Early events in cartilage repair after subchondral bone microfracture.

The current study investigated healing of large full-thickness articular cartilage defects during the first 8 weeks with and without penetration of the subchondral bone using microfracture in an established equine model of cartilage healing. Chondral defects in the weightbearing portion of the medial femoral condyle were made bilaterally; one defect in each horse was microfractured whereas the contralateral leg served as the control. The expression of cartilage extracellular matrix components (Types I and II collagen and aggrecan) was evaluated using histologic techniques, reverse transcription coupled polymerase chain reaction, in situ hybridization, and immunohistochemistry. This study confirms an increase in Type II collagen mRNA expression in repair tissue as early as 6 weeks after microfracture. Although other matrix mRNA and protein levels changed in concentration and tissue location over the course of the study, no significant differences were seen in microfractured defects. Although the microfracture techniques appear to improve clinical functionality, volume of repair tissue, and augment Type II collagen content, aggrecan content is less than ideal. Therefore, methods to enhance key matrix components such as aggrecan after microfracture may additionally improve repair tissue observed after the procedure.