title: Effect of short-duration low-magnitude cyclic loading versus immobilization on tendon-bone healing after ACL reconstruction in a rat model.
abstract: BACKGROUND
  Successful anterior cruciate ligament reconstruction with use of soft-tissue grafts requires healing between tendon and bone. Little is known about the effect of mechanical load on the cellular and molecular cascade of tendon-to-bone healing. Understanding these mechanical influences has critical implications for postoperative rehabilitation following anterior cruciate ligament reconstruction. The purpose of this study was to test the hypothesis that, compared with perioperative immobilization, short-duration low-magnitude cyclic axial loading would result in impaired tendon-to-bone healing after anterior cruciate ligament reconstruction in a rat model.
  
  
  METHODS
  Fifty-two male Sprague-Dawley rats underwent anterior cruciate ligament reconstruction with use of a flexor digitorum longus autograft. The patellar tendon, capsule, and ligamentous structures were circumferentially released, and an external fixator parallel to the anterior cruciate ligament graft was placed across the knee. Mechanical loading, consisting of cyclic displacement of the femur and tibia constrained to axial translation parallel to the graft, was applied daily. The rats were randomly assigned to immobilization or daily loading, for fourteen or twenty-eight days. Biomechanical, micro-computed tomographic, and histomorphometric analysis was performed on the bone-tendon-bone complexes.
  
  
  RESULTS
  The load measured across the knees during cyclic displacement increased over time (p < 0.05). Load-to-failure testing of the isolated femur-anterior cruciate ligament graft-tibia specimens revealed no significant differences between groups at two or four weeks. By two weeks postoperatively, a greater number of ED1+ inflammatory macrophages (phagocytic cells involved in the initial injury response) were seen at the tendon-bone interface after loading in the cyclically loaded group than in the immobilized group (p = 0.01). Compared with the baseline values, the number of trabeculae was significantly lower after loading for four weeks (p = 0.02).
  
  
  CONCLUSIONS
  Short-duration low-magnitude cyclic axial loading of the anterior cruciate ligament graft in the postoperative period is not detrimental to the strength of the healing tendon-bone interface but appears to be associated with greater inflammation and less bone formation in the tunnel in this rat model.
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, New York, NY, USA. brophyr@wudosis.wustl.edu
authors.forename: Robert H
authors.initials: RH
authors.lastname: Brophy
authors.forename: David
authors.initials: D
authors.lastname: Kovacevic
authors.forename: Carl W
authors.initials: CW
authors.lastname: Imhauser
authors.forename: Mark
authors.initials: M
authors.lastname: Stasiak
authors.forename: Asheesh
authors.initials: A
authors.lastname: Bedi
authors.forename: Alice J S
authors.initials: AJ
authors.lastname: Fox
authors.forename: Xiang-Hua
authors.initials: XH
authors.lastname: Deng
authors.forename: Scott A
authors.initials: SA
authors.lastname: Rodeo
mesh_terms: D000818:Animals; D016118:Anterior Cruciate Ligament / Q000473:pathology / Q000503:physiopathology* / Q000601:surgery*; D001696:Biomechanical Phenomena; D004730:Endothelium, Vascular / Q000473:pathology / Q000503:physiopathology; D016267:External Fixators; D005269:Femur / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D007103:Immobilization; D008264:Macrophages / Q000473:pathology / Q000502:physiology; D008297:Male; D010006:Osteoblasts / Q000473:pathology / Q000502:physiology; D010010:Osteoclasts / Q000473:pathology / Q000502:physiology; D011475:Prosthesis Failure; D051381:Rats; D017207:Rats, Sprague-Dawley; D013710:Tendons / Q000473:pathology / Q000637:transplantation*; D013718:Tensile Strength; D013977:Tibia / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D016474:Weight-Bearing / Q000502:physiology*; D014945:Wound Healing / Q000502:physiology*; D055114:X-Ray Microtomography