Rethinking meniscal ossicles as imaging biomarkers for meniscal turnover, overload, and pathology in pre-clinical knee osteoarthritis research

Despite widespread use of mouse osteoarthritis models, meniscal ossicles (MOs), a distinct feature of murine meniscus, remain incompletely characterized. This study aimed to detail MO morphology and bone features in adult mice with and without obesity and post-traumatic osteoarthritis using MRI and micro-CT with histological correlation. At 16 weeks, male C57BL/6 mice were assigned to a control-fat diet (CFD) or high-fat diet (HFD). At 36 weeks, right knees underwent sham (CFD) or compression-induced injury (HFD with post-traumatic osteoarthritis; HFD-PTOA) with contralateral knees as internal controls. After the endpoint (40 weeks), MRI and micro-CT assessed MOs for bone marrow (BM) cavities, fragmentation, and bone parameters. Histology evaluated OARSI scores and meniscal surface irregularity. Native BM cavities were commonly observed in inner anterior horns, particularly in the lateral meniscus, across groups. Injury was associated with de novo outer-region BM cavities, mainly in the anterior horn of the medial meniscus (AHMM) (CFD vs HFD-PTOA knees; p = 0.018; paired proportion difference = 0.74, 95% CI [-0.27, 0.98]). AHMM fragmentation increased following injury (0% in CFD vs 83% in HFD-PTOA knees; p = 0.015), while posterior horn medial meniscus fragmentation was observed across groups. MO imaging features showed compartment-specific associations with histologic cartilage degeneration and meniscal surface irregularity, mainly in the medial compartment. Mineralization and ossification of MOs with marrow formation differ by anatomical region and phenotype. These characteristics are promising imaging biomarkers for assessment of mineral turnover, joint overload and pathology, and the chondro-osteogenic potential of resident meniscal cells.