The Subchondral Bone Condition During Microfracture Affects the Repair of the Osteochondral Unit in the Cartilage Defect in the Rat Model

Background: Microfracture (MF) is frequently performed as a first-line treatment for articular cartilage defects. Although good clinical outcomes are often obtained in the short term, poor clinical outcomes sometimes occur because of subchondral bone deterioration. The condition of the subchondral bone treated with MF may affect the repair of the osteochondral unit. Purpose: To analyze histological findings of the osteochondral unit after performing MF on subchondral bone in different states-normal, absorption, and sclerosis-in a rat model. Study Design: Controlled laboratory study. Methods: Full-thickness cartilage defects (5.0 x 3.0 mm) were created in the weightbearing area of the medial femoral condyle in both knees of 47 Sprague-Dawley rats. Five MF holes were created within the cartilage defect using a 0.55-mm needle to a depth of 1 mm at 0 weeks (normal group), 2 weeks (absorption group), and 4 weeks (sclerosis group) after the cartilage defect was created. In the left knee, MF holes were filled with beta-tricalcium phosphate (beta-TCP). At 2 and 4 weeks after MF, knee joints were harvested and histologically analyzed. Results: MF holes were enlarged at 2 weeks and further enlarged at 4 weeks in all groups. In the absorption group, osteoclast accumulation around the MF holes and cyst formation were observed. The trabecular bone surrounding the MF holes was thickened in the sclerosis group. The diameter of the MF hole was largest in the absorption group at 2 and 4 weeks after MF compared with the other groups. No subchondral bone cysts were observed after beta-TCP implantation. Pineda scores in all groups were significantly better with beta-TCP implantation than without beta-TCP implantation at 2 and 4 weeks. Conclusion: MF for subchondral bone with bone absorption induced enlargement of the MF holes, cyst formation, and delay of cartilage defect coverage. Implantation of beta-TCP into the MF holes enhanced remodeling of the MF holes and improved repair of the osteochondral unit compared with MF only. Therefore, the condition of the subchondral bone treated with MF affects repair of the osteochondral unit in a cartilage defect.