Effect of printing orientation on mechanical properties of 3D-printed orthodontic aligners.

PURPOSE: The purpose of this study was to assess differences in the fundamental mechanical properties of resin-made three-dimensional (3D) printed orthodontic aligners according to the printing orientation. METHODS: Twenty resin 3D-printed dumbbell-shaped specimens and 20 orthodontic aligners were fabricated and postcured in nitrogen. Half of the specimens and aligners were built in horizontal (H), the other half in vertical (V) directions. The dumbbell-shaped specimens were loaded in a tensile testing machine, while parts of the aligners were embedded in acrylic resin, ground, polished, and then underwent instrumented indentation testing (IIT). Mechanical properties that were assessed included the yield strength (YS), breaking strength (BS), plastic strain (ε), Martens hardness (HM), indentation modulus (EIT), elastic index (ηIT), and indentation relaxation (RIT). Data were analyzed statistically with independent t­tests or Mann-Whitney tests at α = 5%. RESULTS: No significant differences were found between specimens or aligners printed either in a horizontal or a vertical direction (P > 0.05 in all instances). Overall, the 3D-printed aligners showed acceptable mechanical propertied in terms of YS (mean 19.2 MPa; standard deviation [SD] 1.7 MPa), BS (mean 19.6 MPa; SD 1.2 MPa), ε (mean 77%; SD 11%), HM (median 89.0 N/mm2; interquartile range [IQR] 84.5-90.0 NN/m2), EIT (median 2670.5 MPa; IQR 2645.0-2726.0 MPa), ηIT (median 27.5%; IQR 25.9-28.1%), and RIT (mean 65.1%; SD 3.5%). CONCLUSION: Printing direction seemed to have no effect on the mechanical properties of 3D-printed resin aligners, which are promising for orthodontic use.

Effect of heat treatment and nitrogen atmosphere during post-curing on mechanical properties of 3D-printed orthodontic aligners.

OBJECTIVES: Three-dimensional (3D)-printed aligners present a promising orthodontic treatment modality, whose clinical success largely depends on the material's mechanical properties. The aim of this study was to evaluate the mechanical properties of resin-made 3D-printed aligners and assess the effect of two different post-curing conditions. MATERIALS AND METHODS: Forty dumbbell-shaped specimens and 40 resin aligners were 3D-printed and divided into four equal groups according to post-curing conditions: presence or absence of oxygen during post-curing and water heat treatment at 85°C for 15 s or none. Samples from the central incisor of the aligner (n = 5/group) were studied by Attenuated Total Reflection Fourier-transform infrared spectroscopy (ATR-FTIR). The dumbbell-shaped specimens were loaded up to fracture under tensile mode and yield strength, ultimate tensile strength, elastic and plastic strain were calculated. The first mandibular molar area from 3D-printed aligners (n = 10/group) was cut and embedded in acrylic resin and then underwent metallographic grinding and polishing followed by instrumented indentation testing to determine the following mechanical properties: Martens hardness, indentation modulus, elastic index, and indentation relaxation. After descriptive statistics, differences according to each post-curing protocol, as well as their combination, were analyzed with linear regression modeling at a 5% significance level. RESULTS: All groups showed identical ATR-FTIR spectra, while no statistically significant effects were seen for either post-curing protocol (N2 presence and heat treatment) or their combination (P > .05 in all instances). CONCLUSIONS: The mechanical properties of 3D-printed resin aligners were not considerably affected either by post-curing in N2 atmosphere or heat treatment.

Reporting, interpretation, and extrapolation issues (SPIN) in abstracts of orthodontic meta-analyses published from 2000 to 2020.

AIM: To assess the prevalence of and identify factors associated with SPIN in abstracts of orthodontic meta-analyses. MATERIALS AND METHODS: Electronic search was performed within the contents of five orthodontic journals and the Cochrane Database of Systematic Reviews (CDSR) to identify meta-analyses of studies involving humans, from 1 January 2000 until 31 August 2020. Inclusion of SPIN in the abstract of meta-analyses, defined as misleading reporting, misleading interpretation, and inappropriate extrapolation of the findings, was documented. Extent of SPIN and associations with journal and year of publication, type of study, number of authors, continent of authorship, methodologist involvement, funding, and significance of the primary outcome were investigated. RESULTS: One hundred and nine meta-analyses were identified, with the highest proportion being published in the European Journal of Orthodontics (EJO: 31/109; 28.4%). Inclusion of SPIN, in at least one domain, was recorded in nearly half (53/109; 48.6%) of the studies, of which 30 (56.6%) included 2 or more domains of SPIN. Meta-analyses of observational studies presented 1.66 times higher risk for including SPIN in their abstracts compared with interventional ones [95% confidence intervals (CIs): 1.14, 2.40; P = 0.007], after adjusting for a number of predictors. Studies with a large number of authors (≥6) presented 1.76 times higher risk of SPIN (≥6 versus 1-3: 95% CIs: 1.04, 2.97; Wald test, P = 0.021), conditional on the pre-defined predictors. CONCLUSIONS: Flaws in the reporting and interpretation of the findings of abstracts of meta-analyses, as framed by inclusion of SPIN are persistent in orthodontic research, being more prevalent in meta-analyses of observational studies. Consistent, multidirectional efforts should be endorsed to improve the quality of the disseminated research findings.