Mechanical and numerical investigations of biodegradable magnesium alloy screws for fracture treatment

Small fracture treatment includes the use of so-called "Herbert screws". In the past years, novel resorbable materials were introduced as an alternative to the classical titanium implants. The purpose of this study was to evaluate the influence of ongoing resorption/corrosion processes on the mechanical stability screws made from the magnesium alloy MgYREZr®. Our samples consisted of two partly resorbed screws, explanted due to medical reasons after 6 and 12 weeks, respectively, and five unused reference screws. We performed three-point bending tests to determine the stability of all screws. Additionally, with FE-models of the screws based on μCT-scans, we investigated whether any differences in the bending behavior of the screws can be attributed to the reduction of the material volume due to resorption alone. Both partly resorbed screws failed at a lower force than the reference screws (178.6 ± 5.5 N for the reference screws, 72.5 N and 74.5 N for the screw explanted after 6 and 12 weeks, respectively). FE simulations performed with the three different geometries and original material parameters (Young's modulus E_new  = 45 GPa, yield limit σ_new  = 235 MPa) showed that the early fracture could not be attributed to the changed geometry alone. Material parameters for the partly resorbed screws were determined by fitting the numerical to the experimental force-displacement curves (E_6week  = 15 GPa, σ_6week  = 135 MPa and E_12week  = 8 GPa, σ_12week  = 135 MPa, respectively). Our results showed that both geometry of the screws and different material properties contribute to the overall stability. Understanding and controlling these two factors throughout the resorption process could enhance treatment options.