Effect of surface treatment on micro shear bond strength of two indirect composites.

AIM: To determine the effect of surface treatment on micro shear bond strength of two indirect composites.
MATERIALS AND METHODS: Blocks of 2 × 7 × 20 mm dimensions were made from two kinds of resin composites, Gradia and Signum plus. Samples were subjected to secondary curing to complete polymerization. They were divided into five groups: control without any preparation, second group sandblasted with aluminum oxide, third, fourth and fifth groups were lased under a beam of 0.5, 1 and 2 W respectively. Panavia resin cement was placed on the composite blocks using tygon tubes and cured and micro shear bond strength was measured. One sample of each group was observed under electronic microscope. Data was analyzed by two-way ANOVA and Tukey's multiple comparison tests.
RESULTS: For Gradia composite, the sandblasted group showed highest strength (25.7±2.9 MPa) followed by the laser beam of 1 W group (with 23.6± 2.8 MPa). In Signum composite, the laser beam of 1 W (21.4±4.2 MPa) showed the highest strength followed by the sandblasted group (with 19.4±3.2 MPa).
CONCLUSION: Surface treatments using sandblast and laser beam of 1W power along with silane are two effective methods to increase the bond strength of composites.

INTRODUCTION

One of the most important aims of operative dentistry is increasing the long-term prognosis of restorations. Use of indirect restorations are recommended when extensive amount of tooth structure is lost.[1] Indirect techniques and polymerization of composite in a laboratory can decrease gap formation between the restoration and tooth structure. Although fabricating indirect restorations without any gap between tooth and restoration is still impossible, use of a thin layer of cement plays an important role in sealing of margins and prevention of micro leakage and reduction of gap formation.[2]

Polymerization of indirect composite takes place by heat, vacuum, higher amount of light or combination of all; therefore, physical and mechanical properties are improved which can lead to stronger restoration, better polished surface and more resistance to plaque and stain collection.[3-5]

Since polymerization takes place in a laboratory completely and homogenously, few numbers of monomers will be available for chemical bond between composite and resin cement which can reduce the bond strength.[6]

In order to increase bond strength, we should increase the surface roughness of the restoration so that mechanical retention can improve and more number of free carbon bonds on surface can be made available.[78]

There are different methods of creating mechanical and chemical surface treatment of composite and ceramic crowns which include sandblasting, use of silane, etching and laser.

Laser can be used for soft tissue surgery, pit and fissure sealant, sterilization of root tip, change in enamel and dentin surfaces for improving their resistance against caries and facilitate bond to composite resin, curing of composite, bleaching of teeth, reducing dentin hypersensitivity and for drilling dental hard tissue.[910]

The aim of the present study is to evaluate the bond strength of two indirect composites using different surface treatment methods (sandblast, silane, laser beam with three different power energies).

MATERIALS AND METHODS

In this experimental study, two different types of indirect resin composites (Gradia and Signum plus) and one type of resin cement (Panavia) were used.

150 composite blocks with 2 × 7 × 20 mm dimensions were made. Half of composite blocks were made from Gradia and another half were made from Signum plus indirect composites. Split metal mold was used for making these blocks. After primary curing using halogen light curing unit, all samples were post cured again inside laboratory light (Hager werber) for another 10 min.

All samples were polished using 220 - 1200 grit sandpaper under running water to make an even surface. Blocks prepared from each composite were randomly divided into 5 groups.

Group 1: For this group no surface treatment was done. Samples were rinsed with water for 20 s, air dried for 10 s, after that 37% phosphoric acid was applied, rinsed and air dried. One layer of silane (from Panavia kit) was applied on samples and thin dried after 3 minutes using air spray.

Group 2: For this group samples were treated using aluminum oxide particles of 50 μ with 2 mm distance and 80 PSI for 10 s, samples were cleaned in an ultrasonic device for 2 min, air dried and then were treated with silane.

Group 3: In this group surface treatment of composite was done using laser beam of 0/5 W and pulse energy of 25 mJ and air pressure of 11% without water. Surface treatment of this group was done with a mild speed for 3-5 s with 1-2 mm distance from surface. The laser beam used was Er,Cr: YSGG with wave length of 2780 nm (Biolase company, USA).

Time for each pulse was 140 μs with 20 Hz frequency.

Group 4: In this group composite surface was treated with 1 W of 50 mJ pulse energy. The rest of work was similar to previous group.

Group 5: In this group laser beam of 2 W with pulse energy of 100 mJ was used. The rest of work was similar to Group 3.

From each group for both composite samples (before bonding the resin cylinders), one sample was selected for SEM evaluation so that type and amount of surface changes can be evaluated.

Fabrication of resin cylinders

For this step we used tygon tube with internal dimension of 0/7 mm and height of 1mm. The tygon tubes were placed over treated surface of composite blocks and then resin cement (Panavia) was mixed according to manufactures’ instructions and was packed inside tygon tube; each tube was light cured for 40 s, after 1 h tubes around the resin cylinders were cut off using BP blade and removed from around the resin and samples were kept at 37°C temperature for 24 h.

For testing the bond strength, we used micro tensile bond strength tester machine and in order to evaluate micro shear bond strength of samples we converted the tester machine. Speed of 0/5 mm/min was applied till failure occurred.

The results of the study were evaluated using SPSS statistical analysis software and two-way ANOVA test.

For comparison between two groups, Tukey's multiple comparison test was used. P value was considered as 0.05.

RESULTS

Results of the present study are shown in Tables 1 and 2 and Graph 1. The highest amount of microshear bond strength was seen in Gradia composite with sandblasted surface treatment and in Signum Plus with laser beam of 1 W.

Table 1

Microshear bond strength of samples with same surface treatment of two composites

Table 2

Tukey's multiple comparison test

Graph 1

Average and mean deviation of microshear bond strength of different groups

The least amount of strength belonged to surface treatment with 2 W laser beam for both the composites.

Two-way ANOVA showed significant differences between two different types of composite in such a way that Gradia composite showed higher bond strength than Signum plus (P<0).

Different types of surface treatment also showed statistically significant differences. Tukey's multiple comparison test was also used for evaluating bond strength between groups. There was no statistically significant differences between sandblasted and lased (1 W) groups for both the composites, but there were statistically significant differences between these 2 groups and group lased with 2 W.

Composite surface of control group which was polished using sand paper of 220- 1200 grit was even and smooth for both types of composites; but for sandblasted surface of composite, surface roughness was obvious and even throughout of treated surface and no destruction of composite structure was seen.

This even surface roughness had created an increased bond surface by increasing surface roughness, depressions also created on the surface and destruction of composite could also be seen.

For lased surfaces, in addition to creating surface roughness and irregularities, deep depressions could also be seen which could be the reason for subsurface destruction and weak bond. In addition, some deep cracks and undercuts also were seen in lased surfaces.

DISCUSSION

Recently indirect composite restorations have been widely used in the field of operative dentistry due to their good esthetics. Despite many advances in indirect composite resin systems and laboratory polymerization techniques which have caused significant improvement in physical properties of these types of restorations, bond strength of these restorations with resin cement is still a problem.[611]

The reasons for reduced bond strength could be increased degree of conversion and decreased unreacted methacrylate groups available for bond due to secondary polymerization methods like light, high temperature, pressure and electromagnetic methods. One study showed that degree of conversion for light curing was 42-45% and after secondary curing in laboratory was 68 %.[12]

Some studies have shown that composites during laboratory procedure show 25-80% decrease in bond strength.[13]

Surface treatments of composite using chemical or mechanical methods are very essential for indirect composites.[14]

Start Many methods are advised for increasing bond strength like micro abrasion,[15] etching with hydrofluoric acid,[16] increasing surface roughness using sandblast and laser.[17] In the present study, two different types of indirect composite (Gradia and Signum plus) were used and different surface treatment methods like sandblasting, laser and silane methods were used. Silanes are double molecules which cause chemical bonds of silicon dioxides with hydroxyl groups of ceramic surface (si-o-si) and have little ability to copolymerize with organic resin matrix.

In many studies, effect of silane on increasing bond strength of ceramic and composite crowns with resin cement has been shown, which facilitates chemical bond to composite surface.[18]

According to Bailey GH and Davidson CL, using silane with silica base is an essential factor for improving chemical bond. Silane can increase wettability of surface and create hydrophobic surface, which in turn can improve penetration of cement inside resin and decrease void formation.[19] In second group, sandblasting was used for surface treatment which showed the highest amount of bond strength for both types of composite. Swift in 1992 showed that using sandblasting along with silane increases bond strength.[20] Research has shown that the type of effected surface treatment for each material depends on the characteristics and composition and particles present in that material. They recommend that etching should be used for all porcelains containing SiO2 particles, but composite should not be etched because etching cause destructive changes in resin. They conclude that the best method for all studied composites is sandblasting for 10 s along with silane.[21]

Many studies have shown that micromechanical retention of internal surface of indirect composites is also essential for better bond strength between composite and resin cement, which is mainly due to loss of resin matrix and exposure of filler particles as a result of increasing effect of bond with resin.[22] Different factors should be considered in using air abrasion like pressure, particle size, type and hardness of particles, dimension of particles, tip size, distance from surface, angle of contact of particles with surface, time and speed of flow of particles.[21]

In the present study, similar to a study conducted by Burnett,[23] we used a micro etcher device and aluminum oxide particles of 50 μ dimension with a contact angle of 90° for 10 s for the whole surface and 80 psi pressure from a distance of 2 mm from the surface.

Laser is one of the methods of surface treatment used for improving micromechanical retention and bond strength of resin cement to ceramic.

Use of Er: YAg laser has been increasing in the field of dentistry for removing dental caries without damaging the surrounding sound tooth structure. This laser has got FDA (Food and drug administration) approval in 1997 for dental treatment.

Maeda used Er: YAg laser on composite compomer and glass ionomer. He showed that use of one type of laser with specific parameter can have different effects on material.[24]

For three groups of the present study, Er: Cr:YSGG laser with three different powers was used.

Cavalcanti showed that lower amount of laser energy caused lesser amount of changes in this type of ceramic and higher amount of energy caused destruction of material and increased carbonization.[25]

In the present study, results of SEM evaluation showed that exposure of composites to laser beams caused irregularities and surface roughness which do not follow particular pattern.

By increasing the laser power, these changes also increase. But in Gradia composite, increasing the surface roughness and depressions did not necessarily increase bond strength. In sandblasted groups, bond strength was higher than laser group, despite creating more regular and even surface without any under cuts. It can be explained that during sandblasting, removal of resin matrix can facilitate bond between silane and SiO2 filler particles, but laser beams along with creating micromechanical retention and deep undercuts could also cause destruction of crystalline and matrix phase and also separation of these two phases from each other.

In Group 4 of Signum composite where 1 W laser beam was used, it showed higher bond strength compared to sandblasted group. It can be explained that absence of SiO2 filler particles in this material prevented effect of sandblast technique compared to Gradia composite which has got this type of filler.

As it has been mentioned before, one of the most important reasons for improving bond strength after sandblasting and using silane in porcelains containing SiO2 filler particles is creating Si-O-Si bond between silane molecules and this filler, the same can be explained with composites with silica filler particles like Gradia.

Bond strength in laser group of 2 W energy for both types of resin composite was very low.

The differences between these two groups could be due to physical destruction because of increased temperature and increased destruction of chemical structure and crystals present in composite and decrease ability of molecules to bond with silane and or resin cement.

Considering the above-mentioned factors affecting bond strength of indirect composites, it is difficult to draw a precise conclusion, unless more studies to be done under different conditions to see best results.

CONCLUSION

Sandblasting of Gradia composite and lasing Signum Plus composite with 1 W showed highest bond strength.

Use of 2 W laser power can decrease bond strength.

Use of sandblast and laser along with silane for surface treatment can improve bond strength of indirect composites.