Meniscus transplantation using a cryopreserved allograft. Histological and ultrastructural study of the transplanted meniscus.

We performed allogenic meniscus transplantation in rabbits and investigated the regenerative process in the transplanted meniscus with particular respect to its ultra-structure. Under a transmission electron microscope, the chondrocytes and extracellular matrix were degenerated or destroyed in week 0. Fibroblasts were observed 4-8 weeks postoperatively. Immature chondrocytes emerged at 12 postoperative weeks. At 16 weeks, the newly formed chondrocytes and extracellular matrix were virtually normal. Although the diameter of collagen fibrils still varied, their arrangement was dense and the filaments had almost normal banding. This study demonstrated that newly formed chondrocytes may differentiate from fibroblasts 8-12 weeks after transplantation. The fibroblasts synthesized collagen fiber and glycosaminoglycan, indicating that repair of the extracellular matrix may be initiated 4-8 weeks postoperatively, becoming were active after 12 weeks when the newly formed chondrocytes emerge. The extracellular matrix is injured during rapid freezing, cryopreservation, and slow thawing, and repair is delayed due to this deterioration. The extracellular matrix thus needs to be protected from injury as far as possible in order to promote its repair, since it normalizes more slowly than chondrocytes.