Establishment of a Rat Pull-Out Repair Model Demonstrating Fibrous Meniscus-Bone Healing and Suppression of Meniscal Extrusion After Medial Meniscus Posterior Root Tear

BACKGROUND: Medial meniscus posterior root tear (MMPRT) disrupts meniscal hoop tension and is associated with osteoarthritis progression. Although pull-out repair is a standard surgical technique for MMPRT, its precise healing process remains unclear. PURPOSE/HYPOTHESIS: This study aimed to establish a rat model for MMPRT pull-out repair, investigating its biomechanical and histological healing and its effect on medial meniscus extrusion (MME). It was hypothesized that pull-out repair would show reduced MME and restore biomechanical property toward normal. STUDY DESIGN: Controlled laboratory study. METHODS: A total of 100 male Wistar rats were assigned to the repair (MMPRT pull-out repair with external fixation) or nonrepair (unrepaired MMPRT with external fixation) group. Contralateral knees served as controls. Operated knees were immobilized with an external fixator for 2 weeks. Evaluations at 4 and 12 weeks included micro-computed tomography to quantify MME using the meniscal extrusion ratio (MER), biomechanical testing for tensile strength, and macroscopic/microscopic assessments. RESULTS: The repair group showed a significantly lower MER than the nonrepair group at all time points. No difference in maximum load to failure was found between 4 and 12 weeks. At both time points, there was no significant difference between the repair group and the healthy leg. Histological analysis at 12 weeks in the repair group showed ongoing meniscus-bone healing, characterized mainly by fibrovascular interface tissue resembling indirect insertion-like healing, although direct healing was observed in 2 specimens. Immunohistochemically, a collagen type 3-positive matrix was observed along the meniscus-bone interface, indicating persistent remodeling of the fibrous tissue. In the nonrepair group, no spontaneous healing at the posterior root attachment was observed, and scar adhesions to surrounding tissues were present at 12 weeks. CONCLUSION: The authors established a rat model for pull-out repair of the MMPRT combined with external fixation. In this model, the process of meniscus-bone healing was elucidated, showing an indirect healing pattern characterized by fibrovascular interface formation. This model may serve as a preclinical platform to test strategies aimed at enhancing repair stability and promoting meniscus-bone healing. CLINICAL RELEVANCE: This model provides a valuable tool for comprehensively investigating the biological and biomechanical aspects of MMPRT repair. It may serve as a preclinical platform to investigate the healing mechanism and test future modifications of the surgical procedure and postoperative loading/rehabilitation conditions.