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

Abstract

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 on 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.

A porous swelling copolymeric material for improved implant fixation to bone

“…Bone anchors made of the co‐polymeric swelling material have some significant advantages over conventional threaded‐based anchors. As described earlier, while the conventional anchors achieve their fixation strength through a shear‐based mechanism, the swelling anchors do that through friction‐based mechanism 40–42 . Therefore, it is expected that in lower density bone, the relative fixation stress of the swelling anchors compared to conventional anchors increases, making them more desirable for low density bone such as in the application of suture anchors for rotator cuff repair or for fixing suture anchors in osteoporotic bone.…”

“…Therefore, the long‐term fixation strength of the swelling anchors is expected to increase as opposed to that of conventional anchors in which bone resorption may occur due to stress‐shielding. In previous studies 37–42 the swelling anchors were solid, not allowing for osteointegration through migration of bone into the anchor, to occur. To allow for the long‐term benefits of osteointegration, the swelling anchors must possess a porosity to allow bone ingrowth to occur.…”

“…Bone anchors made of this material achieve their fixation to bone by absorbing interstitial fluids and swelling by a controlled amount, thus producing radial stresses in the surrounding bone. [40][41][42][43] These radial stresses results in frictional resistance against pull-out forces which are acting on the anchor during functional activities.…”

“…As described earlier, while the conventional anchors achieve their fixation strength through a shear-based mechanism, the swelling anchors do that through friction-based mechanism. [40][41][42] Therefore, it is expected that in lower density bone, the relative fixation stress of the swelling anchors compared to conventional anchors increases, making them more desirable for low density bone such as in the application of suture anchors for rotator cuff repair or for fixing suture anchors in osteoporotic bone. In addition, due to the difference in fixation mechanism (friction vs. shear), the swelling anchors possess refixation capability compared to conventional anchors that become loose once the maximum pull-out force has been reached.…”