Background: Vena Cava filters are used to prevent pulmonary embolism in patients with deep vein thrombosis who are unresponsive to anticoagulation therapy. Various filter designs exist in the market with different characteristics distinguishing them. An understanding of the characteristics of these filters is desirable in order to develop better designs. Methods: A computational fluid dynamical study of the flow over an unoccluded stainless steel Greenfield Vena Cava filter (Boston Scientific, Watertown, MA) to determine its properties has been performed. Simulation of flow over a filter placed axisymmetrically in a rounded inferior vena cava has been performed at a Reynolds numbers of 1000 and the consequences of the flow (by studying parameters like shear stress and stagnation zones) have been discussed. Furthermore, a new finite element based numerical method has been developed that allows the study of capturing properties of Inferior Vena Cava filters. The key idea is the introduction of a thin-wire-model (TWM) that enables a drastic reduction in the computational cost while still maintaining control on the physics of the problem. This numerical technique has been applied to evaluate the embolus capture characteristic of a Greenfield filter. Results: The flow around the unoccluded filter is found to be steady and laminar at the conditions studied. A recirculation/stagnation zone develops immediately downstream of the filter head. This zone is significantly larger when the central hole is occluded. The shear stress and stagnation zone properties for such a flow over a Greenfield filter are compared with existing literature (in vitro studies). A graph showing the regions wherein clots escape or get captured has been determined by a means of numerical simulations. The data has further been analyzed to determine the probability of clot capture as function of the clot size. Conclusions: The stagnation zone formed behind the head of the Greenfield filter is found to be smaller in size when compared to that of the same filter with the central hole occluded. A map of the shear stress distribution shows a small region having the potential for thrombogenesis. The non-Newtonian properties of blood are not seen to cause much variation in the flow field when compared to the Newtonian model. However variation in the cava size leads to a significant change in the shear stresses. This study also establishes a novel method wherein computational means are used to determine the efficacy of clot capturing of filters. These techniques can further be used to compare the different characteristics among filters.
Perforation of the inferior vena cava by a Greenfield filter
