Wear particle diffusion and tissue differentiation in TKA implant fibrous interfaces

In the context of mechanical loosening, we studied the hypothesis that wear-particle migration in the fibrous membrane under tibial plateaus after total knee arthroplasty can be explained by the pumping effects of the interstitial fluid in the tissue. Further, as a secondary objective we investigated the possibility that interface-tissue differentiation is influenced by interstitial fluid flow and strain, as mechanical effects of interface motions. For comparative reasons, we analyzed a previously published simplified two-dimensional finite-element model, this time assuming biphasic tissue properties. We wanted to determine hydrostatic pressure and flow velocities in the fluid phase, in addition to stresses and strains, for time-dependent loading of the plateau. We found that fluid flow in the interface was extremely slow, except in the periphery. Hence, loosening due to particle-induced bone resorption appears improbable. The results, however, do support the idea that particles migrate with fluid flow, when such flow occurs. Where fibrous tissue tends to be prominent in reality, the fluid is repeatedly extruded and reabsorbed in the model. Where these values are low, fibrocartilage is commonly found. When material properties were varied to subsequently represent fibrocartilage and two stages of mineralization, the strains and fluid velocities is reduced. Fluid pressure, however, did not change. Our results refute the hypothesis that wear particles are pumped through the interface tissue below a TKA but support the hypothesis that interface tissue type and loosening processes are influenced by mechanical tissue variables such as tissue strain and interstitial fluid velocity.