Repair response of the inner and outer regions of the porcine meniscus in vitro

Background: The menisci are essential intra-articular structures that contribute to knee function, and meniscal injury or loss is associated with joint degeneration. Tears of the outer vascularized zone have a greater potential for repair than do tears in the inner avascular region. Objective and Hypothesis: Develop an in vitro explant model to examine the hypothesis that differences exist in the intrinsic repair response between the outer and inner region of the meniscus. Study Design: Controlled laboratory study. Methods: Cylindrical explants were harvested from the outer one third and inner two thirds of medial porcine menisci. To simulate a full-thickness defect, a central core was removed and reinserted immediately. Explants were cultured for 2, 4, or 6 weeks, and meniscal healing was investigated using mechanical testing, histologic analysis, and fluorescence confocal microscopy. Results: Over the 6-week culture period, meniscal explants exhibited migration of cells into the repair site, followed by increased tissue formation that bridged the interface. The repair strength increased significantly over time, with no differences between the 2 regions. Conclusion: The findings show that explants from the avascular inner zone and vascular outer zone of the meniscus exhibit similar healing potential and repair strength in vitro. Clinical Relevance: These findings support the hypothesis that the regional differences in meniscal repair observed clinically are owed to the additional vascular supply of the outer meniscus rather than intrinsic differences between the extracellular matrix and cells from these 2 areas.