Articular cartilage is a load-bearing tissue covering the ends of long bones that normally allows for painless motion of joints. Adult articular cartilage has limited capacity for self-repair when injured. Current surgical treatments for cartilage defects include the transplantation of fresh osteochondral tissue from autologous or allogeneic sources to restore the architecture and characteristics of native tissue at the defect site. Such treatments have a successful clinical history but are limited by tissue availability. Osteochondral tissue engineering aims to fabricate osteochondral grafts in vitro but faces the challenge of integrating the cartilaginous and osseous layers to form a mechanically functional unit. The native osteochondral interface consists of a zone of calcified cartilage (ZCC) that anchors articular cartilage to subchondral bone through an interdigitated zone of intermediate stiffness. Thus, the overall goal of this dissertation was to form a functional osteochondral tissue graft in vitro by integrating articular cartilage to bone through a biomimetic transitional zone of calcified cartilage.

The ZCC at the human osteochondral interface was found to have increased hydraulic conductance, or ease of fluid transport, and undergo changes in thickness and vascularity with increasing stages of osteoarthritis, suggesting that the ZCC is a zone of active tissue remodeling during cartilage repair. Cartilage explants were used to investigate conditions for in vitro calcification of cartilage and the resulting functional biomechanical consequences of matrix calcification. The in vitro calcification of immature articular cartilage in medium supplemented with β-glycerophosphate was dependent on cell viability and occurred only in the deep zone, resulting in a local increase in mechanical stiffness. Calcification of mature cartilage occurred within 3 days in medium with supersaturated calcium and phosphate and was dependent on supplementation with fetuin, an abundant serum protein. Cartilage explants cultured atop devitalized bone underwent fetuin-mediated calcification in the zone adjacent to bone, thereby attaching cartilage to bone through a calcified cartilage interface and achieving integration strengths similar to that of native immature osteochondral tissue.

These studies demonstrate that calcification can be used to integrate cartilage and bone to form a mechanically functional osteochondral graft. Such methods can be useful to provide biological fixation of cartilaginous grafts to bone for the formation of osteochondral grafts suitable for the treatment of large defects or ultimately, for resurfacing damaged or osteoarthritic joints.