Fracture properties of human teeth in proximity to the dentinoenamel junction.

Under conditions of controlled failure, the fracture properties of dentin at and in proximity to the dentinoenamel junction (DEJ) were studied with a work-of-fracture (Wf) technique and by scanning electron microscopy (SEM). In 37 degrees C distilled water, fracture of normal human teeth at the DEJ was a difficult failure mode to investigate. Even with high stress concentration at the DEJ, only a few specimens showed apparent DEJ failure at localized regions. SEM fractography of these localized regions revealed surfaces similar in some respects to surfaces found in other studies of the DEJ of diseased teeth and decalcified or desiccated teeth. SEM fractographs varied depending upon the distance of the fractured surface from the DEJ. The results of this research suggest that the work of fracture of dentin increases for failure in the near vicinity of the DEJ, probably reaching a maximum at the DEJ.

Fracture properties of human enamel and dentin in an aqueous environment.

SEM fractography and work-of-fracture techniques were used to investigate the fracture properties of human enamel and dentin as a function of the temperature of an aqueous environment. Both enamel and dentin were found to be anisotropic with respect to mechanical failure but were found to have little variation in fracture properties with respect to temperatures from 0 degree to 70 degrees C. Fractographs of a naturally fractured tooth were consistent with experimental findings. The results of this research indicate that natural fracturing of teeth probably cannot be explained on the basis of a reduced work of fracture resulting from chewing hot or cold foods. It was also shown that if dentin surfaces can be kept below 100 degrees C during cavity preparation, the restored tooth will not be weakened by an irreversible reduction of its work of fracture.

Mechanical properties of glass-only porcelains prepared by the use of two feldspathic frits with different thermal properties.

Low- and non-leucite-containing commercial porcelains with low firing temperatures have become popular. However, improving the strength of glass porcelains is difficult. The purpose of this study was to determine if dispersed glass particles could be used as a reinforcing agent for an all-glass porcelain. We produced 3 feldspathic glasses (high-fusing, medium-fusing, low-fusing) by melting powders consisting of potassium-feldspar and 0, 5, or 20 mass% Na2O, respectively. For high-fusing, medium-fusing, and low-fusing feldspathic glasses, the deformation temperatures were 945 degrees C, 647 degrees C, and 518 degrees C, and the thermal expansion coefficient values were 8.6 x 10(-6)/degrees C, 10.3 x 10(-6)/degrees C, and 13.4 x 10(-6)/degrees C between 25 degrees C and the glass-transition temperature, respectively. The high-fusing-glass (or medium-fusing-glass) powders were mixed with low-fusing-glass powders before being fired into test specimens. The mean flexural strength and fracture toughness (K1C) of 3 single-glass porcelains ranged from 57 to 63 MPa and from 0.68 to 0.73 MPa m(1/2), respectively, presenting no significant differences in one-way ANOVA. However, the flexural strength of 50% high-fusing-glass + 50% low-fusing-glass porcelain was 114 MPa (p < 0.05) and K1C was 1.2 MPa m(1/2) (p < 0.05). Microcracks were observed with a back-scattered scanning electron microscope and were associated with the high- (or medium-) fusing glass particles, suggesting residual stress in the low-fusing-glass matrix due to a coefficient of thermal expansion mismatch between the dispersed glass particles and the matrix glass. The dispersing glass particles appeared to act as a reinforcing agent for strengthening a glassy porcelain.

Analysis of the "shear" bond strength of pretreated aged composites used in some indirect bonding techniques.

Investigations of the repair of composites have shown that bond strengths can be significantly reduced at an interface involving an aged composite. Brackets placed by some indirect bonding techniques also have an interface involving an aged composite. This study investigated some of the properties of sealant-composite interfaces and parameters affecting these properties independent of other regions and interfaces found in the total enamel-sealant-composite-bracket system using a shear bond-strength test and fractographic analysis. The effects of various pretreatments of 7-day-old composite surfaces on the bond strength and contact angle of a mixed sealant on these pretreated surfaces were determined. Fractured specimens with adhesive failure closest to the point of force application had low bond strengths (17.62 MPa), which could be correlated with surface pretreatment. Preliminary tests showed that acetone pretreatment produced the most consistently strong interfaces with the fewest adhesive failures of this type. Specimens with adhesive failure only in other regions had higher bond strengths (23.41 MPa), which showed no dependence on pretreatment. There was no statistical relationship between contact angle and bond strength. The data coupled with fracture path analysis suggest that: first, the critical region for fracture analysis is that part of the adhesive interface which is closest to the point of force application; second, failure during some dental shear bond strength tests probably occurs as a consequence of tensile stress induced by a bending moment rather than by shear stress; and third, specifying bond strength as breaking load/area may be incorrect.

Fracture properties of human enamel and dentin.

Fracture in enamel is anisotropic with respect ot the orientation of the enamel rods, with Wf for fracture parallel to the rods being 0.3 X 10(2) J/meter2 but on the order of 2.0 X 10(2) J/meter2 for fracture perpendicular to the rods. Fractographs of enamel showed that the enamel rods behaved as integral units during controlled fracture. A model was proposed to explain the fracture properties of enamel involving the assumption that the hydroxyapatite crystals did not fracture but that their orientation determined the fracture properties. Dentin was also anisotropic, with fracture parallel to the dentinal tubules being the strong direction - Wf was 5.5 X 10(2) J/meter2 for fracture parallel to the tubules, but 2.7 X 10(2) J/meter2 for fracture perpendicular to the tubules. Wf for dentin was of the same magnitude as the high strain rate Wf found for bone (2.0 X 10(2) J/meter2 [ref 3]). The data obtained in this investigation indicated that dentin and enamel are brittle substances and that forces occasionally imposed during mastication or bruxism probably are capable of initiating fracture when the normal anatomy of the tooth has been altered by cavity preparation. As consequence, the design of cavity preparations should include considerations to reduce stress concentrations and thus improve the practical fracture strength of tooth structure.

Stress induced phase transformation of a cesium stabilized leucite porcelain and associated properties.

OBJECTIVES: The addition of CS2O to dental porcelains might provide a means for controlling the thermal expansion and toughness of feldspathic porcelains. The purpose of this investigation was to determine for a leucite porcelain the effect of CS2O content upon its coefficient of thermal expansion (alpha), toughness, hardness, and content of low (tetragonal) leucite and high (cubic) leucite. METHODS: In order to determine the amount of low leucite in the specimens, an x-ray calibration curve for low leucite and an internal standard of copper was obtained using quantitative x-ray diffraction techniques. Utilizing a stress induced phase transformation between low and high leucite, an x-ray intensity conversion ratio (r) was determined for high leucite so that the calibration curve for low leucite could be used to determine the amount of high leucite present in the experimental porcelains. Specimens were prepared with various amounts of CS2O (0.0, 0.5, 1.0, 1.5, 2.0 mol%) so that, except for the as-received porcelain (A), all had the same thermal history. Specimens were tested for content of low and high leucite, hardness (Vickers), toughness (indentation-strength method), and coefficient of thermal expansion (alpha) over two temperature ranges (30-500 degrees C and 30-640 degrees C). Fractured surfaces were examined with a scanning electron microscope (SEM). For each property, specimen groups were compared for statistical differences. These comparisons were statistically analyzed using analysis of variance (ANOVA) and Fisher's protected least significant differences (PLSD). RESULTS: Quantitative x-ray examination of abraded and heat-treated specimens demonstrated that a stress induced phase transformation occurred which could be reversed by heat treatment. The conversion ratio was determined as r = 1.93 +/- 0.29. The addition of CS2O lowered the wt% of low leucite from 63 +/- 6% to 0% and increased the amount of high leucite from 0% to 62 +/- 5%. ANOVA showed that the addition of CS2O had a significant effect on alpha (p < 0.0001), hardness (p < 0.005), and toughness (p < 0.0001). CS2O significantly (PLSD) lowered the alpha (p < 0.0001), hardness (p < 0.01), and toughness (p < 0.0001) of a high-content leucite porcelain. SIGNIFICANCE: A stress induced phase transformation of high leucite to low leucite was demonstrated and, as a consequence, suggested the potential for phase transformation toughening. The alpha of a leucite porcelain could be controlled by stabilizing high (cubic) leucite to room temperature with the fraction of high leucite dependent upon the amount of CS2O added. A method was developed to determine the amount of high leucite present in a porcelain.

Optimum particle size distribution for reduced sintering shrinkage of a dental porcelain.

OBJECTIVES: The purpose of this investigation was to develop a systematic method for obtaining an optimum particle size distribution for a given dental porcelain and compaction method that had a low sintering shrinkage. METHODS: Particles of coarse, medium and fine size were obtained from a commercial dental porcelain using a sedimentation method. Particle sizes were chosen so that the smaller particles would fit into the interstices between the next larger size. The different-sized powders were mixed in various proportions and compacted into a cylindrical mold. The specimens were fired and their linear firing shrinkages determined. An equation was developed to predict optimum proportions. The shrinkage of these mixed powders was compared to the shrinkage of the component powders, to the as-received commercial porcelain, and to predictions based on theoretical equations. These comparisons were statistically analyzed using analysis of variance (ANOVA) and Fisher's Protected Least Significant Differences (PLSD). RESULTS: Mixing specific proporations of different-sized particles significantly reduced sintering shrinkage (p = 0.01) for all mixtures except for one trimodal mixture having the smallest particles. These reductions in shrinkage were obtained for the proportions predicted by the developed equation. Sintering shrinkages for predicted proportions of a three-component mixture and two two-component mixtures were significantly less than that of the original powder (p = 0.01). SIGNIFICANCE: A systematic method was developed for producing dental frits with low firing shrinkage. Mixing of different-sized particles produced frits with lower sintering shrinkage. The optimum proportions of the sized particles were given by an equation. The results of this study suggest that optimum particle size distributions for frits are dependent upon compaction methods.

Bond strength of aged composites found in brackets placed by an indirect technique.

The "Thomas" indirect technique for bracket attachment produces an interface not present in direct techniques, that is, an aged composite-sealant interface. Our primary goal was to determine if a weakened interface was produced by a modified (sealant was mixed prior to placement of brackets) Thomas indirect technique when the composite was aged for 7 days. The enamel-bracket system was investigated in vitro by comparison of shear bond strengths for metal and ceramic brackets bonded to bovine teeth by a direct and indirect method. Nearly all specimens failed at the bracket-composite interface and, subsequently, no difference was found between specimens placed by direct or indirect methods. No evidence was found to suggest that an aged composite would predispose the enamel-bracket system to fail at the sealant-composite interface. The ceramic brackets used in this investigation had lower bond strengths then metal ones, but the breaking loads were similar.