Effect of four different surface treatments on shear bond strength of three porcelain repair systems: An in vitro study.

BACKGROUND: Ceramic fracture in metal ceramic restorations are serious and pose an aesthetic and functional dilemma both for the patients and the dentist. This has created a demand for the development of practical repair options which do not necessitate the removal and remake of entire restorations. AIM: To evaluate and compare the effect of four different surface treatments on shear bond strength of metal ceramic specimens with three commercially available porcelain repair systems.
MATERIALS AND METHODS: Specimens were fabricated with a base-metal ceramic alloy and divided into three groups, to evaluate three porcelain repair systems. Each group was divided into four subgroups based on surface treatment (A) sandblasting, (B) sandblasting followed by etching with 9% HF (Hydrofluoric acid) on surrounding ceramic, (C) Use of a diamond bur on exposed metal followed by etching with 37% H3PO4 and (D) Control groups (D1, D2, D3 for three groups of porcelain repair system which was not subjected to further treatment after finishing with 240 grit silicon carbide paper grinding. Shear bond strength of each group of specimens based on surface treatment were evaluated with a universal testing machine after storing in distilled water for 7 days. One way ANOVA and Tukey-HSD procedure were used to compare the mean values between and among the groups.
RESULTS: The mean shear bond strength of group III (10.402 ± 1.055) were significantly higher than group I (8.647 ± 0.990) and group II (8.099 ± 0.600) for all surface treatments. However the mean values of shear bond strength of sub-group A were significantly higher than sub-group C and D but were not significantly higher than sub-group B.
CONCLUSION: The results of this study suggest that in fractured metal ceramic restorations the exposed metal surface treated with sandblasting or sandblasting and etching the surrounding ceramic surface with HF can increase the shear bond strength of the repaired metal ceramic area. Porcelain repair systems which contain hybrid composites and 4-META as primer had increased bond strength.

INTRODUCTION

Metal-ceramic restorations are widely used in restorative dentistry with a high degree of success. On occasions, fractures do occur in ceramic as a result of trauma, metal flexure, or ceramic fatigue, and a decision on how to rectify the resultant defect needs to be made. Fractured porcelains will affect aesthetics and function of the prostheses, which may warrant patients to seek immediate treatment. One option is to remake the restoration. This is but both expensive and time consuming. Removal and reconstruction of the prostheses is a costly affair, and it is therefore worthy to attempt repair with composite resins intra-orally, especially in less severe cases.[12] An easy alternative is to repair the deficiency using one of the many proprietary porcelain repair systems. However, for the repair to withstand functional loads, the bond between the repair material and remaining restoration must be strong and durable.

Three conditions which are usually suggested for repair of metal ceramic restorations are: Fracture in porcelain with no metal exposure, fracture with both porcelain and metal exposure and fracture with substantial metal exposure. Mechanical and chemical bonding methods have been recommended to enhance the bond strength of these composite resins to metal ceramic restorations.[34] Various surface treatments to increase the mechanical bonding like roughening with diamond bur[5] and air abrasion with aluminum oxide[67] can be used to condition the surfaces of the both metal and ceramic. Acid etching with hydrofluoric acid[8] acidulated phosphate fluoride[4] or ammonium hydrogen bifluoride[5] can be done in ceramics. Chemical bonding using adhesive primers[910] and silane coupling agents[11] enhance bonding after initial mechanical surface treatments. Silane coupling agents which are generally available in the repair systems are not able to bond to the porcelain.[12] The improvement in adhesion of composites to base metals and ceramic can be made by the addition of adhesive primers to various composite formulations to enhance bonding to sandblasted base metal and ceramic surfaces.[13]

The objective of this study was to evaluate the effect of four different surface treatments on shear bond strength of three different porcelain repair systems with a metal ceramic alloy.

MATERIALS AND METHODS

A silicone mold was prepared from machined stainless steel, which was fabricated to be 8 mm in diameter and 2 mm height on one half and 4 mm height for the other half, creating a semicircular ring, 2 mm in depth on the surface of the wax pattern for standardization of porcelain thickness of 2 mm. Wax pattern were made and a sprue of 3 mm uniform length was attached to the wax patterns.

Wax patternswere then invested in a phosphate bonded investment (Bellasun, BEGO- Bremer Goldschlägerei Wilh. Herbst GmbH and Co. KG) and casting was done using Nickel chromium alloy (4 all alloy, Ivoclar vivadent). Two stage burn out casting procedure was followed according to manufacturer's instruction and castings were then divested and the sprues were left on the underside of the sample to help in porcelain application. The remaining investment was removed by use of sandblaster with 50 μm aluminum oxide.

After sand blasting, the samples were ultrasonically cleaned for 3 min in distilled water and opaque porcelain was applied uniformly in layers, followed by application of dentin porcelain. The porcelain surfaces were sintered at recommended temperatures to obtain metal ceramic samples.

The metal porcelain samples were embedded in polymethyl methacrylate material (DPI, chemical cure) with the help of standard 2 cm × 2 cm stainless steel cylinder for the purpose of standardization. Mounted samples were uniformly sandblasted to a flat surface following a 50 stroke, figure eight hand grinding pattern on 180 and 240 grit wet silicon carbide papers to simulate surface characteristics produced by clinical grinding procedure.[10]

After treatment, each sample was ultrasonically cleaned in solution of 99.8% methanol and distilled water for 15 min to remove any trapped residue. A total of 120 specimens were divided into three groups of 40 specimens each. Each group was divided into four subgroups of 10 samples each. Samples from all the three study groups and control group were examined under scanning electron microscope (FEI-Field Emission Ionization, Model Quanta 200, Czechoslovakia) after surface treatment before the bonding procedure [Figures 1 and 2].

Figure 1

Mean shear bond strength between different subgroups for each of the study groups

Figure 2

Mean shear bond strength between different groups for each of the study subgroups

Summary of treatment subgroups:

D = Controlgroup

A = Sandblasting

B = Sandblasting + Etching with 9% HF-Hydrofluoric acid

C = Mechanical roughening with diamond bur + Etching with 37% phosphoric acid.

The controlgroup was not subjected to further treatment after finishing with 240 grit silicon carbide paper grinding. The control group was divided into D1, D2, and D3 for use with the three groups of porcelain repair systems.

Group A specimens were sandblasted using an abrasive tool with 50 μm aluminum oxide particle with a pressure of 5 kg/cm2 at a distance of 5 mm away from the surface with circling motion of 6 mm diameter for 20 s. After sandblasting, the specimen was ultrasonically cleaned for 15 min in a distilled water bath.

Group B specimenswere etched with 9% HF acid gel (Ultradent, Salt Lake City Utah) for 1 min as recommended by the manufacturer. After etching, the specimens were thoroughly rinsed for 30 s using an air water spray to ensure that all the acid was removed from the surface.

Group C specimens were mechanically roughened with a SF-Superfine 101 Shofu diamond bur for 30 s to create an undercut and roughness followed by etching with 37% phosphoric acid (Total etch, Ivoclarvivadent) for 1 min followed by air water spray.

After surface treatment, the bonding surface of the specimens were then masked with water proof plastic adhesive tape (3M) approximately 0.1 mm thick and a 6 mm diameter hole was punched approximating the standardized area of bonding.

Three brands of porcelain repairmaterials were chosen and the manufacturer's directions were followed for each system during the bonding procedure. The composite restorative material of each of the repair systems were placed in a cylinder of 2 mm height. Excess composite was removed from around the capsule with a fine sable brush before polymerization. The composite cylinder was then polymerized with a visible light curing unit (Blue phase, Ivoclar Vivadent) with light directed approximately 45° from the intersection of bonding sites and composite cylinder. Four 40 s polymerization sequences divided equally around the circumference of the composite cylinder were completed. The total polymerization time for each specimen was 160 s. The diameter of the bonded cylinder was 6.0 mm.

Ten specimens in each Subgroup were stored in a distilled water bath at 37°C for 7 days before determination of shear bond strength of the bonded composite cylinder.[14] The specimens were mounted in a custom fixture for determination of shear bond strength after the designated storage time. The specimens were aligned in a universal testing machine (Llyod) with the shearing rod against and parallel to the bonding surfaces. The bonded composite cylinders were subject to continuous loading at 5 mm/min until fracture occurred. The load values were converted into bond strength (MPa) for each of the cross sectional area of the samples.

Mean and standard deviation were estimated from specimens for each study group [Table 1]. Mean values were compared between different study groups by using one way ANOVA followed by Tukey-HSD procedure. P < 0.05 was considered as the level of significance.

Table 1

Mean standard deviation and test of significance of mean value between subgroups, for each study group

Statistical analysis

Level of significance @ 5% (0.05)

Power @ 90%

The following methods of statistical analysis have been used in this study:

All data were analyzed by using the statistical program of social science version 18.0 (SPSS Inc., Chicago, USA).

One way ANOVA

Tukey-HSD procedure

RESULTS

For Group I, the mean value of shear bond strength in subgroup A (8.647 ± 0.990) were significantly higher than subgroup C (5.720 ± 0.395) and subgroup D1 (4.288 ± 0.990), but not significantly higher than subgroup B (8.107 ± 1.262). For Group II, the mean value of shear bond strength in subgroup A (8.099 ± 0.600) were significantly higher than subgroup B (7.033 ± 0.973), subgroup C (4.938 ± 0.600) and subgroup D2 (3.691 ± 0.292). For Group III, the mean value of shear bond strength in subgroup A (10.402 ± 1.056) were significantly higher than subgroup C (7.737 ± 0.73) and subgroup D3 (5.787 ± 0.417) but not significantly higher than subgroup B (9.852 ± 0.631) [Table 1].

For subgroup A, the mean value of shear bond strength of Group III (10.402 ± 1.056) were significantly higher than Group I (8.647 ± 0.990) and Group II (8.099 ± 0.600). For subgroup B, the mean value of shear bond strength of Group III (9.852 ± 0.631) were significantly higher than Group I (8.107 ± 1.262) and Group II (7.033 ± 0.973), also mean value of Group I were significantly higher than Group II. For subgroup C, the mean value of shear bond strength of Group III (7.737 ± 0.73) were significantly higher than Group I (5.720 ± 0.395) and Group II (4.938 ± 0.600), also mean value of Group I were significantly higher than Group II. For subgroup D, the mean value of shear bond strength of Group III (5.787 ± 0.417) were significantly higher than Group I (4.288 ± 0.990) and Group II (3.691 ± 0.292), also mean value of Group I were significantly higher than Group II [Table 1].

DISCUSSION

In the present study, three porcelain repair systems with three different bonding agents were selected. All three porcelain systems contain hybrid composite, Studies have shown that for repair purpose, the use of hybrid type composite resin results in higher bond strength than that of microfilled composite resin.[7]

When the mean shear bond strength of the three test groups were compared Group III had higher bond strength values for all the four types of surface treatments [Figure 1]. The higher values of Group III could be attributed to the presence of adhesive primer containing 4-META (4-methacryloxyethyl trimellitate anhydride) in the adhesive system. 4-META is a bipolar molecule with a methacrylate group at one end and a trimellitate anhydride group at the other. Trimellitate anhydride in the presence of water produces two negatively charged carboxylic groups. The methacrylate group chemically copolymerizes with resin composite. The carboxyl group chemically bonds to metallic oxides of ceramic, enhancing the bond strength.[13] The bond strength value of Group I and Group II were less than that of GroupIII. This could be because of the absence of any adhesive primer in the adhesive system in both groups and the silane coupling agents which is provided by the manufacturer in these groups are not able to bond to the metal surface as much as they do to porcelain. This could be explained on the basis of silica content which is very low in base metal alloy (1.5 wt%) as compared to silica in the feldspathic ceramic (50-60 wt%).[1415161719] So the bond strength obtained was mainly contributed by chemical bonding between ceramic and silane coupling agent.

The mean shear bond strength values for Group II was less then Group I. This could be because of the presence of acetone in the adhesive system which has higher vapor pressure (184 mm Hg at 20°C) as compared to ethanol (43.9) and water (17.5)[1819] which is present in the Group III and Group I. A higher vapor pressure results in easier solvent evaporation. So during clinical application more amount of the solvent evaporates which inhibits the wettability and consequent adhesion.

When comparing the shear bond strength values of four different subgroups of surface treatment (A, B, C, D) [Table 1] the mean bond strength of subgroup A [sandblasting] was not significantly higher than subgroup B (sandblasting + etching 9% HF), but it was significantly higher than subgroup C (mechanical roughening + etching 37% phosphoric acid) and subgroup D (control group) [Figure 2].

The higher values for subgroup A and subgroup B is in agreement with other studies.[32021] An increase in surface energy allows a solid to draw a wetting medium on to its surface more readily thus increasing its wettability and adhesion. The structure of micrograph is similarto the results reported previously.[172223]

Surface treatment by a coarse diamond point and total-etch bonding regimen provides highest bond strength.[202425] In the present study, the mean shear bond strength values of subgroup C were lower since the bond strength obtained by mechanical roughening with a diamond bur on the exposed metal created surface irregularities for bonding.

Subgroup D specimens (control), who were subjected to the basic surface treatment of wet silicon carbide paper grinding only, exhibited the lowest bond strength in spite of using adhesive and silane coupling agents. This explains the importance of surface treatment.

Etching of the porcelain blocks with hydrofluoric acid holds promise in the repair of fractured porcelain with composite resin at chairside.[119] The bond strength of subgroup A (8.0-10.4 Mpa) and the bond strength of subgroup B (7.0-9.8 Mpa) were compared to that of acid etched enamel which has been reported to be in the range of 16-20 Mpa.[21] When a bond strength of such a magnitude can be obtained by sandblasting only, the use of caustic and potentially harmful hydrofluoric acids can be eliminated for intra oral repair of metal ceramics.

In vitro evaluation is the first step of testing any material to examine the properties and potential that it possesses. Because our study tested only shear bond strength of porcelain repair material to metal ceramic, it is suggested that other aspects of the bond, such as effect of different mechanical test designs, mode of failure and micro leakage be studied for a more comprehensive evaluation of these porcelain repair systems.

CONCLUSION

Within the limitation of this study, the results were statistically evaluated and following observations were made:

Surface treatment with sandblasting exhibitedthe highest shear bond strength (8.0-10.4 Mpa) followed by combined sandblasting + etching 9% hydrofluoric acid (7.0-9.8 Mpa), followed by roughening with diamond bur + etching 37% phosphoric acid and control group.

The mean shear bond strength of Group III was significantly higher than other groups forall the four surface treatments.

Porcelain repair systems which contain hybrid composites and 4-META as primer had increased bond strength with metal ceramics.