Fixation strength of swellable bone anchors in low-density polyurethane foam

This study represents a natural extension of our previous efforts in the design and development of a new class of swellable bone anchors, which absorb body fluids and achieve fixation by an expansion-fit mechanism, Specifically, this study investigates (i) correlations between the optimal swelling strain for highest fixation strength and the foam (or bone) density, and (ii) the influence of a threaded surface an the fixation strength of the swellable implant, For this purpose, the immediate and the final (after swelling) fixation strengths of two variations of the swellable bone anchor designs (a smooth anchor and a screw anchor) were measured in two different foams (used to simulate bone) with different densities. The amount of swelling was varied systematically for each foam and anchor design combinations. This study indicates that the screw swellable anchors have higher initial fixation strength than smooth swellable anchors, but the final fixation strengths of both anchors are quite similar. Further, it is observed that the optimal swelling strain decreased with increasing foam density, Both the smooth and screw swellable anchors were also found to exhibit higher fixation strengths than the metallic screws of similar geometry. (C) 2001 John Wiley & Sons, Inc.