Our study evaluated tendon-to-bone healing in a dog model. Twenty adult mongrel dogs had a transplantation of the long digital extensor tendon into a 4.8-millimeter drill-hole in the proximal tibial metaphysis. Four dogs were killed at each of five time-periods (two, four, eight, twelve, and twenty-six weeks after the transplantation), and the histological and biomechanical characteristics of the tendon-bone interface were evaluated.
Serial histological analysis revealed progressive re-establishment of collagen-fiber continuity between the bone and the tendon. A layer of cellular, fibrous tissue was noted between the tendon and the bone, along the length of the bone tunnel; this layer progressively matured and reorganized during the healing process. The collagen fibers that attached the tendon to the bone resembled Sharpey fibers. High-resolution radiographs showed remodeling of the trabecular bone that surrounded the tendon.
At the two, four, and eight-week time-periods, all specimens had failed by pull-out of the tendon from the bone tunnel. The strength of the interface was noted to have significantly and progressively increased between the second and the twelfth week after the transplantation. At the twelve and twenty-six-week time-periods, all specimens had failed by pull-out of the tendon from the clamp or by mid-substance rupture of the tendon. The progressive increase in strength was correlated with the degree of bone ingrowth, mineralization, and maturation of the healing tissue, noted histologically.CLINICAL RELEVANCE
Since the site at which a graft is fixed to bone is, mechanically, the weakest area in the early post-transplant period, knowledge of the healing process (for example, of semitendinosus and gracilis tendon grafts in the reconstruction of the anterior cruciate ligament) will lead to a better understanding of how to improve the initial fixation of graft to bone. It also can aid in the identification of methods to improve the biological healing response of tendon grafts. In addition, the findings of the current study can help clinicians in the planning of early weight-bearing or range-of-motion rehabilitation modalities for patients who have had a reconstruction of a ligament and aid in the decision of when to remove an internal fixation device that has been used for the attachment of a graft. We recommend that a healing ligament be protected for at least eight weeks after a reconstruction.