An In Silico Model to Examine the Interaction Between Implant Degradation and Fracture Healing Under Mechanical Loading.

Biodegradable implants are promising for fracture fixation but they have not been applied to the load-bearing skeletal sites. A critical issue is how implant degradation and fracture healing affect each other under mechanical loading. To address this issue, we first developed a finite element model of a long bone fracture fixed with a Zn alloy-based screw-plate system, where implant degradation and bone healing were simulated based upon the continuum damage mechanics and mechano-regulated tissue differentiation algorithm, respectively. For comparison, non-degradable Ti alloy implant with normal bone healing and non-healing fracture with normal implant degradation were served as two reference controls. In terms of the effect of implant degradation on bone healing, the results indicated that implant degradation resulted in a greater volume of newly formed bone within the callus (16% for the degradable implant vs 12% for the non-degradable implant) and a better biomechanical recovery of the fractured bone (bone stiffness fraction: 107% vs 95%) at week 8. Regarding the effect of bone healing on implant degradation, fracture healing led to a significant decrease in the degradation rate of the implant (implant stiffness fraction at week 4: 8% for non-healing vs 40% for healing) and an increase in the overall period from 4 to 8 weeks for a complete degradation of the implant. These results together suggest that implant degradation and fracture healing significantly affect each other under mechanical loading. The in silico model developed here may provide a valuable platform to consider interactions between material degradation and bone healing when designing biodegradable implants for orthopaedic internal fixation at the load-bearing sites.