Mechanical and electrochemical characterization of 3D printed orthodontic metallic appliances after in vivo ageing
Spiros Zinelis, Georgios Polychronis, Frixos Papadopoulos, Christos Kokkinos, Anastasios Economou, Nearchos Panayi, Spyridon N. Papageorgiou, Theodore Eliades
Abstract
Three-dimensional (3D) printing technology is a promising technique for fabricating custom orthodontic metallic appliances. Aim of this study was to assess the effect of intraoral aging on the mechanical / electrochemical properties of 3D-printed orthodontic metallic appliances. Twelve molar orthodontic distalization appliances 3D-printed from cobalt chromium (Co-Cr) alloy were retrieved after intraoral use and twenty blocks fabricated under similar conditions were used as control. The samples’ microstructural / elemental composition assessment was assessed with SEM/EDS, while their mechanical properties (modulus of elasticity [EIT], Martens hardness [HM] and the elastic index [ηIT]) were measured by instrumented indentation testing. Finally, the samples’ electrochemical features were assessed with a potentiostat-connected cell arrangement in terms of open circuit potential (OCP), corrosion potential (Ez), current density (I300) and breaking potential (Epit). Results were analyzed by t-test / Mann-Whitney test (α = 0.05). The used Co-Cr alloy was found to have a highly homogenous structure with no significant differences between retrieved and new specimens in HM (4037.7 ± 215.6 vs 4090.9 ± 259.8 N/mm2), EIT (120.0 ± 13.2 vs 123.8 ± 12.9 GPa), or nIT (28.4 ± 2.6 vs 28.6 ± 2.9 %) (P > 0.05 in all instances). Metallic surfaces retained the same oxidation tendency and oxide dissolution rate in passive region in both groups (P > 0.05 for OCP, Ez, and I300). However, intraorally-aged specimens had a significantly lower breakdown potential due to degraded protection efficacy of surface oxide (P = 0.003 for Epit). The tested 3D-printed Co-Cr orthodontic appliances present clinically-acceptable mechanical properties that remained unaffected by intraoral ageing, which however degraded the protection of surface oxide against pitting corrosion.