Biomechanical design considerations of a 3D-printed tibiotalocalcaneal nail for ankle joint fusion

Tibiotalocalcaneal (TTC) arthrodesis treatment using intramedullary nails faces significant challenges due to inadequate bone integration and mechanical stability. This study developed a novel 3D-printed long titanium TTC intramedullary nail incorporating diamond lattice structures and differential thread leads to enhance biological fixation and compression. Four 3D-printed TTC nails (5 mm diameter, 70 mm length) with solid (TTC 1), lattice structure (TTC 2), lattice with longitudinal ribs (TTC 3), and lattice with both longitudinal and transverse ribs (TTC 4) were designed and manufactured. The lattice region featured a diamond array (70% porosity, 650 μm pore size, 1.2 mm unit length) with 2.5 mm thickness surrounding a 2.5 mm solid core. Static four-point bending tests assessed mechanical strength following ASTM F1264 protocols. Six skeletally mature Yorkshire pigs underwent TTC arthrodesis using TTC 1, 2, and 4 designs. Outcomes were evaluated using radiographic imaging and micro-CT analysis at 12 weeks post-surgery. All 3D-printed nails demonstrated acceptable precision with errors below 5% for straightness, circularity, and pitch distance. Mechanical testing revealed fracture strengths of 2387.33 ± 32.88 N, 435.00 ± 50.00 N, 849.17 ± 63.98 N, and 1133.67 ± 81.28 N for TTC 1-4, respectively. The differential thread design achieved significant compression ratios (81-82.5%) at fusion sites. Micro-CT analysis showed significantly higher bone formation in lattice designs (TTC 2: 145.37 ± 37.35 mm³, TTC 4: 137.81 ± 9.52 mm³) compared to the solid design (TTC 1: 28.085 ± 3.21 mm³). However, TTC 2 experienced two implant fractures, while TTC 4 maintained structural integrity while promoting substantial bone growth. This study concluded that titanium 3D printing technology can be applied for manufacturing long TTC intramedullary nails with surface lattice design but reinforcing ribs need to be added to provide enough mechanical strength.