Disease Model for Meniscus Degeneration: Clinical Inflammatory and Degenerative Tear Model

Abstract

The meniscus is a fibrocartilaginous intra-articular structure of the knee that functions as a load bearing surface, offering protection of articular cartilage and stability of the knee joint. The meniscus degenerates due to multiple factors, such as trauma, overuse, overweight, and osteoarthritis (OA). While meniscus degeneration is common and present in multiple forms, current treatment options are extremely limited to partial meniscectomy and meniscus allograft transplantation. Current procedures fall short of restoring meniscus functions and preventing OA progression. Many hurdles remain to improve current treatment, and such hurdles stem from the little known pathogenesis and absence of a disease model. This study aims to devise a disease model for meniscus degeneration. First, we sought to evaluate inflammatory changes and related degeneration of the meniscus, along with cartilage and synovium, against polyethylene wear particles in a unicompartmental knee arthroplasty model, thereby establishing a clinical inflammation model. Secondly, we sought to characterize the degeneration process of meniscus root tears in osteoarthritic knees, with emphasis on fibrocartilage and calcification, thereby establishing a clinical degenerative tear model. In chapter I, we hypothesized that ultra-high molecular weight polyethylene (UHMWPE) particles per se would interact with intra-articular tissue, which by acting as inflammatory reservoirs, would subsequently induce OA changes in vitro and in vivo. UHMWPE particles were generated by a previously published micro-cutting process method. In vitro experiment was carried out using an inverted culture system. Three cell types were used: chondrocytes, meniscal fibrochondrocytes, and synoviocytes. Each cell type was cultured with two different concentrations of UHMWPE particles. Pro-inflammatory cytokine production, phagocytosis, and apoptosis were analyzed. In vivo experiment was done by injecting two concentrations of UHMWPE particles into normal and murine OA model knee joints. In vitro experiment showed that pro-inflammatory cytokine and mediator (IL-1β, IL-6, TNF-α, Nitric Oxide, and Prostaglandin E2) production, phagocytosis of particles, and apoptosis were increased in all cell types. In vivo experiment showed degeneration of cartilage and meniscus, as well as synovitis after UHMWPE particle injection. UHMWPE wear particles per se exert toxic effects in cartilage, synovium, and meniscus of the knee joint resulting in pro-inflammatory cytokine release, phagocytosis of particles and apoptosis. This study suggests a clinical inflammatory model based on UKA for meniscus degeneration. In chapter II, our purpose was to characterize degeneration of medial meniscus posterior root tears among osteoarthritic knees with emphasis on fibrocartilage and calcification. Samples of medial meniscus posterior roots were harvested from cadavers and patients during knee replacement surgery and grouped as follows: normal reference, no tear, partial tear, and complete tear. Degeneration was analyzed with histology, immunohistochemistry, and PCR. Uniaxial tensile tests were performed on specimens with and without fibrocartilage. Quantifiable data was statistically analyzed by Kruskal-Wallis test with Dunn’s comparison test. Thirty, 28, and 42 samples harvested from 99 patients were allocated in no tear, partial tear, and complete tear group, respectively. Modified Bonar tendinopathy scores for each group were 3.97, 9.31, and 14.15, respectively, showing higher degree of degeneration associated with tear extent (p<0.05 for all groups). Characterization of root matrices revealed increase in fibrocartilage according to tear extent. Tear margins revealed fibrocartilage in 59.3% of partial tear and 76.2% of complete tear samples with distinctive cleavage-like morphology. Root tears with similar morphology were induced within fibrocartilaginous areas during uniaxial tensile testing. Even in the no tear group, 56.6% of samples showed fibrocartilage in the anterior margin of root, adjacent to the meniscus. Increased staining area of calcification and expression of ENPP1 gene were observed in the complete tear group compared to no tear group (p<0.0001). Fibrocartilage and calcification increased in medial meniscus posterior roots, associated with the degree of tear. Both findings, which impair the ligament’s resistance to tension, may play a pivotal role during pathogenesis of degenerative meniscal root tears in OA knees.