title: Amplitude and strength of muscle contraction are reduced in experimental tears of the rotator cuff.
abstract: BACKGROUND
  Chronic tendon tears lead to retraction, fatty infiltration, and atrophy of the respective muscle. These muscle changes are decision-making criteria in rotator cuff tear management.
  
  
  OBJECTIVE
  To investigate the functional implications of these morphological changes in a sheep rotator cuff tear model.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  The authors established chronic retraction of the musculotendinous unit accompanied with fatty infiltration and atrophy of the infraspinatus muscle in 20 sheep. The contractile force and passive tension of the muscle as a function of its length were measured and the active work capacity determined.
  
  
  RESULTS
  After tendon release and chronic retraction (by 5.7 ± 0.9 cm), fatty infiltrated and atrophied infraspinatus muscles (with a density of 22.4 ± 10.4 Hounsfield units [HU] and a cross-sectional area of 65% ± 16% of the contralateral control side) had a mean contractile amplitude and strength of 2.7 ± 0.4 cm and 235 ± 71 N compared with the contralateral control shoulder of 4.1 ± 0.7 cm and 485 ± 78 N (P < .05), respectively. The mean active work of the muscle was 2.8 ± 0.9 N·m for retracted and 8.8 ± 2.4 N·m for control muscles (P < .05). The correlation of total active work to fatty infiltration (r = 0.78, P < .001) was significant.
  
  
  CONCLUSIONS
  Chronic tendon tears are associated not only with retraction, fatty infiltration, and atrophy but also with loss of strength and contractile amplitude. The functional changes can only indirectly and approximately be predicted by computed tomography imaging findings.
  
  
  CONCLUSIONS
  The current criteria (atrophy, retraction, and fatty infiltration) may help to quantify the structural reparability of a chronically retracted musculotendinous unit after rotator cuff tendon tear but may only approximately predict the remaining function of the muscle.
authors.affiliation: Balgrist University Hospital, University of Zürich, Zürich, Switzerland.
authors.forename: Dominik C
authors.initials: DC
authors.lastname: Meyer
authors.forename: Christian
authors.initials: C
authors.lastname: Gerber
authors.forename: Brigitte
authors.initials: B
authors.lastname: Von Rechenberg
authors.forename: Stephan H
authors.initials: SH
authors.lastname: Wirth
authors.forename: Mazda
authors.initials: M
authors.lastname: Farshad
mesh_terms: D000273:Adipose Tissue / Q000473:pathology; D000818:Animals; D002908:Chronic Disease; D023421:Models, Animal; D009119:Muscle Contraction / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology*; D009133:Muscular Atrophy / Q000209:etiology / Q000503:physiopathology; D017006:Rotator Cuff / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology*; D000070636:Rotator Cuff Injuries; D012756:Sheep; D014057:Tomography, X-Ray Computed