Staging of patellar tendon autograft healing after posterior cruciate ligament reconstruction. A biomechanical and histological study in a sheep model.

The prerequisite for adequate rehabilitation after cruciate ligament reconstruction with a patellar tendon autograft is a thorough knowledge of the biologic healing processes. It is of interest to determine whether distinct phases similar to those in wound healing can be differentiated. It is also important to assess the magnitude of the biomechanical loading capacity and the duration of the healing processes. One posterior cruciate ligament in each of 48 skeletally mature sheep was replaced with a free patellar tendon autograft. Immediate rehabilitation without immobilization followed. Four phases of healing were demonstrated using the histologic condition of the autograft as a guide. The biomechanical data were correlated with the morphologic data. During the necrotic phase, a maximum of necrotic tissue was seen two weeks postoperatively. The strength of the graft is initially limited by the surgical fixation strength; however, later the intraarticular portion becomes the strength-limiting factor. During the revitalization phase, which is characterized by revascularization and proliferation of fibroblasts, and during the following phase, collagen formation, an increase in maximum stress is seen while the elastic modulus remains constant. Only in the remodeling phase is an increase in elastic modulus seen, owing to longitudinal alignment of the collagen bundles. One year after implantation, the autograft achieves approximately 50% of the material properties of the control. Even after two years, the autograft reaches only a maximum stress of 60% and an elastic modulus of 70% of the control. Ligamentization of the autograft could not be demonstrated in this study, but degeneration was seen in the core region of the graft during late remodeling.