Accuracy and margin quality of advanced 3D-printed monolithic zirconia crowns

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 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.

Bond Strength of Milled and Printed Zirconia to 10-Methacryloyloxydecyl Dihydrogen Phosphate (10-MDP) Resin Cement as a Function of Ceramic Conditioning, Disinfection and Ageing

This study aimed to assess the suitability of printed zirconia (ZrO2) for adhesive cementation compared to milled ZrO2. Surface conditioning protocols and disinfection effects on bond strength were also investigated. ZrO2 discs (n = 14/group) underwent either alumina (Al2O3) airborne particle abrasion (APA; 50 µm, 0.10 MPa) or tribochemical silicatisation (TSC; 110 µm Al2O3, 0.28 MPa and 110 µm silica-modified Al2O3, 0.28 MPa), followed by disinfection (1 min immersion in 70% isopropanol, 15 s water spray, 10 s drying with oil-free air) for half of the discs. A resin cement containing 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) was used for bonding (for TSC specimens after application of a primer containing silane and 10-MDP). Tensile bond strength was measured after storage for 24 h at 100% relative humidity or after 30 days in water, including 7500 thermocycles. Surface conditioning significantly affected bond strength, with higher values for TSC specimens. Ageing and the interaction of conditioning, disinfection and ageing also impacted bond strength. Disinfection combined with APA mitigated ageing-related bond strength decrease but exacerbated it for TSC specimens. Despite these effects, high bond strengths were maintained even after disinfection and ageing. Adhesive cementation of printed ZrO2 restorations exhibited comparable bond strengths to milled ZrO2, highlighting its feasibility in clinical applications.

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.

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.

Tooth morphology, internal fit, occlusion and proximal contacts of dental crowns designed by deep learning-based dental software: A comparative study

OBJECTIVES

This study compared the tooth morphology, internal fit, occlusion, and proximal contacts of dental crowns automatically generated via two deep learning (DL)-based dental software systems with those manually designed by an experienced dental technician using conventional software.

METHODS

Thirty partial arch scans of prepared posterior teeth were used. The crowns were designed using two DL-based methods (AA and AD) and a technician-based method (NC). The crown design outcomes were three-dimensionally compared, focusing on tooth morphology, internal fit, occlusion, and proximal contacts, by calculating the geometric relationship. Statistical analysis utilized the independent t-test, Mann-Whitney test, one-way ANOVA, and Kruskal-Wallis test with post hoc pairwise comparisons (α = 0.05).

RESULTS

The AA and AD groups, with the NC group as a reference, exhibited no significant tooth morphology discrepancies across entire external or occlusal surfaces. The AD group exhibited higher root mean square and positive average values on the axial surface (P < .05). The AD and NC groups exhibited a better internal fit than the AA group (P < .001). The cusp angles were similar across all groups (P = .065). The NC group yielded more occlusal contact points than the AD group (P = .006). Occlusal and proximal contact intensities varied among the groups (both P < .001).

CONCLUSIONS

Crowns designed by using both DL-based software programs exhibited similar morphologies on the occlusal and axial surfaces; however, they differed in internal fit, occlusion, and proximal contacts. Their overall performance was clinically comparable to that of the technician-based method in terms of the internal fit and number of occlusal contact points.

CLINICAL SIGNIFICANCE

DL-based dental software for crown design can streamline the digital workflow in restorative dentistry, ensuring clinically-acceptable outcomes on tooth morphology, internal fit, occlusion, and proximal contacts. It can minimize the necessity of additional design optimization by dental technician.