Cartilage repair by osteochondral graft in the knees of pigs - A histological study

Purpose: To investigate the mechanism of free autogenous osteochondral graft in full thickness cartilage defect in the knees of pigs aged three to four months old and weighing 8.5 to 11.5 kg, by histological examination. Methods: Full thickness cartilage defects were created on the medial femoral condyles of both knees. The defects on the right knee served as control study. The defects on the left knee were grafted with free autogenous osteochondral grafts. Biopsies were performed in 4, 8 and 12 weeks and the specimens were examined for predominant tissues, structure characteristics and cell changes as well as the healing of bone and cartilage between the graft and host tissue. Results: The control lesions showed extensive fibrous tissues with increased neovascularization, but no cartilage. With osteochondral grafts, bone remodeling took place in four weeks; and cartilage healing by direct bonding in approximately 12 weeks. There were transient degenerative changes of chondrocyte cellularity and morphology and increased neovascularization in four to eight weeks, which improved by 12 weeks. Conclusions: The mechanism of free autogenous osteochondral grafts started off with bone remodeling in as early as four weeks, which in turn provided better graft stability and promoted neovascularization and nutrition to the cartilage. The healing of hyaline cartilage by direct bonding to the host tissue took place in 12 weeks. There were transient interval changes of chondrocyte cellularity and morphology that warrant postoperative protection of the knees for 8 to 12 weeks.