Journal of Dental Medicine

Statement of Problem: Dental porcelain is one of the best materials ever used in dentistry. Excellent tissue compatibility, esthetics, very low solubility in oral fluids, high compressive strength, and the lowest bacterial plaque accumulation on the glazed porcelain are some of its advantages. Porcelain brittleness due to its low tensile strength, impact strength and the occlusal attrition of opposing teeth enamel are some of its disadvantages. The most important mechanical properties of dental porcelains are their flexural strength that is the ability of a porcelain to resist fracture when loaded from above. Different ways have been proposed to eliminate porcelain brittleness and develop its flexural strength, among which baking porcelain on a metal framework is more commonly used.
Purpose: Considering that restoration and replacement of teeth by porcelain fused to metal restorations is still the most commonly used technique, the aim of the present study was to measure the flexural strength of a porcelain bonded to metal powder, D4 Dentin, and to compare it with Vita Dentin powder.
Materials and Methods: In this experimental study, a stainless steel box, 25x5x2mm was made according to ISO 6872: 1995 (E) Standard and filled with D4 Dentin powder and Vita Dentin (VMK 68N) and baked in Vita furnace. Then 10 blocks of D^ Dentin and 10 of Vita Dentin were made. Samples were placed on the three point bending test machine and force was applied the middle of each block. To analyze the data, t- student test and co-variance analysis were used.
Results: Mean flexural strength of D4 Dentin was 53.40±1.35 MPa and that of Vita Dentin was 53.86+7.38 MPa. The difference was not statistically significant.

Conclusion: According to ISO 6872: 1995 (E) standard, the flexural strength of all D4 Dentin samples were greater than 50 Mpa.

Background and Aims: The aim of this in vitro study was to compare shear bond strength of metal brackets bonded to dental porcelain on the basis of presence or absence of silane, type of acid [hydrofluoric acid (HF) or phosphoric acid (H3PO4)] and roughness of porcelain surface (glazed or deglazed) within mouth-like environment.
Materials and Methods: Eighty glazed ceramic disks were randomly divided into 8 groups of 10 disks: group 1 [HF+silane], group 2 [deglazed+HF+silane], group 3 [HF], group 4 [deglazed+HF], group 5 [H3PO4+silane], group 6 [deglazed+H3PO4+silane], group 7 [H3PO4], group 8 [deglazed+H3PO4]. Then the brackets were bonded and thermocycled. After that, shear bond strength test was done using the Zwick device and the type of bond failure was determined under stereomicroscope at 4X magnification. 3-way ANOVA and Kruskal-Wallis were used for statistical analyses.
Results: The shear bond strength for the test groups were as follows: group (1):13.05±7.7 MPa , group (2):25.16±10.66 MPa, group (3):6.7±5.86 MPa, group (4):15.39±8.97 MPa, group (5):12.76±7.91 MPa, group (6):13.57±7.85 MPa, group (7):0.54±0.67 MPa, group (8): 9.34±6.52 MPa. The type of bond failure in all groups was adhesive failure except for group 2. No significant difference in the interaction between (glazed or deglazed), (presence or absence of silane), and type of acid was found (P>0.05).
Conclusion: Under the conditions of this study, the best clinical method was the use of 37% phosphoric acid and silane that resulted in the optimal clinical strength and adhesive bond failure.

Background and Aims: The bond strength between veneer ceramics and zirconia frameworks is the weakest component in the layered ceramics. Due to the possible effect of adding pigments to the core materials on the bond strength between core and veneer as well as the introduction of new ceramic materials in dentistry, the aim of this study was to compare the zirconia core-veneer microtensile bond strength using two ceramic veneers with or without coloring the core.
Materials and Methods: In this experimental study, 12 disc-shaped specimens were fabricated using a manually aided design- manually aided manufactured (MAD/MAM) zirconia core (Zirkonzahn). Two veneering ceramics of Ceram Kiss and Zirkonzahn ICE were also used to veneer the specimens. Half of the Zirkonzahn discs were remained white and the others were colored by shade A2.Then, the discs were cut into microbars (30 for each group) and the microtensile bond strength of the core-veneer was calculated. The specimens were assessed using scanning electron microscopy (SEM) and the data were analyzed using two-way ANOVA and Student's t-test.
Results: Significant differences with respect to veneer layer were found (P<0.001). No significant differences were seen among colored and uncolored cores (P=0.69).
Conclusion: According to the apparent effect of veneering ceramics on the core-veneer bond strength, careful selection of these agents is essential to achieve adequate bond strength between core and veneer to prevent delaminating and chipping failures of zirconia veneered restorations.

Background and Aims: The aim of this study was to evaluate the quality of an experimental hydrofluoric acid (HF) for preparation of porcelain and to compare it with two commercial hydrofluoric acids in Iranian trademark.

Materials and Methods: A- Evaluation of etch pattern of experimental HF using scanning electron microscope (SEM): 6 feldespathic discs were divided into 3 groups. Each group was etched with related HF (experimental, Ultradent and Kimia) for 1 minute. SEM images were recorded at 3 magnifications. B- Bond strength test: 18 feldespathic discs were considered for each acidic group. Then the porcelain surfaces were etched and bonded to composite with unfilled resin. Consequently, the microshear test was done. C- Microleakage test: 54 discs were divided into 3 groups (n=18). Then the porcelain surfaces were etched and bonded to composite with unfilled resin and finally observed under stereomicroscope. The data were analyzed with one-way ANOVA and Smirnov tests.

Results: SEM analysis showed no difference between groups in terms of etch pattern. Microshear bond strength values for experimental, Kimia, and Ultradent HF were 28.53 (±4.92), 28.21 (±6.61), and 26.14 (±7.61) MPa, respectively. There was no significant difference between the bond strength of test groups (P<0.05). Furthermore, no significant difference was found between the microleakage of test groups (P>0.05).

Conclusion: Quality of experimental HF in terms of etch pattern, microshear bond strength and microleakage of composite/porcelain interface was similar to that of two commercial hydrofluoric acids.