Engineering Modular Components of an Osteochondral Implant

Osteoarthritis remains a poorly manageable outcome of traumatic cartilage injury as cartilage is unable to self-repair. Transplantation of an engineered, osteochondral tissue plug has been proposed as a better way to achieve repair of cartilage defects. Chitosan-based materials that support both chondrogenesis and osteogenesis while promoting rapid tissue integration may provide a viable solution. Bone marrow MSCs were co-encapsulated in glycosaminoglycan-chitosan microcapsules (300-400 µm diameter) with either hydroxyapatite (HAP) microgranules or methacrylated hyaluronan-collagen hydrogels and cultured in either osteogenic or chondrogenic induction media respectively for 4 weeks. Capsule cultures were analyzed to evaluate cell growth and distribution, matrix composition and gene expression. In chondrogenic cultures the effects of hypoxia and medium perfusion were also evaluated, while osteogenic capsules fused into 3D constructs and subjected to compressive testing. In chondrocytic capsules, quantitative assays showed that hyaluronan-collagen inclusion enhanced cell growth and matrix (s-GAG & collagen II) deposition. Growth was further promoted by both perfusion and maintenance in hypoxic (2% oxygen) conditions. PCR results showed that collagen incorporation, perfusion and hypoxia together combined to enhance expression of collagen II, aggrecan and SOX-9 by up to 10-fold. Osteoinduced MSCs deposited significant bone-like mineral in microcapsule interiors, including mineralization of capsule membranes. Compressive testing showed that mineralization increased capsule construct mechanical properties by at least 3 orders of magnitude. HAP-containing capsules had compressive yield strengths of 10 MPa and compressive moduli of 45 GPa. These results demonstrate that MSCs in GAG-chitosan capsules can be cultured to generate in vitro constructs with physical and biological properties approaching those of cartilage and bone and may be suitable for in vivo testing of an osteochondral plug.