Formation of biphasic constructs containing cartilage with a calcified zone interface.

The zone of calcified cartilage is the mineralized region of articular cartilage that anchors the hyaline cartilage to the subchondral bone and serves to disperse mechanical forces across this interface. In an attempt to mimic this zonal organization, we have developed the methodology to form biphasic constructs composed of cartilaginous tissue anchored to the top surface of a bone substitute (porous calcium polyphosphate, CPP) with a calcified interface. To accomplish this, chondrocytes were selectively isolated from the deep zone of bovine articular cartilage, placed on top of the CPP substrate, and grown in the presence of beta-glycerophosphate (10 mM, beta-GP). By 8 weeks, cartilage tissue had formed with two zones: a calcified region adjacent to the CPP substrate and a hyaline-like zone above. Little or no mineralization occurred in the absence of beta-GP. The mineral that formed in vitro was identified as hydroxyapatite, similar in composition and crystal size to that found in vivo. The tissue stiffness was seven times greater, and the interfacial shear properties at the cartilage-CPP interface were at least two times greater in the presence of this mineralized zone within the in vitro-formed cartilage than in tissue lacking a mineral zone. In conclusion, developing a biphasic construct with a calcified zone at the tissue-biomaterial interface resulted in significantly better cartilage load-bearing (compressive) properties and interfacial shear strength, emphasizing the importance of the presence of a mineralized zone in bioengineered cartilage. Because failure due to shear occurred at the cartilage-CPP interface instead of the tidemark, as occurs with osteochondral tissue, further study is required to optimize this system so that it more closely mimics the native tissue.