Fracture-surface analysis of dental ceramics

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

STATEMENT OF PROBLEM

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

PURPOSE

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

MATERIAL AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Shade, Aging and Spatial-Dependent Variation of Elastoplastic and Viscoelastic Characteristics in a Dental, Submicron Hybrid CAD/CAM Composite

This article reports the elastoplastic and viscoelastic response of an industrially cured CAD/CAM resin-based composite (Brilliant Crios, Coltene) at different scales, spatial locations, aging conditions, and shading. Mechanical tests were performed at the macroscopic scale to investigate material strength, elastic modulus, fracture mechanisms and reliability. An instrumented indentation test (IIT) was performed at the microscopic level in a quasi-static mode to assess the elastic and plastic deformation upon indentation, either by mapping transverse areas of the CAD/CAM block or at randomly selected locations. A dynamic-mechanical analysis was then carried out, in which chewing-relevant frequencies were included (0.5 to 5 Hz). Characteristics measured at the nano- and micro-scale were more discriminative in identifying the impact of variables as those measured at macro scale. Anisotropy as a function of the spatial location was identified in all shades, with gradual variation in properties from the center of the block to peripheral locations. Depending on the scale of observation, differences in shade and translucency are very small or not statistically significant. The aging effect is classified as low, but measurable on all scales, with the same pattern of variation occurring in all shades. Aging affects plastic deformation more than elastic deformation and affects elastic deformation more than viscous deformation.

Validation of the Orr theory in dental resin-based composites: A fractographic approach

OBJECTIVES

The present study aimed to assess the applicability of Orr's equation on light-cured resin-based composites (RBCs) tested in 3- and 4-point bending tests, as well as assess the operator reliability of this method.

METHODS

Fracture mirrors of 320 specimens (n = 320) made of four RBCs that had failed in the two tests were analyzed microscopically and consequently two radii of each specimen were measured by two operators, each measuring twice. The mirror constant A was calculated using Orr's equation. The results of the two operators were compared using a t-Test and further assessment of the data was performed using a MANOVA, Tukey's post hoc test and regression analysis.

RESULTS

With the exception of 11 specimens, the measurement of the fracture mirror radii was carried out successfully. The calculation of the mirror constant proved successful with a high coefficient of determination (R² > 0.98). The calculated mirror constants were operator-independent if the results of more than one measurement was taken into consideration. The ratio of the mirror constant A to the fracture toughness K_Ic lay between 2.1 and 2.4 for three of the four RBC. The fourth RBC had a ratio of 1.6 and was the only one where different mirror constants were observed as a function of the testing method.

SIGNIFICANCE

The present study proves the applicability of brittle fracture mechanics on RBCs despite them often being regarded as not brittle due to their also plastic behavior.

Pre-cementation treatment of glass-ceramics with vacuum impregnated resin coatings

OBJECTIVE

The study aimed to investigate the effectiveness of a vacuum impregnation process to eliminate the porosity at the ceramic-resin interface to optimize the reinforcement of a glass-ceramic by resin cementation.

METHODS

100 leucite glass-ceramic disks (1.0 ± 0.1 mm thickness) were air-abraded, etched with 9.6 % HF acid, and silanated. Specimens were randomly allocated to 5 groups (n = 20). Group A received no further treatment (uncoated control). Groups B and D were resin-coated under atmospheric pressure, whereas groups C and E were resin-coated using vacuum impregnation. The polymerized resin-coating surfaces of specimens in groups B and C were polished to achieve a resin thickness of 100 ± 10 µm, while in groups D and E no resin-coating modification was performed prior to bi-axial flexure strength (BFS) determination. Optical microscopy was undertaken on the fracture fragments to identify the failure mode and origin. Comparisons of BFS group means were made by a one-way analysis of variance (ANOVA) and post-hoc Tukey test at α = 0.05.

RESULTS

All resin-coated sample groups (B-E) showed a statistically significant increase in mean BFS compared with the uncoated control (p < 0.01). There was a significant difference in BFS between the ambient and vacuum impregnated unpolished groups (D and E) (p < 0.01), with the greatest strengthening achieved using a vacuum impregnation technique.

SIGNIFICANCE

Results highlight the opportunity to further develop processes to apply thin conformal resin coatings, applied as a pre-cementation step to strengthen dental glass-ceramics.

Effect of the prosthetic index on stress distribution in Morse taper connection implant system and peri-implant bone: a 3D finite element analysis

Background

The combination of a prosthetic index with Morse taper connection was developed, with the purpose of making prosthetic procedures more precise. However, the presence of the index may compromise the mechanical performance of the abutment. The aim of this study is to evaluate the effect of prosthetic index on stress distribution in implant–abutment-screw system and peri-implant bone by using the 3D finite element methodology.

Methods

Two commercial dental implant systems with different implant–abutment connections were used: the Morse taper connection with platform switching (MT-PS) implant system and the internal hex connection with platform matching (IH-PM) implant system. Meanwhile, there are two different designs of Morse taper connection abutment, namely, abutments with or without index. Consequently, three different models were developed and evaluated: (1) MT-PS indexed, (2) MT-PS non-indexed, and (3) IH-PM. These models were inserted into a bone block. Vertical and oblique forces of 100 N were applied to each abutment to simulate occlusal loadings.

Results

For the MT-PS implant system, the maximum stress was always concentrated in the abutment neck under both vertical and oblique loading. Moreover, the maximum von Mises stress in the neck of the MT-PS abutment with index even exceed the yield strength of titanium alloy under the oblique loading. For the IH-PM implant system, however, the maximum stress was always located at the implant. Additionally, the MT-PS implant system has a significantly higher stress level in the abutment neck and a lower stress level around the peri-implant bone compared to the IH-PM implant system. The combined average maximum stress from vertical and oblique loads is 2.04 times higher in the MT-PS indexed model, and 1.82 times for the MT-PS non-indexed model than that of the IH-PM model.

Conclusions

MT-PS with index will cause higher stress concentration on the abutment neck than that of without index, which is more prone to mechanical complications. Nevertheless, MT-PS decreases stress within cancellous bone and may contribute to limiting crestal bone resorption.

Fracture analysis of one/two-piece clinically failed zirconia dental implants

OBJECTIVES

Analyzing factors that may have led to fracture of zirconia implants by macro/micro-fractography.

METHODS

Six one-piece and ten two-piece full-ceramic zirconia implants from two manufacturers, Z-Systems and CeraRoot, were retrieved after clinical failure. The time-to-failure ranged from 3 to 49 months. Optical and scanning electron microscopy (SEM) were used to analyze the fracture planes at the macro- and microscopic level. Treatment planning, surgical protocol, fracture-origin location and characteristic fracture features were assessed.

RESULTS

The fracture of all implants seemed to have been primarily due to overload in bending mode, while the fracture-initiation sites varied for the one- and two-piece implants. The fracture of all one-piece implants originated in the constriction region between two threads in the endosseous implant part. For two-piece implants, the abutment neck, internal abutment-implant connections and inner threads were found to be the main fracture-initiation sites. Surface defects at the root area for one-piece implants and damages at the abutment surface for two-piece implants were connected to the fracture origins. Importantly, the clinical failures of implants were often found to result from combined effects related to patient aspects, treatment planning/protocols, a high bending moment at the weakest link, implant-surface conditions and specific implant designs.

SIGNIFICANCE

This study provided information to be considered for future optimization of treatment planning and the surgical protocol for zirconia implants. Optimization of the surface conditions and the zirconia-starting powder were also suggested.

Comparison of porcelain veneer fracture in implant-supported fixed full-arch prostheses with a framework of either titanium, cobalt-chromium, or zirconia: An in vitro study

Objectives

The aim of this study was to compare porcelain veneer strength on screw‐retained implant‐supported fixed full‐arch prostheses with a framework of either milled titanium (Ti), cobalt–chromium (CoCr), and yttria‐stabilized zirconia (Y‐TZP) in an in vitro loading model.

Materials and Methods

Fifteen screw‐retained maxillary implant‐supported full‐arch prostheses (FDP), five each of Ti, CoCr, and Y‐TZP frameworks with porcelain veneers were included. All FDPs were subjected to thermocycling before loading until fracture of the veneer. The load was applied at the distal fossa of the occlusal area of the pontic replacing 24. Fracture loads were analyzed, and the fracture quality was assessed. Statistical analysis on the fracture load was performed using Kruskal–Wallis test. The statistical significance was set at p < .05.

Results

There was no statistical significance found between the groups regarding fracture load. The highest and lowest load was seen within the CoCr FDP, varying between 340 and 1484 N. Different types of fracture appearances were seen. The Y‐TZP FDPs had a higher number of fractures locally in the loaded area while CoCr and Ti more often showed cracks in the anterior region, at a distance from the loaded area.

Conclusions

Within the limitations of this study, the conclusion was that framework material may affect the fracture behavior of maxillary full‐arch bridges; however, there were no differences in veneer fracture strength when frameworks of Ti, CoCr, or Y‐TZP were compared.

Influence of the foundation substrate on the fatigue behavior of bonded glass, zirconia polycrystals, and polymer infiltrated ceramic simplified CAD-CAM restorations

This study evaluated the influence of distinct substrates on the mechanical fatigue behavior of adhesively cemented simplified restorations made of glass, polycrystalline or polymer infiltrated-ceramics. CAD/CAM ceramic blocks (feldspathic - FEL; lithium disilicate - LD; yttria-stabilized zirconia - YZ; and polymer-infiltrated ceramic network - PICN) were shaped into discs (n = 15, Ø = 10 mm; thickness = 1.0 mm), mimicking a simplified monolithic restoration. After, they were adhesively cemented onto different foundation substrates (epoxy resin - ER; or Ni-Cr metal alloy - MA) of the same shape (Ø = 10 mm; thickness = 2.0 mm). The assemblies were subjected to fatigue testing using a step-stress approach (200N-2800 N; step-size of 200 N; 10,000 cycles per step; 20 Hz) upon the occurrence of a radial crack or fracture. The data was submitted to two-way ANOVA (α = 0.05) to analyze differences considering 'ceramic material' and 'type of substrate' as factors. In addition, a survival analysis (Kaplan Meier with Mantel-Cox log-rank post-hoc tests; α = 0.05) was conducted to obtain the survival probability during the steps in the fatigue test. Fractographic and finite element (FEA) analyzes were also conducted. The factors 'ceramic material', 'type of substrate' and the interaction between both were verified to be statistically significant (p < .001). All evaluated ceramics presented higher fatigue failure load (FFL), cycles for failure (CFF) and survival probabilities when cemented to the metallic alloy substrate. Among the restorative materials, YZ and LD restorations presented the best fatigue behavior when adhesively cemented onto the metallic alloy substrate, while FEL obtained the lowest FFL and CFF for both substrates. The LD, PICN and YZ restorations showed similar fatigue performance considering the epoxy resin substrate. A more rigid foundation substrate improves the fatigue performance of adhesively cemented glass, polycrystalline and polymer infiltrated-ceramic simplified restorations.

Fractal analysis at varying locations of clinically failed zirconia dental implants

OBJECTIVES

Previous studies have shown that the fracture toughness of ceramics can be determined from the fractal dimensions (D) of their fracture surfaces and that the surface should be leveled to obtain an accurate D measurement. This study was to determine the effects of leveling operations and distance from the failure origin on the D values.

METHODS

Twelve clinically failed zirconia implants from four different manufacturers: Axis Biodental (n=7), Z-Systems (n=3), Straumann (n=1), and Swiss Dental Solutions (n=1) were obtained from one of the authors and thoroughly cleaned. Epoxy replicas were made of three locations along the crack path in the center region of each fracture surface (near origin (O), hackle (H), and near compression curl (CC)) using a light body polyvinyl siloxane impression material. Surfaces were scanned in ScanAsyst mode with a scan size of 5μm×5μm and a scan rate of 0.592Hz using the atomic force microscope. The surface scans were then leveled using 1st order flattening operation in the AFM analysis software. The height data before and after the operation were imported into a custom MathCAD script, and FRACTALS software was used to determine the D value by Minkowski Cover algorithm, which was shown previously to be the algorithm with the highest precision. A Wilcoxon signed-rank test, two-way repeated-measures ANOVA, and one-way repeated-measures ANOVA were performed as detailed below.

RESULTS

The data were not normally distributed (S-W p≤0.05), so a non-parametric repeated measures test (Wilcoxon signed-rank test) was selected. The median D values before and after leveling were 2.161 and 2.174, respectively. There was a significant difference before and after leveling (p<0.001). The two-way repeated-measures ANOVA showed no significant difference among the D values for different implant brands (p=0.66) and scanning locations on the fracture surface (p=0.83). After eliminating the implant brand as a factor, the data passed normality and equal variance tests (S-W p=0.88, BF p=0.15). The mean D values and standard deviations from the three locations (O, H, CC) were 2.183±0.031, 2.179±0.024, and 2.175±0.018, respectively. One-way repeated measures ANOVA showed no significant effect of scanning location (p=0.74).

SIGNIFICANCE

The leveling operation successfully removed the tilt without decreasing surface tortuosity, as it increased the D values significantly. The fractal dimension was the same at the three locations on the fracture surfaces. This means that hackle and compression curl regions can be used to determine fracture toughness when the failure origin has been lost.

Endodontic Access Effect on Full Contour Zirconia and Lithium Disilicate Failure Resistance

Objectives:

To evaluate the effect of endodontic access on the failure load resistance of both adhesively and conventionally luted, full-contour monolithic yttria-stabilized zirconium dioxide (Y-TZP) and adhesively luted lithium disilicate (LD) crowns cemented on prepared teeth.

Methods and Materials:

Seventy-two human maxillary molars were prepared per respective guidelines for all-ceramic crowns with one group (n=24) restored with LD and the other (n=48) receiving Y-TZP crowns. Preparations were scanned using computer-aided design/computer-aided milling (CAD/CAM) technology, and milled crowns were sintered following manufacturer recommendations. All LD crowns and half (n=24) of the Y-TZP crowns were adhesively cemented, while the remaining Y-TZP specimens were luted using a conventional glass ionomer cement (GIC). One LD group, one Y-TZP adhesive group, and one GIC-luted group (all n=12) then received endodontic access preparations by a board-certified endodontist: the pulp chambers were restored with a dual-cure, two-step, self-etch adhesive and a dual-cure resin composite core material. The access preparations were restored using a nano-hybrid resin composite after appropriate ceramic margin surface preparation. After 24 hours, all specimens were loaded axially until failure; mean failure loads were analyzed using Mann-Whitney U test (α=0.05)

Results:

Endodontic access did not significantly reduce the failure load of adhesively luted LD or Y-TZP crowns, but Y-TZP crowns with GIC cementation demonstrated significantly less failure load.

Conclusions:

These initial findings suggest that endodontic access preparation may not significantly affect failure load resistance of adhesively luted Y-TZP and LD crowns. Definitive recommendations cannot be proposed until fatigue testing and coronal seal evaluations have been accomplished.

Modulus of Elasticity of Two Ceramic Materials and Stress-Inducing Mechanical Deformation following Fabrication Techniques and Adhesive Cementation Procedures of a Dental Ceramic

Statement of Problem

Fabrication technique, precementation, and cementation operative procedures can induce significant modification of the stressing patterns throughout the thickness of some classes of dental ceramic materials.

Objectives

To estimate, by means of the deflection test, residual stress in restorative dental ceramic following fabrication technique, precementation, and resin cement coating procedures and to relate it to the elastic property of the ceramic material tested.

Materials and Methods

From IPS e.max® Press, lithium disilicate heat-pressed glass-ceramic (elastic modulus of 95 ± 5 GPa) disc-shaped specimens (n = 10) were made according to the manufacturer's instructions. One surface of the specimens was polished to provide accurate baseline profilometric measurements (reference surface). Deflection measurements were performed after polishing and annealing alumina air-particle abrasion of the unpolished surface followed by resin cement coating of the alumina air-particle abraded surface. The specimens were reprofiled at 24, 48, and 168 hrs after coating. The Friedman test followed by Dunn's multiple comparison test was employed to identify significant differences (p < 0.05). To compare the difference in mean of maximum mechanical deflection, after cement coating at 0 hr, between two different ceramic materials (IPS e.max Press and Vitadur Alpha (result from another study)), Student's t-test for unpaired data was performed.

Results

Baseline profilometric measurements identified a convex form on the polished surface of the ceramic discs with a mean of maximum mechanical deflection of 4.45 ± 0.87 μm. A significant reduction in convexity of the polished specimens was characterized after alumina air-particle abrasion of the unpolished surface. The mean deflection significantly increased after resin cement coating and did not change over the time investigated.

Conclusions

The precementation treatment, namely, alumina air-particle abrasion and cementation procedure of IPS e.max® Press glass-ceramic disc-shaped specimens generates stress that induced mechanical deformation. However, a dental ceramic material with higher elastic modulus (stiffer) would minimize stress-inducing mechanical deformation.

Shear Bond Strength of Ceramic Bonded to Different Core Materials and Their Pattern of Failure: An In Vitro Study

Introduction

In metal-ceramic restoration, most of the bond failures between the ceramic layer and the metal coping is the chipping of the ceramic layer, thus exposing the metal surface, which compromises the aesthetics. Hence, this leads to the introduction of zirconia-based restorations in dentistry. However, even zirconium coping has the common complication of delamination or porcelain chipping from the zirconium core. Hence, the shear bond strength between the commonly used core materials and ceramic requires investigation to facilitate the materials in clinical use for longevity. Therefore, this study was conducted to compare the shear bond strength between different core materials and ceramic layering to find out the best core material for ceramic bonding.

Materials and methods

A total number of 45 samples were made as per ISO standardization (base 5 mm diameter and 1 mm thickness, step with 4 mm diameter and 4 mm in length). These samples were divided into three groups, Group A: Nickel-chromium, Group B: Cobalt-chromium, and Group C: Zirconium. Ceramic layering was layered on the top surface of each sample until an ideal height of 4 mm was obtained, and it was subjected to shear bond strength using a universal testing machine with a 50-KN load cell. This was followed by analyzing the nature of the fracture pattern using scanning electron microscopy (SEM).

Results

 There were no significant differences found for the shear bond strength among group A and group B. The zirconium (group C), however, had significantly lower values than both group A and group B. The microscopic examination also revealed that the failure between the coping and the ceramic layer primarily occurred near the interface with the residual veneering porcelain remaining on the core.

Conclusions

It was found that the shear bond strength of the metal-ceramic group is better than the zirconium ceramic group, however, the fracture between the copings and the ceramic layering is found to be similar for both adhesive and cohesive failure.

The Influence of Mixing Methods on the Compressive Strength and Fluoride Release of Conventional and Resin-Modified Glass Ionomer Cements

Objective

To evaluate the compressive strength and fluoride ion release of conventional and resin-modified glass ionomer cement mixing methods (hand mix and mechanical mix) compared to ready-to-use ones.

Materials and Methods

Two conventional glass ionomer cements (GICs) (Fuji II and Fuji II Caps), two resin-modified GICs (Fuji II LC and Fuji II L Caps), and one ready-to-use GIC (Ionoseal, Voco) were used. For the compressive strength test, cylindrical specimens (6 mm × 4 mm) of each group were prepared. The test was performed in a universal testing machine (EMIC DL2000). For the fluoride release test, specimens were prepared in the form of discs and placed in deionized/distilled water, which were replaced daily for 15 days. The fluoride ion release readings were performed on an electrode (Orion 96-09) connected to a digital ion analyzer (Quimis 0400ISE). The compressive strength data were analyzed with one-way ANOVA, and the ion release data were submitted to repeated measures ANOVA (material vs. time) and Holm-Sidak post test (α = 5%).

Results

The one-way ANOVA showed statistical difference between the tested materials (p < 0.001). Ionoseal showed the highest values of compressive strength (p < 0.001). Mechanical manipulation increased the compressive strength only for conventional GIC, and resin-modified GIC did not present any statistical difference. Conventional GIC (mechanical mix) showed higher fluoride release on first day than the other groups tested.

Conclusion

There was influence of the mixing methods of the materials on the compressive strength and fluoride release pattern of the glass ionomer cements.

Fracture Resistance of Various Thickness e.max CAD Lithium Disilicate Crowns Cemented on Different Supporting Substrates: An In Vitro Study

PURPOSE

To investigate the influence of abutment material properties on the fracture resistance and failure mode of lithium disilicate (IPS e.max) CAD/CAM (computer-aided design/manufacturing) crowns on traditionally and minimally prepared simulated tooth substrates.

MATERIALS AND METHODS

Thirty lithium disilicate (IPS e.max) CAD/CAM crowns were divided into three groups (n = 10): TD: traditional thickness crowns cemented on Paradigm MZ100 abutments; MD: minimal thickness crowns cemented on Paradigm MZ100 abutments; ME: minimal thickness crowns cemented on e.max abutments. The 3Shape system was used to scan, design and mill all abutments and crowns with a die space set to 40 µm. Traditional thickness crowns were designed based on manufacturer guidelines with 1.5 mm occlusal thickness and 1.0 mm margins. Minimal thickness crowns were designed with 0.7 mm occlusal thickness and 0.5 mm margins. MZ100 composite and e.max abutments were selected to simulate dentin and enamel substrates, respectively, based on their elastic-modulus. Variolink Esthetic was used to cement all samples following manufacturer's instructions. A universal testing machine was used to load all specimens to fracture with a 3 mm radius stainless steel hemispherical tip at a crosshead speed 0.5 mm/minute along the longitudinal axis of the abutment with a 1 mm thermoplastic film placed between the loading tip and crown surface. Data was analyzed using ANOVA and Bonferroni post hoc assessment. Fractographic analysis was performed with scanning electron microscopy (SEM).

RESULTS

The mean fracture load (standard deviation) was 1499 (241) N for TD; 1228 (287) N for MD; and 1377 (96) N for ME. Statistically significant difference between groups did not exist (p = 0.157, F = 1.995). In groups TD and MD with low e-modulus abutments, the dispersion of a probability distribution (coefficient of variation: CV) was statistically higher than that of group ME with high e-modulus abutments. SEM illustrated larger micro-fracture dimensions in Group MD than Group ME.

CONCLUSION

Minimal thickness e.max crowns did not demonstrate statistical difference in fracture resistance from traditional thickness crowns. Fracture mechanisms of minimal thickness e.max crowns may be affected by the e-modulus of the substrate. Minimal thickness e.max crowns may be a viable restorative option when supported by high e-modulus materials.

Advancements in all-ceramics for dental restorations and their effect on the wear of opposing dentition

Tooth wear is a process that is usually a result of tooth to tooth and/or tooth and restoration contact. The process of wear essentially becomes accelerated by the introduction of restorations inside the oral cavity, especially in case of opposing ceramic restorations. The newest materials have vastly contributed toward the interest in esthetic dental restorations and have been extensively studied in laboratories. However, despite the recent technological advancements, there has not been a valid in vivo method of evaluation involving clinical wear caused due to ceramics upon restored teeth and natural dentition. The aim of this paper is to review the latest advancements in all-ceramic materials, and their effect on the wear of opposing dentition. The descriptive review has been written after a thorough MEDLINE/PubMed search by the authors. It is imperative that clinicians are aware of recent advancements and that they should always consider the type of ceramic restorative materials used to maintain a stable occlusal relation. The ceramic restorations should be adequately finished and polished after the chair-side adjustment process of occlusal surfaces.

Ten-year survival of pressed, acid-etched e.max lithium disilicate monolithic and bilayered complete-coverage restorations: Performance and outcomes as a function of tooth position and age

STATEMENT OF PROBLEM

Long-term clinical data on the survival of pressed lithium disilicate glass-ceramic restorations and the effect that different technical and clinical variables have on survival are lacking.

PURPOSE

The purpose of this clinical study was to examine the 10-year survival of pressed lithium disilicate glass-ceramic restorations and the relationship between clinical parameters on outcomes.

MATERIAL AND METHODS

Five hundred and fifty-six patients, ranging in age from 17 to 97 years, from a private clinical practice were enrolled. All participants required single-tooth replacement or repair in any area of the mouth, including single crowns, 3-unit fixed partial dentures, cantilevered anterior restorations, and foundation restorations. Together, the longevity of 1960 complete-coverage restorations was studied. Participants were offered the options of gold, conventional metal-ceramic, or lithium disilicate restoration. Participants who chose glass-ceramic restorations were included in the study. The overall survival of the glass-ceramic restorations was assessed by using clinical factors determined at recall, and the effect of various clinical parameters was evaluated by using Kaplan-Meier survival curves to account for attrition bias and other reasons for failure. The statistical significance of differences between parameters was determined using the log-rank test (α=.05).

RESULTS

A total of 556 patients electing lithium disilicate restorations were evaluated. The mean age of patients at the time of restoration placement was 62 years, with a range of 17 to 97 years. Men comprised 39.5% of the patients, and women, 60.5%. Many patients required more than one restoration. Seven failures (bulk fracture or large chip requiring replacement) were recorded for the 1960 complete-coverage lithium disilicate restorations, with the average time of failure being 4.2 years. The total time at risk computed for the units was 5113 years, providing an estimated failure risk of 0.14% per year. The 10-year estimated cumulative survival was 99.6% (95% confidence : 99.4-99.8).The estimated cumulative survival rate of 1410 monolithic and 550 bilayered e.max complete-coverage restorations was 96.5% and 100%, respectively, at 10.4 and 7.9 years (P<.05). Seven failures were recorded for the monolithic complete-coverage restoration units placed. The total time at risk for these monolithic units was 3380 years, providing an estimated risk of 0.2% per year. Failures were primarily in molar teeth (5 of 7) and occurred in both arches (3/2). No failures were recorded for the bilayered complete-coverage restorations. The total time at risk computed for the bilayered units was 1733 years, providing an estimated risk of 0% per year.

CONCLUSIONS

Pressed lithium disilicate restorations in this study survived successfully over the 10.4-year period studied with an overall failure rate below 0.2% per year and were primarily confined to molar teeth. The risk of failure at any age was minimal for both men and women.

Chipping of veneering ceramic on a lithium disilicate anterior single crown: Description of repair method and a fractographic failure analysis

OBJECTIVE

This article presents a retrospective analysis of an anterior single crown that showed chipping of the veneering ceramic, the clinical stages of intraoral repair made in composite resin, and fractographic analysis of the causes of failure.

CLINICAL CONSIDERATIONS

The ceramic chipping occurred in the incisal and labial surfaces of the crown, 1 year after installation. Clinical examination revealed the presence of occlusal interference, which was probably responsible for chipping. Vinyl-polysiloxane impression was made from the patient, and epoxy replica was produced. The replica was gold coated and inspected under the optical microscopy and scanning electron microscope (SEM) for descriptive fractography. Optical microscopy and SEM images showed that chipping initiated at the incisal edge, where it is possible to note an area of damage accumulation. At the labial surface, multiple arrest lines with their convex sides facing the incisal edge were observed. The fractured area was repaired intraorally with composite resin, and the patient's occlusion was checked and monitored.

CONCLUSION

According to the fractographic analysis, occlusal interference was related to ceramic chipping in the incisal edge. Intraoral repair technique with composite resin was indicated for this moderate chipping.

CLINICAL SIGNIFICANCE

Retrieval analysis of chipping ceramic delivers better understanding of the failure origin and could prevent future failures. Intraoral repair is a practical and conservative technique and may be performed in a single clinical session without requiring the removal of prosthesis.

An evidence-based evaluation of contemporary dental ceramics

Abstract: Based on data from three recently published laboratory surveys with large numbers of different types of contemporary ceramic restorations, specific indications and contra-indications are given. The indications are based on longevity data, aesthetic expectations, tooth position, level of parafunctional activity, tooth reduction requirements, and potential wear of the opposing dentition.

CPD/Clinical Relevance: This article provides an evidence-based guide for clinicians to use when placing contemporary ceramic restorations. The article details which specific ceramic restorations are indicated in specific clinical situations, based on data from laboratory surveys and clinical parameters.

Fatigue failure load of an adhesively-cemented lithium disilicate glass-ceramic: Conventional ceramic etching vs etch & prime one-step primer

OBJECTIVES

To evaluate the effect of different glass-ceramic surface treatments and aging on the fatigue failure load of a lithium disilicate glass-ceramic adhesively cemented to a dentin analogue material.

METHODS

One hundred and twenty (120) disc-shaped lithium disilicate specimens (Ø=10mm, thickness=1.5mm) were produced and randomly allocated (n=20) into 6 groups, considering 2 study factors: "surface treatment" in 3 levels (SIL-silane application only; HF5+SIL-5% hydrofluoric acid etching and silane application; ME&P-etching with an one-step ceramic primer), and "storage" in 2 levels (baseline-storage for 7 days; aging-storage for 90 days+12,000 thermal cycles). Ceramic discs were adhesively cemented to discs of a dentin analogue material (Ø=10mm, thickness=2.0mm) following the manufacturers' instructions. The fatigue failure load was determined by the staircase approach (250,000 cycles; 20Hz; initial load=1050N [∼70% of mean load-to-failure]; step size=52.5N [5% of initial load]). Micro-morphologic, fractographic, and atomic force microscope analysis were also performed. Fatigue failure load data were evaluated by one-way ANOVA, Bonferroni and t-tests for independent samples.

RESULTS

HF5+SIL presented higher fatigue failure load in both conditions (baseline and aging); ME&P presented intermediary mean values, while the SIL group presented the worst performance. All groups had a statistically significant decrease in the fatigue performance after aging.

SIGNIFICANCE

Hydrofluoric acid followed by silane application showed the best fatigue performance for an adhesively-cemented lithium disilicate ceramic. Aging negatively influenced the fatigue performance for all tested groups.

Analysis of flexural strength and contact pressure after simulated chairside adjustment of pressed lithium disilicate glass-ceramic

STATEMENT OF PROBLEM

Research evaluating load-to-failure of pressed lithium disilicate glass-ceramic (LDGC) with a clinically validated test after adjustment and repair procedures is scarce.

PURPOSE

The purpose of this in vitro study was to investigate the effect of the simulated chairside adjustment of the intaglio surface of monolithic pressed LDGC and procedures intended to repair damage.

MATERIAL AND METHODS

A total of 423 IPS e.max Press (Ivoclar Vivadent AG) disks (15 mm diameter, 1 mm height) were used in the study. The material was tested by using an equibiaxial loading arrangement (n≥30/group) and a contact pressure test (n≥20/group). Specimens were assigned to 1 of 14 groups. One-half was assigned to the equibiaxial load test and the other half underwent contact pressure testing. Testing was performed in 2 parts, before glazing and after glazing. Before-glazing specimens were devested and entered in the test protocol, while after-glazing specimens were devested and glazed before entering the test protocol. Equibiaxial flexure test specimens were placed on a ring-on-ring apparatus and loaded until failure. Contact pressure specimens were cemented to epoxy resin blocks with a resin cement and loaded with a 50-mm diameter hemisphere until failure. Tests were performed on a universal testing machine with a crosshead speed of 0.5 mm/min. Weibull statistics and likelihood ratio contour plots determined intergroup differences (95% confidence bounds).

RESULTS

Before glazing, the equibiaxial flexural strength test and the Weibull and likelihood ratio contour plots demonstrated a significantly higher failure strength for 1EC (188 MPa) than that of the damaged and/or repaired groups. Glazing following diamond-adjustment (1EGG) was the most beneficial post-damage procedure (176 MPa). Regarding the contact pressure test, the Weibull and likelihood ratio contour plots revealed no significant difference between the 1PC (98 MPa) and 1PGG (98 MPa) groups. Diamond-adjustment, without glazing (1EG and 1PG), resulted in the next-to-lowest equibiaxial flexure strength and the lowest contact pressure. After glazing, the strength of all the groups, when subjected to glazing following devesting, increased in comparison with corresponding groups in the before-glazing part of the study.

CONCLUSIONS

A glazing treatment improved the mechanical properties of diamond-adjusted IPS e.max Press disks when evaluated by equibiaxial flexure and contact pressure tests.

The effect of hydrofluoric acid concentration on the fatigue failure load of adhesively cemented feldspathic ceramic discs

OBJECTIVE

This study investigated the influence of hydrofluoric acid (HF) etching at different concentrations on the fatigue failure load of adhesively cemented feldspathic ceramic discs (Vita Mark II). Besides, their effect on the micromorphology of ceramic surface was investigated.

METHODS

Eighty ceramic discs (ϕ=10 mm; thickness=1.5 mm) were cemented to epoxy supporting discs (ϕ=10 mm; thickness=2.0mm) using different surface conditioning methods (n=20): nonetched control (CTRL), or etched for 60s with different HF concentrations: 1% (HF1), 5% (HF5), or 10% (HF10). All the ceramic discs received a silane application (Monobond Plus). The epoxy discs were etched with 10% HF for 60s and received a primer coating (Multilink Primer A+B). Adhesively cementation was performed (Multilink Automix), and the assemblies (ceramic discs/epoxy discs) were subjected to cyclic loads in water by a staircase approach (500,000 cycles; 20Hz; initial load=290N; step size=30N). Fatigue failure load data were analyzed using 1-way ANOVA and post-hoc Tukey's tests (α=.05).

RESULTS

Mean failure load of the HF5 group (255.0±23.0N) was significantly lower; HF1 group (301.7±71.0N) presented intermediate values, and the highest values were achieved in CTRL (351.7±13.4N) and HF10 (341.7±20.6N) groups. All the failures were radial cracks starting from the bonding surface.

SIGNIFICANCE

In terms of fatigue failure load, etching with 1% and 5% HF had a deleterious effect on the fatigue behavior of an adhesively cemented feldspathic ceramic, while 10% HF had no negative influence.

Stress distribution pattern of screw-retained restorations with segmented vs. non-segmented abutments: A finite element analysis

Background. Screw-retained restorations are favored in some clinical situations such as limited inter-occlusal spaces. This study was designed to compare stresses developed in the peri-implant bone in two different types of screw-retained restorations (segmented vs. non-segmented abutment) using a finite element model. Methods. An implant, 4.1 mm in diameter and 10 mm in length, was placed in the first molar site of a mandibular model with 1 mm of cortical bone on the buccal and lingual sides. Segmented and non-segmented screw abutments with their crowns were placed on the simulated implant in each model. After loading (100 N, axial and 45° non-axial), von Mises stress was recorded using ANSYS software, version 12.0.1. Results. The maximum stresses in the non-segmented abutment screw were less than those of segmented abutment (87 vs. 100, and 375 vs. 430 MPa under axial and non-axial loading, respectively). The maximum stresses in the peri-implant bone for the model with segmented abutment were less than those of non-segmented ones (21 vs. 24 MPa, and 31 vs. 126 MPa under vertical and angular loading, respectively). In addition, the micro-strain of peri-implant bone for the segmented abutment restoration was less than that of non-segmented abutment. Conclusion. Under axial and non-axial loadings, non-segmented abutment showed less stress concentration in the screw, while there was less stress and strain in the peri-implant bone in the segmented abutment.

ADM guidance-Ceramics: guidance to the use of fractography in failure analysis of brittle materials

OBJECTIVES

To provide background information and guidance as to how to use fractography accurately, a powerful tool for failure analysis of dental ceramic structures.

METHODS

An extended palette of qualitative and quantitative fractography is provided, both for in vivo and in vitro fracture surface analyses. As visual support, this guidance document will provide micrographs of typical critical ceramic processing flaws, differentiating between pre- versus post sintering cracks, grinding damage related failures and occlusal contact wear origins and of failures due to surface degradation.

RESULTS

The documentation emphasizes good labeling of crack features, precise indication of the direction of crack propagation (dcp), identification of the fracture origin, the use of fractographic photomontage of critical flaws or flaw labeling on strength data graphics. A compilation of recommendations for specific applications of fractography in Dentistry is also provided.

SIGNIFICANCE

This guidance document will contribute to a more accurate use of fractography and help researchers to better identify, describe and understand the causes of failure, for both clinical and laboratory-scale situations. If adequately performed at a large scale, fractography will assist in optimizing the methods of processing and designing of restorative materials and components. Clinical failures may be better understood and consequently reduced by sending out the correct message regarding the fracture origin in clinical trials.

Microstructural characterization of dental zinc phosphate cements using combined small angle neutron scattering and microfocus X-ray computed tomography

OBJECTIVE

To characterize the microstructure of two zinc phosphate cement formulations in order to investigate the role of liquid/solid ratio and composition of powder component, on the developed porosity and, consequently, on compressive strength.

METHODS

X-ray powder diffraction with the Rietveld method was used to study the phase composition of zinc oxide powder and cements. Powder component and cement microstructure were investigated with scanning electron microscopy. Small angle neutron scattering (SANS) and microfocus X-ray computed tomography (XmCT) were together employed to characterize porosity and microstructure of dental cements. Compressive strength tests were performed to evaluate their mechanical performance.

RESULTS

The beneficial effects obtained by the addition of Al, Mg and B to modulate powder reactivity were mitigated by the crystallization of a Zn aluminate phase not involved in the cement setting reaction. Both cements showed spherical pores with a bimodal distribution at the micro/nano-scale. Pores, containing a low density gel-like phase, developed through segregation of liquid during setting. Increasing liquid/solid ratio from 0.378 to 0.571, increased both SANS and XmCT-derived specific surface area (by 56% and 22%, respectively), porosity (XmCT-derived porosity increased from 3.8% to 5.2%), the relative fraction of large pores ≥50μm, decreased compressive strength from 50±3MPa to 39±3MPa, and favored microstructural and compositional inhomogeneities.

SIGNIFICANCE

Explain aspects of powder design affecting the setting reaction and, in turn, cement performance, to help in optimizing cement formulation. The mechanism behind development of porosity and specific surface area explains mechanical performance, and processes such as erosion and fluoride release/uptake.

The Strengthening Mechanism of Resin Cements on Porcelain Surfaces

All-ceramic crowns bonded with resin cements have increased performance, and two theories have been proposed. Marquis (1992) suggested that the resin modified defects by crack healing, while Nathanson (1993) proposed that resin polymerization shrinkage strengthened porcelains. Both theories imply a sensitivity of strengthening to defect size. The hypothesis tested was that resin strength enhancement is independent of defect severity. We ground 200 porcelain discs to remove imperfections and indented 120 to create a large defect. Discs were tested dry, wet, and after being coated with 75–100 μm of resin cement in bi-axial flexure. Disc strength with and without indentations was increased significantly when coated with 2 resin cements. Both cements significantly increased the strength independent of defect population, and the hypothesis was accepted. It is proposed that the combination of surface pre-treatment and cement moved the fracture origin from the porcelain/cement interface to the cement surface, consistent with resin strength enhancement independent of defect severity.

Resin Elasticity and the Strengthening of All-ceramic Restorations

Resin luting of all-ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three-point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain discs were air-abraded. One group acted as a control, and 3 additional groups were coated with 120 ± 20 μm of each resin prior to bi-axial flexure testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R2 = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.

3D statistical failure analysis of monolithic dental ceramic crowns

For adhesively retained ceramic crown of various types, it has been clinically observed that the most catastrophic failures initiate from the cement interface as a result of radial crack formation as opposed to Hertzian contact stresses originating on the occlusal surface. In this work, a 3D failure prognosis model is developed for interface initiated failures of monolithic ceramic crowns. The surface flaw distribution parameters determined by biaxial flexural tests on ceramic plates and point-to-point variations of multi-axial stress state at the intaglio surface are obtained by finite element stress analysis. They are combined on the basis of fracture mechanics based statistical failure probability model to predict failure probability of a monolithic crown subjected to single-cycle indentation load. The proposed method is verified by prior 2D axisymmetric model and experimental data. Under conditions where the crowns are completely bonded to the tooth substrate, both high flexural stress and high interfacial shear stress are shown to occur in the wall region where the crown thickness is relatively thin while high interfacial normal tensile stress distribution is observed at the margin region. Significant impact of reduced cement modulus on these stress states is shown. While the analyses are limited to single-cycle load-to-failure tests, high interfacial normal tensile stress or high interfacial shear stress may contribute to degradation of the cement bond between ceramic and dentin. In addition, the crown failure probability is shown to be controlled by high flexural stress concentrations over a small area, and the proposed method might be of some value to detect initial crown design errors.

The effect of silane applied to glass ceramics on surface structure and bonding strength at different temperatures

PURPOSE

To evaluate the effect of various surface treatments on the surface structure and shear bond strength (SBS) of different ceramics.

MATERIALS AND METHODS

288 specimens (lithium-disilicate, leucite-reinforced, and glass infiltrated zirconia) were first divided into two groups according to the resin cement used, and were later divided into four groups according to the given surface treatments: G1 (hydrofluoric acid (HF)+silane), G2 (silane alone-no heat-treatment), G3 (silane alone-then dried with 60℃ heat-treatment), and G4 (silane alone-then dried with 100℃ heat-treatment). Two different adhesive luting systems were applied onto the ceramic discs in all groups. SBS (in MPa) was calculated from the failure load per bonded area (in N/mm²). Subsequently, one specimen from each group was prepared for SEM evaluation of the separated-resin–ceramic interface.

RESULTS

SBS values of G1 were significantly higher than those of the other groups in the lithium disilicate ceramic and leucite reinforced ceramic, and the SBS values of G4 and G1 were significantly higher than those of G2 and G3 in glass infiltrated zirconia. The three-way ANOVA revealed that the SBS values were significantly affected by the type of resin cement (P<.001). FIN ceramics had the highest rate of cohesive failure on the ceramic surfaces than other ceramic groups. AFM images showed that the surface treatment groups exhibited similar topographies, except the group treated with HF.

CONCLUSION

The heat treatment was not sufficient to achieve high SBS values as compared with HF acid etching. The surface topography of ceramics was affected by surface treatments.

Fractographic principles applied to Y-TZP mechanical behavior analysis

The purpose of this study was to evaluate the use of fractography principles to determine the fracture toughness of Y-TZP dental ceramic in which KIc was measured fractographically using controlled-flaw beam bending techniques and to correlate the flaw distribution with the mechanical properties. The Y-TZP blocks studied were: Zirconia Zirklein (ZZ); Zirconcad (ZCA); IPS e.max ZirCad (ZMAX); and In Ceram YZ (ZYZ). Samples were prepared (16mm×4mm×2mm) according to ISO 6872 specifications and subjected to three-point bending at a crosshead speed of 0.5mm/min. Weibull probability curves (95% confidence bounds) were calculated and a contour plot with the Weibull modulus (m) versus characteristic strength (σ0) was used to examine the differences among groups. The fractured surface of each specimen was inspected in a scanning electron microscope (SEM) for qualitative and quantitative fractographic analysis. The critical defect size (c) and fracture toughness (KIc) were estimated. The fractured surfaces of the samples from all groups showed similar fractographic characteristics, except ZCA showed pores and defects. Fracture toughness and the flexural strength values were not different among the groups except for ZCA. The characteristic strength (p<0.05) of ZZ (η=920.4) was higher than the ZCA (η=651.1) and similar to the ZMAX (η=983.6) and ZYZ (η=1054.8). By means of quantitative and qualitative fractographic analysis, this study showed fracture toughness and strength that could be correlated to the observable microstructural features of the evaluated zirconia polycrystalline ceramics.

Esthetic Prosthetic Restorations: Reliability and Effects on Antagonist Dentition

Recent advances in ceramics have greatly improved the functional and esthetic properties of restorative materials. New materials offer an esthetic and functional oral rehabilitation, however their impact on opposing teeth is not welldocumented. Peer-reviewed articles published till December 2014 were identified through Pubmed (Medline and Elsevier). Scientifically, there are several methods of measuring the wear process of natural dentition which enhances the comparison of the complicated results. This paper presents an overview of the newly used prosthetic materials and their implication on antagonist teeth or prostheses, especially emphasizing the behavior of zirconia restorations.

Effect of etching time and resin bond on the flexural strength of IPS e.max Press glass ceramic

OBJECTIVE

To evaluate the effect of hydrofluoric acid (HFA) etching time and resin cement bond on the flexural strength of IPS e.max(®) Press glass ceramic.

METHODS

Two hundred and ten bars, 25mm×3mm×2mm, were made from IPS e.max(®) Press ingots through lost-wax, hot-pressed ceramic fabrication technology and randomly divided into five groups with forty-two per group after polishing. The ceramic surfaces of different groups were etched by 9.5% hydrofluoric acid gel for 0, 20, 40, 60 and 120s respectively. Two specimens of each group were selected randomly to examine the surface roughness and 3-dimensional topography with atomic force microscope (AFM), and microstructure was analyzed by the field emission scanning electron microscope (FE-SEM). Then each group were subdivided into two subgroups (n=20). One subgroup of this material was selected to receive a thin (approximately 0.1mm) layer of resin luting agent (Variolink N) whereas the other subgroup remained unaltered. Half of subgroup's specimens were thermocycled 10,000 times before a 3-point bending test in order to determine the flexural strength. Interface between resin cement and ceramic was examined with field emission scanning electronic microscope.

RESULTS

Roughness values increased with increasing etching time. The mean flexural strength values of group 0s, 20s, 40s, 60s and 120s were 384±33, 347±43, 330±53, 327±67 and 317±41MPa respectively. Increasing HF etching times reduced the mean flexural strength (p<0.05). However, the mean flexural strength of each group, except group 0s, increased significantly to 420±31, 435±50, 400±39 and 412±58MPa after the application of dual-curing resin cement. In the present investigation, no significant differences after thermocycling on the flexural strengths were evident.

SIGNIFICANCE

Overtime HF etching could have a wakening effect on IPS e.max(®) Press glass ceramic, but resin cement bonding to appropriately etched surface would strengthen the dental ceramic.

Effect of porcelain and enamel thickness on porcelain veneer failure loads in vitro

STATEMENT OF PROBLEM

Bonded porcelain veneers are widely used esthetic restorations. Although high success and survival rates have been reported, failures occur. Fracture is the most common failure mode. Fractures range from incomplete cracks to the catastrophic. Minimally invasive or thin partial veneers have gained popularity.

PURPOSE

The aim of this study was to measure the influences of porcelain veneer thickness and enamel substrate thickness on the loads needed to cause the initial fracture and catastrophic failure of porcelain veneers.

MATERIAL AND METHODS

Model discoid porcelain veneer specimens of varying thickness were bonded to the flattened facial surfaces of incisors, artificially aged, and loaded to failure with a small sphere. Individual fracture events were identified and analyzed statistically and fractographically.

RESULTS

Fracture events included initial Hertzian cracks, intermediate radial cracks, and catastrophic gross failure. Increased porcelain, enamel, and their combined thickness had like effects in substantially raising resistance to catastrophic failure but also slightly decreased resistance to initial Hertzian cracking. Fractographic and numerical data demonstrated that porcelain and tooth enamel behaved in a remarkably similar manner. As porcelain thickness, enamel thickness, and their combined thickness increased, the loads needed to produce initial fracture and catastrophic failure rose substantially. Porcelain veneers withstood considerable damage before catastrophic failure.

CONCLUSIONS

Increased enamel thickness, increased porcelain thickness, and increased combined enamel and porcelain thickness all profoundly raised the failure loads necessary to cause catastrophic failure. Enamel and feldspathic porcelain behaved in a like manner. Surface contact damage occurred initially. Final catastrophic failure followed flexural radial cracking. Bonded porcelain veneers were highly damage tolerant.

Influence of Fine Threads and Platform-Switching on Crestal Bone Stress Around Implant—A Three-Dimensional Finite Element Analysis

The aims of this study were to investigate the effect of implant fine threads on crestal bone stress compared to a standard smooth implant collar and to analyze how different abutment diameters influenced the crestal bone stress level. Three-dimensional finite element imaging was used to create a cross-sectional model in SolidWorks 2007 software of an implant (5-mm platform and 10 mm in length) placed in the premolar region of the mandible. The implant model was created to resemble a commercially available fine thread implant. Abutments of different diameters (5.0 mm: standard, 4.5 mm, 4.0 mm, and 3.5 mm) were loaded with a force of 100 N at 90° vertical and 40° oblique angles. Finite element analysis was done in COSMOSWorks software, which was used to analyze the stress patterns in bone, especially in the crestal region. Upon loading, the fine thread implant model had greater stress at the crestal bone adjacent to the implant than the smooth neck implant in both vertical and oblique loading. When the abutment diameter decreased progressively from 5.0 mm to 4.5 mm to 4 mm and to 3.5 mm the thread model showed a reduction of stress at the crestal bone level from 23.2 MPa to 15.02 MPa for fine thread and from 22.7 to 13.5 MPa for smooth collar implant group after vertical loading and from 43.7 MPa to 33.1 MPa in fine thread model and from 36.9 to 20.5 MPa in smooth collar implant model after oblique loading. Fine threads increase crestal stress upon loading. Reduced abutment diameter that is platform switching resulted in less stress translated to the crestal bone in the fine thread and smooth neck.

Tooth wear against ceramic crowns in posterior region: a systematic literature review

The objective of this systematic review was to assess tooth wear against ceramic crowns in posterior region in vitro and in vivo. An electronic PubMed search was conducted to identify studies on tooth wear against ceramic crowns in posterior region. The selected studies were analyzed in regard to type of crowns, natural antagonist, measuring protocol and outcome. From a yield of 1 000 titles, 43 articles were selected for full-text analysis; finally, no in vitro and only five in vivo studies met the inclusion criteria. As there is heterogeneity in design, used measuring method, ceramics and analysis-form, a meta-analysis was not possible. Results of these studies are very controversial which makes a scientifically valid comparison impossible. This review indicated that some all-ceramic crowns are as wear friendly as metal-ceramic crowns. Up to now, it has been impossible to associate tooth wear with any specific causal agent. The role of ceramic surface treatment that might be responsible for the changing in rate of tooth wear seems undetermined as yet through clinical trials. The literature reveals that studies on this topic are subject to a substantial amount of bias. Therefore, additional clinical studies, properly designed to diminish bias, are warranted.