The Critical-Size Osteochondral Defect for Single-Hole Microfracture Repair in Rats: A Center-Initiated, Perforation-Anchored Healing Pattern With Limited Spatial Coverage

Background: Microfracture is widely used for cartilage repair, but the maximum defect size amenable to healing from a single perforation remains undefined. Furthermore, the spatial propagation of tissue regeneration originating from a solitary microfracture channel has not been quantitatively characterized. Purpose: To determine the critical-size osteochondral defect for effective single-hole microfracture repair and to characterize the spatial pattern of tissue regeneration in a rat model. Study Design: Controlled laboratory study. Methods: Cylindrical full-thickness osteochondral defects (diameter: 1, 1.5, or 2 mm; depth: 1 mm) were created in the distal femurs of Sprague-Dawley rats and treated with a standardized central single-hole microfracture (diameter: 0.3 mm; depth: 3 mm). Animals were evaluated at 4 and 8 weeks postoperatively. Outcomes included International Cartilage Regeneration & Joint Preservation Society (ICRS) macroscopic assessment (ICRS scoring), subchondral bone microarchitecture analysis (micro–computed tomography), histological evaluation (safranin O–fast green staining, modified O’Driscoll scoring), and immunohistochemical quantification of collagen type I and collagen type II (COL II). Results: At 8 weeks, 1- and 1.5-mm defects demonstrated improved filling, more complete subchondral bone reconstruction, and greater cartilage-like matrix maturation with higher COL II signal than those of the 2-mm defect group (all P < .05), but with no significant difference between these 2 groups ( P > .05). Spatial analysis revealed that reparative tissue (high safranin O–fast green staining and COL II signal) was concentrated around the microfracture tract and significantly diminished toward the periphery ( P < .05), consistent with a center-initiated, perforation-anchored spatial repair pattern. Conclusion: In this rat model, single-hole microfracture demonstrated a center-initiated, perforation-anchored healing pattern, supporting that approximately 1.5 mm is the critical defect gap. Clinical Relevance: This perforation-centered spatial repair pattern suggests limited effective coverage of a single microfracture channel and may help inform microfracture strategy for cartilage defects.