Novel Zirconia Materials in Dentistry

Biomimetic Tissue Engineering Strategies for Craniofacial Applications

Biomimetics is the science of imitating nature's designs and processes to create innovative solutions for various fields, including dentistry and craniofacial reconstruction. In these areas, biomimetics involves drawing inspiration from living organisms/systems to develop new materials, techniques, and devices that closely resemble natural tissue structures and enhance functionality. This field has successfully demonstrated its potential to revolutionize craniofacial procedures, significantly improving patient outcomes. In dentistry, biomimetics offers exciting possibilities for the advancement of new dental materials, restorative techniques, and regenerative potential. By analyzing the structure/composition of natural teeth and the surrounding tissues, researchers have developed restorative materials that mimic the properties of teeth, as well as regenerative techniques that might assist in repairing enamel, dentin, pulp, cementum, periodontal ligament, and bone. In craniofacial reconstruction, biomimetics plays a vital role in developing innovative solutions for facial trauma, congenital defects, and various conditions affecting the maxillofacial region. By studying the intricate composition and mechanical properties of the skull and facial bones, clinicians and engineers have been able to replicate natural structures leveraging computer-aided design and manufacturing (CAD/CAM) and 3D printing. This has allowed for the creation of patient-specific scaffolds, implants, and prostheses that accurately fit a patient's anatomy. This review highlights the current evidence on the application of biomimetics in the fields of dentistry and craniofacial reconstruction.

In Vitro Aging and Fracture Tests on Differently Veneered Partially Stabilized Zirconia Anterior Crowns

Objectives: To evaluate the fracture resistance of veneered incisor crowns made from highly translucent zirconia frameworks. Materials and Methods: Ninety-six all-ceramic single crowns were based on either a coping with minimum wall thickness or a cutback framework fabricated from highly translucent zirconia (5Y-PSZ). Each one-third of the specimens was finalized with different veneering ceramics using standardizations and glaze firing. Crowns were luted to cobalt-chromium dies with MDP-containing composite cement. Half of the entire sample underwent artificial aging (chewing simulation and thermocycling) before fracture load tests were conducted using a 6 mm steel sphere applied in a 90° angle to the oral aspect of the crowns with 1.3 mm distance to the incisal edge. Besides descriptive presentation of recorded forces at first damage (F1d) and fracture (Fu), Kruskal Wallis and Mann-Whitney U tests were used to analyze data at α = 0.05. Results: Directly after manufacturing, incisor crowns of all test groups showed sufficient mean fracture resistances. After artificial aging, crack formation was observed in a high percentage for fully veneered crowns of all test groups, but only for one veneering ceramics with cutback crowns. Mean test forces of unaged crowns were F1d,mean ≥ 422 N | Fu,mean ≥ 749 N (fully veneered) and F1d,mean ≥ 644 N | Fu,mean ≥ 706 N (cutback) dropped significantly to F1d,mean ≥ 131 N | Fu,mean ≥ 223 N (fully veneered) and F1d,mean ≥ 324 N | Fu,mean ≥ 524 N (cutback) . Conclusions: Within the limitations of this laboratory study, 5Y-PSZ based anterior crowns can be a viable treatment option. Framework design, choice of the veneering ceramics and artificial aging show relevant effects on the fracture resistances. Concerted veneering ceramics should be used and partially veneering of the zirconia frameworks should be favored over full veneers.

Polishing systems for modern aesthetic dental materials: a narrative review

Objectives To review the current literature surrounding chairside polishing systems for resin composites, zirconia and lithium disilicate restorations.Methods A literature search was undertaken and databases were hand-searched for the most relevant articles.Discussion The current marketplace contains a wide variety of polishing systems, each with different abrasive compounds and number of steps. Current efforts are aimed at reducing the number of steps required for polishing to improve clinical effectiveness. Reduced step systems showed some comparable results to the more traditional multi-step protocols, but the most effective results were achieved with the use of polishing paste as an additional step.Conclusions Based on the current available literature, the use of material-specific polishing systems is effective for chairside polishing of direct and indirect restorative materials. However, it is important to emphasise that, for optimum outcomes, it is essential to follow manufacturers' recommendations for each step, with particular considerations of the handpiece speed, time spent per step and use of adjunct water coolant.

A Comprehensive Review of Techniques for Enhancing Zirconia Bond Strength: Current Approaches and Emerging Innovations

The increasing use of zirconia in dental restorations necessitates a comprehensive understanding of effective bonding techniques to ensure long-term clinical success. Zirconia's unique chemical composition presents challenges in achieving a durable bond as it lacks the glass phase necessary for traditional etching and silanization processes. This review evaluates current methods and emerging innovations for enhancing zirconia bond strength to resin cements. Our findings emphasize the importance of mechanical surface treatments such as air-particle abrasion and tribochemical silica-coating, which significantly improve micromechanical retention. Laser irradiation, while less commonly used, also shows promise in enhancing bond strength without compromising zirconia's structural integrity; 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) primers emerged as critical agents in forming stable P-O-Zr bonds, particularly when used with resin cements containing 10-MDP. However, variations in formulation and application methods impact their overall efficacy. Resin cement demonstrated superior bond strength compared to conventional cement, but clinical outcomes varied, highlighting the importance of cement-primer compatibility and strict procedural adherence. Emerging technologies such as polymer-infiltrated ceramic networks (PICNs) and additive manufacturing (AM) of zirconia offer potential for future advancements, although they require further research to address mechanical and aesthetic challenges. In conclusion, while established methods such as sandblasting and the use of MDP primers remain reliable, ongoing research into novel materials and techniques continues to offer opportunities for enhancing zirconia bonding. Clinicians must balance effectiveness, procedural complexity, and clinical practicality when selecting the most appropriate bonding protocols for zirconia restorations.

Effect of sintering cycle on the strength and translucency of multilayered zirconia

PURPOSE

A newly introduced sintering protocol promises to offer higher translucency while not significantly compromising the flexural strength of the material. However, the effect of the novel sintering protocol has not been thoroughly validated. The purpose of this study was to measure and compare the effect of two sintering protocols on the translucency and flexural strength of two multilayered zirconia materials.

MATERIALS AND METHODS

Two types of multilayered zirconia materials (ZirCAD Prime and Prime esthetic) were selected. Presintered disk specimens were obtained from Translucent, Gradient, and Dentin layers (n = 20). The disks were allocated to 2 groups: standard sintering protocol (peak temperature 1500°C) and high translucency sintering protocol (peak temperature 1600°C). After the sintering process, 10 specimens from each group were randomly selected. The optical values (L*, a*, b*) were measured and used to assess translucency using the relative translucency parameter (RTP00) and translucency differences (ΔRTP00). Then, all 20 specimens were tested for biaxial flexural strength. The outcomes were analyzed. The analysis of variance is used to analyze any significant effects on translucency and flexural strength. Then, any significant difference in the translucency and flexural strength between all pairs of materials was analyzed using Bonferroni-corrected Student's t-test (α = 0.05).

RESULTS

The high translucency sintering protocol significantly decreased biaxial strength in the Prime translucent and dentine layer, Prime esthetic translucent, and gradient layer. RTP00 was significantly reduced in the Prime gradient and Prime esthetic gradient layer when sintered with a high translucency protocol. The lowest ΔRTP00 was observed in the Prime dentine layer, while the highest ΔRTP00 was observed in the Prime esthetic dentin layer.

CONCLUSIONS

High translucency protocol significantly lowers the biaxial flexural strength of both multilayered materials, but the alteration in translucency is within clinically acceptable thresholds (TAT00 = 2.62).

Evaluation of zirconia ceramics fabricated through DLP 3d printing process for dental applications

Zirconia ceramics are versatile materials with remarkable properties such as a high thermal resistance, high fracture strength, and low thermal conductivity. They are chemically inert and highly wear- and corrosion-resistant, making them ideal for a wide range of applications in the aerospace, automotive, and biomedical fields. In dentistry, zirconia ceramics are used for veneers, crowns, bridges, and implants because of their biocompatibility. Despite the various benefits of zirconia ceramics, they are difficult to process because of their high hardness and brittleness. Additive manufacturing (AM) has proven to be a viable alternative to conventional fabrication processes, particularly for the processing of difficult-to-cut materials. AM of ceramics has gained significant attention in recent years because of its flexibility and ability to produce customized geometries rapidly and economically. In this study, the digital light processing (DLP) technique was employed to 3D print yttria-stabilized zirconia. The fabricated zirconia was evaluated and characterized for use in dental applications. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) were performed on the green body to assess the decomposition of the additives in the slurry and determine the debinding temperatures. The as-built parts were subjected to debinding and sintering to obtain fully dense zirconia parts. The parts tended to shrink after sintering; therefore, the shrinkage ratios were evaluated and found to be 1.2817, 1.2900, and 1.3388 in the x-, y-, and z-directions, respectively. The average density after sintering was 6.031 g/cc. The flexural strength determined using four-point bending tests was 451.876 MPa, and the tensile and compressive strengths were 143 MPa and 298.4 MPa, respectively.

Enhancing Fracture Toughness of Dental Zirconia through Incorporation of Nb into the Surface

BACKGROUND

Our previous study found that the addition of pentavalent cations like niobium (Nb) to yttria-stabilized zirconia increased fracture toughness but also raised the coefficient of thermal expansion (CTE), and opacity also increased undesirably. A new surface treatment is required to boost fracture toughness without altering CTE or translucency.

METHODS

The surfaces of pre-sintered 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 4.2 mol% yttria-stabilized partially stabilized zirconia (4.2Y-PSZ) were treated with a Nb sol solution containing Nb2O5 nanoparticles. After drying and sintering, a high-Nb-content surface layer formed with a depth of approximately 1 mm.

RESULTS

The Nb content in this surface layer matched that of a bulk material with 1 mol% Nb2O5. The tetragonality of the surface zirconia increased, enhancing the surface fracture toughness without changing the CTE or translucency.

CONCLUSIONS

Adding Nb near the surface improved the fracture toughness without affecting the CTE or translucency. This method could strengthen zirconia prostheses, allowing more reliable dental restorations.

Mechanical and optical properties of a borosilicate glass used to improve the finishing of 3Y-TZP restorations

Borosilicate glass was developed to enhance the mechanical behavior and smoothness of dental zirconia as an alternative to conventional glaze. This study assessed the mechanical and optical properties of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) coated with borosilicate glass or a commercial glaze fired for an extended period of time. Disc-shaped 3Y-TZP zirconia specimens (Zpex, Tosoh) were sintered at 1550°C for 2 hours. The specimens were divided into three groups: as-sintered (control, C); commercial glaze (G); and borosilicate glass (SL). The glaze and borosilicate glass were applied over the zirconia and fired for 20 minutes at 950°C and 1200°C, respectively. Biaxial flexural strength, fractography, X-ray diffraction (XRD), roughness (Ra and Rz), fracture toughness (Vickers indentation method), color difference (∆E00), and translucency (TP00) analyses were conducted. The t-test or the one-way ANOVA and Tukey's tests were used to analyze the data (α = 0.05). Flexural strength data were subjected to the Weibull analysis. The SL group exhibited the highest flexural strength (1025.8 MPa), whereas the C (859.41 MPa) and G (816.0 MPa) groups exhibited similar values. The SL group also had the highest characteristic strength. The fracture origin in all groups was on the zirconia surface. XRD analysis revealed that the specimens from the SL group contained tetragonal, cubic, and monoclinic phases. The SL group presented the lowest surface roughness. Fracture toughness in the SL group was lower than in the C group, but similar to that observed in the G group. The translucency and color differences observed in the G and SL groups were similar. Borosilicate glass enhanced the flexural strength of 3Y-TZP, promoted the smoothest surface, and exhibited optical properties similar to those of the glaze.

Critical evaluations on the crystallographic properties of translucent dental zirconia ceramics stabilized with 3-6 mol% yttria

OBJECTIVES

This study aimed to determine the crystalline phase composition of 3-6 mol% yttria-stabilized zirconia (3-6YSZ), specifically investigating the presence of tetragonal (t), cubic (c), and/or additional yttria-rich tetragonal (t') phase.

METHODS

Laboratory-fabricated specimens comprising 3-5YSZ, resembling translucent dental zirconia ceramics (TZ specimens), and a blend of 3YSZ and 8YSZ, representing a c-phase reference, were prepared. Additionally, 25 dental zirconia products stabilized with 3-6 mol% yttria were analyzed. Whole X-ray diffraction (XRD) patterns were obtained for Rietveld analysis, complemented by fine scanning in the 2θ region from 72º to 76º for qualitative phase analysis. Moreover, yttria concentrations in each specimen were determined using X-ray fluorescence (XRF) spectroscopy.

RESULTS

In the 2θ region from 72º to 76º, TZ and dental zirconia product specimens displayed four peaks attributed to t- and t'-phases, but the c-phase peak was absent. Rietveld analysis of the whole XRD patterns, utilizing a t-t' model, demonstrated the t-phase fraction ranging from 86 mass% in 3YSZ to 11 mass% in 6YSZ. Rietveld analysis appeared reliable, as the yttria contents calculated based on lattice parameters aligned well with those measured by XRF. This study established that dental 3-6YSZ consisted of yttria-lean t- and yttria-rich t'-phases.

SIGNIFICANCE

The present study enhances understanding of the crystalline structure of dental zirconia ceramics. Future crystallographic analyses of these ceramics should consider the presence of t- and t'-phases.

Effect of calcination on minimally processed recycled zirconia powder derived from milling waste

OBJECTIVE

To assess the influence of calcination process on the properties of minimally processed recycled 3Y-TZP, and to compare it with its commercial counterpart.

METHODS

Non-milled 3Y-TZP waste was collected, fragmented and ball-milled to a granulometric < 5 µm. Half of the recycled powder was calcined at 900 °C. Recycled 3Y-TZP disks were uniaxially pressed and sintered to create two recycled groups: 1) Calcined and 2) Non-calcined to be compared with a commercial CAD/CAM milled 3Y-TZP. The microstructure of experimental groups was assessed through density (n = 6), scanning electron microscopy (n = 3) and energy-dispersive X-ray spectroscopy (n = 3); and the crystalline content was evaluated through X-ray diffraction (XRD) (n = 3). Optical and mechanical properties were investigated through reflectance tests (n = 10), and Vickers hardness, fracture toughness (n = 5), and biaxial flexural strength tests (n = 16), respectively. Fractographic analysis was performed to identify fracture origin and crack propagation. Statistical analyses were performed through ANOVA followed by Tukey´s test, and by Weibull statistics.

RESULTS

Particle size distribution of recycled powder revealed an average diameter of ∼1.60 µm. The relative density of all experimental groups was > 98.15 % and XRD analysis exhibited a predominance of tetragonal-phase in both recycled groups, which were similar to the crystallographic pattern of the control group. Cross-section micrographs presented flaws on the non-calcined group, and a more homogeneous microstructure for the calcined and commercial groups. Commercial samples showed lower contrast-ratio and higher translucency-parameter than the recycled groups, where non-calcined presented higher translucency-parameter and lower contrast-ratio than its calcined counterpart. The commercial group presented higher fracture toughness and characteristic strength than the recycled groups. Moreover, the calcined group exhibited higher hardness, characteristic strength, and probability of survival at higher loads than the non-calcined group. Fractographic analysis depicted the presence of microstructural flaws in the non-calcined group, which may have acted as stress-raisers and led to failures at lower flexural strengths values.

SIGNIFICANCE

The calcination process improved the microstructure, optical, and mechanical properties of the recycled 3Y-TZP.

Fatigue methods for evaluating translucent dental zirconia

OBJECTIVE

To investigate fatigue methods for the evaluation of translucent zirconia and to associate in vitro failures with clinically reported ones.

DATA

Studies published in English that used fatigue tests on dental translucent zirconia.

SOURCES

Two databases (MEDLINE/PubMed and Scopus) were electronically searched without any restriction on year of publication.

STUDY SELECTION

A total of 4555 studies were identified. After removal of duplicates (78) and irrelevant articles (4316) that did not meet the inclusion criteria, 161 articles were considered eligible based on their titles and abstracts. These articles were fully read, leading to the inclusion of 41 studies in the review.

RESULTS

The most widely used fatigue method for evaluation of translucent zirconia was step-stress (18 articles), followed by staircase (seven articles), and step-wise (two articles). Most studies had been conducted in a wet environment with the use of a stainless steel piston to apply load to cemented structures on a dentin-like substrate. Most fracture analyses indicated the fracture originated on the cementation or contact surface where the load was applied. Moreover, studies that utilized anatomical structures (dental crowns) reported fractures starting at the cervical margin of the crowns.

CONCLUSION

Most studies used the step-stress method. Only three studies reported failures similar to those found in clinical trials that used translucent zirconia restorations.

CLINICAL SIGNIFICANCE

The study findings can assist on correlating clinical failures to the ones observed in vitro.

Effect of toothbrushing on surface roughness, gloss, and topography of polished and glazed ultra-translucent zirconia

PURPOSE

This study evaluated the effect of toothbrushing cycles on surface roughness (Ra), gloss (GU), and morphology of two zirconia finishing and polishing protocols.

MATERIALS AND METHODS

An ultra-translucent zirconia disc was sectioned into rectangular plates (12 mm × 7 mm × 3 mm) and divided into two groups according to the polishing and finishing system used (diamond rubber abrasive/DRA or glazing/GLA). Bovine enamel (BEN) plates with the same dimensions were used as a Control. Specimens of zirconia and enamel were analyzed for Ra and GU (n = 11) and surface morphology by scanning electron microscopy (n = 3) before toothbrushing (baseline) and after 15,000 and 30,000 toothbrushing cycles. Ra and GU data were analyzed by ANOVA two-way and post-hoc Tukey's test (α = 0.05), while the surface morphology was analyzed qualitatively.

RESULTS

The Ra decreased significantly after 30,000 toothbrushing cycles for DRA and GLA zirconia ceramics. DRA showed a higher GU at the baseline, after 15,000 and 30,000 toothbrushing cycles than GLA and BEN. Toothbrushing polished the zirconia, creating a smooth surface, while no changes were observed for BEN.

CONCLUSIONS

The increase in toothbrushing cycles (30,000) changed the surface roughness of DRA and GLA zirconia ceramics. DRA zirconia presented the highest GU, which did not change with toothbrushing.

Characterization of a hydrothermally aged experimental alumina-toughened zirconia composite

OBJECTIVES

To assess the effects of different aging protocols on chemical, physical, and mechanical properties of an experimental ATZ composite compared to a zirconia.

METHODS

Disc-shaped specimens were obtained through uniaxial pressing of commercial powders (Tosoh), ATZ comprised of 80%ZrO2/20%Al2O3 (TZ-3YS20AB) and 3Y-TZP (3Y-SBE). The specimens of each material were divided into different groups according to the aging protocol: immediate, autoclave aging and hydrothermal reactor aging. The aging protocols were performed at 134 ºC for 20 h at 2.2 bar. Crystalline evaluations were performed using X-Ray Diffraction. The nanoindentation tests measured the elastic modulus (Em) and hardness (H). Biaxial flexural strength was performed, and Weibull statistics were used to determine the characteristic strength and Weibull modulus. The probability of survival was also determined. The Em and H data were analyzed by one-way ANOVA and Tukey test.

RESULTS

Diffractograms revealed the presence of monoclinic phase in both materials after aging. The hydrothermal reactor decreased the Em for ATZ compared to its immediate condition; and the H for both ATZ and 3Y-TZP regarding their immediate and autoclave aging conditions, respectively. The aging protocols significantly increased the characteristic strength for ATZ, while decreased for 3Y-TZP. No difference regarding Weibull modulus was observed, except for 3Y-TZP aged in reactor. For missions of up to 500 MPa, both materials presented a high probability of survival (>99 %) irrespective of aging condition.

SIGNIFICANCE

The synthesized ATZ composite exhibited greater physical and microstructural stability compared to 3Y-TZP, supporting potential application of the experimental material for long-span reconstructive applications.

Effect of different surface treatments and adhesive cementation on the surface topography and flexural strength of translucent and ultra-translucent monolithic zirconia

PURPOSE

To evaluate the effect of different surface treatments and adhesive cementation on the miniflexural strength (MFS) of monolithic zirconia.

MATERIALS AND METHODS

Two-hundred and forty (240) sintered bars of translucent zirconia (ZT) and ultra-translucent zirconia (ZUT) were obtained (8 mm ×2 mm ×1 mm). The bars were divided into 16 groups (n = 15) according to the factors "Zirconia" (ZT and ZUT), "Cementation" (Cem) and "surface treatment" (Ctrl:Control, Al:Aluminum oxide/Al2O3 50 µm, Si:Silica/SiO2 coated alumina particles oxide 30 µm, Gl:Glazing+hydrofluoric acid). Half of the bars received an adhesive layer application, followed by application of resin cement and light curing. The surface roughness was measured in non-cemented groups. All the bars were subjected to the MFS test (1.0 mm/min; 100 kgf). Scanning electron microscopy was used for qualitative analyses. MFS data (MPa) and roughness (µm) were statistically evaluated by three-way and two-way ANOVA respectively and Tukey's test (5%).

RESULTS

The surface treatment and the interaction were significant for roughness. Glazing promoted less roughness compared to silicatization. Regarding MFS, only the zirconia and surface treatment factors were significant. For ZT, the sandblasted groups had an increase in MFS and glazing reduced it. There was no difference between the groups without cementation for the ZUT; however, ZUT.Si/Cem, and ZUT.Al/Cem obtained superior MFS among the cemented groups.

CONCLUSIONS

Sandblasting increases the flexural strength for ZT, while glaze application tends to reduce it. Applying resin cement increases the flexural strength of ZUT when associated with sandblasting. Sandblasting protocols promote greater surface roughness.

The impact of aging and thickness on flexural strength of various zirconia ceramics

BACKGROUND

Effects of the aging process on the flexural strength of Y-TZP and different Y-PSZ ceramics of different thicknesses were investigated.

METHODS

300 disc-shaped samples (12 mm diameter, 0.8 and 1.5 mm thicknesses) were made from 5 different zirconia materials 3Y-TZP LA, 4Y-PSZ, 5Y-PSZ, 3 + 5Y-PSZ and 4 + 5Y-PSZ. Experimental groups were artificially aged in an autoclave at 134 °C, 2 bar pressure for 1 and 5 h; control groups were not subjected to any treatment. Microstructural analysis was conducted using Scanning Electron Microscopy, and X-Ray Diffraction analysis determined the crystalline phase content. The impact of aging on flexural strength was investigated with the use of the biaxial flexural strength test. Data were analyzed using three-way ANOVA tests with a significance level of p < 0.05, applying Bonferroni correction for multiple comparisons.

RESULTS

Statistically significant differences in flexural strength were observed among the materials and the material thicknesses (p < 0.05), while there were no significant differences among the aging times (p > 0.05). The highest mean flexural strength values were recorded in the case of the 3 Y-TZP-1.5 mm-5 h group (744.1 ± 61.2 MPa), which was attributed to phase-transformation toughening. The lowest values were observed in the case of the 5 Y-PSZ-1.5 mm-5 h (338.3 ± 34.8 MPa) group.

CONCLUSIONS

Both material type and thickness significantly affect the flexural strength of zirconia ceramics, whereas aging time does not; thus, material selection and thickness are crucial considerations for clinicians.

Adsorption of oral antibacterial agents on zirconia surfaces with different crystal systems

Zirconia ceramics are widely used as dental prosthetics owing to their high biocompatibility, excellent mechanical strength, and aesthetic properties similar to color tones of natural teeth. However, there exists a growing demand for the facile attachment of antibacterial properties in long-term dental restoration. Thus, in this study, we evaluated the adsorption ability of cetylpyridinium chloride (CPC) and benzalkonium chloride (BKC)-quaternary amines widely used as antibacterial substances in commercial toothpaste and other oral care products-onto zirconia surfaces with tetragonal and monoclinic crystal structures. Although tetragonal zirconia has been widely used in dental prosthetic materials such as crowns etc., monoclinic zirconia has also been used under oral conditions because of long-term implantation. When antibacterial molecule loading on zirconia powders under simulated oral conditions, it was revealed that monoclinic zirconia adsorbed approximately five times more CPC and BKC per unit area compared with that of tetragonal zirconia. Moreover, in tetragonal zirconia, the adsorption amounts of both CPC and BKC increased slightly with growing Y2O3 content as a stabilizer. This phenomenon was attributable to the formation of complexes between rare earth elements (REE) such as Y2O3 in zirconia and quaternary amines such as CPC and BKC. In this study, the antibacterial molecular adsorption ability of dental zirconia was observed, and new advantages of zirconia in dental applications were discovered.

Evaluation of vertical marginal discrepancy and load-to-failure of monolithic zirconia and lithium disilicate laminate veneers manufactured in different thicknesses

OBJECTIVES

This study aimed to evaluate the feasibility of monolithic zirconia laminate veneers (MZLV) compared to lithium disilicate laminate veneers (LDLV).

MATERIALS AND METHODS

Sixty resin replicas, each prepared with depths of 0.5 mm, 0.7 mm, and 1 mm, were produced using a 3D printer from acrylic teeth. Laminate veneers of these thicknesses were milled from pre-sintered monolithic zirconia (3rd generation) and lithium disilicate blocks. The intaglio surface of MZLV was treated with air abrasion using 110 μm diameter silica-modified aluminium oxide particles and ceramic primer, while LDLV was etched with etchant gel and treated with the ceramic primer before cementation with resin cement. Vertical marginal discrepancy (VMD) was assessed using a stereomicroscope, and a load-to-failure test was conducted using a universal testing machine. Failure modes were evaluated macroscopically on fractured surfaces. Data were analysed statistically using Two-way ANOVA and Bonferroni correction (α = 0.05).

RESULTS

LDLV samples exhibited significantly larger VMD compared to MZLV samples across all thicknesses, especially in cervical, palatal, and mean data. Within the LDLV group, load-to-fracture values for 0.7 mm and 1.0 mm thicknesses were similar, whereas for 0.5 mm thickness, it was significantly lower. In the MZLV group, load-to-fracture values were lower for 0.7 mm and 1.0 mm thicknesses compared to LDLV, but higher for 0.5 mm thickness.

CONCLUSIONS

Material choice and restoration thickness significantly influence laminate veneer restorations' success. MZLV generally exhibits superior vertical marginal fit compared to LDLV, with varying load-to-failure values across different thicknesses. Clinical management of debonding in MZLV is simpler compared to restoration fracture in LDLV.

CLINICAL RELEVANCE

Considering clinical factors, MZLV may be a preferable option to LDLV for this restoration with the thickness of 0.5 mm.

Does the internal surface treatment technique for enhanced bonding affect the color, transparency, and surface roughness of ultra-transparent zirconia?

PURPOSE

To investigate the effects of different surface treatments and thicknesses on the color, transparency, and surface roughness of ultra-transparent zirconia.

METHODS

A total of 120 Katana ultra-translucent multi-layered zirconia specimens were divided into 12 groups according to the thickness (0.3, 0.5, and 0.7 mm) and surface treatment (control, airborne particle abrasion [APA], lithium disilicate coating, and glaze on). Color difference (ΔE00) and relative translucency parameter (RTP00) were calculated using a digital spectrophotometer. The surface roughness (Ra, Rq, Sa, and Sq) was measured using a non-contact profile scanner. The surface morphologies and microstructures of the samples were observed using a tungsten filament scanning electron microscope. Statistical analyses were performed by one-way and two-way analysis of variance (ANOVA) followed by post hoc multiple comparisons and Pearson's correlation (α = 0.05).

RESULTS

The results showed that the surface treatment, ceramic thickness, and their interactions had significant effects on ΔE00 and RTP00 (p < 0.001). The surface treatment significantly altered the micromorphology and increased the surface roughness of the ceramic samples. APA exhibited the lowest transparency, largest color difference, and highest surface roughness. Zirconia with 0.3 mm and 0.7 mm thicknesses showed strong negative correlations between Sa and RTP00.

CONCLUSIONS

The three internal surface treatments significantly altered the surface roughness, color difference, and transparency of ultra-transparent zirconia. As the thickness increased, the influence of the inner surface treatment on the color difference and transparency of zirconia decreased.

CLINICAL IMPLICATIONS

For new zirconia internal surface treatment technologies, in addition to considering the enhancement effect on the bonding properties, the potential effects on the color and translucency of high-transparency zirconia should also be considered. Appropriately increasing the thickness of zirconia restorations helps minimize the effect of surface treatment on the optical properties.

Analysis of a Regression Model for Creating Surface Microgeometry after Machining Zirconia YML Used for Dental Application

This article focuses on research in the machining of zirconia crowns for dental implants. Its goal is to find the most suitable cutting parameters that significantly affect the final surface roughness of the crowns for dental implants. This study conducts investigations and experiments to specify the cutting parameters that achieve the optimal surface roughness of zirconia crowns for dental implants. The experiments were designed to precisely determine the cutting parameters that influence the surface roughness of zirconia crowns. The results of this study provide important insights for improving the manufacturing process of zirconia crowns with the specified most suitable cutting parameters. This research contributes to the enhancement of zirconia crown manufacturing techniques and the improvement in the quality and effectiveness of dental implants.

Functionally graded bi-material interface for Porcelain Veneered Zirconia dental crowns: A study using viscoelastic finite element analysis

OBJECTIVES

During the manufacturing of Porcelain Veneered Zirconia (PVZ) dental crowns, the veneer-core system undergoes high-temperature firing cycles and gets fused together which is then, under a controlled setting, cooled down to room temperature. During this cooling process, the mismatch in thermal properties between zirconia and porcelain leads to the development of transient and residual thermal stresses within the crown. These thermal stresses are inherent to the PVZ dental crown systems and render the crown structure weak, acting as a precursor to veneer chipping, fracture, and delamination. In this study, the introduction of an intermediate functionally graded material (FGM) layer at the bi-material interface is investigated as a potentially viable alternative for providing a smoother transition of properties between zirconia and porcelain in a PVZ crown system.

METHODS

Anatomically correct 3D crown models were developed for this study, with and without the FGM layer modeled at the bi-material interface. A viscoelastic finite element model was developed and validated for an anatomically correct bilayer PVZ crown system which was then used for predicting residual and transient stresses in the bilayer PVZ crown. Subsequently, the viscoelastic finite element model was further extended for the analysis of graded sublayers within the FGM layer, and this extended model was used for predicting the residual and transient stresses in the functionally graded PVZ crown, with an FGM layer at the bi-material interface.

RESULTS

The study showed that the introduction of an FGM layer at the bi-material interface has the potential to reduce the effects from transient and residual stresses within the PVZ crown system relative to a bilayer PVZ crown structure. Furthermore, the study revealed that the FGM layer causes stress redistribution to alleviate the stress concentration at the interfacial surface between porcelain and zirconia which can potentially enhance the durability of the PVZ crowns towards interfacial debonding or fracture.

SIGNIFICANCE

Thus, the use of an FGM layer at the bi-material interface shows a good prospect for enhancing the longevity of the PVZ dental crown restorations by alleviating the abrupt thermal property difference and relaxing thermal stresses.

Effect of a metal artifact reduction algorithm on dehiscence and fenestration detection around zirconia implants with cone beam computed tomography

OBJECTIVE

To assess the efficacy of the metal artifact reduction algorithm (MARA) of the Cranex 3D cone beam computed tomography (CBCT) device in the detection of peri-implant dehiscence and fenestration around zirconia implants.

STUDY DESIGN

In total, 60 implants were placed in bovine ribs. Dehiscence and fenestration defects were created around the implants, after which 60 CBCT images were obtained with and 60 without activation of MARA. Three radiologists examined the images for the presence of defects. The area under the curve (AUC) from receiver operating characteristic analysis, sensitivity, and specificity were calculated to assess the ability to discriminate the presence vs absence of bone defects. One-way analysis of variance was employed to analyze outcome measures. The significance level was established at 5% (α = 0.05).

RESULTS

AUC values indicated excellent discrimination of dehiscence on images with MARA activation and an excellent to outstanding range of discrimination with MARA deactivation. For fenestration, MARA activation and deactivation both led to outstanding discrimination. Sensitivity and specificity values revealed that activation of MARA was helpful in distinguishing the presence vs. absence of dehiscence, while both MARA conditions were helpful for fenestration. However, there were no statistically significant differences between MARA activation and deactivation for any outcome measure (P >.05).

CONCLUSION

CBCT is suitable for detecting peri-implant defects, but MARA application does not significantly affect peri-implant dehiscence and fenestration detection.

The influence of hygroscopic expansion of resin supporting dies on the fracture resistance of ceramic restorations during thermal cycling

OBJECTIVES

To evaluate the hygroscopic expansion characterization of resin composite dies during thermal cycling, and their influence on the fracture resistance of dental ceramic materials as well as the effect of pre-immersion on these measurements.

METHODS

Disc-shaped specimens (φ = 15.0 mm, h = 1.2 mm) and anatomical crown dies of four resin composites (epoxy, Z350, P60, G10) were fabricated. Disc-shaped samples were continuously soaked in distilled water and the volume expansion was measured at different time point by Archimedes method. Disc-shaped samples were pre-immersed for 0, 7, or 30 days, elastic modulus and hardness were measured using Nanoindentation test; thermal cycling (TC) test was performed (5 °C-55 °C, 104 cycles), and volume expansion during TC was measured. Four kinds of resin die with pre-immersion for 0, 7, or 30 days were cemented to 5Y-Z crown, or epoxy dies without pre-immersion were cemented to 5Y-Z, 3Y-Z and lithium disilicate glass (LDG) crowns, and load-to-failure testing was performed before and after TC. Finite element analysis (FEA) and fractography analysis were also conducted.

RESULTS

The hygroscopic expansion was in the order: epoxy > Z350 > P60 > G10. Except for G10, the other three resin composites exhibited different degrees of hygroscopic expansion during TC. Only the elastic modulus and hardness of epoxy decreased after water storage. However, only the fracture loads of 5Y-Z and LDG crowns supported by epoxy dies were significantly decreased after TC. FEA showed a stress concentration at the cervical region of the crown after volume expansion of the die, leading to the increase of the peak stress at the crown during loading.

SIGNIFICANCE

Only the hygroscopic expansion of epoxy dies caused by TC led to the decrease in the fracture resistance of the 5Y-Z and LDG crown, which may be related to the decrease in the elastic modulus of the epoxy die and the tensile stress caused by it.

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.

Forensic study of the mechanical properties of prosthetic materials: Submersion in mangrove environment - A pilot study

INTRODUCTION

It is crucial for dental surgeons to use the mechanical properties of dental prosthetic materials to correlate the submersion time in a mangrove environment.

OBJECTIVE

This study aimed to submerge dental prosthetic materials, such as acrylic resin and zirconia, contained within acrylic resin disks in a mangrove environment, and analyze the alterations in mechanical parameters, such as surface roughness and microhardness, to estimate submergence time in similar forensic situations.

MATERIALS AND METHODS

A total of 6 specimens was embedded in acrylic resin disks numbered from 1 to 6. The materials were polished for initial parameter readings a day before submersion, and new readings were obtained 1after submersion. All specimens were subjected to surface roughness analysis, in addition to Knoop microhardness analysis for acrylic resin and Vickers microhardness analysis for zirconia. After the experiment, the data were computed for statistical comparation of the materials properties different parameters.

RESULTS

There was a significant increase in surface roughness and Knoop microhardness was reduced in the Acrylic Resin samples (p< 0.05); however, no statistically significant differences were observed in the roughness or Vickers microhardness values of the zirconia samples.

CONCLUSION

Zirconia prosthetics were more resistant to degradation when submerged in a mangrove environment compared to acrylic resin ones; however, owing to the obstacles inherent in this study, we suggest further research on the properties of prosthetic materials submerged in mangroves or other environments, which could bolster the work of dental professionals in forensic medical institutes.

Strength and phase transformation of different zirconia types after chairside adjustment

STATEMENT OF PROBLEM

Limited evidence is available for the effect of chairside adjustment using diamond instruments on different types of zirconia.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of simulated adjustments on the biaxial flexural strength and phase transformation of 3 different zirconia types.

MATERIAL AND METHODS

Three zirconia types from the same manufacturer (Katana; Kuraray) were used: High Translucency (3Y-PSZ), Super Translucent Multi Layered (4Y-PSZ), and Ultra Translucent Multi Layered (5Y-PSZ). Thirty disk-shaped specimens (Ø14×1.2 mm) were fabricated according to the International Organization for Standardization (ISO) Standard 6872 from different zirconia types (N=90). Specimens were either left without adjustment (NA), adjusted with Dialite ZR finishing and polishing system (Brasseler) (APol), or adjusted with course diamond instruments only (ADia). The specimens were distributed into 9 groups (n=10): group 3Y-PSZ/NA, group 3Y-PSZ/APol, group 3Y-PSZ/ADia, group 4Y-PSZ/NA, group 4Y-PSZ/APol, group 4Y-PSZ/ADia, group 5Y-PSZ/NA, group 5Y-PSZ/APol, and group 5Y-PSZ/ADia. The biaxial flexural strength of each specimen was measured by using a universal testing machine (Model 4411; Instron) and according to ISO 6872. X-ray diffraction analysis was conducted to quantify the monoclinic phase transformation. Scanning electron microscopy images were obtained to evaluate the fracture pattern. Statistical analysis was performed by using analysis of variance (ANOVA) and Tukey multiple comparison tests for pairwise comparisons (α=.05).

RESULTS

The mean biaxial flexural strengths ranked from the highest to the lowest were for 3Y-PSZ, 4Y-PSZ, and 5Y-PSZ under any test condition (P=.007). Chairside adjustment with a diamond instrument significantly decreased the flexural strength of all zirconia types (P<.05). No statistically significant difference was found between the effect of APol and ADia on the strength of zirconia 3Y-PSZ (P=.603), 4Y-PSZ (P=.993), and 5Y-PSZ (P=.660). Phase transformation did not occur in the 5Y-PSZ groups. ADia groups had significantly higher phase transformation values regardless of zirconia type (P<.05).

CONCLUSIONS

The biaxial flexural strength of zirconia decreased significantly after chairside adjustment with diamond instruments regardless of the yttria percentage. Adjustment with the Dialite ZR finishing and polishing system caused less tetragonal to monoclinic phase transformation than adjustment with a course-grit diamond instrument.

Optical properties of monolithic zirconia fabricated with nanoparticle jetting

STATEMENT OF PROBLEM

Excellent optical properties are essential for esthetic dental materials. However, the translucency and color masking ability of zirconia fabricated with nanoparticle jetting (NPJ), a type of printed zirconia, are unknown.

PURPOSE

The purpose of this in vitro study was to evaluate the translucency and color masking ability of zirconia fabricated using NPJ.

MATERIAL AND METHODS

A total of 90 specimens with thicknesses of 1.5, 1.0, and 0.5 mm were fabricated using high translucent milled zirconia (HT), low translucent milled zirconia (LT), and NPJ. CIELab values (L*, a*, and b*) of the specimens over 7 backgrounds, black, white, VitaB1, VitaA2, VitaA4, gold alloy (Au), and titanium (Ti), were obtained using a spectral radiometer. The relative translucency parameter (RTP) and color difference (∆E) of specimens over VitaB1, VitaA4, Au, and Ti were determined using VitaA2 as the control with the CIEDE2000 color difference equation. The normality of the data distribution was determined using the Shapiro-Wilk test. Differences among groups were analyzed using 2-way analysis of variance and the Student-Newman-Keuls (SNK) post hoc test (α=.05). The ∆E of specimens was analyzed according to perceptibility (∆E=0.8) and acceptability (∆E=1.8) thresholds using the 1 sample t test. The correlation between RTP and ∆E and RTP/∆E and thickness was examined using the Pearson correlation analysis.

RESULTS

Statistically significant differences were observed in translucency and color masking ability among HT, LT, and NPJ (P<.05). The RTP value was the lowest for zirconia fabricated with NPJ (P<.001) and highest for HT (P<.001). Monolithic zirconia fabricated with NPJ had lower ∆E values than those of HT and LT for the same thickness and background (P<.05). A positive correlation was found in RTP and ∆E (P<.001). A negative correlation was observed in RTP and thickness (P<.001) and ∆E and thickness across a constant background (P<.001).

CONCLUSIONS

Zirconia fabricated with NPJ was less translucent and had a greater color masking ability for discolored backgrounds than HT and LT.

Effect of Coffee Thermocycling on Color Stability and Translucency of CAD-CAM Polychromatic High Translucent Zirconia Compared With Lithium Disilicate Glass Ceramic

AIMS AND OBJECTIVES

To evaluate the effect of coffee thermocycling on color stability and translucency of CAD-CAM polychromatic high translucent zirconia compared with lithium disilicate glass ceramic.

METHODS

Sixteen rectangular plates (14 × 16 × 1.0 mm) of two ceramic materials (IPS E.max CAD (IEC), IPS E.max ZirCAD Prime [IZP]) were prepared. Each specimen was measured for color coordinates using a spectrophotometer following 30,000 cycles of coffee thermocycling. CIELAB formula was used to determine color and translucency differences (ΔE and ΔTP). The means of ΔE and ΔTP were compared using independent samples t-test and were evaluated using their respective 50%:50% perceptibility and acceptability thresholds (PT and AT). One-way analysis of variance was performed to evaluate the translucency parameter (TP) and surface roughness (Ra) of each material.

RESULTS

Mean ΔE values of IEC (4.69) and IZP (4.64) were higher than the AT (ΔE ≤ 2.7) with no significant difference found between the two groups (p = 0.202). Considering the TP, only IEC showed a statistically significant increase in TP value (p < 0.001). However, the mean ΔTP of IEC (3.25) remained within the range of acceptability (1.3 < ΔTP ≤ 4.4).

CONCLUSIONS

Within the limitations of this current study, the color stability of all materials was clinically affected by coffee thermocycling. In terms of translucency, only lithium disilicate glass ceramic was influenced by coffee thermocycling. High translucent zirconia had superior translucency stability compared to lithium disilicate glass ceramic.

Effects of plasma surface treatment on the bond strength of zirconia with an adhesive resin luting agent

The purpose of this study was to evaluate the effect of the atmospheric pressure plasma treatment as a surface treatment method on the contact angle and shear bond strength (SBS) of zirconia ceramics and the failure mode between the self-adhesive resin luting agent and zirconia. The zirconia specimens were divided into eight groups based on the surface treatment method: alumina blasting, air plasma, argon plasma (AP), Katana cleaner, ozonated water, ozonated water+AP, Katana cleaner+AP, and tap water+AP. The contact angles, SBS, and fracture modes were tested. AP treatment significantly reduced the contact angle (p<0.0001). The combination of AP and other cleaning methods showed a higher bond strength and more mixed fractures. Our findings indicate that using atmospheric pressure plasma with argon gas, combined with other cleaning methods, results in a stronger bond than when using alumina blasting alone.

Enhancement of Biocompatibility of High-Transparency Zirconia Abutments with Human Gingival Fibroblasts via Cold Atmospheric Plasma Treatment: An In Vitro Study

The objective of this study was to explore the effects of cold atmospheric plasma (CAP) treatment on the biological behavior of human gingival fibroblasts (HGFs) cultured on the surface of high-transparency zirconia. Two types of zirconia, 3Y-ZTP and 4Y-PSZ, were subjected to a CAP treatment for various treatment durations. Analyses of the physical and chemical properties of 3Y-ZTP and 4Y-PSZ were conducted using scanning electron microscopy, contact angle measurements, and X-ray photoelectron spectroscopy, both before and after CAP treatment. The biological responses of HGFs on both surfaces were assessed using CCK-8 assay, confocal laser scanning microscopy, and real-time PCR. Initially, the oxygen and hydroxyl contents on the surface of 4Y-PSZ exceeded those on 3Y-ZTP. CAP treatment enhanced the surface hydrophilicity and the reactive oxygen species (ROS) content of 4Y-PSZ, while not altering the surface morphology. After CAP treatment, HGFs' adhesion on 4Y-PSZ was superior, with more pronounced effects compared to 3Y-ZTP. Notably, HGFs counts and the expression of adhesion-related genes on 4Y-PSZ peaked following the CAP exposures for 30 s and 60 s. Consequently, this study demonstrates that, following identical CAP treatments, 4Y-PSZ is more effective in promoting HGFs adhesion compared to traditional 3Y-ZTP zirconia.

An Approach to Improve Specimen Processing for the Flexural Strength Testing of Zirconia

Measuring the flexural strength of restorative materials such as zirconia is crucial for providing proper indications for clinical applications and predicting performance. Great variations in specimen preparation for flexural strength measurements exist among laboratories. The aim was to evaluate how the processing method, surface treatment, and test method of the specimens affect the flexural strength of zirconia. Zirconia specimens (VITA YZ HT) (n = 270) were processed using CAD/CAM or were conventionally milled with three different surface treatments (machined, ground, polished) and were measured with three-point bending (non-chamfered/chamfered) or biaxial flexural strength test. Weibull statistics were conducted. The mean flexural strength values ranged from 612 MPa (conventional, machined, three-point bending non-chamfered) to 1143 MPa (CAD/CAM, polished, biaxial flexural strength). The highest reliability is achieved when specimens are prepared using thoroughly controllable processing with CAD/CAM and subsequently polished. Higher strength values are achieved with the biaxial flexural strength test method because the stress concentration in relation to the effective volume is smaller. Polishing reduces surface microcracks and therefore increases the strength values.

In silico nonlinear dynamic finite-element analysis for biaxial flexural strength testing of CAD/CAM materials

PURPOSE

The aim of this study was to establish and assess the validity of in silico models of biaxial flexural strength (BFS) tests to reflect in vitro physical properties obtained from two commercially available computer-aided design/computer-aided manufacturing (CAD/CAM) ceramic blocks and one CAD/CAM resin composite block.

METHODS

In vitro three-point bending and BFS tests were conducted for three CAD/CAM materials (n = 10): Katana Zirconia ST10 (raw material: super-translucent multilayered zirconia, ST10; Kuraray Noritake Dental, Niigata, Japan), Katana Zirconia HT10 (raw material: highly translucent multilayered zirconia, HT10; Kuraray Noritake Dental), and Katana Avencia N (AN; Kuraray Noritake Dental). Densities, flexural moduli, and fracture strains were obtained from the in vitro three-point bending test and used as an input for an in silico nonlinear finite element analysis. The maximum principal stress (MPS) distribution was obtained from an in silico BFS analysis.

RESULTS

The elastic moduli of AN, HT10, and ST10 were 6.513, 40.039, and 32.600 GPa, respectively. The in silico fracture pattern of ST10 observed after the in silico evaluation was similar to the fracture pattern observed after the in vitro testing. The MPS was registered in the center of the tensile surface for all three specimens. The projections of the supporting balls were in the form of a triple asymmetry.

CONCLUSIONS

The in silico approach established in this study provided an acceptable reflection of in vitro physical properties, and will be useful to assess biaxial flexural properties of CAD/CAM materials without wastage of materials.

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.

Emerging trends and clinical recommendations for zirconia ceramic crowns: a concise review

Introduction A little over ten years ago, zirconia crowns made their debut in the field of dentistry. Despite early problems with the chipping of veneering porcelain, clinical studies have demonstrated excellent performance. It is essential for a ceramic crown to have good aesthetic qualities, in addition to having good mechanical characteristics. The exceptional mechanical qualities of zirconia crowns and the simplicity with which they may be machined, employing computer-aided design and computer-aided manufacturing schemes, are primarily responsible for the widespread use of these materials in clinical settings. New ceramic-based materials, including monolithic zirconia, zirconia-containing lithium disilicate ceramics, and graded glass/zirconia/glass, have recently been launched in the field of dentistry. These newly discovered zirconia crown materials stem from varied technological approaches, each likely to lead to additional clinical advancements. At this point, it seems imperative to offer a concentrated report on the newer developments, along with essential clinical recommendations for best clinical outcomes with zirconia crowns.Types of studies This review article is a consolidation of several case studies, cohort studies and systematic reviews, as well as experimental and observational randomised control trials and other peer-reviewed articles.Results On reviewing, a concise list of clinical recommendations is generated, demonstrating that monolithic zirconia offers some clinical advantages over veneered zirconia crowns.Conclusion This review article discloses various clinical revelations and in-office recommendations for favourable usage of zirconia ceramic crowns that can lead to better patient outcomes and long-term clinical success rates.

The Acoustic Emission Testing in the Evaluation of Fracture Toughness of Brittle Materials

AIM

Evaluating the fracture resistance of dental ceramics is essential for assessing their behavior. This study aimed to validate a custom load-to-fracture test for assessing fracture strength compared to a conventional method.

MATERIALS AND METHODS

Acoustic emission testing, a non-destructive (ND) lab test, was employed to evaluate the fracture toughness (FT) of brittle materials by capturing sound waves generated by crack formation in failing samples. A total of 130 samples, divided into three types (glass sheets, zirconia sheets, and monolithic zirconia crowns), were tested. The fracture loads were measured using both custom and conventional methods.

RESULTS

The mean fracture loads for glass sheets were 650.46 N ± 110.38 (custom) compared to 691.41 N ± 155.92 (conventional). For zirconia sheets, the values were 95.25 N ± 7.78 (custom) vs 112.75 N ± 31.26 (conventional). Monolithic zirconia crowns showed mean fracture loads of 1108.99 N ± 327.89 (custom) compared to 1292.52 N ± 271.42 (conventional). Statistically significant differences were evident in all three types, indicating lower values with custom testing for all samples.

CONCLUSION

The custom testing demonstrated an advantage in identifying cracks at lower loads, thereby enhancing the accuracy of fracture load values. Despite its limitations, the study suggests that the custom setup could be a viable alternative to conventional fracture load testing of brittle materials. However, further testing with more materials is recommended to enhance the results' accuracy and generalizability.

CLINICAL SIGNIFICANCE

The findings indicate that the custom load-to-fracture test can provide more accurate measurements of FT in dental ceramics, which is crucial for predicting their clinical performance and longevity. How to cite this article: Haddad C, Eng JG, Zoghbi AE. The Acoustic Emission Testing in the Evaluation of Fracture Toughness of Brittle Materials. J Contemp Dent Pract 2024;25(7):617-623.

Advances in ceramics for tooth repair: From bench to chairside

OBJECTIVES

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

DATA, SOURCES AND STUDY SELECTION

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

CONCLUSIONS

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

CLINICAL SIGNIFICANCE

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

Effect of the coloring liquid shade and dipping time on the color, transparency, and flexural strength of monolithic zirconia

STATEMENT OF PROBLEM

The impact of different coloring liquid shades and dipping times on the color, transparency, and flexural strength of monolithic zirconia ceramics is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effects of different coloring liquid shades (A2, 3M2, and 5M2) and dipping times (no dipping, 30 seconds, 60 seconds, and 90 seconds) on the color difference (ΔE00), relative translucency parameter (ΔRTP00), and 3-point flexural strength (σ) of monolithic zirconia ceramics.

MATERIAL AND METHODS

Yttria-stabilized zirconia (3Y-TZP, 3 mol%) was cut into Ø16×1.2-mm plates (n=10) and 25×4×1.2-mm bars (n=15), which were colored using 3 shades of coloring liquid at 4 dipping times. Color coordinates were measured on a gray background by using a digital spectrophotometer with an integrating sphere attachment. The color and translucency differences were evaluated using 50:50% perceptibility (PT00 and TPT00) and acceptability (AT00 and TAT00) thresholds. The 3-point flexural strengths of the bar-shaped specimens were measured using a universal testing machine and analyzed using the Weibull distribution to calculate the Weibull modulus (m) and characteristic fracture strength (σ0). The data were analyzed with the 2-way ANOVA, Kruskal-Wallis, and LSD post hoc tests (α=.05).

RESULTS

Both shade and dipping time significantly affected the color and translucency of monolithic zirconia (P<.001). The ΔE00 was above the PT00 for all groups, with only 3M2-90 and A2-60 being below the AT00. The main cause of color differences was the difference in lightness. Only A2 showed ΔRTP00 below the TPT00 (A2-30 (ΔRTP00=0.26), A2-60 (ΔRTP00=0.29), and A2-90 (ΔRTP00=0.46)). All experimental groups showed translucency differences below TAT00. In addition, only the dipping time had a significant effect on the flexural strength of zirconia (P<.001).

CONCLUSIONS

The optical properties of monolithic zirconia ceramics were affected by the shade and dipping time of the coloring liquid. The mismatch in lightness was the main reason for the color difference. The dipping time affects the flexural strength of monolithic zirconia, whereas the shade of the coloring liquid did not seem to influence flexural strength.

Brittle-Ductile Threshold in Lithium Disilicate under Sharp Sliding Contact

Computer-aided design (CAD)/computer-aided manufacturing (CAM) milling and handpiece grinding are critical procedures in the fabrication and adjustment of ceramic dental restorations. However, due to the formation of microfractures, these procedures are detrimental to the strength of ceramics. This study analyzes the damage associated with current brittle-regime grinding and presents a potential remedy in the application of a safer yet still efficient grinding regime known as "ductile-regime grinding." Disc-shaped specimens of a lithium disilicate glass-ceramic material (IPS e.max CAD) were obtained by cutting and crystallizing the lithium metasilicate CAD/CAM blanks (the so-called blue blocks) following the manufacturer's instructions. The discs were then polished to a 1 µm diamond suspension finish. Single-particle micro-scratch tests (n = 10) with a conical diamond indenter were conducted to reproduce basic modes of deformation and fracture. Key parameters such as coefficient of friction and penetration depth were recorded as a function of scratch load. Further, biaxial flexure strength tests (n = 6) were performed after applying various scratch loads to analyze their effects on ceramic strength. Scanning electron microscopy (SEM) and focused ion beam (FIB) were used to characterize surface and subsurface damage. Statistical analysis was performed using one-way analysis of variance and Tukey tests. While the SEM surface analysis of scratch tracks revealed the occurrence of both ductile and brittle removal modes, it failed to accurately determine the threshold load for the brittle-ductile transition. The threshold load for brittle-ductile transition was determined to be 70 mN based on FIB subsurface damage analyses in conjunction with strength degradation studies. Below 70 mN, the specimens exhibited neither strength degradation nor the formation of subsurface cracks. Determination of the brittle-ductile thresholds is significant because it sets a foundation for future research on the feasibility of implementing ductile-regime milling/grinding protocols for fabricating damage-free ceramic dental restorations.

Incorporating Stud Attachments in the Bar Design for an Implant-Supported Overdenture

Bone resorption following tooth loss might compromise retention, stability, and support of conventional removable prostheses, and for this reason, implant-supported overdentures are suggested as a viable alternative for completely edentulous patients. Bars, telescopic attachments, or stud attachments have been used to provide retention through a different mechanism of action based on specific design characteristics. The purpose of this report is to thoroughly describe the applied protocol for the fabrication of an implant overdenture supported by two bars incorporating stud attachments. A 67-year-old male patient presented to the Postgraduate Clinic of the National and Kapodistrian University in Athens seeking dental rehabilitation. The remaining teeth were characterized with poor prognosis, mainly due to their periodontal status. The proposed treatment plan included the placement of four implants in the maxilla and two implants in the mandible and the fabrication of implant-supported overdentures. The diagnostic stages revealed adequate prosthetic space that would enable the fabrication of a bar substructure for the maxillary overdenture. To combine the benefits of bars and stud attachments, two bars with four attachments were fabricated. Evaluation of the delivered prosthesis revealed adequate retention, support, and stability achieved with minimal palatal coverage. Patient's reported satisfaction and quality of life were increased. Recall appointments at one, six, and twelve months did not reveal any adverse effects or patient's complaints. According to the present case report, different types of attachments may be used after careful study of each case. More studies are needed to report on different aspects of the chosen treatment plan.

Effect of the CAD/CAM Milling Protocol on the Fracture Behavior of Zirconia Monolithic Crowns

Although advancements in CAD/CAM technology allow for more personalized treatments, it is not clear how modifications in the CAD/CAM milling process could affect the restoration surface conditions and their mechanical behavior. The objective of this study was to evaluate the effect of different CAD/CAM milling protocols on the topography and fracture behavior of zirconia monolithic crowns (3Y-PSZ) subjected to a chewing simulation. Monolithic 3Y-PSZ premolar crowns were milled using three protocols (n = 13) (slow (S), normal (N), and fast (F)). Crowns were cemented on a dentin analog abutment and subjected to mechanical aging (200 N, 2 Hz, 1,500,000 cycles, 37 °C water). Surviving crowns were subjected to compressive load test and analyzed using fractography. Fracture load data were analyzed with two-parameter Weibull analysis. The surface topography of the crowns was examined with a stereomicroscope and a 3D non-contact profiler. All crowns survived the chewing simulation. Crowns milled using the F protocol had the greatest characteristic fracture load, while crowns produced with the S protocol showed high Weibull modulus. Groups N and S had a more uniform surface and detailed occlusal anatomy than group F. The CAD/CAM milling protocol affected the topography and mechanical behavior of 3Y-PSZ monolithic crowns.

Fatigue and failure mode analyses of glass infiltrated 5Y-PSZ bonded onto dentin analogues

The purpose of this study was to evaluate the fatigue survival of 5Y-PSZ zirconia infiltrated with an experimental glass and bonded onto dentin analogues. Disc-shaped specimens of a 5Y-PSZ (Katana UTML Kuraray Noritake) were cemented onto dentin analogs (NEMA G10) and divided into four groups (n = 15): Zctrl Group (control, without infiltration); Zglz Group (Glaze, compression surface); Zinf-comp Group (Experimental Glass, compression surface); Zinf-tens Group (Experimental Glass, tension surface). Surface treatments were varied. Cyclic fatigue loading, oblique transillumination, stereomicroscope examination, and scanning electron microscopy were performed. Fatigue data were analyzed (failure load and number of cycles) using survival analysis (Kaplan-Meier and Log-Rank Mantel-Cox). There was no statistically significant difference in fatigue survival between the Zglz, Zctrl, and Zinf-comp groups. The Zinf-tens group presented a significantly higher failure load when compared to the other groups and exhibited a different failure mode. The experimental glass effectively infiltrated the zirconia, enhancing structural reliability, altering the failure mode, and improving load-bearing capacity over more cycles, particularly in the group where the glass was infiltrated into the tensile surface of the zirconia. Glass infiltration into 5Y-PSZ zirconia significantly enhanced structural reliability and the ability to withstand loads over an increased number of cycles. This approach has the potential to increase the durability of zirconia restorations, reducing the need for replacements and save time and resources, promoting efficiency in clinical practice.

The effect of combining primers and cements from different cement systems on the bond strength between zirconia and dentin

OBJECTIVE

The aim of this study was to evaluate the influence of combining primers and cements from two different resin cement systems on the microtensile bond strength (μTBS) between zirconia and human dentin.

MATERIALS AND METHODS

A total of 120 specimens of zirconia cemented to dentin were allocated into eight groups based on cement type (RelyX Ultimate or Panavia V5) and primers (Tooth Primer, Clearfil Ceramic Primer and Scotchbond Universal Adhesive) combinations, applied to dentin or ceramic surfaces. Following artificial aging with 5000 thermocycles, μTBS tests were conducted. Statistical analysis was performed using One-way ANOVA and Tukey's post hoc tests (p ≤ 0.05), and failure modes were assessed.

RESULTS

The Panavia V5 cement system demonstrated the highest bond strength (19.4 ± 4.4 MPa), significantly higher than the other groups except when RelyX cement was used with Panavia primers (16.9 ± 3.7 MPa). Cohesive fractures within the cement layer were the predominant failure mode.

CONCLUSIONS

The combination of primers from different adhesive cement system brands may significantly affect the bonding effectiveness. Therefore, using products from a single product line of the same adhesive cement system, and following the manufacturer's recommendations for indications and use, is crucial for a more predictable clinical outcome.

Development of ZTA (80% Al2O3/20% ZrO2) pre-sintered blocks for milling in CAD/CAM systems

The present work aims to develop a production method of pre-sintered zirconia-toughened-alumina (ZTA) composite blocks for machining in a computer-aided design and computer-aided manufacturing (CAD-CAM) system. The ZTA composite comprised of 80% Al2O3 and 20% ZrO2 was synthesized, uniaxially and isostatically pressed to generate machinable CAD-CAM blocks. Fourteen green-body blocks were prepared and pre-sintered at 1000 °C. After cooling and holder gluing, a stereolithography (STL) file was designed and uploaded to manufacture disk-shaped specimens projected to comply with ISO 6872:2015. Seventy specimens were produced through machining of the blocks, samples were sintered at 1600 °C and two-sided polished. Half of the samples were subjected to accelerated autoclave hydrothermal aging (20h at 134 °C and 2.2 bar). Immediate and aged samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Optical and mechanical properties were assessed by reflectance tests and by biaxial flexural strength test, Vickers indentation and fracture toughness, respectively. Samples produced by machining presented high density and smooth surfaces at SEM evaluation with few microstructural defects. XRD evaluation depicted characteristic peaks of alpha alumina and tetragonal zirconia and autoclave aging had no effect on the crystalline spectra of the composite. Optical and mechanical evaluations demonstrated a high masking ability for the composite and a characteristic strength of 464 MPa and Weibull modulus of 17, with no significant alterations after aging. The milled composite exhibited a hardness of 17.61 GPa and fracture toughness of 5.63 MPa m1/2, which remained unaltered after aging. The synthesis of ZTA blocks for CAD-CAM was successful and allowed for the milling of disk-shaped specimens using the grinding method of the CAD-CAM system. ZTA composite properties were unaffected by hydrothermal autoclave aging and present a promising alternative for the manufacture of infrastructures of fixed dental prostheses.

Color- and strength-graded zirconia: Strength, light transmission, and composition

STATEMENT OF PROBLEM

Natural teeth are graded in terms of translucency and strength. Graded zirconia materials are now available with a higher yttria content on the top or in the enamel zone to increase surface translucency and a lower yttria content on the bottom or in the dentin zone to increase strength. Such materials could provide multiple advantages over uniform materials and reduce the need for porcelain veneering in anterior artificial crowns; however, studies on the properties of these materials are lacking.

PURPOSE

The purpose of this in vitro study was to measure and compare the biaxial flexural strength, percentage light transmission, elemental content, and phase content of zones within and among color graded zirconia blocks and color- and strength-graded zirconia blocks.

MATERIAL AND METHODS

Disks of a color graded material (Katana STML) and a strength- and color-graded material (IPS e.max ZirCAD Prime) were fabricated (Zircom Furnace). Biaxial flexural strength was measured using a piston-on-3-ball test in a universal testing machine (n=10). Absolute light transmission was measured with a spectrophotometer. Elemental compositions were quantified for 3 zones in each of the 2 zirconia materials using X-ray fluorescence analysis. Zirconia phase fractions were quantified for 3 zones using X-ray diffraction analysis (XRD). Where appropriate, 2-way ANOVA, 1-way ANOVA, and Tukey multiple pairwise comparison testing were used to determine which of the 6 zones differed from one another (α=.05).

RESULTS

The color-graded zirconia exhibited gradients in light transmission and differences in phase content in the 3 zones measured. The color- and strength-graded zirconia exhibited gradients in light transmission, biaxial flexural strength, elemental composition, and phase content in the 3 zones measured. The bottom, dentin, or intaglio layer of a strength graded zirconia material was substantially stronger than all other zones of either material (P<.05). The top, enamel, zones of both materials possessed high light transmissibility (P<.05). The 2 materials differed with respect to biaxial strength (P<.001), light transmission (P<.02), elemental composition, and phase composition overall, as well as in most zone-by zone comparisons (P<.05). The performance and composition of the color graded material was consistent with it being a 5Y material throughout. The performance and composition of the color and strength graded material was consistent with it having a 3Y bottom zone and a 5Y top zone.

CONCLUSIONS

A strength-graded and color-graded zirconia material offers potential advantages in both strength and translucency.

Effect of using different strips on reducing the most common error in panoramic imaging: A randomised controlled trial on palatoglossal air space shadow

INTRODUCTION

Panoramic radiography quality can be impaired by some errors such as positioning errors. Palatoglossal air space shadow error is one of the most common positioning errors and it is due to the tongue not sticking to the roof of the palate. Techniques used to deal with this error might help prevent unnecessary radiation to patients and save them time and money. The study aimed to investigate the effects of using celluloid matrix and edible tapes (fruit leather and chewing gum) on reducing the palatoglossal air space shadow error in panoramic imaging.

METHODS

In our study, 270 patients referred to the Department of Radiology were randomised into three groups: a control group, a celluloid matrix group and an edible tapes group. Before panoramic imaging, all patients were instructed to adhere their tongues to the roof of their mouths, with the distinction that for the celluloid matrix and edible tapes groups, patients were asked to place celluloid tapes, fruit leathers, or chewing gums on their tongues before doing so. The routine imaging process was then performed, and the results were compared across groups to evaluate the incidence of palatoglossal air space shadow error.

RESULTS

The number of error-free images in each fruit leather, chewing gum and celluloid tape group were significantly higher than the control group (all cases P < 0.05). The chances of error-free images in the fruit leather groups were the highest (9.57 times). The age (P = 0.136) and gender (P = 0.272) of patients had no significant effect on the results of interventions.

CONCLUSION

The application of fruit leathers, chewing gums and celluloid tapes reduced the palatoglossal air space shadow error of panoramic imaging.

Effect of femtosecond laser induced surface patterns on the flexural strength of monolithic zirconia

To investigate how patterns generated by femtosecond (fs) laser and femtosecond laser power affect the surface roughness (Ra) and biaxial flexural strength (BFS) of monolithic zirconia. Eighty disk-shaped zirconia specimens were divided into eight subgroups (n = 10): Control (C), airborne-particle abrasion (APA), 400 mW fs laser (spiral [SP(400)], square [SQ(400)], circular [CI(400)]), and 700 mW fs laser ([SP(700)], [SQ(700)], [CI(700)]). Ra values were calculated by using a surface profilometer. One additional specimen per group was analyzed with scanning electron microscopy and x-ray diffractometry. BFS values were obtained by using the piston-on-3-ball test. One-way ANOVA and either Tukey's HSD (BFS) or Tamhane's T2 (Ra) tests were used to evaluate data (α = 0.05). Regardless of the pattern and power, fs laser groups had higher Ra than C and APA, while SP groups had lower Ra than CI and SQ groups (p ≤ 0.004). For each pattern, Ra increased with higher laser power (p < 0.001), while the laser power did not affect the BFS (p ≥ 0.793). CI and SQ groups had lower BFS than the other groups (p ≤ 0.040), whereas SP groups had similar BFS to C and APA (p ≥ 0.430). Fs laser microstructuring with spiral surface pattern increased the Ra without jeopardizing the BFS of zirconia. Thus, this treatment might be an option to roughen tested zirconia.

Anterior Esthetic Rehabilitation with Computer-Aided Design/Computer-Aided Manufacturing Zirconia: A Case Report

In layered zirconia prosthesis, the choice of zirconia composition, framework design, and shade is closely related to the properties of the abutments. This interdependence emphasizes how crucial it is to take a deliberate and tailored approach to meet the unique needs of every therapeutic circumstance. To successfully treat anterior teeth and achieve restorations that look natural, challenges such as incorrect size and shape, atypical gingival contour, and unsightly hues need to be resolved. Ceramodetal restorations have occasionally allowed for the most appealing, authentic replication of natural teeth, despite its proven strength, endurance, and improved aesthetics. Due to their superior cosmetic results, metal-free materials have been used for anterior rehabilitation. Materials like dental zirconia, which offers excellent aesthetics and desired mechanical characteristics, have emerged in response to the increased need for visually appealing and metal-free alternatives. In this case study context, experiences in the clinic using multilayer zirconia prostheses intended exclusively for anterior teeth are discussed.

Evaluation of diamond rotary instruments marketed for removing zirconia restorations

STATEMENT OF PROBLEM

The high strength of zirconia makes the removal of zirconia restorations challenging and time consuming. Whether diamond rotary instruments marketed for removing zirconia restorations are more efficient is unclear.

PURPOSE

The purpose of this in vitro study was to compare the efficiency of diamond rotary instruments specifically marketed to cut zirconia with the efficiency of a conventional diamond rotary instrument.

MATERIAL AND METHODS

Two diamond rotary instruments marketed to cut zirconia (JOTA Zirkon Cut Z838L [JOT] and Intensiv ZirconCut Zr02/10 [IZC]) and a conventional diamond rotary instrument (Intensiv FG 334/6 [IFG]) were tested on 2 zirconia materials: 3Y-TZP (IPS ZirCAD LT) and a multilayered 4Y-TZP (IPS ZirCAD MT Multi). Zirconia specimens (2 mm) were cut under water cooling using a force of 2 N or 6 N. Cutting times and maximum temperatures at the tip of the diamond rotary instruments were recorded. The surface roughness before and after use was measured, and the elemental composition was analyzed.

RESULTS

Overall, cutting times were shorter for IFG (85 seconds) and IZC (100 seconds) than for the JOT (182 seconds). Cutting times were shorter for MT zirconia than for LT zirconia. Higher temperatures (2 N: 24.6 °C, 6 N: 36.7 °C) and lower surface roughness occurred with higher cutting loads. Impurities of diamond particles were seen for JOT. The diamond particle embedding materials were either nickel alloys (IFG and JOT) or a resin material (IZC).

CONCLUSIONS

Diamond rotary instruments marketed for cutting zirconia did not perform better or generate less heat compared with a conventional diamond rotary instrument. A load of 2 N with sufficient water cooling is recommended for cutting zirconia to avoid an extensive temperature increase.

Effect of Luting Materials on the Accuracy of Fit of Zirconia Copings: A Non-Destructive Digital Analysis Method

The marginal accuracy of fit between prosthetic restorations and abutment teeth represents an essential aspect with regard to long-term clinical success. Since the final gap is also influenced by the luting techniques and materials applied, this study analyzed the accuracy of the fit of single-tooth zirconia copings before and after cementation using different luting materials. Forty plaster dies with a corresponding zirconia coping were manufactured based on a single tooth chamfer preparation. The copings were luted on the plaster dies (n = 10 per luting material) with a zinc phosphate (A), glass-ionomer (B), self-adhesive resin (C), or resin-modified glass-ionomer cement (D). The accuracy of fit for each coping was assessed using a non-destructive digital method. Intragroup statistical analysis was conducted using Wilcoxon signed rank tests and intergroup analysis by Kruskal-Wallis and Mann-Whitney U tests (α = 0.05). Accuracy of fit was significantly different before/after cementation within A (0.033/0.110 µm) and B (0.035/0.118 µm; p = 0.002). A had a significantly increased marginal gap compared to C and D, and B compared to C and D (p ≤ 0.001). Significantly increased vertical discrepancies between A and B versus C and D (p < 0.001) were assessed. Of the materials under investigation, the zinc phosphate cement led to increased vertical marginal discrepancies, whereas the self-adhesive resin cement did not influence the restoration fit.

Recent advances in dental zirconia: 15 years of material and processing evolution

OBJECTIVES

The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

METHODS

A literature search was performed in May/2023 using Web of Science Core Collection with the term "dental zirconia". The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

RESULTS

The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

SIGNIFICANCE

The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore imperative for dental clinicians and laboratory technicians to get the needed knowledge on zirconia material science, to follow manufacturers' instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Effect of connector height and retainer occlusal thickness on the fracture resistance of posterior 4-unit monolithic 5Y-TZP fixed partial dentures after thermomechanical aging

STATEMENT OF PROBLEM

The connector height and retainer occlusal thickness of fixed partial dentures (FPDs) may affect restoration longevity.

PURPOSE

The purpose of this in vitro study was to determine and compare the fracture resistance of 4-unit monolithic 5% yttria tetragonal zirconia polycrystal (5Y-TZP) FPDs made with different connector heights and retainer occlusal thicknesses after thermomechanical aging.

MATERIAL AND METHODS

Forty test metal dies were duplicated from a master metal die containing 2 anatomic abutment preparations of the mandibular right first premolar and second molar for a 4-unit FPD. The dies were divided into 2 groups of 20 each for the fabrication of 4-unit FPDs, with 2-mm and 4-mm uniform connector heights at all 3 connectors, resulting in 6.3-mm2 and 12.6-mm2 connector areas. Each of these groups was further divided into 2 subgroups based on the occlusal thickness of the 2 retainers of 1 mm and 2 mm (n=10). Polyvinyl siloxane impressions of the test metal dies were made and poured in Type V dental stone. Laboratory scans were performed on all the stone dies, and 40 5Y-TZP FPDs (Ceramill Zolid FX) were designed and fabricated. Subsequently, all the FPDs were luted on to the metal dies with a self-adhesive resin cement. The FPDs were preloaded (400 000 mechanical cycles; 4000 thermocycles) using a mastication simulator and tested for axial compressive strength. Two-way analysis of variance (ANOVA) was used to examine the effect of connector and occlusal thicknesses on the fracture load (α=.05). The data were further assessed using the post hoc Tukey HSD multiple comparison test (α=.05).

RESULTS

The mean fracture load values were between 737 N and 1563 N. Significant differences in the mean fracture load were found between the connector heights (601 N; P<.001) and occlusal thicknesses (225 N; P=.002), but the interaction of the 2 factors was not significant (P=.132) The Tukey post hoc analysis showed significant differences between the connector thicknesses groups (P<.01), but the occlusal thicknesses were found to be similar for the same connector height (P=.609) CONCLUSIONS: Connector height and occlusal retainer thickness influenced the fracture load of 4-unit monolithic 5Y-TZP FPDs after thermomechanical aging.