Chitosan-glycerol phosphate/blood implants elicit hyaline cartilage repair integrated with porous subchondral bone in microdrilled rabbit defects

OBJECTIVE: We have previously shown that microfractured ovine defects are repaired with more hyaline cartilage when the defect is treated with in situ-solidified implants of chitosan-glycerol phosphate (chitosan-GP) mixed with autologous whole blood. The objectives of this study were (1) to characterize chitosan-GP/blood clots in vitro, and (2) to develop a rabbit marrow stimulation model in order to determine the effects of the chitosan-GP/blood implant and of debridement on the formation of incipient cartilage repair tissue. METHODS: Blood clots were characterized by histology and in vitro clot retraction tests. Bilateral 3.5 x 4 mm trochlear defects debrided into the calcified layer were pierced with four microdrill holes and filled with a chitosan-GP/blood implant or allowed to bleed freely as a control. At 1 day post-surgery, initial defects were characterized by histomorphometry (n=3). After 8 weeks of repair, osteochondral repair tissues between or through the drill holes were evaluated by histology, histomorphometry, collagen type II expression, and stereology (n=16). RESULTS: Chitosan-GP solutions structurally stabilized the blood clots by inhibiting clot retraction. Treatment of drilled defects with chitosan-GP/blood clots led to the formation of a more integrated and hyaline repair tissue above a more porous and vascularized subchondral bone plate compared to drilling alone. Correlation analysis of repair tissue between the drill holes revealed that the absence of calcified cartilage and the presence of a porous subchondral bone plate were predictors of greater repair tissue integration with subchondral bone (P<0.005), and of a higher total O'Driscoll score (P<0.005 and P<0.01, respectively). CONCLUSIONS: Chitosan-GP/blood implants applied in conjunction with drilling, compared to drilling alone, elicited a more hyaline and integrated repair tissue associated with a porous subchondral bone replete with blood vessels. Concomitant regeneration of a vascularized bone plate during cartilage repair could provide progenitors, anabolic factors and nutrients that aid in the formation of hyaline cartilage.