Additive Manufacturing of Zirconia Ceramic and Its Application in Clinical Dentistry: A Review

Fracture resistance, marginal and internal adaptation of innovative 3D-printed graded structure crown using a 3D jet printing technology

PURPOSE

This in vitro study aimed to create a graded structured dental crown using 3D printing technology and investigate the fracture resistance and the adaptation of this new design.

MATERIALS AND METHODS

A dental crown with a uniform thickness of 1.5 mm was designed, and the exported stereolithography file (STL) was used to manufacture 30 crowns in three groups (n = 10), solid (SC), bilayer (BL), and multilayer (ML) crowns using  3D jet printing technology. Marginal and internal gaps were measured using the silicone replica technique. Crowns were then luted to a resin die using a temporary luting agent and the fracture resistance was measured using a universal testing machine. One-way ANOVA and Tukey post hoc tests were used to compare the fracture resistance and the adaptation of crowns at a significance level of 0.05.

RESULTS

Mean marginal and internal gap of the ML group were 80 and 82 mm, respectively; which were significantly (p < 0.05) smaller than BL (203 and 183 mm) and SC (318 and 221 mm) groups. The SC group showed the highest mean load at fracture (2330 N) which was significantly (p < 0.05) higher than the BL (1716 N) and ML (1516 N) groups.

CONCLUSION

3D jet printing technology provides an opportunity to manufacture crowns in a graded structure with various mechanical properties. This study provided an example of graded structured crowns and presented their fracture resistance. SC group had the highest fracture resistance; however, ML had the best marginal and internal adaptation.

Accuracy, adaptation and margin quality of monolithic zirconia crowns fabricated by 3D printing versus subtractive manufacturing technique: A systematic review and meta-analysis of in vitro studies

OBJECTIVE

The purpose of this systematic review and meta-analysis was to evaluate the accuracy (trueness and precision), marginal and internal adaptation, and margin quality of zirconia crowns made by additive manufacturing compared to subtractive manufacturing technology.

METHODS

The investigation adhered to the PRISMA-ScR guidelines for systematic reviews and was registered at the Prospero database (n°CRD42023452927). Four electronic databases, including PubMed, Scopus, Embase, and Web of Science and manual search was conducted to find relevant studies published until September 2023. In vitro studies that assessed the trueness and precision, marginal and internal adaptation, and margin quality of printed crowns compared to milled ones were included. Studies on crowns over implants, pontics, temporary restorations, laminates, or exclusively experimental materials were excluded.

RESULTS

A total of 9 studies were included in the descriptive reporting and 7 for meta-analysis. The global meta-analysis of the trueness (P<0.74,I2=90 %) and the margin quality (P<0.61,I2=0 %) indicated no significant difference between the root mean square of printed and milled zirconia crowns. The subgroup analysis for the printing system showed a significant effect (P<0.01). The meta-analysis of the crown areas indicated no significant difference in most of the areas, except for the marginal (favoring milled crowns) and axial (favoring printed crowns) areas. For precision and adaptation, both methods showed a clinically acceptable level.

CONCLUSIONS

Additive manufacturing technology produces crowns with trueness and margin quality comparable to subtractive manufacturing. Both techniques have demonstrated the ability to produce crowns with precision levels, internal discrepancy, and marginal fit within clinically acceptable limits.

CLINICAL SIGNIFICANCE

3D printing emerges as a promising and potentially applicable alternative method for manufacturing zirconia crowns, as it shows trueness and margin quality comparable to restorations produced by the subtractive method.

Advances in zirconia-based dental materials: Properties, classification, applications, and future prospects

OBJECTIVES

Zirconia (ZrO2) ceramics are widely used in dental restorations due to their superior mechanical properties, durability, and ever-improving translucency. This review aims to explore the properties, classification, applications, and recent advancements of zirconia-based dental materials, highlighting their potential to revolutionize dental restoration techniques.

STUDY SELECTION, DATA AND SOURCES

The most recent literature available in scientific databases (PubMed and Web of Science) reporting advances of zirconia-based materials within the dental field is thoroughly examined and summarized, covering the major keywords "dental zirconia, classification, aesthetic, LTD, applications, manufacturing, surface treatments".

CONCLUSIONS

An exhaustive overview of the properties, classifications, and applications of dental zirconia was presented, alongside an exploration of future prospects and potential advances. This review highlighted the importance of addressing challenges such as low-temperature degradation resistance and optimizing the balance between mechanical strength and translucency. Also, innovative approaches to improve the performances of zirconia as dental material was discussed.

CLINICAL SIGNIFICANCE

This review provides a better understanding of zirconia-based dental biomaterials for dentists, helping them to make better choice when choosing a specific material to fabricate the restorations or to place the implant. Moreover, new generations of zirconia are still expected to make progress on key issues such as the long-term applications in dental materials while maintaining both damage resistance and aesthetic appeal, defining the directions for future research.

Effect of wall thickness on shape accuracy of hollow zirconia artificial teeth fabricated by a 3D printer

PURPOSE

This study aimed to analyze how the wall thickness of 3D-printed hollow zirconia teeth affects shape accuracy.

METHODS

Datasets with measurement points were created for different artificial teeth resembling the mandibular right first molar (Geomagic Design X, 3D Systems). Reference distances were 9.8 mm for mesio-distal direction (M-D), 10.9 mm for bucco-lingual direction (B-L), 7.0 mm for MB-BB and DB-BB, and 4.5 mm for ML-LB and DL-LB. The outer geometry was identical for all artificial teeth with wall thicknesses of 0.30, 0.50, 0.75, and 1.00 mm. Twenty zirconia teeth were fabricated using a 3D printer (CeraFab 7500 Dental, Lithoz) for each group and sintered before support removal. After performing analog distance measurements using a micrometer screw, the digital distance measurements and angular deviations between measurement points on 3D scans were analyzed. Possible effects were investigated using nonparametric ANOVA, followed by Tukey's honest significant difference (HSD) test for multiple comparisons.

RESULTS

The shape accuracy was acceptable for artificial teeth with wall thicknesses of ≥0.5 mm. The largest distance deviation was observed for a wall thickness of 0.3 mm. In particular, DB-BB showed a median deviation of >56.2 µm, which is significantly larger than that for other test groups, ranging from 7.4-9.5 µm (P < 0.05). In most cases, angular deviations were the largest for teeth with 0.3-mm wall thickness (11.6°) and remained below 5.0° for the other test groups.

CONCLUSIONS

Acceptable accuracy was obtained for artificial teeth with wall thicknesses of at least 0.5 mm.

Advances in ceramics for tooth repair: From bench to chairside

OBJECTIVES

To give a comprehensive review of advancement in dental ceramics, fabrication methods, and the challenges associated with clinical application.

DATA, SOURCES AND STUDY SELECTION

Researches on chemical composition, biomechanical behaviors, optical properties, bonding strategies and fabrication methods were included. The search of articles was independently conducted by two authors in the PubMed, Scopus, Medline and Web of Science.

CONCLUSIONS

Dental ceramics have shown significant advancements in terms of esthetics and function. However, improving fracture toughness without compromising optical properties remains a challenge. Repairing fractured zirconia or glass-matrix ceramic prostheses with the same material is difficult due to the sintering process. Developing innovative bonding techniques that provide strong and long-lasting bonding strength between ceramics and tooth structures poses a recurring obstacle.

CLINICAL SIGNIFICANCE

Despite the emergence of dental ceramics and fabrication techniques, certain limitations such as susceptibility to brittleness and fracture still exist. Therefore, the current review provided valuable information around the advanced dental ceramics in tooth repair. The laboratory test data and the clinical outcome are also presented in details, aiming to guide clinicians in making informed decisions regarding ceramic restorations.

Trueness, physical properties, and surface characteristics of additive-manufactured zirconia crown

OBJECTIVE

This study aimed to conduct a comparison of trueness and physical and surface properties among five distinct types of additive manufactured (AM) zirconia crowns and zirconia crowns produced using the subtractive manufacturing (SM).

MATERIAL AND METHODS

Zirconia crowns were fabricated using five distinct techniques, each varying in the method of slurry transfer and photocuring source. Each experimental group utilized either one of the four digital light processing (DLP)-based techniques (DLP spreading, DLP spreading gradation, DLP vat and DLP circular spreading) or the stereolithography (SLA)-based technique (SLA spreading). The control (CON) group employed SM. To assess accuracy, trueness was measured between the scan and reference data. To analyze the physical properties, voids were examined using high-energy spiral micro-computed tomography scans, and the crystal structure analysis was performed using X-ray diffraction (XRD). Surface roughness was assessed through laser scanning microscopy.

RESULTS

Differences in the trueness of internal surfaces of crowns were found among the groups (P < 0.05). Trueness varied across the measurement surfaces (occlusal, lateral, and marginal) in all the groups except for the DLP spreading gradation group (P < 0.05). Voids were observed in all AM groups. All groups showed similar XRD patterns. All AM groups showed significantly greater surface roughness compared to the CON group (P < 0.001).

CONCLUSION

The AM zirconia crowns showed bubbles and a rougher surface compared to the SM crowns. All groups exhibited typical zirconia traits and trueness levels within clinically acceptable limits, suggesting that current zirconia AM techniques could be suitable for dental applications.

Assessment of the trueness of additively manufactured mol3% zirconia crowns at different printing orientations with an industrial and desktop 3D printer compared to subtractive manufacturing

OBJECTIVES

This study endeavours to investigate the effect of printing orientation on the trueness of additively manufactured molar zirconia crowns. The areal surface roughness and the characteristics of the marginal regions of the crowns were also considered.

METHODS

Twelve molar crowns were manufactured at 0°, 45°, and, 90° printing orientations in a Lithoz and AON zirconia printer, respectively. Twelve milled crowns were used as a comparison. Samples were scanned and analysed in metrology software to determine the trueness of the groups. Regions of interest were defined as the margins, intaglio surface and contact points. Areal surface roughness and print layer thickness were further analysed using a confocal laser scanning microscope.

RESULTS

The results indicate that there are clear differences between the investigated desktop (AON) and industrial (Lithoz) 3D printer. The 45° Lithoz group is the only sample group showing no significantly different results in trueness for all regions analysed compared to the milled group. Areal surface roughness analysis indicates that the print layers in the marginal regions are within clinically tolerable limits and surface characteristics.

CONCLUSIONS

The printing orientation for zirconia crowns is critical to trueness, and differences are evident between different AM apparatuses. Considerations for design and orientation between different apparatuses should therefore be considered when utilising direct additive manufacturing processes. The areal surface roughness of the marginal regions is within acceptable clinical limits for all manufacturing processes and print orientations considered.

CLINICAL SIGNIFICANCE

The materials and apparatuses for additive manufacturing of zirconia crowns are now clinically acceptable from the perspective of the trueness of a final crown for critical functional surfaces and areal surface roughness of the marginal regions.

Optimized 3D printed zirconia-reinforced leucite with antibacterial coating for dental applications

OBJECTIVES

This study aims to produce by robocasting leucite/zirconia pieces with suitable mechanical and tribological performance, convenient aesthetics, and antibacterial properties to be used in dental crown replacement.

METHODS

Leucite pastes reinforced with 12.5%, 25%, and 37.5% wt. ZrO2 nanoparticles were prepared and used to print samples that after sintering were characterized in terms of density, shrinkage, morphology, porosity, mechanical and tribological properties and translucency. A coating of silver diamine fluoride (SDF) and potassium iodide (KI) was applied over the most promising material. The material's antibacterial activity and cytotoxicity were assessed.

RESULTS

It was found that the increase of ZrO2 reinforcement up to 25% enhanced both microhardness and fracture toughness of the sintered composite. However, for a superior content of ZrO2, the increase of the porosity negatively affected the mechanical behaviour of the composite. Moreover, the composite with 25% ZrO2 exhibited neglectable wear in chewing simulator tests and induced the lowest wear on the antagonist dental cusps. Although this composite exhibited lower translucency than human teeth, it was three times higher than the ZrO2 glazed material. Coating this composite material with SDF+KI conferred antibacterial properties without inducing cytotoxicity.

SIGNIFICANCE

Robocasting of leucite reinforced with 25% ZrO2 led to best results. The obtained material revealed superior optical properties and tribomechanical behaviour compared to glazed ZrO2 (that is a common option in dental practice). Moreover, the application of SDF+KI coating impaired S. aureus proliferation, which anticipates its potential benefit for preventing pathogenic bacterial complications associated with prosthetic crown placement.

Influence of glazing and aging on the marginal, axial, axio-occlusal, and occlusal fit of 3-unit monolithic zirconia restorations fabricated using additive and subtractive techniques

STATEMENT OF PROBLEM

Studies are sparse on how glazing and aging influence the fit of additively fabricated monolithic zirconia restorations.

PURPOSE

The purpose of this in vitro study was to assess the effect of glazing and aging on the fit of 3-unit monolithic zirconia restorations fabricated using different techniques.

MATERIAL AND METHODS

A total of 32 monolithic zirconia restorations were fabricated for a typodont model by using 4 distinct techniques (subtractive fabrication [SF], stereolithography [SLA], digital light processing [DLP], and lithography-based ceramic manufacturing [LCM]). The silicone replica approach was adopted to measure the discrepancy values for premolar and molar abutments after sintering, glazing, and 1 year of aging. The silicone replicas were sliced into mesiodistal and buccopalatal cross-sections, and digital micrographs of the cross-sections were made with a ×80 stereomicroscope. An inherent measuring program was run to record the discrepancy values (µm). Repeated-measures 2-way ANOVAs with the Bonferroni post hoc test were used to statistically analyze the acquired data. (α=.05).

RESULTS

From the repeated measures 2-way ANOVAs, both the glazing×fabrication technique and the aging×fabrication technique interactions were not statistically significant (P>.05). Glazing significantly influenced premolar abutment marginal (P=.022) and occlusal (P=.007) discrepancy values, as well as molar abutment marginal discrepancy values (P=.047). Aging had a statistically significant effect on premolar abutment marginal (P=.008) and occlusal (P=.011) discrepancy values, as well as molar abutment occlusal discrepancy values (P=.039). In both the glazing and aging data, for all areas of interest, statistically significant differences were detected among the fabrication techniques (P<.05). The LCM group had the lowest discrepancy values, followed by the SLA, SF, and DLP groups.

CONCLUSIONS

The LCM and SLA groups outperformed the other groups in terms of fit accuracy. The glazing and aging procedures altered the discrepancy values. The marginal discrepancy values of all groups were below the threshold of clinical acceptability (<120 µm).

Effect of firing time and wall thickness on the biaxial flexural strength of 3D-printed zirconia

OBJECTIVES

To evaluate the effect of accelerated firing on 3D-printed zirconia.

METHODS

To check if formulae provided by ISO 6872 can be extended to thin samples, finite element analyses were carried out in advance of fabricating 3-mol% yttria-stabilized tetragonal zirconia polycrystal discs by milling and by 3D-printing. Four groups (n = 38 each) of 3D-printed specimens were produced with two nominal thicknesses (0.6 mm and 1.2 mm) and two firing strategies (long: 51 h, accelerated: 14.5 h). In the milled group (thickness 1.2 mm, n = 30), a standard firing program (9.8 h) was selected. Biaxial flexural strength tests were applied and mean strength, characteristic strength, and Weibull modulus were calculated for each group. Differences were analyzed using Welch ANOVA and Dunnett-T3 post-hoc tests.

RESULTS

Maximum tensile stresses occurring during biaxial strength testing can be calculated according to ISO 6872 for thin samples with b > 0.3 mm. Variability of measured strengths values was smaller for milled zirconia compared with 3D-printed zirconia. The 1.2-mm-thick 3D-printed samples had significantly decreased strength after accelerated firing than after long firing. However, for the 0.6-mm-thick samples, comparable mean biaxial strength values of about 1000 MPa were measured for both firing protocols.

SIGNIFICANCE

At the moment, long fabrication time for zirconia restorations is a major drawback of 3D-printing when compared with milling technology. This investigation showed that the strength of 0.6-mm-thick zirconia discs fabricated by 3D-printing was not impaired by accelerated firing. Thus, overnight firing of thin-walled 3D-printed zirconia restorations could be possible.

The Effect of Sintering on Zirconia Manufactured via Suspension-Enclosing Projection Stereolithography for Dental Applications: An In Vitro Study

BACKGROUND

Zirconia is a widely used material in the dental industry due to its excellent mechanical and aesthetic properties. Recently, a new 3D printing process called suspension-enclosing projection stereolithography (SEPS) was introduced to fabricate zirconia dental restorations. However, the effect of the sintering time and temperature on the properties of zirconia produced via SEPS has not been fully investigated.

METHODS

Zirconia slurries were prepared with varying percentages of zirconia powders and 3D printing resins, and 5Y-TZP (5 mol% yttria-stabilized zirconia) (n = 40) and 3Y-TZP (3 mol% yttria-stabilized zirconia) (n = 40) bar specimens were fabricated via SEPS manufacturing. The specimens were sintered at different temperatures and dwell times, and their flexural strength, density, and phase composition were measured. The viscosity of the slurries was also measured. Statistical analysis was performed using Welch's ANOVA and Kruskal-Wallis tests to evaluate the impact of the sintering conditions.

RESULTS

Significant differences in flexural strength (p < 0.01) were observed between the 5Y-TZP samples, with those sintered at 1530 °C for 120 min showing an average strength of 268.34 ± 44.66 MPa, compared to 174.16 ± 42.29 MPa for those sintered at 1450 °C for 120 min. In terms of density, significant differences (p < 0.01) were noted for the 3Y-TZP specimens, with an average density of 6.66 ± 0.49 g/cm3 for samples sintered at 1530 °C for 120 min, versus 5.75 ± 0.55 g/cm3 for those sintered at 1530 °C for 10 min. X-ray diffraction confirmed the presence of a predominantly tetragonal phase in both materials.

CONCLUSIONS

Zirconia printed via SEPS manufacturing can be sintered at a higher temperature with shorter dwell times, thereby producing high density samples. Different sintering conditions can be used to fully sinter 3D-printed zirconia for potential dental applications.

Clinical Effectiveness of 3D-Milled and 3D-Printed Zirconia Prosthesis-A Systematic Review and Meta-Analysis

BACKGROUND

Additive manufacturing (three-dimensional (3D) printing) has become a leading manufacturing technique in dentistry due to its various advantages. However, its potential applications for dental ceramics are still being explored. Zirconia, among ceramics, has increasing popularity and applications in dentistry mostly due to its excellent properties. Although subtractive manufacturing (3D milling) is considered the most advanced technology for the fabrication of zirconia restorations, certain disadvantages are associated with it.

METHODS

A systematic review was piloted to compare the clinical performance of zirconium crowns that were fabricated using three-dimensional (3D) milling and 3D printing. A meta-analysis was performed, and studies published up to November 2022 were identified. The terms searched were "Zirconium crowns", "3D printing", "CAD/CAM" (Computer-Aided Design and Computer-Aided Manufacturing), "Milling", "dental crowns", and "3D milling". The characteristics that were compared were the year in which the study was published, study design, age of the patient, country, the number of crowns, the type of crown fabrication, marginal integrity, caries status, and outcomes. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used to structure this systematic review. Out of eleven hundred and fifty titles identified after a primary search, nine articles were included in the quantitative analysis. The research question based on PICO/PECO (Participant, Intervention/exposure, Comparison, and Outcome) was "Do 3D-printed and milled (P) zirconia crowns and FDPs (I) have a better survival rate (O) when conventional prosthesis is also an option (C)"? The data collected were tabulated and compared, and the risk of bias and meta-analysis were later performed. Only nine articles (clinical research) were selected for the study. Since there were no clinical studies on the 3D printing of zirconium crowns, six in vitro studies were considered for the comparison. Zirconium crowns in the milling group had an average minimum follow-up of 6 months.

RESULTS

A moderate risk of bias was found, and survival was significant. A high heterogeneity level was noted among the studies. Marginal integrity, periodontal status, and survival rate were high. Linear regression depicted no statistical correlation between the type of cement used and the survival rate.

CONCLUSIONS

It can be concluded that the milled crowns had a higher performance and satisfactory clinical survival.

The state of additive manufacturing in dental research - A systematic scoping review of 2012-2022

Background/purpose

Additive manufacturing (AM), also known as 3D printing, has the potential to transform the industry. While there have been advancements in using AM for dental restorations, there is still a need for further research to develop functional biomedical and dental materials. It's crucial to understand the current status of AM technology and research trends to advance dental research in this field. The aim of this study is to reveal the current status of international scientific publications in the field of dental research related to AM technologies.

Materials and methods

In this study, a systematic scoping review was conducted using appropriate keywords within the scope of international scientific publishing databases (PubMed and Web of Science). The review included related clinical and laboratory research, including both human and animal studies, case reports, review articles, and questionnaire studies. A total of 187 research studies were evaluated for quantitative synthesis in this review.

Results

The findings highlighted a rising trend in research numbers over the years (From 2012 to 2022). The most publications were produced in 2020 and 2021, with annual percentage increases of 25.7% and 26.2%, respectively. The majority of AM-related publications in dentistry research originate from Korea. The pioneer dental sub-fields with the ost publications in its category are prosthodontics and implantology, respectively.

Conclusion

The final review result clearly stated an expectation for the future that the research in dentistry would concentrate on AM technologies in order to increase the new product and process development in dental materials, tools, implants and new generation modelling strategy related to AM. The results of this work can be used as indicators of trends related to AM research in dentistry and/or as prospects for future publication expectations in this field.

Description of Poly(aryl-ether-ketone) Materials (PAEKs), Polyetheretherketone (PEEK) and Polyetherketoneketone (PEKK) for Application as a Dental Material: A Materials Science Review

Poly(aryl-ether-ketone) materials (PAEKs), a class of high-performance polymers comprised of polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), have attracted interest in standard dental procedures due to their inherent characteristics in terms of mechanical and biological properties. Polyetheretherketone (PEEK) is a restorative dental material widely used for prosthetic frameworks due to its superior physical, mechanical, aesthetic, and handling features. Meanwhile, polyetherketoneketone (PEKK) is a semi-crystalline thermoplastic embraced in the additive manufacturing market. In the present review study, a new way to fabricate high-performance polymers, particularly PEEK and PEKK, is demonstrated using additive manufacturing digital dental technology, or 3-dimensional (3D) printing. The focus in this literature review will encompass an investigation of the chemical, mechanical, and biological properties of HPPs, particularly PEEK and PEKK, along with their application particularly in dentistry. High-performance polymers have gained popularity in denture prosthesis in advance dentistry due to their flexibility in terms of manufacturing and the growing interest in utilizing additive manufacturing in denture fabrication. Further, this review also explores the literature regarding the properties of high-performance polymers (HPP) compared to previous reported polymers in terms of the dental material along with the current advancement of the digital designing and manufacturing.

Edge chipping damage in lithium silicate glass-ceramics induced by conventional and ultrasonic vibration-assisted diamond machining

OBJECTIVES

Diamond machining of lithium silicate glass-ceramics (LS) induces extensive edge chipping damage, detrimentally affecting LS restoration functionality and long-term performance. This study approached novel ultrasonic vibration-assisted machining of pre-crystallized and crystallized LS materials to investigate induced edge chipping damage in comparison with conventional machining.

METHODS

The vibration-assisted diamond machining was conducted using a five-axis ultrasonic high-speed grinding/machining machine at different vibration amplitudes while conventional machining was performed using the same machine without vibration assistance. LS microstructural characterization and phase development were performed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. Machining-induced edge chipping depths, areas and morphology were also characterized using the SEM and Java-based imaging software.

RESULTS

All machining-induced edge chipping damages resulted from brittle fractures. The damage scales, however, depended on the material microstructures; mechanical properties associated with the fracture toughness, critical strain energy release rates, brittleness indices, and machinability indices; and ultrasonic vibration amplitudes. Pre-crystallized LS with more glass matrix and lithium metasilicate crystals yielded respective 1.8 and 1.6 times greater damage depths and specific damage areas than crystallized LS with less glass matrix and tri-crystal phases in conventional machining. Ultrasonic machining at optimized amplitudes diminished such damages by over 50 % in pre-crystallized LS and up to 13 % in crystallized LS.

SIGNIFICANCE

This research highlights that ultrasonic vibration assistance at optimized conditions may advance current dental CAD/CAM machining techniques by significant suppression of edge chipping damage in pre-crystallized LS.

A Novel Approach for Powder Bed Fusion of Ceramics Using Two Laser Systems

The one-step AM process is considered the goal many researchers seek in the field of Additive Manufacturing (AM) of high-technology ceramics. Among the several AM techniques, only Powder Bed Fusion (PBF) can directly print high-technology ceramics using one step. However, the PBF technique faces numerous challenges to efficiently be employed in the PBF of ceramics. These challenges include the formation of cracks, generated thermal stress, effective laser-powder interaction, and low acquired relative density. This study developed a new preheating mechanism for ceramic materials using two laser systems to surpass beyond these challenges and successfully print ceramics with a single-step AM method. One laser is used to preheat the powder particles before the second laser is utilised to complete the melting/sintering process. Both lasers travel along the same scanning path. There is a slight delay (0.0001 s) between the preheating laser and the melting/sintering laser to guarantee that the melting/sintering laser scans a properly preheated powder. To further facilitate testing of the preheating system, a numerical model has been developed to simulate the preheating and melting process and to acquire proper process parameters. The developed numerical model was shown to determine the correct process parameters without needing costly and time-consuming experiments. Alumina samples (10 × 10 × 6 mm³) were successfully printed using alumina powder as feedstock. The surface of the samples was nearly defect-free. The samples' relative densities exceeded 80%, the highest reported relative density for alumina produced by a single-step AM method. This discovery can significantly accelerate the transition to a one-step AM process of ceramics.

Effect of maximum support attachment angle on intaglio surface trueness of anatomic contour monolithic prostheses manufactured by digital light processing and zirconia suspension

STATEMENT OF PROBLEM

Support structures are essential for the quality of resin-based prostheses made by the digital light processing (DLP), but few studies have evaluated the effect of support structure on the accuracy of zirconia-based anatomic contour prostheses.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of maximum support attachment angle (MSA) on the intaglio surface trueness of anatomic contour prostheses made by DLP and compare the trueness of 2-unit anatomic contour prostheses with that of those produced by milling.

MATERIAL AND METHODS

Anatomic contour single-unit prostheses were manufactured using DLP and a suspension with 3-mol% yttria-stabilized zirconia. Four different conditions of MSA values to the vertical axis of the object (50, 55, 60, and 65 degrees) were applied (n=10). After printing, postprocessing, and sintering, all successfully produced prostheses were evaluated for intaglio surface trueness by considering the root mean square (RMS). Using the MSA showing the highest trueness, the 2-unit prostheses made by DLP (DLP group) were compared with milled (MIL group) prostheses in terms of intaglio accuracy (n=10). One-way analysis of variance and a post hoc pairwise comparison or independent t test were used for trueness analysis (α=.05).

RESULTS

Three MSA groups (50, 55, and 60 degrees) were successfully produced with significant differences between the trueness of the single-unit prostheses for the groups with different MSA values (P<.05). The highest trueness was in the 50-degree MSA group. The 2-unit prostheses of the DLP group with 50-degree MSA showed significantly lower trueness than those of the MIL group (P<.05); however, the RMS values of both groups were lower than 50 μm.

CONCLUSIONS

The intaglio surface trueness of anatomic contour DLP-generated prostheses can be improved by changing the MSA. The 50-degree MSA was beneficial for the accuracy of both single-unit and 2-unit DLP-generated prostheses, produced within clinically acceptable limits.

Recent Advances on 3D-Printed Zirconia-Based Dental Materials: A Review

Zirconia-based materials are widely used in dentistry due to their biocompatibility and suitable mechanical and tribological behavior. Although commonly processed by subtractive manufacturing (SM), alternative techniques are being explored to reduce material waste, energy consumption and production time. 3D printing has received increasing interest for this purpose. This systematic review intends to gather information on the state of the art of additive manufacturing (AM) of zirconia-based materials for dental applications. As far as the authors know, this is the first time that a comparative analysis of these materials' properties has been performed. It was performed following the PRISMA guidelines and using PubMed, Scopus and Web of Science databases to select studies that met the defined criteria without restrictions on publication year. Stereolithography (SLA) and digital light processing (DLP) were the techniques most focused on in the literature and the ones that led to most promising outcomes. However, other techniques, such as robocasting (RC) and material jetting (MJ), have also led to good results. In all cases, the main concerns are centered on dimensional accuracy, resolution, and insufficient mechanical strength of the pieces. Despite the struggles inherent to the different 3D printing techniques, the commitment to adapt materials, procedures and workflows to these digital technologies is remarkable. Overall, the research on this topic can be seen as a disruptive technological progress with a wide range of application possibilities.

Manufacturing and Characterization of Dental Crowns Made of 5-mol% Yttria Stabilized Zirconia by Digital Light Processing

We herein report manufacturing of dental crowns made of 5-mol% yttria partially stabilized zirconia (5Y-PSZ) with desired mechanical properties, optical translucency and dimensional accuracy using digital light processing (DLP). To this end, all processing parameters were carefully controlled and optimized. First, 5Y-PSZ particles with a bimodal distribution were prepared via calcination of as-received granules and subsequent ball-milling and then used to formulate 5Y-PSZ suspensions with a high solid loading of 50 vol% required for high densification after sintering. Dispersant content was also optimized. To provide high dimensional accuracy, initial dimensions of dental crowns for 3D printing were precisely determined by considering increase and decrease in dimensions during photopolymerization and sintering, respectively. Photopolymerization time was also optimized for a given layer thickness of 50 μm to ensure good bonding between layers. A multi-step debinding schedule with a slow heating rate was employed to avoid formation of any defects. After sintering at 1500 °C for 2 h, 5Y-PSZ could be almost fully densified without noticeable defects within layers and at interfaces between layers. They had high relative densities (99.03 ± 0.39%) with a high cubic phase content (59.1%). These characteristics allowed for achievement of reasonably high mechanical properties (flexural strength = 625.4 ± 75.5 MPa and Weibull modulus = 7.9) and % transmittance (31.4 ± 0.7%). In addition, 5Y-PSZ dental crowns showed excellent dimensional accuracy (root mean square (RMS) for marginal discrepancy = 44.4 ± 10.8 μm and RMS for internal gap = 22.8 ± 1.6 μm) evaluated by the 3D scanning technique.

Evaluation of the Marginal Fit of Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) Zirconia Bridges Fabricated With Different Firing Cycles of Veneering Porcelain: An In Vitro Study

Introduction Zirconia provides adequate mechanical strength to be used as a framework for all ceramic prostheses. Such prostheses must be covered with suitable porcelain to obtain good aesthetic results. The aim To study the effect of the firing cycle numbers of veneering ceramics (one cycle, two cycles, and three cycles) on the marginal fit of computer-aided design (CAD) and computer-aided manufacturing (CAM) zirconia bridges. Materials and methods The sample consisted of 30 full ceramic zirconia bridges, designed by CAD/CAM on a metal bridge model that was designed for this purpose. The sample was divided into three groups (N = 10); group A underwent a single firing cycle, group B underwent two cycles, and group C underwent three cycles. The copper model of the bridge was prepared to be a three-unit bridge, and the impressions of the metal models were taken to make zirconia cores. After that, the veneering ceramic layer and the micro-marginal gap were measured (in microns) on both the buccal and lingual surfaces of each bridge in the sample using the replica technique. A one-way ANOVA test was used to detect statistically significant differences between the groups. Results There were no significant differences between the studied groups in binary comparison; however, the arithmetic mean values of the marginal gap in group C were greater than all the studied groups. Conclusion Within the limitations of the current work, we found that increasing the number of firing cycles of zirconia cores affects the marginal fit; thus, it is recommended to follow the two firing cycle protocol for better adaptation of the CAD/CAM zirconia bridges.

Fracture toughness and hardness of in-office, 3D-printed ceramic brackets

OBJECTIVES

Three-dimensional (3D) printing technology is a promising manufacturing technique for fabricating ceramic brackets. The aim of this research was to assess fundamental mechanical properties of in-office, 3D printed ceramic brackets.

MATERIALS AND METHODS

3D-printed zirconia brackets, commercially available polycrystalline alumina ceramic brackets (Clarity, 3 M St. Paul, MN) and 3D-printed customized polycrystalline alumina ceramic ones (LightForce™, Burlington, Massachusetts) were included in this study. Seven 3D printed zirconia brackets and equal number of ceramic ones from each manufacturer underwent metallographic grinding and polishing followed by Vickers indentation testing. Hardness (HV) and fracture toughness (K1c) were estimated by measuring impression average diagonal length and crack length, respectively. After descriptive statistics calculation, group differences were analysed with 1 Way ANOVA and Holm Sidak post hoc multiple comparison test at significance level α = .05.

RESULTS

Statistically significant differences were found among the materials tested with respect to hardness and fracture toughness. The 3D-printed zirconia proved to be less hard (1261 ± 39 vs 2000 ± 49 vs 1840 ± 38) but more resistant to crack propagation (K1c = 6.62 ± 0.61 vs 5.30 ± 0.48 vs 4.44 ± 0.30 MPa m^1/2 ) than the alumina brackets (Clarity and Light Force respectivelty). Significant differences were observed between the 3D printed and the commercially available polycrystalline alumina ceramic brackets but to a lesser extent.

CONCLUSIONS

Under the limitations of this study, the 3D printed zirconia bracket tested is characterized by mechanical properties associated with advantageous orthodontic fixed appliances traits regarding clinically relevant parameters.

Digital Light Processing of Zirconia Suspensions Containing Photocurable Monomer/Camphor Vehicle for Dental Applications

This study reports the utility of solid camphor as a novel diluent in photocurable hexanediol diacrylate (HDDA) monomer to manufacture 4 mol% yttria partially stabilized zirconia (4Y-PSZ) components for dental applications by digital light processing (DLP). The use of a 65 wt% HDDA-35 wt% camphor solution allowed 4Y-PSZ suspensions to have reasonably low viscosities (1399 ± 55.8 mPa·s at a shear rate of 75 s^-1), measured by a cone/plate viscometer, at a high solid loading of 48 vol%, where 4Y-PSZ particles prepared by calcination of as-received 4Y-PSZ granules, followed by a ball-milling process, were used with assistance of a dispersant. These 4Y-PSZ suspensions could be successfully applied to our custom-made DLP machine for manufacturing 4Y-PSZ components. To this end, several processing parameters, including layer thickness of 4Y-PSZ suspension, UV illumination time for layer-by-layer photocuring process, and initial dimensions of 4Y-PSZ objects, were tightly controlled. As sintering temperature increased from 1300 °C to 1500 °C, relative density and grain size of 4Y-PSZ objects increased, and cubic phase content also increased. Thus, after sintering at the highest temperature of 1500 °C for 3 h, high mechanical properties (biaxial flexural strength = 911 ± 40.7 MPa, hardness = 1371 ± 14.4 H_v) and reasonably high optical transmittance (translucency parameter = 7.77 ± 0.32, contrast ratio = 0.809 ± 0.007), evaluated by a spectrophotometer, were obtained due to a high relative density (97.2 ± 1.38%), which would be useful for dental applications.

Physiomechanical and Surface Characteristics of 3D-Printed Zirconia: An In Vitro Study

The objective of this study is to examine the physiomechanical and surface properties of 3D-printed zirconia in comparison to milled zirconia. A total of 80 disc-shaped (14 × 1.5 ± 0.2 mm) specimens (20 milled and 60 3D-printed (at three different orientations; horizontal, vertical, and tilted)) were manufactured from 3-mol% yttria-stabilized tetragonal zirconia. Five specimens per group were evaluated for crystalline phase, grain size, density, porosity, surface roughness, wettability, microhardness, and SEM analysis of the surface. Biaxial flexural strength (BFS) was measured (n = 15) followed by Weibull analysis and SEM of fractured surfaces. Statistical analysis was performed using one-way ANOVA and Tukey’s post hoc test at α = 0.05. All groups showed a predominant tetragonal phase, with a 450 nm average grain size. There was no significant difference between groups with regards to density, porosity, and microhardness (p > 0.05). The tilted group had the highest surface roughness (0.688 ± 0.080 µm), significantly different from the milled (p = 0.012). The horizontal group presented the highest contact angle (89.11 ± 5.22°), significantly different from the milled and tilted (p > 0.05). The BFS of the milled group (1507.27 ± 340.10 MPa) was significantly higher than all other groups (p < 0.01), while vertical and tilted had a similar BFS that was significantly lower than horizontal (p < 0.005). The highest and lowest Weibull modulus were seen with tilted and milled, respectively. Physical properties of all groups were comparable. The surface roughness of the tilted group was higher than milled. The horizontal group had the highest hydrophobicity. Printing orientations influenced the flexural strength of 3D-printed zirconia. Clinical implications: This study demonstrates how the printing orientation affects the physiomechanical characteristics of printed zirconia.

3D Printing of Bioinert Oxide Ceramics for Medical Applications

Three-dimensionally printed metals and polymers have been widely used and studied in medical applications, yet ceramics also require attention. Ceramics are versatile materials thanks to their excellent properties including high mechanical properties and hardness, good thermal and chemical behavior, and appropriate, electrical, and magnetic properties, as well as good biocompatibility. Manufacturing complex ceramic structures employing conventional methods, such as ceramic injection molding, die pressing or machining is extremely challenging. Thus, 3D printing breaks in as an appropriate solution for complex shapes. Amongst the different ceramics, bioinert ceramics appear to be promising because of their physical properties, which, for example, are similar to those of a replaced tissue, with minimal toxic response. In this way, this review focuses on the different medical applications that can be achieved by 3D printing of bioinert ceramics, as well as on the latest advances in the 3D printing of bioinert ceramics. Moreover, an in-depth comparison of the different AM technologies used in ceramics is presented to help choose the appropriate methods depending on the part geometry.

Additive Manufacturing of Lithium Disilicate with the LCM Process for Classic and Non-Prep Veneers: Preliminary Technical and Clinical Case Experience

BACKGROUND

ceramic veneers, crowns, and other types of restorations are often made using either the press heating technique or the subtractive method. The advent of lithography-based ceramic manufacturing (LCM) allows for the manufacturing of such restorations in an additive way.

METHODS

this concept paper describes the first clinical experience in the application of LCM lithium disilicate restorations in vivo for the manufacturing of classic veneers for a patient with severe tooth wear. The applied restorations were analyzed in terms of their marginal fit in metrology software (Geomagic Control X, 3D Systems). Furthermore, the feasibility of 3D printing of non-prep veneers with a 0.1 mm thickness was tested.

RESULTS

the classic LCM lithium disilicate veneers were tried in the mouth cavity and demonstrated adequate esthetics and a sufficient marginal fit of 100 µm. Furthermore, the non-prep veneers with a 0.1 mm thickness could be successfully printed using LCM technology and also demonstrated an adequate fit on the model in vitro.

CONCLUSIONS

the described technical approach of lithium disilicate 3D printing with LCM technology may pose a valid alternative to subtractive and analog manufacturing and be a game-changing option with the use of additive chairside ceramic fabrication.

Wear Behavior between Aesthetic Restorative Materials and Bovine Tooth Enamel

Tooth enamel wear occurs because of daily mastication and occlusion. This study investigated the wear behavior of bovine teeth against aesthetic restorative materials in vitro. Abrader specimens were fabricated using four tooth-colored restorative materials (zirconia, lithium disilicate glass ceramic, dental porcelain, and resin composite), with bovine tooth enamel as a control. Flattened bovine tooth enamel was used as the substrate specimen. These materials were characterized by Vickers hardness tests and surface roughness measurements. Two-body wear tests between the abrader and substrate specimens were performed, and the worn topographies were evaluated using a contour-measuring instrument and 3D laser microscope. The restorative materials and bovine tooth enamel had similar surface roughness but different hardness and wear behaviors. Bovine teeth showed the largest wear in tooth–tooth contact as the abrader and substrate specimens. Compared to bovine teeth, zirconia, lithium disilicate glass ceramic, and dental porcelain showed greater hardness and less wear on their surfaces, and less substrate wear of the opposite tooth enamel. The lowest hardness resin composite showed intermediate wear on its surface, resulting in the lowest substrate wear. Accordingly, dentists should pay attention to the selection of restorative materials to reconstruct their morphologies owing to different wear behaviors.

Evaluation of intaglio surface trueness, wear, and fracture resistance of zirconia crown under simulated mastication: a comparative analysis between subtractive and additive manufacturing

PURPOSE

This in-vitro analysis aimed to compare the intaglio trueness, the antagonist's wear volume loss, and fracture load of various single-unit zirconia prostheses fabricated by different manufacturing techniques.

MATERIALS AND METHODS

Zirconia crowns were prepared into four different groups (n = 14 per group) according to the manufacturing techniques and generations of the materials. The intaglio surface trueness (root-mean-square estimates, RMS) of the crown was measured at the marginal, axial, occlusal, and inner surface areas. Half of the specimens were artificially aged in the chewing simulator with 120,000 cycles, and the antagonist's volume loss after aging was calculated. The fracture load for each crown group was measured before and after hydrothermal aging. The intaglio trueness was evaluated with Welch's ANOVA and the antagonist's volume loss was assessed by the Kruskal-Wallis tests. The effects of manufacturing and aging on the fracture resistance of the tested zirconia crowns were determined by two-way ANOVA.

RESULTS

The trueness analysis of the crown intaglio surfaces showed surface deviation (RMS) within 50 µm, regardless of the manufacturing methods (P = .053). After simulated mastication, no significant differences in the volume loss of the antagonists were observed among the zirconia groups (P = .946). The manufacturing methods and simulated chewing had statistically significant effects on the fracture resistance (P < .001).

CONCLUSION

The intaglio surface trueness, fracture resistance, and antagonist's wear volume of the additively manufactured 3Y-TZP crown were clinically acceptable, as compared with those of the 4Y- or 5Y-PSZ crowns produced by subtractive milling.

[Manufacturing of zirconia restorations by means of additive fabrication. Part I]

Additive fabrication or layer-by-layer synthesis technologies is one of the most dynamically developing areas of digital production. Modern additive technologies can be used to fabricate zirconia-based restorations. The first part of this article will present the fabrication of zirconia restorations using additive technologies such as stereolithography, digital light processing, selective laser sintering, selective laser melting and inkjet printing, as well as the advantages and disadvantages of the mentioned technologies.

Systematic Compounding of Ceramic Pastes in Stereolithographic Additive Manufacturing

In this paper, stereolithographic additive manufacturing of ceramic dental crowns is discussed and reviewed. The accuracy of parts in ceramic processing were optimized through smart computer-aided design, manufacturing, and evaluation. Then, viscous acrylic resin, including alumina particles, were successfully compounded. The closed packing of alumina particles in acrylic pastes was virtually simulated using the distinct element method. Multimodal distributions of particle diameters were systematically optimized at an 80% volume fraction, and an ultraviolet laser beam was scanned sterically. Fine spots were continuously joined by photochemical polymerization. The optical intensity distributions from focal spots were spatially simulated using the ray tracing method. Consequently, the lithographic conditions of the curing depths and dimensional tolerances were experimentally measured and effectively improved, where solid objects were freely processed by layer stacking and interlayer bonding. The composite precursors were dewaxed and sintered along effective heat treatment patterns. The results show that linear shrinkages were reduced as the particle volume fractions were increased. Anisotropic deformations in the horizontal and vertical directions were recursively resolved along numerical feedback for graphical design. Accordingly, dense microstructures without microcracks or pores were obtained. The mechanical properties were measured as practical levels for dental applications.