The interstitial fluid pressure response during stress-relaxation of articular cartilage due to viscosity and porous media effects: a computational study
Paschke, Brandon James
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Articular cartilage is a complex material made of several fluid and solid components. A model that fully describes the responses of cartilage is required to accurately create a cartilage replacement that can be used in cases of injury or disease. Modeling of articular cartilage has proven difficult and currently no constitutive law fully describes its solid and fluid responses. Many of the current models describe the interstitial fluid as inviscid, even though it is known that proteoglycan migration within cartilage causes a viscous response within interstitial fluid. The goal of this research was to create a viscous fluid porous media model that better captures the compressive resistance of cartilage created by migration of interstitial fluid during cartilage compression. Through the creation of this model it was possible to capture the experimental magnitudes of fluid pressure within cartilage during unconfined slow compression simulations. As part of this model, a porous media approximation was used, which demonstrates that small variations in the solid matrix, comprised of collagen fibers, can cause large variations in system response. Magnitudes of mean pressure values, after 150 seconds of compression, for the viscous fluid porous media model bound the values found in experimental testing. Limitations of the fluid model are that system relaxation isn't captured and the slope increase of pressures during compression for experiments don't match those of the fluid model. A main conclusion drawn from the model is that viscosity of interstitial fluid plays a large role in creating compressive resistance within articular cartilage. Another takeaway is that the porous media approximation greatly impacts the magnitude of fluid pressurization, which creates a need to accurately represent the solid matrix within cartilage.