Dimensional accuracy and clinical adaptation of ceramic crowns fabricated with the stereolithography technique

Marginal gap measurement of ceramic single crowns before cementation: A systematic review

STATEMENT OF PROBLEM

Different instruments have been used to measure the marginal gaps of crowns in vitro. However, a comprehensive systematic review is lacking.

PURPOSE

The purpose of this systematic review was to evaluate the existing literature on the instruments used for the in vitro marginal gap measurement of ceramic single crowns before cementation and to determine whether the crown material and method of fabrication influenced the marginal gap.

MATERIAL AND METHODS

The search was conducted in 2024 across the EBSCO Host, Scopus, PubMed, and Web of Science databases by following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and predefined eligibility criteria. Eligible articles were screened to evaluate 6 instruments for measuring crown marginal gaps: direct view microscopy, scanning electron microscopy, impression replica, cross-sectioning, microcomputed tomography, and 3-dimensional (3D) superimposition. The normality of the data was assessed by using the Kolmogorov-Smirnov test, and the differences in mean marginal gap were statistically evaluated using the Welch ANOVA (α=.05).

RESULTS

Ninety-two articles were included, with 77 documenting single measurement instruments and 15 using a combination of 2 or more measurement instruments. Direct view microscopy was the most used instrument and appeared in 31 (40%) of the studies. No significant differences in mean marginal gap (F=2.09, P=.077) were found across the 6 measurement instruments. Across all studies, excluding those using 3D superimposition, the mean ±standard deviation number of marginal gap measurements per crown was 34.3 ±50.6. Among the 77 studies using a single measurement instrument, 64 used computer-aided design and computer-aided manufacturing (CAD-CAM) technology to fabricate the crowns. CAD-CAM crowns had a mean ±standard deviation marginal gap of 78.9 ±28.6 µm (n=64) compared with 71.6 ±29.5 µm (n=13) for crowns manufactured using conventional methods. Zirconia and lithium disilicate were the most researched materials. Zirconia crowns recorded a mean ±standard deviation marginal gap of 69.4 ±34.2 µm for 972 crowns, which was significantly different (P=.045) from lithium disilicate with a mean ±standard deviation marginal gap of 92.2 ±42.5 µm for 602 crowns.

CONCLUSIONS

Direct view microscopy was the most used marginal gap measurement instrument for ceramic single crowns before cementation, and CAD-CAM was the most used crown fabrication method. No significant differences in mean marginal gap were found among the 6 marginal gap measurement instruments.

Accuracy of additive and subtractive methods for interim tooth-supported restorations: An in vitro comparison of occlusal, axial, and intaglio surfaces

STATEMENT OF PROBLEM

Subtractive and additive manufacturing methods have been commonly used for interim tooth-supported restorations. However, the accuracy of the occlusal, axial, and intaglio surfaces remains unclear.

PURPOSE

The purpose of this in vitro study was to compare the accuracy of 1 subtractive and 2 additive manufacturing methods among occlusal, axial, and intaglio surfaces in the production of interim tooth-supported restorations.

MATERIAL AND METHODS

Interim crowns simulating a maxillary first molar were designed and manufactured using the following 3 methods: milling from polymethylmethacrylate (PMMA), stereolithography (SLA), and digital light projection (DLP), with 13 specimens per group. The manufacturing processes were standardized, and all specimens were scanned under consistent conditions with an intraoral scanner. The standard tessellation language (STL) files of the specimens were exported, and analyzed using a metrology software program. The analysis was performed for the overall crown and for the axial surfaces, nonfunctional and functional cusps, central fossa, margin, and intaglio. The root mean square (RMS) deviations were calculated, and color maps were generated. Statistical analyses including the Welch 1-way analysis of variance (ANOVA) with the Games-Howell post hoc tests (α=.05) and the 2-way ANOVA with the Bonferroni correction tests (α=.05), were performed to assess the impact of technology and crown surface on accuracy.

RESULTS

The milled PMMA method demonstrated significantly greater precision than the SLA or DLP methods (P<.001), with no significant difference in trueness between milled PMMA and SLA (P=.239). Milled PMMA and SLA methods exhibited the highest accuracy for the marginal area, whereas DLP showed the lowest. The intaglio surface accuracy was similar among all methods (P>.05).

CONCLUSIONS

Subtractive and additive manufacturing methods demonstrated comparable accuracy on the intaglio surface, remaining within clinically acceptable limits. Milled PMMA exhibited better occlusal accuracy.

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.

Effect of Axis Change on Shrinkage Rate of 3D-Printed Bioceramic Zirconia Fabricated via Digital Light Processing

Isotropic shrinkage is critical for producing dimensionally accurate prostheses using zirconia. However, the anisotropic shrinkage of 3D-printed zirconia limits its utility in clinical applications. We aimed to evaluate the impact of specimen axis alterations on the shrinkage of digital light processing (DLP)-printed zirconia. Cubes measuring 10 × 10 × 10 mm3 (similar in size to molar crowns) and cuboids measuring 10 × 10 × 20 mm (similar in size to a three-unit bridge) were manufactured using a DLP 3D printer. Zirconia specimens were pre-sintered at 1300 °C and 1400 °C. The Z-axis of some specimens was switched to the X-axis before the final sintering procedure. The X-axis, Y-axis, and Z-axis lengths of the green body, pre-sintered block, and fully sintered block were measured using digital calipers. The 3D-printed specimens showed lower shrinkage and higher deviation than the milled specimens, whose shrinkage rate was 26%. The shrinkage rates of the 3D-printed cubic specimens were 19.9% (length), 20.0% (width), and 21.99% (height), while those of the cuboidal specimens were 20.26%, 19.72%, and 21.81%, respectively. For the 3D-printed specimens, which shrink anisotropically during sintering, the axis change step had no significant impact on the shrinkage rate. In all groups, the shrinkage rate along the building direction during printing significantly exceeded that along the gravity direction during sintering.

Factors Affecting the Color Change of Monolithic Zirconia Ceramics: A Narrative Review

Zirconia restorations are widely used in dentistry due to their high esthetic expectations and physical durability. However, zirconia's opaque white color can compromise esthetics. Therefore, zirconia is often veneered with porcelain, but fractures may occur in the veneer layer. Monolithic zirconia restorations, which do not require porcelain veneering and offer higher translucency, have been developed to address this issue. Zirconia exists in three main crystal phases: monoclinic, tetragonal, and cubic. Metal oxides such as yttrium are added to stabilize the tetragonal phase at room temperature. 3Y-TZP contains 3 mol% yttrium and provides high mechanical strength but has poor optical properties. Recently, 4Y-PSZ and 5Y-PSZ ceramics, which offer better optical properties but lower mechanical strength, have been introduced. This review examines the factors affecting the color change in monolithic zirconia ceramics. These factors are categorized into six main groups: cement type and color, restoration thickness, substrate color, sintering, aging, and zirconia type. Cement type and color are crucial in determining the final shade, especially in thin restorations. Increased restoration thickness reduces the influence of the substrate color while the sintering temperature and process improve optical properties. These findings emphasize the importance of material selection and application processes in ensuring esthetic harmony in zirconia restorations. This review aims to bridge gaps in the literature by providing valuable insights that guide clinicians in selecting and applying zirconia materials to meet both esthetic and functional requirements in restorative dentistry.

Effect of aging on the marginal fit of milled and printed zirconia crowns: an in-vitro study

BACKGROUND

The additive manufacturing is one of the promising methods for fabrication of zirconia based restorations: However, studies on the marginal fit of zirconia restorations fabricated with this technique are sparse. This in vitro study was to measure and compare the marginal fit of milled and printed zirconia based crowns.

METHODS

Twenty-two human premolars were prepared to receive zirconia crowns. Each tooth preparation was scanned by a laboratory scanner. Digitally designed crowns were divided into two groups (n = 11) according to the method of fabrication either via 3D with a commercial 3Y-TZP slurry or a 5-axis milling machine using a multilayer zirconia blank. The marginal gaps were measured before, after cementation, and after thermomechanical accelerated aging. Kolmogorov-Smirnov and Shapiro-Wilk tests were conducted. Mann-Whitney U test was used to compare between the two techniques. Friedman's test was used to compare between marginal gap distances before, after cementation and after aging. Dunn's test was used for pair-wise comparisons when Friedman's test is significant. The significance level was set at P ≤ 0.05.

RESULTS

Marginal discrepancies between the groups showed significant variations. In comparison to milled, 3D printing demonstrated a statistically significant greater marginal gap distance before, after cementation, and after aging (P < 0.001, Effect size = 2.361), (P = 0.011, Effect size = 1.28), and (P = 0.014, Effect size = 1.234), per respective.

CONCLUSION

Significant differences were found between the two technologies used for fabricating zirconia crowns. The least discrepancies values were found with the milling technique.

Do 3D printed and milled tooth-supported complete monolithic zirconia crowns differ in accuracy and fit? A systematic review and meta-analysis of in vitro studies

STATEMENT OF PROBLEM

Additive (3-dimensional printing) and subtractive (milling) methods are digital approaches to fabricating zirconia restorations. Comparisons of their resultant fabrication accuracy and restoration fit are lacking.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the accuracy and fit of monolithic zirconia crowns fabricated by 3-dimensional printing and milling.

MATERIAL AND METHODS

The PubMed (Medline), Scopus, Embase, Web of Science, Cochrane Library, and Google Scholar databases were searched up to August 2023. Eligible records were included, and the standardized mean difference (SMD) analyzed 4 outcomes: marginal fit, intaglio fit, trueness, and precision. Publication bias was analyzed with Trim-and-fill, the Egger regression test, and Begg funnel plot. Methodological quality was rated using the QUIN tool.

RESULTS

A total of 15 publications were found eligible out of the initial 6539 records. The 3-dimensional printing group demonstrated a lower marginal fit (SMD=1.46, 95% CI=[0.67, 2.26], P<.001; I2=83%, P<.001) and trueness (SMD=0.69, 95% CI=[0.20, 1.18], P=.006; I2=88%, P<.001) and a significantly higher precision (SMD=-2.19, 95% CI=[-2.90, -1.48], P<.001; I2=56%, P=.045). The intaglio fit did not differ significantly across the study groups (SMD=0.77, 95% CI=[-0.22, 1.77], P=.127; I2=87%, P<.001).

CONCLUSIONS

Given the high degree of heterogeneity, it can be cautiously concluded that while 3-dimensional printing led to greater precision, the outcomes of the 2 accuracy and adaptation parameters most crucial to the longevity of the restorations-trueness and marginal fit-showed the superiority of the milling technique.

In vitro fracture and fatigue resistance of monolithic zirconia crowns fabricated by stereolithography

STATEMENT OF PROBLEM

Stereolithography has been used to print zirconia ceramic crowns with acceptable dimensional accuracy and fracture force. However, studies that compared the fatigue resistance of zirconia crowns fabricated by stereolithography are lacking.

PURPOSE

The purpose of this in vitro study was to compare the fracture and fatigue resistance of monolithic zirconia crowns printed by stereolithography apparatus (SLA) and digital light processing (DLP) with those of zirconia crowns milled by computer numerical control (CNC).

MATERIAL AND METHODS

A total of 120 crowns were fabricated (n=40/material) and underwent 0, 104, 105, or 106 dynamic loading cycles of 30 to 300 N in artificial saliva, followed by a static fracture loading test (n=10). After fracture, 1 crown from each group was selected for fractography analysis by scanning electron microscopy (SEM). Data were statistically analyzed through 2-way ANOVA and post hoc analysis for multiple comparisons (α=.05).

RESULTS

The 2-way ANOVA results showed that the mean ±standard deviation force at fracture was the highest for CNC (before fatigue loading: 5154 ±568 N, 104: 5735 ±1231 N, 105: 5523 ±797 N, and 106: 6007 ±1258 N), followed by DLP (before fatigue loading: 3381 ±612 N, 104: 4046 ±1146 N, 105: 2929 ±559 N, and 106: 3223 ±739 N), and the lowest for SLA (before fatigue loading: 2956 ±598 N, 104: 2757 ±421 N, 105: 3326 ±391 N, and 106: 3103 ±246 N) (P<.01). The fracture force of the crowns was not significantly affected by the number of fatigue cycles (P>.05). Fractography analysis showed that the number of arrest lines increased for crowns of all 3 materials. SEM images also showed the steps of SLA and DLP from their layer-by-layer printing and small cracks between layers of SLA after 106 loading cycles.

CONCLUSIONS

The fracture force of monolithic zirconia crowns milled by CNC was significantly higher than that of zirconia crowns printed by stereolithography. Zirconia crowns printed by SLA and DLP could withstand typical clinical conditions, and their fracture and fatigue resistance exceeded the clinically estimated average occlusal forces.

Shear bond strength of vat photopolymerization additive-manufactured zirconia to veneering ceramic

PURPOSE

To evaluate the shear bond strength (SBS) of stereolithography (SLA), digital light processing (DLP) manufactured, and computer numerical control (CNC) milled zirconia to veneering ceramic.

MATERIALS AND METHODS

Rectangular shaped zirconia substrates (10 × 5 × 5 mm3) were manufactured through SLA, DLP, and CNC technology separately. Their surface roughness was measured and the surface topography was analyzed by atomic force microscope (AFM). Then the veneering ceramic (5 × 5 × 5 mm3) was applied to carry out the SBS test. Failure modes were examined by a scanning electron microscope (SEM). The data of SBS and roughness were statistically analyzed with one-way ANOVA followed by S-N-K post hoc comparisons (a = 0.05).

RESULTS

The surface roughness of the SLA group (0.38 ± 0.03 µm) and the DLP group (0.37 ± 0.04 µm) were both significantly higher than the CNC group (0.16 ± 0.00 µm) (p < 0.001). AFM results revealed irregular surface of SLA and DLP zirconia. No significant difference was found in SBS value of the three groups (p = 0.253). SEM image showed different failure modes including cohesive, adhesive, and mixed failure.

CONCLUSION

The bonding ability of SLA and DLP zirconia to veneering ceramic were comparable with that of CNC zirconia.

Accuracy, fit, and marginal quality of advanced additively manufactured and milled zirconia 3-unit fixed dental prostheses

STATEMENT OF PROBLEM

Advanced additive manufacturing (AM) of zirconia is an emerging technology that can explore the limitations of traditional computer-aided design and computer-aided manufacturing (CAD-CAM) milling techniques. However, a comprehensive evaluation of their differences in producing zirconia restorations, especially multi-unit restorations, is lacking.

PURPOSE

The purpose of this in vitro study was to compare the accuracy, fit, marginal quality, and surface roughness of zirconia 3-unit fixed dental prostheses (FDPs) by using advanced AM and 2 CAD-CAM milling materials.

MATERIAL AND METHODS

Based on the same CAD model, 30 3-unit posterior FDPs (n=10) were manufactured by using AM and 2 CAD-CAM milling materials (VT and UP). The accuracies of the total, intaglio, occlusal, axial, and marginal regions were calculated separately by comparing the scanned model with the design model by using 3-dimensional (3D) deviation analysis. The silicone layer was scanned to evaluate the marginal and intaglio fit in 3 dimensions. A 3D laser microscope was used for surface roughness detection, marginal quality assessment, and marginal defect measurement. The data were analyzed using ANOVA and the Tukey post hoc test (α=.05).

RESULTS

Compared with CAD-CAM milling, the AM group had higher accuracy and smaller positive deviations on the axial and intaglio regions (P<.001). Different manufacturing methods showed no statistically significant effect on the mean intaglio fit (P>.05), and all were within the clinically acceptable range (<100 µm). The intaglio gap was significantly higher than the target parameter in the occlusal regions. AM-fabricated FDPs had significantly higher surface roughness than milled ones, yet showed better margin quality with fewer marginal defects CONCLUSIONS: Compared with CAD-CAM milling, the advanced additively manufactured zirconia 3-unit FDPs provided better accuracy, improved margin quality, and clinically acceptable fit, but higher surface roughness, and may be a promising alternative for clinical applications.

Zirconia crowns manufactured using digital light processing: Effects of build angle and layer thickness on the accuracy

OBJECTIVES

This study investigated the effects of build angle and layer thickness on the trueness and precision of zirconia crowns manufactured using digital light processing (DLP) technology.

MATERIALS AND METHODS

Single crowns were fabricated from zirconia using DLP technology. The crowns were manufactured with three different representative build angles (0°, 45°, and 90°) and two different layer thicknesses (30 μm and 50 μm). After printing, the specimens were non-contact-scanned, and their accuracy was assessed using a 3D analysis software. Root mean square (RMS) values were used to determine trueness and precision. Color maps were generated to detect deviations within the specimens. Statistical analyses were conducted using two-way ANOVA.

RESULTS

Build angle and layer thickness significantly affected trueness and precision (p < 0.05). At a 30-μm layer thickness, the crowns printed at angles of 0° (32.2 ± 3.2 μm) and 45° (33.9 ± 2.4 μm) demonstrated the best marginal trueness compared to those in other groups (p < 0.05). Notably, those printed at an angle of 90° exhibited the best intaglio surface trueness (37.4 ± 4.0 μm). At a 50-μm layer thickness, the crowns printed at an angle of 90° exhibited the lowest accuracy concerning marginal and intaglio surface aspects (27.7 ± 8.2 μm).

CONCLUSIONS

Both the build angle and layer thickness significantly affected the dimensional accuracy of DLP-printed zirconia crowns, with the 30-μm layer thickness offering superior trueness. Optimal results were achieved using build angles of 0° and 45° in conjunction with thinner layers, minimizing marginal defects.

CLINICAL SIGNIFICANCE

All zirconia crowns produced at different build angles and layer thicknesses satisfied clinical requirements. Specific combinations of these factors realized the fabrication of single crowns that possessed the highest accuracy.

Dimensional accuracy and clinical adaptation of monolithic zirconia crowns fabricated with the nanoparticle jetting technique

STATEMENT OF PROBLEM

The nanoparticle jetting (NPJ) technique is a recently developed additive manufacturing method that may have useful dental applications. The manufacturing accuracy and clinical adaptation of zirconia monolithic crowns fabricated with NPJ are unknown.

PURPOSE

The purpose of this in vitro study was to compare the dimensional accuracy and clinical adaptation of zirconia crowns fabricated with NPJ and those fabricated with subtractive manufacturing (SM) and digital light processing (DLP).

MATERIAL AND METHODS

Five standardized typodont right mandibular first molars were prepared for ceramic complete crowns, and 30 zirconia monolithic crowns were fabricated using SM, DLP, and NPJ (n=10) with a completely digital workflow. The dimensional accuracy at the external, intaglio, and marginal areas was determined by superimposing the scanned data and computer-aided design data of the crowns (n=10). Occlusal, axial, and marginal adaptations were evaluated by using a nondestructive silicone replica and dual scanning method. The 3-dimensional discrepancy was evaluated to determine clinical adaptation. Differences among test groups were analyzed by using a MANOVA and the post hoc least significant difference test for normally distributed data or the Kruskal-Wallis test with Bonferroni correction for nonnormally distributed data (α=.05).

RESULTS

Significant differences were found in the dimensional accuracy and clinical adaptation among the groups (P<.001). The NPJ group had a lower overall root mean square (RMS) value for dimensional accuracy (22.9 ±1.4 μm) than the SM (27.3 ±5.0 μm) and DLP (36.4 ±5.9 μm) groups (P<.001). The NPJ group had a lower external RMS value (23.0 ±3.0 μm) than the SM group (28.9 ±5.4 μm) (P<.001) and equivalent marginal and intaglio RMS values than the SM group. The DLP group had larger external (33.3 ±4.3 μm), intaglio (36.1 ±10.7 μm), and marginal (79.4 ±12.9 μm) deviations than the NPJ and SM groups (P<.001). With regard to clinical adaptation, the marginal discrepancy was smaller in the NPJ group (63.9 ±27.3 μm) than in the SM group (70.8 ±27.5 μm) (P<.001). No significant differences were found between the SM and NPJ groups in terms of the occlusal (87.2 ±25.5 and 80.5 ±24.2 μm, respectively) and axial (39.1 ±19.7 and 38.4 ±13.7 μm, respectively) discrepancies. The DLP group had larger occlusal (239.0 ±60.1 μm), axial (84.9 ±29.1 μm), and marginal (140.4 ±84.3 μm) discrepancies than the NPJ and SM groups (P<.001).

CONCLUSIONS

Monolithic zirconia crowns fabricated using NPJ have higher dimensional accuracy and clinical adaptation than those fabricated using SM or DLP.

Trueness and precision of digital light processing fabricated 3D printed monolithic zirconia crowns

OBJECTIVES

The present study aimed to evaluate the trueness and precision of monolithic zirconia crowns (MZCs) fabricated by 3D printing and milling techniques.

METHODS

A premolar crown was designed after scanning a prepared typodont. Twenty MZCs were fabricated using milling and 3D-printing techniques (n = 10). All the specimens were scanned with an industrial scanner, and the scanned data were analyzed using 3D measurement software to evaluate the trueness and precision of each group. Root mean square (RMS) deviations were measured and statistically analyzed (One-way ANOVA, Tukey's, p ≤ 0.05).

RESULTS

The trueness of the printed MZC group (140 ± 14 μm) showed a significantly higher RMS value compared to the milled MZCs (96 ± 27 μm,p < 0.001). At the same time, the precision of the milled MZCs (61 ± 17 μm) showed a significantly higher RMS value compared to that of the printed MZCs (31 ± 5 μm,p < 0.001).

CONCLUSIONS

The Fabrication techniques had a significant impact on the accuracy of the MZCs. Milled MZCs showed the highest trueness, while printed MZCs showed the highest precision. All the results were within the clinically acceptable error values.

CLINICAL SIGNIFICANCE

Although the trueness of the milled MZCs is higher, the manufacturing accuracy of the 3D-printed MZCs showed clinically acceptable results in terms of trueness and precision. However, additional clinical studies are recommended. Furthermore, the volumetric changes of the material should be considered.

The physical-mechanical properties of 3D-printed versus conventional milled zirconia for dental clinical applications: A systematic review with meta-analysis

AIM OF STUDY

This systematic review aimed to compare the physical-mechanical properties of 3D-printed (additively manufactured (AM)) zirconia compared to conventionally milled (subtractive manufactured: SM) zirconia specimens.

MATERIALS AND METHODS

A thorough search of Internet databases was conducted up to September 2023. The search retrieved studies that evaluated AM zirconia specimens and restorations regarding the physical-mechanical properties and mechanical behavior of zirconia. The main topic focused on 3Y-TZP. However, records of 4YSZ and 5YSZ were also included to gather more comprehensive evidence on additively manufactured zirconia ceramic. The quality of studies was assessed using the ROB2 tool, Newcastle Ottawa scale, and the Modified Consort Statement. Of 1736 records, 57 were assessed for eligibility, and 38 records were included in this review, only two clinical trials meet the inclusion criteria and 36 records were laboratory studies. There were no signs of mechanical complications and wear to antagonists with short-term clinical observation. SM thin specimens ≤1.5 mm showed statistically significant higher flexural strength than AM zirconia (p ≤ 0.01), while thicker specimens showed comparable outcomes (p > 0.5). The fracture resistance of dental restorations was dependent on the aging protocol, restoration type, and thickness. The bond strength of veneering ceramic to zirconia core was comparable.

CONCLUSIONS

The results pooled from two short-term clinical trials showed no signs of mechanical or biological complications of additively manufactured 3Y-TZP zirconia crowns. The flexural strength might depend on the specimens' thickness, but it showed promising results to be used in clinical applications, taking into account the printing technique and orientation, material composition (yttria content), solid loading, and sintering parameters. 3D-printed restorations fracture resistance improved when adhered to human teeth. The veneering ceramic bond was comparable to milled zirconia specimens. Long-term RCTs are recommended to confirm the mechanical behavior of 3D-printed restorations.

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.

Mechanical properties of 3D printed prosthetic materials compared with milled and conventional processing: A systematic review and meta-analysis of in vitro studies

STATEMENT OF PROBLEM

Three-dimensional (3D) additive manufacturing (AM) is an evolving technology in dentistry, proposed as an alternative to subtractive milling manufacture (MM) or conventional processing. However, a systematic review of the use of AM technology instead of milling or conventional processing is lacking.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the mechanical properties of 3D printed prosthetic materials compared with MM and conventional techniques.

MATERIAL AND METHODS

An electronic search of the literature was conducted on the MEDLINE (via PubMed), Scopus, and Web of Science databases. The inclusion criteria were in vitro studies published in the last 5 years, in English or Italian, and with 3D AM printed dental prosthetic materials. Data extraction was focused on dental prosthetic materials (ceramics, polymers, and metals) and their mechanical properties: flexural strength, fracture load, hardness, roughness, removable partial denture (RPD) fit accuracy, trueness, marginal discrepancy, and internal fit. Data considered homogenous were subjected to meta-analysis using the Stata17 statistical software program (95% confidence interval [CI]; α=.05). Since all variables were continuous, the Hedge g measure was calculated. A fixed-effects model was used for I2=0%, while the statistical analysis was conducted using a random-effects model with I2>0%.

RESULTS

From a total of 3624 articles, 2855 studies were selected, and 76 studies included after full-text reading. The roughness of AM-printed ceramics generally increased compared with that of conventional processing while the marginal discrepancy was comparable both for ceramics and polymers. The flexural strength, hardness, and fracture load of AM-printed polymers were statistically lower than those of the conventional group (P<.05). No significant difference was detected in terms of hardness, roughness, marginal discrepancy, fracture load, trueness, or internal fit between the AM and MM techniques (P>.05). Milling techniques showed significantly higher values of flexural strength (Hedge g=-3.88; 95% CI, -7.20 to -0.58; P=.02), also after aging (Hedge g=-3.29; 95% CI, -6.41 to -0.17; P=.04), compared with AM printing.

CONCLUSIONS

AM is comparable with MM in terms of mechanical properties, in particular with polymeric materials. The flexural strength of AM-printed prostheses is lower than with conventional and MM techniques, as are the parameters of hardness and fracture load, while the marginal discrepancy is similar to that of MM and conventional techniques. AM prostheses are commonly used for interim crowns and fixed partial dentures, as their rigidity and fracture resistance cannot support mastication forces for extended periods. More comparative studies are needed.

Accuracy and adaptation of one-piece endodontic crowns fabricated through 3D printing and milling

STATEMENT OF PROBLEM

High-level evidence regarding the accuracy and adaptation of 1-piece endodontic crowns fabricated by using 3-dimensional (3D) printing technology is lacking.

PURPOSE

The purpose of this in vitro study was to compare the accuracy and adaptation of 1-piece endodontic crowns produced through 3D printing and computer-numerical-control milling technology and to explore the influence of trueness on 1-piece endodontic crown adaptation.

MATERIAL AND METHODS

One-piece endodontic crowns were prepared for a typodont right mandibular first molar, scanned with a 3Shape E3 scanner, and designed with a computer-aided design software program. Two types of 1-piece endodontic crowns were fabricated: 3D printed by using resin and zirconia slurry and milled from Grandio and zirconia blocks. A reverse engineering software program was used to superimpose 4 groups of crowns with the reference crowns used for accuracy analysis. Microcomputed tomography was used to measure 1-piece endodontic crown adaptation. The correlation between trueness and adaptation was evaluated through the Spearman correlation test (α=.05).

RESULTS

Milled resin-based 1-piece endodontic crowns demonstrated better trueness on marginal and occlusal surfaces compared with 3D printed ones (P<.001). However, no significant difference was observed in the trueness of intaglio surfaces between the 2 groups (P>.05). The milled group exhibited better adaptations than the printed one (P<.05). For zirconia 1-piece endodontic crowns, no significant differences were found in trueness or adaptation between the milled and printed groups (P>.05). Notably, the trueness of the axial wall had the greatest impact on overall crown adaptation, with its adaptation closely linked to the trueness of each area, particularly the axial wall.

CONCLUSIONS

Milled resin-based 1-piece endodontic crowns exhibited higher levels of trueness and adaptation compared with 3D printed ones, while 3D printed zirconia 1-piece endodontic crowns were comparable with milled ones.

Additive manufacturing of dental ceramics in prosthodontics: The status quo and the future

PURPOSE

This review aims to summarize the available technologies, material categories, and prosthodontic applications of additive manufacturing (AM) dental ceramics, evaluate the achievable accuracy and mechanical properties in comparison with current mainstream computer-aided design/computer-aided manufacturing (CAD/CAM) subtractive manufacturing (SM) methods, and discuss future prospects and directions.

STUDY SELECTION

This paper is based on the latest reviews, state-of-the-art research, and existing ISO standards on AM technologies and prosthodontic applications of dental ceramics. PubMed, Web of Science, and ScienceDirect were amongst the sources searched for narrative reviews.

RESULTS

Relatively few AM technologies are available and their applications are limited to crowns and fixed partial dentures. Although the accuracy and strength of AM dental ceramics are comparable to those of SM, they have the limitations of relatively inferior curved surface accuracy and low strength reliability. Furthermore, functionally graded additive manufacturing (FGAM), a potential direction for AM, enables the realization of biomimetic structures, such as natural teeth; however, specific studies are currently lacking.

CONCLUSIONS

AM dental ceramics are not sufficiently developed for large-scale clinical applications. However, with additional research, it may be possible for AM to replace SM as the mainstream manufacturing technology for ceramic restorations.

Influence of additives and binder on the physical properties of dental silicate glass-ceramic feedstock for additive manufacturing

OBJECTIVES

The aim of the study was to investigate the impact of organic additives (binder, plasticizer, and the cross-linking ink) in the formulation of water-based feedstocks on the properties of a dental feldspathic glass-ceramic material developed for the slurry-based additive manufacturing technology "LSD-print."

MATERIAL AND METHODS

Three water-based feldspathic feedstocks were produced to study the effects of polyvinyl alcohol (AC1) and poly (sodium 4-styrenesulfonate) (AC2) as binder systems. A feedstock without organic additives was tested as the control group (CG). Disc-shaped (n = 15) and bar (n = 7) specimens were slip-cast and characterized in the green and fired states. In the green state, density and flexural strength were measured. In the fired state, density, shrinkage, flexural strength (FS), Weibull modulus, fracture toughness (KIC), Martens parameters, and microstructure were analyzed. Disc-shaped and bar specimens were also cut from commercially available CAD/CAM blocks and used as a target reference (TR) for the fired state.

RESULTS

In the green state, CG showed the highest bulk density but the lowest FS, while the highest FS in the green state was achieved with the addition of a cross-linking ink. After firing, no significant differences in density and a similar microstructure were observed for all slip-cast groups, indicating that almost complete densification could be achieved. The CAD/CAM specimens showed the highest mean FS, Weibull modulus, and KIC, with significant differences between some of the slip-cast groups.

SIGNIFICANCE

These results suggest that the investigated feedstocks are promising candidates for the slurry-based additive manufacturing of restorations meeting the class 1a requirements according to DIN EN ISO 6871:2019-01.

3D-printed versus conventionally milled zirconia for dental clinical applications: Trueness, precision, accuracy, biological and esthetic aspects

OBJECTIVES

This systematic review aimed to compare the clinical outcome, internal gap, trueness, precision, and biocompatibility of 3D-printed (AM) compared to milled (SM) zirconia restorations.

DATA SOURCE

A thorough search of Internet databases was conducted up to September 2023. The search retrieved studies compared AM zirconia to SM zirconia restorations regarding clinical outcome, fit, trueness, precision, and biocompatibility.

STUDY SELECTION

Of 1736 records, only 59 were screened for eligibility, and 22 records were included in this review. The quality of studies was assessed using the revised Cochrane risk-of-bias tool (ROB2), and the Modified Consort Statement. One clinical study exhibited a low risk of bias. All laboratory studies revealed some bias concerns. Short-term observation showed 100 % survival with no signs of periodontal complications. 3D-printed zirconia crowns showed statistically significant lower ΔE and a better match to adjacent teeth (p ≤ 0.5). The fit, trueness, and precision vary with the printing technique and the tooth surface.

CONCLUSIONS

3D-printed zirconia crowns provide better aesthetic color and contour match to adjacent natural teeth than milled crowns. Both 3D printing and milling result in crowns within the clinically acceptable internal and marginal fit. Except for nanoparticle jetting, the marginal gap of SM crowns was smaller than AM crowns, however, both were clinically acceptable. Laminate veneers might be more accurately produced by 3D printing. 3D-printed axial surface trueness was better than milled axial surfaces. Long-term RCTs are recommended to confirm the clinical applicability of 3D-printed restorations.

CLINICAL SIGNIFICANCE

Internal fit and gap, precision, and trueness are fundamental requirements for successful dental restorations. Both techniques produce restorations with clinically acceptable marginal and internal fit. Axial surfaces and narrow or constricted areas favored 3D-printed than conventionally milled zirconia.

Accuracy of the intaglio surface of 3D-printed hybrid resin-ceramic crowns, veneers and table-tops: An in vitro study

OBJECTIVES

The present study aims to examine the influence of the build angle on the accuracy (trueness and precision) of 3D printed crowns, table-tops and veneers with a hybrid resin-ceramic material.

METHODS

One crown, on table-top and one veneer were printed in five different build angles (0°, 30°, 45°, 60°, 90°) (n = 50) with the digital light processing (DLP) system (Varseo XS, Bego) using hybrid resin (Varseo Smile Crownplus A3, Bego). All printed restorations were scanned using the laboratory scanner (D2000, 3Shape) and matched onto the initial reference design in metrology software (Geomagic Control X, 3D Systems). The root mean square error (RMSE) was calculated between the scanned and reference data. The data was statistically analyzed using the Tukey multiple comparison test and Wilcoxon multiple comparison test.

RESULTS

The crown group showed higher trueness at 30° (0.021 ± 0.002) and 45° (0.020 ± 0.002), and table-tops at 0° (0.015 ± 0.001) and 30° (0.014 ± 0.001) (p < 0.0001). Veneers demonstrated higher trueness at 30° (0.016 ± 0.002) (p < 0.0001). All three restoration types demonstrated the lowest trueness at a 90° build angle and portrayed deviations along the z axis. The veneer and table-top groups showed the lowest precision at 90° (veneers: 0.021 ± 0.008; table-tops: 0.013 ± 0.003). The crown group portrayed the lowest precision at 45° (0.017 ± 0.005) (p < 0.0001).

CONCLUSION

The build angle of DLP-printed hybrid resin-ceramic restorations influences their accuracy.

CLINICAL SIGNIFICANCE

Considering the build angle is important to achieve a better accuracy of 3D-printed resin-ceramic hybrid restorations. This may help predict or avoid the interference points between a restoration and a die and minimize the clinical adjustments.

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).

Accuracy of zirconia crowns manufactured by stereolithography with an occlusal full-supporting structure: An in vitro study

STATEMENT OF PROBLEM

Additive manufacturing is emerging as an alternative method of fabricating dental restorations, but the support design needs to be optimized.

PURPOSE

The purpose of this in vitro study was to evaluate the 3-dimensional trueness and adaptations of zirconia crowns manufactured by stereolithography (SLA) with an occlusal full-supporting structure, compared with those SLA-printed with pillar supports, and those made by milling.

MATERIAL AND METHODS

A zirconia abutment was prepared, and an anatomic contour crown was designed. The crowns were manufactured by SLA and milling (n=6). For SLA manufacturing, a full-supporting base and pillar supports were designed. The 3-dimensional (3D) trueness of the fabricated crowns was characterized by 3D deviation analysis. The adaptations of crowns in the SLA-base and milling groups were measured by using a triple-scan method. Color-difference maps and the root mean square (RMS) values were used to characterize the 3D trueness. One-way analysis of variance (ANOVA) and Tukey post hoc test were used to analyze the difference in RMS values among the 3 groups, and Student t test was used to analyze the difference in cement-gap width between the milling group and the SLA group with the full-supporting base (α=.05).

RESULTS

The 3D deviation analysis showed that in the external area, the RMS value of the SLA-pillar group was significantly higher than that of the SLA-base and the milling groups (P<.05). In the intaglio area, the milling group showed a lower RMS value than the 2 SLA groups (P<.05). The color-difference maps showed the SLA-base group had smaller positive errors at the cusp inclines than the SLA-pillar group. No statistically significant difference was found in adaptations between the SLA-base and milling groups (P>.05).

CONCLUSIONS

The occlusal full-supporting base provided improved support in fabricating the crowns, and no remnants were left after removal. The zirconia crowns manufactured by SLA with an occlusal full-supporting structure had good external 3D trueness and clinically acceptable adaptation.

Research on the low-temperature degradation of dental zirconia ceramics fabricated by stereolithography

STATEMENT OF PROBLEM

Stereolithography is a promising method of fabricating zirconia ceramics with high strength and accuracy. However, studies of the aging effects on zirconia ceramics fabricated by this technique are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the aging effects on the crystalline content, microstructure, and mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) printed by stereolithography apparatus (SLA) and digital light processing (DLP) compared with those of zirconia milled by computer numerical control (CNC).

MATERIAL AND METHODS

Bar-shaped specimens were fabricated after layer-by-layer printing, debinding, and sintering by SLA and DLP. Specimens milled and sintered by CNC were used as controls (n=24/material). The specimens were divided into 12 groups (n=6) and aged (0/5/10/15 hours, 134 °C, 0.2 MPa), after which the crystalline content, microstructure, and mechanical properties were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and 3-point bend tests. The flexural strength and monoclinic (m) phase content were statistically evaluated (α=.05).

RESULTS

The XRD results showed that an m peak was not detected for any of the tested materials before aging. The m-phase content was the highest for SLA (5/10/15 hours: 19.64%/34.76%/41.88%), followed by DLP (5/10/15 hours: 9.62%/21.76%/28.43%) and CNC (5/10/15 hours: 2.29%/7.77%/7.66%). The SEM images showed zirconia grain fragments for DLP and grain pullout for SLA, while surface defects were not obvious for CNC. Within the materials, the flexural strength was the highest for SLA after aging for 5 hours (1010.3 MPa), followed by 10 hours (913.06 MPa) and 15 hours and 0 hours, which exhibited no difference (0/15 hours: 776.71/814.28 MPa) (P<.001). The flexural strength for CNC and DLP did not significantly change after aging for 5 hours, 10 hours, and 15 hours (P>.05). The flexural strength for CNC was always more than 1200 MPa, and that for DLP was approximately 800 MPa before and after aging.

CONCLUSIONS

Although the m-phase content for SLA and DLP increased with the aging time, the mechanical properties did not significantly decrease, indicating the stability of both materials.

Surface Texture Designs to Improve the Core-Veneer Bond Strength of Zirconia Restorations Using Digital Light Processing

Veneered zirconia ceramics are widely used for dental restorations. However, the relatively poor bonding strength between the ceramic core and veneer porcelain remains a common problem in clinical applications. To address this issue, this study focused on enhancing the core-veneer bond strength of zirconia restorations through the implementation of surface textures using digital light processing (DLP) technology. The light intensity was precisely tuned to optimize mechanical strength and minimize light scattering. Subsequently, hexagonal or square grids were printed on the surface of the zirconia ceramic core. Following veneering procedures, the shear bond strength (SBS) test was conducted using a universal testing machine. Dates were compared using analysis of variance (ANOVA) and the least significant difference (LSD) test. Furthermore, optical microscopy and scanning electron microscopy (SEM) were used to examine the failure modes and observe the cross-sectional structures, respectively. The results indicated that the presence of a 0.09 mm high hexagon grid led to a significant 21% increase in the SBS value. However, grids with heights of 0.2 and 0.3 mm showed less improvement, owing to the formation of large defects at the interface during the fusion process. This study demonstrated the potential of DLP technology in preparing zirconia ceramics with complex structures and high mechanical strength, thereby offering promising solutions for overcoming challenges associated with 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.

Influence of surface finishing and printing layer orientation on surface roughness and flexural strength of stereolithography-manufactured dental zirconia

OBJECTIVE

To evaluate the effect of surface finishing and printing layer orientation on the surface roughness and flexural strength of three-dimensionally (3D) printed 3 mol% yttria-stabilized zirconia manufactured by stereolithography (SLA).

METHODS

Ninety bar-shaped zirconia specimens (1 mm x 1 mm x 12 mm) were 3D-printed via SLA. After debinding and sintering, they were randomly divided according to the printing layer orientation: parallel (PR) or perpendicular (PD) to the tensile surface for bending test. Each group was submitted to a surface finishing protocol (n=15/group): unpolished (subgroup 0), with polished tensile surface (subgroup 1), and with polished lateral and tensile surfaces (subgroup 3). Roughness of tensile surface was determined using a contact sensor and surface morphology was analyzed under Scanning Electron Microscopy (SEM). Flexural strength, apparent elastic modulus, and Weibull parameters were assessed using a 3-point bending test. Fractured specimens were examined to identify failure origins. Finite element analysis was used to evaluate tensile stress peaks and failure risk.

RESULTS

PR orientation exhibited higher strength, higher apparent elastic modulus, higher maximum principal stress peaks, and lower failure risk. For both layer orientations, groups with polished lateral and tensile sides (PR3 and PD3) were the strongest. SEM revealed that polishing led to changes in defect type, location, and size.

SIGNIFICANCE

SLA zirconia shows different mechanical properties according to surface roughness and defects. Orienting the printed layers parallel to the tensile side improves its mechanical performance. Polishing can significantly improve its flexural strength. It is necessary to reduce the final product's surface roughness and large pores for its best performance.

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.

Effect of dimensional variations on the manufacturing process and the 3D shrinkage ratio of stereolithographic dental alumina ceramics

The purpose of the present study was to evaluate the influence of height and length variations of alumina ceramics manufactured by stereolithography on deformations caused by the manufacturing process and on the 3D shrinkage ratio to control the final dimensions and improve the adaptation of stereolithographic ceramic dental prostheses. Two different U-shaped models were designed with variable heights or lengths. The specimens were manufactured by stereolithography and were scanned using a microtomographic device before and after the heat treatment. Dimensional variations were measured using inspection software. The number and surface of layers of alumina ceramic influenced the reliability of the stereolithography manufacturing but did not influence the 3D shrinkage ratio. The larger the layer surface, the larger the deformation of the ceramic. Dental ceramics manufactured by stereolithography with smallest layer surface are the most reliable. This helps in the selection of the build orientation.

Fit, Precision, and Trueness of 3D-Printed Zirconia Crowns Compared to Milled Counterparts

Precise fit of a crown and accurate reproduction of the digital design are paramount for successful treatment outcomes and preservation of clinician and technician time. The study aimed to compare the internal fit, marginal adaptation, precision, and trueness of 3D-printed zirconia crowns compared to their milled counterpart. A total of 20 monolithic 3 mol% yttria stabilized-zirconia crowns (n = 10) were made using computer-assisted design (CAD) followed by additive (3D-printed) and subtractive (milled) manufacturing. Digital scanning of the master die with and without a fit checker followed by image superimposition, and analysis was performed to evaluate internal and marginal adaptation in four areas (occlusal, axial, marginal, and overall). ISO 12836:2015 standard was followed for precision and trueness evaluation. Statistical analysis was achieved using a t-test at α = 0.05. Internal fit and marginal adaptation revealed no significant difference between the two test groups (p > 0.05). The significant difference in trueness (p < 0.05) was found between the two groups in three areas (occlusal, axial, and internal). The best and worst trueness values were seen with 3D-printed crowns at occlusal (8.77 ± 0.89 µm) and Intaglio (23.90 ± 1.60 µm), respectively. The overall precision was statistically better (p < 0.05) in the 3D-printed crowns (9.59 ± 0.75 µm) than the milled (17.31 ± 3.39 µm). 3D-printed and milled zirconia crowns were comparable to each other in terms of internal fit and marginal adaptation. The trueness of the occlusal and axial surfaces of 3D-printed crowns was better, whereas the trueness of fitting surface of milled crowns was better. 3D-printed crowns provided a higher level of precision than milled crowns. Although the internal and marginal fit of both production techniques were comparable, 3D printing of zirconia produced more precise crowns.

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.

Accuracy of additive manufacturing in stomatology

With the rapid development of the three-dimensional (3D) printing technology in recent decades, precise and personalized manufacturing has been achieved gradually, bringing benefit to biomedical application, especially stomatology clinical practice. So far, 3D printing has been widely applied to prosthodontics, orthodontics, and maxillofacial surgery procedures, realizing accurate, efficient operation processes and promising treatment outcomes. Although the printing accuracy has improved, further exploration is still needed. Herein, we summarized the various additive manufacturing techniques and their applications in dentistry while highlighting the importance of accuracy (precision and trueness).

Surface characteristics and flexural strength of Porous-Surface designed zirconia manufactured via stereolithography

PURPOSE

To design and fabricate zirconia bars with porous surfaces using stereolithography and evaluate their surface characteristics and flexural strengths.

METHODS

Five groups of zirconia bars (20 mm × 4 mm × 2 mm) with interconnected porous surfaces were designed and manufactured: (i) 400-μm pore size and 50% porosity (D400-P50 group), (ii) 400-μm pore size and 30% porosity (D400-P30 group), (iii) 200-μm pore size and 50% porosity (D200-P50 group), (iv) 200-μm pore size and 30% porosity (D200-P30 group), and (v) 100-μm pore size and 30% porosity (D100-P30 group). Zirconia bars without a porous surface (NP) were used as controls. The surface topographies and pore structures were investigated using scanning electron microscopy and three-dimensional laser microscopy. The printed porosity was calculated using the Archimedes method. Fifteen specimens from each group were subjected to a three-point bending test according to the ISO 6872:2015 standard. A Weibull analysis was performed, and the fractured surfaces were examined using scanning electron microscopy.

RESULTS

Zirconia bars with porous surfaces were designed and successfully manufactured. The designed pore size, porosity, and shape of the printed pores were approximately achieved for all the porous surfaces. The flexural strength of the control group was significantly higher than those of the groups with porous surfaces (p < 0.001). For the same porosity, groups with a pore size of 400 μm exhibited a lower flexural strength than the other groups (p<0.001). Additionally, for the same pore-size design, the flexural strengths of group D400-P50 and D400-P30 exhibited no significant differences (p = 0.150), while the flexural strengths of D200-P30 were significantly higher than that of D200-P50 group (p = 0.043). The control group and D400-P50 group had higher Weibull moduli than the other groups. The fractography of the specimens with porous surfaces indicated more than one crack origin, mainly owing to defects, including pores and cracks.

CONCLUSION

Zirconia bars with porous surfaces were successfully designed and fabricated using the stereolithography technique. Although porous surfaces may be advantageous for osteogenesis, the porous-surface design can reduce the flexural strength of the printed zirconia bars. By reducing the pore size, controlling the porosity, and improving the printing accuracy, a higher strength can be achieved. This article is protected by copyright. All rights reserved.

Additive Manufacturing of Dental Ceramics: A Systematic Review and Meta-Analysis

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the effect of using additive manufacturing (AM) for dental ceramic fabrication in comparison with subtractive manufacturing (SM), and to evaluate the effect of the type of AM technology on dental ceramic fabrication.

MATERIALS AND METHODS

A search was conducted electronically in MEDLINE (via PubMed), EBSCOhost, Scopus, and Cochran Library databases, and also by other methods (table of contents screening, backward and forward citations, and grey literature search) up to February 12, 2022, to identify records evaluating additive manufacturing of ceramics for dental purposes in comparison with subtractive manufacturing. A minimum of 2 review authors conducted tstudy selection, quality assessment, and data extraction. Quality assessment was performed with Joanna Briggs Institute tool, and the quantitative synthesis was performed with the Comprehensive Meta-Analysis program (CMA, Biostat Inc). Hedges's g for effect size was calculated, with 0.2 as small, 0.5 as medium, and 0.8 as large. Heterogeneity was assessed with I² and prediction interval (PI) statistics. Publication bias was investigated with funnel plots and grey literature search. Certainty of evidence was assessed with the Grading of Recommendations: Assessment, Development, and Evaluation (GRADE) tool.

RESULTS

A total of 28 studies were included for the qualitative and quantitative synthesis; 11 in vitro studies on accuracy, 1 in vivo study on color, and 16 in vitro studies on physical and mechanical properties. Meta-analysis showed overall higher accuracy for SM compared with AM, with medium effect size (0.679, CI: 0.173 to 1.185, p = 0.009) and also for marginal (g = 1.05, CI: 0.344 to 1.760, p = 0.004), occlusal (g = 2.24, CI: 0.718 to 3.766, p = 0.004), and total (g = 4.544, CI: -0.234 to 9.323, p = 0.062) with large effect size; whereas AM had higher accuracy than SM with small effect size for the external (g = -0.238, CI: -1.215 to 0.739), p = 0.633), and internal (g = -0.403, CI: -1.273 to 0.467, p = 0.364) surfaces. For technology, self-glazed zirconia protocol had the smallest effect size (g = -0.049, CI: -0.878 to 0.78, p = 0.907), followed by stereolithography (g = 0.305, CI: -0.289 to 0.9, p = 0.314), and digital light processing (g = 1.819, CI: 0.662 to 2.976, p = 0.002) technologies. Flexural strength was higher for ceramics made by SM in comparison to AM with large effect size (g = -2.868, CI: -4.371 to -1.365, p < 0.001). Only 1 study reported on color, favoring ceramics made through combined AM and SM.

CONCLUSIONS

Subtractive manufacturing had better overall accuracy, particularly for the marginal and occlusal areas, higher flexural strength, and more favorable hardness, fracture toughness, porosity, fatigue, and volumetric shrinkage; whereas AM had more favorable elastic modulus and wettability. Both methods had favorable biocompatibility. All studies on accuracy and mechanical properties were in vitro, with high heterogeneity and low to very low certainty of evidence. There is a lack of studies on color match and esthetics.

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.

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

Additive manufacturing (AM) has many advantages and became a valid manufacturing technique for polymers and metals in dentistry. However, its application for dental ceramics is still in process. Among dental ceramics, zirconia is becoming popular and widely used in dentistry mainly due to its outstanding properties. Although subtractive technology or milling is the state of art for manufacturing zirconia restorations but still has shortcomings. Utilizing AM in fabricating ceramics restorations is a new topic for many researchers and companies across the globe and a good understanding of AM of zirconia is essential for dental professional. Therefore, the aim of this narrative review is to illustrate different AM technologies available for processing zirconia and discus their advantages and future potential. A comprehensive literature review was completed to summarize different AM technologies that are available to fabricate zirconia and their clinical application is reported. The results show a promising outcome for utilizing AM of zirconia in restorative, implant and regenerative dentistry. However further improvements and validation is necessary to approve its clinical application.

Optimized Zirconia 3D Printing Using Digital Light Processing with Continuous Film Supply and Recyclable Slurry System

Stereolithography (SL) can fabricate complex ceramic parts layer by layer using computer-aided design (CAD) models. The traditional SL system utilizes a vat filled with ceramic slurry with a high solid content, which for ceramics contributes to several limitations and operational difficulties, and further renders it nonrecyclable mainly due to the presence of printed residue and its high viscosity. In this study, we utilized a continuous film supply (CFS) system integrated with a tape-casting type digital light processing (DLP) printer to fabricate zirconia prototypes with a solid content of 45 volume percent (vol.%). Various printing and postprocessing parameters were studied for optimization, to achieve a relative density of 99.02% ± 0.08% with a microhardness of 12.59 ± 0.47 GPa. Slurry reusability was also demonstrated by printing with recycled slurry to produce consistent relative density values in the range of 98.86% ± 0.02% to 98.94% ± 0.03%. This method provides new opportunities for material recycling and the fabrication of dense complex ceramic products, reducing the consumption of the material.