Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review

Over 14-year survival of pressed e.max lithium disilicate glass-ceramic complete and partial coverage restorations in patients with severe wear: A prospective clinical study

STATEMENT OF PROBLEM

Long-term clinical data are lacking on the comparative survival of adhesively luted lithium disilicate glass-ceramic complete and partial coverage restorations in patients with severe wear and the effect that different clinical variables have on their survival.

PURPOSE

The purpose of this study was to examine the 14-year survival of pressed e.max lithium disilicate glass-ceramic complete and partial coverage restorations in patients with severe wear and to evaluate associated clinical parameters.

MATERIAL AND METHODS

Patients demonstrating severe wear and requiring single unit defect-specific partial or complete coverage restorations were recruited in a clinical private practice and received lithium disilicate restorations. The effect of various clinical parameters was evaluated using Kaplan-Meier survival curves to account for attrition bias and other causes of failure. The statistical significance of differences between parameters was determined by using the log rank test (α=.05).

RESULTS

A total of 53 participants (24 men and 29 women) requiring 662 lithium disilicate restorations were evaluated. The mean age of the participants at the time of restoration placement was 60 (range 25 to 85 years). Of 662 units, 551 were anterior and posterior complete and 111 were posterior partial coverage restorations. Six failures (bulk fracture or large chip) requiring replacement occurred with the average time to failure of 4.5 (range 2 to 8) years, yielding a crude estimate of annual failure of 0.1% (cumulative monitoring period of 4650 years, overall survival rate of 98.6%). Of the 551 complete coverage restorations, 4 failed (0 anterior, 4 posterior), yielding a crude estimate of 0.1% annual failure with the survival function time at 14 years. Of the 111 partial coverage restorations, 2 failed, yielding a crude estimate of 0.4% annual failure with the survival function time at 12 years. The only statistically significant difference was seen between anterior crowns and posterior onlays, with no failures in anterior crowns (P<.001). Other clinical variables had no significant effect on survival (P>.05).

CONCLUSIONS

Pressed e.max lithium disilicate partial and complete coverage restorations both showed high survival rates in patients with severe wear over 14 years with an overall yearly failure rate of 0.1%. Risk of failure at any age was minimal for both men and women. All anterior crowns survived. The highest annual failure rate (0.4%) was for posterior onlay partial coverage restorations.

Exploring the influence of placing bi-directional E-glass fibers as protective layer under a CAD-CAM resin composite on the fracture pattern

OBJECTIVES

To investigate the influence of the presence and position of bidirectional E-glass fibers under a CAD-CAM resin composite on the fracture pattern evaluated both after quasi-static mechanical loading and after fatigue.

METHODS

Rectangular specimens (10 mm-long, 5 mm-large and 4.2 mm-thick) were prepared and divided into four groups (n = 30/group). The control group (C-Group) consisted of a 4.2 mm-thick layer of monolithic CAD/CAM resin composite resin (Cerasmart, GC). In the 3 other groups including the placement of a fiber layer (F-Groups), the CAD/CAM resin composite layer was reduced to 3-, 2- and 1-mm thickness (F3-, F2- and F1-Groups, respectively). Two bonded layers of bidirectional E-glass FRC (Dentapreg, ADM A.S.) were bonded underneath and a light-curable resin composite base (Gaenial Posterior, GC) was then added to reach a total thickness of 4.2 mm for all samples. In each group, half of the specimens (n = 15) were submitted to quasi-static mechanical loading to failure in a universal testing machine. The other half (n = 15) was subjected to cyclic isometric stepwise loading until failure or completion of 105000 cycles (5000 cycles at 500 N, followed by five stages of 20000 cycles at 750 N, 1000 N, 1250 N, 1500 N, and 1750 N). The data were analyzed by Weibull statistics for quasi-static loading, and by the Kaplan-Meier product limit estimation procedure after fatigue. All fractured specimens were studied using light and electron microscopy techniques, and the types of fracture were determined.

RESULTS

For quasi-static mechanical loading, significant differences were observed for Weibull modulus and characteristic strength between groups, with values ranging from 10.8 to 22.4 for the former and from 2336.6 to 2974.7 for the latter. Also, survival after stepwise fatigue revealed statistically significant differences between groups (p < 0.05), the lowest values of cycles before failure being observed for F1-Group - Median = 61223 (50415; 65446) - as compared to the other groups - C-Group: Median = 89005 (86189; 98195); F3-Group: Median = 85198 (77279; 87860); F2-Group: Median = 89306 (87454; 97024). Both in quasi-static loading and after fatigue, the observation of fracture modes revealed major differences. While all fractures were vertical (split) in C-Group, the majority of the specimens in F-Groups presented some degree of horizontal deflection of the crack. In all deviated fractures, fractographic analysis confirmed a toughening effect of the fiber layer.

SIGNIFICANCE

The present in vitro work tends to show that the fracture pattern of CAD-CAM resin composites is favorably affected by the presence and position of an underlying bidirectional E-glass fiber layer. The placement of E-glass fibers under a CAD-CAM resin composite may therefore represent an interesting strategy to reduce the risk of catastrophic restoration failure, which could be integrated in the development of the new generation of indirect materials, possibly in 3D-printing approaches.

Should finishing, polishing or glazing be performed after grinding YSZ ceramics? A systematic review and meta-analysis

The present systematic review and meta-analysis aimed to assess the characteristics and consequences of post-processing methods after grinding procedures in YSZ ceramics on its surface roughness and flexural strength. The protocol of this review was made prospectively and is available online in the PROSPERO database (link). Literature searches on PubMed/MEDLINE, EMBASE, Lilacs, Web of Science and Scopus were conducted on December 2022 to select in vitro studies written in English, without publishing-date restrictions, that considered surface characteristics and mechanical properties of YSZ ceramics submitted to grinding and subsequent post-processing surface treatments as an attempt to revert the effect induced by grinding. Two authors independently selected the studies, extracted the data and assessed the risk of bias. Mean differences (Rev-Man 5.1, random effects model, α= 0.05) were obtained by comparing flexural strength and surface roughness values of ground surfaces with at least one post-processing surface treatment (global analysis). Subgroup analyses were performed considering the most prevalent categories of post-processing methods. A total of 33 (out of 4032) studies were eligible and included in the analysis. In the global analysis, ground surfaces showed higher flexural strength than when post-processing methodologies were employed (p< 0.0001). The subgroup analysis showed that only polishing was able to enhance the flexural strength after grinding (p= 0.001); however, when other protocols were used, the ground surface was always superior in terms of flexural strength (p< 0.0001). Post-processing techniques in both the global and sub-group analyses were able to reduce the surface roughness after grinding in YSZ ceramics (p< 0.00001). High heterogeneity was found in all the meta-analyses. Concerning the risk of bias analysis, the included studies had mixed scores for the considered factors. In conclusion, in terms of improving flexural strength and restoring surface roughness after grinding, polishing protocols can be considered the best indication as post-processing treatment after YSZ ceramics adjustments/grinding.

Incidence of endodontic therapy after complete or partial coverage glass-ceramic restorations with a follow-up time of over 30 years: A prospective clinical study

STATEMENT OF PROBLEM

Long-term clinical data are lacking on the comparison of the incidence of endodontic therapy in adhesively luted complete and partial coverage glass-ceramic restorations, as well as on the effect of technique and clinical variables.

PURPOSE

The purpose of this prospective clinical study was to assess the long-term incidence of teeth requiring endodontic therapy after receiving either complete or partial coverage glass-ceramic restorations.

MATERIAL AND METHODS

Participants requiring single anterior complete, posterior complete, or posterior partial (inlay or onlay) coverage restoration, or a combination of these on a vital tooth were recruited from a clinical private practice. Only the participants who chose glass-ceramic partial and complete coverage restorations without the need of endodontic therapy were included in the study. The overall clinical performance of these glass-ceramic restorations was assessed by clinical factors determined at recall. The effect of various clinical parameters (type of restoration, dental arch, tooth position in the dental arch, age and sex of participant, and ceramic thickness) 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 by using the log rank test (α=.05).

RESULTS

A total of 1800 participants requiring 4511 glass-ceramic anterior and posterior restorations were evaluated. The mean age of the participants at the time of restoration placement was 62 (range 20 to 99 years, 710 men and 1090 women). Of 4511 restorations, 1476 were anterior complete coverage, 2119 posterior complete coverage, and 916 posterior partial coverage. Endodontic therapy after restoration placement was needed for 76 restorations (10 anterior complete, 50 posterior complete, and 16 posterior partial). The total time at risk was 50 436 years providing an estimated need for endodontic therapy risk of 0.15% per year. The estimated 35-year cumulative survival was 97.36%. The majority of endodontic treatments (67%, 52/76) occurred in the first 5 years. The estimated cumulative survival of anterior complete coverage, posterior complete coverage, posterior partial inlay, and posterior partial onlay restorations was 98.89% (n=1476, 10 endodontic treatments), 96.38% (n=2119, 50 endodontic treatments), 96.78% (n=553, 11 endodontic treatments), and 98.53% (n=363, 5 endodontic treatments), respectively. Statistically significant differences occurred between anterior complete coverage, posterior complete coverage, and posterior partial coverage inlay restorations, with a higher incidence in posterior complete coverage and posterior partial inlay restorations (P<.05). First molars had the highest rate of endodontic therapy after restoration in both arches. Age and restoration thickness were significant factors, recording statistically higher number of endodontic treatments in participants >52 years and restorations with all surfaces ≥1 mm (P<.05). Other clinical variables, dental arch and sex of the participants, were not significantly related to endodontic treatments (P>.05).

CONCLUSIONS

The clinical performance of 4511 units over 30 years in service was excellent, with the estimated cumulative survival of 97.36%. Posterior complete coverage and posterior partial inlay restorations had a significantly higher need for endodontic therapy than anterior complete coverage restorations. Their overall clinical performance relative to endodontic treatment was excellent with a cumulative survival of 96.38% and 96.78% over 30 years. There was no difference in the endodontic treatment rate between posterior complete and partial coverage restorations. Thickness of the restoration affected the endodontic treatment rate, with ≥1 mm resulting in higher incidence. The age of the participants influenced the endodontic treatment rate, with higher incidence in the >52-year age group. Other confounding clinical variables did not have a significant effect on the endodontic treatment rate.

Two-body wear resistance and fatigue survival of new Y-TZP and ATZ ceramics made with a new slip-casting method

BACKGROUND

Dental zirconium oxide restorations are milled from pre-sintered blocks or disks which are produced either with high isostatic pressure (HIP) or, simpler, a slurry technique. The objective was to perform a fatigue test and an in vitro wear simulation of two ceramics, yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramic and a hybrid zirconium oxide-aluminum oxide ceramic, (ATZ) both produced either the classical way using high isostatic pressure (HIP, control) or with a slurry technique.

MATERIALS AND METHODS

Ten discs/group were subjected to a cyclic biaxial fatigue test using a staircase approach under water at 37 °C in a dynamic universal testing machine. The 2-body wear test was performed on eight lapped 12 mm thick cylindrical samples subjected to spherical (ø 6 mm) leucite ceramic antagonists in a CS-4 chewing simulator at 49 N force and 0.7 mm lateral movement for 600 k cycles and 4167 thermal cycles (5-55 °C). Volumetric wear was calculated based on laser-scanned surfaces. Selected samples of both tests were viewed in SEM.

RESULTS

All the ceramic specimens produced using the HIP method survived up to 1.2 M cycles with the maximum load of the equipment (1000 N) loading the specimens up to 1527 MPa. The fatigue limit stress at 1.2 M cycles for the Slurry ATZ samples was 946 MPa. For the Slurry Y-TZP samples the fatigue limit stress at 1.2 M cycles was 658 MPa. At 600 k cycles, all zirconium oxide ceramics showed no measurable wear and had a highly polished appearance. The leucite ceramic antagonists wear developed in a linear way. There was no difference between the materials produced with the slurry and the HIP process. ATZ ceramic produced significantly more wear than 3Y- TZP ceramic.

CONCLUSIONS

The HIP method provided higher fatigue strength than the Slurry manufacturing method. All HIP ceramics surpassed the limit threshold (1527 MPa) of the testing machine. The tested ceramics did not show any measurable wear but had worn the leucite reinforced glass ceramic antagonists for a considerable amount.

Biaxial flexural strength and phase transformation characteristics of dental monolithic zirconia ceramics with different sintering durations: An in vitro study

STATEMENT OF PROBLEM

Zirconia is a polymorphic metastable material which can react through a phase transformation from tetragonal to monoclinic when exposed to mechanical, physical, or chemical stimuli.

PURPOSE

The purpose of this in vitro study was to investigate the fracture strength and phase structure of different high-translucency zirconia ceramics depending on the changes in sintering duration and thermocycling.

MATERIAL AND METHODS

Two monolithic zirconia ceramics, Katana (KAT) and NexxZr (NEX), were used to prepare disk-shaped specimens (n=66). The sintering temperature was 1500 °C, and 3 different sintering durations were tested: 1 hour, 2 hours (recommended by the manufacturer), and 3 hours. Thermocycling was applied to half the specimens. Fracture strength was calculated, and the specimens were analyzed with an X-ray diffractometer (XRD) to determine the level of the phase transformation. The normal distribution of the data was analyzed by using the Shapiro-Wilk test. Two-way ANOVA was used to compare multiple groups (α=.05). The Fisher least significant difference test was applied to identify significant differences in fracture strength. The paired-specimen t test was applied to perform intragroup comparisons.

RESULTS

Sintering duration significantly affected the fracture strength of KAT (P=.007). For nonthermocycled specimens, the fracture strength of NEX was significantly higher than that of KAT (P<.001). Thermocycling had a significant effect on fracture strength depending on sintering duration and zirconia ceramic interaction (P=.046).

CONCLUSIONS

The sintering duration only affected the KAT zirconia, and the fracture strength of KAT decreased when sintered for 3 hours. Thermocycling decreased the fracture strength of both zirconias, except when the sintering duration was 2 hours for NEX. The fracture strength was higher for NEX than for KAT. Tetragonal-monoclinic phase transformation was not found for either zirconia according to the XRD analysis.

Restoration's thickness and bonding tooth substrate are determining factors in minimally invasive adhesive dentistry

Purpose To explore fracture strength and failure behaviour of minimally invasive CAD-CAM composite resin overlay restorations.Methods Eighty bi- and tri-layer cylindrical overlay model including the restoration bonded over bovine tooth dentin (Groups D) and enamel-dentin (Groups E) were assembled (diameter 9 mm). Restorations were milled from CAD-CAM composite resin blocks (Brilliant Crios, Coltène/Whaledent AG) in different thicknesses (0.5mm, 1mm, 1.5mm, 2mm) and equally distributed in four Groups D and four Groups E (n=10). All specimens were submitted to an Hertzian load-to-failure contact test with spherical indenter. Critical loads were recorded in Newton and data were analysed using Kruskal-Wallis test for multiple and Mann-Whitney test for 2-samples comparisons (p < 0.05). Fragments were examined using SEM. The stress distribution for specimens with restorations of 0.5 mm and 2 mm was also investigated with FEA.Results For all specimens, the mean static loads in Newton increased with an increase in restoration thickness. On contrary, restorations with the same thickness displayed higher resistance values when bonded over enamel than dentin, except for the 2-mm thick restorations. A damage competition was detected between cone/median cracks originating at the loading contact area of the restorations and radial cracks beginning at their inner surface, with the former prevailing in restorations bonded on enamel and the latter being dominant for restorations bonded on dentin.Conclusions For bonded ultra-thin resin composite restorations (0.5 mm to 1.5 mm) enamel as bonding substrate assures higher critical loads to fracture than dentin. This influence gradually decreases as restoration thickened.

Mechanical stability of dental CAD-CAM restoration materials made of monolithic zirconia, lithium disilicate, and lithium disilicate-strengthened aluminosilicate glass ceramic with and without fatigue conditions

STATEMENT OF PROBLEM

Studies investigating the mechanical stability of lithium disilicate-strengthened aluminosilicate glass ceramic that do not require sintering after milling compared with other computer-aided design and computer-aided manufacturing (CAD-CAM) materials are lacking.

PURPOSE

The purpose of this in vitro study was to investigate the flexural strength of CAD-CAM zirconia, lithium disilicate, and lithium disilicate-strengthened aluminosilicate glass ceramics with and without fatigue conditions.

MATERIAL AND METHODS

Specimens (N=90, n=15) (12×4×3 mm) from the following CAD-CAM materials were prepared and polished: lithium disilicate glass ceramic (IPS e.max CAD); lithium disilicate-strengthened aluminosilicate glass ceramic (N!ce); and zirconium dioxide ceramic (IPS e.max ZirCAD). All specimens were divided into 2 subgroups: immediate testing without aging and simulation of aging by using a mastication simulator for 1 200 000 cycles (5 °C-55 °C). Thereafter, flexural strength testing was performed by using a universal testing machine (1 mm/min) on nonaged and aged specimens. The data were evaluated by using nonparametric 2-way ANOVA and Wilcoxon rank post hoc tests (α=.05).

RESULTS

Both the material type and aging significantly affected the results (P<.001). The interaction was not significant (P>.05). Under nonaged conditions, zirconium dioxide ceramic (1136 ±162 MPa) showed significantly higher mean ±standard deviation flexural strength (P<.001) than lithium disilicate (304 ±34 MPa) and lithium disilicate-strengthened aluminosilicate glass ceramic (202 ±17 MPa). The glass ceramic groups were also significantly different from each other (P<.001). After aging, zirconium dioxide (1087.9 ±185.3 MPa) also presented significantly higher mean ±standard deviation flexural strength (P<.001) than lithium disilicate (259 ±62 MPa) and lithium disilicate-strengthened aluminosilicate glass ceramic (172 ±11 MPa) (P<.001). Aging significantly decreased the flexural strength of lithium disilicate (14.6%) (P=.03) and lithium disilicate-strengthened aluminosilicate glass ceramic (14.5%) (P=.01) but had minimal effect on the zirconium dioxide ceramic (4.3%) (P=.29).

CONCLUSIONS

Among the tested CAD-CAM materials, the mechanical performance of lithium disilicate-strengthened aluminosilicate glass ceramic was comparable with that of lithium disilicate and considerably lower than that of zirconia. Aging decreased the flexural strength of both lithium disilicate and lithium disilicate-strengthened aluminosilicate glass ceramic.

Influence of CAD/CAM milling, sintering and surface treatments on the fatigue behavior of lithium disilicate glass ceramic

This paper reports on the process-fatigue relation of lithium disilicate glass ceramic (LDGC) using low-cycle, high-load Hertzian indentations with a rigid indenter to simulate teeth grinding/clenching of LDGC restorations with different surface asperities obtained in CAD/CAM milling, sintering, polishing and glazing. The maximum contact stresses were evaluated as functions of the number of load cycles and surface treatments using the Hertzian model. Indentation-induced surface damage was viewed using scanning electron microscopy (SEM) to understand the relationships among microstructures, surface asperities, crack morphology and propagation. Different processes and surface treatments significantly affected the maximum contact stresses of indented LDGC surfaces (ANOVA, p < 0.05), which were all significantly reduced with the number of cycles (ANOVA, p < 0.05). Quasi-plastic deformation was dominant in single-cycle indentation of all processed and treated surfaces. In higher cycle indentations, inner cone cracks were formed on all surfaces; median and transverse cracks were formed on the roughest surfaces processed by CAD/CAM milling and sintering. Ring cracks, fretting, pulverization, micro-bridges, surface smearing and wedging, and edge chippings were also propagated on all surfaces. The process-fatigue relation provides an understanding of the mechanical functions of surface asperities produced in different processes and treatments. It indicates that the mechanically assisted growth of surface asperities with different roughness strongly affected the indentation-induced surface damage. Finally, the smoothest surfaces produced by CAD/CAM milling, polishing and sintering sustained the highest contact stresses and the least fatigue damage at higher cycles, ensuring their superior fatigue performance compared to other processed LDGC surfaces.

10.9-year survival of pressed acid etched monolithic e.max lithium disilicate glass-ceramic partial coverage restorations: Performance and outcomes as a function of tooth position, age, sex, and the type of partial coverage restoration (inlay or onlay)

STATEMENT OF PROBLEM

Long-term clinical data on the survival of pressed lithium disilicate glass-ceramic when used with partial coverage restorations and the effect that different technical and clinical variables have on survival are sparse.

PURPOSE

The purpose of this clinical study was to determine the 10.9-year survival of pressed lithium disilicate glass-ceramic partial coverage restorations and associated clinical parameters on outcomes.

MATERIAL AND METHODS

Individuals requiring single unit defect-specific partial coverage restorations in any area of the mouth were recruited in a clinical private practice. Participants were offered the options of partial coverage cast gold or glass-ceramic (lithium disilicate) restorations. Only participants that chose glass-ceramic partial coverage restorations were included in the study. The overall survival of the glass-ceramic restorations was assessed by the clinical factors (participant's age, sex, dental arch, tooth position in dental arch, type of partial coverage restoration, and ceramic thickness) determined at recall. The effect of this clinical parameters was evaluated by using Kaplan-Meier survival curves accounting for attrition bias and other reasons for failure. The statistical significance of differences between parameters was determined by using the log rank test (α=.05).

RESULTS

A total of 304 participants requiring 556 lithium disilicate restorations were evaluated. The mean age for the participant at the time of restoration placement was 62 with a range of 20 to 99 years, 120 were men and 184 were women. A total of 6 failures (bulk fracture or large chip) requiring replacement were recorded with the average time to failure of 2.4 (0.8-9.2) years. The total time at risk computed for these units was 1978.9 years providing an estimated failure risk of 0.3% per year. The 10-year estimated cumulative survival was 95.6%. The estimated cumulative survival of inlays (n=246) and onlays (n=305) were 93.9% and 98.3%, at 9.9 and 9.8 years, respectively (P<.05). Of the 6, there were 3 failures recorded for the partial coverage inlay restorations. The total time at risk for these inlays was 786.79 years providing an estimated risk of 0.38% per year. The other 3 failures recorded occurred for the partial coverage onlay restorations. The total time at risk for the onlays was 1032.17 years providing an estimated risk of 0.29% per year. The failures occurred in the molar region only. There were no failures recorded for the anterior partial coverage inlays (n=5). The total time at risk computed for the anterior units was 21.55 years providing an estimated risk of 0% per year. There was no statistically significant difference in the survival of partial coverage restorations among men and women, different age groups, or position in the dental arch. The thickness of the restoration had no influence on the survival of glass-ceramic partial coverage restorations.

CONCLUSIONS

Pressed lithium disilicate defect-specific partial coverage restorations reported high survival rate over the 10.9-year period with an overall failure rate of 0.3% per year and limited to the molar teeth. Risk of failure at any age was minimal for both men and women.

Comparison of 16.9-year survival of pressed acid etched e.max lithium disilicate glass-ceramic complete and partial coverage restorations in posterior teeth: Performance and outcomes as a function of tooth position, age, sex, and thickness of ceramic material

STATEMENT OF PROBLEM

Long-term clinical data are lacking on the comparison of the survival of adhesively luted pressed e.max lithium disilicate glass-ceramic complete and partial coverage restorations in posterior dentitions and the effect that different technical and clinical variables have on their survival.

PURPOSE

The purpose of this clinical study was to examine and compare the 16.9-year survival of posterior pressed e.max lithium disilicate glass-ceramic complete and partial coverage restorations and associated clinical parameters on the outcome.

MATERIAL AND METHODS

Patients requiring either single-unit posterior defect-specific partial coverage or complete coverage restorations were recruited in a clinical private practice. The participants were offered the options of direct restorations, partial coverage cast gold, or glass-ceramic (lithium disilicate) restorations. Those requiring complete coverage restorations were given the options of complete cast gold, metal-ceramic, or glass-ceramic restorations. Only participants who chose glass-ceramic partial and complete coverage restorations were included in the study. The overall survival of the glass-ceramic restorations was assessed by the clinical factors determined at recall. The effect of various clinical parameters (type of restoration, dental arch, tooth position in the dental arch, age and sex of participant, and ceramic thickness) 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 738 participants requiring 2392 lithium disilicate restorations in posterior teeth were evaluated. The mean age of the participants at the time of restoration placement was 62 (range: 20-99 years, 302 men and 436 women). Of 2392 units, 1782 were complete and 610 were partial coverage restorations. A total of 22 failures (bulk fracture or large chip) requiring replacement were recorded with the average time to failure 3.5 (0.02-7.9) years. The total time at risk computed for these units was 13227.9 years, providing an estimated failure risk of 0.17% per year. The 16.9-year estimated cumulative survival was 96.49%. The estimated cumulative survival of posterior complete (n=1782) and posterior partial coverage restorations (n=610) was 96.75% at 10.5 years and 95.27% at 16.9 years (P<.05). Of the 22, 16 failures were recorded for the complete coverage restorations. The total time at risk for these restorations was 10144.5 years, providing an estimated risk of 0.16 per year. The other 6 failures recorded occurred for the partial coverage restorations. The total time at risk for these restorations was 3083.5 years, providing an estimated risk of 0.19% per year. No statistically significant difference was found in the survival of posterior complete and partial coverage restorations among men and women, different age groups, or posterior tooth position in the dental arch (P>.05). The thickness of the restoration also had no influence on the survival of glass-ceramic posterior restorations (P>.05).

CONCLUSIONS

Pressed e.max lithium disilicate complete and partial coverage restorations showed high survival rates in posterior teeth over a 16.9-year period, with an overall failure rate of 0.17% per year. Risk of failure at any age was low for both men and women. No statistically significant difference was found in the survival of complete and partial coverage restorations, and none of the confounding variables, including the thickness of the restoration, appeared to have a significant effect on survival.

Fracture load and microcrack comparison of crowns manufactured from tooth-shaped and traditional blocks

This study intended to analyze microcracks and fractographic markings on the surface of all ceramic crowns after milling and compare the fracture loads. 90 crowns were manufactured from two feldspathic (Priticrown-Pr and Vita Mark II-Vi) and a lithium disilicate (EmaxCAD-Em) blocks (n = 30). Two groups (n = 15) were prepared for each ceramic. In the first group, crowns were analyzed twice via the fluorescent penetrant method for microcrack detection, after the manufacturing process and thermal cycles. The load to fracture test was applied at a crosshead speed of 0.5 mm/min until catastrophic failure. Second group crowns were directly cemented onto the Co-Cr dies following the manufacturing process and loaded to fracture. Fractographic markings were analyzed through scanning electron microscope. Spearman correlation analysis, Kruskal-Wallis H test, Mann-Whitney U test, and Wilcoxon Signed Rank test were applied (α = .05). Fracture loads of Em crowns were higher than other groups (p < .05), with and without the aging procedure. Except for second group Pr (r = -.532), no significant relationship was found between microcrack numbers and fracture loads (p > .05). Thermal cycling did not affect microcrack numbers and fracture loads (p > .05). Tooth-shaped multilayered Pr blocks did not provide an advantage in terms of microcrack and fracture loads.

Fracture Resistance of Monolithic Zirconia Crowns on Four Occlusal Convergent Abutments in Implant Prosthesis

Adjusting implant abutment for crown delivery is a common practice during implant installation. The purpose of this study was to compare the fracture resistance and stress distribution of zirconia specimens on four occlusal surface areas of implant abutment. Four implant abutment designs [occlusal surface area (SA) SA100, SA75, SA50, and SA25] with 15 zirconia prostheses over the molar area per group were prepared for cyclic loading with 5 Hz, 300 N in a servo-hydraulic testing machine until fracture or automatic stoppage after 30,000 counts. The minimum occlusal thickness of all specimens was 0.5 mm. Four finite element models were simulated under vertical or oblique 10-degree loading to analyze the stress distribution and peak value of zirconia specimens. Data were statistically analyzed, and fracture patterns were observed under a scanning electron microscope. Cyclic loading tests revealed that specimen breakage had moderately strong correlation with the abutment occlusal area (r = 0.475). Specimen breakage differed significantly among the four groups (P = 0.001). The lowest von Mises stress value was measured for prosthesis with a smallest abutment occlusal surface area (SA25) and the thickest zirconia crown. Thicker zirconia specimens (SA25) had higher fracture resistance and lowest stress values under 300 N loading.

Fracture Resistance of Monolithic Zirconia Crowns in Implant Prostheses in Patients with Bruxism

The aim of this study is to determine the minimum required thickness of a monolithic zirconia crown in the mandibular posterior area for patients with bruxism. Forty-nine full zirconia crowns, with seven different occlusal thicknesses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 mm, were made by using a computer-aided design/computer-aided manufacturing system (CAD/CAM). Seven crowns in each group were subjected to cyclic loading at 800 N and 5 Hz in a servohydraulic testing machine until fracture or completion of 100,000 cycles. Seven finite element models comprising seven different occlusal thicknesses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 mm were simulated using three different loads of vertical 800 N, oblique 10 degrees 800 N, and vertical 800 N + x N torque (x = 10, 50, and 100). The results of cyclic loading tests showed that the fracture resistance of the crown was positively associated with thickness. Specimen breakage differed significantly according to the different thicknesses of the prostheses (p < 0.01). Lowest von Mises stress values were determined for prostheses with a minimal thickness of 1.0 mm in different loading directions and with different forces. Zirconia specimens of 1.0 mm thickness had the lowest stress values and high fracture resistance and under 800 N of loading.

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.

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

OBJECTIVES

The purpose of this study was to evaluate the effects of artificial aging on thin glass-ceramic occlusal premolar veneers, adhesively bonded to dentin, by examining the changes caused by artificial aging using spectral domain optical coherence tomography (SD-OCT). In addition, the development of cracks in the ceramic veneers and their possible influence on the behavior of the ceramic restorations were examined.

METHODS

In total, 48 extracted sound upper premolars were prepared in the dentin for occlusal veneers milled from lithium disilicate ceramic blocks (IPS e.max CAD, Ivoclar Vivadent, Liechtenstein). All restorations were adhesively bonded using resin cement (Variolink Esthetic DC, Ivoclar Vivadent). Specimens were 3-dimensionally and 2-dimensionally imaged by SD-OCT (Telesto II, Thorlabs GmbH, Germany), then subjected to thermal-dynamic loading in a chewing simulator with 1,200,000 cycles at a load of 10 kg. Specimens were 2D and 3D imaged again after the artificial aging. Finally, they were subjected to quasi-static loading using a universal testing machine until failure occurred and later examined microscopically to assess the mode of failure. ANOVA test was performed for statistical analysis of data and Tukey's post-hoc test was used to compare the groups at 5% significance level. Chi-Square Test and Fischer's Exact Test of Independence were conducted to test the association between nominal variables.

RESULTS

No changes or irregularities were observed in the cement layer or tooth substrate after the aging process. However, wear of the ceramic was noticed at the surface of contact with the antagonist during the test. The development of cracks was detected in 23% of the specimens. Cracks did not affect the fracture strength (p > 0.05) but influenced the mode of failure (p ≤ 0.001).

SIGNIFICANCE

Optical coherence tomography allows an easy and non-invasive method to internally scan teeth and restorations. Development of cracks in the ceramic did not affect the fracture strength of the restorations but might lead to a more catastrophic type of failure.

Evaluating dental zirconia

OBJECTIVES

To survey simple contact testing protocols for evaluating the mechanical integrity of zirconia dental ceramics. Specifically, to map vital material property variations and to quantify competing damage modes.

METHODS

Exploratory contact tests are conducted on layer structures representative of zirconia crowns on dentin.

RESULTS

Sharp-tip micro- and nano-indentations were used to investigate the roles of weak interfaces and residual stresses in veneered zirconia, and to map property variations in graded structures. Tests with blunt sphere indenters on flat specimens were used to identify and quantify various critical damage modes in simulated occlusal loading in veneered and monolithic zirconia.

SIGNIFICANCE

Contact testing is a powerful tool for elucidating the fracture and deformation modes that control the lifetimes of zirconia dental ceramics. The advocated tests are simple, and provide a sound physical basis for analyzing damage resistance of anatomically-correct crowns and other complex dental prostheses.

Fracture strength testing of crowns made of CAD/CAM composite resins

PURPOSE

The purpose of this study was to ascertain whether computer aided design/computer aided manufacturing (CAD/CAM) composite resin crowns have sufficient strength to withstand the bite force of the molar teeth. The null hypothesis was that the fracture strength of CAD/CAM composite resin crowns is lower than the average maximum bite force of the molar tooth.

METHODS

The crowns, which shape is the right maxillary first molar, were fabricated using four CAD/CAM blanks made of composite resins (Block HC: HC, KZR-CAD HR: HR, KZR-CAD HR2: HR2, Avencia Block: AVE) and one CAD/CAM blank made of lithium disilicate glass-ceramic (IPS e.max CAD: IPS), which was used as a control. Fracture strength of fabricated crowns bonded to metal abutment and biaxial flexural strength of the materials were evaluated.

RESULTS

The results of fracture strength test and biaxial flexural strength test showed different tendencies. The fracture strength of CAD/CAM composite resin crowns except HC ranged from 3.3kN to 3.9kN, and was similar to that of IPS (3.3kN). In contrast, biaxial flexural strength of CAD/CAM composite resins ranged from 175MPa to 247MPa, and was significantly lower than that of IPS (360MPa).

CONCLUSIONS

All CAD/CAM composite resin crowns studied presented about 3-4 times higher fracture strength than the average maximum bite force of the molar tooth (700-900N), which result leads to the conclusion that CAD/CAM composite resin crowns would have sufficient strength to withstand the bite force of the molar teeth.

Residual stress of porcelain-fused to zirconia 3-unit fixed dental prostheses measured by nanoindentation

OBJECTIVE

To evaluate the residual stress (nanoindentation based on hardness) of fatigued porcelain-fused to zirconia 3-unit fixed dental prostheses (FDP) with different framework designs.

METHODS

Twenty maxillary 3-unit FDP replacing second-premolar (pontic) were fabricated with conventional framework-design (even-thickness of 0.5mm and 9mm² connector area) and modified framework-design (thickness of 0.5mm presenting lingual collar connected to proximal struts and 12mm² connector area). Connector marginal ridges were loaded and the fractured and suspended FDPs were divided (n=3/each) into: (1) Fractured zirconia even-thickness (ZrEvenF); (2) Suspended zirconia even-thickness (ZrEvenS); (3) Fractured zirconia with modified framework (ZrModF); (4) Suspended zirconia with modified framework (ZrModS); (5) Non-fatigued FDP with conventional framework design (Control). The FDPs were nanoindented at 0.03mm (Region of Interest (ROI) 1), 0.35mm (ROI 2) and 1.05mm (ROI 3) distances from porcelain veneer outer surface with peak load 4000μN. The Linear Mixed Analysis of Variance (ANOVA) Model on ranks and Least Significant Difference Test on ranks (95%) were used.

RESULTS

Highest rank hardness values were found for Control group and ZrModS, whereas the lowest values were found in ZrModF. Statistical differences (p=0.000) were found among all groups except for comparison between ZrModS and Control group (p=0.371). Hardness between ROIs were statistically significant different (p<0.001) where ROI 1 presented the lowest values.

SIGNIFICANCE

Framework-design modification did not influence the residual stress of porcelain-fused to zirconia fatigued 3-unit FDP. Whereas fractured FDPs showed the highest residual stress compared to suspended and control FDPs. Residual stress increased as nanoindented away from framework.

Effects of porcelain thickness on the flexural strength and crack propagation in a bilayered zirconia system

OBJECTIVE

This study evaluated the influence of porcelain (VM9, VITA Zahnfabrik, Germany) thickness on the flexural strength and crack propagation in bilayered zirconia systems (YZ, VITA Zahnfabrik, Germany).

MATERIAL AND METHODS

Thirty zirconia bars (20.0x4.0x1.0 mm) and six zirconia blocks (12.0x7.5x1.2 mm) were prepared and veneered with porcelain with different thickness: 1 mm, 2 mm, or 3 mm. The bars of each experimental group (n=10) were subjected to four-point flexural strength testing. In each ceramic block, a Vickers indentation was created under a load of 10 kgf for 10 seconds, for the propagation of cracks.

RESULTS

The results of flexural strength were evaluated by One-way ANOVA and Tukey's test, with a significance level of 5%. The factor "thickness of the porcelain" was statistically significant (p=0.001) and the l-mm group presented the highest values of flexural strength. The cracks were predominant among the bending specimens with 1 and 2 mm of porcelain, and catastrophic failures were found in 50% of 3-mm-thick porcelain. After the indentation of blocks, the most severe defects were observed in blocks with 3-mm-thick porcelain.

CONCLUSION

The smallest (1 mm) thickness of porcelain on the zirconia infrastructure presented higher values of flexural strength. Better resistance to defect propagation was observed near the porcelain/ zirconia interface for all groups. Higher flexural strength was found for a thinner porcelain layer in a bilayered zirconia system. The damage caused by a Vickers indentation near and far the interface with the zirconia shows that the stress profiles are different.

Dental Ceramics for Restoration and Metal Veneering

A survey of the development of dental ceramics is presented to provide a better understanding of the rationale behind the development and clinical indications of each class of ceramic material. Knowledge of the composition, microstructure, and properties of a material is critical for selecting the right material for specific applications. The key to successful ceramic restorations rests on material selection, manufacturing technique, and restoration design, including the balancing of several factors such as residual stresses, tooth contact conditions, tooth size and shape, elastic modulus of the adhesives and tooth structure, and surface state.

Interfacial modulus mapping of layered dental ceramics using nanoindentation

PURPOSE

The aim of this study was to test the modulus of elasticity (E) across the interfaces of yttria stabilized zirconia (YTZP) / veneer multilayers using nanoindentation.

MATERIALS AND METHODS

YTZP core material (KaVo-Everest, Germany) specimens were either coated with a liner (IPS e.max ZirLiner, Ivoclar-Vivadent) (Type-1) or left as-sintered (Type-2) and subsequently veneered with a pressable glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent). A 5 µm (nominal tip diameter) spherical indenter was used with a UMIS CSIRO 2000 (ASI, Canberra, Australia) nanoindenter system to test E across the exposed and polished interfaces of both specimen types. The multiple point load – partial unload method was used for E determination. All materials used were characterized using Scanning Electron Microscopy (SEM) and X – ray powder diffraction (XRD). E mappings of the areas tested were produced from the nanoindentation data.

RESULTS

A significantly (P<.05) lower E value between Type-1 and Type-2 specimens at a distance of 40 µm in the veneer material was associated with the liner. XRD and SEM characterization of the zirconia sample showed a fine grained bulk tetragonal phase. IPS e-max ZirPress and IPS e-max ZirLiner materials were characterized as amorphous.

CONCLUSION

The liner between the YTZP core and the heat pressed veneer may act as a weak link in this dental multilayer due to its significantly (P<.05) lower E. The present study has shown nanoindentation using spherical indentation and the multiple point load - partial unload method to be reliable predictors of E and useful evaluation tools for layered dental ceramic interfaces.

Joining Veneers to Ceramic Cores and Dentition with Adhesive Interlayers

Adhesive joining of veneers to cores offers potential simplicity and economy in the fabrication of all-ceramic crowns. We tested the hypothesis that resin-based adhesives can be used for such fabrication without compromising mechanical integrity of the crown structure. A simple test procedure for quantifying this hypothesis was proposed. A model glass veneer layer 1 mm thick (representative of porcelain), adhesively bonded onto a glass-like core substrate (ceramic or dental enamel), was loaded at its top surface with a hard sphere (occlusal force) until a radial crack initiated at the veneer undersurface. The critical loads for fracture, visually observable in the transparent glass, afforded a measure of the predisposition for the adhesive to cause veneer failure in an occlusal overload. Two adhesives were tested, one a commercial epoxy resin and the other a relatively stiff in-house-developed composite. The results confirmed that stiffer adhesives provide higher resistance to failure.

Dental prostheses mimic the natural enamel behavior under functional loading: A review article

Alumina- and zirconia-based ceramic dental restorations are designed to repair functionality as well as esthetics of the failed teeth. However, these materials exhibited several performance deficiencies such as fracture, poor esthetic properties of ceramic cores (particularly zirconia cores), and difficulty in accomplishing a strong ceramic–resin-based cement bond. Therefore, improving the mechanical properties of these ceramic materials is of great interest in a wide range of disciplines. Consequently, spatial gradients in surface composition and structure can improve the mechanical integrity of ceramic dental restorations. Thus, this article reviews the current status of the functionally graded dental prostheses inspired by the dentino-enamel junction (DEJ) structures and the linear gradation in Young's modulus of the DEJ, as a new material design approach, to improve the performance compared to traditional dental prostheses. This is a remarkable example of nature's ability to engineer functionally graded dental prostheses. The current article opens a new avenue for recent researches aimed at the further development of new ceramic dental restorations for improving their clinical durability.

Biaxial flexural strength of bilayered zirconia using various veneering ceramics

PURPOSE

The aim of this study was to evaluate the biaxial flexural strength (BFS) of one zirconia-based ceramic used with various veneering ceramics.

MATERIALS AND METHODS

Zirconia core material (Katana) and five veneering ceramics (Cerabien ZR; CZR, Lava Ceram; LV, Cercon Ceram Kiss; CC, IPS e.max Ceram; EM and VITA VM9; VT) were selected. Using the powder/liquid layering technique, bilayered disk specimens (diameter: 12.50 mm, thickness: 1.50 mm) were prepared to follow ISO standard 6872:2008 into five groups according to veneering ceramics as follows; Katana zirconia veneering with CZR (K/CZR), Katana zirconia veneering with LV (K/LV), Katana zirconia veneering with CC (K/CC), Katana zirconia veneering with EM (K/EM) and Katana zirconia veneering with VT (K/VT). After 20,000 thermocycling, load tests were conducted using a universal testing machine (Instron). The BFS were calculated and analyzed with one-way ANOVA and Tukey HSD (α=0.05). The Weibull analysis was performed for reliability of strength. The mode of fracture and fractured surface were observed by SEM.

RESULTS

It showed that K/CC had significantly the highest BFS, followed by K/LV. BFS of K/CZR, K/EM and K/VT were not significantly different from each other, but were significantly lower than the other two groups. Weibull distribution reported the same trend of reliability as the BFS results.

CONCLUSION

From the result of this study, the BFS of the bilayered zirconia/veneer composite did not only depend on the Young's modulus value of the materials. Further studies regarding interfacial strength and sintering factors are necessary to achieve the optimal strength.

Comparative study of flexural strength test methods on CAD/CAM Y-TZP dental ceramics

Clinically, fractures are the main cause of computer-aided design and computer-aided manufacturing (CAD/CAM) 3 mol%-yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) all-ceramic dental restorations failure because of repetitive occlusal loading. The goal of this work is to study the effect of test methods and specimen's size on the flexural strength of five ceramic products. Both bi-axial flexure test (BI) and uni-axial flexure tests (UNI), including three-point flexure test (3PF) and four-point flexure test (4PF), are used in this study. For all five products, the flexural strength is as follows: BI > 3PF > 4PF. Furthermore, specimens with smaller size (3PF-s) have higher values than the bigger ones (3PF). The difference between BI and UNI resulted from the edge flaws in ceramic specimens. The relationship between different UNI (including 3PF-s, 3PF and 4PF) can be explained according to Weibull statistical fracture theory. BI is recommended to evaluate the flexural strength of CAD/CAM Y-TZP dental ceramics.

Fracture resistance of monolithic zirconia crowns with different occlusal thicknesses in implant prostheses

STATEMENT OF PROBLEM

The use of monolithic zirconia crowns in implant prostheses is increasing, especially when the interdental space is insufficient. However, fractures have been reported in clinical practice.

PURPOSE

The purpose of this study was to determine the minimal thickness of a complete zirconia crown used for an implant prosthesis in the posterior dental region.

MATERIAL AND METHODS

Fifty complete zirconia crowns were produced using a computer-aided design/computer-aided manufacturing technique. In each group, 5 crowns of varying thicknesses (0.4, 0.5, 0.6, 0.7, and 0.8 mm) were subjected to cycles of vertical and 10-degree oblique compressive loading at 5 Hz and 300 N in a servohydraulic testing machine. Five finite element models comprising 5 different occlusal thicknesses (0.4, 0.5, 0.6, 0.7, and 0.8 mm) were simulated at 2 loading angles (0 and 10 degrees) and 3 loading forces (300, 500, and 800 N). Data were statistically analyzed, and fracture patterns were observed with a scanning electron microscope.

RESULTS

Cyclic loading tests revealed that the fracture resistance of the specimens was positively associated with prosthesis thickness (P<.01). Low von Mises stress values were obtained for prostheses with a minimal thickness of 0.7 mm under varying loading directions and forces.

CONCLUSIONS

Zirconia prostheses with a minimal thickness of 0.7 mm had a high fracture resistance and the lowest stress values. Therefore, dentists and laboratory technicians should carefully choose the optimum thickness of zirconia prostheses.

The effect of a fiber reinforced cavity configuration on load bearing capacity and failure mode of endodontically treated molars restored with CAD/CAM resin composite overlay restorations

OBJECTIVES

To evaluate the fracture strength and the mode of failure of endodontically treated molars restored with CAD/CAM overlays with fiber reinforced composite build-up of the pulp chamber.

METHODS

40 Devitalized molars were cut over the CEJ and divided into five groups (n=8). The pulp chamber area was filled with: group 0 (control), no resin build-up; group 1, hybrid composite build-up (G-aenial posterior, GC); group 2, as in group 1 but covered with 3 nets of bi-directional E-glass fibers (EverStickNET, Stick Tech Ltd.); group 3, a FRC resin (EverX posterior, GC); group 4, as in group 3 but covered by the bi-directional fibers. The crowns were restored with CAD-CAM composite restorations (Lava Ultimate, 3M ESPE). Maximum fracture loads were recorded in Newton and data were analyzed using Kruskal-Wallis test (p<0.05). Fragments were analyzed using SEM.

RESULTS

The mean static loads in Newton were: group 0: 2448 (546); group 1: 2817 (873); group 2: 2128 (952); group 3: 2429 (1091); group 4: 2577 (833). No significant differences were found between the groups (p>0.05). All specimens fractured in a catastrophic way, under the CEJ. The main crack evolved in the corono-apical direction. In groups 2 and 4 secondary fracture paths with apico-coronal direction were detected close to the bi-directional fibers' layer.

CONCLUSIONS

For the restoration of endodontically treated molars, the incorporation of FRCs did not influence the load-bearing capacity of the tooth-restoration complex. The SEM analysis showed a low ability of the bi-directional fibers net in deviating the fracture but this effect was not sufficient to lead more favorable fracture patterns, over the CEJ.

CLINICAL SIGNIFICANCE

The use of FRCs to reinforce the "core" of devitalized molars against vertical fractures under static loads seems useless when the thickness of the CAD/CAM composite overlay restoration is high.

Fracture load of ceramic restorations after fatigue loading

STATEMENT OF PROBLEM

A clinician must decide what ceramic coping and veneer material to prescribe based on the amount of tooth reduction possible and the desired esthetic outcome of the restoration.

PURPOSE

The purpose of this in vitro study was to compare the fracture strength of monolithic and bilayered lithium disilicate (IPS e.max) and zirconia (LAVA) crowns at clinically relevant thicknesses after load cycling.

MATERIAL AND METHODS

Crowns (n=8) were fabricated from 6 groups: 1.2-mm monolithic lithium disilicate, 1.5-mm monolithic lithium disilicate, 1.5-mm bilayered lithium disilicate with hand-layered veneer, 0.6 mm monolithic zirconia, 1.2-mm bilayered zirconia with hand-layered veneer, and 1.2-mm bilayered zirconia with milled veneer (dimension represents thickness at the occlusal pit). Crowns were cemented to identical milled resin dies with resin-modified glass ionomer cement. Cemented crowns were stored at 37°C for 24 hours and load cycled for 200,000 cycles at 25 N at a rate of 40 cycles/minute. The ultimate fracture load for each specimen was measured in a universal testing machine. Data were analyzed with a 1-way ANOVA and Tukey honest significant difference post hoc analysis (α=.05).

RESULTS

Mean ±SD fracture load values were 1465 ±330 N for monolithic lithium disilicate (1.2-mm thickness) and 2027 ±365 N (1.5-mm thickness) and 1732 ±315 N for bilayered hand-veneered lithium disilicate (1.5-mm thickness). Fracture loads were 1669 ±311 N for monolithic zirconia crowns (0.6mm thickness), 2625 ±300 N for zirconia milled-veneered (1.2-mm thickness), and 2655 ±590N for zirconia hand-veneered crowns (1.2mm thickness). One-way ANOVA showed a statistically significant difference among the groups (P<.01). Veneered zirconia crowns showed the highest fracture strength, 1.2-mm hand veneered zirconia was similar to that of 1.5-mm monolithic zirconia, and all other groups were not statistically different.

CONCLUSIONS

Crowns of 1.2-mm bilayered zirconia had higher fracture loads than 0.6-mm zirconia or 1.2-mm lithium disilicate monolithic crowns.

Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations

STATEMENT OF PROBLEM

Preparation designs and ceramic thicknesses are key factors for the long-term success of minimally invasive premolar partial coverage restorations. However, only limited information is presently available on this topic.

PURPOSE

The purpose of this in vitro study was to evaluate the fracture resistance and failure modes of ceramic premolar partial coverage restorations with different preparation designs and ceramic thicknesses.

MATERIAL AND METHODS

Caries-free human premolars (n=144) were divided into 9 groups. Palatal onlay preparation comprised reduction of the palatal cusp by 2 mm (Palatal Onlay Standard), 1 mm (Palatal-Onlay-Thin), or 0.5 mm (Palatal Onlay Ultrathin). Complete-coverage onlay preparation additionally included the buccal cusp (Occlusal Onlay Standard; Occlusal Onlay Thin; Occlusal Onlay Ultrathin). Labial surface preparations with chamfer reductions of 0.8 mm (Complete-Veneer-Standard), 0.6 mm (Complete-Veneer-Thin), and 0.4 mm (Complete Veneer Ultrathin) were implemented for complete veneer restorations. Restorations were fabricated from a pressable lithium disilicate ceramic (IPS-e.max-Press) and cemented adhesively (Syntac-Classic/Variolink-II). All specimens were subjected to cyclic mechanical loading (F=49 N, 1.2 million cycles) and simultaneous thermocycling (5°C to 55°C) in a mouth-motion simulator. After fatigue, restorations were exposed to single-load-to-failure. Two-way ANOVA was used to identify statistical differences. Pair-wise differences were calculated and P-values were adjusted by the Tukey-Kramer method (α=.05).

RESULTS

All specimens survived fatigue. Mean (SD) load to failure values (N) were as follows: 837 (320/Palatal-Onlay-Standard), 1055 (369/Palatal-Onlay-Thin), 1192 (342/Palatal-Onlay-Ultrathin), 963 (405/Occlusal-Onlay-Standard), 1108 (340/Occlusal-Onlay-Thin), 997 (331/Occlusal-Onlay-Ultrathin), 1361 (333/Complete-Veneer-Standard), 1087 (251/Complete-Veneer-Thin), 883 (311/Complete-Veneer-Ultrathin). Palatal-onlay restorations revealed a significantly higher fracture resistance with ultrathin thicknesses than with standard thicknesses (P=.015). Onlay restorations were not affected by thickness variations. Fracture loads of standard complete veneers were significantly higher than thin (P=.03) and ultrathin (P<.001) restorations.

CONCLUSIONS

In this in vitro study, the reduction of preparation depth to 1.00 and 0.5 mm did not impair fracture resistance of pressable lithium-disilicate ceramic onlay restorations but resulted in lower failure loads in complete veneer restorations on premolars.

Hertzian contact response and damage tolerance of dental ceramics

OBJECTIVES

To determine the contact response and damage tolerance or strength degradation of a range of dental CAD/CAM ceramic materials including novel polymer-infiltrated-ceramic-network (PICN) materials by means of spherical indentations at various loads and indenter radii.

METHODS

The seven tested materials included Mark II, PICN test materials 1 and 2, In-Ceram Alumina, VM 9, In-Ceram YZ (Vita Zahnfabrik, Bad Saeckingen, Germany) and IPS e.max CAD, (Ivoclar Vivadent, Schaan, Liechtenstein). To evaluate the damage tolerance and role of indenter size, indentations with tungsten carbide spheres (0.5mm and 1.25mm radius) were placed on bending bars with varying loads (1.96-1000N). The indented bending bars were subsequently loaded to fracture in three-point bending. The contact induced damage was analyzed by light microscopy (LM) and SEM. The spherical contact response was measured on polished surfaces.

RESULTS

The initial strengths for the individual materials were found to reduce above specific indentation loads, which were a function of the indenter radius. Employing a 0.5mm radius sphere resulted in the following strength degrading loads and ordering of materials: VM9 (98N)<MarkII-PICN1 (147N)<ICAlumina-e.maxCAD (300N)<PICN2-YTZP (500N). For the materials indented with the 1.25mm sphere, higher loads were required for the onset of strength degradation: VM9 (190N)<MarkII (300N)<PICN1 (400N)<e.maxCAD (500N)<ICAlumina (700N)<PICN2 (1000N)<YTZP (above 1000N). Two different damage modes were observed by SEM and LM - brittle cone cracking and plastic deformation. The PICN materials exhibited elastic-plastic behavior with creep. In contrast YTZP showed entirely elastic behavior upon loading with both spheres.

SIGNIFICANCE

This study aims to emulate the likely clinical behavior of contact loading by opposing cusps to dental restorative ceramic materials by utilizing spherical indentations at various loads and sphere diameters.