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 tot al knee arthroplasty can be explained by the pumping effects of the interst itial fluid in the tissue. Further, as a secondary objective we investigate d the possibility that interface-tissue differentiation is influenced by in terstitial fluid flow and strain, as mechanical effects of interface motion s. For comparative reasons, we analyzed a previously published simplified t wo-dimensional finite-element model, this time assuming biphasic tissue pro perties. We wanted to determine hydrostatic pressure and flow velocities in the fluid phase, in addition to stresses and strains, for time-dependent l oading of the plateau. We found that fluid flow in the interface was extrem ely slow, except in the periphery. Hence, loosening due to particle-induced bone resorption appears improbable. The results, however, do support the i dea that particles migrate with fluid flow, when such flow occurs. Where fi brous tissue tends to be prominent in reality, the fluid is repeatedly extr uded and reabsorbed in the model. Where these values are low, fibrocartilag e is commonly found. When material properties were varied to subsequently r epresent fibrocartilage and two stages of mineralization, the strains and f luid velocities is reduced. Fluid pressure, however, did not change. Our re sults refute the hypothesis that wear particles are pumped through the inte rface tissue below a TKA but support the hypothesis that interface tissue t ype and loosening processes are influenced by mechanical tissue variables s uch as tissue strain and interstitial fluid velocity. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.