This study represents a natural extension of our previous efforts in the de
sign and development of a new class of swellable bone anchors, which absorb
body fluids and achieve fixation by an expansion-fit mechanism, Specifical
ly, this study investigates (i) correlations between the optimal swelling s
train for highest fixation strength and the foam (or bone) density, and (ii
) the influence of a threaded surface an the fixation strength of the swell
able 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 wa
s varied systematically for each foam and anchor design combinations. This
study indicates that the screw swellable anchors have higher initial fixati
on strength than smooth swellable anchors, but the final fixation strengths
of both anchors are quite similar. Further, it is observed that the optima
l swelling strain decreased with increasing foam density, Both the smooth a
nd screw swellable anchors were also found to exhibit higher fixation stren
gths than the metallic screws of similar geometry. (C) 2001 John Wiley & So
ns, Inc.