Mechanical model of an early stage osteoarthritis

Chiravuri, Venkat
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University of Delaware
Osteoarthritis (OA) is a common degenerative disease which affects the articular surfaces and adjacent tissues of human synovial joints. More than 20 million Americans are estimated to have OA. OA changes the entire joint; morphologically, biochemically, structurally and biomechanically. The early stages of OA are characterized by a roughened articular surface, with possible partial thickness defects. This may worsen with fibrillation and eventually full thickness damage on both surfaces resulting in bone on bone contact. Animal models are commonly used to study the pathophysiology of OA, providing us the insight into cartilage degeneration. There have been numerous surgery-based models of OA, which induce OA by some sort of trauma to the knee such as meniscectomy or anterior cruciate ligament transection. To overcome some of the problems inherent in these models, a mechanically induced model of uni-compartmental OA was developed that altered the mechanical loading of the knee of New Zealand white rabbit. The overall purpose of the research project was to develop a model of early-stage OA, to look for early adaptation in the cartilage and subchondral bone and to assess possible treatments. The research objectives of the current thesis are to determine changes in the knee articular cartilage and subchondral bone from tissue that underwent the mechanically induced OA model. To further interpret the experimental results of the animal model, a mathematical model is formulated that is used to predict the joint load and stress state at the articular surfaces. One of the specific purposes of the research is to determine the changes in material properties of the articular cartilage, mineral and trabecular structure of the subchondral bone and histology of the femoral and tibial joint surfaces in tissue under two different loading time periods of 2 and 4 weeks. The contralateral knees are to be used as internal controls. Correlations with the cartilage material properties to the subchondral bone changes are made. The second specific aim of the research is to formulate an analytical model of the knee joint of the New Zealand white rabbit under the mechanically induced loading model of OA that incorporates accurate three-dimensional geometry, external joint loading, deformable articular contact and non-linear ligament properties. The model is implemented to determine the loads and stresses on the tibial and femoral articular surfaces given the changing articular cartilage material properties at the 2 and 4 week loading time periods and in the contralateral control. The results of the testing showed that the properties of cartilage as well as the trabecular bone morphology were altered. For articular cartilage, aggregate moduli and permeability values showed significant changes towards an OA-like degeneration that increased with the time of loading. The bone mineral density and concentration was altered also in the subchondral bone with exposure to the altered loading. Histological evaluation showed surface damage only and loss of proteoglycan, but little full thickness damage. The results of the analytical model showed increased stresses in the femur which have led to the changes in the properties of both the cartilage and the trabecular bone morphology. The results in general indicate signs of early stage OA, formulating in a new mechanical model for early stage OA.