Evaluation of the dimensional accuracy of thermoformed appliances taken from 3D printed models with varied shell thicknesses: An in vitro study.

OBJECTIVE: Clinicians make numerous decisions when 3D printing models for fabrication of thermoformed appliances, including printing solid or hollow models. While hollow models can reduce resin use, models intended for thermoformed appliance fabrication must be printed with sufficient thickness to withstand thermoforming. The aim of the study was to determine for hollow 3D printed orthodontic models if there is an effect of shell thickness on the dimensional accuracy of retainers thermoformed upon them as compared with solid models and, if so, to identify the minimum shell thickness that ensures dimensional accuracy of the thermoformed retainer under the conditions investigated. MATERIAL AND METHODS: Thermoformed appliances were fabricated on 3D printed models of six shell thicknesses: 1.0mm, 1.5mm, 2.0mm, 2.5mm, 3.0mm, and solid (n=10/group). The models were scanned before and after thermoforming. Thermoformed appliances were captured by two methods: scanning a polyvinylsiloxane casting of the appliance and scanning the appliance interior surface (intaglio surface). Each model-appliance pair was compared using superimposition software. A generalized linear model and post-hoc Tukey contrasts (α=0.05) were applied to compare each thickness. RESULTS: Model thickness has a statistically significant effect on dimensional accuracy of thermoformed appliances. Appliances fabricated on 1.0mm and 1.5mm models displayed poor accuracy, with a statistically significantly lower percentage of data points within tolerance (±0.250mm) than appliances fabricated on models printed at 2.0mm thickness and greater. CONCLUSIONS: 3D printed model thickness affects the dimensional accuracy of a thermoformed retainer. To ensure minimal deformation and promote clinical utility of the thermoformed appliance, models should be printed with a minimum shell thickness of 2.0mm for the materials investigated.

Effect of print layer height on the assessment of 3D-printed models.

INTRODUCTION: Many variables can affect the accuracy of 3D-printed orthodontic models, and the effects of different printing parameters on the clinical utility of the printed models are just beginning to be understood. The objective of this study was to investigate the effect of print layer height on the assessment of 3D-printed orthodontic models with the use of the American Board of Orthodontics Cast-Radiograph Evaluation grading system. METHODS: Twelve cases were scanned using a desktop model scanner and 3D-printed using a stereolithography-based printer at three different layer heights (25, 50, and 100-µm; n = 12 per group). All models were scored by eleven graders using the Cast-Radiograph Evaluation grading system. All models were scored a second time, at least two weeks later. RESULTS: No statistically significant effects of print layer height were found on the scoring of the models for any of the grading metrics or total score. 3D-printed models of each layer height were highly positively correlated with stone models for the total score, with the strongest correlation found with models printed at 100-µm. CONCLUSIONS: 100-µm layer height 3D-printed models are potentially clinically acceptable for the purposes of evaluation of treatment outcomes, diagnosis and treatment planning, and residency training.

Comparison of automated grading of digital orthodontic models and hand grading of 3-dimensionally printed models.

INTRODUCTION: Emerging workflows in orthodontics enable automated analysis of digital models and production of physical study models from digital files for the evaluation of treatment outcomes. The objective of this study was to compare the automated assessment of digital orthodontic models and the hand grading of 3D-printed models with the use of the American Board of Orthodontics cast-radiograph evaluation (ABO CRE) system. METHODS: Plaster models from 15 cases were scanned with the use of a desktop model scanner to create digital models from which physical models were produced with the use of a stereolithography-based 3D printer. All digital models from each case were graded with the use of an automated software tool (SureSmile), and 3D-printed models were scored by hand with the use of the ABO CRE grading system. All hand-graded models were scored a second time at least 2 weeks later. RESULTS: SureSmile gave statistically significantly higher scores to alignment and rotations (P < 0.001), overjet (P < 0.001), occlusal contacts (P < 0.001), and total score (P < 0.001). Hand grading scored higher in buccolingual inclination (P < 0.001). No significant differences were found in marginal ridges, occlusal relationships, and interproximal contacts. CONCLUSIONS: Scores assessed in an automated manner by SureSmile are generally significantly greater than those assessed by hand grading.

Silver diamine fluoride and bond strength to enamel in vitro: A pilot study.

PURPOSE: To evaluate if pre-treatment with silver diamine fluoride (SDF) adversely affects the bond strength of orthodontic brackets to enamel. METHODS: 30 extracted non-carious permanent molar teeth were embedded in acrylic resin cylinders with buccal surfaces exposed and randomly divided equally into two groups. The experimental enamel surfaces were treated with 38% SDF applied for 1 minute between phosphoric acid etch and metal orthodontic bracket bonding with Transbond XT Light Cure Adhesive. Control groups were treated with 37% phosphoric acid etch followed by bonding. All samples were subjected to 500 thermocycles between 5°C and 55°C prior to shear load testing. Mean values and standard deviations of shear bond strengths for each group were analyzed using a general linear model at P< 0.05. Characteristics of bond failure were also recorded via Adhesive Remnant Index (ARI) and analyzed using an ordinal logistic regression at P< 0.05. RESULTS: No significant difference in shear bond strength to enamel was observed between the control and experimental groups (P= 0.65). Comparison of ARI did demonstrate a significant difference between the groups (P= 0.013); SDF significantly altered the characteristic of bond failure, resulting in more adhesive remaining bonded to enamel after failure. No silver staining of treated surfaces was observed. CLINICAL SIGNIFICANCE: The application of SDF to etched non-carious enamel in vitro prior to orthodontic bracket bonding does not adversely affect bond strength.

A surface-tethered spheroid model for functional evaluation of 3T3-L1 adipocytes.

In order to effectively treat obesity, it must be better understood at the cellular level with respect to metabolic state and environmental stress. However, current two-dimensional (2D) in vitro cell culture methods do not represent the in vivo adipose tissue appropriately due to the absence of complex architecture and cellular signaling. Conversely, 3D in vitro cultures have been reported to have optimal results mimicking the adipose tissue in vivo. The main aim of this study was to examine the efficacy of a novel conjugate of a genetically engineered polymer, elastin-like polypeptide (ELP) and a synthetic polymer, polyethyleneimine (PEI), toward creating a 3D preadipocyte culture system. We then used this 3D culture model to study the preadipocyte differentiation and adipocyte maintenance processes when subjected to various dosages of nutritionally relevant free fatty acids with respect to total DNA and protein content, cell viability, and intracellular triglyceride accumulation. Our results showed that 3T3-L1 preadipocytes cultured on the ELP-PEI surface formed 3D spheroids within 72 h, whereas the cells cultured on unmodified tissue culture polystyrene surfaces remained in monolayer configuration. Significant statistical differences were discovered between the 3D spheroid and 2D monolayer culture with respect to the DNA and protein content, fatty acid consumption, and triglyceride accumulation, indicating differences in cellular response. Results indicated that the 3D culture may be a more sensitive modeling technique for in vitro adipocyte culture and provides a platform for future evaluation of 3D in vitro adipocyte function.

Use of three-dimensional spheroids of hepatocyte-derived reporter cells to study the effects of intracellular fat accumulation and subsequent cytokine exposure.

Non-alcoholic fatty liver disease (NAFLD) is a family of liver diseases associated with obesity. Initial stage of NAFLD is characterized by a fatty liver, referred to as steatosis, which progresses in some individuals to non-alcoholic steatohepatitis (NASH) and liver failure. In order to study and treat the many liver diseases such as NAFLD, an improved in vitro cellular model is needed. Several studies in the past have attempted to elucidate these mechanisms using primary hepatocytes or relevant hepatoma cell lines in two-dimensional (2D) monolayer in vitro cultures. These 2D planar culture systems, unfortunately, do not represent the complex architecture of hepatic tissue in vivo. Therefore, we have engineered an elastin-like polypeptide (ELP)-polyethyleneimine (PEI) copolymer and shown that ELP-PEI coated surfaces influenced H35 rat hepatoma cell morphology to create 3D spheroids. Our reporter cell model recapitulates many cellular features of the human disease, including fatty acid uptake, intracellular triglyceride accumulation, decreased proliferation, decreased liver-specific function, and increased reactive oxygen species accumulation. Finally, to demonstrate the utility of the reporter cells for studying transcriptional regulation, we compared the transcriptional dynamics of nuclear factor κB (NFκB) in response to its classical inducer (tumor necrosis factor-α, TNF-α) under lean and fatty conditions in both 2D and 3D culture configurations. We found that, in 3D spheroids, linoleic acid treatment activated NFκB at earlier time points during the development of steatosis, but suppressed the TNF-mediated NFκB activation at later time points. These studies therefore provide a good starting point to evaluate such relationships observed during NAFLD in a 3D in vitro cell culture.