title: Effect of immediate and delayed high-strain loading on tendon-to-bone healing after anterior cruciate ligament reconstruction.
abstract: BACKGROUND
  We previously demonstrated, in a rat anterior cruciate ligament (ACL) graft reconstruction model, that the delayed application of low-magnitude-strain loading resulted in improved tendon-to-bone healing compared with that observed after immediate loading and after prolonged immobilization. The purpose of this study was to determine the effect of higher levels of strain loading on tendon-to-bone healing.
  
  
  METHODS
  ACL reconstruction was carried out in a rat model in three randomly assigned groups: high-strain daily loading beginning on either (1) postoperative day one (immediate-loading group; n = 7) or (2) postoperative day four (delayed-loading group; n = 11) or (3) after prolonged immobilization (immobilized group; n = 8). Animals were killed two weeks after surgery and micro-computed tomography (micro-CT) and biomechanical testing of the bone-tendon-bone complex were carried out.
  
  
  RESULTS
  The delayed-loading group had greater tissue mineral density than either the immediate-loading or immobilized group (mean [and standard deviation], 813.0 ± 24.9 mg/mL compared with 778.4 ± 32.6 mg/mL and 784.9 ± 26.4 mg/mL, respectively; p &lt; 0.05). There was a trend toward greater bone volume per total volume fraction in both the immobilized and the delayed-loading group compared with the immediate-loading group (0.24 ± 0.03 and 0.23 ± 0.06 compared with 0.20 ± 0.05; p = 0.06). Trabecular thickness was greater in the immobilized group compared with the immediate-loading group (106.5 ± 23.0 μm compared with 72.6 ± 10.6 μm; p &lt; 0.01). There were no significant differences in failure load or stiffness between the immobilized group and either high-strain cyclic-loading group.
  
  
  CONCLUSIONS
  Immediate application of high-strain loading appears to have a detrimental effect on healing in this rat model. Any beneficial effects of delayed loading on the healing tendon-bone interface (after a brief period of immobilization) may be offset by the detrimental effects of excessive strain levels or by the detrimental effects of stress deprivation on the graft.
  
  
  CONCLUSIONS
  The timing and magnitude of mechanical load on a healing rat ACL reconstruction graft may have important implications for postoperative rehabilitation. Avoidance of exercises that cause high graft strain in the early postoperative period may lead to improved tendon-to-bone healing in humans.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, Yale Physicians Building, 800 Howard Avenue, Floor 1, New Haven, CT 06519.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Packer
authors.affiliation: Department of Orthopaedic Surgery, University of Michigan, 24 Frank Lloyd Wright Drive, Lobby A, Ann Arbor, MI 48106.
authors.forename: Asheesh
authors.initials: A
authors.lastname: Bedi
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Alice J
authors.initials: AJ
authors.lastname: Fox
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Selom
authors.initials: S
authors.lastname: Gasinu
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Carl W
authors.initials: CW
authors.lastname: Imhauser
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Mark
authors.initials: M
authors.lastname: Stasiak
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Xiang-Hua
authors.initials: XH
authors.lastname: Deng
authors.affiliation: Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for S.A. Rodeo: rodeos@hss.edu.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Rodeo
mesh_terms: D000818:Animals; D059549:Anterior Cruciate Ligament Reconstruction; D001842:Bone and Bones / Q000502:physiology*; D004195:Disease Models, Animal; D006614:Hindlimb / Q000502:physiology; D008297:Male; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D013314:Stress, Mechanical; D013710:Tendons / Q000502:physiology*; D013718:Tensile Strength / Q000502:physiology; D013997:Time Factors; D014945:Wound Healing / Q000502:physiology*