Development of a novel rat patella explant model to study osteochondral tissue changes in post-traumatic osteoarthritis

<p dir="ltr">Osteoarthritis (OA) is a multifactorial disease that affects all the joint tissues and can permanently change the joint environment. The specific details of how OA initiation and progression occur, are still largely unknown. Age-related OA is associated with subchondral bone changes, which occur at the later stages of disease. Post traumatic osteoarthritis (PTOA) is a subset of OA that occurs specifically secondary to joint injury such as rupture of the cruciate ligaments in the knee. In contrast to age related OA, the subchondral bone changes take place early in the PTOA process. Also, PTOA tends to affect a much younger cohort. Pro-inflammatory responses to injury occur following the initial insult in this kind of injury. Moreover, biomechanical factors such as altered mechanical stimulation and/or overload are important factors in the acute and chronic degenerative process. This thesis will describe the development of a novel rat patellar explant model to study the interconnection between bone and cartilage in the initiation and progression of PTOA. OA disease mechanisms have been studied extensively over many decades, typically using either<i> in vivo</i> or <i>in vitro</i> model systems. More recently <i>ex vivo</i> models have been used to overcome the limitations of standard <i>in vitro</i> models and simplify the complexity of<i> </i><i>in</i><i> </i><i>v</i><i>ivo</i><i> </i>ones. Thus, we aimed to develop a novel patellar explant model system to study bone-cartilage crosstalk in PTOA. Specifically, we sought to assess the explant response to inflammatory and mechanical stimuli, as well as mechanical overload and microdamage, in bone and cartilage compartments. Other osteochondral explant models such as femoral head, or cylindrical cored specimens have been used before, and are useful in capturing certain aspects of tissue dynamics. However, these approaches are limited by bone damage that occurs during the process of tissue isolation, such a tissue harvesting. Hence, the proposed patellar explant model is an optimal model to answer our research question, on the contribution of subchondral bone damage in PTOA (in Chapter 2). Therefore, to assess and optimize our explant system, an initial study was carried out comparing our patellar model against the gold standard femoral head explant. We first compared the culture conditions and tissue viability of both explant models in a long-term culture system. Then, to validate the ability of our explant to serve as a damage response model, pro-inflammatory cytokine treatment was used so as to recapitulate the early stages of PTOA. In addition, we carried out an optimization study to determine the most appropriate cytokine regime to use in this scenario. Then, since mechanical stimulation is critical to joint health and function, we next (in Chapter 3) focussed on whether controlled mechanical stimulation of our explant model, in a bioreactor system, could induce appropriate tissue responses in the osteochondral compartments. And if so, would they be representative of those seen in situ. While mechanical compression is a major consideration in the process of bone-cartilage crosstalk because of injury as well as in the process of cartilage breakdown. Here, two distinct regimes were developed and used to determine how this contributes to the release of metabolic mediators involved in extracellular matrix degradation and cellular death. Lastly, (in Chapter 4) a mechanical overload model was developed to induce subchondral bone microdamage and to assess cell damage and death and how this contributes to PTOA. In conclusion, through this research, we were able to characterise a novel osteochondral patellar explant model and demonstrate that it can be used to study the role of inflammatory and mechanical factors as a result of injury, and how they affect joint tissue function. This novel explant model was found to be capable of recapitulating the anabolic and catabolic changes that take place in the initial stages of injury and disease, in a controlled environment, and can serve as a test-bed and/or screening system for novel treatments for PTOA.</p>