The natural history of the anterior cruciate ligament autograft of patellar tendon origin.

A rabbit model for ACL reconstruction using autogenous patellar tendon (PT) was used to study graft viability, its response to new physical forces, and the intrasynovial milieu. The autograft was assessed grossly, histologically, and biochemically with respect to time. Histologic observations demonstrated that autografts were centrally acellular with a peripheral rim of cells at 2 weeks, and had a central focal proliferation of cells at 3 weeks and cellular homogeneity by 4 weeks postoperation. Necrosis followed by cellular proliferation suggested that a population of cells other than the native PT fibroblasts may be inhabiting the graft. Graft sequestration experiments demonstrated that autografts are repopulated by cells of extrinsic origin after transplantation. Autografts showed a gradual assumption of the microscopic properties of normal ACL; by 30 weeks posttransplant the tissue characteristics were ligamentous in appearance. Histologic changes paralleled the biochemical metamorphosis: Type III collagen was not observed in PT; however, a gradual increase in its concentration was seen in the grafts; by 30 weeks its concentration (10%) was the same as in normal ACL. Similarly, glycosaminoglycan content increased from its normally low level in PT to that found in native ACL. Collagen-reducible cross-link analysis revealed that grafted tissue changed from the normal PT pattern of low dihydroxylysinonorleucine and high histidinohydroxymerodesmosine to the opposite pattern seen in normal ACL by 30 weeks. These data suggest that PT autografts undergo a process of "ligamentization" when placed in the ACL environment, and that cells responsible for this metamorphosis are of extragraft origin.