EVALUATION OF SHEAR BOND STRENGTH OF COMPOSITE TO DENTIN REWETTED WITH A DESENSITIZER- AN IN-VITRO STUDY

Aim: The aim of the study was to evaluate the shear bond strength of composite to dentin with 2 different dentin bonding agents after dry dentin is rewetted with a desensitizer. Materials and Methods: Sixty human mandibular molars were taken and the occlusal surface was ground to expose flat dentinal surface. The samples were divided into two major groups Group I and Group II based on the bonding agent used. In Group I Gluma Bond5 was used and in Group II Adper Single Bond 2 was used. Each major group is subdivided into three subgroups a, b and c with ten samples each. Sub Group ‘a’ is moist dentin group. Sub Group ‘b’ is dry dentin group, Sub Group ‘c’ is rewetted dentin group. The desensitizer used is GLUMA Desensitizer. All these samples were thermocycled and the shear bond test was performed using Instron Universal Testing Machine. Results: Dry dentin rewetted with GLUMA Desensitizer followed by GLUMA Bond5 showed highest shear bond strength than that of other subgroups. The data was analysed using ANOVA and Tukeys honestly significant test. Conclusion: Desensitizer used as a rewetting agent increases the shear bond strength of composite on dry dentin.


INTRODUCTION
Adhesive dentistry has been replacing conventional dental amalgam because of its high aesthetic property, minimal tooth preparation and satisfactory longevity during function and it helps evade mercury toxicity inherent with dental amalgam. The adhesive property of composite restorations to enamel is based on a strong and durable micromechanical bond between enamel and bonding agent. [1] Acid etching is a prerequisite to achieve this bond. Conventionally, enamel is etched, rinsed and dried to expose the partially demineralised enamel rods. The bonding resin penetrates the exposed enamel rods and retains the composite to the enamel. [2] Compared to enamel bonding, dentin bonding is complex due to its structure, rich organic matrix and moisture content. [3] Dentinal bonding is also through demineralising of the dentinal matrix and hybridizing it with the resin bonding agent. [4] Often clinical situations require restoration involving enamel and dentin. The use of air syringe to dry enamel also desiccates the dentin with resultant collapse of the weak dentinal collagen scaffold. This affects the penetration of bonding resin to achieve hybridization and bonding. Only moisture in dentin preserves the micromorphological integrity of collagen. Hence, water chasing bonding agents are introduced that would displace the moisture and establish satisfactory penetration into the enamel and dentin for retention. But most studies show that capillary penetration of bonding agent into the dried enamel establishes strongest bond strength to enamel. [5] Hence, clinicians prefer drying of the enamel to gain maximum retention. To overcome the problem associated with bonding to dehydrated dentin the suggested practice is to rewet the inadvertently dried dentin with water. [6] Care should be taken while drying or rewetting of dentin so that dentin should not be over wet or over dried. Over wetting can cause phase separation between the hydrophobic and hydrophilic components. Moreover, even remnant water can be one of the cause for postoperative sensitivity often encountered with resin composite restoration. [7] To preserve collagen as well as to avoid or minimise postoperative sensitivity, a balanced application of rewetting agents are used on the dry dentin. A few studies have shown that application of desensitizing agents may resolve the problem of post-operative sensitivity and also maybe a satisfactory substitute for water as rewetting agent. [8] Hence the purpose of this study was to evaluate the shear bond strength of a selected composite to dentin rewetted with the selected desensitizer and when used with two different dentin bonding agents. Sixty freshly extracted, caries-free human permanent molars were debrided using periodontal curettes and were stored in distilled water until use. Flat dentin surface was created on the occlusal surface of extracted teeth with slowspeed diamond disk under continuous water cooling. Then, each tooth was mounted in a chemically cured acrylic resin, such that 3-4 mm of the crown will be exposed. (Figure1).

Figure 1: Exposing the Dentin
The specimens were divided into two major groups of thirty teeth each, depending upon different bonding agents used as Group I and Group II (Table  1). Group I: GLUMA Bond5 group and Group II: Adper Single Bond 2 adhesive group. These two groups were subdivided into three subgroups a, b, and c consisting of ten teeth in each sub group. In Group Ia after etching and rinsing, the dentin is made visibly moist by gently blotting with absorbent paper. Then GLUMA Bond5 is applied in two coats and polymerised for 20 seconds as per manufacturers' instructions. In Group Ib after etching the dentin is dried with air syringe to desiccate the substrate. Then, GLUMA Bond5 is applied as in group Ia.
In Group Ic after etching and desiccating the substrate as in group Ib, the dentin is rewetted with GLUMA Desensitizer which was applied in two coats and allowed to penetrate for 30-60 seconds and dried by applying a stream of compressed air until the fluid film had disappeared and the surface is no longer shiny and then rinsed thoroughly with water according to manufacturer's instruction. The excess water was blot dried and this was followed by application of GLUMA Bond5 as in previous sub groups. For the subgroups in Group II, the procedures similar to the subgroups in Group I were followed except that the bonding agent used was Adper Single Bond 2 as per manufacturer's instructions instead of GLUMA Bond5.

Fabrication of composite cylinder:
Tygon tubes of 2mm height and internal diameter of 5mm were placed over the prepared dentin surface. Composite resin was filled in to the tygon tube in single increment and was polymerized for 20 seconds using light curing unit as per the procedure followed by Vibha et al in 2016 [9] . The tube was then cut with No. 11 BP blade and removed. (Figure 2). The specimens were stored in distilled water for 24 hours at 37 0 C.

Figure 2: Fabrication of Composite cylinder
Thermocycling procedure: The prepared specimens were then thermo-cycled in water baths set to 8 0 C and 48 0 C. The specimens were thermo-cycled for 2500 cycles with dwell time of 30 seconds at the set low and high temperatures with a transfer time of 10 seconds.

Shear bond strength evaluation:
The thermo-cycled specimens were subjected to shear bond strength evaluation using Instron Universal Testing Machine set at a crosshead speed of 1mm/minute. (Figure 3) The specimens were debonded using a knife edge at the tooth restoration. The break load values were recorded through a computer connected to it. The shear bond values were obtained in MegaPascal (MPa).

STATISTICAL ANALYSIS
The results were entered into a Microsoft excel sheet for calculation of statistics. One way analysis of variance (ANOVA) was used to evaluate the shear bond strength of composite on dentin. Tukey's Honestly Significant test was used to identify the significant pairs at 5%.  Table 2 shows moist dentin proved better strength than that of dry dentin. Dry dentin when rewetted with a GLUMA Desensitizer showed markedly more strength than that of dry and moist dentin. Dry dentin rewetted with GLUMA Desensitizer followed by GLUMA Bond5 and Charisma Smart showed more strength than that of other subgroups (Graph 1). There was a significant difference in shear bond strength between the six subgroups using ANOVA.

DISCUSSION:
Composites are extensively used for both anterior and posterior restorations. Hybridization theory by Nakabayashi (1991) is the commonly accepted technique for dentin adhesion. After etching of dentin, collagen fibril network with inter and intrafibrillar micro-porosities are exposed. [10] Low viscosity monomers when applied on the etched dentin surface diffuse into the demineralised dentin and form a resin -dentin interdiffusion zone. When this is polymerized, there will be entanglement of the fibrils by the resin which creates a hybrid layer of resin-reinforced dentin. Hybrid layer thus formed establishes the bonding mechanism for many adhesive systems . [10] Various methods of drying the dentin have been experimented in the past with varying degree of success . [11] Optimum moisture in dentin preserves the micro morphological integrity of collagen and formation of sufficiently bonding hybrid zone. The dentin is hence generally kept visibly moist to achieve optimum bonding, by gentle mopping with absorbent paper than air drying during restorative procedure using resin composite. [12] Dehydration of dentin would result in the collapse of the collagen scaffold which otherwise helps in the formation of the hybrid zone that retain the restoration. [10] Hence, inadvertent over drying can contribute to insufficiency of bonding of resin composite to dentin. [12][13][14] The dentin could also get dehydrated because of the environmental conditions. [15]. In addition post-operative sensitivity following composite restorations in posterior teeth is a common problem experienced by clinicians. [16,17] A lot of innovative techniques have been discussed in the literature to enhance the bond strength of composite to enamel and dentin. [18] It has been demonstrated that rewetting with water revives the collapsed dentin matrix and returns the bond strength. [6] It has also been observed that remnant water can also be one of the cause for postoperative sensitivity. [7] Further, if the dentin is over wet there is a phase separation between the hydrophobic and hydrophilic components that will end up with a blister or globule formation termed the 'over-wet' phenomenon. [7] Hence, in order to achieve optimum bond and reduce the postoperative discomfort it has been suggested to rewet the dry dentin with dentin desensitizers such as Vivasens, System P, Denshield, Sensodent K [19,20] Many proprietary rewetting agents are also available in the market such as Gluteraldehyde in water, MS coat, Tubilicid, Hurriseal and Protect [19,20] . Gluma has an established effect of tubular occlusion and desensitizing open and exposed dentin. [21] Therefore, in this study we have compared the shear bond strength of composite on dentin with two different bonding agents applied over application of 'GLUMA Desensitizer'.
In this study when GLUMA Desensitizer is used to rewet dry dentin along with GLUMA Bond5, it showed the best strength. Previous studies have also demonstrated that application of GLUMA Desensitizer after conventional etching of dentin has improved the efficacy of dentin bonding system. [22][23][24][25] Schupbach et al in 1997 resolved that the increase in bond strength may be due to the synergistic action of Glutaraldehyde and 2-hydroxyl ethyl methacrylate (HEMA) [21] In this study GLUMA Desensitizer is used over dry dentin as a wetting agent and the result was highly favorable.
According to this study, dry dentin rewetted with GLUMA Desensitizer along with bonding agent GLUMA Bond5 which is from the same manufacturer and Charisma Smart composite resin which is also from the same manufacturer showed highest bond strength. Unexpectedly, this group showed better bond strength than the conventional moist bonding procedure. This is in concurrence with similar studies by Soares et al in 2006 and Ritter et al in 2000 which also showed rewetting with desensitisers enhanced bonding than conventional moist dentin bonding [22][23][24] Contrary to this study, Lehmann and Degrange showed conventional moist bonding to be superior than rewetting [19] This can be due to change in composition of different rewetting agents used.
Dry dentin bonding demonstrated the least bond strength in this study. This is in concurrence with many other studies where similar result was achieved. [12][13][14] Yet another observation in this study was the achievement of higher bond strength when using same manufacturers' product. This is not an accidental finding as many studies of properties between inter and intra manufacturers products have revealed similar results [ 26] CONCLUSION: Within the limitations of this study, it was observed that the shear bond strength of composite on dentin is more when dry dentin is rewetted with a GLUMA Desensitizer than conventional moist dentin bonding procedure. This gives a new insight that it may be acceptable to dry the etched enamel and dentin to achieve high bond strengths through dried enamel and desensitizer rewetted dentin.
It was also observed that products from same manufacturer demonstrated better result than mixing products of different manufacturers.