Healing of the patellar tendon autograft after posterior cruciate ligament reconstruction— a process of ligamentization?

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Abstract
Forty-eight skeletally mature sheep underwent poste rior cruciate ligament reconstruction with free patellar tendon autografts in one knee; the contralateral knee served as a control. Immediate rehabilitation without immobilization followed. Autograft healing was evalu ated by histologic, roentgenographic, and biomechani cal techniques up to 2 years postoperatively. After implantation, the autograft tissue underwent necrosis and degeneration, followed by a gradual healing proc ess comprising revascularization, cellular migration, and formation of an extracellular matrix. The autograft bone pegs were osseously incorporated by 6 weeks. After an initial loss of strength, the material properties of the operated knee recovered to only about one-third that of the control. Better alignment of the collagen fiber bundles resulted in increased material properties, up to approximately 50% of the control at 52 weeks. After 2 years, the autograft tissue was found to differ structur ally and mechanically from a ligament, suggesting that the autograft may never approach normal ligament characteristics. Degenerative alterations, the wide spread presence of type III collagen, and abnormal accumulations of glycosaminoglycans in the autograft correlated with a maximum stress of 60% and an elastic modulus of 70% of the control. Although ligamentiza tion was not seen, the staging of autograft healing into different phases based on distinct morphologic mani festations (necrosis, revitalization, collagen formation, and remodeling) and correlating with changing mechan ical properties may provide a rationale for rehabilitation protocols with a realistic evaluation of the loading ca pacity of the replacement tissue.