title: Quantitative histology and ultrastructure fail to explain weakness of immobilized rabbit Achilles' tendons.
abstract: OBJECTIVE
  To test the hypothesis that mechanical weakness caused by immobilization is the result of characteristic histologic and ultrastructural changes in rabbit Achilles' tendons.
  
  
  METHODS
  Single-blind, randomized controlled trial.
  
  
  METHODS
  University animal care facility.
  
  
  METHODS
  Twenty-three New Zealand rabbits.
  
  
  METHODS
  Twenty weight-matched rabbits underwent unilateral hind leg immobilization. The Achilles' tendons of immobilized and contralateral legs were harvested after 4 or 8 weeks. For ultrastructural outcomes, 6 normal Achilles' tendons of 3 rabbits served as controls.
  
  
  METHODS
  Light microscopic assessments were made on the tendons for cross-sectional area of the tendon, number of tenocytes, adipocytes and blood vessels, roundness of nuclei, area of intense metachromasia, and area of spatially aligned collagen fibers. Transmission electron microscopy (TEM) measured mean collagen fibril diameter and density.
  
  
  RESULTS
  Light microscopic assessment failed to reveal a statistical difference in any of the outcome measures between immobilized and contralateral tendons. TEM did not show a statistical difference in mean fibril diameter between the immobilized groups (4 wk, 113.8+/-1.6 nm; 8 wk, 113.5+/-1.4 nm) compared with their respective contralateral tendons (4 wk, 111.4+/-1.4 nm; 8 wk, 116.2+/-1.8 nm) and normal controls (111.8+/-2.0 nm). Eight-week contralateral fibrils were statistically larger than 4-week contralateral fibrils, which was attributed to a training effect of the leg opposite of the casted leg.
  
  
  CONCLUSIONS
  Our results add to the recent literature about the absence of characteristic histologic or collagen fibril size markers that could explain the mechanical weakness of immobilized tendons. Further research using biochemical, gene expression, and functional imaging markers of tendon is needed to pinpoint the alterations responsible for the mechanical weakness. Such markers would be crucial for the rehabilitation and secondary prevention of tendon injuries.
authors.affiliation: Bone and Joint Research Laboratory, University of Ottawa, Ottawa, ON, Canada.
authors.forename: Jian
authors.initials: J
authors.lastname: Zhou
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Koike
authors.forename: Hans K
authors.initials: HK
authors.lastname: Uhthoff
authors.forename: Guy
authors.initials: G
authors.lastname: Trudel
mesh_terms: D000125:Achilles Tendon / Q000648:ultrastructure*; D000818:Animals; D002452:Cell Count; D003094:Collagen / Q000648:ultrastructure; D004195:Disease Models, Animal; D005260:Female; D005500:Follow-Up Studies; D018476:Hypokinesia / Q000473:pathology*; D046529:Microscopy, Electron, Transmission; D011817:Rabbits; D012149:Restraint, Physical