Towards customisable fully composite bone fracture fixators

Bone fractures can affect people of all ages, leading to an unpleasant and painful experience and possibly disabilities. Open reduction internal fixation (ORIF) with metal plates and screws provides excellent fracture fixation stability; however, the implants often cause soft-tissue adhesion and lack patient-specific customisability due to their inherent rigidity. Research is therefore ongoing to obtain a customisable, fully composite bone fracture fixation system. In this work, the substitution of metal screws with thiol-ene composite ones was investigated for use in AdhFix procedures. AdhFix offers an innovative system for customisable bone fixation patches (BFPs). A thiol-ene composite based on triazine-trione derivatives and inorganic fillers is applied topologically over the bone fracture and around pre-inserted screws. Screws are tightened, then the composite is cured within seconds using a dental lamp. The prototype formulation employing hydroxyapatite (HA) as a filler was found to be unsuitable for small screw manufacturing. Three pathways have therefore been explored for improving the mechanical performance of the composite: employing different fillers, reduction of porosity, and compatibilization of the fillers. Applying a vacuum degassing step yielded a significant reduction in porosity, with a consequent enhancement in mechanical characteristics of the final material. The use of hydroxyapatite whiskers (HA-w) as fillers yielded the highest modulus (9.4 (0.3) GPa, +40%) and a higher strength (81 (5) MPa, +37%). Substitution of HA with silanized bioactive glass yielded a material with lower modulus (5.4 (0.1) GPa, -20%), but significantly higher strength (111 (8) MPa, +88%) and toughness (+500%). The composite containing 36 wt% of silanised bioactive glass was used to manufacture screws and BFPs, which were tested on porcine model fractures. Results showed that smaller composite screws (⌀ 1.5 mm) were the strength limiting factor, while larger (⌀ 2.5 mm) ones achieved strength comparable with prototype BFPs fixated with metal screws. Even though the ⌀ 1.5 mm fully composite BFPs have lower strength than the ones using metal screws, they still have superior strength than k-wires, which yield at 80 N. Furthermore, thanks to the use of bioactive glass, a first step has been made to achieve complete resorbability.