Fatigue testing of two porcelain–zirconia all-ceramic crown systems

ObjectiveTo evaluate the mouth-motion step-stress fatigue behavior of two porcelain–zirconia all-ceramic crown systems.MethodsThe average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing proximal walls by 1.5 mm and occlusal surface by 2.0 mm. The CAD-based tooth preparation was made by rapid prototyping and used as a master die to fabricate all-ceramic crowns with 1.0 mm porcelain veneered on 0.5 mm Y-TZP cores (LAVA veneer + LAVA frame, 3M/ESPE, and Vita veneer + CERCON frame, Dentsply). Crowns were cemented on aged (60 days in water) composite (Z100, 3M/ESPE) reproductions of the die. Three crowns from the LAVA group were subjected to single cycle load to failure for stress profile design; remainder subjected to step-stress mouth-motion fatigue (three step-stress profiles). All mechanical testing was performed by sliding a WC indenter of 6.25 mm diameter 0.7 mm lingually down the mesio-distal cusp. Master Weibull curves and reliability for missions of 50,000 cycles at 200 N load were calculated (Alta Pro 7, Reliasoft).ResultsSingle load to failure showed fractures through the zirconia core. Reliability for a 200 N × 50K cycle mission was not significantly different between systems. In fatigue, failure occurred by formation of large chips within the veneer originating from the contact area without core exposure.ConclusionsLAVA and CERCON ceramic systems present similar fatigue behavior; fatigue loading of both systems reproduces clinically observed failure modes.     

Zirconia-reinforced lithium silicate crowns: Effect of thickness on survival and failure mode

ObjectiveTo evaluate the reliability and failure mode of zirconia-reinforced lithium silicate (ZLS) molar crowns of different thicknesses.MethodsMonolithic ZLS molar crowns (0.5 mm, 1.0 mm, and 1.5 mm thickness) were modeled and milled using a CAD/CAM system (n = 21/group). Crowns were cemented on dentin-like epoxy resin replicas with a resin cement. The specimens were subjected to single load-to-failure test for step-stress profiles designing. Mouth-motion step-stress accelerated-life test was performed under water by sliding an indenter 0.7 mm lingually down on the distobuccal cusp until specimen fracture or suspension. Use level probability Weibull curves and reliability were calculated and plotted. Polarized-light optical microscope and scanning electron microscope (SEM) were used to characterize fracture patterns.ResultsIrrespective of crown thickness, beta (β) values were higher than 1 and fatigue accelerated failures. While 0.5 mm ZLS crowns exhibited a significant reduction in the probability of survival at 200 N, 300 N and 400 N mission loads (69%, 41% and 19%, respectively), no significant difference was observed between 1.0 mm and 1.5 mm crowns. Both thicknesses have maintained the survivability at approximately 90%. Failure primarily comprised bulk fracture where radial cracks originated from the cementation surface beneath the indenter loading trail and propagated towards the cervical margin.Significance1.5 mm- and 1.0 mm-thickness monolithic ZLS crowns presented higher probability of survival compared to 0.5 mm crowns. Bulk fracture was the chief failure mode, regardless of thickness.     

Long-term clinical evaluation of direct resin composite restorations in vital vs. endodontically treated posterior teeth — Retrospective study up to 13 years

ObjectivesThis retrospective study evaluated and compared the survival rate of Class II posterior direct resin based composite (RBC) restorations made in vital teeth (VT) and endodontically treated teeth (ETT). The influence of risk factors on the long-term performance of restorations was also investigated.MethodsPatients (n = 245) receiving RBC posterior restorations between 2004 and 2012 were selected. A total of 597 restorations (485 in VT, 112 in ETT) with minimum 2.5–3 mm remaining cusp thickness, made with the same brand of RBC and adhesive, were evaluated using the USPHS criteria. Data were analyzed with Mann–Whitney, Chi-square and Fisher’s Exact Test, Extended Cox-regression and Kaplan–Meier analysis (p < 0.05). Relative risk ratio was estimated for each evaluated parameter.ResultsThe mean observation period was 8.6 ± 2.3 years. An annual failure rate in VT and ETT of 0.08% and 1.78%, respectively, was detected. The reasons of failures included restoration fracture, secondary caries in VT; vertical root fracture, cusp fracture, restoration fracture, secondary caries and loss of adhesion in ETT. Significantly better performance was observed in RBCs of VT for each evaluated parameter. Among the evaluated risk factors only occlusal stress affected negatively the survival of RBC in ETT (Hazard Ratio 37.1; CI_95% 8.4–163.7).SignificanceAlthough, there is significant difference in the success rate of RBCs in VT (98.97%) and ETT (76.8%), the long-term (6–13 years) durability of Class II RBCs with 2.5–3 mm cusp thickness in ETT is also clinically acceptable. The presence of occlusal stress decreases the survival of RBCs in ETT.     

Fatigue resistance of ultrathin CAD/CAM ceramic and nanoceramic composite occlusal veneers

ObjectiveThe fracture resistance of different ultrathin occlusal computer-aided design/computer-aided manufacturing (CAD/CAM) veneers was investigated under cyclic mechanical loading to restore combined enamel-dentin defects.MethodsEighty-four molars were reduced occlusally until extensive dentin exposure occurred with a remaining enamel ring. Twenty-four molars were ground flat for examination of highly standardized specimens, of which 8 were treated with uniformly flat 0.3 mm IPS Empress CAD and 0.3 and 0.5 mm IPS e.max CAD restorations. Sixty-four molars were anatomically prepared until dentin exposure and were restored using occlusal veneers with fissure/cusp thicknesses of 0.3/0.5 mm from 3 different dental CAD/CAM materials: IPS Empress CAD, IPS e.max CAD and Lava Ultimate CAD/CAM. Teeth were etched with 37% phosphoric acid, and occlusal veneers were bonded using an adhesive luting system (Syntac Primer, Adhesive, Heliobond and Variolink II). Specimens were placed under cyclic mechanical loading in a chewing simulator (1 million cycles at 50 N) and were examined for cracks after each cyclic loading sequence. The anatomical 0.3/0.5 mm IPS e.max CAD specimens experienced an additional 1 million cycles at 100 N. Kaplan–Meier survival curves and log-rank tests were used for data analysis.ResultsAll highly standardized and 0.3/0.5 mm IPS e.max CAD specimens tolerated cyclic loading. One anatomical Lava Ultimate CAD/CAM and 10 IPS Empress CAD specimens showed cracks.SignificanceUltrathin occlusal veneers of lithium disilicate ceramic and nanoceramic composite showed remarkably high fracture strength under cyclic mechanical loading. These veneers might be a tooth substance preserving option for restoring combined dentin–enamel defects.     

Microstructure,topography, surface roughness,fractal dimension,internal and marginal adaptation of pressed and milled lithium-disilicate monolithic restorations

PurposeTo characterize the effect of two processing techniques (pressing and CAD/CAM — Computer Aided Design/Computer Aided Machining) of lithium-disilicate (LD) based crowns on the microstructure, topography, roughness, fractal dimension, internal and marginal adaptation.MethodsOne-hundred identical preparations for monolithic crowns were made with dentin analogue material (G10 epoxy resin). One of the preparations was scanned and a monolithic crown in lithium-disilicate was planned in the CAD system. Fifty crowns were milled in a wax pattern and subjected to pressing (IPS e.max Press), while 50 crowns were machined at CAD/CAM (IPS e.max CAD) and posteriorly crystallized. Internal and marginal adaptation were assessed via replica technique at four manufacturing moments (Milled wax pattern; LD^PRESS; Milled LD^CAD; Crystallized LD^CAD) and considering 5 regions (margin, cervico-axial angle, axial wall, axial-occlusal angle and occlusal wall). Complementary analysis considering microstructure and topography, roughness and fractal dimension were performed in Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).ResultsThe processing technique resulted in different ceramic microstructure, topography, roughness and fractal dimension, whereas CAD/CAM lead to smoother, more homogeneous but more complex topography features (higher fractal dimension) in comparison to the pressing technique (P < 0.05). Regarding marginal and internal fit, LD^PRESS crowns showed to be more adapted at the margin, while LD^CAD were more adapted at the occluso-axial angle; other regions were not statistically different (α = 0.05).ConclusionsCAD/CAM and the pressing techniques for manufacturing LD crowns lead to completely different ceramic surface characteristics and affect crown adaptation at the margin and at occluso-axial angle.     

Clinical performance of occlusal onlays made of lithium disilicate ceramic in patients with severe tooth wear up to 11 years

ObjectivesEvaluation of survival and complication rate of monolithic occlusal onlays made of lithium disilicate ceramic used in patients with severe tooth wear up to 11 years of clinical service.MethodsIn a prospective non-randomized clinical study 7 patients (4 male, 3 female; median age: 44.3 ± 6.56 years old) were restored full mouth with a total of 103 adhesively bonded occlusal onlays made of lithium disilicate ceramic (IPS e.max Press, Ivoclar Vivadent, Schaan, Liechtenstein). All restorations were examined during annual recall visits using periodontal parameters according to the modified United States Public Health Service (USPHS) criteria: (a) marginal discoloration, (b) secondary caries, (c) marginal integrity, (d) surface texture, (e) restoration fracture, and (f) occlusal wear, rating with Alpha, Bravo and Charlie over an observation period up to 11 years (68–139 months; median: 94.9 ± 26.1 months). Data was statistically analyzed using the Kaplan–Meier estimation.ResultsMonolithic lithium disilicate occlusal onlays presented a 100% survival rate. Four restorations within one patient (3.9%) presented marginal discoloration, one after 60 and three after 108 months (all rated Bravo). One restoration (1%) showed a marginal crack formation (technical complication) after 120 months, rated Bravo. No biological complication, debonding or secondary caries could be found and tested periodontal parameters showed excellent results.SignificanceBased on the analyzed data up to 11 years, monolithic occlusal onlays made of lithium disilicate ceramic can be considered as a reliable treatment option for full-mouth rehabilitations in patients with severe tooth wear.     

Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach

ObjectiveThe aim of this study was to analyze and to compare the fracture type and the stress at failure of clinically fractured zirconia-based all ceramic restorations with that of morphologically similar replicas tested in a laboratory setup.MethodsReplicas of the same shape and dimensions were made for 19 crowns and 17 fixed partial dentures, all made of veneered zirconia frameworks, which fractured during intra-oral service. The replicas were statically loaded by applying axial load in a universal testing machine. The principles of fractography were used to identify the location and the dimensions of the critical crack and to estimate the stress at failure. Failure was classified according to origin and type (P < 0.05 was considered significant).ResultsClinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2 ± 9 MPa), defects at core veneer interface (27.7 ± 6 MPa), the generation of Hoop stresses (884.3 ± 266 MPa), radial cracking (831 MPa), or fracture of the connector (971 ± 343 MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4 ± 34.8 MPa) or by fracture of the connector (1098.9 ± 259 MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F = 6.8, P < 0.01).SignificanceWithin limitations of this study, careful design of fracture strength test would lead to more clinically relevant data. The performance of zirconia veneered restorations could be further improved with careful design considerations.     

Fracture strength of CAD/CAM composite and composite-ceramic occlusal veneers

PurposeTo determine the effect of material type and restoration thickness on the fracture strength of posterior occlusal veneers made from computer-milled composite (Paradigm MZ100) and composite-ceramic (Lava Ultimate) materials.Methods60 maxillary molars were prepared and restored with CAD/CAM occlusal veneer restorations fabricated from either Paradigm MZ100 or Lava Ultimate blocks at minimal occlusal thicknesses of 0.3, 0.6, and 1.0 mm. Restorations were adhesively bonded and subjected to vertical compressive loading. The maximum force at fracture and mode of failure were recorded. 2-Way ANOVA was used to identify any statistically significant relationships between fracture strength and material type or thickness. Spearman's rank correlation coefficient was used to analyze mode of failure with regard to fracture strength.ResultsThe average maximum loads (N) at fracture for the Paradigm MZ100 groups were 1620 ± 433, 1830 ± 501, and 2027 ± 704 for the material thicknesses of 0.3, 0.6, and 1.0 mm, respectively. The Lava Ultimate groups fractured at slightly higher loads (N) of 2078 ± 605, 2141 ± 473, and 2115 ± 462 at the respective 0.3, 0.6, and 1.0 mm thickness.Statistical analyses revealed that, while no significant difference existed among the various restoration thicknesses in terms of fracture strength (P > 0.05), the material type was found to be influential (P = 0.04). The maximum load at fracture (N) for Lava Ultimate averaged over all thicknesses (2111 ± 500) was significantly higher than that of the Paradigm MZ100 (1826 ± 564). No correlation between mode of failure and fracture strength was found.ConclusionsUnder the conditions of this study, the maximal loads at fracture for these “non-ceramic” occlusal veneer restorations were found to be higher than human masticatory forces. Occlusal veneers made from the two materials tested are likely to survive occlusal forces regardless of restoration thickness, with those fabricated from the composite-ceramic hybrid material being more likely to survive heavier loads.     

High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings — A new fabrication mode for all-ceramic restorations

ObjectivesWith this in vitro study the fracture strength of zirconia-based crown copings being veneered with a CAD/CAM generated high-strength ceramic cap by sintering is compared with anatomically identical zirconia-based crowns, which were either overpressed or veneered by the layering technique for completion.MethodsA 1.2 mm, 360° chamfer preparation was performed on a second maxillary molar and was dublicated 15 times in a cobalt–chromium-alloy. A sample of 45 zirconia copings was produced and divided into three groups. In the first group (VT) zirconia copings received conventional veneering in layering technique, in the second group the veneering porcelain was pressed over the zirconia coping (PT), and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented conventionally onto their dies and tested in the universal testing machine until clinical failure. The fracture load data were compared by a one-way analysis of variance and a multiple comparison posthoc test (α < 0.05).ResultsSpecimens from group VT showed a mean (S.D.) fracture load of 3700.39 (1238.72) N, group OT 3523.73 (1181.11) N and group ST 6262.67 (2257.42) N. The difference between groups VT/OT and ST were statistically significant (P < 0.001).SignificanceThe new CAD/CAM-fabricated bilayered restorations (ST) were superior to the present techniques (VT and OT) in terms of fracture load and offer the possibility to produce cost-effective crowns and fixed partial dentures with a potential lower risk of chippings.     

Wear resistance of crowns made from different CAM/CAD materials

ObjectivesThe aim of this laboratory study was to evaluate the wear resistance of crowns made from current computer-aided design and computer-aided manufacture (CAD/CAM) materials. In addition, the abrasion of the steatite antagonist against these materials was compared.MethodsIdentically shaped crowns of lithium disilicate, zirconia-reinforced lithium disilicate and a polymer-infiltrated ceramic network (PICN) were fabricated with an occlusal thickness of 1.5 mm and a lateral wall thickness of 1.2 mm (n = 8). The crowns were cemented with a dual-polymerizing luting resin on composite resin dies. Using spherical steatite antagonists, all specimens were loaded with 49 N for 1,200,000 cycles in a mastication simulator with additional thermocycling. After 120,000, 240,000, 480,000, 960,000, and 1,200,000 cycles, precision impressions were made and investigated with a laser scanning microscope. The vertical and volume substance loss was measured. Additionally, the substance loss of the antagonists was evaluated after 1,200,000 loading cycles.ResultsNo significant difference (p > 0.05) was found in the median volume loss of the test materials after 1,200,000 cycles (lithium disilicate: 0.405 mm³, PICN: 0.362 mm³, zirconia-reinforced lithium disilicate: 0.340 mm³). The vertical substance loss of PICN (157 μm) was significantly lower (p ≤ 0.05) than that of lithium disilicate (201 μm) and zirconia reinforced lithium disilicate (191 μm). However, the substance loss of steatite against zirconia-reinforced lithium disilicate (0.191 mm³) was significantly lower (p ≤ 0.05) than against lithium disilicate (0.296 mm³) and PICN (0.531 mm³).SignificanceAll three CAD/CAM materials showed wear resistance that seems appropriate for clinical application. Also, the abrasion of the antagonist looks promising.     

Influence of bonding surface and bonding methods on the fracture resistance and survival rate of full-coverage occlusal veneers made from lithium disilicate ceramic after cyclic loading

ObjectivesThe purpose of this laboratory study was to evaluate the influence of bonding method and type of dental bonding surface on fracture resistance and survival rate of resin bonded occlusal veneers made from lithium disilicate ceramic after cyclic loading.MethodsFourty-eight extracted molars were divided into three groups (N = 16) depending on the preparation: within enamel, within dentin/enamel or within enamel/composite resin filling. Lithium disilicate occlussal veneers were fabricated with a fissure-cusp thickness of 0.3–0.6 mm. Restorations were etched (5% HF), silanated and adhesively luted using a dual-curing luting composite resin. Test groups were divided into two subgroups, one using a only a self-etching primer, the other additionally etching the enamel with phosphoric acid. After water storage (37 °C; 21 d) and thermocycling (7500 cycles; 5–55 °C), specimens were subjected to dynamic loading in a chewing simulator (600,000 cycles; 10 kg/2 Hz). Surviving specimens were loaded until fracture using a universal testing machine.ResultsAll specimens survived artificial aging, several specimens showed some damage. ANOVA revealed that enamel etching provided statistically significantly (p ≤ 0.05) higher fracture resistance than self-etching when bonding to enamel and dentin. Self-etching provided statistically significant (p ≤ 0.05) higher fracture resistance for the enamel-composite group than for the enamel group. Enamel etching provided statistically significant (p ≤ 0.05) higher fracture resistance for the enamel and dentin group than for groups enamel and enamel-composite.SignificanceEtching enamel improved the fracture resistance of occlusal veneers when bonding to dentin and enamel and increased the survival rate when bonding to enamel.     

Fracture resistance and failure modes of polymer infiltrated ceramic endocrown restorations with variations in margin design and occlusal thickness

Purpose

The purpose of this in vitro study was to assess the effect of varying the margin designs and the occlusal thicknesses on the fracture resistance and mode of failures of endodontically treated teeth restored with polymer infiltrated ceramic endocrown restorations.

Fracture toughness of cross-linked and non-cross-linked temporary crown and fixed partial denture materials

ObjectivesTemporary crowns and fixed partial dentures are exposed to considerable functional loading, which places severe demands on the biomaterials used for their fabrication (= temporary crown & bridge materials, t-c&b). As the longevity of biopolymers is influenced by the ability to withstand a crack propagation, the aim of this study was to investigate the fracture toughness of cross-linked and non-cross-linked t-c&bs.MethodsFour different t-c&bs (Luxatemp AM Plus, Protemp 3 Garant, Structur Premium, Trim) were used to fabricate bar shaped specimens (2 mm × 5 mm × 25 mm, ISO 13586). A notch (depth 2.47 mm) was placed in the center of the specimen using a diamond cutting disc and a sharp pre-crack was added using a razor blade. 60 specimens per material were subjected to different storage conditions (dry and water 37 °C: 30 min, 60 min, 4 h, 24 h, 168 h; thermocycling 5–55 °C: 168 h) prior to fracture (3-point bending setup). The fracture sites were inspected using SEM analysis. Data was subjected to parametric statistics (p = 0.05).ResultsThe K_IC values varied between 0.4 and 1.3 MPa m^0.5 depending on the material and storage time. Highest K_IC were observed for Protemp 3 Garant. Fracture toughness was significantly affected by thermocycling for all dimethacrylates (p < 0.05) except for Structur Premium. All dimethacrylates showed a linear-elastic fracture mechanism, whereas the monomethacrylate showed an elasto-plastic fracture mechanism.SignificanceDimethacrylates exhibit a low resistance against crack propagation immediately after curing. In contrast, monomethacrylates may compensate for crack propagation due to plastic deformation. However, K_IC is compromised with increasing storage time.     

Evaluation of crack propagation in dental composites by optical coherence tomography

ObjectivesThe purpose of this study was to image the sites of fracture initiation and slow crack propagation in a fiber reinforced composite, using the optical coherence tomography (OCT) technique.MethodsBar specimens (2 mm × 3 mm × 25 mm) of fiber reinforced composite were mechanically and thermally cycled to emulate oral conditions. The interior of these samples was analyzed prior to and after loading, using OCT. The device used was a home-built Fourier domain OCT setup working at 800 nm with 6 μm spatial resolution.ResultsIntact specimens after load cycling were analyzed. It was clearly seen that OCT images provide an insight into crack propagation, which is not seen by the naked eye.SignificanceBy using OCT the possibility of analyzing the fracture propagation quantitatively, and in depth, was added, opening up possibilities to quantitative studies.     

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

ObjectiveTo investigate the impact of high-speed sintering, layer thickness and artificial aging in a chewing simulator on the fracture load (FL) and two-body wear (2BW) of 4Y-TZP crowns.Methods4Y-TZP crowns (Ceramill Zolid HT+, Amann Girrbach AG) in three different layer thicknesses (0.5, 1.0, 1.5; N = 192, n = 64/group) were manufactured using CAD/CAM technology and sintered at 1580 °C (high-speed sintering) or 1450 °C (control group). Specimens were polished in two-steps and bonded to standardized CoCr abutments with Multilink Automix (Ivoclar Vivadent). 2BW after 6000 thermo- and 1,200,000 chewing-cycles employing enamel antagonists was determined using best fit machining. FL was tested before and after artificial aging. Univariate ANOVAs, post hoc Scheffé, unpaired t-, Kruskal–Wallis- and Mann–Whitney-U-test were computed (p < 0.05).ResultsHigh-speed sintering resulted in less 2BW of the zirconia than the control group (p = 0.013). High-speed sintering (p = 0.001–0.006) and an increase in layer thickness (p < 0.001–0.012) resulted in higher FL values, while artificial aging led to a reduction of FL (p < 0.001).SignificanceAs high-speed sintering resulted in less two-body wear of the zirconia and comparable or even higher fracture load results than the control group, this cost- and time efficient alternative presents promising mechanical results.     

Three-dimensional trueness analysis of ceramic crowns fabricated using a chairside computer-aided design/manufacturing system: An in vitro study

PurposeThis study analyzed the trueness of polymer-infiltrated ceramic and glass ceramic crowns manufactured using the chairside computer-aided design/manufacturing (CAD/CAM) system.MethodsThe master model designs crowns using a CAD program after acquiring a digital impression with an intraoral scanner. Vita Enamic (VE), Vita Suprinity (VS), and IPS e.max CAD (IPS) were used to manufacture 10 crowns each (total: 30 crowns), using the chairside CAD/CAM system (inLab MC XL). Trueness was evaluated by superimposing the CAD data on the scan data using a three-dimensional program. The Kruskal–Wallis H test, a nonparametric test, and the Mann–Whitney U test were performed by applying the significance level (0.05/3 = 0.016), which was adjusted by post-analysis Bonferroni testing.ResultsThere was a significant difference in the trueness between the samples (p < 0.05). However, there was no statistically significant difference in the outer surface trueness between the samples (p > 0.05).ConclusionsThese findings show that the milling accuracy of VE is better than that of VS and IPS.     

Occlusal geometrical considerations in all-ceramic pre-molar crown failure testing

Objectives

To evaluate the influence of occlusal geometry of all-ceramic pre-molars, namely cusp angle and associated notch radius, on the scatter of load to failure tests.

Methods

Forty-five all-ceramic upper pre-molar crowns with three zirconia core thicknesses (0.4, 0.6 and 0.8 mm) were broken on dental implant abutments oriented in three angulations (0°, 15°, and 30°). The crowns were loaded using a 4 mm diameter steel cylindrical bar placed along the midline fissure at a crosshead speed of 1 mm min⁻¹. The scatter of the failure load was evaluated using Weibull analysis. The cusp angle of each crown was critically evaluated to determine the cusp angle and effective radius of the fissure notch root. The relationship between failure load and cusp angle was compared with that between failure load and effective radius as well as notch induced stress concentration by considering R² values of fitted trend lines with these relationships.

Results

The fracture load differences either between abutment angulations or zirconia thicknesses were not clearly revealed in this study. Except for the group of 30° abutment angulation, the crowns present high scatter of failure loads with low Weibull modulus. However, a simple dependence between fracture load and effective cusp angle was observed.

Significance

Occlusal geometry is an important issue that affects the degree of stress concentration and should be understood by both technician and clinician for appropriate design and material selection of all-ceramic crowns.     

Translational research on clinically failed zirconia implants

Objectives

To provide fractographic analysis of clinically fractured zirconia implants recovered with their cemented crown. To calculate bending moments, corresponding stress and crack onset location on the implant’s fracture surface using a mathematical model integrating spatial coordinates of the crown-implant part and occlusal loading obtained from 2D and 3D images.

Examination of ceramic/enamel interfacial debonding using acoustic emission and optical coherence tomography

ObjectiveThis study investigates monitored micro-crack growth and damage in the ceramic/enamel adhesive interface using the acoustic emission (AE) technique with optical coherence tomography (OCT) under fatigue shear testing.MethodsShear bond strength (SBS) was measured first with eight prepared ceramic/enamel adhesive specimens under static loads. The fatigue shear testing was performed with three specimens at each cyclic load according to a modified ISO14801 method, applying at 80%, 75%, 70%, and 65% of the SBS to monitor interface debonding. The number of cycles at each load was recorded until ceramic/enamel adhesive interface debonding occurred. The AE technique was used to detect micro-crack signals in static and fatigue shear bond tests.ResultsThe results showed that the average SBS value in the static tests was 18.07 ± 1.72 MPa (mean ± standard deviation), expressed in Newton's at 56.77 ± 5.40 N. The average number of fatigue cycles in which ceramic/enamel interface damage was detected in 80%, 75%, 70% and 65% of the SBS were 41, 410, 8141 and 76,541, respectively. The acoustic behavior varied according to the applied load level. Events were emitted during 65% and 70% fatigue tests. A good correlation was observed between the crack location in OCT images and the number of AE signal hits.SignificanceThe AE technique combined with OCT images as a pre-clinical assessment tool to determine the integrity of cemented load bearing restored ceramic material. Sustainable cyclic load stresses in ceramic/enamel bonded specimens were substantially lower than the measured SBS. Predicted S–N curve showed that the maximum endured load was 10.98 MPa (about 34.48 N) passing 10⁶ fatigue cyclic.     

Effect of margin design on fracture load of zirconia crowns

Zirconia‐based restorations are showing an increase as the clinicians’ preferred choice at posterior sites because of the strength and esthetic properties of such restorations. However, all‐ceramic restorations fracture at higher rates than do metal‐based restorations. Margin design is one of several factors that can affect the fracture strength of all‐ceramic restorations. The aim of this study was to assess the effect of preparation and crown margin design on fracture resistance. Four groups of bilayer zirconia crowns (with 10 crowns in each group) were produced by hard‐ or soft‐machining technique, with the following four different margin designs: chamfer preparation (control); slice preparation; slice preparation with an additional cervical collar of 0.7 mm thickness; and reduced occlusal thickness (to 0.4 mm) on slice preparation with an additional cervical collar of 0.7 mm thickness. Additionally, 10 hard‐machined crowns with slice preparation were veneered and glazed with feldspathic porcelain. In total, 90 crowns were loaded centrally in the occlusal fossa until fracture. The load at fracture was higher than clinically relevant mastication loads for all preparation and margin designs. The crowns on a chamfer preparation fractured at higher loads compared with crowns on a slice preparation. An additional cervical collar increased load at fracture for hard‐machined crowns.