The Effect of Time-Dependent Prestresses at the Bone-Screw Interface under Bending Load

The viscoelastic properties of bone significantly influence the mechanical stability of bone-screw complex. This study aims to evaluate the changes in stiffness of a bone-screw model under different bending displacements and pre-stress conditions over time. Using a finite element model, we applied bending displacements of 12,24, and 48 μm were applied to the head of the screw. The stiffness was predicted at time intervals of 1,61,121, and 181 seconds to assess the impact of stress relaxation. The bone was modeled with viscoelastic properties, and the screw was considered rigid. The results showed a decrease in stiffness with increasing displacement. At 12 μ m, the initial stiffness was approximately 6.41 kN/mm, decreasing slightly over time due to stress relaxation. At higher displacements (24 and 48 μ m), the stiffness reduction was more pronounced, especially in the absence of pre-stress, highlighting the significant role of initial thread engagement. Pre-stress conditions led to a stiffness increase of up to 70% compared to the free-stressed state. The study demonstrates that both displacement and pre-stress significantly affect the stiffness of the bone-screw interface. Increased displacement reduces stiffness, while pre-stress raises it. Understanding these interactions is crucial for optimizing orthopedic implant design and surgical techniques to improve patient satisfaction and outcomes.