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Bond characteristics of porcelain

fused to cast and milled titanium

Karina Andrea Novaes Olivieri 1

Maximiliano Piero Neisser 2 Marco Antônio Bottino 2 Milton Edson Miranda 3

1PhD, Post-graduated, Department of Dental Materials and Prosthodontics, Faculty of Odontology of São José dos Campos, UNESP, Brazil and Associated Professor, Department of Prosthodontics, Faculty of Odontology São Leopoldo Mandic – Campinas- Brazil.

2PhD, Associated Professor, Department of Dental Materials and Prosthodontics, Faculty of Odontology of São José dos Campos, UNESP, Brazil.

3PhD, Associated Professor, Department of Prosthodontics, Faculty of Odontology São Leopoldo Mandic, Campinas, Brazil.

Received for publication: November 29, 2004 Accepted: November 17, 2005

Correspondence to: Karina Andrea Novaes Olivieri

Rua: Mauro de Próspero, 500, Bloco 57, Apto 36

Bragança Paulista – SP CEP: 12913-045 Phone: (19) 8128-1414

E-mail: kaolivieri@ig.com.br

Abstract

Metalloceramic restorations combine the aesthetic properties of ceramic materials with the high strength of metals. The titanium has excellent biocompatibility, good mechanical properties and low density, and has been recently used for metalloceramic prosthesis. The purpose of this study was to evaluate the shear bond strength of these two materials, and also analyze their bonding interface using Scanning Electron Microscopy (SEM). Thirty-six specimens were prepared and divided in three groups: Group 1- gold alloy (Degudent U-Degussa) + Vita Omega 900 ceramic(Vita) (control group); Group 2 – milled commercially pure titanium (cpTi-Dentaurum) + Titankeramik ceramic (Vita); Group 3 – cast and milled commercially pure titanium (cpTi-Dentaurum) + Titankeramik ceramic(Vita). The shear bond strength mechanical assay was performed in an Instron 4301 machine with capacity for 500Kg. After the test completion, surfaces were evaluated using SEM. The numerical results, put in tables, were: G1= 40,55MPa (+/- 4,8), G2= 63,54 MPA (+/- 1,73) and G3= 68,17 MPA (+/- 1,19). The statistical analysis (ANOVA) showed no significant statistically differences between groups G2 and G3 and the values were larger (significant statiscally) than G1. It is concluded that the titanium alloys is a good alternative to gold alloys to metalloceramic restorations emphasizing that it is cheaper, biocompatible and is has a low density. Key Words:

shear bond strength, gold alloy, titanium, ceramic, scanning electronic microscopy.

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Groups Metals Porcelains Specimens

1 Gold Alloy Vita Omega 900 N=12 2 cp-Titanium milled Titankeramik- Vita N=12 3 cp-Titanium cast plus milled Titankeramik - Vita N=12

TOTAL= 36

Table 1- Groups, metals, porcelains and specimes. Introduction

Since its introduction into clinical dentistry about 35 years ago, the metal-ceramic or porcelain-fused-to metal technique has become increasingly popular1. These restorations are used in fixed or simple prosthesis for many years, joining the natural aesthetic of a fragile material as a porcelain, with the durability and marginal adaptation of the cast metal2. Metalloceramic restorations were made possible by the following developments: ceramics and alloys that form a strong bond; ceramics and alloys with matching coefficients of thermal expansion; low fusing ceramic materials and alloys that resist deformation at the ceramic fusing temperature3. The metal should not deform and cause breakage of porcelain; second, it must remain permanently bonded to the porcelain and, finally, it must not change the color of the porcelain. Others important factors will include accuracy, biologic compatibility, and tarnish resistance. It porcelain is not flex and break, the alloy must resist plastic deformation. The porcelain must have a high yield strength and have resilience and a high modulus of elasticity2 .

Because of weight, high strength to weight ratio, low modulus of elasticity, and excellent corrosion resistance, titanium and some of its alloys have been important materials for the aerospace industry since the 1950s. Now, with the additional advantages of excellent biocompatibility, good local spot weldability, and easy shaping and finishing by a number of mechanical and electrochemical processes, these materials are finding uses in dental applications, such as implants and restorative castings4-7 .

Bonding porcelain to dental alloys is accomplished during porcelain firing, a sintering process8. Bonding is thought to result from mechanical interlocking9, Van der Waals forces, and chemical bonding10. All three mechanisms requiring wetting of the metal surface with porcelain during sintering. To facilitate wetting, McLean2 recommended firing an initial air brushed layer of opaque porcelain 20ºC higher than the manufacturer’s recommended opaque firing temperature. The contribution to the various bonding mechanisms has been debated, but a chemical bond is considered necessary to achieve the bond strengths for clinical dentistry11-12. The purpose of this investigation was to determine the bond characteristics of porcelain fused to cast and milled titanium and compare titanium/porcelain shear bond strength with

those a conventional gold/porcelain system. Scanning electron microscopy was used to characterize the mechanism of fracture as well as the nature of the porcelain-metal interface.

Material and Methods

Commercially pure titanium (cp-titanium) (Tritan- Dentaurum-Germany) and a gold alloy (Degudent U – Degussa-Hülls S.A.- Germany) (control group) were used in this study. The commercially pure titanium was divided in two groups: cp-titanium milled group and cp-cp-titanium cast and milled group. To gold alloy was fused the Vita Omega 900 (Vita-Germany) (Table 1).

The metal structures of the cp-titanium cast plus milled specimens (group 3) and the alloy specimens (group 1) were obtained from 12 wax templates in cylinder form with 3 cm of length and 0,5cm of diameter. It was made four bars in gold alloy and four in cp-titanium. These bars were milled in smaller cylinders as see Figure 1. It was followed the manufactures’ recommendations to obtain the metal structure of the specimens in regard of casting of the alloys.

The investment used in the cast of the gold alloy was Ceramvest (Polidental-Brazil) that is used in high fusion. To cp-titanium it was used the Rematitan Plus (Dentaurum-Germany). The lost-wax technique was applied to obtain the metal part of the specimens. After the final investment setting, models were placed in an EDG furnace (model EDGCON 3P – 3000- Brazil) for wax elimination under temperatures between 600O and 900O C for 90 minutes.

The gold alloy casting was made in a centrifuge by induction (model DUCATRON – Series 3 – France) using an argon gas shielding and a digital optical infrared pyrometer (M-GULTAN- Pirograt- IS-3D- Germany) for temperature control. The cp-titanium casting was made in a centrifuge by induction (Rematitan Autocast – Dentaurm - Germany) using an argon gas shielding and a digital optical infrared pyrometer (M-GULTAN- Pirograt- IS-3D- Germany) for temperature control. The metal structures were separated from their investments and submitted to sandblasting with glass microballs and aluminum oxide subsequently.

The metal structures of the cp-titanium milled specimens (group 2) were made from the tooling of titanium bars obtained directly from the manufacturer (Figure 1).

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All these metal structures were milled to fit in the disposable used in the shear bond strength and to standardize all the specimens.

To application of the respective porcelains it was used a metal/ Teflon® disposable that had ten orifices to appose the metal structures. This disposable had a manual adjustment to allow the application of the ceramic in two layers of 2mm each one, as see in Figures 2 and 3.

It was followed the recommendations of the manufactures to application of the porcelains and to the temperatures´ fusing. The Figures 4 and 5 show the readies specimens (metal \ porcelain) to be submitted to shear bond strength.

The shear bond strength was made in an universal testing machine (Instron 4301) with 500kg of load and a crosshead speed set at 0,50mm/min. To this test was used an external cylinder disposable with a plan adaptation on one side (A)

Fig. 1 – Metal structure of the specimens and respective dimensions. Fig. 2 – Disposable used to porcelain application

Fig. 3 - Disposable used to porcelain application and metal structures.

Fig. 4 – Specimens and respective dimensions.

Fig. 5 – Specimens to the shear bond strength. Fig. 6 –Disposable to shear bond strength: A) external cylinder; B) internal cylinder.

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and, in its interior, other cylinder with the same form apposed, the internal cylinder (B). Both cylinders had a hole of 4mm of diameter in plan surfaces in where the specimens were inserted (Figure 6)13.

Two-way analysis of variance (ANOVA) was used to detected significant differences among the conditions studied.

Post test analysis of the fracture surfaces of the shear samples was performed using a scanning electron microscopy (SEM) (JEOL – JSM – 5600LV – England). This analysis was used to assess the mechanisms of failure and the nature of the interface between the titanium and porcelain. Results

The statistics analysis from the shear bond strength test are showed in Table 2 and Figure 7.

The Figures 8, 9 and 10 show the fracture surfaces of the specimens after shear bond strength test.

Discussion

Metalceramic restorations are currently popular in restorative dentistry and combine the natural esthetics of a brittle material such as a porcelain, with the durability and marginal fit of a metal casting10.

If titanium’s distinct advantages are to be used, for aesthetic crows and bridges, the ability to apply a porcelain veneer becomes important. Because of titanium’s high affinity for oxygen, porcelain firing should take place below 800ºC in order to prevent excess oxide formation. Moreover, since little or no residual stress due to thermal mismatch should exist in the final Ti/porcelain bond interface, the significant discrepancies already noted in their thermal coefficients of expansion will have to be modified to more closely match. It appears that an interfacial oxide layer, some 100 to 1000 nm thick, forms during firing; and the thicker this layer becomes, the weaker the bonding between the porcelain and the titanium6. Titanium has been used in metal-porcelain restorations

Fig. 7 – Graphic of the statistics analysis of the shear bond strength

test (MPa). Fig. 8 – Gold Alloy: specimens after shear bond strength test; A,

metal , B, porcelain e C, fracture line ( opaque of porcelain).

Fig.9 - Milled titanium/Titankeramik Vita: specimens after shear bond strength test; A, metal/porcelain, B, metal/porcelain/adhesive, C, porcelain and D, porcelain/fracture line.

Fig.10 – Cast and milled titanium/Titankeramik Vita: specimens after shear bond strength test; A, metal/porcelain/adhesive, B, metal/ adhesive, C, metal/porcelain/adhesive and D, porcelain.

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because of its several advantages, such as good corrosion resistance, excellent biocompatibility, low weight, low thermal conductivity and reasonable price. The success of the porcelain-fused-to-alloy restoration depends acutely on the success of the strong bonding between porcelain and the metal structure. Acceptable restorations require metals and porcelains to be chemically, thermally, mechanically and esthetically compatible. Chemical compatibility implies a bond strong enough to resist both transient and residual thermal stresses and mechanical forces encountered in clinical function6.

The metal/porcelain system used as a control group in this study was the gold/porcelain because its physical and chemical properties were very studied in the dentistry literature defining as excellent in their qualities1,14-15. The titanium was used as a test group, in different conditions of treatment, because it’s a metal that has a optimal physical and chemical properties and its very used in Implantodonty. So, it choose to test its bond strength with the porcelain simulating metalceramic restorations.

The porcelain was chose instead of resin because its mechanical properties is better; beyond of the porcelain show more resistance in regard of wear, more stable in regard of color and it is the election material to use in Venners crows3. A critical requirement for adhesion is thermal compatibility between the ceramic and metal. If the two materials contract at different rates during cooling, strong residual stresses will form across the interface. If these stresses are strong enough the porcelain will crack or separate from the metal. Even if the stresses are less strong and do not cause immediate failure, they can still weaken the bond. To avoid these problems the porcelains and metal alloys are formulated to have closely matched thermal expansion coefficients. Typically the porcelains have coefficients of thermal expansion coefficients between 13.0 and 14.0 x 10-6/ºC and the metals between 13.5 and 14.5 x 10-6/ºC. The difference of 0.5 x 10-6/ºC in the thermal expansion between the metal and

porcelain causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain. This condition puts the ceramic under slight residual compression, which makes it less sensitive to applied tensile forces3.

The theory of ceramic-to-metal bonding has focused in four possible mechanisms: (1) Van der Waals’s forces, (2) compressive forces generated in thermal expansion, (3) mechanical bonding with surface geometry, and (4) chemical bonding with the oxides in the ceramic and the metal16. Most investigators agree that the adherence is result of a combination of the above forces. However, there is disagreement as to the relative importance to each force and likewise as to the technique variables that potentially influence overall bond strength11,17.

The oxide layer theory states that the bond of porcelain to metal is achieve by the presence of an adherence oxide layer at the porcelain-metal interface. Hence, the requirement for porcelain adherence is two-fold: First, there must be an oxide present at the interface, and second, the oxide must be adherent to the metal. On the one hand, if an oxide layer is lacking, or is of insufficient thickness to prevent complete dissolution by the fusion porcelain, the porcelain comes into direct contact with the alloy surface, and the adherence is poor. This situation is observed in the fusing of porcelain to some noble metal alloys where the surface becomes covered with a metallic layer deposited by diffusional creep and no surface oxide is present18. This situation is also observed when the first-formed oxide on the surface of a gold alloy is dissolved by hydrofluoric acid, and the alloy surface is sufficiently depleted in the oxidizable elements such that reformation of the surface oxide is impossible19. On the other hand, if the oxide is not adherent to the metal substrate, the resultant porcelain-metal bond will again be poor, as has been demonstrated in Mackert Jr et al.18 study.

Fusion of porcelain on titanium was not without difficulties. Launtenschaler and Monaghan20 attempted to bond a low-fusing porcelain to cast titanium at approximately 800ºC. The use of a low-firing temperature cycle (800ºC) was shown to prevent excess oxidation of the titanium. Wang et al.7 studied a specific porcelain to cast titanium and found better results to this porcelain in regard to conventional porcelain. In our study it was used specific porcelains to cast and milled titanium.

The shear bond strength is considered the method more trust in the bond evaluation because it concentrates the load in the interface of the materials21-23. In this study, the results showed averages very near among titanium groups (G2= 63,54 MPA (+/- 1,73) and G3= 68,17 MPA (+/- 1,19) differing from control group (gold alloy) (G1= 40,55MPa (+/- 4,8)). Drummond et al.24 observed a significant difference between the bond strength of the porcelain-metal composites: 51.12 11,29) MPa for the porcelain-gold system and 31.83 (+/-Statistics Analysis Average 40,55 63,54 68,17 Standard deviation 4,83 1,73 1,19 Variance Coefficient 11,92 2,72 1,74 Minimum 32,34 60,28 66,70 Median 40,47 63,43 67,97 Maximum 46,55 69,91 69,80 G3(CMVITA)** (MPa) G1 (Au) (MPa) G2(MVITA)* (MPa)

* MVITA – milled group - Titankeramik

** CMVITA – cast plus milled group – Titankeramik

Table 2 - Descriptive statistics analysis of the shear bond strength test (MPa).

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3.65) MPa for the porcelain-non-precious system. This values are similar that were found in this study.

Lubovich and Goodkind25, using a shear bond strength, found greater values in the bond strength to non-precious alloys when they compared with gold alloys. Chong and Beech26 and Drumond et al.24 observed the opposite from the above authors, to gold alloy 20.0 MPa and 51.1 MPa, respectively, and to non-precious alloys, 9.6 MPa and 31.8MPa, respectively.

According to Anusavice et al.27 the mastigatory load in central incisors is between 6.5 to 17,5 MPa and in posterior teeth, 30MPa. So, it can conclude that the metalloceramic restorations using titanium/porcelain system is totally viable in the clinical practice. Over there, it found greater values to the titanium/porcelain system when compared to gold alloy/ porcelain system.

The scanning electronic microscopy (SEM) was made in this study to evaluate the interface, after shear bond test, of the specimens. It observed mixture failures, cohesive and adhesives. The mixture failures could indicate the presence of the residual stress in the porcelains in the cohesive failures regions. Any stress of residual tension inside porcelain could decrease the external load necessary to cause the fracture. The microgaphies suggested no differences between cast and milled titanium. In both it was detected high amount of porcelain in the metal superficies after shear bond test, confirming, then, the greater values in the test to titanium group15.

According to Craig’s failures classification between porcelain and metal3, it was suggest the failure type IV (metal/ metal oxides) with remains ceramic in the metal region to show the most failures observed in this study.

Considering the results from the shear bond strength it can conclude:a) the groups G2 and G3, titanium groups, had the greater values of shear bond strength when compared with the control group;b) the microgaphies suggested no differences between cast and milled titanium. In both it was detected high amount of porcelain in the metal superficies after shear bond test, confirming, then, the greater values in the test. It is concluded that the titanium alloys is a good alternative to gold alloys to metalloceramic restorations emphasizing that it is cheaper, biocompatible and is has a low density.

Acknowledgments

The authors thank FAPESP (# 00/08975-0) for the financial support an the Professor Mário Fermando de Góes, Ph. D, Dental Materials Department, and Vinícius Hipólito, post graduated student, by the help with scanning electron microscopy.

This article is based on the Ph.D. degree thesis of K.A.N. Olivieri, which was presented at the 81st General Session of the IADR- Göteborg, Sweden (June 25-28, 2003) as a poster.

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