Effect of nitrogen atmosphere during 3D printing on mechanical properties of orthodontic aligners.

The aim of this study was to investigate the effect of low oxygen environment during printing on the mechanical properties of 3D printed aligners. Thirty-six 3D printed rectangular (20 x 20 x 10 mm) specimens were fabricated using the SprintRay Pro 55 printer and equally divided into two groups. Eighteen of them were built under normal atmosphere conditions during printing (CON) and the rest by continuously purging pure (NIT), thereby eliminating the oxygen content. Both groups underwent post-curing in an illuminated chamber. Three samples from each group were analyzed by attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). The remaining specimens underwent metallographic grinding/polishing followed by nano-indentation testing. The mechanical properties measured were: Martens Hardness (HM), indentation modulus (E(IT)), and elastic index (eta(IT)). Statistically significant differences between groups were assessed using Mann-Whitney tests (alpha = 0.05). Both groups showed identical ATR-FTIR spectra and complete C = C conversion. No statistically significant differences in the parameters tested were seen. The medians (interquartile ranges) were HM CON: 68.0 N/mm(2) (65.0-71.0 N/mm(2)), NIT: 73.0 N/mm(2) (67.0-83.0 N/mm(2)); E(IT) CON: 1999.0 MPa (1952.0-2154.0 MPa); NIT: 2047.0 MPa (1702.0-2104.0 MPa); eta(IT) CON: 28.0% (27.2%-29.0%) and NIT: 28.9% (27.1%-31.4%). The results of this study showed that elimination of oxygen during printing does not affect the mechanical properties of 3D printed aligners.