A porous swelling copolymeric material for improved implant fixation to bone

Abstract

Most metallic commercial bone anchors, such as screws and suture anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, potentially leading to failure, which are particularly evident in low‐density bone. These include stress‐shielding resulting from mechanical properties mismatch; lack of mechanically induced remodeling and osteointegration; and when the pullout force on the anchor, during functional activities, exceeds their pullout strength, catastrophic failure occurs leaving behind large bone defects that may be hard to repair. To overcome these deficiencies, we introduced in this study a porous swelling co‐polymeric material and studied its swelling and compressive mechanical characteristics as bone anchor under different configurations. Porosity was achieved by adding a non‐dissolvable agent (NaCl) during the process of polymerization, which was later dissolved in water, leaving behind a porous structure with adequate porosity for osteointegration. Three different groups of cylindrical samples of the swelling co‐polymer were investigated. Solid, fully porous, and partially porous with a solid core and a porous outer layer. The results of the swelling and simple compression study show that the partially porous swelling co‐polymer maintains excellent mechanical properties matching those of cancellous bone, quick swelling response, and an adequate porous outer layer for mechanically induced osteointegration. These suggest that this material may present an effective alternative to conventional bone anchors particularly in low‐density bone.

Fixation strength of swelling copolymeric anchors in artificial bone

“…After dissolving these crystals through desalination in water following polymerization, a porous structure with an average pore size of 300 µm, shown in the past to be adequate for effective bone ingrowth, 38,39 was produced. 40 Results from a previous study on the swelling and mechanical properties of solid, partially porous, and fully porous samples showed that increased porosity reduces strength. 40 Therefore, in the present study, three groups of samples were studied, solid, partially porous, and fully porous.…”

“…40 Results from a previous study on the swelling and mechanical properties of solid, partially porous, and fully porous samples showed that increased porosity reduces strength. 40 Therefore, in the present study, three groups of samples were studied, solid, partially porous, and fully porous. All samples were cylindrical, 8 mm in diameter and 8 mm long.…”

“…It was then found that a volume ratio of 80% MMA to 20% AA produced adequate swelling (4%-6% by volume) without compromising strength. [20][21][22][23] To enhance the material's osteointegration properties, we recently reported on a method 36 to generate a porous morphology in the swelling material. This method consisted of adding NaCl crystals with an average diameter of 300 µm (crystal sizes vary from 100 to 400 µm) during the process of polymerization.…”

“…Therefore, porous swelling anchors allowing for osteointegration may lead to short‐term and long‐term superior fixation, particularly in low‐density bone compared to conventional anchors. Results from a swelling and mechanical testing study conducted on solid, fully porous, and partially porous bone anchors indicated that as the percentage of porosity in the anchor increased the swelling ratio increased but the mechanical strength of the anchor decreased 36,37 …”

“…Results from a swelling and mechanical testing study conducted on solid, fully porous, and partially porous bone anchors indicated that as the percentage of porosity in the anchor increased the swelling ratio increased but the mechanical strength of the anchor decreased. 36,37 The goal of this study was to determine the fixation strength of solid, porous, and partially porous swelling bone anchors using artificial bone of variable density. We also characterized and compared the failure modes produced in the surrounding bone during pull-out testing between swelling anchors and a conventional cancellous bone screw.…”