EVALUATION OF THE SHEAR BOND STRENGTH OF THE UNION BETWEEN TWO CoCr-ALLOYS AND A DENTAL CERAMIC AVALIAÇÃO DA RESISTÊNCIA AO CISALHAMENTO DA UNIÃO ENTRE DUAS LIGAS A BASE DE CoCr E UMA CERÂMICA

1Graduate Student, Department of Dental Materials and Prosthodontics, São José dos Campos Dental School, UNESP, São José dos Campos, São Paulo, Brazil. 2DDS, MSc, PhD Student, Department of Dental Materials and Prosthodontics, São José dos Campos Dental School, UNESP, São José dos Campos, São Paulo, Brazil. 3DDS, MS, PhD, Associate Professor, Department of Dental Materials and Prosthodontics, São José dos Campos Dental School, UNESP, São José dos Campos, São Paulo, Brazil.


INTRODUCTION
Currently, the options for prosthetic rehabilitation have increased significantly with the popularization of metal-free prosthesis or the rank of osseointegrated implants, presenting satisfactory results concerning esthetics or function.However, the conventional metal-ceramic fixed partial dentures are still of great importance in the dental clinic because of their versatility, being viable where other therapeutical modalities are not, as well as for its accessible cost.Therefore, fixed partial dentures with metallic structures of noble or alternative alloys still have its place guaranteed in the everyday dental practice.
The oral rehabilitation, besides reestablishing function and esthetics, must respect the patient's financial capacity.Therefore, the use of noble alloys for fabrication of ceramometal dentures should be avoided in many cases because of their high cost 4 .
The use of alternative metal alloys is an unquestionable need, and the use of CoCr and NiCr alloys in particular have permitted high quality treatment for a large number of patients with limited financial means in developing countries such as Brazil 1 .These and other non-precious alloys have been developed and have become superior to the gold-based metals in several aspects, including hardness, elasticity modulus and tensile strength 1,8,26 .Thinner infrastructures became viable, with a smaller volume of material, but with the necessary rigidity for extensive fixed partial dentures 26 .
The currently most used non-precious alloys are the nickel-chromium based alloys with or without beryllium.However, their biocompatibility has been questioned concerning possible damages to the health of the patient and professionals involved in the fabrication of prosthesis caused by long exposure to Ni and Be 2,14 .The allergic effect of Ni and the toxic potential of Ni and Be for the laboratory technician still causes divergences among professionals of the area about the use of these materials 8,21 .The stability of NiCr alloys is lower in acidic solutions or in the presence of the acidic nature of the plaque, which modifies the release of Ni 24,25 .Thus, there is a tendency of NiCr alloys to be substituted by more biocompatible ones.
An option to nickel-chromium alloys is the cobaltchromium alloy, an alternative that does not sacrifice the physical properties of the metal ceramic systems 8,14 .Studies in animals substantially show that the CoCr alloys are relatively well tolerated 8 , being therefore more biocompatible than the NiCr alloys.However, the metal/ceramic complex does not have all its performances and properties completely defined.As a consequence, more studies on the subject are necessary.
The success of the ceramometal restorations depends on the strength and integrity of the adhesion between metal and porcelain, the most susceptible site for occurrence of cracks 4,5,7,17,18 .The cracks generally progress through the metal/ceramic interface or through the covering porcelain 5,23 .Therefore, the resistance of this fragile area must be evaluated.
Evaluation of the resistance of the interface between two cobalt-chromium alloys and ceramics is performed by the shear test, which allows evaluation of the metal/ceramic adhesion by inducing tension on the interface, so that the crack initiates at the most fragile point 7,16 .
Based on the importance of the integrity of the metal/ ceramic interface and the scarcity of works evaluating this interface in cobalt-chromium alloys, the purpose of this study was to verify the shear bond strength of the interface between two CoCr alloys and a ceramic.

MATERIAL AND METHOD
Twenty-two ceramometal specimens were made in standardized cylindrical format.For creation of the metallic portion of the specimen, a device was used to standardize the waxed parts.This device is composed of two milled metal parts.The first structure is a metallic ring that served as a matrix placed on a flat surface where the wax was melted.The purpose of the second structure is to remove the wax standardized by the first.Twenty-two wax molds were made to obtain, through the lost wax casting technique, the correspondent metallic portion of the specimens in two types of cobalt-chromium alloys (Wirobond C, Bego; Remanium 2000, Dentaurum), whose composition is presented in Table 1.After unmounting and removal of the sprue, the cylinders were finished in order to refine and finish its forms, since all specimens must have accurate dimensions to allow a perfect relation with the shear device.
The metallic structures were prepared for application of the covering ceramics (Vita Omega 900, Vita Zahnfabrik), first with aluminum oxide spurt (100µm) for 10 seconds at a distance of 2cm and at 45°.The structures were cleaned with tap water, isopropyl alcohol and then dried.
Application of the ceramics was performed according to the manufacturer's instructions, on the upper surface of the cylinders with the aid of a Teflon matrix to standardize its dimensions.The metallic structures were placed in each of the ten orifices of the Teflon device, and 4mm of ceramic material were applied.The specimens were carefully removed and fired as recommended by the manufacturer.Contraction of the ceramic was compensated with a second firing, allowing achievement of the samples' final dimensions as described in Figure 1.
After preparation of the specimens, they were divided into experimental groups as described in Table 2.
For accomplishment of the mechanical shear bond test, a steel device was made (Figure 2).This device is composed of two independent pieces.The first one (A) is cylindrical with a flat adaptation on its walls to allow introduction of the second part (B) in its interior.The second part, also cylindrical with one flat wall, served as a piston during mechanical evaluation.On the flat sides there is a perforation with 4-mm diameter to allow introduction of the specimens in both parts simultaneously.The metallic portion is lodged in part A and the ceramic portion in part B. The set was placed in a universal testing machine (EMIC) and on the upper cylindrical prolongation of part B, load was applied at a constant speed of 0.5mm/min until the samples were cracked.This way, the maximum load supported by the metal/ ceramic interface was obtained.

RESULTS
The means and standard deviations obtained from the data of the shear bond strength test for each alloy investigated can be seen in Figure 3. Analysis of variance was performed for the studied groups.No statistically significant difference was observed between groups at a significance level of 5% (p=0.05).

DISCUSSION
Nowadays, the use of alloys, especially NiCr with and without Be, represents the largest amount of alloys used in ceramometal prosthesis.On the other hand, questions about toxic and allergenic elements and even on the carcinogenic potential of these alloys have been raised 8,[12][13][14]21 . Thebiological characteristics of these alloys should also be taken into consideration when selecting the material 12,13 , considering that their use could harm the health of patients and professionals involved in fabrication of the prosthesis.The Ni is considered one of the most common causes of allergic dermatitis 1 , appearing in researches as a component with the higher allergenic 8,13 and toxic potential together with Be 10 .
Therefore, the CoCr alloys were developed to become an option to the Ni-based alloys, and are considered secure substitutes for clinical use 8 with favorable physical and mechanical properties 8,14 .Thus, the performance of the metalceramic interface of these alloys must be evaluated and, in

FIGURE 2 -
FIGURE 2-Device for the mechanical test: A) external part; B) internal part; C) specimen.The arrow indicates the orifices where the specimens were placed

FIGURE 1 -
FIGURE 1-Specimen project and respective dimensions

FIGURE 3 -
FIGURE 3-Graphic representation of the means and standard deviations of shear resistance data (MPa) according to the alloy used

TABLE 2 -
Experimental groups division