Repair of osteochondral defects with biphasic cartilage-calcium polyphosphate constructs in a sheep model

There has been interest in developing novel biological treatments to repair focal cartilage defects. We have developed a method of forming biphasic constructs ("osteochondral"-type plug) in vitro consisting of cartilaginous tissue, formed on and anchored to the intended articulation surface of a porous ceramic substrate. The purpose of this study was to evaluate the biochemical and biomechanical properties and morphology of in vitro-formed biphasic constructs 3 and 9 months after implantation into 4mm diameter full thickness osteochondral defects in the trochlear groove of sheep stifles. The implants withstood loading in vivo up to 9 months with evidence of fusion to adjacent native cartilage and fixation by bone ingrowth into the ceramic substrate. The cartilage layer was eroded from those implants that were proud to the joint surface. Control implants (ceramic only) had fibrous tissue on the articulating surface after implantation for 3-4 months. Neither the cellularity nor proteoglycan content of the implanted cartilage, when it remained, changed significantly between 3 and 9 months and the collagen content increased slightly. The elastic equilibrium modulus of the cartilage improved with time with the greatest improvement (10-fold) occurring early during the first 3-4 months after implantation. This study suggests that biphasic constructs may be suitable to repair joint defects as the implants were maintained up to 9 months in sheep. Importantly the mechanical properties of the implanted cartilage improved significantly after implantation suggesting that cartilage can mature in vivo after implantation. The results indicate that further study of this treatment approach is warranted to attempt to overcome the technical surgical difficulties identified in this study.