Elution behavior of a 3D-printed, milled and conventional resin-based occlusal splint material

Effect of layer thickness and polishing on wear resistance of additively manufactured occlusal splints

OBJECTIVE

To evaluate the effect of polishing and layering thickness on the wear resistance of 3D-printed occlusal splint materials.

METHODS

Specimens with 3 different layer thicknesses (50, 75, 100 µm) were produced in the form of a disc 3 mm thick using V-Print splint resin on a 3D-printer with digital light processing technology. (n = 16 for each thickness) All specimens were washed and cured according to the manufacturer's instructions. Half of the specimens of each layer thickness were polished with silicon carbide papers. All specimens were subjected to 120.000 cycles of a chewing simulator for 2-body wear tests. Before and after the wear test, the specimens were scanned with a laser scanner, and the images were overlaid using a 3D analysis program and the volume loss was calculated. The wear patterns of the specimens were examined under a scanning electron microscope. Statistical evaluation was performed using a Shapiro-Wilk test, 2-way ANOVA, 1-way ANOVA, and Tukey post hoc test (α = 0.05).

RESULTS

While polishing had a significant effect (p = 0.003) on the wear volume of the occlusal splints, layer thickness (p = 0.105) and their interaction between polishing and layer thickness (p = 0.620) did not significantly affect the wear volume. Regardless of the polishing, the lowest mean wear was observed for D50 (0.064 mm3), followed by D75 (0.078 mm3), and D100 (0.096 mm3). However, a significant difference was observed only between polished D50 and unpolished D100.

CONCLUSION

The polished 3D-printed occlusal splint resin showed higher wear resistance than the unpolished one, regardless of the layer thickness.

CLINICAL SIGNIFICANCE

Since different layer thicknesses of 50 µm and greater had no effect on the wear resistance of the material, a layer thickness of 100 µm may be preferred for faster printing. However, polishing occlusal splints may reduce the amount of wear and improve clinical performance.

An organotypic model of oral mucosa cells for the biological assessment of 3D printed resins for interim restorations

STATEMENT OF PROBLEM

Three-dimensionally (3D) printed resins have become popular as a new class of materials for making interim restorations. However, little is known about how the fabrication parameters can influence biological compatibility with oral tissues.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of the postpolymerization time on the cytotoxicity of resins for printing interim restorations by using a 3D organotypic model of the oral mucosa.

MATERIAL AND METHODS

Cylindrical specimens were prepared with conventional acrylic resin (AR), computer-aided design and computer-aided manufacture (CAD-CAM) resin (CC), composite resin (CR), and 2 resins for 3D printing (3DP) marketed as being biocompatible. The 3DPs were submitted to postpolymerization in an ultraviolet (UV) light chamber for 1, 10, or 20 minutes (90 W, 405 nm). Standard specimens of the materials were incubated for 1, 3, and 7 days in close contact with an organotypic model of keratinocytes (NOK-Si) in coculture with gingival fibroblasts (HGF) in a 3D collagen matrix, or directly with 3D HGF cultures. Then, the viability (Live/Dead n=2) and metabolism (Alamar Blue n=6) of the cells were assessed. Spectral scanning of the culture medium was performed to detect released components (n=6) and assessed statistically with ANOVA and the Tukey post hoc test (α=.05).

RESULTS

Severe reduction of metabolism (>70%) and viability of keratinocytes occurred for 3DP resin postpolymerized for 1 minute in all periods of analysis in a time-dependent manner. The decrease in cell metabolism and viability was moderate for the 3D culture of HGFs in both experimental models, correlated with the intense presence of resin components in the culture medium. The resins postpolymerized for 10 and 20 minutes promoted a mild-moderate cytotoxic effect in the period of 1 day, similar to AR. However, recovery of cell viability occurred at the 7-day incubation period. The 3DP resins submitted to postpolymerization for 20 minutes showed a pattern similar to that of CR and CC at the end of the experiment.

CONCLUSIONS

The cytotoxic potential of the tested 3DP resins on oral mucosa cells was influenced by postprinting processing, which seemed to have been related with the quantity of residual components leached.

Mechanical and biological properties of polymer materials for oral appliances produced with additive 3D printing and subtractive CAD-CAM techniques compared to conventional methods: a systematic review and meta-analysis

OBJECTIVES

The aim of this review was to analyze mechanical and biological properties of resin materials used with subtractive or additive techniques for oral appliances fabrication and compare them to those conventionally manufactured.

MATERIALS AND METHODS

The protocol was registered online at Open Science Framework (OSF) registries ( https://osf.io/h5es3 ) and the study was based on the preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P). An electronic search was conducted on MEDLINE (via PubMed), Scopus, and Web of Science from 1 February 2022 to 1 May 2022.

INCLUSION CRITERIA

in vitro and in vivo studies published in the last 10 years, with CAD-CAM or 3D printed resins for occlusal splints. Data considered homogenous were subjected to meta-analysis (95% confidence interval [CI]; α = 0.05) with Stata17 statistical software. Since all variables were continuous, the Hedge g measure was calculated. A fixed-effects model was used for I2 = 0%, while statistical analysis was conducted using a random-effects model with I2 > 0%.

RESULTS

13 studies were included after full-text reading. The mechanical properties most studied were wear, flexural strength, surface hardness and surface roughness, while only 1 study investigated biological properties, performing the XTT viability assay. For the meta-analysis, only surface roughness, volume loss, and flexural strength were selected. Considering surface roughness, the subtractive specimen had a lower average value compared to traditional ones (Hedge's g with 95% CI = -1.25[ -1.84, - 0.66]). No significant difference was detected in terms of volume loss (P > 0.05) between the groups (Hedge's g with 95% CI = -0.01 [-2.71, - 2.68]). While flexural strength was higher in the control group (Hedge's g with 95% CI = 2.32 [0.10-4.53]).

CONCLUSION

3D printed materials showed properties comparable to conventional resins, while milled splint materials have not shown better mechanical performance compared with conventional heat-cured acrylic resin. Polyetheretherketone (PEEK) have great potential and needs to be further investigated. Biological tests on oral cell populations are needed to confirm the long-term biocompatibility of these materials.

CLINICAL RELEVANCE

The use of "mixed splints" combining different materials needs to be improved and evaluated in future research to take full advantage of different characteristics and properties.

Analysis of Gingival Fibroblasts Behaviour in the Presence of 3D-Printed versus Milled Methacrylate-Based Dental Resins-Do We Have a Winner?

Computer-aided design and computer-aided manufacturing (CAD/CAM) techniques are based on either subtractive (milling prefabricated blocks) or additive (3D printing) methods, and both are used for obtaining dentistry materials. Our in vitro study aimed to investigate the behavior of human gingival fibroblasts exposed to methacrylate (MA)-based CAD/CAM milled samples in comparison with that of MA-based 3D-printed samples to better elucidate the mechanisms of cell adaptability and survival. The proliferation of human gingival fibroblasts was measured after 2 and 24 h of incubation in the presence of these samples using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and the membrane integrity was assessed through the lactate dehydrogenase release. The level of reactive oxygen species, expression of autophagy-related protein LC3B-I, and detection of GSH and caspase 3/7 were evaluated by fluorescence staining. The MMP-2 levels were measured using a Milliplex MAP kit. The incubation with MA-based 3D-printed samples significantly reduced the viability, by 16% and 28% from control after 2 and 24 h, respectively. There was a 25% and 55% decrease in the GSH level from control after 24 h of incubation with the CAD/CAM milled and 3D-printed samples, respectively. In addition, higher levels of LC3B-I and MMP-2 were obtained after 24 h of incubation with the MA-based 3D samples compared to the CAD/CAM milled ones. Therefore, our results outline that the MA-CAD/CAM milled samples displayed good biocompatibility during 24-h exposure, while MA-3D resins are proper for short-term utilization (less than 24 h).

Bisphenol A release from CAD/CAM splint materials

This study aimed to investigate the bisphenol A (BPA) release from four CAD/CAM splint materials: three polycarbonate-based (DD BioSplint C, Splint Plus Biostar, Temp Premium Flexible) and one polymethylmethacrylate-based (Temp Basic) material. From each material, ten cylindrical samples (n = 40) were immersed in high-performance liquid chromatography (HPLC) grade water following ISO 10993-12 and incubated for 24 h in an incubation shaker at 37°C and 112 rpm. Following BPA derivatization, analysis was performed by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). After 24 h of incubation, all investigated materials released significant amounts of BPA compared to water blanks. The material-dependent elution increased in the following order: DD BioSplint C < Splint Plus Biostar < Temp Basic < Temp Premium Flexible. Subtracting extraneous BPA, the concentrations ranged between 2.27 ng/mL and 12.65 ng/mL. After extrapolating the concentrations in relation to the average surface area of occlusal splints, the amount of BPA per mL exceeded the Tolerable Daily Intake (TDI) set by the European Union for a person weighing 70 kg by 1.32-6.16 times. Contrary to the release from previously investigated materials, BPA elution from CAD/CAM splint materials was highly elevated. Considering the increasing adaptation of CAD/CAM techniques, elution from them may represent a relevant BPA source in daily dental practice.

Digital manufacturing techniques and the in vitro biocompatibility of acrylic-based occlusal device materials

OBJECTIVES

Material chemistry and workflow variables associated with the fabrication of dental devices may affect the biocompatibility of the dental devices. The purpose of this study was to compare digital and conventional workflow procedures in the manufacturing of acrylic-based occlusal devices by assessing the cytotoxic potential of leakage products.

METHODS

Specimens were manufactured by 3D printing (stereolithography and digital light processing), milling, and autopolymerization. Print specimens were also subjected to different post-curing methods. To assess biocompatibility, a human tongue epithelial cell line was exposed to material-based extracts. Cell viability was measured by MTT assay while Western blot assessed the expression level of selected cytoprotective proteins.

RESULTS

Extracts from the Splint 2.0 material printed with DLP technology and post-cured with the Asiga Flash showed the clearest loss of cell viability. The milled and autopolymerized materials also showed a significant reduction in cell viability. However, by storing the autopolymerized material in dH2O for 12 h, no significant viability loss was observed. Increased levels of cytoprotective proteins were seen in cells exposed to extracts from the print materials and the autopolymerized material. Similarly to the effect on viability loss, storing the autopolymerized material in dH2O for 12 h reduced this effect.

CONCLUSIONS/CLINICAL RELEVANCE

Based on the biocompatibility assessments, clinical outcomes of acrylic-based occlusal device materials may be affected by the choice of manufacturing technique and workflow procedures.

In Vitro Cytotoxic and Inflammatory Response of Gingival Fibroblasts and Oral Mucosal Keratinocytes to 3D Printed Oral Devices

The purpose of this study was to examine the biocompatibility of 3D printed materials used for additive manufacturing of rigid and flexible oral devices. Oral splints were produced and finished from six printable resins (pairs of rigid/flexible materials: KeySplint Hard [KR], KeySplint Soft [KF], V-Print Splint [VR], V-Print Splint Comfort [VF], NextDent Ortho Rigid [NR], NextDent Ortho Flex [NF]), and two types of PMMA blocks for subtractive manufacturing (Tizian Blank PMMA [TR], Tizian Flex Splint Comfort [TF]) as controls. The specimens were eluted in a cell culture medium for 7d. Human gingival fibroblasts (hGF-1) and human oral mucosal keratinocytes (hOK) were exposed to the eluates for 24 h. Cell viability, glutathione levels, apoptosis, necrosis, the cellular inflammatory response (IL-6 and PGE2 secretion), and cell morphology were assessed. All eluates led to a slight reduction of hGF-1 viability and intracellular glutathione levels. The strongest cytotoxic response of hGF-1 was observed with KF, NF, and NR eluates (p < 0.05 compared to unexposed cells). Viability, caspase-3/7 activity, necrosis levels, and IL-6/PGE2 secretion of hOK were barely affected by the materials. All materials showed an overall acceptable biocompatibility. hOK appeared to be more resilient to noxious agents than hGF-1 in vitro. There is insufficient evidence to generalize that flexible materials are more cytotoxic than rigid materials. From a biological point of view, 3D printing seems to be a viable alternative to milling for producing oral devices.

Prolonged post-washing in ethanol decreases bond strength of additively manufactured crown materials

OBJECTIVES

This study aimed to investigate the effect of post-washing duration and crown thickness on the bond strength between additively manufactured crown materials and dental cement in vitro.

METHODS

Rectangular-shaped specimens of two thicknesses (1.5 and 2.0 mm) were additively manufactured from permanent VarseoSmile Crown (VC) and long-term temporary NextDent (ND) materials. The specimens were post-washed (n = 160) in ethanol for 5 min, 10 min, 1 h, and 8 h then cemented with dual-cure resin cement. Twenty PMMA (TC) were milled as a control. A chevron-notch test was performed to measure the maximum load until failure (N). Interfacial bond strength (J/m2) was calculated and statistically analysed. The mode of failure was analysed by scanning electron microscopy (SEM).

RESULTS

There was a significant difference in the bond strength between all groups (p < 0.01). VC at 1.5mm thickness post-washed for 10 min showed the highest mean bond strength (1.77 ±0.96 J/m2) while VC at 2.0mm thickness post-washed for 8 h showed the lowest (0.22 ±0.10 J/m2). Exposure to ethanol for 8 h resulted in lower bond strength. Within the type of material, there were no differences in bond strength between the thicknesses when post-washed for the same duration.

CONCLUSIONS

Prolonged post-washing of AM crown materials can significantly decrease the bond strength to resin cement. There were no differences between the permanent and long-term temporary AM materials. When post-washed for 5 min, AM materials observed comparable or higher bond strength values compared to PMMA.

CLINICAL SIGNIFICANCE

The output of this research serves as a guide for dental practitioners, emphasising the importance of adhering to correct post-washing procedures for optimal bond strength of additively manufactured crown materials.

Cellular responses to 3D printed dental resins produced using a manufacturer recommended printer versus a third party printer

PURPOSE

The aim of this study was to evaluate the influence of different 3D dental resins, using a manufacturer recommended printer and a third-party printer, on cellular responses of human gingival cells.

MATERIALS AND METHODS

Three NextDent resins (Denture 3D+, C&B MFH and Crowntec) were used to produce specimens on printers NextDent 5100 (groups ND, NC and NT, respectively) and Phrozen Sonic Mini 4K (groups PD, PC and PT, respectively). Human gingival fibroblasts were cultured and biocompatibility was evaluated on days 1, 3 and 7. IL-6 and IL-8 concentrations were evaluated at 3 days using ELISA. Surface roughness was evaluated by a contact profilometer. SEM and fluorescence micrographs were analyzed at days 1 and 7. Statistical analyses were performed using SPSS and mean differences were tested using ANOVA and post-hoc Tukey tests (P < .05).

RESULTS

There was an increase in cellular viability after 7 days in groups PC and PT, when compared to group PD. ND group resulted in higher concentration of IL-6 when compared to PT group. SEM and fluorescence micrographs showed less adhesion and thinner morphology of fibroblasts from group PD. No significant differences were found regarding surface roughness.

CONCLUSION

The use of different printers or resins did not seem to influence surface roughness. NextDent 5100 and Phrozen Sonic Mini 4K produced resins with similar cellular responses in human gingival fibroblasts. However, Denture 3D+ resin resulted in significantly lower biocompatibility, when compared to C&B MFH and Crowntec resins. Further testing is required to support its long-term use, required for complete dentures.

Validation of a human saliva model for the determination of leachable monomers and other chemicals from dental materials

This study aimed to prove the validity of a mixture of chemicals, including salts, small organic molecules, mucin, and α-amylase, as saliva surrogate ("artificial saliva") for assessing leakage of methacrylate monomers and other constituents from dental materials. To achieve this, we developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantification of 2-hydroxyethyl methacrylate (HEMA), triethylene glycol dimethacrylate (TEGDMA), diurethane dimethacrylate (UDMA), bisphenol A glycerolate dimethacrylate (BisGMA), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), bisphenol A (BPA), and five homologues of ethoxylated bisphenol A dimethacrylate (BisEMA EO2-6) in unstimulated and artificial saliva, and compared their concentrations in the two saliva media following either spiking with a mixture of the compounds or incubation of test specimens of printed biomaterials. Test specimens were immersed in unstimulated/artificial saliva, incubated at 37 °C for 24 h, and saliva aliquots were extracted with methanol and subsequently analyzed by LC-MS/MS. The method was validated with regard to matrix effects, linearity, selectivity, lower limits of quantification (LLOQ), precision, bias and combined measurement uncertainty (u'). The performance characteristics of the method were comparable for unstimulated and artificial saliva samples. The combined u' for individual chemicals at a concentration of 10 × LLOQ were within the range of 5.3-14 % for unstimulated saliva and 6.9-16 % for artificial saliva, except for the BisEMA homologues. Combined u' for the latter were 27-74 % in unstimulated saliva, and 27-79 % in artificial saliva. There was no detectable release of BPA from the test specimens, and the TPO concentrations were mainly below the LLOQ. TEGDMA and UDMA were detected in the highest quantities, and at comparable concentrations in the unstimulated and artificial saliva. For all BisEMA homologues, the release was higher in unstimulated saliva than in artificial saliva. The study showed that the artificial saliva model can be a suitable replacement for native saliva, but might underestimate leakage of more lipophilic methacrylates.

Biaxial Flexural Strength of Printed Splint Materials

One therapeutical alternative in the treatment of functional disorders is the use of printed oral splints. The mechanical properties of these materials are highly essential to their clinical effectiveness, and their performance may vary depending on factors such as cleaning, post-polymerization, or their orientation during construction. The objective of this in vitro investigation is to evaluate the effectiveness of the selected materials in terms of their biaxial flexural strength in relation to the criteria listed above. Splint materials were used in the printing of 720 discs. The printing process was carried out in different orientations in relation to the building platform. Either an automatic or manual cleaning process was performed on the samples. For post-polymerization, either an LED or Xenon light was utilized. A piston-on-three-ball test was used to measure the biaxial flexural strength (BFS) of the materials after they were stored in water for either 24 h or 60 days. The homogeneity of the data was controlled by employing the Levene method, and the differences between the groups were analyzed using the ANOVA and Bonferroni methods. After being stored for twenty-four hours, the mean BFS ranged anywhere from 79 MPa to 157 MPa. Following a period of sixty hours, the BFS exhibited a substantial drop and revealed values that ranged from 72 to 127 MPa. There was no significant difference that could be identified between the materials or between the various cleaning processes. The results of post-polymerization showed that the LED light produced higher means than the Xenon light did. In terms of position, the mean values varied greatly, with 0°'s mean value being 101 MPa, 45°'s mean value being 102 MPa, and 90°'s mean value being 115 MPa. The use of a build orientation of 90° and post-polymerization with LED light resulted in significantly increased biaxial flexural strength. According to this study, this design should be implemented in order to ensure that splint materials have the highest possible strength.

3D-printed and conventional provisional single crown fabrication on anterior implants: A randomized clinical trial

OBJECTIVES

The present study aims to compare provisional single crowns on anterior implants made using conventional PMMA and 3D-printed workflows. The study assessed the occurrence of failures, color variation, signs of early deterioration, operating time, and patients' satisfaction with the treatment through a randomized controlled trial.

METHODS

This study was conducted as a randomized controlled trial, following the SPIRIT and CONSORT guidelines. Patients were included in the study after meeting the eligibility criteria and were randomly assigned to one of two groups (conventional and 3D-printed). FDI criteria, visible plaque index (VPI), bleeding on probing (BOP), and color variation were considered as the primary outcomes. Operating time and patient satisfaction were also assessed as secondary outcomes. Fisher's exact test was performed to analyze the association between the primary and secondary outcomes and the study groups. Mann-Whitney test was used to compare the mean VAS satisfaction scores between the conventional PMMA and 3D-printed groups (STATA 14™, with an α = 0.05).

RESULTS

A total of 42 provisional single crowns (n = 21) were made for 33 patients. Only the fracture parameter (FDI) showed a statistically significant difference, with 3D-printed provisionals exhibiting higher rates of catastrophic failures compared to conventional ones (p = 0.05). Although the operating time for the 3D-printed group was shorter (p < 0.001), no statistical difference observed in patients' satisfaction regarding esthetics, phonetics, chewing, or comfort.

SIGNIFICANCE

3D-printed and conventional PMMA provisional single crowns showed comparable clinical performance, except for the observed fracture types. Although 3D-printed provisional restorations showed a shorter operating time, overall patients' satisfaction was not affected.

Multi-peak Light-emitting Diode Curing Units and Polymerization of 3D-printed Crown and Bridge Resin

PURPOSE

This study aimed to evaluate the substitutive and additive efficacy of multi-peak light-emitting diode (LED) curing units for post-curing of a three-dimensional- (3D-) printed crown and bridge resin.

METHODS

A total of 792 disc- and 180 bar-shaped specimens were printed with a crown and bridge resin (NextDent C&B MFH) and post-cured using two LED curing units (VALO Cordless and Bluephase N G4) in fast and standard modes. Conventional post-curing (LC-3D Print Box, Group PC) was compared with LED-only curing (Groups V1, V2, B1, and B2) and LED-combined curing (Groups PV1, PV2, PB1, and PB2) in terms of microhardness, flexural strength, degree of conversion (DC), and CIE L*a*b* color and translucency parameters. Cytotoxicity of the resin eluates was evaluated using the WST-1 assay. Temperature increases on the resin surface were measured with infrared thermography. Data were statistically analyzed using ANOVA and Kruskal-Wallis tests (α=0.05).

RESULTS

The microhardness and flexural strength in Groups V1, V2, B1, and DC in all LED-only groups were lower than in other groups (p<0.05). Larger color disparities existed between Group PC and all LED-only groups than between Group PC and the others (p<0.05). There was no significant difference in cytotoxicity among the groups. The temperature increase was lowest in Groups V1 and PV1 during light curing (p<0.05).

CONCLUSIONS

Post-curing by multi-peak LED curing units was not as effective as the conventional post-curing device. Additional post-curing by LED curing units did not improve the material properties.

Recent Advances in 3D Printing of Polymers for Application in Prosthodontics

Contemporary mass media frequently depict 3D printing as a technology with widespread utilization in the creation of dental prosthetics. This paper endeavors to provide an evidence-based assessment of the current scope of 3D printing's integration within dental laboratories and practices. Its primary objective is to offer a systematic evaluation of the existing applications of 3D-printing technology within the realm of dental prosthetic restorations. Furthermore, this article delves into potential prospects, while also critically examining the sustained relevance of conventional dental laboratory services and manufacturing procedures. The central focus of this article is to expound upon the extent to which 3D printing is presently harnessed for crafting dental prosthetic appliances. By presenting verifiable data and factual insights, this article aspires to elucidate the actual implementation of 3D printing in prosthetic dentistry and its seamless integration into dental practices. The aim of this narrative review is twofold: firstly, to provide an informed and unbiased evaluation of the role that 3D printing currently plays within dental laboratories and practices; and secondly, to instigate contemplation on the transformative potential of this technology, both in terms of its contemporary impact and its future implications, while maintaining a balanced consideration of traditional dental approaches.

Mechanical Properties of 3D-Printed Occlusal Splint Materials

Data regarding the mechanical properties of three-dimensionally (3D) printed materials for occlusal splint manufacturing are scarce. The aim of the present study was to evaluate the flexural strength and surface hardness of modern 3D-printed occlusal splint materials and compare them with two control groups, namely, milled and conventional cold-polymerized occlusal splint materials. A total of 140 rectangular specimens were manufactured for the present study. The specimens were prepared in accordance with the International Organization for Standardization standards (ISO 20795-1:2013). Five 3D-printed (NextDent Ortho Rigid, Dental LT Clear, Dentona Flexisplint, Cosmos Bite Splint, and ProArt Print Splint), one milled (ProArt CAD Splint), and one cold-polymerized (ProBase Cold) occlusal splint materials were used to determine flexural strength and surface hardness values. The three-point flexure test was used for the determination of flexural strength values, while Vickers hardness was measured to determine surface hardness. Ten specimens (n = 10) of each material were tested using these procedures. One-way ANOVA and Tukey's post-hoc test were used to analyze the obtained results (α = 0.05). The values of flexural strength ranged from 46.1 ± 8.2 MPa to 106 ± 8.3 MPa. The Vickers hardness values ranged from 4.9 ± 0.5 VHN to 20.6 ± 1.3 VHN. Significant differences were found among the tested materials (p < 0.0001). The milled and cold-polymerized materials yielded higher values for both flexural strength (only one 3D-printed resin had comparable results to cold-polymerized acrylics) and surface hardness. There are differences in the mechanical properties of the various tested occlusal splint materials. The flexural strength of most of the 3D-printed materials and their surface hardness values are still inferior when compared to the milled or cold-polymerized materials.

Degree of conversion and residual monomer elution of 3D-printed, milled and self-cured resin-based composite materials for temporary dental crowns and bridges

The aim of this work was to investigate the elution of residual monomers as a function of the manufacturing process, which are CAD/CAM manufacturing, self-curing and 3D printing. The experimental materials used consisted of the base monomers TEGDMA, Bis-GMA and Bis-EMA and 50 wt.% fillers. Additionally, a 3D printing resin without fillers was tested. The elution of the base monomers into the different media (water, ethanol and ethanol/water (75/25 vol. %)) at 37 °C over a period of up to 120 d as well as the degree of conversion (DC) by FTIR were investigated. No monomer elution could be detected in water. Most residual monomers in both other media were released from the self-curing material whereas the 3D printing composite released relatively little. The CAD/CAM blanks released hardly any quantitatively detectable amounts of monomers. Relative to the base composition, TEGDMA eluted less than Bis-GMA and Bis-EMA. DC did not correlate with residual monomer release; thus, leaching was determined not only by the amount of residual monomers present but by further factors as possibly network density and structure. The CAD/CAM blanks and the 3D printing composite showed similar high DC but lower residual monomer release from the CAD/CAM blank, likewise the self-curing composite and the 3D printing resin exhibited similar DC but different monomer elution. In terms of residual monomer elution and DC, the 3D printing composite seems promising as a new material class for the use as temporary dental crowns and bridges.

Leaching components and initial biocompatibility of novel bioactive restorative materials

OBJECTIVES

Bioactive restorative materials were developed on the premise that direct restorations should not only serve the purpose of reconstructing dental hard tissue defects but also exhibit biological features that prevent secondary caries development, without having adverse effects on the host cells. This study focuses on assessing the in vitro biocompatibility of two novel bioactive restorative materials.

METHODS

Specimens of the bioactive restorative materials, Cention Forte (CF) and ACTIVA BioACTIVE RESTORATIVE (AB), a glass ionomer cement/glass hybrid (EQUIA Forte HT, EF) and an established nanohybrid composite (Venus Diamond, VD) were produced and finished. The specimens were eluted in water and methanol and the resulting eluates were analyzed via gas chromatography-mass spectrometry. hGF-1 cells were exposed to eluates prepared in cell culture medium. Cellular ATP levels, oxidized glutathione concentration, caspase-3/7 activity and the inflammatory response (IL-6 and PGE₂ levels) were determined. Microscopic images were taken to examine the cell morphology.

RESULTS

Methyl methacrylate and 2-Hydroxyethyl methacrylate were the main monomers detected in CF and AB eluates. All materials inhibited cell proliferation and led to significantly reduced ATP-levels. The cells exhibited a healthy morphology in the presence of CF and AB. Cells exposed to VD showed increased oxidized glutathione levels. Only EF led to enhanced caspase-3/7 activity. CF and AB caused IL-6 levels to increase, while EF and AB led to enhanced PGE₂ levels.

SIGNIFICANCE

CF and AB are promising materials from a biological point of view and seem to have improved bioactive properties compared to glass ionomer cements.

Conditional Mitigation of Dental-Composite Material-Induced Cytotoxicity by Increasing the Cure Time

Light-cured composite resins are widely used in dental restorations to fill cavities and fabricate temporary crowns. After curing, the residual monomer is a known to be cytotoxic, but increasing the curing time should improve biocompatibility. However, a biologically optimized cure time has not been determined through systematic experimentation. The objective of this study was to examine the behavior and function of human gingival fibroblasts cultured with flowable and bulk-fill composites cured for different periods of time, while considering the physical location of the cells with regard to the materials. Biological effects were separately evaluated for cells in direct contact with, and in close proximity to, the two composite materials. Curing time varied from the recommended 20 s to 40, 60, and 80 s. Pre-cured, milled-acrylic resin was used as a control. No cell survived and attached to or around the flowable composite, regardless of curing time. Some cells survived and attached close to (but not on) the bulk-fill composite, with survival increasing with a longer curing time, albeit to <20% of the numbers growing on milled acrylic even after 80 s of curing. A few cells (<5% of milled acrylic) survived and attached around the flowable composite after removal of the surface layer, but attachment was not cure-time dependent. Removing the surface layer increased cell survival and attachment around the bulk-fill composite after a 20-s cure, but survival was reduced after an 80-s cure. Dental-composite materials are lethal to contacting fibroblasts, regardless of curing time. However, longer curing times mitigated material cytotoxicity exclusively for bulk-fill composites when the cells were not in direct contact. Removing the surface layer slightly improved biocompatibility for cells in proximity to the materials, but not in proportion to cure time. In conclusion, mitigating the cytotoxicity of composite materials by increasing cure time is conditional on the physical location of cells, the type of material, and the finish of the surface layer. This study provides valuable information for clinical decision making and novel insights into the polymerization behavior of composite materials.

Influence of Pre-Treatment and Artificial Aging on the Retention of 3D-Printed Permanent Composite Crowns

The aim of this in vitro study is to investigate the bonding properties of a 3D-printable permanent composite material in comparison to milled composite materials. The tested materials are 3D-printed BEGO VarseoSmile Crown plus (VA1_ab, VA1_nt, VA2_ab, VA2_nt), Vita Enamic (EN1, EN2), and 3M Lava Ultimate (UL1, UL2) (N = 64; n = 8). For this purpose, all crowns are luted to polymer tooth stumps #46 (FDI) using dual-curing luting composite, strictly according to the manufacturer’s instructions. VA1_ab and VA2_ab are additionally airborne-particle abraded. 4 groups (VA2_ab, VA2_nt, EN2, UL2) are artificially aged (1,200,000 cycles, 50 N, 10,000 thermocycles), whereby no specimen has failed. All 64 specimens undergo pull-off testing until retention loss. The mean forces of retention-loss is 786.6 ± 137.6 N (VA1_nt, *), 988.6 ± 212.1 N (VA2_nt, *, Ɨ), 1223.8 ± 119.2 N (VA1_ab, Ɨ, ǂ), 1051.9 ± 107.2 N (VA2_ab, *, Ɨ), 1185.9 ± 211.8 N (EN1, Ɨ, ǂ), 1485.0 ± 198.2 N EN2, ǂ), 1533.8 ± 42.4 N (UL1, ǂ), and 1521.8 ± 343.4 N (UL2, ǂ) (one-way ANOVA (Scheffé method); p < 0.05; *, Ɨ, ǂ: group distribution). No characteristic failure modes can be detected. In conclusion, all of the pull-off forces reflect retention values that seem to be sufficiently high for clinical use. Additional airborne-particle abrasion of VA does not result in significantly better retention but can be recommended.

Effects of resin materials dedicated for additive manufacturing of temporary dental restorations on human gingival keratinocytes

OBJECTIVE

This study investigated the effect of eluates of conventional and 3D-printed resin materials for manufacturing temporary dental restorations on gingival keratinocytes.

METHODS

Three-dimensional (3D)-printed resin materials: 3Delta temp (Deltamed), NextDent MFH (Nextdent), Freeprint temp (Detax), GC temp (GC), were compared to Grandio disc (Voco) and Luxatemp (DMG). Human gingival keratinocytes (IHGKs) were exposed to eluates of the materials and XTT assays were performed at 24 h, 48 h, 72 h, or 144 h. For quantification of the proinflammatory response, the protein amount of IL-6 and 8 was determined in the supernatants using ELISA. One-way ANOVA with post hoc analysis was used to compare differences in cell viability and IL-6 and IL-8 levels between groups.

RESULTS

At 24 h, and more remarkably at 48 h, a significant decrease in cell viability occurred for the 3D-printed materials compared to the untreated IHGKs, but also compared to Grandio disc and Luxatemp. Except for the expression of IL-8 in presence of the eluate of Grandio disc at 24 and 48 h, all tested materials caused attenuation of IL-6 and 8 from IHGKs for any observation period.

CONCLUSIONS

The materials for additive manufacturing affect cell proliferation differently than the subtractive manufactured material Grandio disc and the conventional material Luxatemp.

CLINICAL SIGNIFICANCE

In comparison to conventional and subtractive manufactured restorations, 3D printed temporary restorations might induce more negative effects on the gingival and probably also on pulpal health since viability and the proinflammatory response of oral keratinocytes are more intensively affected by these materials.

Cytotoxicity of 3D‐printed, milled, and conventional oral splint resins to L929 cells and human gingival fibroblasts

Objectives

Evidence on the biocompatibility of three‐dimensional (3D)‐printed and milled resins for oral splints is limited. This in vitro study assessed the influence of the manufacturing method on the cytotoxicity of oral splint resins on L929 cells and human gingival fibroblasts (GF1).

Materials and Methods

Standardized specimens of four 3D‐printed, two‐milled, one‐thermoformed, and one‐pressed splint resin were incubated with L929 and GF1 cells for 24 h. Immunofluorescence and WST‐8 assay were performed to evaluate cytotoxic effects. One‐way analysis of variance and Tukey's multiple comparison test were applied with the variables “splint resin” and “manufacturing method” (p < .05).

Results

Immunofluorescence showed attachment of L929 and GF1 cells to the splint resins. Irrespective of the manufacturing method, the WST‐8 assay revealed significant differences between splint resins for the viability of L929 and GF1 cells. L929 cells generally showed lower viability rates than GF1 cells. The evaluation of cell viability by the manufacturing method showed no significant differences between 3D printing, milling, and conventional methods.

Conclusions

The cytotoxic effects of 3D‐printed, milled, and conventional oral splint resins were similar, indicating minor influence of the manufacturing method on biocompatibility. Cytotoxicity of the resins was below a critical threshold in GF1 cells. The chemical composition might be more crucial than the manufacturing method for the biocompatibility of splint resins.

Resin-based composite materials: elution and pollution

Pollution arises from all human activity and the provision of oral healthcare using resin-based composite restorative materials (RBCs) should be considered. This paper aims to provide a comprehensive review of the potential pollutant risk to the environment from the chemical compounds found in resin-based restorative materials, by including: 1) the principal pollutant compounds present in the resin matrix; 2) the degradation process of RBCs and its consequences; 3) the methods used for the detection and quantification of monomer elution and RBC microparticles; and 4) a review of the release mechanisms of eluates and RBC microparticles into the environment.

RBCs are pollutants by virtue of the compounds created during the degradation processes. These are in the form of the constituent eluted monomers and microparticles. Their impact on the environment and biodiversity is unknown. These materials are currently one of the main direct-placement restorative materials and their success is unquestionable when used and maintained correctly. Mitigation strategies for reducing the impact of pollution on the environment should be considered and implemented by all stakeholders and processes in the supply chain, from manufacturing, clinical use and waste management.

All the constituent components of resin-based composites have the potential to act as environmental pollutants as a consequence of their breakdown and subsequent elution.

Microparticles have a pollution potential as they are easily dispersed in solution and have an increased surface area that potentiates the elution of monomers.

Strategies to reduce their pollution impact should include: a) development of innovative direct-placement restorative materials; b) minimising waste; and c) providing good-quality preventive dentistry that minimises restoration failure and requirement for replacement.

Cytotoxicity of polymers intended for the extrusion-based additive manufacturing of surgical guides

Extrusion-based printing enables simplified and economic manufacturing of surgical guides for oral implant placement. Therefore, the cytotoxicity of a biocopolyester (BE) and a polypropylene (PP), intended for the fused filament fabrication of surgical guides was evaluated. For comparison, a medically certified resin based on methacrylic esters (ME) was printed by stereolithography (n = 18 each group). Human gingival keratinocytes (HGK) were exposed to eluates of the tested materials and an impedance measurement and a tetrazolium assay (MTT) were performed. Modulations in gene expression were analyzed by quantitative PCR. One-way ANOVA with post-hoc Tukey tests were applied. None of the materials exceeded the threshold for cytotoxicity (< 70% viability in MTT) according to ISO 10993-5:2009. The impedance-based cell indices for PP and BE, reflecting cell proliferation, showed little deviations from the control, while ME caused a reduction of up to 45% after 72 h. PCR analysis after 72 h revealed only marginal modulations caused by BE while PP induced a down-regulation of genes encoding for inflammation and apoptosis (p < 0.05). In contrast, the 72 h ME eluate caused an up-regulation of these genes (p < 0.01). All evaluated materials can be considered biocompatible in vitro for short-term application. However, long-term contact to ME might induce (pro-)apoptotic/(pro-)inflammatory responses in HGK.

Additively Manufactured Surgical Implant Guides: A Review

Static computer assisted implant surgery (s-CAIS) is an integral part of the digital workflow in implant dentistry and provides the link between the virtual planning environment and surgical field. The accuracy of s-CAIS is influenced by many cumulative factors including the fit of the template which is related to the manufacturing process. This critical review provides an overview of the current research on additively manufactured surgical implant guides.

The Biological Effects of 3D Resins Used in Orthodontics: A Systematic Review

Three-dimensional (3D) resin medical-dental devices have been increasingly used in recent years after the emergence of digital technologies. In Orthodontics, therapies with aligners have gained popularity, mainly due to the aggressive promotion policies developed by the industry. However, their systemic effects are largely unknown, with few studies evaluating the systemic toxicity of these materials. The release of bisphenol A and other residual monomers have cytotoxic, genotoxic, and estrogenic effects. This systematic review aims to analyze the release of toxic substances from 3D resins used in Orthodontics and their toxic systemic effects systematically. The PICO question asked was, "Does the use of 3D resins in orthodontic devices induce cytotoxic effects or changes in estrogen levels?". The search was carried out in several databases and according to PRISMA guidelines. In vitro, in vivo, and clinical studies were included. The in vitro studies' risk of bias was assessed using the guidelines for the reporting of pre-clinical studies on dental materials by Faggion Jr. For the in vivo studies, the SYRCLE risk of bias tool was used, and for the clinical studies, the Cochrane tool. A total of 400 articles retrieved from the databases were initially scrutinized. Fourteen articles were included for qualitative analysis. The risk of bias was considered medium to high. Cytotoxic effects or estrogen levels cannot be confirmed based on the limited preliminary evidence given by in vitro studies. Evidence of the release of bisphenol A and other monomers from 3D resin devices, either in vitro or clinical studies, remains ambiguous. The few robust results in the current literature demonstrate the absolute need for further studies, especially given the possible implications for the young patient's fertility, which constitutes one of the largest groups of patients using these orthodontic devices.

Effect of post-polymerization with autoclaving treatment on monomer elution and mechanical properties of 3D-printing acrylic resin for splint fabrication

OBJECTIVES

To evaluate the effect of post-treatment autoclaving on monomer elution and mechanical properties of three-dimensionally (3D) printed resin for splint fabrication.

METHODS

Photopolymer resin specimens (Dental LT Clear) were 3D-printed and processed according to the manufacturer's instructions. The specimens were randomly divided to different post-treatment protocols: water storage, autoclaving at different temperatures (121 °C or 132 °C), times (4 or 30 min) and no treatment as a control. The elution of UDMA, HEMA, and EGDMA monomers was determined using high-performance liquid chromatography (HPLC) by immersing the specimens in 75% ethanol for 72 h. The flexural modulus, surface microhardness and linear dimensional changes were measured. The monomer elution and flexural modulus were statistically analyzed using Welch's ANOVA followed by Dunnett's T3 tests, while the surface microhardness and dimensional changes were analyzed using one-way ANOVA followed by Bonferroni tests (α = 0.05).

RESULTS

The overall monomer elution concentrations were significantly highest for the control group and lowest for specimens treated in an autoclave at 132 °C for 4 min. The flexural modulus was not significantly different between all groups. The surface microhardness was significantly higher for all autoclaved groups than the control and water storage groups. The linear expansion was significantly higher after post-treatment autoclaving in contrast to water storage.

SIGNIFICANCE/CONCLUSIONS

Post-polymerization autoclave treatment of the 3D-printed resin reduced monomer elution and improved surface microhardness without deteriorating the flexural modulus. Post-treatment with an autoclave at 132 °C for 4 min can be recommended for 3D-printed resin for splint fabrication.

Dynamic fatigue of 3D-printed splint materials

OBJECTIVES

Printed splints may be an alternative treatment for functional disorders. In addition to the selection of materials, the influence of cleaning or polymerisation can affect the dynamic behaviour and fatigue limit of printed materials.

MATERIAL AND METHODS

96 discs (n = 6 per group, 16mmx2mm) were printed (P30+ DLP-printer, Straumann, CH; 100 μm layer) from splint materials (M1: Luxaprint OrthoPlus, DMG, G; M2: V-Print Splint, Voco, G). Specimens were either automatically cleaned (C1: Straumann P Wash, Straumann, CH) or manually cleaned (C2: Voco Pre-/Main-Clean protocol, Voco, G). Post polymerisation was performed with LED (P1: Cure, Straumann, CH) or Xenon light (P2: Otoflash N171, Ernst Hinrichs Dental, G). The flexural fatigue limit was determined under cyclic loading in terms of a staircase approach with a piston-on-3-ball-test according to ISO 6872 after 24 h or 60 days water storage (37 °C). Specimens were preloaded with 50N and dynamic force was applied for 10⁵ loadings per step (f = 3Hz; steps 1: 50N-100N, 2: 50N-150N, 3: 50N-200N, 4: 50N-250N; F1000, Prematec, G; water at 37 °C).

STATISTICS

Kaplan Maier Log Rank (Mantel-Cox) test, ANOVA, Pearson correlations, Levene-test (α = 0.05, SPSS 26.0, IBM, Armonk, NY, USA)).

RESULTS

Mean survival cycles after 24 h of storage varied between 40388 (M1C2P2) and 195140 (M2C2P1) cycles and after 60 d decreased to 14022 (M1C2P2) and 173237 (M2C1P1). Kaplan Maier Log Rank test revealed significant differences between the material combinations. For M1 cleaning (Pearson: 0.346, p = 0.016) and for M2 polymerisation (Pearson: 0.616, p = 0.000) significantly influenced the number of loading cycles. Intermediate effects were found for material (p = 0.026), cleaning (p = 0.024) and polymerisation (p = 0.000) as well as the combination of material and polymerisation (p = 0.008).

CONCLUSIONS

The results show that the number of possible loading cycles of additively manufactured splint specimens depends on the type of material, their cleaning and post-polymerisation.

CLINICAL SIGNIFICANCE

Materials, cleaning and post-polymerisation of additive manufacturing processes should be matched to improve dynamic loading performance of splint materials.

3D Printing in Digital Prosthetic Dentistry: An Overview of Recent Developments in Additive Manufacturing

Popular media now often present 3D printing as a widely employed technology for the production of dental prostheses. This article aims to show, based on factual information, to what extent 3D printing can be used in dental laboratories and dental practices at present. It attempts to present a rational evaluation of todays´ applications of 3D printing technology in the context of dental restorations. In addition, the article discusses future perspectives and examines the ongoing viability of traditional dental laboratory services and manufacturing processes. It also shows which expertise is needed for the digital additive manufacturing of dental restorations.