Porcelain-fused-to-metal system is the most common indirect restoration in dentistry. ¹ Precious and nonprecious metal alloys are used in this regard. Noble alloys used in the infrastructure of these restorations have an excellent bond to the porcelain, good mechanical properties, and high biocompatibility. However, due to the high price of these alloys, their uses are limited. ²

Nickel–chrome alloys, which were cheaper than gold, were firstly introduced for partial veneers, bridges, and custom frames. However, many adverse reactions have been reported following the use of these alloys. Nickel in particular has played a major role in causing these reactions. ³ Nickel and beryllium alloys are carcinogens in animals. ⁴ , ⁵ Toxic and allergic reactions and issues in metabolic processes have been reported in some cases. ⁶ , ⁷ Beryllium vapors from metal spills can cause conjunctivitis, dermatitis, and bronchitis. ⁸ , ⁹ Cobalt–chromium (Co–Cr) alloys were later developed to eliminate the toxic effects. ¹⁰ , ¹¹ , ¹²

Among nonprecious alloys for metal-porcelain restorations, Co–Cr alloys have become very popular due to their good mechanical properties, such as high coefficient of elasticity, resistance to permanent deformation, and low toxicity. ¹³ , ¹⁴ , ¹⁵

The half-life of metal-porcelain restorations depends on various factors, including the bond strength between porcelain and the underlying metal structure. ¹⁶ Proper oxidation of the metal surface is required for a stable bond between the metal alloy and porcelain to prevent the porcelain from denaturation. ¹⁷ Nonprecious metal alloys are easily oxidized, and a thick oxide layer is formed on the alloy during porcelain production. The high thickness of the oxide layer causes problems in the bond between the porcelain and the alloy due to the formation of cracks inside the oxide layer. ¹⁸ To solve this problem, attempts have been made to modify the components of the metal alloy ¹⁹ or methods of preparing the alloy surface, such as degassing, ²⁰ increasing the firing temperature of the porcelain opaque layer, ²¹ and the use of abrasives. ²²

The durability of restorations can also be affected by intraoral conditions, such as the chewing cycle and temperature changes that occur with food intake. ¹⁶ To make laboratory conditions more similar to clinical conditions, some laboratory studies have used a combination of mechanical and thermal cycles. ²³ , ²⁴ , ²⁵ In general, during mechanical cycles, a force is repeatedly applied to the specimens that mimic the chewing cycles, ²⁶ while in the thermal cycle, sudden and extreme thermal changes are applied to mimic the oral condition. ²⁷ Another way to reinforce the bond between alloy and porcelain is to use bonding agents (metal conditioners). ²⁸

The most frequent issue with metal-ceramic restorations is the fracture of veneering porcelain and its chipping from metal-ceramic restorations. ²⁹ Due to the frequency of porcelain chipping from metal-ceramic restorations according to recent clinical studies, the importance of shear bond strength between metal and porcelain has increased. ³⁰ However, previous studies have not reached a certain solution to this issue. Hence, this study was designed to investigate and compare the effect of two different metal bonds on the shear strength of metal–porcelain bonds while the oxide layer exists on the alloy surface. The null hypothesis was that the two tested metal bonds do not affect the shear bond strength of metal–porcelain bonds.

Study procedure

This in vitro study was approve in research and ethics committee of Isfahan (NO: 50545). In the present in vitro study, Co–Cr metal alloy (Sintron – Soft Metal, Amann Girrbach) was used. According to the manufacturer's instructions, 33 disk-shaped samples with a diameter of 10 mm and a thickness of 2.5 mm were milled by CAD/CAM machine (IMES-ICORE 340). The bonding surface of the metal samples was polished with silicon carbide paper with a particle size of 400 nm under heavy cooling. Then, the bonding surface of the samples was subjected to airborne-particle abrasion by 110 alumina particles at a pressure of 4 bar for 10 s with a distance of 5 mm from the nozzle of the device and an angle of 45° to the sample surface. Ultrasonic purification was then performed using isopropyl alcohol (70%), and the samples were allowed to dry at room temperature. Then, the samples were subjected to the oxidation process Figure 1under the recommended conditions in the furnace (Koosha Fan Pars Model: Auto term 300) at 910°C for 20 min. Figure 1

Samples after oxidation procedure.

Data analysis of the present study indicated that the mean shear bond strength value was significantly different among the three study groups.

Table 2shows the mean, standard deviation, minimum, and maximum shear strength of samples. According to Tukey's tests, statistically significant differences were obtained for the shear strength between C and MSh samples (P = 0.001), C and MC samples (P = 0.018), and MSh and MC samples (P = 0.405).{Table 2}

Table 3and Table 4compare the mean shear bond strength of the test materials between groups using the one-way ANOVA and pairwise comparison using Tukey's honest significant difference post hoc test.{Table 3}{Table 4}

Figure 4shows the mean shear bond strength within the three groups. The mean shear bond strength was significantly different between the three groups. Figure 4

Mean shear bond strength (MPa) between the three groups. C: Samples without metal conditioner; Msh: Samples with Shofu metal conditioner; MC: Samples with Creation metal conditioner; SD: Standard deviation

The most important metal–porcelain bonding mechanisms in PFM (Porcelain Fused to Metal) restorations are van der Waals forces, micromechanical bonding forces, and chemical bonding. To establish a chemical bond between the two components, electrons must be transferred between the crystalline part of the porcelain and the metal oxide. The presence of an oxide layer is necessary to strengthen the bond between the metal and the porcelain. Where the coupling part is made of noble metals, due to the fact that these metals do not oxidize, small amounts of base and nonprecious metals (usually indium and tin) are added to the alloy, which forms an oxide layer during the firing process. On the other hand, when in these restorations, the coupling is made of base alloys, a thick oxide layer is formed, which leads to fracture during porcelain firing processes and can reduce the bond strength. ³¹

In the present study, we investigated the bond strength between the metal and the porcelain in PFM restorations with chromium–cobalt alloys when/when not using metal conditioners. Both metal conditioners had a significant effect on increasing the bond strength compared to the control group.

A metal conditioner is usually a paste, powder, or liquid that is mixed together. In titanium metal restorations, it has been suggested to use metal conditioner for bonding to porcelain, but in Cr–Co metal restorations, the manufacturer's instruction is different for various kinds of ceramics. Metal conditioners of different brands have different chemical compositions, which makes them have different effects on the metal–porcelain bond. However, almost all kinds of metal conditioners reinforce the junction between the metal and the porcelain by chemical bonding. The main elements of both metal conditioners used in this study were Si and Ti. Studies have shown that the silica element in the metal conditioner compound absorbs metal oxides that form on the metal surface during porcelain, the cooking process. ³² Ti also acts as an oxygen scavenger in its composition and plays an important role in preventing the formation of an oxide layer on the metal surface during the cooking process, since cracks, formed inside the oxide layer, endanger the bonding. ¹⁸ , ³² , ³³

According to the results of the present study, metal conditioners that have Si and Ti in their chemical composition increase the strength of the metal–porcelain bond and control the thickness of the oxide layer.

In previous studies, ³⁴ metal conditioners were gold-based and made for gold alloys, or that tungsten was the main component. Tungsten does not have a significant role in controlling oxide layer thickness, and its role is to prevent the oxide layer from being washed by APEC porcelain and make the oxide layer hard and impermeable.

In some studies on titanium alloy restorations, it has been concluded that the use of metal conditioners on a metal surface that has already undergone the airborne-particle abrasion process increases the bond strength between the metal and the porcelain due to the significant wettability of the metal surface. ³³ A characteristic of the present study was surface treatment and airborne-particle abrasion of the samples. The use of metal conditioners may also be more effective for Cr–Co metals if subjected to airborne-particle abrasion, but confirmation of this hypothesis requires further investigation in future studies.

A study by Yoo et al. ³⁵ examined the claims of metal conditioner manufacturers that these materials are coefficient of thermal expansion (CTE) balancing agents and confirmed that if Cr–Co and porcelain are selected in a way that their CTEs are not compatible, the higher bond strength will be obtained in the group that used metal conditioner. The bond strength between the metal and the porcelain in PFM restorations, according to ISO 9693, is higher than 25 MPa and the mean bond strength obtained is lower than this value in the group that did not use metal conditioner. Therefore, it is likely that the use of metal conditioner is essential in restorations with metal base (specifically in this study, soft metal chrome cobalt alloy).

Another factor that influences the success of porcelain-metal restorations is the resistance of the metal–porcelain bond to thermal, mechanical, and chemical stresses in the mouth. In this study, the simulation of thermal stresses in the mouth by placing samples in thermal cycles has been performed. Totally, 3000 thermal cycles were applied to the samples, which is approximately equivalent to 2.5 years. ²⁵

There are many methods for measuring the bond strength, the best of which is the shear test, flexural 3 points, and flexural 4 points. The shear test was chosen because it exerts a force on the area of the initial contact between the metal and the porcelain and, unlike flexural tests, the coefficient of elasticity of the metal does not affect the result. ³⁶

The present survey was limited to the lack of complete simulation of the oral environment regarding sample shape and other characteristics and difficulty in accessing different brands of metal conditioners. Therefore, it is recommended for future studies resemble oral conditions more accurately. It is also suggested to investigate the effect of both airborne-particle abrasion and metal conditioners together on bond strength.

According to the results of this study and considering the limitations, the use of Creation (USA) and Shofu (Japanese) metal conditioners to increase the bond strength between Ceramill Sintron and porcelain (Vita VMK Master) was approved.

Financial support and sponsorship

This study was supported by Isfahan University of Medical Sciences (Grant No: 399393)

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.