Fixation strength of conformal additively manufactured Ti6Al4V implants in large animal model

Additive manufacturing (AM) enables patient-specific lattice-based implants with porosity engineered to encourage bone ingrowth and to mimic bone's mechanical stiffness. The strength of the bone-implant interface can be measured through a destructive 'push-out' testing. The aim of this study is to explore the effect of implant-bone stiffness ratio (gamma) on the push-out force using numerical simulation and a small experimental study. Numerical simulations of an implant-bone interface during a push-out test showed a fundamental change of failure mode for gamma ranging from 0.1 to 10. For the geometry considered, the largest push-out forces were predicted for gamma approximate to 0.7, essentially doubling the push-out force compared to a solid titanium implant. The experimental and simulation results also demonstrated that using an intermediate stiffness metal implant lattice geometry, gamma approximate to 1.35, does not significantly improve the peak force of the push-out test compared to the solid implant. For the experimental study, critical-sized defects were simulated via robotic bone resection in the right lateral distal femur of a group of similar to 2.5-year-old healthy sheep, and then solid or lattice-based Ti6Al4V implants inserted. The femurs were harvested 6 months after implantation. Nine of the implanted femurs (six solid and three lattice-based) were used for fixation testing. The experimental study showed no significant difference in push-out force between a solid and moderately stiff lattice metal implant as indicated by the numerical simulation.