Finite element modeling of articular cartilage

The ends of all diarthrodial joints are covered by a thin layer of articular cartilage (AC) which plays an essential role in the functioning of the joint. The AC provides for almost frictionless motion of the articulating surfaces, reduces contact stresses by distribution of the loads, and dissipates energy for the relatively stiff and brittle bone underneath. Approximately 30% of Dutch Warmblood horses suffers to some degree from Osteochondrosis (OC), a disease that affects the developing AC layer and can lead to severe lameness. The onset and development of OC is strongly correlated to local mechanical loads in the joint and therefore Wageningen University started a project to gain more insight in the role of mechanical loading in the pathogenesis and aetiology of OC. Due to the composition and structure of AC and depending on the loading case, a valid finite element description of the mechanical behavior of AC can be a challenging task. Much of the complicated mechanical behavior of AC can be modeled with a porous solid saturated with a fluid representing the solid extracellular matrix and interstitial fluid respectively. A more subtle effect is the osmotic swelling pressure due to the negative fixed charges on the matrix and the freely moving ions in the interstitial fluid. Finally, most of the strain rate dependent behavior and the compression tension nonlinearity of AC is best modeled by incorporation of the collagen fibre structures. Of course, one has to realise that these parameters are a function of depth and that they all change, depending on each other, undermechanical loading.