Current status of zirconia restoration

An introduction of biological performance of zirconia with different surface characteristics: A review

Zirconia (ZrO₂) ceramic is widely used in dentistry as a clinical dental biomaterial. In this review, we are focusing on and summarizing the biological performance of zirconia under different surface characteristics. We have included an initial tissue cell attachment study on zirconia and bacterial adhesion on zirconia. Our results suggest that surface modifications applied on zirconia may change the interfacial surface characteristics e.g. surface roughness, surface free energy, and chemistry of zirconia. The modifications also result in advanced biological performance of zirconia, including enhanced tissue cell attachment and reduction of bacterial adhesion. The recent laboratory research has provided many interesting modification methods and showed clinically interesting and promising outcomes. A few of the outcomes are validated and have been applied in clinical dentistry.

Impact of high-speed sintering, layer thickness and artificial aging on the fracture load and two-body wear of zirconia crowns

OBJECTIVE

To investigate the impact of high-speed sintering, layer thickness and artificial aging in a chewing simulator on the fracture load (FL) and two-body wear (2BW) of 4Y-TZP crowns.

METHODS

4Y-TZP crowns (Ceramill Zolid HT+, Amann Girrbach AG) in three different layer thicknesses (0.5, 1.0, 1.5; N=192, n=64/group) were manufactured using CAD/CAM technology and sintered at 1580°C (high-speed sintering) or 1450°C (control group). Specimens were polished in two-steps and bonded to standardized CoCr abutments with Multilink Automix (Ivoclar Vivadent). 2BW after 6000 thermo- and 1,200,000 chewing-cycles employing enamel antagonists was determined using best fit machining. FL was tested before and after artificial aging. Univariate ANOVAs, post hoc Scheffé, unpaired t-, Kruskal-Wallis- and Mann-Whitney-U-test were computed (p<0.05).

RESULTS

High-speed sintering resulted in less 2BW of the zirconia than the control group (p=0.013). High-speed sintering (p=0.001-0.006) and an increase in layer thickness (p<0.001-0.012) resulted in higher FL values, while artificial aging led to a reduction of FL (p<0.001).

SIGNIFICANCE

As high-speed sintering resulted in less two-body wear of the zirconia and comparable or even higher fracture load results than the control group, this cost- and time efficient alternative presents promising mechanical results.

Translucency, flexural strength, fracture toughness, fracture load of 3-unit FDPs, Martens hardness parameter and grain size of 3Y-TZP materials

OBJECTIVE

This investigation tested pre-shaded 3Y-TZP materials on optical, mechanical and structural properties and calculated correlations between these properties.

METHODS

Seven A2-shaded 3Y-TZP zirconia materials were investigated on translucency (T) via UV-vis-spectrophotometer, fracture load of 3-unit FDPs (FL), biaxial flexural strength (FS), Chevron-Notch Beam (CNB), fracture toughness (K_IC) and Martens parameter (hardness: HM and indentation modulus: E_IT). FL, FS and K_IC were measured in a universal testing machine. The grain size was evaluated by scanning electron microscopy (SEM). Data was analyzed using one-way ANOVA followed by post hoc Scheffé, Kruskal-Wallis-, Mann-Whitney-U- and Pearson-test (p<0.05).

RESULTS

For translucency, negative correlations were found with results of facture load (R=-0.444, p<0.001) and K_IC (R=-0.503, p<0.001). While a positive correlation was found between translucency and flexural strength (R=0.238, p=0.019), between fracture load and E_IT (R=0.227, p<0.029), between fracture load and K_IC (R=0.362, p<0.001) as well as between fracture load and the grain size (R=0.598, p=0.007). While the grain size positively correlated with E_IT (R=0.534, p=0.017) as well as E_IT with HM (R=0.720, p<0.001).

SIGNIFICANCE

Despite of being based on the same raw material, tested zirconia materials significantly differed regarding optical, mechanical (except biaxial flexural strength and Martens hardness) and structural properties. Materials with highest optical properties were those with lowest mechanical properties (CER, COP).

Effect of ceramic material type on the fracture load of inlay-retained and full-coverage fixed dental prostheses

Objective: Ceramic inlay-retained fixed partial denture (IRFPD) is a conservative prosthetic option but the mechanical durability of new high strength zirconia reinforced glass ceramic FPDs is not investigated. The purpose of this study was to compare fracture load of 3-unit ceramic FPDs. Materials and methods: Extracted premolars and molars (N = 64) were used to create three test groups (IRFPDs) and one control group (full coverage FPD) (n = 8). The teeth were embedded in PMMA resin with a mesiodistal distance of 6 mm. Premolars had a distal and molars had a mesial inlay preparation (width: 3 mm; height: 4 mm) in the test groups. IRFPDs were made from a zirconia reinforced lithium silicate (VS) or a monolithic zirconia. Zirconia IRFPDs received 2 types of surface treatments: sandblasting (Zr-IRFPD) or internal coating with feldspathic porcelain (ZrC-IRFPD). Control group was made from monolithic zirconia with the same connector size and zirconia surfaces were sandblasted (Zr-FPD). All restorations were cemented using a resin luting cement. After 5000 thermo-cycles, fracture load values (N) were determined with a universal testing machine at a crosshead speed of 0.75 mm/min. Data were analyzed using 1-way ANOVA and Tukey`s post hoc test (p ˂ .05). Result: Fracture load (mean ± SD) of Zr-FPDs, Zr-IRFPDs and ZrC-IRFPDs were 672 ± 183, 672 ± 123 and 638 ± 59, respectively, being not statistically different (p > .05). VS-IRFPD exhibited statically lower values (391 ± 136). The predominant mode of failure was fracture at the connector area in all groups. Conclusion: The fracture load of 3-unit IRFPD was significantly affected by types of ceramics but the retainer design and surface treatment in Zr groups did not show a significant effect.

Dentistry 4.0 Concept in the Design and Manufacturing of Prosthetic Dental Restorations

The paper is a comprehensive but compact review of the literature on the state of illnesses of the human stomatognathic system, related consequences in the form of dental deficiencies, and the resulting need for prosthetic treatment. Types of prosthetic restorations, including implants, as well as new classes of implantable devices called implant-scaffolds with a porous part integrated with a solid core, as well as biological engineering materials with the use of living cells, have been characterized. A review of works on current trends in the technical development of dental prosthetics aiding, called Dentistry 4.0, analogous to the concept of the highest stage of Industry 4.0 of the industrial revolution, has been presented. Authors’ own augmented holistic model of Industry 4.0 has been developed and presented. The studies on the significance of cone-beam computed tomography (CBCT) in planning prosthetic treatment, as well as in the design and manufacture of prosthetic restorations, have been described. The presented and fully digital approach is a radical turnaround in both clinical procedures and the technologies of implant preparation using computer-aided design and manufacturing methods (CAD/CAM) and additive manufacturing (AM) technologies, including selective laser sintering (SLS). The authors’ research illustrates the practical application of the Dentistry 4.0 approach for several types of prosthetic restorations. The development process of the modern approach is being observed all over the world. The use of the principles of the augmented holistic model of Industry 4.0 in advanced dental engineering indicates a change in the traditional relationship between a dentist and a dental engineer. The overall conclusion demonstrates that it is inevitable and extremely beneficial to implement the idea of Dentistry 4.0 following the assumptions of the authors’ own, holistic Industry 4.0 model.

Comparing the Repair of Veneered Zirconia Crowns with Ceramic or Composite Resin: An in Vitro Study

STATEMENT OF PROBLEM

Current techniques for repairing porcelain-chipped restorations have several limitations. With advances in CAD/CAM technology, the combination of resin cements and high-strength ceramic materials might offer new options for repairing the chipping of veneering ceramic.

PURPOSE

The purpose of this study is to compare the load-to-failure of veneered zirconia crowns repaired by different materials.

MATERIAL AND METHODS

Veneered zirconia crowns were made on aluminum dies (n = 10/group). Feldspathic porcelain (Vita VM9, Vident) was applied to the zirconia coping (Vita In-Ceram YZ, Vident) in a cylindrical shape (Ø 10.5 mm, height 7.5 mm). A bevel cut on the porcelain veneer (45 degree, 3 mm width) was made at one side of each crown to simulate porcelain chipping. The crowns were then divided into four different groups according to the repair materials: 1. Conventional resin composite (A; Tetric EvoCeram, Ivoclar Vivadent); 2. Flowable resin composite (B; G-aenial Universal Flo, GC america); 3. CAD/CAM milled feldspathic ceramic (C; Vita Trilux Forte, Vident); 4. CAD/CAM milled lithium disilicate glass-ceramic (D; IPS e.max CAD, Ivoclar Vivadent). Resin cement (Multilink Automix, Ivoclar Vivadent) was used to cement the CAD/CAM ceramic materials to the beveled crowns. Each crown underwent 5000 cycles of thermocycling. The strength test was performed on an Instron universal testing machine by loading force on the center of repaired part to record load-to-failure. Data were analyzed by ANOVA and Tukey HSD post-hoc tests (α = 0.05).

RESULTS

Mean loads-to-failure (in Newton +/- SD) of repaired veneered zirconia crowns were: Gr. A: 660.0 ± 200.5; Gr. B: 681.7 ± 175.9; Gr. C: 1236.0 ± 188.8; Gr. D: 1536.3 ± 286.1. Catastrophic failure was the most dominant failure mode in every group. Few specimens exhibited cohesive failure. Only one specimen in group D had adhesive failure.

CONCLUSIONS

Within the limitation of the study, veneered zirconia crowns repaired with CAD/CAM ceramic materials have significantly higher load-to-failure than veneered crowns repaired with resin composite (p ≤ 0.05).

CLINICAL IMPLICATIONS

Traditionally, porcelain-chipped restorations are often repaired with resin composite and bonding technique. Repairing chipped porcelain with CAD/CAM ceramics fitting the fractured parts can be alternative option with potential advantages. More well-designed studies are necessary to justify this novel repair technique.

Fracture Resistance of Zirconia-Reinforced Lithium Silicate Ceramic Crowns Cemented with Conventional or Adhesive Systems: An In Vitro Study

In recent years, Zirconia-reinforced Lithium Silicate ceramic (ZLS), combining lithium-silicate and zirconia features, has shown to have excellent mechanical and aesthetic characteristics. Thus, the aim of this study was to compare the fracture strength of ZLS single crowns cemented with two different cementation techniques. Twenty crowns were realised and cemented on teeth replicas achieved from an extracted premolar human tooth. The samples were divided into two groups of 10 specimens each, Glass-ionomeric cement (GIC) group and Self-Adhesive Resin Cement (ARC) group. The mechanical test was performed using a universal testing machine. The specimens were then evaluated with a scanning electron microscope (SEM) to identify for all crowns and related abutments the pattern of fracture after the breaking point. The data obtained were statistically analysed. The mean fracture toughness values and standard deviations (±SD) were 2227 ± 382 N and 3712 ± 319 N respectively for GIC and ARC groups. In fact, t-test showed a statistically significant difference between the two groups (p < 0.001). Moreover, the SEM results demonstrated portions of abutments still attached to the crown fragments in the ARC group, whilst these were not present in the GIC group. Within the limitations of this study, these results suggest the use of adhesive cementation for ZLS crowns, which significantly increase the compressive strength of ZLS restorations compared to GIC.

Laser-Milled Microslits Improve the Bonding Strength of Acrylic Resin to Zirconia Ceramics

Heightened aesthetic considerations in modern dentistry have generated increased interest in metal-free “zirconia-supported dentures.” The lifespan of the denture is largely determined by the strength of adhesion between zirconia and the acrylic resin. Thus, the effect on shear bond strength (SBS) was investigated by using an acrylic resin on two types of zirconia ceramics with differently sized microslits. Micromechanical reticular retention was created on the zirconia surface as the novel treatment (microslits (MS)), and air-abrasion was used as the control (CON). All samples were primed prior to acrylic resin polymerization. After the resin was cured, the SBS was tested. The obtained data were analyzed by using multivariate analysis of variance(α = 0.05). After the SBS test, the interface failure modes were observed by scanning electron microscopy. The MS exhibited significantly higher bond strength after thermal cycles (p < 0.05) than the CON. Nevertheless, statistically comparisons resulted in no significant effect of the differently sized microslits on SBS (p > 0.05). Additionally, MS (before thermal cycles: 34.8 ± 3.6 to 35.7 ± 4.0 MPa; after thermal cycles: 26.9 ± 3.1 to 32.6 ± 3.3 MPa) demonstrated greater SBS and bonding durability than that of CON (before thermal cycles: 17.3 ± 4.7 to 17.9 ± 5.8 MPa; after thermal cycles: 1.0 ± 0.3 to 1.7 ± 1.1 MPa), confirming that the micromechanical retention with laser-milled microslits was effective at enhancing the bonding strength and durability of the acrylic resin and zirconia. Polycrystalline zirconia-based ceramics are a newly accessible material for improving removable prosthodontic treatment, as the bond strength with acrylic resin can be greatly enhanced by laser milling.

Toughening of highly translucent zirconia by monoclinic ZrO2 and SiO2 particle coating

The mechanical properties of highly translucent partially stabilized zirconia (PSZ) need to be improved; however, improvement of mechanical properties often decreases translucency. To overcome this problem, a monoclinic ZrO₂ (mZrO₂)/SiO₂ dispersion was prepared and applied as a coating material for PSZ. The influence of surface treatment by the mZrO₂/SiO₂ dispersion on the surface topography, crystallography, and mechanical properties of highly translucent PSZ was investigated in this study. Following the treatment, the mechanical strength of highly translucent PSZ improved by 170% compared with control, for the best mZrO₂/SiO₂ dispersion ratio and heating temperature condition, while maintaining its translucency. The proposed coating is promising for improving the mechanical properties of highly translucent PSZ.

Tissue Engineering and Regenerative Medicine: Achievements, Future, and Sustainability in Asia

Exploring innovative solutions to improve the healthcare of the aging and diseased population continues to be a global challenge. Among a number of strategies toward this goal, tissue engineering and regenerative medicine (TERM) has gradually evolved into a promising approach to meet future needs of patients. TERM has recently received increasing attention in Asia, as evidenced by the markedly increased number of researchers, publications, clinical trials, and translational products. This review aims to give a brief overview of TERM development in Asia over the last decade by highlighting some of the important advances in this field and featuring major achievements of representative research groups. The development of novel biomaterials and enabling technologies, identification of new cell sources, and applications of TERM in various tissues are briefly introduced. Finally, the achievement of TERM in Asia, including important publications, representative discoveries, clinical trials, and examples of commercial products will be introduced. Discussion on current limitations and future directions in this hot topic will also be provided.

The effect of various mechanical and chemical surface conditioning on the bonding of orthodontic brackets to all ceramic materials

Background/purpose

Increasing the bond strength between the orthodontic brackets and all-ceramic materials is one of the challenges facing orthodontists. The purpose of this study is to assess the shear bond strength (SBS) of metal brackets to two types of all ceramic materials using various surface mechanical and chemical conditioning methods.

Materials and methods

Sixty ceramic blocks were prepared using two types of all ceramic materials (IPS e.max and VITA Suprinity® PC) and treated with 3 surface treatments; surface etching with 9.6% hydrofluoric acid (HFA) for 2 mins; surface roughening with Sof-Lex finishing discs; and surface roughening with Sof-Lex finishing discs and etching with HFA. Metal brackets were attached to the surface of the ceramic blocks using light cure orthodontic adhesive. Samples were subjected to 2000 thermo-cycles (5-50 °C) and the SBS was assessed using Instron machine. The adhesive remnant index (ARI) was evaluated under light microscope. Descriptive and group comparison were calculated using Two-way ANOVA, Post-hoc Tukey's and Chi-square tests and significance level set at (P < 0.05).Results: surface roughening of both ceramic materials with Sof-Lex discs and HFA resulted in a significant increase in SBS compared to other experimental groups (P < 0.05). However, VITA Suprinity ceramic prepared with Sof-Lex discs only showed the lowest SBS. The distribution of the ARI scores was significantly different between the groups (P < 0.05).

Conclusion

Surface preparation of all ceramic materials with Sof-Lex discs and hydrofluoric acid combination produces the highest SBS to metallic orthodontic brackets.

Mechanical behavior of endocrowns fabricated with different CAD-CAM ceramic systems

STATEMENT OF PROBLEM

The mechanical behavior of ceramic endocrowns is unclear.

PURPOSE

The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic endocrowns made by using different computer-aided design and computer-aided manufacturing (CAD-CAM) systems.

MATERIAL AND METHODS

Sixty mandibular human molars were endodontically treated, prepared for endocrowns, and divided into 4 groups (n=15) according to the following various ceramic systems: leucite-based glass-ceramic (LC group), lithium disilicate-based glass-ceramic (LD group), glass-ceramic based on zirconia-reinforced lithium silicate (LSZ group), and monolithic zirconia (ZR group). After adhesive bonding, the specimens were subjected to thermomechanical loading and then to fracture resistance testing in a universal testing machine. The failure mode of the specimens was qualitatively evaluated. Three-dimensional FEA was performed to evaluate the stress distribution in each group. Data were analyzed by using a 1-way ANOVA and the Tukey HSD test (α=.05).

RESULTS

Statistically significant differences among the groups were observed (P<.05). The outcomes of the LC, LD, and LSZ groups were similar (1178 N, 1935 N, and 1859 N) but different from those of the ZR group (6333 N). The LC and LD groups had a higher ratio of restorable failures, while LSZ and ZR had more nonrestorable failures. Fractographic analysis indicated a regular failure pattern in the ZR group and irregular failure patterns in the other groups. Three-dimensional FEA revealed similar values and stress pattern distributions among the groups.

CONCLUSIONS

The mechanical performance of monolithic zirconia was better than that of the other ceramic endocrowns considered in this research; however, monolithic zirconia presented a higher rate of catastrophic tooth structure failure.

Enhanced mechanical properties of ZrO2-Al2O3 dental ceramic composites by altering Al2O3 form

OBJECTIVES

This study evaluates the difference in physical and mechanical properties of ZrO₂ ceramics, commonly used in dental applications, altered by three different forms of Al₂O₃ content; microparticles (m), nanoparticles (n), and microfiber (f).

METHODS

Three different types of ZrO₂-Al₂O₃ composites were formed using microparticle (m), nanoparticle (n), or microfibre (f) forms of Al₂O₃. The physical and mechanical properties such as sintering shrinkage, relative density, Vickers hardness, fracture toughness, and biaxial strength were evaluated. A Weibull analysis was performed to assess the strength reliability of the specimens. All data were calculated using the t-test and ANOVA.

RESULTS

The sintering shrinkage and relative density of all ceramic composite groups were decreased with the addition of Al₂O₃. The mechanical properties of ZrO₂-Al₂O₃ (f) composite were higher than that of ZrO₂-Al₂O₃ (m) composite and ZrO₂-Al₂O₃ (n) composite. The maximum hardness, fracture toughness, and biaxial flexural strength were observed for 10 vol% of Al₂O₃ fibre. When the content of Al₂O₃ fibre in the matrix was increased above 20 vol%, agglomeration occurred and resulted in a decrease of hardness and toughness. The Weibull modulus value of the ZrO₂-Al₂O₃ (f) composite was the lowest compared to that of other groups. However, characteristic strength (σ₀) of ZrO₂-Al₂O₃ (f) the highest value.

SIGNIFICANCE

The current study demonstrated that the addition of right amount of Al₂O₃ microfibre into the ZrO₂ matrix enhanced the mechanical properties of ZrO₂-Al₂O₃ (f) composite, which would be favourable for dental applications.

Evaluation of Stress Distribution in Tooth-Supported Fixed Dental Prostheses Made of Translucent Zirconia with Variations in Framework Designs: A Three-Dimensional Finite Element Analysis

PURPOSE

To evaluate the influence of the framework designs on the stress distribution within tooth-supported partially veneered fixed dental prostheses (FDPs) made of translucent zirconia under simulated loads using a three-dimensional finite element analysis (3D-FEA).

MATERIAL AND METHODS

For a linear 3D-FEA, simplified 3D solid models of prepared abutment teeth (first premolar and first molar) with different 3-unit FDPs were created. The models with different FDP designs-monolithic zirconia (control); semi-monolithic zirconia with 0.3 mm veneer thickness (SM0.3); semi-monolithic zirconia with 0.5 mm veneer thickness (SM0.5); semi-monolithic zirconia with 0.5 mm veneer thickness supported with cap design (SMC), and semi-monolithic zirconia with 0.5 mm veneer thickness supported with wave design (SMW)-were analyzed using 3D-FEA. The elastic properties of the components (bone, dentine, cement, translucent zirconia, and veneering porcelain) were obtained from the published data for FEA. Simulated static loading forces (300 N) were applied at 10° oblique direction over six points in the occlusal surfaces of the FDPs. Maximum principal stress, shear stress, and safety factor were calculated and analyzed among the different models.

RESULTS

Semi-monolithic with cap design showed the smallest maximum principal stress levels in the veneering porcelain compared to all other models (SM0.3, SM0.5, SMW). The SM0.3 had lower maximum principal stress levels in the veneering porcelain compared to SM0.5. Regarding stresses in the zirconia framework, all models had comparable results in maximum principal tensile stresses, except SMW had a lower value. Maximum principal stress levels were located in the veneer component of SM0.3, SM0.5, and SMW, whereas, such levels were observed in the cervical areas of the zirconia frameworks of SMC and control. The SM0.3 had the highest maximum shear stress levels at the zirconia-veneer interface, while SMW had the lowest shear values. The 3D-FEA models with different FDP designs showed different minimum safety factor levels.

CONCLUSIONS

Framework and veneer designs play a significant role in the stress distribution of the partially veneered zirconia FDPs under loading. The FDPs with zirconia frameworks with cap design minimize the maximum principal tensile stress in the veneering porcelain. The FDPs with 0.3-mm-veneering porcelain show low maximum principal tensile stress in the veneering porcelain, but highest maximum shear stress at the zirconia-veneer interface. The FDPs with wave design of zirconia frameworks minimize the maximum shear stress considerably.

Monolithic Zirconia FPD on Modified Titanium Bonding Bases in Limited Interocclusal Distance: A Case Report

A 62-year-old male patient sought treatment for missing maxillary teeth. A diagnostic cast demonstrated that the interocclusal distance was insufficient. A 5-unit screw-retained implant-supported fixed partial denture (FPD) was used to restore missing maxillary teeth. The restoration of multiple missing teeth using an implant-supported FPD is challenging when the interocclusal distance is limited due to lack of retention and inadequate esthetics. In this case, a hexagonal, screw-retained, and subgingivally located titanium-based zirconia implant-supported FPD with a conical abutment base was used for restoration to overcome the limited interocclusal distance. This implant-supported FPD, consisting of CAD/CAM-designed monolithic zirconia cemented to a titanium bonding base in the laboratory, is expected to facilitate predictable retention and adequate esthetics as well as provide ease of retrieval.

Effect of thermal and mechanical cycles on shear bond strength of zirconia core to porcelain veneer under different surface treatments

Background. Due to the fragile nature of all-ceramic restorations, it is necessary to provide an appropriate (core) infrastructure to support the veneering porcelain. The veneer detachment and chipping are disadvantages of these restorations. Several techniques have been proposed to minimize these problems. This study evaluated the effect of thermal and mechanical cycles on the shear bond strength of zirconia core to porcelain veneer under different surface treatments.

Methods. Sixty disk-like zirconium samples were randomly divided into three groups. The first group was polished and veneered with porcelain, without additional surface treatments. The two other groups were subjected to different surface treatments (modified aluminum oxide by silica and activator‒aluminum oxide and primer) and veneering with porcelain. Half of the samples in each group were subjected to 6000 thermal cycles and 20,000 masticatory cycles of 50 N to imitate the intraoral conditions; the other half were placed in distilled water at 37°C until the shear strength test. Each sample was then buried using PMMA in a mounting jig so that the gap between the core and the veneer could be placed upward. Then, they were exposed to shear stress using a universal testing machine at a rate of 1 mm/min until fracture. The maximum force leading to the fracture was recorded.

Results. Comparison of the groups showed that the highest shear bond strength was related to the samples treated with aluminum oxide and primer, without applying thermal and masticatory cycles, which indicated no significant difference from the group treated with aluminum oxide and primer, with thermal and masticatory cycles. The lowest shear bond strengths were related to the polished samples without surface treatment by applying thermal and masticatory cycles (P=0.001), which indicated no significant difference from the untreated group without thermal and masticatory cycles.

Conclusion. Based on the results, treatment with aluminum oxide and primer increased the shear bond strength of zirconia core to porcelain veneer. Thermocycling and masticatory cycles failed to reduce the shear bond strength in all the three groups significantly

Surrounding Tissue Response to Surface-Treated Zirconia Implants

Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), which are partially stabilized zirconia, have been used for fabricating dental implants. This study investigated the soft tissue attachment, the collagen fiber orientation to zirconia at different surface conditions, and the bone response using implantation experiments in animals. The zirconia implant surfaces were treated with ultraviolet irradiation (UV), a combination of large-grit sandblasting and hydrofluoric acid etching (blastedHF), and a combination of blastedHF and UV (blastedHF+UV). The surface treated with blastedHF and blastedHF+UV appeared rough and hydrophilic. The surface treated with blastedHF+UV appeared to be superhydrophilic. Subsequently, tapered cylindrical zirconia implants were placed in the alveolar sockets of the maxillary molars of rats. The bone-to-implant contact ratio of blastedHF and blastedHF+UV implants was significantly higher than that of the non-treated controls and UV-treated implants. The four different surface-treated zirconia implants demonstrated tight soft tissue attachments. Perpendicularly oriented collagen fibers towards zirconia implants were more prominent in blastedHF and blastedHF+UV implants compared to the controls and UV-treated implants. The area of the soft tissue attachment was the greatest with the perpendicularly oriented collagen fibers of blastedHF+UV-treated implants. In conclusion, blastedHF+UV treatment could be beneficial for ensuring greater soft-tissue attachment for zirconia implants.

Influence of CAD/CAM Fabrication and Sintering Procedures on the Fracture Load of Full-Contour Monolithic Zirconia Crowns as a Function of Material Thickness

OBJECTIVE

The purpose of this in vitro study was to analyze the effect of computer-aided design/computer-aided manufacturing (CAD/CAM) fabrication and sintering procedures on the fracture load of monolithic zirconia crowns with different material thicknesses adhesively seated to methacrylate dies fabricated with stereolithography technology.

METHOD

Monolithic zirconia crowns were fabricated from inCoris TZI C material with a chairside CAD/CAM system (CEREC MCXL) comprising three material thicknesses (0.5/1.0/1.5 mm, n=8 each). Two CAD/CAM fabrication procedures (milling, MI; grinding, GR), two chairside sintering procedures (superspeed, SS; speedfire sintering, SF), and one labside sintering procedure (classic, CL) were evaluated. In total, 144 crowns were fabricated. Restorations were adhesively seated to methacrylate dies fabricated with SLA technology. Thermomechanical cycling (TCML) was performed before fracture testing. Loading forces until fracture were registered and statistically analyzed with one-way analysis of variance (ANOVA), post hoc Scheffé test, and three-way ANOVA (α=0.05).

RESULTS

Test groups showed statistically significant differences (p<0.05). The highest mean value was found for 1.5-mm crowns of group GR_SF with 3678.6 ± 363.9 N. The lowest mean value was found for group 0.5-mm crowns of group MI_SF with 382.4 ± 30.7 N. There was a significant three-way interaction effect between thickness, sintering, and processing [F(4,126)=9.542; p<0.001; three-way ANOVA, significance level α=0.05].

CONCLUSIONS

CAD/CAM fabrication and sintering procedures influence the maximum loading force of monolithic zirconia crowns with different material thicknesses. A material thickness of 0.5 mm should be considered as a critical thickness for monolithic zirconia crown restorations.

High-translucent yttria-stabilized zirconia ceramics are wear-resistant and antagonist-friendly

OBJECTIVES

To evaluate two-body wear of three zirconia ceramics stabilized with 3, 4 and 5mol% yttria and to compare their wear behavior with that of a lithium-disilicate glass-ceramic.

METHODS

Sixteen rectangular-shaped specimens made from three grades of zirconia ceramics and a lithium-disilicate glass-ceramic were polished and dynamically loaded in a chewing simulator (2kg vertical load, 2.1Hz) under water at 90°C for 1.2×10⁶ cycles (about 7 days) in the ball-on-plate mode against steatite antagonists. Surface roughness was measured before and after wear testing. Wear tracks were scanned with a non-contact 3D profilometer and super-impositions were used to determine wear loss of the antagonists. Wear surfaces were imaged by SEM. XRD and micro-Raman spectroscopy were used to characterize phase transformation and stress status in the worn and unworn areas of the zirconia ceramics.

RESULTS

Independent of fracture toughness, strength and aging-susceptibility, the three zirconia ceramics showed a similar and limited amount of wear (∼10μm in depth) and were more wear-resistant than the lithium-disilicate glass-ceramic (∼880μm in depth). Abrasive wear without obvious cracks was observed for all investigated zirconias, whereas the glass-ceramic with a lower fatigue threshold and high susceptibility to surface dissolution exhibited significant abrasion, fatigue and corrosion wear. All three zirconia ceramics yielded a lower antagonist wear than the glass-ceramic and no significant differences were found between the zirconia ceramics.

SIGNIFICANCE

In the context of this study, high-translucent zirconia ceramics stabilized with a higher yttria content, recently introduced in the dental field, were as wear-resistant and antagonist-friendly as conventional high-strength zirconia and suitable for monolithic restorations.

Choice of resin cement shades for a high-translucency zirconia product to mask dark, discolored or metal substrates

PURPOSE

The aim was to study the masking ability of high-translucency monolithic zirconia and provide guidance in selecting resin luting cements in order to mask discolored substrates.

MATERIALS AND METHODS

160 high-translucency zirconia specimens were divided into 32 groups depending on their thickness and shades. Using five shades of try-in paste, the specimens were luted onto the sub strates (Co-Cr, precious-metal, opaque porcelain-sintered Co-Cr, opaque porcelain-sintered precious-metal, and 5M3-shade zirconia). All CIELAB color parameters were measured and statistically analyzed.

RESULTS

Zirconia shade and thickness and try-in paste shade affected CIELAB color parameters (P=.000) in different substrates groups, and there were interactions among these factors (P=.000). All five try-in paste shades can be chosen to achieve ΔE values of zirconia with 1.2 - 1.5 mm for masking dark-tooth-like 5M3-shade and zirconia with 1.5 mm for masking precious-metal groups < 2.6. Only suitable try-in paste shades were used, can ΔE values that less than 2.6 be achieved when applied translucent monolithic zirconia with 0.7-1.0 mm for masking dark-tooth-like 5M3-shade and zirconia with 0.7 - 1.2 mm for masking precious-metal groups.

CONCLUSION

Choosing suitable resin cement shades is necessary for high-translucency monolithic zirconia to achieve ideal masking ability (ΔE < 2.6) on the dark-tooth.

A comparative evaluation of the effect of liners on the shear bond strength of veneered zirconia block: An in vitro study

Aim:

This study aims to evaluate the effect of lithium disilicate glass–ceramic liner, silicon dioxide based liner, and glass–ceramic interlayer on the shear bond strength (SBS) of a commercially available veneered zirconia block and to study fractographic behavior of the samples using universal testing machine, scanning electron microscope (SEM).

Setting and Design:

In vivo – comparative study.

Materials and Methods:

60 samples were fabricated from VITA (vita zahnfabrik. Bad sackingen, Germany) zirconia discs. Samples were divided into 4 groups with 15 samples each. First is the control group, second is lithium disilicate glass–ceramic liner group, third is silicon dioxide based liner, and fourth is glass–ceramic interlayer group. SBS of samples was recorded using universal testing machine. Samples were further analyzed for fractographic behavior using SEM.

Statistical Analysis Used:

One Way ANOVA test, and Chi-Square test.

Results:

The intergroup comparison of mean SBS (Mpa) was done using the post hoc Bonferroni test. The mean SBS (Mpa) was significantly more among lithium disilicate and glass–ceramic interlayer groups in comparison to silicon dioxide-based liner group. Fractographic analysis was done using the Chi-square test.

Conclusion:

It was concluded that maximum SBS was obtained for lithium disilicate liner. Maximum adhesive failures were found with lithium disilicate liner, and silicon dioxide-based liner group showed cohesive failures.

"CAD-on" Interfaces - Fracture Mechanics Characterization

PURPOSE

To apply fracture mechanics methodology to determine the interfacial fracture toughness of the interfaces present in "CAD-on" crowns consisting of CAD/CAM milled lithium disilicate veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal framework.

MATERIALS AND METHODS

The notchless triangular prism specimen fracture toughness test was used to determine interfacial fracture toughness. Four groups, each consisting of (6 × 6 × 6 × 12) mm prisms (n = 22), were produced. Half-size [(6 × 6 × 6 × 6) mm] specimens of IPS e.max CAD and IPS e.max ZirCAD were approximated under vibration with Crystal Connect fusing glass and sintered according to manufacturer's guidelines to obtain the following three interfaces: (1) e.max CAD/Crystal Connect/e.max CAD (Group I); (2) Zir CAD/Crystal Connect/Zir CAD (Group II); and (3) Zir CAD/Crystal Connect/e.max CAD (Group III). For Group IV (control, based on the "press-on" veneering technique), half-size [(6 × 6 × 6 × 6) mm] IPS e.max ZirCAD prisms were coated with ZirLiner and pressed with IPS e.max ZirPress ingots to obtain (6 × 6 × 6 × 12) mm prisms. All specimens were tested using a computer controlled material testing machine. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α = 0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured surfaces were characterized under a scanning electron microscope.

RESULTS

All experimental groups demonstrated a cohesive mode of failure in the fusing glass layer. The number and size of defects appeared to correlate with the variability of fracture toughness values. There were no significant differences between the fracture toughness of the "CAD-on" interfaces (p = 0.052). The results suggested that the fracture toughness of Crystal Connect limited the interfacial fracture toughness values. The "CAD-on" fracture toughness value (Group III) was significantly greater than that of the ZirPress "press-on" control (Group IV) (p < 0.001).

CONCLUSION

The "CAD-on" process results in stronger bonding between veneer and framework, compared to conventional veneering. The clinical use of "CAD-on" crowns could therefore be advocated. The selection of any restorative material requires a thorough analysis of advantages, limitations and results from clinical studies to inform the clinical decision in a case-by-case approach.

Biocompatibility of Polymer and Ceramic CAD/CAM Materials with Human Gingival Fibroblasts (HGFs)

Four polymer and ceramic computer-aided design/computer-aided manufacturing (CAD/CAM) materials from different manufacturers (VITA CAD-Temp (polymethyl methacrylate, PMMA), Celtra Duo (zirconia-reinforced lithium silicate ceramic, ZLS), IPS e.max CAD (lithium disilicate (LS₂)), and VITA YZ (yttrium-tetragonal zirconia polycrystal, Y-TZP)) were tested to evaluate the cytotoxic effects and collagen type I secretions on human gingival fibroblasts (HGFs). A total of 160 disc-shaped samples (Ø: 10 ± 2 mm; h: 2 mm) were milled from commercial blanks and blocks. Direct-contact cytotoxicity assays were evaluated at 24, 48, and 72 h, and collagen type I (COL1) secretions were analysed by cell-based ELISA at 24 and 72 h. Both experiments revealed statistically significant differences (p < 0.05). At 24 and 48 h of contact, cytotoxic potential was observed for all materials. Later, at 72 h, all groups reached biologically acceptable levels. LS₂ showed the best results regarding cell viability and collagen secretion in all of the time evaluations, while Y-TZP and ZLS revealed intermediate results, and PMMA exhibited the lowest values in both experiments. At 72 h, all groups showed sharp decreases in COL1 secretion regarding the 24-h values. According to the results obtained and the limitations of the present in vitro study, it may be concluded that the ceramic materials revealed a better cell response than the polymers. Nevertheless, further studies are needed to consolidate these findings and thus extrapolate the results into clinical practice.

Comparison of Shear Strength of Metal and Ceramic Orthodontic Brackets Cemented to Zirconia Depending on Surface Treatment: An In Vitro Study

Objectives Due to the high demand for all-ceramic restorations, monolithic zirconia restorations are nowadays frequently used. With the demand for adult orthodontic treatments, orthodontists need to be mindful of the quality of their brackets bonding to this type of material, as it requires special conditioning. This study aimed to compare different surface treatments of zirconia when bonding metal or ceramic orthodontic brackets. The objectives are to compare the shear bond strength; the amount of adhesive remaining on the surface of the material; the incidence of adhesive, cohesive, and mixed failures; and the occurrence of zirconia fractures.

Materials and Methods Forty monolithic blocks of zirconia of a diameter of 10 mm and a length of 10 mm were prepared and randomly divided into two groups ( n = 20): metallic or ceramic brackets. Each group was subsequently divided into two subgroups ( n = 10) depending on the surface preparation (laser treatment or airborne particle abrasion): SMB (airborne particle abrasion, metal bracket), SCB (airborne particle abrasion, ceramic bracket), LMB (laser; metal bracket), and LCB (laser, ceramic bracket). The samples were tested for shear bond strength using a universal testing machine. The adhesive remnant index and the occurrence of zirconia fractures and different types of failures were assessed by optical and electron microscopy.

Statistical Analysis Results were analyzed using analysis of variance.

Results The differences were significant between the metallic (SMB, LMB) and ceramic (SCB, LCB) bracket groups with regard to shear bond strength, with respectively 23.29 ± 5.34 MPa, 21.59 ± 4.03 MPa, 20.06 ± 4.05 MPa, and 17.55 ± 3.88 MPa. In terms of surface treatment, no statistical differences were found between the different groups.

Conclusion Metal brackets have a greater bond strength than ceramic brackets when cemented to zirconia. The surface treatment of zirconia surface has no influence on the shear bond strength.

Microstructural, physical, and optical characterization of high-translucency zirconia ceramics

STATEMENT OF PROBLEM

The microstructural, physical, and optical properties of newly available highly-translucent zirconia ceramics have not been thoroughly investigated.

PURPOSE

The purpose of this in vitro study was to evaluate the microstructural, physical, and optical properties of current zirconia ceramics with different degrees of translucency and to compare the optical properties with lithium disilicate glass-ceramic at different thicknesses.

MATERIAL AND METHODS

Disk-shaped specimens (N=108) were prepared from 3 different monolithic zirconia ceramics: VITA YZ HT (YZ-HT), VITA YZ ST (YZ-ST), and VITA YZ XT (YZ-XT) and lithium disilicate glass-ceramic (IPS e-max CAD LT [IPS]) with a diameter of 14 mm and 3 different thicknesses (0.5, 1.0, and 1.5 mm ±0.02 mm). The microstructural features (grain size, elemental composition, phase identifications, and quantifications), physical properties (Vickers hardness and fracture toughness), and optical properties (translucency parameter and contrast ratio) were investigated. Data were analyzed using 1-way ANOVA, followed by the post hoc Tukey tests or Kruskal-Wallis and Mann-Whitney U tests (α=.05).

RESULTS

Differences in grain size, phase assemblages, fracture toughness, translucency parameter, and contrast ratio were found to be significant (P<.05). The average grain size was different for the 3 zirconia specimens with the following order: YZ-XT>YZ-ST>YZ-HT. The fracture toughness of YZ-HT was higher than that of YZ-ST and YZ-XT (P<.05). IPS with 0.5-mm thickness had the highest TP followed by YZ-XT and YZ-ST. The CR values were in the range of 0.54 to 0.91 (YZ-HT), 0.43 to 0.61 (YZ-ST), 0.29 to 0.45 (YZ-XT), and 0.27 to 0.53 (IPS), and all decreased with an increase in thickness.

CONCLUSIONS

Based on the results of the present study, the microstructural, physical, and optical properties of zirconia ceramics were affected by the changes in chemical composition and sintering parameters.

Adhesion to Zirconia: A Systematic Review of Current Conditioning Methods and Bonding Materials

Background. Reliable bonding between resin composite cements and high strength ceramics is difficult to achieve because of their chemical inertness and lack of silica content that makes etching impossible. The purpose of this review is to classify and analyze the existing methods and materials suggested to improve the adhesion of zirconia to dental substrate by using composite resins, in order to explore current trends in surface conditioning methods with predictable results. Methods. The current literature, examining the bond strength of zirconia ceramics, and including in vitro studies, clinical studies, and a systematic review, was analyzed. The research in the literature was carried out using PubMed and Cochrane Library databases, only papers in English, published online from 2013 to 2018. The following keywords and their combinations were used: Zirconia, 3Y-TZP, Adhesion, Adhesive cementation, Bonding, Resin, Composite resin, Composite material, Dentin, Enamel. Results. Research, in PubMed and Cochrane Library databases, provided 390 titles with abstracts. From these, a total of 93 publications were chosen for analysis. After a full text evaluation, seven articles were discarded. Therefore, the final sample was 86, including in vitro, clinical studies, and one systematic review. Various adhesive techniques with different testing methods were examined. Conclusions. Airborne-particle abrasion and tribo-chemical silica coating are the pre-treatment methods with more evidence in the literature. Increased adhesion could be expected after physico-chemical conditioning of zirconia. Surface contamination has a negative effect on adhesion. There is no evidence to support a universal adhesion protocol.

Effect of etching with potassium hydrogen difluoride and ammonium hydrogen difluoride on bonding of a tri-n-butylborane initiated resin to zirconia

The purpose of the current study was to evaluate the effect of etching with potassium hydrogen difluoride (KHF₂) and ammonium hydrogen difluoride (NH₄HF₂) on the bond strength of a self-polymerizing methyl methacrylate resin (MMA-TBB) bonded to zirconia. Zirconia disks were prepared using the following surface treatment: no treatment, alumina blasting, and etching with KHF₂ or NH₄HF₂. The specimens were bonded with the MMA-TBB. The shear bond (Ø=5 mm) strength was measured. The surface free energies of the specimens were determined by measuring contact angles. The KHF₂ and NH₄HF₂ groups exhibited higher shear bond strength and surface free energy than did the alumina blasting and no treatment groups. Compared with alumina blasting, etching with KHF₂ and NH₄HF₂ exhibited superior bonding ability of mechanical retention to zirconia.

Fatigue survival and failure resistance of titanium versus zirconia implant abutments with various connection designs

STATEMENT OF PROBLEM

Data regarding the effect of connection design and abutment material on the fatigue survival and failure resistance of implant abutments are scarce.

PURPOSE

The purpose of this in vitro study was to investigate the effect of connection design and abutment material on the fatigue survival and failure resistance of implant abutment assemblies.

MATERIAL AND METHODS

Three types of implants (n=18, N=54) and 6 groups of abutments (n=9, N=54) with different connection designs-internal conical (IC), internal tri-channel (IT), and external hexagonal (EH)-and abutment materials-titanium (T) and zirconia (Z)-were investigated. All the abutments were restored with identical central incisor crowns. Fatigue testing, including thermal and mechanical aging, was performed in a mastication simulator (Esetron Smart Robotechnologies) for up to 1.2×10⁶ cycles with a load of 50 N at an angle of 45 degrees. Then, the surviving specimens were subjected to failure resistance testing in a universal testing machine (Shimadzu AG-IS; Shimadzu Corp) at a crosshead speed of 1.0 mm/min. The maximum loads to failure (N) were recorded. Survival performance of the specimens throughout the fatigue testing was examined by the Kaplan-Meier survival analysis. The failure loads were analyzed by using the Kruskal-Wallis test followed by the Mann-Whitney U tests with Bonferroni-Holm correction (α=.05).

RESULTS

All the specimens of groups ICT, ITT, ITZ, and EHT survived fatigue testing, whereas 2 specimens from group ICZ and 3 specimens from EHZ failed. Statistically significant differences were found among the groups, based on the results of maximum failure loads (P<.05). The highest mean failure load was obtained in the ICT group (1069 ±182 N), followed by the ITT (926 ±197 N), EHT (873 ±126 N), ITZ (568 ±81 N), EHZ (311 ±45 N), and ICZ (287 ±63 N) groups.

CONCLUSIONS

Abutment material and connection design affected the fatigue survival of implant abutment assemblies. Implant abutment assemblies with a titanium-titanium interface revealed higher failure resistance than the implant abutment assemblies with a titanium-zirconia interface.

Influence of different veneering techniques and thermal tempering on flexural strength of ceramic veneered yttria partially stabilized tetragonal zirconia polycrystalline restoration

Background

Different technique for ceramic veneering and thermal tempering process are expected to be a reason for alteration in strength of ceramic veneered zirconia. This study evaluates the effect of different veneering technique and varied thermal tempering process on flexural strength of ceramic veneered zirconia.

Material and Methods

Ceramic veneered zirconia bars (25 mm length, 4 mm width, 0.7&1.0mm of zirconia & ceramic thickness) were prepared from zirconia block (e.max® ZirCAD), sintered at 1500°C for 4 hours, and veneered with ceramics with different techniques including CAD-fused using e.max CAD® (C), Pressed-on using e.max® Zirpress (P), and layering using e.max® ceram (L), with different tempering process through fast (F), medium (M), and slow (L) cooling (n=15). The specimens were determined for flexural strength on a universal testing machine. ANOVA and Bonferroni's multiple comparisons were used to determine for significant difference (α=0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristics strength (σc). The interfaces were microscopically examined. The phase transformation of zirconia was determined using X ray diffraction.

Results

The mean±sd (MPa), m, σc of flexural strength were 922.06±83.45, 12.78, 958.32 for CF, 924.26±74.64, 14.28, 959.62 for CM, 930.25±92.42, 11.83, 970.83 for CS, 518.29±59.97, 10.11, 542.97 for PF, 516.50±67.51, 8.75, 539.17 for PM, and 520.51±42.38, 14.59, 544.51 for PS, 604.36±64.09, 11.28, 630.67 for LF, 583.81±56.95, 11.67, 609.81 for LM, 547.33±52.23, 12.19, 569.36 for LS. The flexural strength was significantly affected by veneering technique (p<0.05). No significant effect on flexural strength upon tempering process was evidenced (P >0.05). Phase transformation from t→m related with veneering and tempering procedure.

Conclusions

Strength of ceramic veneered zirconia associated with different veneering techniques, but not directly related with tempering process. CAD-on ceramic veneering zirconia is benefit for enhancing the strength of ceramic bilayer and was recommended as a method for ceramic veneering zirconia. Key words:CAD-CAM, cooling process, flexural strength, thermal tempering, zirconia.

Current status on lithium disilicate and zirconia: a narrative review

Background

The introduction of the new generation of particle-filled and high strength ceramics, hybrid composites and technopolymers in the last decade has offered an extensive palette of dental materials broadening the clinical indications in fixed prosthodontics, in the light of minimally invasive dentistry dictates. Moreover, last years have seen a dramatic increase in the patients’ demand for non-metallic materials, sometimes induced by metal-phobia or alleged allergies. Therefore, the attention of scientific research has been progressively focusing on such materials, particularly on lithium disilicate and zirconia, in order to shed light on properties, indications and limitations of the new protagonists of the prosthetic scene.

Methods

This article is aimed at providing a narrative review regarding the state-of-the-art in the field of these popular ceramic materials, as to their physical-chemical, mechanical and optical properties, as well as to the proper dental applications, by means of scientific literature analysis and with reference to the authors’ clinical experience.

Results

A huge amount of data, sometimes conflicting, is available today. Both in vitro and in vivo studies pointed out the outstanding peculiarities of lithium disilicate and zirconia: unparalleled optical and esthetic properties, together with high biocompatibility, high mechanical resistance, reduced thickness and favorable wear behavior have been increasingly orientating the clinicians’ choice toward such ceramics.

Conclusions

The noticeable properties and versatility make lithium disilicate and zirconia materials of choice for modern prosthetic dentistry, requiring high esthetic and mechanical performances combined with a minimal invasive approach, so that the utilization of such metal-free ceramics has become more and more widespread over time.

Comparative Effectiveness of Multiple Laser Scanning and Conventional Techniques on Zirconia Shear Bond Strength

This study aimed to compare the impact of different laser scanning with that of conventional methods on zirconia surface treatment through evaluation of shear bond strength (SBS) values. One hundred and thirty-two sintered zirconia cubic-samples were prepared and randomly divided into six study groups: milling control (without surface treatment); grinding; sandblasting; and three-times, four-times, and five-times laser scanning groups. The treatment process for the first three groups was performed before the zirconia coating, while the last three groups were treated after zirconia coating with veneer slurry through a spraying technique. In the current study, the surface roughness Ra, contact angle measurement, phase transformation, topography and interfaces, SBS in unaged and aged conditions, and fracture mode patterns of zirconia cores were investigated. The results were analyzed using laser confocal scanning microscopy, drop analyzer, X-ray diffractometry (XRD), scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), universal testing machine and stereomicroscope. The results indicated that three-times laser scanned specimens presented higher Ra values than the other studied groups. The minimum contact angle value was detected in the mentioned group, while the control group presented the highest value. The XRD showed phase transformation from tetragonal to monoclinic t–m following grinding and sandblasting. However, the laser scanned specimens and the control group preserved the structural integrity of the zirconia core, presenting the tetragonal phase only. The highest SBS values were recorded in specimens treated with three-times laser scanning in the unaged and aged conditions. A mixed fracture was a common fracture pattern among the studied groups. The results confirmed that SBS could be optimized through three-times laser scanning and it provided better adhesion between zirconia and the veneer ceramic material. Multiple scanning processes of more than three times are not recommended for zirconia surface treatment.

Effect of Immediate Dentin Sealing on the Bond Strength of Monolithic Zirconia to Human Dentin

Objective:

This study evaluated the shear bond strength (SBS) of pretreated monolithic zirconia surfaces bonded to human dentin following immediate dentin sealing (IDS) using two different self-adhesive resin luting agents.

Methods and Materials:

Sixty intact human third molars were collected, stored, sectioned appropriately, and molded according to ISO 29022:2013, resulting in 120 dentin specimens. Ceramic cylindrical specimens were fabricated using CAD/CAM technology and sintered as recommended (final bonding area A=2.56 mm2). Specimens were randomly assigned to eight groups (15≥n≥14) depending on dentin conditioning method (IDS or delayed dentin sealing [DDS]), zirconia surface pretreatment (airborne particle abrasion [APA] with 50 μm Al2O3 particles at 3 bar for 10 seconds or tribochemical silica coating [TBC] with 30 μm CoJet particles at 2.8 bar for 10 seconds), and adhesive luting agent type (Panavia F2.0 [PAN] or PermaCem Dual Smartmix [PER]). Bonded specimens were water-stored (37°C, 24 hours) and subjected to SBS testing (50-kgF load cell, 1 mm/min). Fracture type was evaluated with stereomicroscopy. Data (MPa) were statistically analyzed using three-way analysis of variance (α=0.05).

Results:

All factors significantly affected SBS values (p&lt;0.001). Dentin conditioning method presented the greatest effect. Mean SBS values ranged from 12.603 MPa (PER-APA-DDS) to 40.704 MPa (PER-TBC-IDS). Based on the fracture type, adhesive failures at the luting agent–zirconia interface were the least common.

Conclusion:

Bonding strategies for monolithic zirconia restorations could potentially benefit from IDS, regardless of the adhesive luting agent system used.

Fracture Toughness Analysis of Ceramic and Resin Composite CAD/CAM Material

Objectives:

To evaluate and compare the fracture toughness of dental CAD/CAM materials of different material classes intended for in-office milling (glass ceramics, hybrid, resin composites) and the influence of aging on this property.

Methods and Materials:

The fracture toughness (critical intensity factor, KIc) values of 9 CAD/CAM restorative materials (Ambarino High-Class, Brilliant Crios, Cerasmart, exp. CAD/CAM composite, Katana Avencia, Lava Ultimate, VITA Enamic, IPS Empress CAD, and IPS e.max CAD) were determined using the SEVNB method in a four-point bending setup. Twenty bending bars of each material with a 4 × 3 cross and a minimum length of 12 mm were cut out of CAD/CAM milling blocks. Notching was done starting with a pre-cut and consecutive polishing and v-shaping with a razor blade, resulting in a final depth of v-shaped notches of between 0.8 and 1.2 mm. Half of the specimens were selected for initial fracture toughness measurements. The others were thermocycled in distilled water for 30,000× (5/55°C; 30-second dwell time) before testing. Specimen fracture surfaces were analyzed using confocal laser scanning microscopy.

Results:

All specimens for each material fractured into two fragments and showed the typical compression curl and brittle failure markings. Comparing initial KIc values, lithium disilicate ceramic IPS e.max CAD showed significantly the highest and leucite-reinforced ceramic IPS Empress CAD significantly the lowest KIc values (p&lt;0.001). All tested CAD/CAM materials with a resin component ranged in the same KIc value group (p&gt;0.999-0.060). After thermal cycling, the highest KIc values were measured for lithium disilicate ceramic IPS e.max CAD, followed by resin composite materials Ambarino High-Class (p&lt;0.001-0.006) and hybrid material VITA Enamic (p&lt;0.001-0.016), while the significantly lowest values were reflected for the resin composite materials Cerasmart, LAVA Ultimate (p&lt;0.001-0.006), and Katana Avencia (p&lt;0.001-0.009). The roughness of the fracture surfaces varied depending on the microstructure of the respective material. The ceramic surfaces showed the smoothest surfaces. The fracture surface of VITA Enamic revealed microstructural inhomogeneities and microcracks. For CAD/CAM resin composite materials, crack paths through the matrix and interfaces of matrix and fillers could be observed at the microstructure level.

Conclusions:

The materials tested show differences in fracture toughness typical for the class they belong to. With one exception (Ambarino High-Class), thermocycling affected the fracture toughness of materials with a resin component negatively, whereas the leucite and lithium disilicate ceramic showed stability.

Wear Characteristics of Dental Ceramic CAD/CAM Materials Opposing Various Dental Composite Resins

The aim of this study was to evaluate the wear properties of opposed dental ceramic restorative CAD/CAM materials and several posterior direct restorative composite resins. Three kinds of dental ceramics CAD/CAM materials (monolithic zirconia, lithium disilicate, leucite) and four dental composite resins—that is, MI Gracefil, Gradia Direct P, Estelite Σ Quick, and Filtek Supreme Ultra—were used in this study. For each of the 12 groups (three ceramics × four composite resins), five each of a canine-shaped ceramic specimen and a cuboidal shape opposing composite resin were prepared. All of the specimens were tested in a thermomechanical loading machine (50 N, 100,000 cycles, 5/55 °C). Wear losses of ceramic specimens and composite resin specimens were evaluated using a three-dimensional profiling system and an electronic scale, respectively. Statistical analyses were performed using the Kruskal–Wallis test and Mann–Whitney U test with Bonferroni’s correction. Zirconia showed significantly less volumetric loss than lithium disilicate or leucite regardless of composite resin type (p > 0.05/3 = 0.017), and that Estelite Σ Quick showed significantly more weight loss than Filtek Supreme Ultra, MI Gracefil, or Gradia Direct P regardless of ceramic type (p > 0.05/6 = 0.083). Zirconia showed less volumetric loss than lithium disilicate or leucite. Some composite resins opposing ceramics showed considerable weight loss.

Trade-off between fracture resistance and translucency of zirconia and lithium-disilicate glass ceramics for monolithic restorations

High strength and translucency are generally not coincident in one restorative material and there is still a continuous development for a better balance between these two properties. Zirconia and lithium-disilicate glass-ceramics are currently the most popular alternatives for monolithic restorations. In this work, the mechanical properties and more important, the slow crack growth (SCG) resistance, which rules long-term durability, were thoroughly studied for three zirconia ceramics stabilized by 3, 4 and 5 mol% yttria in comparison to lithium-disilicate glass-ceramic. Translucency versus strength maps revealed that the more translucent zirconia compositions (i.e. with higher yttria contents) fill the gap between the standard 3 mol% yttria stabilized zirconia (3Y-TZP) and lithium-disilicate. Moreover, increasing yttria content did not always result in lower strength, as values for 3 mol% and 4 mol% yttria were the same. Independent on the yttria contents, all zirconia showed similar relative susceptibility to SCG under static and cyclic conditions and were significantly more SCG-resistant than lithium-disilicate glass ceramic. A concern with higher yttria contents (5 and 4 mol%) however could lie in the higher sensitivity to defects, resulting in a larger scatter in strength. STATEMENT OF SIGNIFICANCE: In addition to the common investigations on the generally reported strength, toughness and translucency, V-K_I diagrams (crack velocity versus stress-intensity factor) from fast fracture to threshold for three newly developed zirconia were directly measured by double torsion methods under static and cyclic loading conditions. The crack-growth mechanisms were analyzed in depth. Results were compared with another popular dental ceramic, namely lithium-disilicate glass-ceramic, revealing the pros and cons of polycrystalline and glass-ceramics in terms of long-term durability. This is the first time that V-K_I curves are compared for the major ceramic and glass-ceramic used for dental restorations. Strength versus translucency maps for different CAD/CAM dental restorative materials were described, showing the current indication range for zirconia ceramics.

Surface characterization of commercially available yttria-stabilized tetragonal zirconia polycrystalline in water and Hanks' solution using X-ray photoelectron spectroscopy

To elucidate the mechanism of adhesion of soft and hard tissues to yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP), Y-TZP and titanium disks were immersed in ultrapure water and in Hanks' solution, and the changes in the surface compositions and chemical states were characterized using X-ray photoelectron spectroscopy. After immersion in ultrapure water for 60 days, the concentration of hydroxyl groups on the Y-TZP surface increased. In addition, only phosphate ions were incorporated into the surface during immersion in Hanks' solution, while other ions did not react. On the other hand, the surface of Ti was also hydrated in ultrapure water; however, calcium phosphate formed on it during immersion in Hanks' solution. Therefore, the reactivity of Y-TZP with electrolytes was lower than that of Ti. We conclude that the formation of the phosphate on the Y-TZP surface in physiological conditions possibly enhances the adhesion of soft and hard tissues to Y-TZP.

Optical properties and flexural strength of translucent zirconia layered with high-translucent zirconia

This study investigated the optical properties and flexural strength of translucent TZP layered with high-translucent PSZ using resin cement of various shades. Zirconia specimens (translucent TZP; Zpex and high-translucent PSZ; ZpexSmile) were 13 mm in diameter, layered at thickness ratios of 0.3/0.7, 0.5/0.5, and 0.7/0.3 mm (ZpexSmile/Zpex), and then luted using resin cement of 3 shades. Monolithic specimens of both were used as controls. CIE L*a*b* color coordinates and translucency parameter (TP) were evaluated as optical properties. Biaxial flexural strength was also determined as a mechanical evaluation. The a* and b* values of layered specimens varied depending on the shade of cement. TP values were not affected by shade of cement and thickness ratio. The biaxial flexural strength was intermediate value between both monolithic specimens. The layering method of zirconia with various translucencies using resin cement of different shades can improve color expression while maintaining clinically sufficient flexural strength.

The influence of altering sintering protocols on the optical and mechanical properties of zirconia: A review

OBJECTIVE

As a result of advancements in chairside technology and speed sintering techniques and increased esthetic demands of patients, efforts have been made to produce monolithic zirconia restorations that are highly translucent, strong, and dense. While methods for processing zirconia are well known, there is a tendency to modify the process parameters with the aim of decreasing the overall processing time and, in particular, the sintering time. This review provides clinicians with scientific evidence of the effects of altering sintering parameters used for dental zirconia on its microstructure, phase transformation, and mechanical and optical properties.

MATERIALS AND METHODS

A systematic search of Embase and Medline using Boolean operators was performed to locate relevant articles.

RESULTS

Eleven articles were selected for this review. The following characteristics of monolithic zirconia have been confirmed to be affected by alterations in sintering: the microstructure, mechanical properties, optical properties, wear behavior, and low thermal degradation.

CONCLUSIONS

The alteration of sintering parameters has been found to alter the grain size, wear behavior, and translucency of zirconia. There is a lack of clinical studies that investigate the influence of altering sintering parameters or methods on the clinical performance of monolithic zirconia restorations.

CLINICAL SIGNIFICANCE

Alteration of sintering parameters alters the microstructural, mechanical, and optical properties of zirconia. This will consequently impact the clinical performance of zirconia prostheses. Future clinical investigations are encouraged to support these in vitro findings.

Effect of different combinations of surface treatment on adhesion of resin composite to zirconia

Purpose: The purpose of this laboratory study was to evaluate the effect of different surface treatment combinations on resin zirconia bonding. Materials and methods: One hundred and five pre-sintered zirconia quadrangles were prepared out of zirconia blocks, polished, then sintered and divided into five groups (n=21). Group I (control): samples were untreated, group II: grit-blasting with 50 µm alumina particles, group III: grit-blasting with 100 µm alumina particles, group IV: Er,Cr:YSGG laser, and group V: selective infiltration etching technique. Microstructural analysis was performed using scanning electron microscopy, atomic force microscopy, a diffractometer, and a profilometer. Cylinders of composite resin were luted with Panavia resin composite cementand Clearfil ceramic primer. Shear bond strength (SBS) was determined using a universal testing machine. Results: SBS results were analyzed using one-way ANOVA followed by Tukey post hoc tests for multiple comparisons. The level of significance was set to 0.05. SBS values of the studied groups II, III, IV, and V were 16.2±1.8 MPa, 15.7±3.7 MPa, 14.8±3.4 MPa, and 16.8±3.0 MPa, respectively. All values were significantly higher than the control group (10.48±1.80 MPa), but without a significant difference between them. Group III exhibited the roughest surface, and Group I had a more significantly reduced surface roughness value than any other group. Group III presented the highest significant increase of tetragonal to monoclinic phase transformation (13%). Conclusion: The use of grit-blasting with greater particles size enhanced SBS with resin composite cement, but induced a higher amount of monoclinic phase transformation. The use of primer based on adhesive monomer with the resin cement is required to enhance the bonding efficiency. The use of laser enhanced the surface roughness and the bonding ability to zirconia.