Development of Polymer-Metal Screw System for Bone Fracture Repair: Combining Strength and Biocompatibility

Bone screws and plates are either made of metals (such as titanium grade 5 alloy or magnesium alloy) or polymers (such as polylactide-co-glycolide, poly-l-lactide, or polyurethane) and are used for the treatment of bone fractures. However, these metallic/polymeric orthopedic products also exhibit several complications, including bone tissue damage, stress shielding, metallosis, and inadequate stability of polymers. We aimed to create a polymer-metal screw system to fix the problems associated with conventional screws in fracture surgery, and in this context, the polymer-metal screw system was designed and manufactured by the authors. The screw system consists of a biomedical grade polymer, either polylactic acid (PLA) or polyurethane (PU) along with a metallic insert made of titanium grade 5 (Ti6Al4V) alloy. The screw's design is protected by a patent publication number US2022/0000529A1. Two sets of polymeric screws were manufactured via injection molding from PLA and PU pellets. The Ti6Al4V metallic insert was placed in both screw sets. The screws were evaluated for the torque required to insert the screw into a polyurethane block and the strength required to pull out the screw from the block using a tension-torque universal testing machine. Also, its bending strength was evaluated and compared with those of titanium screws and polymeric screws without an inner metallic insert. To investigate the proposed screw's ability against bending force under in vivo cyclic bending loads, a bending fatigue test was also performed. The biocompatibility of the screws was evaluated via in vitro cell culture on human osteosarcoma cells and in vivo animal studies on Sprague-Dawley rats, as per ISO 10993. Follow-up investigations were conducted on days 7 and 14 after implantation. The polymeric outer screw exhibited optimum results for driving torque (266.14 ± 46.93 N·mm), pullout force (158.66 ± 9.75 N), and bending (static and fatigue) tests. The PLA outer screw exhibited a static bending strength of 153.8 ± 9.6 MPa, which was approximately 89.9% lower than that of the titanium screw. However, with the inclusion of an inner metallic insert, the bending strength of the PLA outer screw rose significantly from 153.8 ± 9.6 MPa to 531.7 ± 32.5 MPa, thereby reducing the difference with the titanium screw by approximately 65.1%. The screw with an inner metallic insert demonstrated a fatigue strength of 211.8 MPa and a fatigue life of 70,143 cycles. In vitro cell culture studies showed the cytocompatibility of both polymeric screws with the human osteosarcoma cell line. Cell adhesion on the surface of the screw and its morphology were similar to the control untreated cells, as depicted by fluorescence microscopy. No acute systemic toxicity was observed for both PLA and PU screw samples in in vivo studies on rats as biomarkers for hepatotoxicity, and nephrotoxicity biomarkers were observed within the normal range by diagnostic tests. Histopathological evaluation showed better bone growth around the PLA and PU screw implant sections compared to sections of commercially available titanium bone screws. The authors have developed a polymer-metal screw system that addresses the drawbacks of current metallic and polymeric orthopedic products. The screw system has been mechanically characterized, evaluated for biological safety, and shown to be compatible with both in vitro and in vivo evaluations. Overall, the polymer-metal screw system is a promising new orthopedic product with the potential to improve the treatment of bone fractures.