:etiology / Q000473:pathology / Q000503:physiopathology*; D017006:Rotator Cuff / Q000503:physiopathology; D000070636:Rotator Cuff Injuries; D013708:Tendon Injuries / Q000150:complications / Q000503:physiopathology*