Current ceramic materials and systems with clinical recommendations: A systematic review

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

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

OBJECTIVES

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

MATERIAL AND METHODS

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

RESULTS

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

SIGNIFICANCE

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

Three Self-Adhesive Resin Cements and Their Influence on the Marginal Adaptation of Zirconia-Reinforced Lithium Silicate Single Crowns: An In Vitro Scanning Electron Microscope Evaluation

Background: In everyday dentistry, monolithic single crowns can be cemented with self-adhesive resin cements. The aim of this in vitro study was to evaluate how the marginal adaptation of full monolithic zirconia-reinforced lithium silicate (ZLS) single crowns is influenced by three different self-adhesive resin cements. Methods: Forty-five typodont teeth fully prepared for full monolithic crowns were divided into three groups (fifteen each) for the use of three different self-adhesive resin cements. A fourth control group (Temp-bond) was created by taking five teeth from each group before cementation with self-adhesive resin cements. All forty-five abutments were scanned using a Primescan intra-oral scanner (IOS), followed by computer-aided design (CAD) and computer-aided manufacturing (CAM) of zirconia-reinforced lithium silicate (ZLS) full crowns using a four-axis machine. Initially, the crowns of the control group were fixed to the abutments using Temp-bond, and the marginal gap was evaluated using a scanning electron microscope (SEM). After removing the control group crowns from the abutments, fifteen crowns in each group were cemented using a different self-adhesive resin cement and observed under SEM for evaluation of the marginal gap. A Kolmogorov-Smirnov test was performed, indicating no normal distribution (p < 0.05), followed by Mann-Whitney tests (α = 0.05). Results: The total mean marginal gap of the temp-bond control group was significantly lower compared to all three groups of self-adhesive resin cement (p < 0.0005). The total mean marginal gap of the G-cem ONE group was significantly lower compared to the TheraCem group (p < 0.026) and RelyX U200 group (p < 0.008). The total mean marginal gap of the TheraCem group was significantly higher than the G-cem ONE group (p < 0.026) but showed no significant difference with the RelyX U200 group (p > 0.110). Conclusions: All four groups showed a clinically acceptable marginal gap (<120 microns). Although all three groups of self-adhesive resin cement showed a significant increase in the marginal gap compared to the temp-bond control group, they were within the limits of clinical acceptability. Regarding the marginal gap, in everyday dentistry, it is acceptable to use all three self-adhesive resin cements, although the G-cem ONE group exhibited the lowest marginal gap for ZLS single crowns.

Comparison of Blue and Infrared Light Transmission Through Dental Tissues and Restorative Materials

OBJECTIVES

The depth of cure using blue-light photocuring units (BL) is limited by tooth structure and qualities of the restorative material through which the activating wavelength must pass. Recent developments incorporate an infrared (IR) activated upconversion (UC) fluorescence of a lining agent filled with nanocrystals of NaYF4 and doped with YB+3 and Tm+3 that emit both blue and violet light locally at the interface of the liner and restorative resin. The purpose of this study was to evaluate the BL and 975 nm infrared (IR) light power transmission through dental tissues and restorative materials.

METHODS AND MATERIALS

Power transmissions of the IR laser (975 nm) and a monowave blue-only light-curing unit (Bluephase 16i) through dental tissues (enamel, dentin, and enamel/dentin junction, or DEJ), eight (8) various dental resin composites, and eight (8) dental ceramics, each at four thicknesses (1, 2, 3 and 4 mm) were evaluated (n=5) using a thermopile sensor (PM10, Coherent Inc) connected to a laser power meter (Fieldmate, Coherent Inc). Power transmission values of each light source and restorative material were subjected to analysis of variance and Tukey test at a pre-set alpha of 0.05.

RESULTS

A linear correlation (r=0.9884) between the supplied current and emitted IR power of the laser diode was found, showing no statistical power reduction with increased distances (collimated beam). For tooth tissues, the highest power transmissions for both light sources were observed using 1.0 mm enamel while the lowest values were found for 2.0 mm dentin and an association of 2.0 mm DEJ and 1.0 mm dentin. The only group where IR demonstrated significantly higher transmission when compared to BL was 1.0 mm enamel. For all resin composites and dental ceramics, increased thickness resulted in a reduction of IR power transmission (except for EverX Posterior fiber-reinforced composite and e.max HT ceramic). IR resulted in higher transmission through all resin composites, except for Tetric EvoCeram White. The highest BL transmission was observed for SDR Flow, at all thicknesses. Higher IR/BL ratios were observed for EverX Posterior, Herculite Ultra, and Lava Ultimate, while the lowest ratio was observed for Tetric EvoCeram White. Reduced translucency shades within the same material resulted in lower power ratio values, especially for BL transmission. Higher IR/BL ratios were observed for e.Max LT, VitaVM7 Base Dentin, and e.max CAD HT, while the lowest values were found for VitaVM7 Enamel and Paradigm C.

CONCLUSION

IR power transmission through enamel was higher when compared to blue light, while no difference was observed for dentin. The power transmission of IR was higher than BL for resin composites, except for a high value and low chroma shade. Fiber-reinforced resin composite demonstrated the highest IR/BL power transmission ratio. A greater IR/BL ratio was observed for lower translucency ceramics when compared to high translucency.

Quantum mechanical analysis of yttrium-stabilized zirconia and alumina: implications for mechanical performance of esthetic crowns

BACKGROUND

Yttrium-stabilized zirconia (YSZ) and alumina are the most commonly used dental esthetic crown materials. This study aimed to provide detailed information on the comparison between yttrium-stabilized zirconia (YSZ) and alumina, the two materials most often used for esthetic crowns in dentistry.

METHODOLOGY

The ground-state energy of the materials was calculated using the Cambridge Serial Total Energy Package (CASTEP) code, which employs a first-principles method based on density functional theory (DFT). The electronic exchange-correlation energy was evaluated using the generalized gradient approximation (GGA) within the Perdew (Burke) Ernzerhof scheme.

RESULTS

Optimization of the geometries and investigation of the optical properties, dynamic stability, band structures, refractive indices, and mechanical properties of these materials contribute to a holistic understanding of these materials. Geometric optimization of YSZ provides important insights into its dynamic stability based on observations of its crystal structure and polyhedral geometry, which show stable configurations. Alumina exhibits a distinctive charge, kinetic, and potential (CKP) geometry, which contributes to its interesting structural framework and molecular-level stability. The optical properties of alumina were evaluated using pseudo-atomic computations, demonstrating its responsiveness to external stimuli. The refractive indices, reflectance, and dielectric functions indicate that the transmission of light by alumina depends on numerous factors that are essential for the optical performance of alumina as a material for esthetic crowns. The band structures of both the materials were explored, and the band gap of alumina was determined to be 5.853 eV. In addition, the band structure describes electronic transitions that influence the conductivity and optical properties of a material. The stability of alumina can be deduced from its bandgap, an essential property that determines its use as a dental material. Refractive indices are vital optical properties of esthetic crown materials. Therefore, the ability to understand their refractive-index graphs explains their transparency and color distortion through how the material responds to light..The regulated absorption characteristics exhibited by YSZ render it a highly attractive option for the development of esthetic crowns, as it guarantees minimal color distortion.

CONCLUSION

The acceptability of materials for esthetic crowns is strongly determined by mechanical properties such as elastic stiffness constants, Young's modulus, and shear modulus. YSZ is a highly durable material for dental applications, owing to its superior mechanical strength.

Examination of the effect of aging process on marginal fit and fractute strength of temporary crowns prepared from different materials

Aim

The aim of this study is to investigate the effect of the aging process on the marginal fit and fracture resistance of temporary crowns prepared using different materials.

Materials and method

The steel die to represent the maxillary first premolar used in this study was produced on a CNC turning machine to include an anatomical occlusal surface. A total of 160 epoxy resin dies were obtained by taking impressions with conventional impression methods on the metal die. Epoxy resin dies were randomly divided into four groups. Temporary crowns were prepared for each group from poly acrylic resin (Vita CADTemp®), bis-acryl composite resin (Protemp 4), poly methyl methacrylate (PMMA; Imident) and poly ethyl methacrylate (PEMA; Dentalon Plus) restorative materials. Half of the specimens (n = 20) in each group (n = 40) were randomly separated and the aging process was applied 5000 times in the device. Marginal gap measurements on epoxy resin dies were made using a stereomicroscope. The fracture strength test of the specimens was performed by using the Instron Universal Test Device. Jamovi 2.2.5 statistical program was used for statistical analysis.

Results

When compared to temporary crowns prepared from all other materials, poly acrylic resin (Vita CADTemp ®) temporary crowns observed significantly lower marginal gap values (59,05 μm) regardless of the aging process, and a significantly higher fracture resistance (478,44 N) in the presence of aging process (p < .05 for each). While the highest marginal gap value was detected in PMMA (Imident) (120.36 μm) temporary crowns with aging process, the lowest marginal gap value was observed in poly acrylic resin (Vita CADTemp®) (59.05 μm) crowns without non-aging process. The marginal fit and fracture resistance of all temporary crowns were negatively affected by the aging process.

Conclusion

Our findings revealed the superiority of poly acrylic resin (Vita CADTemp®) crowns to the temporary crowns prepared from all other materials in terms of the significantly lower marginal gap in the absence of aging process, and the significantly higher fracture resistance in the presence of aging process. Marginal fit and fracture resistance values for all materials were found to be within clinically acceptable limits.

Repair bond strength of a new self-adhesive composite resin to three different resin-matrix ceramic CAD-CAM materials

This study aimed to evaluate the shear bond strength of two flowable composite resins to resin-matrix ceramic CAD-CAM materials.Fifty-four plates obtained from Lava Ultimate (LU), Cerasmart (CS), and Vita Enamic (VE) CAD-CAM blocks were assigned to nine groups: N0: Nova Compo SF (NCSF), N1: Silane/Single Bond Universal (SBU)/NCSF, N2: SBU/NCSF, N3: Silane/G-Premio Bond (GPB)/NCSF, N4: GPB/NCSF, G1: Silane/SBU/G-aenial Universal Injectable Composite (GUIC), G2: SBU/GUIC, G3: Silane/GPB/GUIC, G4: GPB/GUIC. After the repair procedures, shear bond strength values were analyzed. Silane pre-application decreased bond strength in most LU and CS groups but increased it in VE. NCSF performed better than GUIC in all CAD-CAM's with similar adhesive protocols. SBU in combination with NSCF had the highest bond strength among all repair procedures in LU and CS. Silane-containing universal adhesives in combination with self-adhesive resin composites should be used to repair resin-matrix ceramic materials.

Translucency of recent zirconia materials and material-related variables affecting their translucency: a systematic review and meta-analysis

BACKGROUND

Recent forms of translucent zirconia material have been developed, offering a wide range of options and varieties for enhancing aesthetics, making it a preferred choice in the field of prosthetic dentistry. However, there is insufficient understanding regarding the recent types of zirconia materials and their optical behavior. Understanding the variables that influence the translucency of zirconia and identifying strategies to enhance its esthetics are crucial.

PURPOSE

The current systemic review highlights a comprehensive understanding of different zirconia generations in relation to their optical characteristics and evaluates material-related variables affecting their translucency.

METHODS

The present review studied in-vitro studies that evaluated the optical characteristics of different yttria content of yttria stabilized materials. The topics explored were: (1) the different zirconia material generations and their optical behavior; (2) material-related factors that affect their translucency. The research was restricted to online publication in the English language from July 1, 2010, to July 31, 2023, using PubMed, Scopus, and Science Direct resources. The search key terms and their combinations were "zirconia," "translucent zirconia," "cubic zirconia," "highly translucent zirconia," "yttria partially stabilized zirconia," "monolithic zirconia," "translucency," "optical properties," and "light transmission."

RESULTS

The data obtained from fifty-three studies addressed the optical characteristics of various zirconia generations. They reported that changing yttria content had a significant impact on translucency. Different kinds of zirconia ceramics of the same generation have varying translucencies. Achieving optimum aesthetics with monolithic zirconia is challenging due to factors related to material aspects such as the presence of additives, point defects, microstructure, thickness, phase distribution, and sintering conditions.

CONCLUSIONS

Newly developed monolithic dental zirconia ceramics have improved aesthetics and translucency. However, additional research is necessary to evaluate their performance and long-term durability.

TRIAL REGISTRATION

This systematic review was registered in PROSPERO, under number CRD42023474482.

Flexural Strength and Morphological Study of Different Multilayer Zirconia Dental Materials

Nowadays, yttria (Y3+)-stabilized ZrO2 (Y-TZP) is the most commonly used material in dental prosthetics. Y-TZP dental ceramics are mainly stabilized via the addition of 3 mol% yttrium oxide (Y2O3). These ceramics exhibit excellent mechanical properties, including high flexural strength, fracture toughness, elastic modulus, etc. Some manufacturers have recently introduced a new class of dental materials with multilayer composition with the aim of combining the advantages of adding more or less Y2O3 to the ceramic composition in one Y-TZP material. The flexural strength values of multilayer Y-TZP may vary depending on the dimensions of the specimen, layer distributions, and especially the layer exposed on the maximum tension side, i.e., loading configuration. Although previous studies have examined the flexural strength of separate Y-TZP layers, capturing the flexural strength of multilayer Y-TZP is still challenging. However, one should keep in mind that multilayer flexural strength is important for clinical indications. The objective of this study is to compare the flexural strength of three distinct multilayer translucent Y-TZP materials made up of layers with different Y3+ contents. Rectangular samples (2 mm × 2 mm × 16 mm) were prepared from CAD/CAM discs using the milling machine Programill PM7 (Ivoclar Vivadent AG). Milled bars were tested for flexural strength in a three-point bending test (ISO 6872:2015) using a universal testing machine (Inspekt Duo 5kN; Hegewald & Peschke, Nossen, Germany) at a crosshead speed of 0.5 mm/min. Representative samples of each type of material were selected for quantitative and qualitative analysis of the microstructure. Representative samples of each type of material were selected for structural, mechanical, and microstructural analyses.

Evaluation of stress distribution on an endodontically treated maxillary central tooth with lesion restored with different crown materials: A finite element analysis

Objectives

The biomechanical response of teeth with periapical lesions that have been restored using various substructure materials, as well as the stress mapping in the alveolar bone, has not been thoroughly described. In this context, the objective of this study is to investigate the structural stress distributions on root canal-treated maxillary right central incisors with lesions restored using different crown materials under linear static loading conditions through finite element analysis (FEA).

Methods

In the study, five FEA models were utilised to represent healthy teeth and teeth restored with different substructure materials: (A) a healthy tooth, (B) a lesioned, root canal-treated, composite-filled tooth, (C) a lesioned, fiber-posted, zirconia-based crown, (D) a tooth with lesions, a fiber post, and Ni-Cr infrastructure crown, (E) a tooth with a lesion, a fiber post, and an IPS E-max infrastructure crown. A force of 100 N was applied at an angle of 45° to the long axis of the tooth from 2 mm cervical to the incisal line on the palatal surface. Deformation behaviour and maximum equivalent stress distributions on the tooth sub-components, including the bony structure for each model, were simulated.

Results

Differences were observed in the stress distributions of the models. The maximum stress values of the models representing the restorations with different infrastructures varied, and the highest value was obtained in the model of the E-max crown (Model E: 136.050 MPa). The minimum stress magnitudes were obtained from Model B the composite-filled tooth (80.39 MPa); however, it was observed that the equivalent stresses in all the models showed a similar distribution for all components with varying magnitudes. In periapical lesion areas, low stresses were observed. In all models, the cervicobuccal collar region of the teeth had dense equivalent stresses.

Conclusion

Different restorative treatment methods applied to root canal-treated teeth with periapical lesions can impact the stress in the alveolar bone and the biomechanical response of the tooth. Relatively high stress values in the cortical bone at the cervical line of the tooth have been observed to decrease towards the apical region. This observation may suggest a potential healing effect by reducing pressure in the periapical lesion area.

Clinical significance

Composite resin restorations can be considered the first-choice treatment option for the restoration of root canal-treated teeth with lesions. In crown restorations, it would be advantageous to prefer zirconia or metal-supported prostheses in terms of biomechanics.

3D quantitative analysis and SEM qualitative analysis of natural antagonist enamel opposing CAD-CAM monolithic zirconia or lithium disilicate tooth-supported crowns versus enamel opposing natural enamel

PURPOSE

This study aimed to evaluate the maximum vertical wear, volume wear, and surface characteristic of antagonist enamel, opposing monolithic zirconia or lithium disilicate crowns.

MATERIALS AND METHODS

The study comprised 24 participants (n = 12), who were randomly allocated to receive either a 5 mol% Y-TZP or a lithium disilicate crown in positions which would oppose the natural first molar tooth. The contralateral first molar along with its antagonist was considered as the enamel opposing natural enamel control. Data collection was performed using an intraoral scanner and polyvinylsiloxane impression. The means of the maximum vertical loss and the volume loss at the occlusal contact areas of the crowns and the various natural antagonists were measured by 3D comparison software. A scanning electron microscope was subsequently used to assess the wear characteristics.

Results

The one-year results from 22 participants (n = 11) indicated no significant differences when comparing the zirconia crown's antagonist enamel (40.28 ± 9.11 µm, 0.04 ± 0.02 mm3) and the natural enamel wear (38.91 ± 7.09 µm, 0.04 ± 0.02 mm3) (P > .05). Also, there is no significant differences between lithium disilicate crown's antagonist enamel (47.81 ± 9.41 µm, 0.04 ± 0.02 mm3) and the natural enamel wear (39.11 ± 7.90 µm, 0.04 ± 0.02 mm3) (P > .05).

Conclusion

While some studies suggested that monolithic zirconia caused less wear on opposing enamel than lithium disilicate, this study found similar wear levels to enamel for both materials compared to natural teeth.

Preparation of multifunctional ceramic foams for sound absorption, waterproofing, and antibacterial applications

Using porous materials for sound absorption is an effective approach to alleviating noise pollution, although their hydrophilic properties potentially cause concerns regarding public safety and health risks. This work provides a facile strategy for establishing a multifunctional ceramic system by using sponges as the sintering template, adjusting the pore structure of ceramic foams by varying the ceramic slurry weights and fluorinating the sintered ceramic foams via hydrolysis and condensation processes to provide low surface energy. The obtained porous ceramic foams demonstrate sound-absorbing, waterproof, and antibacterial properties. The results reveal that the increase in ceramic slurry weight decreases the pore size and porosity due to the formation of more compact structures, and the decrease in porosity compromises the sound absorption performance. In the middle-range sound frequency, the maximum sound absorption coefficient reached 0.92. In addition, the fluorination of the rough ceramic surfaces endows the ceramic foams with waterproof properties, which enables them to float on water and display the silver mirror phenomenon. In addition, due to the waterproof property reducing the contact area between the ceramic surface and the bacterial suspension, as well as the lipophilic fluorine chain disrupting the bacterial structures, these ceramic foams exhibited antibacterial rates above 95%. In addition, the mechanisms underlying the sound-absorbing, waterproof, and antibacterial properties of these porous ceramic foams are elucidated. Therefore, this work provides a facile approach to developing a multifunctional ceramic system. Their practical features make these ceramic foams more significant in the field of noise reduction.

Hydrofluoric acid concentration and etching time affect differently the microstructure and surface properties of pressed lithium disilicate glass ceramics

OBJECTIVE

To evaluate the effect of different hydrofluoric acid concentrations and etching times on the surface, chemical composition and microstructure of lithium disilicate.

MATERIAL AND METHODS

Ninety specimens of pressed lithium disilicate (LDS) were obtained (IPS e.max Press, Rosetta SP and LiSi Press). The specimens of each material were divided in two groups according to the hydrofluoric acid concentration: 5% and 10% (n = 15/group), and subdivided according to the etching time: 20, 40 and 60 s (n = 5/group). Crystalline evaluations and chemical composition were performed through x-ray diffraction (XRD) and energy-dispersive x-ray spectroscopy (EDS), respectively. Microstructural analyses were performed by scanning electron microscope (SEM), surface roughness (Ra), and material thickness removal evaluation. Thickness removal and Ra data were analyzed by ANOVA and Tukey test (p < 0.05).

RESULTS

XRD demonstrated characteristic peaks of lithium disilicate crystals, lithium phosphate and of a vitreous phase for all materials. EDS identified different compositions and SEM confirmed different surface responses to acid etching protocols. Material and etching time influenced Ra and material thickness removal (p < 0.05).

CONCLUSION

Hydrofluoric acid concentration and etching time affect the surface characteristics of LDS differently. LiSi Press presented higher resistance to hydrofluoric acid etching compared to e.max Press and Rosetta SP.

CLINICAL SIGNIFICANCE

Applying the appropriate etching protocol is pivotal to avoid excessive material removal and to prevent jeopardize the mechanical and optical properties of the material.

Influence of aging process and restoration thickness on the fracture resistance of provisional crowns: A comparative study

Background

The advancement of digital dentistry enhanced the fabrication of indirect provisional restorations utilizing durable materials, yet the performance of provisional crowns fabricated with various techniques, and different thickness remains unknown. Thus, this in-vitro study aimed to evaluate the influence of restoration thickness and aging on the fracture behavior of provisional crowns fabricated using different techniques.

Methods

A dentiform maxillary first molar was prepared using a highly filled epoxy resin material to construct identical die replicas. Four groups of provisional crowns were fabricated: Group 1 was milled at 1.5 mm occlusal thickness; Group 2 was milled at 0.9 mm thickness; Group 3 was 3D-printed at 1.5 mm occlusal thickness; and Group 4 was 3D-printed at 0.9 mm occlusal thickness. Eight crowns from each group were subjected to a thermocycling process for 5000 cycles between baths held at 5 °C and 55 °C with a dwell time of 30 s and transfer time of 5 s. All crowns (aged and non-aged (control)) were loaded for fracturing using a universal testing machine at a 0.5 mm/min crosshead speed. Data were analyzed using a two-way analysis of variance and multiple comparisons at (α = 0.05).

Results

The maximum mean force load was found in the non-aged milled group (M1.5) at 1706.36 ± 124.07 N; the minimum mean force load was recorded for the aged 3D-printed group (3D0.9) at 552.49 ± 173.46 N. A significant difference was observed before and after thermocycling (p < 0.01).

Conclusion

Computer-aided design and manufacture of milled provisional crowns is superior to 3D-printed crowns for fracture resistance.

Endocrowns: Indications, Preparation Techniques, and Material Selection

Endodontic treatment is often necessary in the field of dentistry. As the tooth structure is lost during such treatment, the tooth may become weaker and lose some of its mechanical qualities. Endodontically treated posterior teeth require cuspal coverage because of their anatomical features. Endocrowns are regarded as a suitable choice for restoring teeth that have undergone endodontic treatment. These restorations are recommended when there is a substantial loss of tooth structure, restricted interocclusal space, or a short clinical crown. They are also contraindicated in case of severe loss of tooth structure where adhesion is not applicable. Endocrowns require a specific preparation design that is distinct from the conventional crown. They can be manufactured by two methods: heat pressing or computer-aided design/computer-aided manufacturing (CAD/CAM). Moreover, several materials have been used in fabricating endocrown restoration. Lithium disilicate glass-ceramic is the most recommended material as it possesses excellent mechanical properties and esthetic results with the ability to bond to tooth structure. In conclusion, several kinds of literature recommend using them for molars. Further research is needed to evaluate this technique for premolar and anterior teeth.

Effect of mouth rinses on the stainability of monolithic lithium disilicate glass-ceramics with different surface treatments: An in vitro study

STATEMENT OF PROBLEM

Mouth rinses have been reported to cause tooth surface discoloration. However, information regarding their effect on the stainability of monolithic glass-ceramics with different surface treatments is lacking.

PURPOSE

The purpose of this in vitro study was to assess the effect of mouth rinses on the color change of milled and pressed monolithic lithium disilicate glass-ceramics with different surface treatments.

MATERIAL AND METHODS

Fifty-six Ø12×1.5-mm disk specimens were fabricated using 2 different processing techniques: milling and pressing. Each group was subdivided into 2 subgroups according to surface treatments: glazed and polished. Specimens were then immersed in 2 different types of mouth rinse (n=7): chlorhexidine (CHX) and Listerine (LST). Color parameters were assessed using a digital spectrophotometer. Color difference (ΔE00) was calculated and compared with perceptibility (ΔE00=0.8) and acceptability (ΔE00=1.8) thresholds. The data were analyzed using ANOVA and Tukey post hoc tests (α=.05).

RESULTS

The color difference (ΔE00) was significantly affected by the type of processing technique, surface treatment, and mouth rinse (P<.001) and their interaction (P=.008). All ΔE00 values were below the selected clinical acceptability threshold (ΔE00=1.8). Milled groups (ΔE00=1.13) showed greater discoloration than pressed groups (ΔE00=0.86). Glazed specimens (ΔE00=0.70) were more resistant to discoloration than polished specimens (ΔE00=1.28) and immersion in CHX (ΔE00=1.09) led to more discoloration than immersion in LST (ΔE00=0.89).

CONCLUSIONS

The color of milled and pressed monolithic lithium disilicate glass-ceramics with different surface treatments was affected by using CHX and LST mouth rinses. Glazed lithium disilicate glass-ceramics showed less staining compared with those that were polished. Specimens immersed in CHX showed more discoloration than those immersed in LST.

A 32-month evaluation of lithium disilicate cantilever resin-bonded fixed dental prostheses to replace a missing maxillary incisor

STATEMENT OF PROBLEM

The absence of a tooth in the esthetic zone can cause emotional and social distress. The use of minimally invasive and visually pleasing lithium disilicate resin-bonded fixed dental prostheses (RBFDPs) may be a suitable option for replacing a missing maxillary incisor. However, the available literature on lithium disilicate cantilever RBFDPs is limited.

PURPOSE

This retrospective multicenter study assessed the survival and success rates of lithium disilicate anterior cantilever RBFDPs with an average follow-up period of 3 years up to 9 years.

MATERIAL AND METHODS

RBFDPs delivered by 3 operators were clinically assessed for survival using a modified United States Public Health Service criteria list. The incidence density was determined for each criterion and operator. The standard error and 95% confidence interval were calculated for each incidence density difference (α=.05 for all analyses).

RESULTS

A total of 108 RBFDPs were evaluated after a mean period of 32.45 months, ranging from 14 days to 111 months. None of the restorations exhibited failure, carious lesions, or fractures during the follow-up period. The primary reasons for reduced success rates were inflammation of the surrounding soft tissues and discoloration, with incidence densities of 0.074 and 0.057 per year, respectively. Significant differences were observed among RBFDPs from different operators for criteria that included adaptation, color match, marginal adaptation, polishability, surface staining, gingival health, and antagonist wear.

CONCLUSIONS

Cantilever lithium disilicate RBFDPs appear to be suitable for short-term restoration. RBFDPs exhibited visible changes after short-term follow-up. However, these changes did not result in failure.

The Influence of Alumina Airborne-Particle Abrasion with Various Sizes of Alumina Particles on the Phase Transformation and Fracture Resistance of Zirconia-Based Dental Ceramics

The surface of zirconia-based dental ceramic restorations require preparation prior to adhesive cementation. The purpose of this study was to assess the influence of airborne-particle abrasion with different sizes of alumina particles (50 μm, 110 μm, or 250 μm) on the mechanical strength of zirconia-based ceramics' frameworks and on the extent of phase transformations. A fracture resistance test was performed. The central surface of the frameworks was subjected to a load [N]. The identification and quantitative determination of the crystalline phase present in the zirconia specimens was assessed using X-ray diffraction. The Kruskal-Wallis one-way analysis of variance was used to establish significance (α = 0.05). The fracture resistance of zirconia-based frameworks significantly increases with an increase in the size of alumina particles used for air abrasion: 715.5 N for 250 μm alumina particles, 661.1 N for 110 μm, 608.7 N for 50 μm and the lowest for the untreated specimens (364.2 N). The X-ray diffraction analysis showed an increase in the monoclinic phase content after air abrasion: 50 μm alumina particles-26%, 110 μm-40%, 250 μm-56%, and no treatment-none. Air abrasion of the zirconia-based dental ceramics' surface with alumina particles increases the fracture resistance of zirconia copings and the monoclinic phase volume. This increase is strongly related to the alumina particle size.

Effect of dynamic loading on fracture resistance of gradient zirconia fixed partial denture frameworks

STATEMENT OF PROBLEM

The new strength-gradient zirconia composed of 3-mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 5-mol% yttria-stabilized tetragonal zirconia polycrystal (5Y-TZP) has been claimed to have superior mechanical properties. However, data on the fracture resistance of 3-unit gradient 5Y-TZP and 3Y-TZP fixed partial denture frameworks are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of dynamic loading on the fracture resistance of gradient zirconia fixed partial denture frameworks.

MATERIAL AND METHODS

Two standardized stainless-steel master dies were designed to simulate a mandibular left second premolar and second molar prepared to receive zirconia frameworks. The frameworks were designed with a 0.6-mm uniform wall thickness. The mesiodistal width of the connectors was 3 ±0.02 mm, and the occlusogingival height was 3 ±0.02 mm. Forty zirconia frameworks were fabricated and divided into 2 groups according to the tested materials (n=20): 3Y-TZP and gradient 5Y-TZP and 3Y-TZP. The frameworks were cemented onto their corresponding dies with a conventional glass ionomer cement. Half of the cemented frameworks in each group underwent 600 000 cycles of dynamic loading in a mastication simulator (n=10). The other half was fractured without dynamic loading (n=10). Fracture resistance measurements (N) for each framework were recorded by using a universal testing machine at a crosshead speed of 1 mm/min. A fractured framework from each group was examined by using a scanning electron microscope (SEM) at ×100 magnification. The data obtained were statistically analyzed by using 2-way ANOVA, the pairwise Tukey honestly significant difference (HSD), and simple main effect tests to detect the difference between group mean values (α=.05).

RESULTS

The mean ±standard deviation of fracture load value before dynamic loading was 1919 ±193 N for the 3Y-TZP group and 908 ±99 N for the gradient 5Y-TZP and 3Y-TZP group. In addition, the mean fracture load value after dynamic loading was 1418 ±163 N for the 3Y-TZP group and 716 ±85 N for the gradient 5Y-TZP and 3Y-TZP group. The interaction between the effects of the zirconia material and dynamic loading on the fracture resistance was statistically significant (P=.002). The 3Y-TZP group had a statistically significant, higher fracture load mean value the gradient 5Y-TZP and 3Y-TZP group before and after dynamic loading (P<.001).

CONCLUSIONS

The fracture resistance of 3Y-TZP frameworks was significantly higher than that of gradient 5Y-TZP and 3Y-TZP frameworks before and after dynamic loading. Dynamic loading significantly reduced the fracture resistance of 3Y-TZP and gradient 5Y-TZP and 3Y-TZP frameworks.

Marginal fit of monolithic versus layered zirconia crowns assessed with 2 marginal gap methods

STATEMENT OF PROBLEM

Zirconia can be used either monolithically or veneered with porcelain. However, whether veneering zirconia affects marginal fit is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the marginal fit of the monolithic and layered zirconia prostheses using 2 different assessment methods.

MATERIAL AND METHODS

An ideal complete crown with a chamfer finish line was prepared on an extracted maxillary central incisor. Two prosthesis designs, a framework and a monolithic design, and 2 marginal fit evaluation methods, the silicone replica and the triple scanning techniques, were used. In the first group, 10 crowns were fabricated with the framework design followed by porcelain veneering, and 10 crowns were fabricated monolithically in the second group. The marginal gap in each group was evaluated with both the silicone replica and triple scan methods. Data were statistically analyzed with a 2-way repeated measures ANOVA (α=.05).

RESULTS

A significant difference was found in the mean marginal gap by design type (P=.003), with the monolithic prostheses having lower mean marginal gaps (31.0 and 84.0 µm). However, both groups showed clinically acceptable marginal fit. No significant difference was found between the assessment methods (P=.092).

CONCLUSIONS

Monolithic zirconia crowns had a better marginal fit than veneered zirconia frameworks. Both the replica and triple scan techniques for marginal gap assessment yielded similar results.

The Influence of Hydrofluoric Acid Temperature and Application Technique on Ceramic Surface Texture and Shear Bond Strength of an Adhesive Cement

All-ceramic restorations are the foundation of modern esthetic dentistry. Clinical approaches for preparation, durability, aesthetics, and repair have been reformed by the idea of adhesive dentistry. The aim of the study and the objective question was to evaluate the impact of heated hydrofluoric acid pretreatment and the application technique's influence on the surface morphology and roughness of leucite-reinforced glass-ceramic materials (IPS Empress CAD, Ivoclar Vivadent), which is fundamental for understanding the adhesive cementation process. Scanning electron microscopy was used to observe the effectiveness of the two HF (Yellow Porcelain Etch, Cerkamed) application techniques and the HF's temperature impact on the surface topography of the ceramic. Based on surface conditioning methods, the adhesive cement (Panavia V5, Kuraray Noritake Dental Inc., Tokyo, Japan) was applied to the conditioned ceramic samples and light-cured. Shear bond strength values were correlated with the micro-retentive surface texture of the ceramic. With universal testing equipment at a 0.5 mm/min crosshead speed, SBS values between the resin cement and the ceramic material were assessed until failure. Analyzing the fractured surfaces of the specimens by digital microscopy, the failure modes were divided into three categories: adhesive, cohesive, and mixed failure. Analysis of variance (ANOVA) was used to statistically analyze the collected data. The results show that alternative treatment methods affected the material's surface characteristics and have an influence on the shear bond strength.

In Vitro Evaluation of marginal adaptation of polyether ether ketone and zirconia copings

Background

Polyether ether ketone (PEEK) has emerged as a new thermoplastic material with potential applications as a restorative material. Aim: This study aimed to evaluate the marginal adaptation of PEEK copings compared to zirconia copings using field emission scanning electron microscopy.

Materials and Methods

A freshly extracted maxillary central incisor was prepared for a full-coverage restoration following standard principles of tooth preparation. The tooth was sent to a laboratory for fabrication of samples using computer-aided design and manufacturing (CAD/CAM). Twenty samples of polyether ether ketone (PEEK) copings (group A) and 20 of zirconia copings were fabricated (group B). The copings were scanned under a field emission scanning electron microscope and measurements were taken at four distinct points. The marginal adaptation over the buccal, lingual, mesial, and distal margins for both groups was evaluated. One-way analysis of variance (ANOVA) and independent t test were applied.

Results

Our findings indicate that PEEK showed better marginal adaptation than zirconia at all measurement points. The mean marginal gap value of the PEEK group was 33.99 ± 8.81 μm and of the zirconia group was 56.21 ± 15.07 μm. On comparing marginal adaptation among the mesial, distal, buccal, and lingual aspects, PEEK showed better adaptation on all four margins, with the best adaptation on the buccal margin that had the lowest mean gap value of 29.27 ± 6.07 μm. The zirconia group adapted best at the distal margin, with a lowest mean gap value of 53.58 ± 15.25 μm (P ≤ 0.05).

Conclusion

PEEK copings had better marginal adaptation and fit compared to zirconia copings. It may have applications as a restorative material in fixed prostheses.

Effect of veneering material type and thickness ratio on flexural strength of bi-layered PEEK restorations before and after thermal cycling

OBJECTIVE

The purpose of this study was evaluating the biaxial strength of bi-layered PEEK restorations before and after aging using different veneering materials in different thickness ratios.

MATERIAL AND METHODS

Ninety specimens of thickness 1.5 mm were divided into three groups according to their veneering material. Group (CAD LD): BioHPP discs veneered with CAD milled lithium disilicate (n=30), group (CAD C): BioHPP discs veneered with CAD milled composite (n=30), and group (LC): BioHPP discs veneered with conventionally layered composite (n=30). Each group was subdivided into 3 subgroups (n=10) according to the different thickness ratios between the core and the veneering material (TC:TV). Subgroup 1: TC:TV=1:0.5, subgroup 2: TC:TV=0.7:0.8, and subgroup 3: TC:TV=0.5:1. Half of the specimens of each subgroup were subjected to thermocycling, and the bi-axial flexural strength of all specimens was tested before and after aging. Three-way ANOVA followed by Bonferroni's post hoc test were used for data analysis. The significance level was set at P ≤ 0.05.

RESULTS

Material, thickness ratio, and aging all had a significant effect on biaxial flexural strength. (LC) group had the highest biaxial flexural strength. TC:TV=0.5:1 showed the lowest biaxial flexural strength. All groups showed significant decrease in biaxial flexural strength after aging.

CONCLUSIONS

Veneering material for PEEK together with the thickness ratio between the core and veneering material greatly affect the flexural strength of bi-layered restorations. Thermocycling negatively impacts the flexural strength of PEEK bi-layered restorations.

CLINICAL SIGNIFICANCE

According to the results of that study, PEEK cores are best veneered with conventionally layered composite with core to veneering thickness ratio being 1:0.5.

Dental Materials Applied to 3D and 4D Printing Technologies: A Review

As computer-aided design and computer-aided manufacturing (CAD/CAM) technologies have matured, three-dimensional (3D) printing materials suitable for dentistry have attracted considerable research interest, owing to their high efficiency and low cost for clinical treatment. Three-dimensional printing technology, also known as additive manufacturing, has developed rapidly over the last forty years, with gradual application in various fields from industry to dental sciences. Four-dimensional (4D) printing, defined as the fabrication of complex spontaneous structures that change over time in response to external stimuli in expected ways, includes the increasingly popular bioprinting. Existing 3D printing materials have varied characteristics and scopes of application; therefore, categorization is required. This review aims to classify, summarize, and discuss dental materials for 3D printing and 4D printing from a clinical perspective. Based on these, this review describes four major materials, i.e., polymers, metals, ceramics, and biomaterials. The manufacturing process of 3D printing and 4D printing materials, their characteristics, applicable printing technologies, and clinical application scope are described in detail. Furthermore, the development of composite materials for 3D printing is the main focus of future research, as combining multiple materials can improve the materials' properties. Updates in material sciences play important roles in dentistry; hence, the emergence of newer materials are expected to promote further innovations in dentistry.

SEM Evaluation of the Marginal Accuracy of Zirconia, Lithium Disilicate, and Composite Single Crowns Created by CAD/CAM Method: Comparative Analysis of Different Materials

(1) Background: The objective of this in vitro study is to evaluate the marginal accuracy of crowns created by CAD/CAM. (2) Methods: A customized chrome-cobalt (Cr-Co) implant abutment simulating a maxillary right first molar was fixed in a hemi-maxillary stone model and scanned. In total, 27 crowns were fabricated, including 9 lithium disilicate crowns, 9 composite crowns, and 9 zirconia crowns. The measurements were determined by scanning electron microscopy. Descriptive analysis was performed using the mean and standard deviation, while the Kruskal-Wallis test was performed to determine whether the marginal discrepancies were significantly different between each group (p < 0.05). (3) Results: The lowest marginal gap value was reported for zirconia (21.45 ± 12.58 µm), followed by composite (44.7 ± 24.96 µm) and lithium disilicate (62.28 ± 51.8 µm). The Kruskal-Wallis tests revealed a statistically significant difference (p-value < 0.05) in the mean marginal gaps between different materials. (4) Conclusions: The proposed digital workflow can be a viable alternative for fixed prosthetic rehabilitations. The best performance in terms of marginal gap was achieved by zirconia crowns, but all three materials demonstrate marginal closure below the clinically accepted threshold value (120 µm). Clinical significance: although significant differences were reported, the investigated CAD/CAM materials showed clinically acceptable marginal gaps.

Effect of Anti-COVID-19 Mouthwashes on Shear Bond Strength of Resin-Matrix Ceramics Repaired with Resin Composite Using Universal Adhesive: An In Vitro Study

Using anti-COVID-19 mouthwashes has become necessary to reduce acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmissions. Resin-matrix ceramic (RMCs) materials that are exposed to mouthwashes may affect the bonding of repaired materials. This research was performed to assess the effects of anti-COVID-19 mouthwashes on the shear bond strengths (SBS) of RMCs repaired with resin composites. A total of 189 rectangular specimens of two different RMCs (Vita Enamic (VE) and Shofu Block HC (ShB)) were thermocycled and randomly divided into nine subgroups according to different mouthwashes (distilled water (DW), 0.2% povidone–iodine (PVP-I), and 1.5% hydrogen peroxide (HP)) and surface treatment protocols (no surface treatment, hydrofluoric acid etching (HF), and sandblasting (SB)). A repair protocol for RMCs was performed (using universal adhesives and resin composites), and the specimens were assessed using an SBS test. The failure mode was examined using a stereomicroscope. The SBS data were evaluated using a three-way ANOVA and a Tukey post hoc test. The SBS were significantly affected by the RMCs, mouthwashes, and surface treatment protocols. Both surface treatment protocols (HF and SB) for both RMCs, whether immersed in anti-COVID-19 mouthwash or not, improved the SBS. For the VE immersed in HP and PVP-I, the HF surface treatment had the highest SBS. For the ShB immersed in HP and PVP-I, the SB surface treatment had the highest SBS.

In-Vitro Investigation of Marginal Adaptation and Fracture Resistance of Resin Matrix Ceramic Endo-Crown Restorations

The aim of this study was to evaluate the fracture resistance and marginal adaptation of endo-crown restorations produced from different resin-matrix ceramics (RMS) and the effects of these materials on marginal adaptation and fracture resistance. Three frasaco models were used by preparing (first) premolar teeth in three different margin preparations: butt-joint, heavy chamfer and shoulder. Each group was further divided into four subgroups according to the type of restorative material used: Ambarino High Class (AHC), Voco Grandio (VG), Brilliant Crios (BC) and Shofu (S) (n = 30). Master models were obtained using an extraoral scanner and fabricated with a milling machine. Marginal gap evaluation was performed with a silicon replica technique using a stereomicroscope. Replicas of the models (n = 120) were produced with epoxy resin. The fracture resistance of the restorations was recorded using a universal testing machine. The data were statistically analyzed using two-way ANOVA, and a t-test was applied for each group. Tukey's post-hoc test was performed to compare significant differences (p ≤ 0.05). The highest marginal gap was observed in VG, and the best marginal adaptation and the highest fracture resistance were found in BC. The lowest fracture resistance in Butt-joint preparation design was found in S. In addition, the lowest fracture resistance value in the heavy chamfer preparation design was found in AHC. The heavy shoulder preparation design displayed the highest fracture resistance values for all materials.

Structural Integrity of Anterior Ceramic Resin-Bonded Fixed Partial Denture: A Finite Element Analysis Study

This study was conducted as a means to evaluate the stress distribution patterns of anterior ceramic resin-bonded fixed partial dentures derived from different materials and numerous connector designs that had various loading conditions imposed onto them through the utilization of the finite element method. A finite element model was established on the basis of the cone beam computed tomography image of a cantilevered resin-bonded fixed partial denture with a central incisor as an abutment and a lateral incisor as a pontic. Sixteen finite element models representing different conditions were simulated with lithium disilicate and zirconia. Connector height, width, and shape were set as the geometric parameters. Static loads of 100 N, 150 N, and 200 N were applied at 45 degrees to the pontic. The maximum equivalent stress values obtained for all finite element models were compared with the ultimate strengths of their materials. Higher load exhibited greater maximum equivalent stress in both materials, regardless of the connector width and shape. Loadings of 200 N and 150 N that were correspondingly simulated on lithium disilicate prostheses of all shapes and dimensions resulted in connector fractures. On the contrary, loadings of 200 N, 150 N, and 100 N with rectangular-shaped connectors correspondingly simulated on zirconia were able to withstand the loads. However, two of the trapezoidal-shaped zirconia connectors were unable to withstand the loads and resulted in fractures. It can be deduced that material type, shape, and connector dimensions concurrently influenced the integrity of the bridge.

Effect of crystal orientation on flexural strength of pressable lithium disilicate glass-ceramics

This study examined the crystal orientation of four kinds of pressable lithium disilicate glass-ceramics and evaluated the effect of crystal orientation on flexural strength. Bar-shaped (24 mm in length, 1.2 mm in thickness, 4.0 mm in width), disk-shaped (12 mm in diameter, 0.5 mm in thickness), and crown-shaped (maxillary first molar) specimens were prepared according to the manufacturer's instructions. Three-point and biaxial flexural strengths were measured for bar- and disk-shaped specimens. Microstructure analysis was performed using X-ray diffractometry and scanning electron microscopy. Three-point flexural strength was improved by parallel crystal orientation along the longitudinal direction of the bar-shaped specimen. There was no relationship between two-dimensional crystal orientation and biaxial flexural strength. The results of this study assumed that biaxial flexural strength was improved by the crystal orientation in the cross-sectional direction. Pressed restorations are expected to possess higher strength than milled restorations due to their crystal orientation.

Effect of resin cement elastic modulus on the biaxial flexural strength and structural reliability of an ultra-thin lithium disilicate glass-ceramic material

OBJECTIVES

Photo- and dual-polymerized resin-based luting agent was evaluated for elastic moduli effects on ultra-thin lithium disilicate (LD) glass-ceramic strengthening, structural reliability, and stress distribution.

MATERIALS AND METHODS

One hundred-sixty LD discs (IPS e.max CAD, Ivoclar/Vivadent) were produced in ultra-thin thicknesses (half with 0.3 mm and the other half with 0.5 mm). The ultra-thin ceramic disks were coated with two different cement types (Variolink Veneer - V and Panavia F 2.0 - P). Two positive control groups were tested following hydrofluoric (HF) acid etching (LDt3, LDt5) and two negative control groups were tested for untreated ceramic (LD 3, LD 5). Biaxial flexural strength (BFS), characteristic strength (σ0) and Weibull modulus (m) were the response variables (n = 20) at the ceramic/resin cement interface (z = 0). Finite element analysis (FEA) was used to calculate maximum principal stress. Data were analyzed using two-way ANOVA, and Tukey's test. Scanning electron microscopy (SEM) was used to analyze the failed specimens using fractography and surface morphology.

RESULTS

The BFS of LD at either thickness was not affected by cement types, as also demonstrated by FEA. Structural reliability significantly improved in the positive control group (LDt5).

CONCLUSION

The cementation of ultra-thin LD with a resin-cement of varying elastic moduli did not influence BFS. LD surface modification by HF acid-etching increased the reliability.

CLINICAL RELEVANCE

Ultra-thin anterior veneer designs made from lithium disilicate have been widely proposed and the apparent success of LD ultra-thin veneers was not influenced by the cement choice in the current studies albeit the elastic moduli luting agents used were of similar values.

Fiber-reinforced composites in dentistry - An insight into adhesion aspects of the material and the restored tooth construct

OBJECTIVE

This review aimed to highlight the insight into adhesion aspects within the components of the glass FRC (i.e., fiber and matrix) and between resin luting material and the glass FRC construction.

METHODS

The fundamentals of semi-interpenetrating polymer network (semi-IPN) based FRCs and their advantages in forming a solid adhesive interface with indirect FRC restoration, dental adhesive, and luting cement are elaborated. The important resin matrix systems and glass fibers used in FRCs are discussed. This is principally based on a survey of the literature over Medline/PubMed, Web of Science, and Scopus databases and a review of the relevant studies and publications in scientific papers in international peer-reviewed journals for the specific topic of biomaterials science. The keywords used for the search approach were: adhesion, fiber-reinforced composite, glass fiber, and semi-interpenetrating polymer network.

RESULTS

The polymer matrix systems of semi-IPN-based FRCs and formation of secondary-IPN layer are pivotal for bonding of multiphasic indirect dental constructs and repair. Additionally, describing areas of indication for FRCs in dentistry, explaining the adhesion aspects of FRC for the cohesion of the material itself, and for obtaining durable adhesion when the FRC construct is luted to tooth and remaining dentition. Current progress in the field of FRC research and future directions are summarized and presented.

SIGNIFICANCE

By understanding the isotropic-anisotropic nature of fibers and the interfacial adhesion within the components of the FRC; between resin cement and the FRC construction, the clinically successful FRC-based multiphasic indirect tooth construct can be achieved. The interfacial adhesion within the components of the FRC and between resin luting material and the FRC construction play a key role in adhesion-based unibody dental restorations.

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Color Assessment of Feldspathic Ceramic with Two Different Thicknesses, Using Multiple Polymeric Cements

The purpose of this study was to evaluate the color changes of feldspathic ceramics CEREC Blocs (Dentsply Sirona, Milford, DE, USA) when cemented with different luting agents, while varying the ceramic thickness. Seventy ceramic discs of feldspathic ceramic (A2 shade) were obtained with 0.5 and 0.8 mm thicknesses. Seventy composite discs (A3 shade) 1 mm in thickness were used as substrates. After being polished and conditioned, the ceramic and composite discs were cemented with different resin cements and a flowable composite: Variolink^® Esthetic Light, Neutral and Warm (Ivoclar Vivadent, Schaan, Liechtenstein); RelyX^TM Veneer B0.5, Translucent and A3 Opaque/yellow shades (3M Oral Care, St. Paul, MN, USA); G-aenial^® Universal Flow A2 (GC Europe, Leuven, Belgium). Color difference (ΔE) was determined using a spectrophotometer. A two-way ANOVA and multiple comparisons were performed using the Bonferroni method with a 95% confidence interval. Variolink^® Neutral showed the highest ΔE (15.12 ± 0.71) and RelyX^TM Veneer A3 the lowest value (1.59 ± 0.33). There are no statistically significant differences between the two ceramic thicknesses for Variolink^® Light (p = 0.230) and RelyX^TM Veneer B0.5 (p = 0.318) cements. The feldspathic ceramic final color is influenced by the cement used and the ceramic thickness. The use of different cements in a thin ceramic has a clinically significant impact on the final esthetic result.

Chemical and mechanical stability of an ion-exchanged lithium disilicate glass in artificial saliva

Multi-component lithium disilicate (LD) glasses were ion-exchanged in a pure or mixed nitrate salt bath. The surface morphologies, mechanical properties, chemical stability and ion leaching of ion-exchanged LD glasses before and after storage in artificial saliva for 21 days were investigated. It can be found that chemical stability of ion-exchanged LD glass was temperature-dependent. The residual compressive stress induced by ion-exchange increased the chemical potential of alkali ions in glass, and the ion-exchanged LD glass, especially 235 °C/64 h group, chemical stability in artificial saliva for 21 days were deteriorated. Back-exchange treatment could relax the stress on the outermost layer of the ion-exchanged LD glass without deteriorating its strengthening effect, and back-exchanged LD glass presented good chemical and mechanical stability in artificial saliva. The results might help to enhance the service stability of ion-exchanged LD glass-ceramics in the oral condition.

Effects of Particle Abrasion Media and Pressure on Flexural Strength and Bond Strength of Zirconia

OBJECTIVES

To compare the effects of particle abrasion medium and pressure on shear bond strength and biaxial flexural strength of three generations of zirconia (Lava Frame, Lava Plus, and Lava Esthetic) with the goal of optimizing the bond to zirconia.

METHODS

280 discs (14 mm diameter; 1 mm thickness) of each zirconia were milled and sintered. Specimens of each material were randomly distributed into 14 groups (n=20); half were tested for shear bond strength and half were tested for biaxial flexural strength. The specimens were particle abraded on one surface by 2 different media (50 μm alumina particles or 50 μm glass beads) for 10 seconds at three different pressures (15, 30, and 45 psi or 0.1, 0.2, 0.3 MPa). Untreated specimens served as positive control. A tube (1.50 mm diameter) filled with dual cured resin cement (Panavia SA) was placed onto the surface and light cured. Specimens were stored in water (37°C for 24 hours) and shear bond strength was measured in a universal testing machine (Instron). Biaxial flexural strength of each specimen was measured according to ISO 6872. Shear bond strength and biaxial flexural strength were compared individually with a 2-way analysis of variance (ANOVA) for factors surface treatment and zirconia composition.

RESULTS

Significant differences were seen between surface treatments (p<0.01), zirconia composition (p<0.01) and their interaction (p<0.01) for both bond strength and flexural strength. With alumina particle abrasion, higher pressure produced higher bonds for Lava Frame and Lava Plus zirconia while the bond of Lava Esthetic declined with increased pressure. Higher pressure (>0.2 MPa or 30 psi) with alumina decreased biaxial flexural strength with Lava Esthetic zirconia.

CONCLUSIONS

Particle abrasion with alumina produced a significantly better combination of bond strength while maintaining biaxial strength of three zirconia materials than particle abrasion with glass beads. The bond strength also depended upon the pressure of particle abrasion and the generation of zirconia used.

Examination of the Shear Force Resistance of Laminate Veneers Adhered with Different Resin Cements

OBJECTIVES: This study investigated the mechanical properties of laminate veneers to determine their resistance to the shear force of three different types of cement used in the restorations. MATERIAL and METHODS: Laminate veneers were prepared using the standard tooth preparation methods for 30 extracted maxillary central incisors. To standardize the depth, guide burs were used to prepare a depth of 0.5 mm from the buccal angle. The samples were allocated into three randomized groups (n = 10): Group A (resin using the total etch method and Variolink Esthetic DC resin cement; Group B (resin using the PANAVIA F2.0 self-etch method; and Group C (self-adhesion bonded with Rely X U200 resin cement). The prepared specimens were stored in distilled water for one week before a shear test was used to determine the resistance of the veneers to the bonding. The obtained data were evaluated statistically. RESULTS: The values of the shear bond strength were statistically significant depending on the type of resin cement that was applied (p ˂ 0.05). The specimens that were cemented using the total-etch method (18.79±4.48 MPa) had the lowest shear force value. The obtained data were evaluated statistically by Tukey multiple comparison test (p > 0.05). CONCLUSIONS: The cement is a highly effective factor in the bonding between fixed prosthetic restorations and the abutment tooth, and the cement used must have sufficient resistance to shear forces.

Baghdadite: A Novel and Promising Calcium Silicate in Regenerative Dentistry and Medicine

For several years, ceramic biomaterials have been extensively utilized to rebuild and substitute for body tissues. Calcium silicates have been proven to exhibit excellent bioactivity due to apatite formation and cell proliferation stimulation, in addition to degradability at levels adequate for hard tissue formation. These ceramics' excellent biological characteristics have attracted researchers. Baghdadite is a calcium silicate incorporating zirconium ions that enhances human osteoblast multiplication and development, increasing mineralization, and ossification. It has currently received much interest in academic institutions and has been extensively explored in the form of permeable frameworks, varnishes, bone adhesive and gap fillings, microparticles, and nanospheres, particularly in a wide range of biomedical applications. This review article aims to summarize and analyze the most recent research on baghdadite's mechanical characteristics, apatite-forming capability, dissolution pattern, and physiochemical qualities as a scaffold for dentofacial tissuè regeneration purposes.

A Scoping Review of Marginal and Internal Fit Accuracy of Lithium Disilicate Restorations

OBJECTIVE

To assess and compare the accuracy of the marginal and internal fit of lithium disilicate crowns and onlays fabricated by conventional and digital methods.

SOURCES

An electronic search was carried out on MEDLINE, Embase, Web of Science and Cochrane Library between 2010 and 2021.

STUDY SELECTION

Seventeen studies published between 2014 & 2021 were included, of which thirteen were in vitro laboratory-based studies; three were in vivo clinical studies and one randomised controlled trial.

DATA

Twelve studies focused on the marginal fit, five focused on the marginal and internal fit. Five studies found that the marginal and internal fit of crowns were more accurate using digital techniques. Five studies noted that there was no difference using either technique and two noted that conventional methods had a more accurate marginal fit.

CONCLUSION

Digital techniques were comparable to conventional methods in terms of accuracy although there was insufficient evidence to indicate that one technique was more accurate than the other with respect to Lithium Disilicate restorations.

CLINICAL SIGNIFICANCE

Digital impressions are reliable and viable alternatives for clinicians compared to conventional impression techniques when restoring teeth with lithium disilicate restorations.

Phase Formation, Mechanical Strength, and Bioactive Properties of Lithium Disilicate Glass-Ceramics with Different Al2O3 Contents

Owing to its excellent mechanical properties and aesthetic tooth-like appearance, lithium disilicate glass-ceramic is more attractive as a crown for dental restorations. In this study, lithium disilicate glass-ceramics were prepared from SiO₂-Li₂O-K₂O-P₂O₅-CeO₂ glass systems with various Al₂O₃ contents. The mixed glass was then heat-treated at 600 °C and 800 °C for 2 h to form glass-ceramic samples. Phase formation, microstructure, mechanical properties and bioactivity were investigated. The phase formation analysis confirmed the presence of Li₂Si₂O₅ in all the samples. The glass-ceramic sample with an Al₂O₃ content of 1 wt% showed rod-like Li₂Si₂O₅ crystals that could contribute to the delay in crack propagation and demonstrated the highest mechanical properties. Surface treatment with hydrofluoric acid followed by a silane-coupling agent provided the highest micro-shear bond strength for all ceramic conditions, with no significant difference between ceramic samples. The biocompatibility tests of the material showed that Al₂O₃-added lithium disilicate glass-ceramic sample was bioactive, thus activating protein production and stimulating the alkaline phosphatase (ALP) activity of osteoblast-like cells.

Evaluation of Effect of Zirconia Surface Treatment with CO2 and Nd:YAG Lasers on Shear Bond Strength between Zirconia Frameworks and Porcelain Veneers

AIM

The purpose of this study was to evaluate the effect of zirconia surface treatment with CO2 and Nd:YAG laser on shear bond strength (SBS) between the zirconia framework and porcelain veneering.

MATERIALS AND METHODS

In this in vitro study, zirconia blocks were converted to 50 cubes and were divided randomly into 5 groups. After sintering (S), porcelain was applied in the control group. The surface treatment of the second to fifth groups included CO2 laser + (S), (S + CO2), Nd:YAG laser + (S), and (S + Nd), respectively. The SBS test was done, and data were analyzed by SPSS16 software. One sample was randomly chosen from each group and the type of failure was examined under scanning electron microscope (SEM). To compare the pairs of means, the least significant difference test was used and the determined significance level was 5% (p < 0.05).

RESULTS

The SBS of S + Nd group was significantly higher than the other ones, except for S + CO2 group. The least amount of SBS belonged to CO2 + S and the highest to S + Nd group. There were no significant differences between the other groups.

CONCLUSION

The bond strength of veneering porcelain to zirconia can be altered by surface treatments. It can also be affected by the type and sequence of laser and sintering application. The effect of Nd:YAG laser on the surface of zirconia, in order to create roughness to increase SBS, is better than that of CO2 laser.

CLINICAL SIGNIFICANCE

Surface treatment of zirconia by certain types of lasers reduces the chipping of the ceramic veneer and increases the success rate of all-ceramic restorations.

Longevity of gold restorations in posterior teeth: A retrospective study up to 10-years

OBJECTIVE

In this retrospective clinical study, we investigated the long-term survival and success of indirect gold restorations, placed by undergraduate students.

MATERIAL AND METHODS

The study included 72 gold restorations in 45 patients, placed in a five-year period between 2009 and 2013. Two experienced dentists performed the evaluation following the FDI criteria after 5 to 10 years. Kaplan-Meier test was used for longevity analysis of success and survival. Data were tested for normality with Shapiro-Wilk test. Cox regression analysis, with significance level appointed at p ≤ .05, was performed using success and survival of restorations as dependent variables. Annual failure rates (AFRs) were calculated.

RESULTS

Overall, 72 indirect gold restorations prepared in posterior teeth were assessed, with an observation time from 63 months to 117 months. The mean service time was 88.1 months and the median 87 months. No failures, but 2 repairs, were observed during the first five years. From 5 to 10 years 1 replacement and 1 more repair occurred. The survival and success rates of gold restorations were calculated at 9 years with 98.6% and 91%, respectively. Restorations placed on premolars had higher survival and success rates compared to molars. Three cases were categorized as repairs: in two cases, a root canal treatment was needed and, in one other, an occlusal perforation of the restoration was observed.

CONCLUSIONS

Posterior indirect gold restorations placed by dental undergraduate students showed a highly predictable clinical performance in terms of success and survival. These results are comparable with survival rates reported in literature for restorations cemented in general dental practices.

CLINICAL SIGNIFICANCE

Casted gold restorations placed from dental students in an undergraduate setting show high survival rates, similar to professional dentists.

Wear behavior and abrasiveness of monolithic CAD/CAM ceramics after simulated mastication

Objectives

To evaluate the wear resistance and abrasiveness of monolithic CAD/CAM ceramics.

Materials and methods

Rectangular-shaped specimens (12 mm × 6.5 mm × 1.5 mm) were sectioned from the following CAD/CAM blocks (n = 10); partially crystallized lithium disilicate (PLD), experimental fully crystallized lithium disilicate (FLD), zirconia-reinforced lithium silicate (ZLS), super-translucent monolithic zirconia (SMZ), and ultra-translucent monolithic zirconia (UMZ). Silicon carbide papers were used to mechanically flatten and polish the surfaces. PLD specimens were subjected to a combined crystallization/glazing firing cycle. Ceramic specimens were mounted to the wear device and tested for 200,000 cycles against human premolars at 20 N force and 2 mm sliding distance. Artificial saliva was used as a lubricant. The teeth were scanned using micro-CT before and after the wear test and the generated models were overlapped to determine the volumetric tooth loss. Before and after the test, specimens’ weights and surface roughness (R_a) values were measured, and the differences were calculated. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were utilized for microstructural and chemical analysis. Statistical analysis was performed using one-way ANOVA or an equivalent test for non-parametric results. Significance level was set at P ≤ 0.05.

Results

The type of ceramic material affected the ceramic and antagonist wear rates (P < 0.001). PLD and ZLS had the highest ceramic and antagonist wear, whereas UMZ and SMZ demonstrated the lowest wear values. The FLD group showed comparable antagonist wear and significantly less ceramic wear than PLD and ZLS.

Conclusions

Monolithic zirconia demonstrated the best wear resistance and least abrasiveness to the antagonist. The experimental lithium disilicate was more wear-resistant than other glass–ceramic groups.

Clinical relevance

Monolithic zirconia is wear-resistant and gentle on the antagonist. In contrast, glass–ceramics are more abrasive to enamel.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-022-04611-w.

The Effect of Feldspathic Thickness on Fluorescence of a Variety of Resin Cements and Flowable Composites

(1) Background: The shade of resin-based materials and ceramic thickness influence the optical color of laminate restorations. The purpose of this study is to evaluate—in vitro—the effect of resin-based cement shade and ceramic thickness on fluorescence of feldspathic laminate veneers; (2) Methods: 180 samples of feldspathic ceramic A2 shade with two different thicknesses (0.5 and 0.8 mm) were obtained. The samples were cemented to composite resin substrates with one of the following materials in different shades (n = 10): resin cement (Variolink Esthetic in Light, Neutral and Warm shades; or RelyX Veneer in B0.5 /white, Translucent and A3 Opaque/yellow opaque shades); flowable composite resin (G-aenial Flo in A2 and A3 shades) or a pre-heated composite resin (Filtek Supreme XTE, A3 body shade). The fluorescence spectra were obtained by means of a spectrofluorometer. Two-way ANOVA, Tukey, and Student’s t-tests were performed (α = 0.05); (3) Results: Fluorescence values were significantly influenced by the resin-based agent tested (p < 0.001), the thickness of ceramic (p < 0.001), and their interaction (p < 0.001). The lowest fluorescence values were achieved by RelyX Veneer resin cement regardless its shade and the ceramic thickness; (4) Conclusions: both the shade of resin-based agent and the feldspathic ceramic thickness influenced the fluorescence of laminate restorations.

How home bleaching agents affect the color and translucency of CAD/CAM monolithic zirconia materials

To assess the effect of 10% carbamide peroxide (CP) and 6% hydrogen peroxide (HP) home bleaching agents on the translucency and color of monolithic zirconia. Ninety disc specimens were fabricated (diameter, 10 mm) from multi-layered (ML), ultra translucent multi-layered (UTML), and super translucent multi-layered (STML) zirconia blocks at three thicknesses (0.4,1,1.5 mm) (n=5). The samples were divided into two subgroups, which were treated with 6% HP (45 min per day) or 10% CP (8 h per day) for 14 days. The color of specimens was measured before bleaching (T0) and after bleaching on the third (T3), seventh (T7), and 14th (T14) day. Color (∆E) and translucency (TP) changes were calculated. The thickness varieties used in the samples and the bleaching agent types used created statistically significant differences only in TP and ∆E₀₀, respectively (p<0.05). Bleaching agents can affect TP and ∆E. Patients who have zirconia restorations should be careful when using home bleaching agents.

Optical Behavior and Surface Analysis of Dental Resin Matrix Ceramics Related to Thermocycling and Finishing

Color preservation of esthetic dental restorative materials in the oral environment represents, besides longevity, a concern, and there is still limited knowledge related to the effect of aging on the optical behavior of resin matrix ceramics. The study analyzed the finishing and thermocycling of resin matrix ceramic material surfaces, in order to assess their consequences on optical properties. Five resin matrix CAD/CAM ceramics, namely a polymer-infiltrated ceramic and four types of nanoparticle-filled resins, were selected for the study, and finished by polishing and glazing. Thermocycling was chosen as the in vitro aging method. Surface microroughness, optical and hardness evaluations were achieved before and after artificial aging. Statistical analyses were performed with IBM SPSS Statistics software at a significance value of p < 0.05. Micro-roughness values increased after thermocycling, but were kept under the clinically accepted values. The optical characteristics of resin matrix ceramics were not significantly modified by thermocycling. Values of the glazed samples became closer to those of the polished ones, after hydrothermal aging, even if the differences were insignificant. Thermocycling significantly decreased the microhardness, mainly for glazed samples. This could be the consequence of glaze removal during thermocycling, which means that glazes provide a surface protection for a limited time.

EFFECT OF GRAIN-BOUNDARY SEGREGATION ON LOW-TEMPERATURE DEGRADATION OF ORAL 3Y-TZP CERAMICS CO-DOPED WITH GEO2 AND TIO2

Purpose: To explore the improvement of its low temperature aging resistance by co-doping oral 3Y-TZP with TiO2 and GeO2. Methods: After the samples of 3Y-TZP co-doped with GeO2 and TiO2 were prepared by mechanical ball milling, drying, and pressureless sintering, the microstructure was studied by scanning transmission electron microscope, energy spectrometer, and high angle annular dark field detector. After treatment in the autoclave, the conversion of all samples was measured by XRD. Results: After mechanical ball milling for 24 hours, the particle size of the composite powder of 3Y-TZP co-doped with GeO2 and TiO2 is uniform, distributed at about 50[Formula: see text]nm, the particle shape is basically spherical, the dispersion is good, but there is a small amount of agglomeration; Tetravalent stabilizers (Ge[Formula: see text], Ti[Formula: see text]) can inhibit the low-temperature aging effect of zirconia materials. With the increase of germanium oxide doping, the hydrothermal degradation resistance of zirconia can be improved. GeO2 doped 3Y-TZP has better low temperature aging resistance than TiO2 doped 3Y-TZP. With the increase of dopant content, the degree of grain boundary segregation increases, but even if the doping amount of Ge[Formula: see text] is small, the degree of grain boundary segregation can be comparable to that of Ti[Formula: see text] with a large doping amount. Conclusion: In this paper, the composite powder of 3Y-TZP co-doped with GeO2 and TiO2 with uniform particle size and good dispersion was prepared by mechanical ball milling. The preparation process is simple, the energy consumption is low, and the cumbersome processes of the co-precipitation method and liquid-phase method are avoided. On the other hand, adding GeO2 and TiO2 can enhance the low-temperature aging resistance of 3Y-TZP ceramics. GeO2 is superior to TiO2 in low-temperature degradation resistance, which can be used as the first choice. This study contributes to the research on doping cations of dental zirconia and provides a reference for dentists to choose a reasonable zirconia material.

The Effect of Y3+ Addition on Morphology, Structure, and Electrical Properties of Yttria-Stabilized Tetragonal Zirconia Dental Materials

Zirconia (ZrO₂), a widely known material with an extensive range of applications, is especially suitable for dental applications. This kind of dental material is produced in the form of blocks or discs (mostly discs—depending on CAD/CAM machines) by cold isostatic pressing (CIP). Such discs are subsequently milled by CAM/CAD technology into a desirable form. Due to the application of CIP, the resulting discs consist of different yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) powders, additives and pigments. The diverse composition of the discs (e.g., different Y³⁺ content) governs material properties, such as hardness, toughness and translucence. The aim of this work was to investigate the impact of Y³⁺ addition on the grains and grain boundaries, on the ZrO₂ phases lattice parameter values and on the electrical equivalent circuit parameters of the prepared Y-TZP samples. The disc-shaped samples were prepared by using CAM/CAD technology. It was observed that the grain size and the grain density were increased by Y³⁺ addition. The sample with the lowest Y³⁺ content was characterized by the highest portion of the tetragonal phase, whilst the disc with the highest Y³⁺ addition consisted mainly of the cubic phase. It was also observed that at the higher Y³⁺ ion concentrations, these ions mainly incorporated the tetragonal phase. Furthermore, conductivity investigations showed that the resistivity of the grains in the samples with the higher Y³⁺ concentrations was decreased as these ions were mainly trapped in the grain boundary. On the other hand, the Y³⁺ trapping increased the capacitance of the grain boundary.

A Review on Surface Finishing Techniques for Difficult-to-Machine Ceramics by Non-Conventional Finishing Processes

Ceramics are advanced engineering materials in which have been broadly used in numerous industries due to their superior mechanical and physical properties. For application, the industries require that the ceramic products have high-quality surface finishes, high dimensional accuracy, and clean surfaces to prevent and minimize thermal contact, adhesion, friction, and wear. Ceramics have been classified as difficult-to-machine materials owing to their high hardness, and brittleness. Thus, it is extremely difficult to process them with conventional finishing processes. In this review, trends in the development of non-conventional finishing processes for the surface finishing of difficult-to-machine ceramics are discussed and compared to better comprehend the key finishing capabilities and limitations of each process on improvements in terms of surface roughness. In addition, the future direction of non-conventional finishing processes is introduced. This review will be helpful to many researchers and academicians for carrying out additional research related to the surface finishing techniques of ceramics for applications in various fields.

Masking ability of different ceramics upon various underlying structures

OBJECTIVE

To evaluate masking ability of different types and thicknesses of ceramics upon various substructures.

MATERIALS AND METHODS

Discs (Ø 12 mm, thickness 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 mm) were prepared from Bruxzir-Anterior® (Bc), Celtra-DUO® (Cc), and Vita-Suprinity® (Vc) (n = 15/group). Substructures (2 mm thickness) were prepared from natural dentine (Ds), tetracycline-stained dentine (Ts), zirconia (Zs), resin composite (Rs), and cast metal (Ms). A spectrophotometer was used to measure the CIE L*a*b* color coordinates of a specimen. Masking ability was determined from the color difference (∆E₀₀ ) of ceramics between before- and after combining with substructure using ANOVA and Bonferroni test (α = 0.05). ∆E₀₀ value was compared with acceptable clinical threshold (∆E₀₀  = 1.8) and an ideal threshold (∆E₀₀  = 0.8).

RESULTS

Type-thickness of ceramics and substructures were significantly affected ∆E₀₀ . Ts significantly influenced ∆E₀₀ more than Rs, Ms, Zs, and Ds. No significant ∆E₀₀ upon Ms and Zs was indicated. Bc completely masked Ts and Rs if thickness was ≥0.6 mm, and Zs, Ms, and Ds if ≥0.4 mm. Cc completely masked Ts and Rs if thickness was >1.6 mm, and Zs, Ms, and Ds if >1.4 mm. Vc completely masked Ts if thickness was >1.4 mm, Rs and Ms if >1.2 mm, and Zs and Ds if >1 mm.

CONCLUSION

Ts is the most influential substructure impact on color appearance, requiring a minimum thickness of 0.6 mm Bc, 1.2 mm Vc, or 1.6 mm Cc to be perfectly masked.

CLINICAL SIGNIFICANCE

Masking ability was significantly influenced by the type and thickness of ceramic material and underlying structure. Monolithic zirconia had a significantly higher masking ability than lithium silicate/phosphate glass ceramic. Ts indicated the most influencial substructure impacting the color appearance of ceramic restoration, which require a minimum thickness of 0.6 mm Bc, 1.2 mm Vc, or 1.6 mm Cc to achieve ideal masking capability.