Effect of Curing Time and Nitrogen Generator Usage on Direct 3D-Printed Aligners Thicknesses—An In Vitro Micro-CT Study

Many variables in the production of direct 3D-printed orthodontic aligners are currently tested to obtain an efficient and safe workflow. The aim of the present study was to investigate the effect of different curing times on the final thickness of the produced aligners. In total, 35 copies of the same aligner with a set thickness of 0.5 mm were directly 3D printed using the instructions provided by the manufacturer. Seven groups (five aligners each) were created. Six groups were post-cured with a nitrogen generator for, respectively, 15 min, 20 min, 25 min, 30 min, 40 min, and 50 min. The last group was cured for 30 min without the nitrogen generator. All aligners were scanned with a micro-CT scanner. To assess local aligner thickness values, a total of 20 reference points were utilised. The comparisons among groups and locations were performed using ANOVA or the Kruskal-Wallis test. Significant differences in thickness were found at various reference points. No impact on curing time was found. No direct association was found between curing time and thicknesses, nor between nitrogen generator usage and thickness. At a 60 degrees printing inclination and following the prescribed centrifugation process, a local thickness increase on the front teeth was observed.