Effects of intraoral aging on mechanical properties of directly printed aligners vs. thermoformed aligners: an in vivo prospective investigation

Objective: The objective of this study was to examine the impact of intraoral aging on the mechanical properties of directly printed aligners (DPA) compared to thermoformed aligners (TA).Materials and methods: This prospective in vivo experiment included three types of aligners: DPAs (group DP) fabricated from Tera Harz TC-85 DAC resin (Graphy, Korea), TA (group INV) made from a polyurethane-based polymer (Align Technology, Inc., CA, USA), and TA (group DUR) made from polyethylene glycol terephthalate based polymer (Scheu-Dental, Germany). Each group was categorized into retrieved (Clin) and unused aligners (Ctr). Thirty patients (10 per group) wore the aligners for 7 days, thereby generating the retrieved samples. Thirty unused aligners were employed as control samples. The following mechanical properties were determined: Martens Hardness (HM), indentation modulus (EIT), elastic index (eta IT), and indentation relaxation (RIT). Intergroup comparisons were conducted using ANOVA/Kruskal-Wallis test. Comparisons between retrieved and control samples were done using Wilcoxon-Mann-Whitney-U/Student's t-test/Welch's test.Results: Statistically significant differences between the groups were found for both control and used samples (P < .001). Pairwise comparisons also revealed significant differences between the samples. The mechanical properties did not differ significantly between unused and retrieved INV- and DUR-aligners, whereas for DP-aligners significant differences for eta IT and RIT were found following intraoral service (P-values .012 and .002, respectively).Conclusions: Group DUR showed generally more favorable mechanical properties compared to DP and INV. The much higher RIT and EIT in DP aligners suggest their higher rigidity and force decay, which could compromise their clinical efficacy.