Background: Modulation of the mechanical environment may profoundly affect the healing tendon graft-bone interface.

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Given the relatively high rate of failure after rotator cuff and other tendon-to-bone repairs, identification of optimal rehabilitation programs postoperatively is an important research goal.

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The optimal mechanical loading regimen for the healing of a tendon graft in a bone tunnel is unknown. We developed a rat model that directly tensions a healing tendon graft, without the use of confounding joint motion. Fifty cycles of either 0, 3, or 6 N of tension were applied to groups daily for 3 or 6 weeks. At 3 weeks the low load (3 N) group had the highest failure load (p ¼ 0.009), but by 6 weeks there were no differences in failure load among groups. At 3 weeks the high load (6 N) group had greater osteoclast activity compared to the immobilized (0 N) group (p < 0.05), and by 6 weeks there were significantly more osteoclasts in the high load group compared to the low load group (p ¼ 0.01). Bone volume fraction was higher in the immobilized group compared to the 3 N load group at 3 weeks (p ¼ 0.014) and 6 weeks (p ¼ 0.007). At 6 weeks, the immobilized group had greater trabecular number compared to both loading groups (p < 0.05). In conclusion, low magnitude loading had a beneficial early effect but continued loading led to poorer new bone formation over time and no beneficial effect at 6 weeks, perhaps due to delayed maturation from cumulative loads. Keywords: tendon healing; tensile load; bone tunnel; osteoclasts; bone ingrowth Successful ACL reconstruction with the use of soft-tissue grafts requires healing between tendon and bone. The native ligament insertion site has a complex structure and composition, with a gradual transition from bone to fibrocartilage and then ligament/tendon, and a corresponding gradation in material properties. 1 The microstructure of the native insertion site is not re-established following surgical attachment of tendon to bone, resulting in poor attachment strength and recurrent laxity in some patients. 2 Both biologic and biomechanical factors affect healing between soft tissue and bone.The mechanical environment at the healing tendon graft-bone interface is affected by joint motion, and understanding how the mechanical environment influences healing has critical implications for the design of postoperative rehabilitation protocols following ACL reconstruction. Previous work in our laboratory using a rat ACL reconstruction model demonstrates that tensile load affects healing at the tendon-bone interface. 3 We found that immediate application of load post-operatively had a detrimental effect on healing when compared to delayed onset of loading. 4 A limitation of these studies is that we applied axial load across the joint via external fixation pins in the femur and tibia, and compliance of the pins limited the ability to apply high levels of strain. Also, tension in the joint capsule and collateral ligaments could inhibit the ability to apply tension to the ACL graft. Lastly, loading of the ACL graft was dependent upon secure graft fixation to bone, which is challenging in this small animal model.In order to address these limitations, we have developed a new model whereby we can directly apply load to a tendon graft in a bone tunnel. We used this model to test the ...