Changes in epiphyseal bone, subchondral bone plate and epiphyseal trabecular bone in surgically and chemically induced rat models of osteoarthritis

Abstract

Purpose: Several experimental animal models have been developed for human osteoarthritis (OA) and used to study the preclinical efficacy of disease and symptom modifying OA drug candidates. The preclinical efficacy has been determined by various microscopic scoring systems and joint pain assessments. The histopathology initiative of Osteoarthritis Research Society International (OARSI) has presented recommendations for OA assessment in order to standardize preclinical efficacy studies. Recommendations for rat samples include the histological analysis of cartilage degeneration and extracellular matrix (ECM) loss, osteophytes, calcified cartilage and subchondral bone, synovium, joint capsule, and growth plate. As subchondral bone has been identified to play an important role in the development and progression of OA, treatment effects on bone tissue have been studied even in more detailed. In this study, we performed a systemic characterization of epiphyseal bone, subchondral bone plate and epiphyseal trabecular bone together with the assessment of knee joint discomfort and pain and degenerative changes in articular cartilage and synovium in four rat OA models. Methods: The study was conducted using male Lewis rats (body weight range 330-380 g). Unilateral OA was induced in their knee joints by applying the following models: 1) intra-articular injection of monoiodoacetate (MIA) at the dose of 1 mg, 2) medial meniscal tear (MMT) combined with medial collateral ligament transection (MCLT), 3) anterior cruciate ligament transection (ACLT) combined with partial medial meniscectomy (pMMx), 4) ACLT. Body weight and OA symptoms were followed in each model during the study. Knee joint discomfort and pain were used as the symptoms of OA. Knee joint discomfort was analyzed as static weight bearing using Incapacitance Tester and knee joint pain as static mechanical allodynia using von Frey monofilaments. Knee joints were harvested at two different time points in each model as follows: in the MIA model at 2 and 4 weeks, in the MMT + MCLT model at 3 and 6 weeks, in the ACLT + pMMx model at 4 and 8 weeks, and in the ACLT model at 5 and 10 weeks. The structure of epiphyseal bone, subchondral bone plate and epiphyseal trabecular bone were analyzed in coronal sections of medial tibial plateau followed by the histological OA assessment as recommended by the OARSI histopathology initiative. This experimental protocol was approved by National Animal Experiment Board, Regional State Administrative Agency for Southern Finland, Hämeenlinna, Finland. Results: Knee joint discomfort was observed in operated hind limbs as decreased static weight bearing during the first week of the study. Knee joint pain was identified in operated and MIA-injected hind limbs as decreased paw withdrawal threshold during the first week and at the end of in-life phase of each model. In the rat MIA model, this knee joint pain was associated with mild synovial inflammation at 2 weeks, cartilage degeneration and ECM loss in superficial layer at 2 weeks and exacerbating down to intermediate layer at 4 weeks, and with the reduction in the amount of epiphyseal and subchondral bone. In the rat MMT + MCLT model, the knee joint pain was observed together with synovial inflammation, cartilage degeneration and the loss of ECM from superficial layer down to tidemark at 3 and 6 weeks, large osteophytes, and with the increased amount of epiphyseal and subchondral bone. In the rat ACLT + pMMx model, the pain of knee joint was identified in association with synovial inflammation, cartilage degeneration and ECM loss from superficial layer down to tidemark at 4 and 8 weeks, and with large osteophytes. In the rat ACLT model, the knee joint pain was associated with mild synovial inflammation, cartilage degeneration and the loss of ECM in superficial layer at 5 weeks and exacerbating down to deep layer at 10 weeks, osteophytes, and with the reduction in the amount of subchondral bone. Conclusions: This study characterized changes in the structure of epiphyseal bone, subchondral bone plate and epiphyseal trabecular bone together with degenerative changes in articular cartilage and synovium, and knee joint discomfort and pain in four rat OA models. The amount of bone decreased in the rat OA models exhibiting from mild to moderate degenerative changes in articular cartilage, whereas the amount of bone remained unchanged or increased in the rat OA models exhibiting from moderate to severe degenerative cartilage changes. These results can be used to design studies for testing the preclinical treatment effects of OA drug candidates on articular cartilage, synovium, epiphyseal bone, and knee joint discomfort and pain.