C.J. Kleverlaan

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

OBJECTIVE The 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. METHODS Replicas 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). RESULTS Clinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2+/-9MPa), defects at core veneer interface (27.7+/-6MPa), the generation of Hoop stresses (884.3+/-266MPa), radial cracking (831MPa), or fracture of the connector (971+/-343MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4+/-34.8MPa) or by fracture of the connector (1098.9+/-259MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F=6.8, P<0.01). SIGNIFICANCE Within 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.

Microtensile bond strength of three simplified adhesive systems to caries-affected dentin

PURPOSE The purpose of the study was to determine the microtensile bond strength of three different simplified adhesive systems to caries-affected dentin. MATERIALS AND METHODS Fifteen extracted human molars with primary carious lesions were ground flat until dentin was exposed. Soft caries-infected dentin was excavated with the help of caries detector dye. On the remaining hard dentin, a standardized smear layer was created by polishing with 600-grit SiC paper. Teeth were divided into three groups and treated with one of the three tested adhesives: Adper Scotchbond 1 XT (3M ESPE), a 2-step etch-andrinse adhesive, Clearfil S3 Bond (Kuraray), a 1-step self-etching or all-in-one adhesive, and Clearfil SE Bond (Kuraray), a 2-step self-etching adhesive. Five-mm-thick composite buildups (Z-250, 3M ESPE) were built and light cured. After water storage for 24 h at 37ºC, the bonded specimens were sectioned into bars (1.0 x 1.0 mm; n = 20 to 30). Microtensile bond strength of normal dentin specimens and caries-affected dentin specimens was measured in a universal testing machine (crosshead speed = 1 mm/min). Data were analyzed using two-way ANOVA and Tukeys post-hoc test (p < 0.05). RESULTS No significant differences in bond strength values to normal dentin between the three adhesives were found. Adper Scotchbond 1 XT and Clearfil S3 Bond showed significantly lower bond strength values to caries-affected dentin. For Clearfil SE Bond, bond strength values to normal and caries-affected dentin were not significantly different. CONCLUSION All the tested simplified adhesives showed similar bond strength values to normal dentin. For the tested 2-step etch-and-rinse adhesive and the all-in-one adhesive, the bond strength values to caries-affected dentin were lower than to normal dentin.

Data Comparison Between Two Dental Spectrophotometers

OBJECTIVES The objective of this study was to clinically test whether the data from two different spectrophotometers, based on spot and surface measurements, can be compared. METHODS Under standardized clinical conditions two devices (Vita Easyshade and Spectro Shade-Micro) were used to record the color of three areas (cervical, middle, and incisal) per tooth for three upper maxillary anterior teeth in 102 participants. Each position was measured three times to attain an average for the CIE L*a*b* coordinates and to attain the corresponding Vita Classical shade tab integrated in the software of both devices. Vita tabs were also described as L*a*b* values using earlier published translations so that color differences (ΔE) could be calculated between them. RESULTS The regression analysis between the two devices showed that the independent correlation coefficients of the L*a*b* values are low. Yet when the suggested shade codes are compared with Vita colors instead of L*a*b*, 40% of the cases were equal and 51% were clinically acceptable. SIGNIFICANCE According to this study the two devices do not give a comparable shade selection output, and thus the exchange of L*a*b* values between the two spectrophotometers cannot be recommended.

Influence of polymerization mode and C-factor on cohesive strength of dual-cured resin cements.

OBJECTIVES The aim of this study is to determine the influence of the C-factor and the mode of polymerization on the cohesive strength of various dual-cure resin cements. METHODS Three curing conditions were tested; chemical curing with free shrinkage conditions (C=0), and constraint shrinkage conditions (C=25), and dual-curing with free shrinkage conditions (C=0). Opaque polyethylene, brass (pretreated with Clearfil SE bond), and transparent polyethylene tubes respectively, were filled with the different cements. The tubes were 20mm long with an inner diameter of 1.6 or 1.8mm. Five cements, DC Core Automix, Panavia F 2.0, Maxcem, Multilink, and RelyX Unicem, were tested with ten specimens per group. The specimens were trimmed to an hour-glass shape with a neck diameter of 1mm, stored in water (37°C, 24h), and subjected to microtensile testing (1mmmin(-1)). SEM analysis was carried out on chemically cured samples of DC Core Automix C=0 and C=25. Data were statistically analyzed (Two-way ANOVA, Tukeys post hoc test, p<0.05). RESULTS Most cements showed no significant differences between the curing modes. A high C-factor negatively influences the cohesive strength of some cements. SEM analysis shows that chemical curing of DC Core Automix in a high C-factor environment leads to more and larger microvoids in the cement. SIGNIFICANCE Constraint shrinkage conditions, i.e. a high C-factor, can negatively influence the physical properties of a dual-cured resin cement, which would clinically be the case in the confined space of a root canal or post space preparation.

Palladium-induced Th2 cytokine responses reflect skin test reactivity

Recently, a crucial role of Th2 responses in nickel allergic contact dermatitis (ACD) was demonstrated. As palladium allergy is an issue of growing interest, the diagnostic potential of Th2 parameters for palladium sensitization was investigated. Palladium (Na2[PdCl4])‐induced lymphocyte proliferation (LPT), Th1 and Th2 cytokine production were correlated with skin test (ST) reactivity in 16 positive and 21 negative controls. Furthermore, the diagnostic potential of these assays was evaluated using receiver operating characteristics (ROC) analysis. For comparison, same experiments were carried out for nickel (NiSO4). Correlation coefficients between palladium ST reactivity and IFN‐γ, LPT, IL‐5, and IL‐13 were 0.34, 0.51, 0.69, and 0.78, and overall test accuracies were 68%, 81%, 89%, and 95%, respectively. Both palladium‐ and nickel‐mediated Th2 responses tightly correlate with ST reactivity, supporting recent findings on the crucial role of Th2 involvement in ACD. Therefore, these assays may have great potential as diagnostic tools for future in vitro sensitization testing.

Fatigue Resistance of Y-TZP/Porcelain Crowns is Not Influenced by the Conditioning of the Intaglio Surface

OBJECTIVES The objective of this study was to investigate the effects of treatments of the intaglio surface of Y-TZP frameworks and luting agents on the fatigue resistance of all-ceramic crowns. METHODS A research design was chosen that attempted to reduce the likelihood of Hertzian cracking and to increase the probability of fracture initiation at the intaglio surface of the framework. Ninety identical preparations were machined in a dentin-like epoxy composite. Each preparation was restored with a Y-TZP framework made by a CAD/CAM system and veneered using feldspathic ceramic. Prior to cementation the intaglio surface of the ceramic was treated using one of four treatments: 1) cleaning with isopropyl alcohol; 2) application of an overglaze; 3) sandblasting with 125 μm aluminum oxide powder; and 4) sandblasting with 30 μm silica powder (CJ). One of three luting cements were used: 1) zinc phosphate; 2) glass ionomer; and 3) adhesive resin cement (PN). All three cements were tested against frameworks that were alcohol cleaned. Only the PN cements were tested against frameworks that had been sandblasted or glazed. Altogether, six groups of 15 specimens each were tested. Fatigue resistance was evaluated using stepwise loads at 1.4 Hz until failure: 5000 cycles at maximum load of 200 N, followed by 10,000 cycles at maximum loads of 800, 1000, 1200, and 1400 N. The cement thickness and failure modes were analyzed using a stereomicroscope and scanning electron microscopy. The results were analyzed using the Kaplan-Meier and Mantel-Cox log rank tests (5%), a one-way analysis of variance, Tukey multiple comparison test, and Weibull nonparametric test. RESULTS The predominant failure mode was chipping of the veneer. The crowns cemented with the adhesive resin cement exhibited chipping failure at higher mean loads than did crowns cemented with cements that usually do not bond strongly with dentin. When the adhesive cement was used, glazing and sandblasting intaglio framework surface treatments exhibited lower mean loads at chipping than did crowns whose intaglio surface was only cleaned with alcohol. Weibull analysis indicated that all specimens had a high ratio of late-to-early failures. CONCLUSIONS The fatigue experiment produced a pattern of failures that is very similar to that observed in clinical trials of Y-TZP crowns that are veneered with feldspathic porcelain. Crowns cemented with an adhesive resin cement exhibited chipping at a significantly higher mean load than those cemented with luting cements that do not usually form strong bonds with dentin. When cemented with adhesive resin cement, glazing or sandblasting the intaglio surface of the framework significantly reduced the mean fatigue loads at which chipping of veneers occurred, as compared to crowns whose intaglio surface had only been cleaned with alcohol. For this cement glazing or sandblasting the intaglio surface of the crown is not recommended.

A comparison of finite element analysis with in vitro bond strength tests of the bracket–cement–enamel system

The aim of this study was to determine the in vitro shear bond strength (SBS) and tensile bond strength (TBS) of 45 metal brackets bonded with Transbond XT to bovine enamel. The SBS was determined by loading the short and the long sides of the bracket base. Testing took place after storage of the specimens for 72 hours in water at 37°C. Fractures were analysed with the adhesive remnant index (ARI) and scanning electron microscope (SEM). The stresses in the system were analysed with finite element (FE) analysis models of the experimental set-up to identify the initial fracture point and the stress distribution at fracture. Statistical analysis of bond strengths was performed using analysis of variance (ANOVA) and the Tukeys post hoc test (P < 0.05). The ARI scores were analysed using Kruskal-Wallis one-way ANOVA on ranks. ANOVA showed significant differences between the three experiments. Loading the short side of the bracket resulted in the highest average bond strength. Tensile loading gave the lowest results. FE models supported the bond strength findings and SEM. FE analysis revealed peak stresses in the cement during loading, confirming that shear testing is sensitive to loading angles. The stress distribution over the bracket-cement-enamel system is not homogeneous during loading. Fractures are initiated at peak stress locations. As a consequence, the size of the bonding area is not predictive of bond strength. The bracket design and the mode of loading may be of greater relevance.

Static and dynamic failure load of fiber-reinforced composite and particulate filler composite cantilever resin-bonded fixed dental prostheses

PURPOSE The aim of this study was to evaluate in vitro the influence of fiber reinforcement and luting cement on the static failure load (SFL) and dynamic failure load (DFL) of simulated two-unit cantilever resin-bonded fixed dental prostheses (RBFDPs). MATERIALS AND METHODS Forty-six particulate filler composite (PFC) beams and 76 fiber-reinforced composite (FRC) beams were prefabricated and subsequently luted (RelyX ARC or Panavia F2.0) onto flat ground bovine enamel. The SFL of the different specimen types was determined with a peel test and the DFL was determined with a rotating cantilever beam fatigue testing device. RESULTS The PFC specimens showed a significantly lower SFL than the FRC specimens. The luting cement showed a significant effect on the SFL of the PFC specimens, but not with FRC. The DFL of PFC specimens was significantly lower than for FRC specimens. The luting cement showed a significant effect on the DFL of the PFC specimens, but not so with FRC. With both the SFL and the DFL tests all PFC beams fractured, leaving the bonded part on the tooth surface, but FRC beams partially debonded from the tooth surface, leaving fibers connected to the enamel surface to a varying extent. Coincidentally, the uncured fibers turned out to be prone to aging, an effect which has been investigated. CONCLUSION Within the limitations of this study, it can be concluded that PFC without fiber reinforcement is not suitable for the fabrication of two-unit cantilever RBFDPs, despite the significant effect of the luting cement, but FRC is suitable.

The influence of varying layer thicknesses on the color predictability of two different composite layering concepts

OBJECTIVE Optical properties of teeth are mimicked by composite layering techniques by combining a relatively opaque layer (dentin) with more translucent layers (enamel). However, the replacing material cannot always optically imitate the tooth when applied in the same thickness as that of the natural tissues. The natural layering composite system is available in 2 concepts: (1) dentin (D) and enamel (E) have the same shade but with different translucencies; (2) D and E have different shades where E is always the same high translucent shade. The objective was to evaluate the influence of varying thicknesses of E and D composites on the overall color and on the translucency for both concepts. METHODS For each concept three composite brands were tested; Concept 1: Clearfil Photo Bright (Kuraray), Herculite XRV Ultra (Kerr), Venus Diamond (Heraeus Kulzer); Concept 2: Amaris (VOCO), CeramX Duo (DENTSPLY) and Point4 (Kerr). Two specimens of each shade (A1-A3) per composite were made of standardized thicknesses with a poly-acrylic mold and Teflon cover, making 36 specimens of wedge-like dimension. The L*a*b* values were measured three times against a white and black background (n=216). Students t-tests revealed significant levels between the average ΔE* values of the 3 areas for each composite. RESULTS Statistically significant differences (p<0.05) were found for all thicknesses and for all shades between the concepts. Concept 2 showed greater variations in ΔE* with increased thicknesses. SIGNIFICANCE Concept 2 composites are more sensitive to layer thickness changes, which implicates less predictability in a daily clinical routine.

The influence of pigments on the slow crack growth in dental zirconia

OBJECTIVES Partially yttria stabilized zirconium oxide was introduced as core material for core-veneered full ceramic dental restorations, because of its biological inertness, high mechanical strength, and toughness. In order to improve the esthetical possibilities pigments in the core are introduced, that might influence the stabilization by yttrium. METHODS Double torsion tests were performed to study the influence of the pigments in the core ceramics on its fracture toughness. RESULTS A significant difference was observed in the stress intensity factor (K(10)) as well as in the R-curve behavior between the ceramic with and without pigment. SIGNIFICANCE The lower stress intensity factor for the ceramic with pigment could affect the clinical performance of dental zirconia restorations with this material.