Effect of Immediate and Delayed High-Strain Loading on Tendon-to-Bone Healing After Anterior Cruciate Ligament Reconstruction

The healing of tendon to bone does not recapitulate the native architecture of the enthesis. Rather, an interposed layer of fibrovascular scar tissue is formed that is biomechanically inferior to the native enthesis and that may predispose tendon-to-bone repairs to an increased rate of failure1-7. While anterior cruciate ligament (ACL) reconstruction generally has favorable results8, incomplete graft-bone healing can lead to graft slippage, resulting in laxity and knee instability. Recent studies have identified unsatisfactory results in up to 25% of patients secondary to residual laxity and the persistence of a postoperative pivot shift9,10. Although many factors contribute to the long-term outcome of ACL reconstruction, secure tendon-to-bone healing is required for a functional graft4,6. There is growing evidence that modulation of the mechanical environment has a critical effect on the healing graft attachment site and its ultimate mechanical integrity11-14. In previous animal studies, our group demonstrated that the delayed onset of low levels of controlled mechanical stimulation (∼2% cyclic axial strain) resulted in improved mechanical and biological parameters of tendon-to-bone healing compared with those found after immediate loading or strict immobilization14. However, the magnitude of mechanical stimulation required for optimal healing of the tendon-bone insertion site is currently undefined. A recent study revealed that the delayed application of postoperative exercise (with presumed higher levels of mechanical load) resulted in a decreased range of motion and worse mechanical properties, compared with those seen after normal cage activity, in a rat rotator cuff repair model15. However, the mechanical load was not directly controlled or quantified. Both the timing and the intensity of postoperative rehabilitation after ACL reconstruction may have important consequences with respect to the healing graft as well as the long-term outcomes of the operation. The purpose of the present study was to determine in a rat model the effect of high levels of controlled axial loading after ACL reconstruction on tendon-to-bone healing. Our hypothesis was that the delayed onset of loading would lead to improved healing (new-bone formation at the tendon-bone interface) and increased graft load to failure compared with those found after either immediate loading or prolonged immobilization.