Development of porous swelling copolymeric bone anchor for orthopedic applications

Bone anchors are crucial in orthopedic surgeries, aiding in fracture fixation, soft tissue reattachment, deformity correction, osteochondral defect repair, and fusion. Existing bone anchors face significant challenges in low-density bones, such as those in the shoulder or osteoporotic conditions. Data from primary arthroscopic rotator cuff repair surgeries between 2007 and 2016 indicate that over 8% of patients required revision procedures, with 38% of those in their 60s needing revision surgery. Currently available clinical bone anchors, such as screws and suture anchors, are made from materials like stainless steel, titanium, PEEK, and resorbable polymers. They often employ threaded mechanisms that rely on shear forces, which can result in low pullout strength in low-density bone as well as gradual loosening over time due to stress shielding. Additionally, catastrophic failures of these threaded anchors leave behind significant bone defects that are difficult to repair. To address these challenges, our group previously introduced a new type of bone anchor made from a co-polymer composed of hydrophobic and hydrophilic components, allowing adjustable proportions to regulate swelling and controlled deterioration of mechanical properties. These anchors establish fixation by absorbing interstitial fluids, generating radial stresses within the surrounding bone to increase frictional resistance against pull-out forces during functional activities. However, the monomers' chemical composition has not been tuned, and no arrangements have been made to incorporate porosity to allow for bone ingrowth. Integrating porosity into swelling copolymeric bone anchors could address the drawbacks of solid anchors noted in prior studies. This approach could facilitate osteointegration by promoting bone migration into the anchor, potentially resulting in long-term fixation. The objective of this research is to develop bone anchors that swell adequately and possess sufficient porosity to facilitate osteointegration, striking a balance where the porosity does not compromise the swelling or mechanical properties of the anchor. Porosity was achieved by adding a non-dissolvable agent (NaCl) during the polymerization process, which was later dissolved in deionized water, leaving behind a porous structure with adequate porosity for potential 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 compression study show that the partially porous swelling co-polymer maintains excellent mechanical properties matching those of cancellous bone, a quick swelling response, and an adequate porous outer layer for mechanically induced osteointegration. Subsequently, we investigated the fixation strength of these swelling copolymeric bone anchors in artificial bone of various densities. The pull-out and subsidence analyses indicate an effective fixation mechanism based on friction, which also minimizes damage following complete failure. The pull-out study states that the implant’s porosity has an inverse relationship with fixation force, while bone density has a direct relationship with fixation force. An important feature of the swelling anchor is their efficient refixation capability. When the force applied to the implant exceeds its passive frictional resistance, the implant starts to move in the bone tunnel, resulting in a slightly reduced but constant dynamic resisting force. Once the applied force is removed, and the implant is no longer displaced under load in the tunnel, the fixation strength is fully recovered. In contrast, in a conventional threaded bone anchor, when the applied force surpasses the maximum fixation strength, the bone between the threads shears, leading to catastrophic implant failure. In the studies mentioned above, swelling characterization was examined using saline solution. Results obtained from recent in vivo studies, where swelling anchors were implanted in the vertebrae of sheep, revealed excessive swelling far beyond what was observed in saline solution. This excessive swelling is likely due to the reaction between the carboxyl group in the swelling bone anchor and the higher alkalinity of body fluids compared to saline solution. Thus, to better simulate in vivo conditions in the subsequent study aimed at refining the swelling bone anchors, fetal bovine serum was utilized. Driven by the need to tailor the material properties of the swelling bone anchor to different bone densities and health conditions, we conducted a study to identify the relationship between monomer ratio and the material properties of the swelling bone anchor. Based on the result of this study, a mathematical relationship using response surface methodology was developed to predict the material properties (swelling and mechanical) based on the ratio of the chemicals. In summary, our findings suggest that the partially porous swelling bone anchor we developed in this research may offer an effective alternative to conventional bone anchors, especially in low-density bone. Furthermore, the composition of these bone anchors can be adjusted to suit various bone density environments.