Evaluation and Comparison of Shear Bond Strength of Three Porcelain Repair Systems in Response to Thermal Stress – An InVitro Study

INTRODUCTION

Porcelain-fused-to-metal restorations being extensively used in fixed partial denture treatment, is the outcome of combined need for esthetic appeal and strength of the restoration. The bond between porcelain and the underlying metal is one of the most important physical properties of ceramo-metal-restorations, as the durability of the prosthesis often depends on the quality of this bond. On occasion fractures1 of ceramic veneer do occur under clinical conditions due to, traumatic occlusion, technical errors, poor abutment preparation, inadequate bond, inappropriate coping design ¹. Several techniques are discussed in the literature for repairing porcelain fractures from porcelain fused-to-metal restorations. Those techniques employing acrylic resins include directly formed acrylic resin facings which are later cemented to the place² and fabrication of a pin only, with an acrylic resin veneer cemented to the labial surface². The technique utilizing porcelain includes fabrication of a pin onlay with a porcelain veneer cemented to the labial surface and fabrication of a pin retained over casting with fused porcelain veneer³. Porcelain-fused-to-metal overcastting is described in the literature to repair fractured porcelain-fused-to-metal restorations⁴ .

To affect the repair intraorally in the case of highly satisfactory complex prosthesis instead of opting for the expense, inconvenience added chair time and possible risk of damage to the prosthesis several porcelain repair systems^{5, 6, 7} are introduced. The refinement of composite resins has resulted in the evaluation of several intraoral porcelain repair systems. Although adequate results with various porcelain repair systems may be obtained in clinical practice, failures do occur. In addition, studies have been conducted to evaluate the effect of thermocycling treatment on the bond strength of the various porcelain repair systems.^6,8,9,10 . Like other restorative materials the porcelain repair systems undergoes thermal stress in the oral cavity. The durability of the bond strength of commercially available porcelain repair materials is important in predicting their clinical success¹¹ . Due to differences in the co-efficient of thermal expansion between porcelain and resin, it would be expected that the repair materials would be affected by thermally induced stresses^12,13,14.the chemistry of newer systems varies from each other. A need was felt to evaluate the shear bond strength of three commercially available porcelain repair systems and changes in bond strength of the same materials when subjected to simulated temperature variations in the oral cavity.

MATERIALS AND METHODS

1. Preparation of the sample

A metal mold was used to prepare test samples of size 8mm in diameter and 4mm in thickness. Disk shaped wax patterns (8mm diameter) were prepared by pouring molten inlay wax (Schuller ULM, Germany) into the mould cavity. In all 30 wax patterns were prepared. These wax patterns were invested in phosphate bonded investment material (Castorit-Super C, Dentaurum, Germany), Nickel-Chromium (Remanium CSe, Dentaurum, Germany) alloy used for casting. All manufacturers' recommendations for the burnout and casting procedures were followed. All castings were then divested and the sprues were retained (to ensure retention of samples when it is embedded in acrylic blocks). Any residual investment material was removed by using a sandblasting unit with (KaVo EWL TYP 5417) aluminium oxide (88-125µm). After sand blasting the samples were ultrasonically cleaned for 3 minutes in a distilled water bath (Sonorex, Super RK 102P).

Porcelain material (Ducera, Dental-Gesellschaft mbH, W-Germany) used to generate the porcelain part of the sample. For all the samples 3mm thickness of porcelain was applied uniformly. Opaque porcelain and body porcelain fired to the metal surface and the manufacturer's instructions were followed accordingly.

In order to fix the samples to the lower cross head of Instron testing machine, the samples were embedded in acrylic block.

2. Preparation of acrylic block

To prepare the acrylic block a special die was made using Easy mix putty rubber base impression material (Reprosil, Dentsply). The desired shape of the block was carved out using modelling wax and an impression of the wax block was made with putty rubber base impression material. A mix of self cure acrylic resin was poured into the impression using dough technique and the samples with retained sprue were embedded in the acrylic block exposing the metal-porcelain interface (Figure 1). The method to test the shear bond strength of 3 porcelain repair systems consisted of fracturing the porcelain part of the samples using Instron Testing Machine, repairing the fractured samples with 3 different repair systems, and then fracturing the repaired samples using the Instron Testing Machine. For some samples the whole porcelain disk was detached. Such samples were discarded and new samples were prepared. 30 such fractured porcelain fused-to-metal samples were obtained.

3. Repair for fractured porcelain-fused-to-metal samples

30 samples were divided randomly into 3 groups, Group A, B and C. 10 samples in each group were repaired with the following different porcelain repair systems.

Group A - Using 3M Scotchbond System (3M Dental Products Div. St.Paul, Minn).

Group B - Using Clearfil Porcelain Bond (Kuraray Co., Ltd., Osaka].

Group C - Using Ceramic Repair System (Ivoclar, Vivadent).

Bonding procedure

For each repair system manufacturer's instructions were followed:

Bonding procedure for 3M scotch Bond System: The exposed porcelain-metal surface roughened (aluminium oxide 88- 120µm) for 10 seconds with 4 bar pressure. Scotch Bond etchant (35% Phosphoric acid) applied to the roughened surface, left for 15 seconds. Then rinsed for 10 seconds and dried for 5 seconds. Silane solution (Relyx ceramic primer) applied to the etched surface and dried. 2 consecutive layers of single bond adhesive coated to the silanated surface. Dried gently with oil free air for 2-5 seconds, then polymerized for 20 seconds. Exposed metal surface coated with opaque (Ivoclar, Vivadent) and polymerized for 20 seconds(Heliolux, Vivadent). To restore the fractured porcelain composite resin(Herculite, Kerr Co., U.S.A.) placed and polymerized for 40 seconds(Heliolux, Vivadent).

Bonding procedure for Clearfil Porcelain Bond: The porcelain-fused-to-metal surface first air abraded with 88- 120µm aluminium oxide particles with 4 bar pressure for 10 seconds. The cleaned porcelain-metal surface acid etched with 40% phosphoric acid for 5 seconds. The etchant then washed and the surface was fully dried with oil free air. One drop of each of the three Clearfil Porcelain Bonding agents (first activator, then catalyst and finally universal liquid) combined in a single mixing well of the disk provided in the kit. The three drops mixed for 5 seconds in the well with applicator sponge. The mixed material applied to the etched surface of the samples with an applicator sponge. With an air syringe and within 30 seconds after application, the applied bonding agent blowed with air for 2 to 3 seconds to remove the volatile substance contained in the Clearfil Porcelain Bond. Opaque (Monoopaque Ivoclar, Vivadent) is applied to the exposed metal surface polymerized for 20 seconds (Heliolux, Vivadent). The composite resin (Herculite, Kerr Co., U.S.A.) material used to build up the original shape and polymerized (Heliolux, Vivadent) for 40 seconds.

Bonding procedure for Ceramic Repair System: The exposed porcelain-metal surface was roughened using 88-120µm aluminum oxide applied with approximately 4 bar pressure for 10 seconds. Total etch (37% phosphoric acid) applied on the ceramic and metal surface using syringe supplied in the kit and allowed to react for 10-15 seconds. Then thoroughly rinsed with water and dried with oil free air. Silane coupling agent (Monobond-S) applied to the etched surface using a brush and allowed to react for 60 seconds. Then dried with oil free air. A layer of 0.5mm thick Monopaque was applied on the conditioned metal surface then polymerized (Heliolux, Vivadent) for 40 seconds. Tetric ceramic composite resin material was used to restore the fractured material and Polymerized (Heliolux, Vivadent) for 40 seconds.

4. Thermocycling treatment procedure

To simulate oral conditions many of the test samples were subjected to various cycles in water, before measuring the bond strength. 10 repaired samples in each group were divided into 2 subgroups. 5 samples in each subgroup subjected to thermocycling (Thermostat, HAAKE C41, Figure 2) treatment. The samples were thermocycled for 2000 cycles at 4°C, 37°C and 60°C (dwell time of 1min) before determination of bond strength.

5. Determination of shear bond strength

The Instron Universal Testing Machine (Figure 3) was used in this study to measure shear bond strength. This machine has two cross heads, upper and lower. The upper crosshead is stationary and the lower cross head is movable. These cross heads are mounted on a hydraulic frame work connected to the recording unit, which records the load applied.

Evaluation of shear bond strength:

Bond strength was measured for both thermocycled and non-thermocycled samples. The acrylic block was positioned into the lower cross head of the Instron testing machine. The shear test was performed with a monobevelled chisel blade. The applied load directed perpendicular to the repaired area with a cross head speed of 5mm / min until fracture occurred.

The load at which the repair material debonded was recorded in Kilo Newton units. This testing procedure was repeated for all 30 samples i.e. 15 thermocycled and 15 non-thermocycled samples. The surface area of the fractured region was measured. The load measured in Kilo Newton converted to MPa using the formula, 

The samples were examined with a Scanning Electron Microscope (Figure 4) in the secondary electron image mode and photomicrographs of the representative areas were obtained to determine the nature of failure of bond.

RESULTS

1. with non-thermocycled samples, the mean shear bond strength of Composite resin bonded to porcelain-metal surface produced by 3 Porcelain repair systems ranged from 28.76±5. lOMPa to 23.49±2.03MPa.

2. The mean shear bond strength with thermocycled samples ranged from 16.52 ±l.27MPa to 14.45±l.94MPa.

3. The mean difference of shear bond strengths before and after thermocycling treatment was found to be 12.2340MPa for 3M Scotchbond system followed by 9.0360MPa for Ceramic repair material and 5.9120MPa forCleariil Porcelain Bond.

4. The change in mean shear bond strength value for the entire repair systems tested was found to be statistically significant. (Group A. t-value - 5.4209. Group B. t-value - 3.1633 and for Group C. t-value - 31.4810) at 5% level of confidence.

DISCUSSION

The popular and increased use of ceramo-metal restorations in dental practice has heralded the occurrence of an increased number of porcelain fracture, accompanied by dire need for an acceptable and long term reliable technique for their repair¹⁵ . Composite resin has been the material of choice for their ease of manipulation and aesthetic property¹⁶.  

Within last few years, several types of porcelain repair systems have been developed for use by the dental profession¹⁷ . The clinician is often left wondering which of these materials gives the optimum repair strength. Therefore, we felt it was necessary to determine which of the commercially available repair system had the best bonding properties.

There are different modes of testing bond strength of porcelain repair systems-shear^8,19 tensile²⁰ , flexural²¹ , compressive or torsion. Shear mode of failure is the most likely method to be experienced clinically. Therefore it was decided to evaluate the bond strength in shear mode. In this study 3M Scotchbond system, Clearfil Porcelain Bond and Ceramic Repair materials were tested. Manufacturer's instructions were followed for bonding procedure. 10 samples for each repair system were used. Out of these, 5 samples were not subjected to thermocycling treatment. These samples were stored in dry bottles to avoid contamination. And the other 5 samples were thermocycled. The shear bond strength tested in an Instron testing machine. The crosshead speed was kept at 5mm/minute. The readings obtained were recorded in Kilo Newtons, and then converted into Mega Pascals (MPa), which denote force per unit area.

Table No.1 to 3 shows the shear bond strength measured for Group A, B and C samples before and after thermocycling treatment. The mean, minimum, maximum and standard deviation values of the shear bond strength are given in Table No. 4.

The mean and standard deviation values obtained for Group A non-thermocycled samples are 28.76MPa and 5.10 respectively, for thermocycled samples the mean and standard deviation values are l6.52MPa and 1.27 respectively. These values vary from minimum of 24.81MPa to maximum of 37.50MPa for non-thermocycled and minimum of 14.37MPa and maximum of 17.65MPa for thermocycled samples.

The mean and standard deviation values obtained for Group B non-thermocycled samples are 2l.38MPa and 2.56 respectively, for thermocycled samples the values are 15.47MPa and 2.76 respectively. These values vary from minimum of 18.90MPa to maximum of 24.67MPa for non-thermocycled and minimum of l2.48MPa and maximum of 18.30MPa for thermocycled samples.

The mean and standard deviation values obtained for Group C non-thermocycled samples are 23.49MPa and 2.03 respectively, for thermocycled samples the values are 14.45MPa and 1.94 respectively. These values vary from minimum of 20.51MPa to maximum of 25.19MPa for non-thermocycled and minimum of 11.30MPa and maximum of 16.40MPa for thermocycled samples.

For the statistical analysis Paired t-test was used to compare the shear bond strength of 3 groups measured for both non-thermocycled and thermocycled samples. The values are given in Table No. 5. This test reveals that there was a significant difference in shear bond strength before and after thermocycling treatment in Group A samples at 5% level of significance (t=5.4209, P<0.05). For Group B samples there was a significant difference in shear bond strength before and after thermocycling treatment at 5% level of significance (t=3.1633, P<0.05). Similarly, there was a significant difference in shear bond strength in Group C samples before and after thermocycling treatment at 5% level of significance (t=3.1481, P<0.05).

To compare the mean difference of shear bond strength for both non-thermocycled and thermocycled samples in Group A, B and C ANOVA test was used and the values obtained are given in Table No. 6. This table shows that the means of shear bond strength of 3 groups differs significantly at 10% level of significance (F=3.4591, P<0.10).

Further, Student's unpaired t-test was used to compare statistically significant difference of shear bond strength before and after thermocycling treatment between the pairs of groups i.e., A to B, A to C and B to C. The values are given in Table No. 7. This test reveals that the mean difference of difference for both non-thermocycled and thermocycled samples between Group A (12.2340MPa) and Group B (5.9120MPa) differs statistically significant at 5% level of significance (t=2.1575, P<0.05). Similarly the mean difference of difference in shear bond strength before and after thermocycling treatment between Group A (12.2340MPa) and Group C (9.0360MPa) differs statistically significant at 10% level of significance (t=1.4060, P<0.10).

Likewise between Group B (5.9120MPa) and Group C (9.0360MPa) the mean difference of difference in shear bond strength before and after thermocycling treatment differs statistically significant at 10% level of significance (t=l.6526, P<0.10).

Following the statistical analysis, the result showed Group A had the greatest bond strength for non-thermocycled and the Group C showed the least bond strength for thermocycled samples.

The photomicrographs obtained from the Scanning Electron Microscope were shown in Figure 5a, 5b, 6a, 6b and 7a, 7b. These photomicrographs showed adhesive type of fracture for both non-thermocycled and thermocycled samples.

In this study the determined shear bond strength for non-thermocycled samples was found to be significantly greater than the thermocycled samples. This finding agrees with the work of many investigators. 

Thomas P. Nowlin et al ¹¹(1981) evaluated the bonding strength of 3 porcelain repair systems in response to thermocycling treatment. They showed that the repair strength was significantly affected by the thermal stress.

R.C. Pratt et al (1989)¹⁴ tested the shear bond strength of 6 porcelain repair systems at 48 hours and 3 months time of interval. Following 3 months water storage the samples were thermocycled at 6°C to 60°C for 500 cycles. The study showed that the water storage and thermocycling significantly lower the mean shear bond strength for all the repair systems tested. However, at the end of 3 months water storage Scotchprime Repair System had the highest mean bond strength compared to other materials tested.

In the study conducted by Abdul-Haq A. Suliman et al in (1993)²² the Clearfil porcelain bond exhibited highest shear bond strength following thermocycling compared to the two other repair systems tested.

CONCLUSION

This in-vitro investigation demonstrated that the 3 porcelain repair systems tested had the optimal shear bond strength without thermocycling treatment. Without thermocycling treatment 3M Scotchbond system showed greater shear bond strength than the Ceramic Repair Material and Clearfil Porcelain Bond. The Clearfil Porcelain Bond material showed least bond strength after thermocycling treatment. The thermocycling treatment yielded the lower bond strength values for all the materials tested. The possible reason for decrease in bond strength could be due to the differences in physical properties, thermal expansion discrepancies of porcelain and composite being prime concern.

Clinical Implication:This study suggested that the porcelain repair products are significantly affected by thermocycling treatment. All the products had a significant loss of bond strength with thermal stress suggesting that porcelain repair technique may be used as an interim clinical procedure.

ACKNOWLEDGEMENT

The authors Acknowledge the help rendered by Mr. Nitin Patel from Ivoclar Vivadent, Mrs. Marlin George from 3M Co. Ltd., and Kuraray Co. Ltd, for providing the materials used in this study.

For being extremely cooperative and permitting the use of Instron Testing Machine and Thermostat, The authors would like to thank Mr.V.K. Gupta of the Shriram Institute for Industrial Research, Bangalore. So also, Prof. Vikram Jayaram, and Mr. Gurulinga (Dept. of Metallurgy, Indian Institute of Sciences, Bangalore) must be specially thanked for providing access to the SEM Study.

And we also acknowledge the support of the Faculty of Dept. of Prosthodontics, Sri Dharmasthala Manjunatheshwara, College of Dental Sciences and Hospital. Sattur, Dharwad, Karnataka. and The Chairman and Dean Of Narsinhbhai Patel Dental College and Hospital. S.P.Shahakar vidyadham, Near Kamana Crossing, Visnagar, District Mehsana, North Gujarat.