Advances in computer-aided design (CAD) / computer-aided manufacturing (CAM) technologies and their ease of application enabled the development of novel treatment concepts for modern prosthodontics. This recent paradigm shift in fixed prosthodontics from traditional to minimally invasive treatment approaches is evidenced by the clinical long-term success of bonded CAD/CAM glass-ceramic restorations. Today, defect-oriented restorations, such as inlays, onlays, and posterior crowns, are predominately fabricated from glass-ceramics in monolithic application. The variety of CAD/CAM ceramic restorative systems is constantly evolving to meet the increased demands for highly aesthetic, biocompatible, and long-lasting restorations. Recently introduced polymer-infiltrated ceramic network CAD/CAM blocks add innovative treatment options in CAD/CAM chairside 1-visit restorations. The material-specific high-edge stability enables the CAD/CAM machinability of thin restoration margins. Full-contour zirconia restorations are constantly gaining market share at the expense of bilayered systems. Advancements in material science and bonding protocols foster the development of novel material combinations or fabrication techniques of proven high-strength zirconia ceramics. CAD/CAM applications offer a standardized manufacturing process resulting in a reliable, predictable, and economic workflow for individual and complex teeth-supported restorations. More evidence from long-term clinical studies is needed to verify the clinical performance of monolithic polymer-infiltrated ceramic network and zirconia teeth-supported minimally invasive and extensive restorations.

CAD/CAM Ceramic Restorative Materials for Natural Teeth.

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.

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

The aim of this study was to evaluate stress distribution in an occlusal veneer according to the restorative material, restoration thickness, and cement layer thickness. A tridimensional model of a human maxillary first molar with an occlusal veneer preparation was constructed using a modeling software of finite element analysis. The model was replicated 9 times to evaluate the factors: restoration thickness (0.6, 1.2, and 1.8 mm) and cement layer thickness (100, 200, and 300 μm). Then, each model received different restorative materials (High Translucency Zirconia - [YZHT], Lithium Disilicate - [LD], Zirconia Reinforced Lithium Silicate - [ZLS], Feldspathic - [F], and Hybrid Ceramic - [HC]), totaling forty-five groups. An axial load (600 N) was applied on the occlusal face for static structural analysis. Solids were considered isotropic, homogeneous, and linearly elastic. Contacts were considered perfectly bonded. Fixation occurred in the dental root and a mechanical static structural analysis was performed. Descriptive statistical analysis and one-way ANOVA (α =10%) were performed for tensile stress peak values in the restoration and cement layer. The difference between groups was compared using the Tukey's test with 10% significance to match the percentage of the mesh convergence test. According to the results, the cement layer thickness did not influence stress distribution in the restoration (p ≥ 0.10). The thicker the restoration, the higher the tensile stress concentration in the restoration. The graphs showed higher stress concentration in the YZHT, followed by LD, F, ZLS, and HC. Also, the restorative material influenced stress concentration on the cement layer, which decreased according to the sequence HC>YZHT>ZLS>LD>F. HC stood out for causing the least stress concentration in the restoration. Cement layer thickness did not interfere in the mechanical performance of the restorations.

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Influence of ceramic material, thickness of restoration and cement layer on stress distribution of occlusal veneers.

Statement of problem The polycrystalline nature of zirconia hinders its ability to bond to tooth structure. Consequently, durable bonding to zirconia has been challenging. In vitro studies have evaluated various methods of bonding to zirconia, but clinical data are sparse. Purpose The purpose of this systematic review was to critically appraise clinical studies investigating the survival rate of resin-bonded zirconia fixed partial dentures (FPDs), inlay-retained zirconia FPDs, and zirconia veneers. Material and methods Searches were performed in MEDLINE, EMBASE, PubMed, Web of Science, Scopus, Cochrane Library, and Google Scholar. Clinical studies of over 12 months duration involving bonded zirconia restorations between 1990 and July 2018 were reviewed. All suitable studies were assessed for quality by using a “Questionnaire for selecting articles on Dental Prostheses”. Results Eight studies were ultimately included. Three studies examined posterior inlay-retained FPDs with estimated survival rates of 12.1% at 10 years, 95.8% at 5 years, and 100% at 20 months. Five studies reviewed anterior, resin-bonded FPDs, all of which had a 3- to 10-year survival rate of 100%. Debonds occurred in all studies, but the prostheses could usually be rebonded. Conclusions With correctly designed buccal and lingual coverage retainers and minimal if any veneering porcelain, zirconia-based, posterior, inlay-retained FPDs seem to have a high clinical survival rate. The role of bonding efficacy in this survival rate is unknown. Anterior, cantilevered, resin-bonded zirconia FPDs seem to have a high clinical survival rate. While these prostheses can debond, fracture of the entire prosthesis is unlikely, so they may be rebonded. To bond zirconia, the use of airborne-particle abrasion with 50-μm alumina (Al2O3) at 0.1 to 0.25 MPa in combination with a phosphate monomer-containing adhesive resin is recommended until further studies become available. Dental dam isolation is also recommended during zirconia bonding.

Clinical efficacy of methods for bonding to zirconia: A systematic review.

Objectives To investigate and compare the mechanical and optical properties of novel strength- and color-gradient multilayered zirconia and the impact of conventional and high-speed sintering. Materials and methods Following zirconia materials were analyzed and compared: the high-speed sintered Katana Zirconia Block STML (4Y-TZP, KZC), the conventionally sintered Katana Zirconia Disc STML (4Y-TZP, KZL) and IPS e. max ZirCAD Prime (5Y-TZP/3Y-TZP, EZL). As control group acted the crystallized lithium disilicate ceramic IPS e. max CAD (ELC). Monolithic single molar crowns were fabricated and half of them were aged in a chewing simulator with human enamel antagonists (1.2 × 106 cycles, 50 N, lateral movement of 0.7 mm, 5/55 °C). The fracture load was tested in a universal testing machine (N = 96/n = 12). The two-body wear was determined using 3D matching of pre- and post-scans (N = 48/n = 12). Translucency (N = 36/n = 10) was evaluated with UV/Vis spectrophotometer. Data was analyzed using Kolmogorov-Smirnov test, one-way ANOVA followed by post-hoc Scheffe test, unpaired t-test and Weibull analysis (p  Results Zirconia groups showed higher fracture load than ELC (p ≤ 0.001). Initially, all zirconia materials ranged in the same values (p > 0.05). After chewing simulation, EZL showed higher fracture load than KZC (p  Conclusions The novel strength and color-gradient multilayered zirconia showed higher mechanical properties than lithium disilicate ceramic. The high-speed sintering of zirconia showed neither a negative impact on the fracture load nor on the two-body wear. However, the optical properties and the reliability of zirconia is lower than those of highly translucent lithium disilicate ceramic.

Comparison between novel strength-gradient and color-gradient multilayered zirconia using conventional and high-speed sintering

Ten-year outcome of zirconia ceramic cantilever resin-bonded fixed dental prostheses and the influence of the reasons for missing incisors.

Objectives To evaluate the efficiency of immediate dentin sealing and the effects of different bonding protocols on the fracture strength of CAD/CAM occlusal veneers bonded to exposed dentin. Methods Ninety-six extracted maxillary premolars were initially divided into three main groups with 32 specimens each: without immediate dentin sealing, immediate dentin sealing/total etching and immediate dentin sealing/selective etching. Teeth were identically prepared in the dentin to receive occlusal veneers of 0.8 mm thickness, milled from lithium disilicate ceramic blocks (IPS e.max CAD). Each main group was later subdivided, according to the pre-cementation surface etching protocol (total/selective), into two subgroups with 16 specimens each. All restorations were adhesively bonded using a resin cement (Variolink Esthetic). Half of the specimens of each subgroup were subjected to thermo-dynamic loading in a chewing simulator with 1,200,000 cycles at 10 kg load. The other half and the surviving specimens were subjected to quasi-static loading until failure. Statistical analysis was performed using three-way ANOVA and Tukey's post-hoc tests. Results All specimens except one survived the artificial aging. A significantly higher fracture strength of restorations (p ≤ 0.001) was obtained when immediate dentin sealing was followed regardless of the etching method with values ranging from a minimum of 1122 ± 336 N to a maximum of 1853 ± 333 N. Neither the pre-cementation treatment nor the artificial aging had a statistical significant effect on the fracture strength. Significance Immediate dentin sealing protocol is recommended whenever dentin is exposed during the preparation for thin glass-ceramic occlusal veneers.

Influence of various bonding techniques on the fracture strength of thin CAD/CAM-fabricated occlusal glass-ceramic veneers

Objectives The purpose of this study was to evaluate the effects of artificial aging on thin glass-ceramic occlusal premolar veneers, adhesively bonded to dentin, by examining the changes caused by artificial aging using spectral domain optical coherence tomography (SD-OCT). In addition, the development of cracks in the ceramic veneers and their possible influence on the behavior of the ceramic restorations were examined. Methods In total, 48 extracted sound upper premolars were prepared in the dentin for occlusal veneers milled from lithium disilicate ceramic blocks (IPS e.max CAD, Ivoclar Vivadent, Liechtenstein). All restorations were adhesively bonded using resin cement (Variolink Esthetic DC, Ivoclar Vivadent). Specimens were 3-dimensionally and 2-dimensionally imaged by SD-OCT (Telesto II, Thorlabs GmbH, Germany), then subjected to thermal-dynamic loading in a chewing simulator with 1,200,000 cycles at a load of 10 kg. Specimens were 2D and 3D imaged again after the artificial aging. Finally, they were subjected to quasi-static loading using a universal testing machine until failure occurred and later examined microscopically to assess the mode of failure. ANOVA test was performed for statistical analysis of data and Tukey’s post-hoc test was used to compare the groups at 5% significance level. Chi-Square Test and Fischer’s Exact Test of Independence were conducted to test the association between nominal variables. Results No changes or irregularities were observed in the cement layer or tooth substrate after the aging process. However, wear of the ceramic was noticed at the surface of contact with the antagonist during the test. The development of cracks was detected in 23% of the specimens. Cracks did not affect the fracture strength (p > 0.05) but influenced the mode of failure (p ≤ 0.001). Significance Optical coherence tomography allows an easy and non-invasive method to internally scan teeth and restorations. Development of cracks in the ceramic did not affect the fracture strength of the restorations but might lead to a more catastrophic type of failure.

Effects of artificial aging and progression of cracks on thin occlusal veneers using SD-OCT.

Background The use of polymers in implant dentistry is gaining popularity, as they might have several advantages due to their shock absorption capacity. Purpose The aim of this study was to evaluate the performance and fracture strength of three resilient shock-absorbing polymer-based materials as well as the widely used ceramics as one-piece screw-retained implant-supported hybrid-abutment-crowns. Materials and methods Forty custom-made CAD/CAM one-piece screw-retained restorations were milled out of 5 different monolithic materials (n = 8); Z: 3Y-TZP zirconia, L: lithium disilicate, P: ceramic-reinforced polyetheretherketone (PEEK), C: nano-hybrid composite resin and E: polymer-infiltrated ceramic-network. Specimens were subjected to dynamic loading for 1,200,000 cycles with integrated thermal cycling. The surviving specimens were subjected to quasi-static loading until failure. Shapiro-Wilk test was used to test for normality, and Levene test was conducted to test the homogeneity of variance. One-way ANOVA test followed by Tukey's post-hoc test were used to detect statistically significant differences between groups. Results All specimens withstood 1,200,000 cycles of thermo-dynamic loading with no signs of cracks or screw loosening. The median values of fracture strength varied from a minimum of 670 N for group E to a maximum of 2645 N for group Z. Conclusions Fracture strength of screw-retained hybrid-abutment-crowns is influenced by the material used. Zirconia showed superior results. However, its fracture strength was comparable to that of PEEK, which additionally enjoys a shock absorbing property.

Christine Yazigi

Papers

Ten-year outcome of zirconia ceramic cantilever resin-bonded fixed dental prostheses and the influence of the reasons for missing incisors.

Influence of various bonding techniques on the fracture strength of thin CAD/CAM-fabricated occlusal glass-ceramic veneers

Effects of artificial aging and progression of cracks on thin occlusal veneers using SD-OCT.

The influence of the restorative material on the mechanical behavior of screw-retained hybrid-abutment-crowns

Clinical evaluation of zirconia cantilevered single-retainer resin-bonded fixed dental prostheses replacing missing canines and posterior teeth.