Biomechanical evaluation of a novel 3D printing tibiotalocalcaneus nail with trilobular cross-sectional design and self-compression effect

The current tibiotalocalcaneal (TTC) nails used in ankle arthrodesis surgery have shortcomings leading to unfavorable clinical failures. This study proposes a novel nail design and fabricated by metal 3D printing that can enhance the global implant stability through finite element (FE) analysis and fatigue testing. A novel titanium nail was designed with trilobular cross-sectional design for increasing anti-rotation stability. This nail has three leads with different, increasing pitches that increase the self-compression effect in the fusion sites. Between the leads, there are two porous diamond microstructure regions that act as a bone ingrowth scaffold. The nail was fabricated by metal 3D printing and implanted into artificial ankle joint to evaluate the self-compression effects. The nonlinear FE analysis was performed models to compare the anti-rotation stability between trilobular nail (Tri-nail) and the conventional circular nail. The static and fatigue four-point bending tests were done to understand the mechanical strength of the novel nail. The experiment of self-compression effect showed that the three lead design provides two stages of significant compression effect, with a pressurization rate as high as 40%. FE simulated results indicated that the Tri-nail group provides significant tangent displacement reduction as well as reduction in the surrounding bone stress value and the stress distribution is more even in the Tri-nail group. Four-point test found that the Tri-nail yielding strength is 12,957 ± 577 N, which is much higher than the approved FDA reference (1026 N). One million cycles using 8% of the yielding strength (1036 N) were accomplished without Tri-nail failure. The proposed novel metal 3D printing Tri-nail can provide enough mechanical strength and is mechanically stable with superior anti-rotation ability and excellent fusion site self-compression effect.