Susanne S. Scherrer

Direct comparison of the bond strength results of the different test methods: A critical literature review

OBJECTIVE The goal of this paper is to undertake a literature search collecting all dentin bond strength data obtained for six adhesives with four tests (shear, microshear, tensile and microtensile) and to critically analyze the results with respect to average bond strength, coefficient of variation, mode of failure and product ranking. METHOD A PubMed search was carried out for the years between 1998 and 2009 identifying publications on bond strength measurements of resin composite to dentin using four tests: shear, tensile, microshear and microtensile. The six adhesive resins were selected covering three step systems (OptiBond FL, Scotch Bond Multi-Purpose Plus), two-step (Prime & Bond NT, Single Bond, Clearfil SE Bond) and one step (Adper Prompt L Pop). RESULTS Pooling results from 147 references showed an ongoing high scatter in the bond strength data regardless which adhesive and which bond test was used. Coefficients of variation remained high (20-50%) even with the microbond test. The reported modes of failure for all tests still included high number of cohesive failures. The ranking seemed to be dependant on the test used. SIGNIFICANCE The scatter in dentin bond strength data remains regardless which test is used confirming Finite Element Analysis predicting non-uniform stress distributions due to a number of geometrical, loading, material properties and specimens preparation variables. This reopens the question whether, an interfacial fracture mechanics approach to analyze the dentin-adhesive bond is not more appropriate for obtaining better agreement among dentin bond related papers.

Low temperature degradation of a Y-TZP dental ceramic

Bars of Y-TZP ceramic for dental restorations were subjected to hydrothermal degradation via in vitro exposure to water steam at 140°C for 7 days. X-ray diffraction, atomic force microscopy and scanning electron microscopy techniques were applied to observe and quantify the tetragonal-monoclinic (t-m) phase transformation associated with the process. Nanoindentation was used to assess the ceramics mechanical properties before and after hydrothermal degradation. Texture associated with martensitic t-m transformation was observed at the grain surface. The t-m transformation followed nucleation-and-growth kinetics, with predominance of the nucleation process. The transformation occurred within a layer of 6 μm below the surface. Mechanical properties deteriorated with hydrothermal degradation, resulting in a 30% reduction of Youngs modulus and hardness. A strong correlation was found between the increasing monoclinic fraction and the decline in mechanical response. It was thus concluded that the emergence of the monoclinic phase and the associated microcracking were the most likely causes for the degradation of mechanical properties.

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic

OBJECTIVES Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test. METHODS The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukeys multiple comparison test (p < or = 0.05). RESULTS For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values. SIGNIFICANCE The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.

Fracture toughness (KIc) of a dental porcelain determined by fractographic analysis

OBJECTIVES Fractographic analysis of indentation cracks is performed following flexure testing as part of the ASTM (1999) standard for fracture toughness, KIc, determination in advanced ceramics. This method depends on the conduciveness of the material towards fractographic interpretation. The purpose of this study was to evaluate the use of fractography in fracture toughness methods with a feldspathic dental porcelain, in which KIc was measured fractographically as well as numerically using two controlled-flaw beam bending techniques. METHODS The following methods for KIc determination were applied to a dental porcelain containing a leucite volume fraction of 15-20%: (1) surface crack in flexure (SCF) (dependent upon fractographic analysis); and (2) indentation strength (IS) at indentation loads of 9.8 and 19.6 N (applying both standard numeric calculations and fractographic analysis). The testing environments were (1) ambient air (IS and SCF) and (2) flowing dry nitrogen (SCF). RESULTS No significant differences were found between numeric and fractographic KIc values for the IS technique at both indentation loads (9.8 and 19.6 N) in ambient air, although KIc values were sensitive to indentation load. Due to the presence of residual stresses, stable crack extension was observed fractographically in all IS specimens, as evidenced by differences between initial (ainitial) and critical (acritical) crack dimensions. For the SCF method, there was a significant difference in toughness between specimens tested in air versus dry nitrogen, however no fractographic evidence for chemically assisted slow crack growth (SCG) was observed. SIGNIFICANCE The SCF method as described by the ASTM standard was applicable to the feldspathic porcelain and produced very comparable results with the numeric toughness calculations of the IS procedure. However, fractographic analysis of the surface crack was somewhat difficult for this glassy ceramic compared with polycrystalline ceramics. Knowledge about stable crack extension or slow crack growth and its fractographic appearance is essential when estimating the toughness from examination of flaw dimensions on fractured surfaces since large calculation errors may occur if these effects are not taken into account.

Microstructural characterization and fracture behavior of a microhybrid and a nanofill composite

OBJECTIVES To characterize the microstructure and composition of two different composites, and to determine their influence on the physical properties and fracture behavior. METHODS The microstructure and composition of a microhybrid (Filtek Z250-Z2) and a nanofill (Filtek Supreme-SU) composite were analyzed using scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS). Filler wt% was determined by thermogravimetric analysis. Hardness (H) and degree of conversion (DC) were evaluated at top and bottom surfaces of 2-mm thick specimens, and the dynamic elastic modulus (E) was determined with ultrasonic waves. Bar specimens (n=30) were subjected to flexure loading and flexural strength (sigmaf) was calculated (MPa). Fractographic analysis (FA) was performed to determine the fracture origin (c) for calculation of fracture toughness (KIc), and these results were compared to those from the single edge notch beam (SENB) method. Results were statistically analyzed using two-way ANOVA, Students t-test and Weibull analysis (alpha=0.05). RESULTS Z2 had higher filler wt%, H, E and DC at 2-mm depth as compared with SU. The fracture behavior (sigmaf and KIc) and the structural reliability (m) of the composites were similar. Results of KIc tested by SENB or calculated from fracture surfaces from flexure testing were similar. SIGNIFICANCE The microstructural organization of the composites determines their physical properties, in spite of the similar filler content. In contrast, the microstructure did not influence the fracture behavior and the structural reliability of these highly filled composites. FA was shown to be a reliable method for determining the KIc of composites.

Clinical outcome of three-unit lithium-disilicate glass-ceramic fixed dental prostheses: up to 8 years results.

OBJECTIVES The purpose of this prospective study was to evaluate the clinical outcome of crown-retained fixed dental prostheses (FDPs) made from a lithium-disilicate glass-ceramic (IPS e.max Press, Ivoclar-Vivadent). METHODS Thirty-six three-unit FDPs were placed in 28 patients. The FDPs replaced teeth in the anterior (16%) and posterior (84%) regions. All teeth were prepared following a standardized protocol. The size of the proximal connector of the FDPs was 12 mm2 (anterior) or 16 mm2 (posterior). FDPs were cemented either with glass-ionomer cement (n=19) or composite resin (n=17). The following parameters were evaluated at baseline, 6 months after cementation and then annually (at abutment and contralateral teeth): probing pocket depth, plaque index, bleeding on probing, and tooth vitality. RESULTS Three FDPs were defined as drop-out. The mean observation period of the remaining 33 FDPs was 86 months (range: 67-98 months): two FDPs in two patients had to be replaced (6%) because of fractures. The 8-year survival rate according to Kaplan-Meier was 93%. In addition, chipping of the veneering material was found in two FDPs (6%). Two abutments (3%) of two restorations had to be treated endodontically; and two FDPs (6%) lost retention and had to be recemented. These complications did not affect the function of the involved restorations clinically. There were no significant differences between the periodontal parameters of the test and control teeth. SIGNIFICANCE Short-span crown-retained three-unit FDPs made from lithium-disilicate glass-ceramic can be used clinically irrespective of an adhesive or conventional cementation.

Fractographic failure analysis of a Procera AllCeram crown using stereo and scanning electron microscopy

OBJECTIVES Presentation of a methodological approach using stereo and scanning electron microscope examination for the failure analysis of an alumina all-ceramic premolar crown (Procera AllCeram). METHODS The recovered part of a fractured Procera alumina crown was examined utilizing first a stereomicroscope and second a scanning electron microscope (SEM). The stereomicroscope analysis was performed at low magnifications with oblique lighting in order to enhance spatial relationships and gross detection of crack features. A preliminary fracture surface map of the stereo observations was drawn and used as a guide for the SEM analysis that followed. Specific sites of interest identified under the stereo microscope were analyzed using the SEM at high magnifications searching for small fracture features such as wake hackle and twist hackle within the veneering ceramic in order to confirm the direction of crack propagation. RESULTS At low magnifications and oblique illumination, the stereomicroscope analysis provided an excellent overview of the fractured topography, showing sites of major interest such as a primary edge chip at a margin, a compression curl indicating the end of the fracture event as well as larger hackle lines distributed over the cracked surface. The greater magnifications with the SEM analysis of the sites of interest showed the presence of wake and twist hackle, indicators of the crack propagation direction. A general map of the fracture events could be reconstructed starting with a primary veneer edge chip at the mesial margin. Hackle and wake hackle of the crack front emanating from this margin arose from hoop stresses and propagated through the full crown thickness towards the distal end of the restoration where the compression curl was located. Additional occlusal surface damage in the form of veneer chipping containing arrest lines and twist hackle running in the opposite direction as the main crack path were observed, but occurred as a secondary event without penetrating the alumina core material. SIGNIFICANCE Stereo and scanning electron microscopy are complementary analysis techniques useful for the mapping and interpretation of the fracture surface. This case examination is intended to guide the clinical researcher in using qualitative (descriptive) fractography as a tool for understanding the failure process in brittle restorative materials, as well as for assessing possible design inadequacies.

Effect of cement film thickness on the fracture resistance of a machinable glass-ceramic

OBJECTIVES The aim of this study was to determine the fracture resistance of a machinable glass-ceramic plate cemented to a resin composite block as a function of the cement film thickness for two types of cement. METHODS Ceramic plates were cemented to resin composite blocks using either zinc phosphate cement or a resin composite cement. For the zinc phosphate cement, the film thickness was 33 +/- 8 microns or 128 +/- 8 microns; for the resin composite cement, the thickness ranged from 26 +/- 11 microns to 297 +/- 48 microns. The elastic modulus was determined for each of the cements. Fracture loads were obtained by using a spherical steel indenter in the center of the glass-ceramic plate. The Weibull distribution was used for the statistical analysis. RESULTS For glass-ceramic plates cemented with zinc phosphate cement, the fracture resistance was independent of the film thickness. When the resin composite cement was used, a gradual decrease of the fracture strength was observed that became statistically significant at a cement thickness of 300 microns or more. The characteristic fracture strength of glass-ceramic plates cemented with the resin composite cement was about 75% higher than when using the zinc phosphate cement. This difference is attributed to the bonding of the resin cement to the ceramic plate and the supporting structure. SIGNIFICANCE The findings of this study suggest that the resistance to fracture due to indentation of the glass-ceramic may not be affected by the cement film thickness as much as previously thought.

Monotonic flexure and fatigue strength of composites for provisional and definitive restorations.

STATEMENT OF PROBLEM Ordinarily, the mechanical strength of composites is characterized by their flexural strength. Information as to the materials fatigue strength is seldom provided. PURPOSE The purpose of this study was to compare the flexural strength and the resistance to fatigue loading of composites and an acrylic resin for provisional and definitive restorations. MATERIAL AND METHODS Artglass, Colombus, and Targis (composites) and Jet, Protemp II, Protemp Garant, and Provipont DC (provisional restorations) were subjected to mechanical tests. Fatigue tests (MPa) (n = 30 specimens/group) were conducted with the rotating-bending cantilever design. Monotonic flexural strength (MPa) (n = 10) was determined in 3-point bending tests. Fatigue resistance was analyzed via the staircase procedure, and flexural strength was examined by use of the 2-parameter Weibull distribution (confidence intervals at 95%). RESULTS The mean fatigue resistances (S(50)) in MPa +/- SD were: Targis, 62.1 +/- 7.0; Artglass, 58.5 +/- 3.7; Colombus, 54.6 +/- 6.2; Provipont DC, 29.5 +/- 3.2; Protemp II, 23.1 +/- 5.3; Jet, 22.8 +/- 8.3; Protemp Garant, 19.6 +/- 4.6. The flexure strengths (Weibulls S(0)) in MPa and their shape parameters (m) were: Colombus, 145.2 (13.1); Targis, 110.3 (7.8); Artglass, 5.9 (5.4); Jet, 150.9 (17.3); Provipont DC, 97.3 (23.8); Protemp II, 57.9 (6.4); Protemp Garant, 54.2 (12.8). The S(50) of Targis was significantly higher than that of Colombus but not different from Artglass. In flexion, the S(0) of Colombus was significantly higher than that of Artglass and Targis. The S(50) ranged between 40% and 60% of the S(0) for the composites and between 15% and 30% for the provisional restorative materials. CONCLUSIONS Correlations between monotonic flexure strength and resistance to fatigue loading were weak. Because fatigue tests are considered more pertinent than monotonic tests as to their predictive value, it is concluded that flexure strength data alone may not provide relevant information for long-term clinical performance. The materials resistance to fatigue loading should also be determined.

Weibull parameters of composite resin bond strengths to porcelain and noble alloy using the Rocatec system.

OBJECTIVE The aim of the study was to determine the Weibull distribution parameters S(0) and m of the tensile bond strength of a composite resin sandwiched between a noble metal alloy and a feldspathic porcelain using the Rocatec system. METHODS Specimens were prepared either as Au-Pt-Pd alloy cylinders alone or as alloy cylinders and a feldspathic porcelain. Surface treatments included 110 microm Al(2)O(3) sandblasting (Rocatec Pre), tribochemical silica coating (Rocatec Plus) and silane application (ESPE-Sil) prior to composite bonding. The tensile bond strengths of ceramic-resin-ceramic and metal-resin-metal specimens were determined before and after thermocycling at 5-60 degrees C for 1000 cycles. Statistical analyses were conducted using two parameter Weibull distributions determined by the characteristic strength (S(0)) at the 63% probability of failure, the Weibull modulus (m) and the 95% confidence intervals. RESULTS With respect to the control group (no Rocatec) (7.5MPa), the Rocatec system significantly augmented the characteristic tensile bond strength S(0) of metal (17-18MPa) and porcelain (20-28MPa) groups. Thermocycling had no effect on the tensile bond strength of the metal samples and increased (not significant) the bond of the porcelain group. The Weibull modulus m for all groups ranged from 2.8 to 3.7. SIGNIFICANCE This study confirmed the efficacy of the Rocatec system for bonding composite resin to metal or ceramic substrates. However, the variability of the strength of the bond is shown in the low Weibull modulus values obtained. This reflect the difficulties in controlling flaw formation and warrants research for future improvement.