Enhancement of Strength and Torque Capacity of Biodegradable MgNdZr Screws via Material Processing

The object of this study is the installation parameters and material state effects of the MgNdZr system biodegradable screws for osteosynthesis. The dependence of torque moments arising at different stages of screw installation into bone on the diameter of the drilled hole is established. The stresses from the applied torque moments calculated by the finite element method are compared with the ultimate strength and bearing stress of magnesium alloys in various structural states which allows the estimate of the safety factor. It is found that increasing the strength of the MgNdZr alloy through severe plastic deformation allows for an increase in the maximum allowable torque and thread allowance up to 65% for the screw rod and up to 30% for the slot surfaces of the screw head. The obtained results allow for determining safe installation modes for locking and cortical screws made of magnesium alloys into bone during operative osteosynthesis. This is achieved by selecting rational ratios between screw and bone hole diameters. The results show that for screws with a 3 mm slot width, the maximum permissible torque increases by 65%, from 0.23 N m to 0.38 N m, when the MgNdZr alloy transitions from cast to severe plastic deformation state.