A two-year clinical trial of enamel wear opposing 5Y-TZP and lithium disilicate crowns.

STATEMENT OF PROBLEM: Clinical information on enamel wear opposing 5 mol% yttria-stabilized tetragonal zirconia polycrystal (5Y-TZP) or lithium disilicate crowns is lacking. Moreover, the effect of surface roughness on the wear of enamel antagonists remains unclear. PURPOSE: The purpose of this clinical study was to quantify the vertical wear and total volume of wear while also measuring the surface roughness of antagonists at 6-month intervals over 2 years. MATERIAL AND METHODS: A tooth-supported first molar 5Y-TZP Lava Esthetic Fluorescent Zirconia crown or an IPS e.max CAD lithium disilicate crown was randomly provided for 24 participants (n=12). The contralateral first molar and opposite teeth were the natural enamel controls. The crowns were fabricated using computer-aided design and computer-aided manufacturing (CAD-CAM) technology and then polished. The data were gathered every 6 months at recall appointments with an intraoral scanner and a polyvinyl siloxane impression. A 3-dimensional (3D) comparative software program was used to measure the maximum vertical and volume loss on opposing enamel, as well as on the enamel control on the opposite side of the mouth. The surface of epoxy resin replicas was analyzed using a scanning electron microscope. The surface roughness of the cusp replica was measured with a 3D laser microscope. The crown's antagonist enamel and natural enamel wear were compared using a 2-way repeated measure ANOVA (α=.05). RESULTS: No significant difference was found in the average volume loss between the crown's antagonist enamel (ZTE 0.042 ±0.005 mm3, LTE 0.048 ±0.006 mm3) and the enamel control (ZCE 0.046 ±0.004 mm3, LCE 0.050 ±0.005 mm3) (P>.05). No significant difference was found in surface roughness between the crown's antagonist enamel (antagonist enamel of zirconia group [ZTE] 0.7 ±0.3 µm, antagonist enamel of lithium disilicate group [LTE] 0.6 ±0.3 µm) and the enamel control (enamel control of zirconia group [ZCE] 0.7 ±0.3 µm, enamel control of lithium disilicate group [LCE] 0.5 ±0.2 µm) in both zirconia and lithium disilicate groups (P>.05). Also, no correlation was found between the amount of wear and the roughness of the surface. The wear patterns observed on the opposing enamel surfaces of the monolithic zirconia and lithium disilicate materials were found to be similar to those seen on natural enamel. CONCLUSIONS: A 2-year clinical investigation found that polished lithium disilicate and 5Y-TZP crowns did not affect enamel wear more than enamel against enamel, contrary to laboratory studies. Another finding revealed that surface roughness did not significantly affect wear.

Effect of dentures and dietary advice on protein intake in older Thai adults with missing posterior occluding teeth.

OBJECTIVE: To compare the effectiveness of dentures with dietary advice and dentures alone on protein intake in older Thai adults. BACKGROUND: Dentures combined with nutritional advice may improve protein intake in older adults with posterior tooth loss. However, evidence is limited, especially in developing countries. METHODS: A randomised-control trial including 69 older adults with less than four pairs of posterior occluding teeth was conducted for 3 months. The experimental group received dentures and dietary advice based on the Health Belief Model (HBM); the control group received dentures alone. Three-day dietary records were collected and protein intake was assessed using INMUCAL-NUTRIENTS V.4.0 software at baseline, 1 month, and 3 months after the intervention. Data were analysed using a t-test, Mann-Whitney U test, chi-square test, and repeated-measure ANOVA. RESULTS: Both groups reported significantly increased protein intake (control group = 0.4, 0.5, 0.6 g kg BW-1 d-1 , intervention = 0.4, 0.6, 0.7 g kg BW-1 d-1 , at baseline, 1 month and 3 months, respectively). However, participants in the experimental group had significantly higher total protein (36.3 vs. 39.8 g/d, P = .032) and animal protein intake (18.9 vs. 23.2 g/day, P = .020) than those in the control group at 3 months after the intervention; plant protein intake was similar (10.9 vs. 11.5 g/day, P = .923). In addition, the HBM scores of the experimental group were higher than those of the control group after the intervention (P < .001). CONCLUSION: Dentures combined with dietary advice significantly increased the total and animal protein intake of older people with posterior tooth loss more than dentures alone. To maintain optimal protein intake, dental practitioners should incorporate dietary advice when providing dental prostheses to older adults with posterior tooth loss.

Optical effect of resin cement, abutment material, and ceramic thickness on the final shade of CAD-CAM ceramic restorations.

STATEMENT OF PROBLEM: High-translucency ceramics can be influenced by the underlying structure, altering the restoration shade. How this affects color matching is unclear. PURPOSE: The purpose of this in vitro study was to investigate the optical effects of cement shade, ceramic type, ceramic thickness, and abutment material on the final color of computer-aided design and computer-aided manufacturing (CAD-CAM) ceramic restorations. MATERIAL AND METHODS: Two shades of resin cement (yellow, translucent), 3 types of background (titanium, white zirconia, yellow zirconia), and 3 types of high-translucent ceramic (each type shade A1 and A3) were used in this study. For the experimental groups, a total of 72 ceramic specimens were produced across 4 groups based on ceramic thickness (1.0 mm, 1.5 mm, and 2.0 mm). For the control groups, each ceramic type and shade was prepared at a thickness of 4 mm. A total of 8 resin cement specimens were produced in 4 groups based on shade by using a plastic mold (12×12×0.2 mm). To demonstrate the effect of implant abutment materials, 3 types of background were fabricated with a thickness of 2 mm. For the experimental groups, 3 specimens (ceramic, cement, and background) were sequentially placed with glycerin in the center of each background specimen. Color measurements of the experimental groups were made with a spectrophotometer and recorded in the Commission Internationale de l'Eclairage Lab coordinate system. The color differences (ΔE) between experimental and control groups were then calculated. The Kruskal-Wallis test (α=.05) was used to analyze the multiple comparisons of ceramic thickness, ceramic type, and abutment material. The Mann-Whitney U test (α=.05) was used to analyze cement shade. RESULTS: Significant differences were found for different ceramic thicknesses, ceramics types, and abutment materials (P≤.001). A clinically acceptable shade (ΔE≤3) was found in 1.5- and 2.0-mm ceramics with the titanium and 2.0-mm ceramics with the yellow zirconia background. However, the color between the experimental group and the control group was similar when using the 2 cement shades (P>.05). CONCLUSIONS: An increase in ceramic thickness could minimize alteration of the final shade. High-translucency ceramics, together with a resin cement, were able to successfully mask titanium with a ceramic thickness of at least 1.5 mm. However, only a ceramic with a thickness of 2.0 mm was able to mask the yellow zirconia background.

Influence of implant abutment material and ceramic thickness on optical properties.

STATEMENT OF PROBLEM: Anterior shade matching is an essential factor influencing the esthetics of a ceramic restoration. Dentists face a challenge when the color of an implant abutment creates an unsatisfactory match with the ceramic restoration or neighboring teeth. PURPOSE: The purpose of this in vitro study was to evaluate the influence of abutment material and ceramic thickness on the final color of different ceramic systems. MATERIAL AND METHODS: Four experimental and control ceramic specimens in shade A3 were cut from IPS e.max CAD, IPS Empress CAD, and VITA Suprinity PC blocks. These specimens had thicknesses of 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm, respectively, for the experimental groups, and 4 mm for the controls. Background abutment specimens were fabricated to yield 3 different shades: white zirconia, yellow zirconia, and titanium at a 3-mm thickness. All 3 ceramic specimens in each thickness were placed in succession on different abutment backgrounds with glycerin optical fluid in between, and the color was measured. A digital spectrophotometer was used to record the specimen color value in the Commission Internationale De L'éclairage (CIELab) color coordinates system and to calculate the color difference (ΔE) between the control and experimental groups. The Kruskal-Wallis test was used to analyze the effect of ceramic thickness on different abutments, and the pair-wise test was used to evaluate within the group (α=.05). RESULTS: The color differences between the test groups and the control decreased with increasing ceramic thickness for every background material. In every case, significant differences were found between 1.0- and 2.5-mm ceramic thicknesses. Only certain 2.5-mm e.max CAD, VITA Suprinity PC, and Empress CAD specimens on yellow-shade zirconia or VITA Suprinity PC on titanium were identified as clinically acceptable (ΔE<3). CONCLUSIONS: Increasing ceramic restoration thickness over the abutment background decreased the color mismatch. Increasing the thickness of ceramic on a yellow-shaded zirconia abutment rather than on titanium or white zirconia yielded a more esthetic color for the whole restoration.

Sectional collapsible complete removable dental prosthesis for a patient with microstomia.

Patients who have unusually small mouths may have difficulty in obtaining dental care and maintaining good oral hygiene. The fabrication of conventional complete removable dental prostheses for an edentulous patient with microstomia is challenging because of the limited access to the oral cavity. Sectional collapsible complete removable dental prostheses were designed as hinged maxillary and mandibular complete dentures that can be folded for denture delivery. This design also prevented denture deflection during function by using the upper part of the prosthesis, minimizing the possibility of breakage.

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.

Effects of Litsea cubeba Essential Oil Incorporated into Denture Soft Lining Materials.

The antimicrobial properties, cell cytotoxicity and surface hardness of soft lining materials (GC soft liner, Viscogel and Coe comfort) incorporated with various concentrations of Litsea cubeba essential oil (LCEO) were evaluated. The minimum inhibitory concentrations of LCEO against Candida albicans and Streptococcus mutans were 1.25% v/v and 10% v/v, respectively. However, when LCEO was incorporated into the three soft lining materials (GC soft liner, Viscogel and Coe comfort), 10% v/v and 30% v/v of LCEO could inhibit the growth of C. albicans and S. mutans, respectively. The extracts of soft lining materials with 10% and 30% v/v LCEO, 2% chlorhexidine, 30% v/v nystatin and no additive were used for cytotoxicity tests on a human gingival fibroblast cell line. There was no significant difference in cell viability in all groups with additives compared to the no additive group (p > 0.05). Surface hardness increased significantly between 2 h and 7 day incubation times in all groups, including the controls (p < 0.05). A higher LCEO concentration had a dose-dependent effect on the surface hardness of all soft lining materials (p < 0.05). However, the surface hardness of materials with additive remained in accordance with ISO 10139-1. LCEO could be used as a natural product against oral pathogens, without having a negative impact on soft lining materials.

Influence of zirconia/glass veneer thickness and implant abutment material on the final shade of implant restorations.

Aim: The aim of this study was to investigate the combined effect of ceramic material, ceramic thickness, and implant abutment background to the final color of restorations. Settings and Design: This was a comparative in vitro study. Materials and Methods: Three different types of monolithic and porcelain-veneered zirconia disc-shaped specimens (Prettau Anterior, VITA YZ ST, and VITA YZ HT) were prepared in A3 shade with two different thicknesses (1 mm and 1.5 mm) (n = 10). Each zirconia material was made of 4-mm thickness as a control specimen of each monolithic zirconia type, and 4-mm thick veneering ceramic (VITA VM9 Base Dentine) was made as a control for veneered zirconia groups. Three simulated implant abutments were fabricated from titanium, white-shaded and yellow-shaded zirconia. The zirconia specimens were placed on different abutment backgrounds, and the color difference (ΔE) between experimental and control specimens was measured. Statistical Analysis Used: The three-way ANOVA and the Scheffé test were used for data analysis (α = 0.05). Results: The mean ΔE values between two thicknesses were significantly different in every background for all zirconia materials. The ΔE values of zirconia specimens on yellow zirconia were lower than those of other abutments. The clinically acceptable ΔE value (ΔE <3) was found in some monolithic zirconia specimens on white-shaded and yellow-shaded abutments, while the ΔE value is approximately 3 or less in all 1.5-mm thick porcelain-veneered zirconia groups. Conclusions: Different zirconia materials on implant abutments affected the final color of restorations. To achieve satisfactory color, the minimum thickness of zirconia restorations should be at least 1.5 mm on yellow zirconia abutment.

Grain size, crystalline phase and fracture toughness of the monolithic zirconia.

PURPOSE: This study evaluated the relationship among translucency, crystalline phase, grain size, and fracture toughness of zirconia. MATERIALS AND METHODS: Four commercial zirconia - Prettau®Anterior® (PA), Prettau® (P), InCorisZI (ZI), and InCorisTZI (TZI)- were selected for this study. The bar specimens were prepared to determine fracture toughness by using chevron notched beam method with four-point bending test. The grain size was evaluated by a mean linear intercept method using a scanning electron microscope. X-ray diffraction and Rietveld refinement were performed to evaluate the amount of tetragonal and cubic phases of zirconia. Contrast ratio (CR) was measured to investigate the level of translucency. RESULTS: PA had the lowest fracture toughness among other groups (P < .05). In addition, the mean fracture toughness of P was significantly less than that of ZI, but there was no difference compared with TZI. Regarding grain size measurement, PA had the largest average grain size among the groups. P obtained larger grain size than ZI and TZI (P < .05). However, there was no significant difference between ZI and TZI. Moreover, PA had the lowest CR value compared with the other groups (P < .05). This means PA was the most translucent material in this study. Rietveld refinement found that PA presented the greatest percentage of cubic phase, followed by TZI, ZI, and P, respectively. CONCLUSION: The different approaches are used by manufacturers to fabricate various types of translucent zirconia with different levels of translucency and mechanical properties, which should be concerned for material selection for successful clinical outcome.

Effects of inter-implant distance on the accuracy of intraoral scanner: An in vitro study.

PURPOSE: Several studies focused on the accuracy of intra-oral scanners in implant dentistry, but the data of inter-implant distances were not widely mentioned. Therefore, this study aimed to evaluate the effect of distance between two implants on the surface distortion of scanned models generated by intra-oral scanners. MATERIALS AND METHODS: Three models with the distances between two fixed scan bodies of 7, 14, and 21 mm were fabricated and scanned with a highly precise D900L dental laboratory scanner as reference models. Fifteen scans were performed with TRIOS3 and CEREC Omnicam intra-oral scanners. Trueness, precision, and angle deviation of the test models were analyzed (α=.05). RESULTS: There was a significant difference among inter-implant distances in both intraoral scanners (P <.001). The error of trueness and precision increased with the increasing inter-implant length, while the angle deviation did not show the same trend. A significant difference in the angle deviation was found among the inter-implant distance. The greatest angle deviation was reported in the 14-mm group of both scanners (P <.05). In contrast, the lowest angle deviation in the 21-mm group of the TR scanner and the 7-mm of the CR scanner was reported (P <.001). CONCLUSION: The inter-implant distance affected the accuracy of intra-oral scanner. The error of trueness and precision increased along with the increasing distance between two implants. However, the distortions were not clinically significant. Regarding angle deviation, the clinically significant angle deviation may be possible when using intra-oral scanners in the partially edentulous arch.

Different Implant Subgingival Depth Affects the Trueness and Precision of the 3D Dental Implant Position: A Comparative in Vitro Study Among Five Digital Scanners and a Conventional Technique.

PURPOSE: The aim of this study was to investigate the effect of implant subgingival depth on the trueness and precision of the 3D implant position, and the effect of digital implant impression techniques on the 3D implant position. MATERIALS AND METHODS: Three resin master models were created with implant analogs submerged 3, 6, and 9 mm from the gingival margin. Four intraoral scanners (TRIOS, DWIO, Omnicam, and TruDef), one laboratory scanner (E3), and a conventional impression technique were used to take impressions of the master models, which resulted in six test models for each depth. These six impression techniques were performed six times for precision assessment. The master models were sent for high-powered micro-focused computed tomography as the gold standard control group. The scan body positions of the test models and their control models were superimposed using reverse-engineering software. The 3D distortion of the implant position in each comparison was measured by linear distortion (dx, dy, dz) and calculated as the global linear distortion (dR). RESULTS: The trueness of the mean dR values at the 3-mm, 6-mm, and 9-mm implant depths was 99 µm, 60.6 µm, and 107 µm, respectively. The least significant difference test of the impression system showed that all the digital impression techniques except the DWIO scanner had better trueness than the conventional impression technique. The 6-mm implant depth exhibited a significantly lower 3D distortion of the implant position than those of the 3-mm and 9-mm implant depths. The E3 scanner had the highest precision, while the conventional impression technique had the lowest precision. All the intraoral scanners except the DWIO scanner showed better precision than the conventional impression technique. CONCLUSION: Most of the intraoral scanners had better trueness and precision than the conventional impression technique for up to 6 mm of implant subgingival depth.

Bonding measurement -Strength and fracture mechanics approaches.

This study investigated the effect of cross-sectional areas on interfacial fracture toughness and bond strength of bilayered dental ceramics. Zirconia core ceramics were veneered and cut to produce specimens with three different cross-sectional areas. Additionally, monolithic specimens of glass veneer were also prepared. The specimens were tested in tension until fracture at the interface and reported as bond strength. Fracture surfaces were observed, and the apparent interfacial toughness was determined from critical crack size and failure stress. The results showed that cross-sectional area had no effect on the interfacial toughness whereas such factor had a significant effect on interfacial bond strength. The study revealed that cross-sectional area had no effect on the interfacial toughness, but had a significant effect on interfacial bond strength. The interfacial toughness may be a more reliable indicator for interfacial bond quality than interfacial bond strength.

Simplified method for determining fracture toughness of two dental ceramics.

This study compared the fracture toughness values (KIC), which were derived from simplified techniques: the indentation fracture (IF), the indentation strength (IS), and fractographic approach to that from a standard testing using surface cracks in flexure (SCF). Forty bar specimens, twenty IPS Empress(®) Esthetic and twenty IPS e.max(®)Ceram were prepared. Ten specimens in each material were tested by IF technique, IS technique and fractographic approach, and additional 10 specimens were tested by the SCF technique. This study showed that the mean KIC derived from fractographic approach were not significantly different from that of the SCF in both materials (p>0.05) whereas the mean KIC from indentation techniques rarely agreed with those of the standard technique. The KIC determination is sensitive to the methods used that affect accuracy. Consequently, test selection should be based on a sound understanding and inherent limitations of each technique.

Tensile strength of bilayered ceramics and corresponding glass veneers.

PURPOSE: To investigate the microtensile bond strength between two all-ceramic systems; lithium disilicate glass ceramic and zirconia core ceramics bonded with their corresponding glass veneers. MATERIALS AND METHODS: Blocks of core ceramics (IPS e.max® Press and Lava™ Frame) were fabricated and veneered with their corresponding glass veneers. The bilayered blocks were cut into microbars; 8 mm in length and 1 mm(2) in cross-sectional area (n = 30/group). Additionally, monolithic microbars of these two veneers (IPS e.max® Ceram and Lava™ Ceram; n = 30/group) were also prepared. The obtained microbars were tested in tension until fracture, and the fracture surfaces of the microbars were examined with fluorescent black light and scanning electron microscope (SEM) to identify the mode of failure. One-way ANOVA and the Dunnett's T3 test were performed to determine significant differences of the mean microtensile bond strength at a significance level of 0.05. RESULTS: The mean microtensile bond strength of IPS e.max® Press/IPS e.max® Ceram (43.40 ± 5.51 MPa) was significantly greater than that of Lava™ Frame/Lava™ Ceram (31.71 ± 7.03 MPa)(P<.001). Fluorescent black light and SEM analysis showed that most of the tested microbars failed cohesively in the veneer layer. Furthermore, the bond strength of Lava™ Frame/Lava™ Ceram was comparable to the tensile strength of monolithic glass veneer of Lava™ Ceram, while the bond strength of bilayered IPS e.max® Press/IPS e.max® Ceram was significantly greater than tensile strength of monolithic IPS e.max® Ceram. CONCLUSION: Because fracture site occurred mostly in the glass veneer and most failures were away from the interfacial zone, microtensile bond test may not be a suitable test for bonding integrity. Fracture mechanics approach such as fracture toughness of the interface may be more appropriate to represent the bonding quality between two materials.

Gap comparison between single crown and three-unit bridge zirconia substructures.

PURPOSE: To compare marginal and internal gaps of zirconia substructure of single crowns with those of three-unit fixed dental prostheses. MATERIALS AND METHODS: Standardized Co-Cr alloy simulated second premolar and second molar abutments were fabricated and subsequently duplicated into type-III dental stone for working casts. After that, all zirconia substructures were made using Lava™ system. Marginal and internal gaps were measured in 2 planes (mesial-distal plane and buccal-palatal plane) at 5 locations: marginal opening (MO), chamfer area (CA), axial wall (AW), cusp tip (CT) and mid-occlusal (OA) using Replica technique. RESULTS: There were significant differences between gaps at all locations. The mean ± SD of marginal gap in premolar was 43.6 ± 0.4 µm and 46.5 ± 0.5 µm for single crown and 3-unit bridge substructure respectively. For molar substructure the mean ± SD of marginal gap was 48.5 ± 0.4 µm and 52.6 ± 0.4 µm for single crown and 3-unit bridge respectively. The largest gaps were found at the occlusal area, which was 150.5 ± 0.5 µm and 154.5 ± 0.4 µm for single and 3-unit bridge premolar substructures respectively and 146.5 ± 0.4 µm and 211.5 ± 0.4 µm for single and 3-unit bridge molar substructure respectively. CONCLUSION: Independent-samples t-test showed significant differences of gap in zirconia substructure between single crowns and three-unit bridge (P<.001). Therefore, the span length has the effect on the fit of zirconia substructure that is fabricated using CAD/CAM technique especially at the occlusal area.

The relationship of maxillary canines to the facial anatomical landmarks in a group of Thai people.

PURPOSE: The objective was to evaluate canine positions, intercanine tip width (ICTW) and width of distal surface of canine (WDC), related to facial landmarks including interalar width (IAW), intercommissural width (ICoW), and distance between left and right projection lines drawn from inner canthus of eyes to alae of the nose (DPICa) in a group of Thai. MATERIALS AND METHODS: One hundred Thai subjects aged 18-35 years were selected. IAW and ICoW were measured on subject's face using digital vernier caliper. Irreversible hydrocolloid impression of the upper arch was taken, and a cast was poured with dental stone. Silicone impression material was used to take imprint of the incisal edge of upper six anterior teeth. DPICa was obtained from the subject's face using custom-made measuring equipment and marked on the silicone incisal imprint. The marks were then transferred from the imprint to the stone cast and measured with digital caliper. The ICTW and WDC were also measured on the stone cast. Pearson's correlation was used to determine the correlation. RESULTS: The results revealed that the correlation between ICTW-ICoW was 0.429 and ICTW-DPICa was 0.573. The correlation between WDC-ICoW was 0.426 and WDC-DPICa was 0.547. However, IAW did not show any correlation with ICTW or WDC (P>.05). CONCLUSION: The correlation between canine position and facial landmarks was found. ICTW and WDC had relationship with ICoW and DPICa. DPICa showed stronger correlation with the position of maxillary canine than that of ICoW.

Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test.

OBJECTIVE: The objective of this study was to test the null hypothesis that the interfacial toughness of each of two types of bonded core-veneer bilayer ceramics is not significantly different from the apparent fracture toughness of the control monolithic glass veneer. METHODS: T-shaped short-bars of a lithia-disilicate glass-ceramic core (LC) and yttria-stabilized polycrystalline zirconia core ceramic (ZC) were prepared according to the manufacturer's recommendations. V-shaped notches were prepared by using 25-mum-thick palladium foil, leaving the chevron-notch area exposed, and the bars were veneered with a thermally compatible glass veneer (LC/GV and ZC/GV). Additionally, we also bonded the glass veneer to itself as a control group (GV/GV). Specimens were kept in distilled water for 30 days before testing in tension. Eight glass veneer bars were prepared for the analysis of fracture toughness test using the indentation-strength technique. RESULTS: The mean interfacial toughness of the LC/GV group was 0.69 MPam(1/2) (0.11), and did not significantly differ from that of the GV/GV control group, 0.74 MPam(1/2) (0.17) (p>0.05). However, the difference between the mean interfacial toughness of the ZC/GV group, 0.13 MPam(1/2) (0.07), and the LC/GV and the GV/GV groups was statistically significant (p<0.05). SIGNIFICANCE: For bilayer all-ceramic restorations with high-strength core materials, the veneering ceramics are the weakest link in the design of the structure. Since all-ceramic restorations often fail from chipping of veneer layers or crack initiation at the interface, the protective effects of thermal mismatch stresses oral prosthesis design should be investigated.

Residual stress in glass: indentation crack and fractography approaches.

OBJECTIVE: To test the hypothesis that the indentation crack technique can determine surface residual stresses that are not statistically significantly different from those determined from the analytical procedure using surface cracks, the four-point flexure test, and fracture surface analysis. METHODS: Soda-lime-silica glass bar specimens (4 mm x 2.3 mm x 28 mm) were prepared and annealed at 650 degrees C for 30 min before testing. The fracture toughness values of the glass bars were determined from 12 specimens based on induced surface cracks, four-point flexure, and fractographic analysis. To determine the residual stress from the indentation technique, 18 specimens were indented under 19.6N load using a Vickers microhardness indenter. Crack lengths were measured within 1 min and 24h after indentation, and the measured crack lengths were compared with the mean crack lengths of annealed specimens. Residual stress was calculated from an equation developed for the indentation technique. All specimens were fractured in a four-point flexure fixture and the residual stress was calculated from the strength and measured crack sizes on the fracture surfaces. RESULTS: The results show that there was no significant difference between the residual stresses calculated from the two techniques. However, the differences in mean residual stresses calculated within 1 min compared with those calculated after 24h were statistically significant (p=0.003). SIGNIFICANCE: This study compared the indentation technique with the fractographic analysis method for determining the residual stress in the surface of soda-lime-silica glass. The indentation method may be useful for estimating residual stress in glass.