Comparison of properties and cost efficiency of zirconia processed by DIW printing, casting and CAD/CAM-milling

Influence of 3D printing orientations on physico-mechanical properties and accuracy of additively manufactured dental ceramics

PURPOSE

This systematic review aims to assess the impact of different 3D printing orientations on the physico-mechanical properties, volumetric change, and accuracy of additively manufactured ceramic specimens, as well as their restorations.

STUDY SELECTION

The web database containing records for building orientation of 3D-printed ceramics until January 2024 was searched, with no language limitations. PRISMA 2020 guidelines were followed, and the risk of bias was evaluated using the modified CONSORT checklist for laboratory studies on dental materials. The RevMan 5.4 of Cochrane collaboration was used for the meta-analysis with α<0.05.

RESULTS

In this systematic review, 35 out of 2967 records were considered. The printing orientations had a significant influence on the mechanical properties of the additively manufactured specimens (P < 0.05), specifically, the horizontally printed specimens exhibited the best mechanical behavior, while the physical properties exhibited controversial results. Additionally, the printing orientations had a significant impact on the volumetric shrinkage and accuracy of molar teeth (P < 0.05).

CONCLUSIONS

The ideal mechanical outcome was observed when ceramic specimens were 3D-printed in a horizontal direction relative to the platform, while vertically printed specimens showed the worst properties. The printing orientations affect the volumetric change of the specimens; however, volumetric change highly depends on the sintering parameters. It is recommended to 3D print ceramic molar crowns at 180° with the occlusal surface facing the printing platform.

CLINICAL SIGNIFICANCE

Several factors affect the additively manufactured restorations throughout the printing procedure. Therefore, printing orientation should be carefully considered before the additive production process of ceramic restorations.

Comparison of the Internal and Marginal Adaptation of Implant-Supported Restorations on Titanium Base Using Various Materials: An In Vitro Study

The adaptation of restorations to the titanium base (TiBase) abutments varies depending on the materials and methods used, playing a crucial role in implant and prosthetic success. This in vitro study aims to compare the internal and marginal fit of a titanium interface among three different milled materials: doped graphene PMMA, single-density zirconia, and dual-density zirconia, used for the rehabilitation of CAD-CAM implant-supported single crowns. A digital method based on the silicone replica technique was employed. The silicone reproduction of each fabricated restoration's inner and basal parts was digitally aligned to the titanium base, measuring the space between them at three predetermined planes: basal, middle, and superior. The material with the worst overall adaptation was dual-density zirconia (0.1 ± 0.03 mm), followed by single-density zirconia (0.09 ± 0.03 mm), and doped graphene PMMA (0.06 ± 0.02 mm). No statistical differences were found in the internal fit, represented by the measurements made at the middle and superior plane, among the materials used (p > 0.05). However, the marginal fit of doped graphene PMMA restorations was statistically better than zirconia restorations (p < 0.05). No significant differences were observed between the marginal fit of both types of zirconia (p > 0.05). Across all three materials, the superior plane showed the best fit, while the marginal plane exhibited the worst.

SLA-3D printing and bioactivity enhancement of zirconia anchor screws for temporomandibular joint disc reduction surgery

A method is proposed for 3D printing and enhancing the surface bioactivity of zirconia ceramic anchor screws, specifically tailored for temporomandibular joint disc reduction surgery. Initially, the challenge posed by the brittleness and processing difficulties of fine ceramic anchor screws was addressed through the application of SLA-3D printing technology. This allowed for an exploration of the forming accuracy and biomechanical properties of the printed anchor screws. According to research findings, the dimensional deviation in the thread processing of 3D printed zirconia screws is approximately 100 μm. When the threaded segment measures 7.0 mm in length, the 3D printed zirconia anchor screw, with a diameter of 2.7 mm, demonstrates comparable maximum axial tensile forces 102.91 N to a titanium screw of 2.0 mm diameter. The maximum vertical tensile force of the zirconia anchor screws exceeds the breaking force of the anchor suture by 21.03 N, fulfilling the requirements for clinical application. Additionally, the application of a ZrO2-PDA-La3+ composite biological coating enhances the surface bioactivity of the 3D printed zirconia anchor screws. PDA ensures reliable adhesion of the biological coating during the implantation process, while La3+ significantly boosts the osteogenic capacity of the zirconia ceramic surface, thereby contributing to the long-term stability of the implant. Ultimately, zirconia anchor screws satisfying basic clinical requirements in terms of mechanical properties and biological activity were successfully developed, offering a novel treatment option for ADDwoR patients, particularly those with metal allergies.

Additive-manufactured ceramics for dental restorations: a systematic review on mechanical perspective

Background

Additive manufacturing (AM) is rapidly expanding as a substitute for conventional heat-pressing and milling techniques for ceramic restorations. However, experimental and clinical evidence on the mechanical properties and performance of the final ceramic products is yet insufficient. This systematic review aimed to update the latest advances in additive manufacturing of restorative ceramics with a focus on their mechanical properties.

Methods

This systematic review was structured using the 5-step methodology based on the research question: what are the mechanical properties of additive-manufactured restorative ceramics in comparison with subtractive manufacturing? The electronic literature search was performed independently by 2 authors in the following databases: PubMed/MEDLINE, Web of Science, and Scopus. Published articles from 2019 to 2023 were screened, analysed and the relevant papers were selected for inclusion in this review.

Results

A total of 40 studies were included. The available ceramics include zirconia, alumina and alumina-zirconia composites, lithium disilicate, porcelain and fluorapatite glass ceramic. The mechanical properties were summarized according to material and technique: density (15 studies), flexural strength (31 studies), fracture toughness (7 studies), Young's modulus (7 studies), hardness (11 studies) and performance (7 studies). Overall, the properties exhibited an upward trend toward the values of conventional techniques. Typical processing defects, including porosity, agglomerates, cracks, surface roughness, and other defects, were also analyzed.

Conclusions

With significant technological advancements, the mechanical properties of AM ceramics have come close to ceramics by conventional manufacturing, whereas their reliability, the influence of printing layer orientations, and long-term performance still need further investigation.

Direct ink writing with dental composites: A paradigm shift toward sustainable chair-side production

OBJECTIVES

To evaluate the dimensional accuracy of occlusal veneers printed using a novel direct ink writing (DIW) system and a clinically approved dental composite.

METHODS

A novel three-dimensional printer was developed based on the extrusion-based DIW principle. The printer, constructed primarily with open-source hardware, was calibrated to print with a flowable resin composite (Beautifil Flow Plus). The feasibility of this technology was assessed through an evaluation of the dimensional accuracy of 20 printed occlusal veneers using a laboratory confocal scanner. The precision was determined by pairwise superimposition of the 20 prints, resulting in a set of 190 deviation maps used to evaluate between-sample variations.

RESULTS

Without material waste or residuals, the DIW system can print a solid occlusal veneer of a maxillary molar within a 20-minute timeframe. Across all the sampled surface points, the overall unsigned dimensional deviation was 30.1 ± 20.2 µm (mean ± standard deviation), with a median of 24.4 µm (interquartile range of 22.5 µm) and a root mean square value of 36.3 µm. The pairwise superimposition procedure revealed a mean between-sample dimensional deviation of 26.7 ± 4.5 µm (mean ± standard deviation; n = 190 pairs), indicating adequate precision. Visualization of the deviation together with the nonextrusion movements highlights the correlation between high-deviation regions and material stringing.

SIGNIFICANCE

This study underscores the potential of using the proposed DIW system to create indirect restorations utilizing clinically approved flowable resin composites. Future optimization holds promise for enhancing the printing accuracy and increasing the printing speed.

Load-bearing capacity, internal accuracy and time-efficiency of heat-pressed, milled and 3D-printed lithium disilicate ultra-thin occlusal veneers

OBJECTIVES

The primary aim of this in vitro study was to compare the load-bearing capacity of lithium disilicate occlusal veneers, fabricated via different manufacturing processes. Secondary objectives included assessing internal accuracy and production time-efficiency.

METHODS

Four fabrication methods for ultra-thin lithium disilicate occlusal veneers on extracted human molars with simulated erosive defects were compared (n = 20/group): CAM: milled lithium disilicate (IPS e.max CAD); HPR: heat-pressed lithium disilicate (IPS e.max Press) out of a milled PMMA template (Ddpmma CAST); 3DP: 3D-printed lithium disilicate (experimental lithium disilicate); PTE: heat-pressed lithium disilicate (IPS e.max Press) out of a 3D-printed template (SilaPrint cast). Internal accuracy was measured prior to thermo-mechanical aging, followed by static loading to measure the load-bearing capacity (Fmax). Fabrication time (time-efficiency) was also recorded. Statistical analysis was performed using the Kruskal-Wallis (KW) test.

RESULTS

No statistically significant differences were found in median load-bearing capacities (Fmax) between the groups (KW p = 0.5902): CAM 1821 N, HPR 1896 N, 3DP 2003 N, PTE 1687 N. Significant differences were found in internal accuracy between the groups that employed printing processes (3DP, PTE) and all other groups in margins (p < 0.001), cusps (p < 0.0018), and fossae (p < 0.0346). The time-efficiency measurements indicated an increase in fabrication time, starting from CAM 67.2 ± 5.8 min, followed by HPR 200.8 ± 33.0 min, PTE 289.2 ± 38.7 min, and peaking with the highest duration observed for 3DP 701.6 ± 8.1 min.

SIGNIFICANCE

The fabrication method of ultra-thin lithium disilicate occlusal veneers does not significantly impact their load-bearing capacity, but affects the clinical fit and adaptation of the veneers.

Effect of artificial aging on fracture toughness and hardness of 3D-printed and milled 3Y-TZP zirconia

PURPOSE

This study aimed to evaluate the impact of artificial aging on the fracture toughness and hardness of three-dimensional (3D)-printed and computer-aided design and computer-aided manufacturing (CAD-CAM) milled 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP).

MATERIALS AND METHODS

Forty bar-shaped specimens (45 × 4 × 3 mm) were prepared using two manufacturing technologies: 3D printing (LithaCon 3Y 210, Lithoz GmbH, Vienna, Austria; n = 20) and milling (Initial Zirconia ST, GC, Japan; n = 20) of 3Y-TZP. The chevron-notch beam method was used to assess the fracture toughness according to ISO 24370. Specimens from each 3Y-TZP group were divided into two subgroups (n = 10) based on the artificial aging process (autoclaving): nonaged and aged. Nonaged specimens were stored at room temperature, while aged specimens underwent autoclave aging at 134°C under 2 bar-pressure for 5 h. Subsequently, the specimens were immersed in absolute 99% ethanol using an ultrasonic cleaner for 5 min. Each specimen was preloaded by subjecting it to a 4-point loading test, with a force of up to 200 N applied for three cycles. Further 4-point loading was conducted at a rate of 0.5 mm/min under controlled temperature and humidity conditions until fracture occurred. The maximum force (Fmax) was recorded and the chevron notch was examined at 30 × magnification under an optical microscope for measurements before the fracture toughness (KIc) was calculated. Microhardness testing was also performed to measure the Vickers hardness number (VHN). A scanning electron microscope (SEM) coupled with an energy dispersive X-ray unit (EDX) was used to examine surface topography and chemical composition. X-ray diffraction (XRD) was conducted to identify crystalline structure. Data were statistically analyzed using two-way ANOVA and Student's t-test with a significance level of 0.05.

RESULTS

The nonaged 3D-printed 3Y-TZP group exhibited a significantly higher fracture toughness value (6.07 MPa m1/2) than the milled 3Y-TZP groups (p < 0.001). After autoclave aging, the 3D-printed 3Y-TZP group maintained significantly higher fracture toughness (p < 0.001) compared to the milled 3Y-TZP group. However, no significant differences in hardness values (p = 0.096) were observed between the aged and nonaged groups within each manufacturing process (3D-printed and milled) independently.

CONCLUSION

The findings revealed that the new 3D-printed 3Y-TZP produced by the lithography-based ceramic manufacturing (LCM) technology exhibited superior fracture toughness after autoclave aging compared to the milled 3Y-TZP. While no significant differences in hardness were observed between the aged groups, the 3D-printed material demonstrated greater resistance to fracture, indicating enhanced mechanical stability.

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.

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.

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.

Clinical longevity of direct and indirect posterior resin composite restorations: An updated systematic review and meta-analysis

OBJECTIVES

To answer the PICO(S) question: Is there a difference in clinical longevity between direct and indirect resin composite restorations placed on permanent posterior teeth?

DATA

Randomized controlled clinical trials (RCTs) investigating direct and indirect resin composite restorations in posterior permanent teeth were considered.

SOURCES

Several electronic databases were searched, with no language or date restrictions. The revised Cochrane Collaboration's tool for assessing risk of bias (RoB-2) was used to analyze the studies; meta-analyses were run and the certainty of evidence was assessed by the GRADE tool. A subgroup meta-analysis was performed for resin composite restorations placed on posterior worn dentition.

STUDY SELECTION

Twenty-three articles were included in qualitative synthesis, while 8 studies were used for meta-analyses. According to the RoB-2 tool, 5 studies were ranked as "low risk", 7 had "some concerns", while 11 papers were rated as "high risk" of bias. There were no statistically significant differences in short-term (p = 0.27; RR=1.54, 95% CI [0.72, 3.33]), medium-term (p = 0.27; RR=1.87, 95% CI [0.61, 5.72]) and long-term longevity (p = 0.86; RR=0.95, 95% CI [0.57, 1.59]). The choice of restorative technique had no influence on short-term survival of resin composite restorations placed on worn dentition (p = 0.13; RR=0.46, 95% CI [0.17, 1.25]). The certainty of evidence was rated as "very low".

CONCLUSIONS

Direct and indirect resin composite restorations may show similar clinical longevity in posterior region, regardless of the observation period or substrate (wear-affected and non-affected dentition). The very low quality of evidence suggests that more long-term RCTs are needed to confirm our results.