Shear Bond Strength of Dental Porcelains to Nickel-chromium Alloys

The continuous technological advance and increasing availability of new base metal alloys and ceramic systems in the market, coupled to the demands of daily clinical practice, have made the constant evaluation of the bond strength of metal/porcelain combinations necessary. This study evaluated the metal/porcelain shear bond strength of three ceramic systems (Duceram, Williams and Noritake) in combination with three nickel-chromium (Ni-Cr) alloys (Durabond, Verabond and Viron). Thirty cast cylinder specimens (15 mm high; 6 mm in diameter) were obtained for each alloy, in a way that 10 specimens of each alloy were tested with each porcelain. Bond strength was measured with an Emic screw-driven mechanical testing machine by applying parallel shear forces to the specimens until fracture. Shear strength was calculated using the ratio of the force applied to a demarcated area of the opaque layer. Mann-Whitney U test was used for statistical analysis of the alloy/ceramic combinations (p<0.05). Viron/Noritake had the highest shear bond sregnth means (32.93 MPa), while Verabond/Duceram (16.31 MPa) presented the lowest means. Viron/Noritake differed statistically from other combinations (p<0.05). Viron/Duceram had statistically significant higher bond strengths than Verabond/Duceram, Verabond/ Williams and Durabond/Noritake (p<0.05). It was also found significant difference (p<0.05) between Verabond/Noritake, Verabond/ Duceram and Durabond/Noritake. No statistically significant difference (p>0.05) were observed among the other combinations. In conclusion, the Noritake ceramic system used together with Viron alloy presented the highest resistance to shear forces, while Duceram bonded to Verabond presented the lowest bond strength. Viron/Duceram and Verabond/Noritake provided intermediate results. The combinations between the Williams ceramic system and Ni-Cr alloys had similar shear strengths among each other.


INTRODUCTION
The alternative alloys for fabrication of metalceramic restorations became more popular in 1960s after the costs of the gold alloys increased.The advantages and properties of base metal alloys have been reported (1,2).Their mechanical properties enable the fabrication of restorations with greater rigidity and less thickness (2).The disadvantages include the potential biologic hazards, difficult handling and uncontrolled chromium oxide formation (3,4).
Nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys are the most used when cost and rigidity are considered.However, these alloys are not universally accepted for casting because they contain beryllium (Be) and other substances that can be harmful (4).It is also difficult to deal with these alloys in the laboratory because of their hardness, rendering necessary accurate casting and preparation procedures prior to firing the porcelain (3).
In Dentistry, the foremost studies proposed the use of feldspathic porcelain (pure) in combination with a framework fabricated in a precious metal such as platinum for preparation of complete crowns (5)(6)(7).This type of porcelain provided better esthetic than that offered by the available resins (methyl methacrylate) for restoration of anterior teeth.
Studies have attempted to explain the nature of the composition that leads to adhesion in the metal/ ceramic interface (3,8).The success of porcelainfused-to-alloy restorations depends on the success of the bond between the ceramic and metal substructure (1).The chemical compatibility between metal and porcelain allows the restoration to resist thermal stress and mechanical forces (9), including a fusing temperature of the porcelain that does not cause distortion of the metal substructure and contraction of the porcelain that can be resisted by the metal (9).
The purpose of this study was to evaluate the shear bond strength of three ceramic systems to three Ni-Cr alloys.
Porcelain build-up and shear strength testing were accomplished using a stainless steel mold, which consisted of two parts: a lower base, with a vertical cylindrical perforation (15 mm high; 6 mm in diameter), and an upper removable portion of semi-circular form, which had a central perforation (3 mm high; 8.4 mm in diameter) that fit perfectly in the upper face of the lower base (2) (Fig. 1).
Thirty standard wax cylinders (15 mm high; 6 mm in diameter) were cast from each alloy.A hightemperature phosphate-bonded investment (Termocast, Polidental Ltd., São Paulo, SP, Brazil) was used and, 45 min after investment (setting at room temperature), the rings were burned-out according to manufacturer instructions.Alloys were melted in individual crucibles with a multiorifice gas-oxygen torch and cast in a broken-arm centrifugal casting machine (Kerr/Sybron, Romulus, MI, USA), with its arm set at four turns.After casting, each ring was bench-cooled and the casting pieces were divested.The specimens were cleaned with 50-μm glass beads in a non-recycling machine (Trijato, Labordental Ltd., São Paulo, SP, Brazil).After casting, each ring was allowed to cool to room temperature and the casting pieces were divested.The removal of casting residues was made with sandblasting with 50 μm glass beads and aluminum oxide mixture with a Trijet machine (Labordental, São Paulo, SP, Brazil).Thirty specimens of each of the three Ni-Cr alloys were obtained.
After the cast pieces were cleaned, the buttons of the specimens were cut with aluminum oxide disks (Dentorium International Inc., New York, NY, USA) and their extremities were shaped to make them parallel to each other.The other surfaces of the specimens were finished to fit in the mold base.After fitting of the specimens, each surface was rubbed with an aluminum oxide stone and blasted with aluminum oxide jets at 80 psi (0.541 MPa).To prevent any possible contamination, the specimens were immersed in distilled water in an ultra-sound machine (Mini Sono Cleaner CA 1470; Kaijo Denki Co. Ltd., Tokyo, Japan) for 10 min.
Porcelain build-up followed two sequences, i.e., one for the opaque and the other for the body, according to the manufacturer's instructions for temperatures and atmospheric conditions.The cylindrical specimens were positioned in the lower half of the mold, with approximately half of its height above that.As the upper part of the mold, with circular form, presents a hole with an 8.4 mm diameter and a 3.0 mm height, it  provided a porcelain layer with similar dimensions for all specimens, that is, 1.2 X 3.00 mm, with an area of 0.2827 cm 2 (Fig. 2).Thirty specimens were obtained from each porcelain system, i.e., 10 specimens of each tested alloy, making up a total of 90 specimens.The procedures for porcelain firing are given in Table 1.
After firing, porcelain finishing was accomplished with diamonded burs and, a second build up of body porcelain was performed.To make those procedures

RESULTS
Shear bond strengths of the combinations of nickel-chromium alloys and ceramic systems are given in the Table 2.
It was also observed statistically significant difference (p<0.05) between Verabond/Noritake, Verabond/Duceram and Durabond/Noritake.However, uniform, the specimens were taken to the oven for the third time, with the same temperatures and atmospheric conditions previously used.
After the applied porcelain area was measured, the lower part of the mold was taken the specimen to an universal testing machine (EMIC model MEM 2000; Emic Equipment and Test Systems Ltd., São José of Pinhais, PR, Brazil).The test configuration was then loaded in shear strength, with generation of forces perpendicular to the ceramic metal interface at a crosshead speed of 0.5 mm/min until failure occurred and failure loads were recorded in MPa.Compressive force was applied to the upper portion of the specimens and stress was produced in the opaque/metal interface.It generated a shear strength that was calculated by dividing the applied force at the failure by the surface area of the opaque layer (Fig. 3).
Data obtained from shear testing were analyzed statistically.Mann-Whitney U non-parametric test was used to verify whether or not there was statistically significant difference among the combinations at 5% significance level.no statistically significant difference (p>0.05) were observed among the other combinations (Table 3).
to point out that few studies have been carried out with the Duceram and Williams systems, mostly comparing these materials with other commercially available ceramic systems.Fernandes Neto ( 17) compared four different alloys in association with three ceramic systems and found that Duceram bonded to Resistal P alloy presented the best results.However, no statistically significant difference was observed when this combination was compared to Duceram/Durabond or to the association between an experimental alloy and Vita VMK 88 or Duceram.In this study, Duceram had an average performance, with lower bond strength than Noritake system.These results are possibly due to the expansion curves and residual contraction of these system (11,18).Fernandes Neto (17) and Yilmaz and Dinçer (9) also found that alloys with titanium could increase the bond strength of dental porcelains.
The findings of this study showed that the Viron alloy had the best performance, followed by Verabond and Durabond.Hammad et al. (13) reported lower bond strength for Vita VMK 88/Durabond alloy when the alloy was submitted to previous oxidation.These observations are in agreement with those of other reports that observed the existence of significant differences among several commercially available brands of Ni-Cr based alloys (12,14,15,18).Therefore, comparisons of the results of studies using different materials are not reliable because of variations in the mechanical properties of the alloys, heating method used for casting or the number of times the alloy was melted (19).In addition, it is important to point out that the strength of the framework does not depend exclusively on the alloy used but also on its design (14,18).Frameworks with sharp and much defined forms commonly have very low fracture strength (14).Therefore, although it is well recognized which general physical properties an ideal alloy should have, it is not possible to make a recommendation to the clinician about which currently available alloys should preferably be used in metal ceramic restorations (20).
In this study, the Noritake ceramic system used together with Viron alloy presented the highest resistance to shear forces, while Duceram bonded to Verabond presented the lowest bond strength.Viron/Duceram and Verabond/Noritake provided intermediate results.
The combinations between the Williams ceramic system and Ni-Cr alloys had similar shear strengths among each other.
Although bond strength data like these can be helpful at clinical level for selection of the most suitable materials to be used, it is important to highlight that, due to the continuous introduction of new metal alloys and ceramic systems, constant studies should be carried out to assess the compatibility of metal/ceramic combinations.This means that, both metal alloys and ceramic systems tested in this study can present higher shear strength if combined with other materials, or tested through techniques specifically developed for each type of alloy or porcelains available.

Figure 1 .
Figure 1.Schematic drawing of the stainless steel mold used for specimen preparation, porcelain application and shear strength testing.A: Lower portion.B: Upper portion.C: Central cylindrical perforation.D: Tested Specimen.E: Porcelain layer.

Figure 3 .
Figure 3. a) Specimen positioned into the mold, after porcelain build-up; b) Specimen being submitted to the shear strength test.

Figure 2 .
Figure 2. Specimens with the opaque (A) and porcelain body after firing (B).