ANOVERVIEWOFSOLVENTSINRESINDENTINBONDING

AN OVERVIEW OF SOLVENTS IN RESIN DENTIN BONDING

1. UTTARANCHAL DENTAL AND MEDICAL RESEARCH INSTITUTE DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS JOURNAL CLUB PRESENTATION TOPIC: AN OVERVIEW OF SOLVENTS IN RESIN DENTIN BONDING Manikandan Ekambaram, Cynthia Kar Yung Yiu , Jukka Pekka Matinlinna International Journal of Adhesion & Adhesives 15 TH SEPTEMBER 2014 PRESENTED BY DR SNEHANGSHU DUTTA PG STUDENT

2. CONTENTS Introduction Role of solvents in resin dentin bonding Solvents commonly incorporated in dental adhesives Problems associated with solvents in dentin bonding Methods to facilitate removal of solvents Conclusion References Cross references

3. INTRODUCTION Contemporary adhesive dentistry widely uses resin adhesives for procedures such as composite restorations and luting of veneers or crowns. Over the past three decades, dental adhesives have evolved to become more user-friendly. They are mainly classified into: Etch-and-rinse adhesives Three-step Two-step Self-etch adhesives Two-step One-step 2. Despite their classification, all adhesives share common components: hydrophobic and hydrophilic methacrylate monomers, volatile solvents, and initiator systems. Newer universal adhesives also contain co-polymers, fillers, and silane. 3.Self-etch monomers are acidic and demineralize dentin similar to etch-and-rinse systems. Solvents are critical in ensuring effective bonding to enamel and dentin, especially dentin, which poses more challenges.

4. ROLE OF SOLVENTS IN RESIN DENTIN BONDING Dental adhesives are co-monomer blends and the monomers of dental adhesives are usually solvated in volatile organic solvents and water. The most commonly used solvents in dental adhesives are ethanol, acetone and water . Commercial adhesive systems may contain up to 80% of solvent by weight. Firstly, the solvents in resin adhesives act as a vehicle that helps in the transportation of the co-monomer blends and the initiators to the tooth. Secondly, these solvents help to dilute the viscous monomers and facilitate its infiltration into demineralized dentin. Thirdly, Tezvergil Mutluay et al. have recently shown that alcohols, including ethanol, propanol and tert-butanol, possessed a matrix metalloproteinases (MMP) inhibitory potential. MMP had been shown to degrade the denuded collagen fibrils of the hybrid layer and caused degradation of the resin-dentin bond over time.

5. H-bonding capacity is an important property of solvent(s) incorporated in dental adhesives. Solvents with a high dielectric constant will have a high H-bonding capacity. Hydrogen bonding capacity would influence the applicability of a dental adhesive to a dry or wet demineralized dentin substrate. Solvents with higher bonding capacity, like water, would be able to re-expand the collapsed demineralized dentin collagen and therefore be applied to dry demineralized dentin surfaces. This property of a solvent is inversely proportional to the vapor pressure, as the solvents with higher vapor pressure have lower H-bonding capacity.

6. SOLVENTS COMMONLY INCORPORATED IN DENTAL ADHESIVES There are different types of solvents incorporated in various resin adhesives for bonding to dentin. The most commonly incorporated solvents in commercial resin adhesives are: acetone, ethanol and water. In some dental adhesives (in particular self-etch adhesives), there is more than one solvent (e.g. ethanol and water). Tert-butanol is another solvent that has been recently incorporated in a commercial adhesive. Few other solvents, like tetrahydrofuran and dimethyl sulfoxide, though have shown promising results in the recent research studies.

7. ACETONE Acetone is a very effective solvent to remove water from the dentin. Hence, it has also been regarded as a water-chaser . Acetone-based adhesives were less viscous than ethanol-based adhesives, as the viscosity of the acetone is three times lower than that of ethanol facilitating its penetration into the demineralized dentin. Tay et al. concluded that it was absolutely necessary to keep the demineralized dentin moist before application of acetone-based water-free dental adhesives. Tay et al. and Jacobsen & Söderholm showed that improved bond strength could be achieved with a wet dentin surface, when acetone-based dental adhesives were used. A recent research study by Hashimoto et al. also demonstrated that infiltration of acetone-based dental adhesives was reduced by 50%, when applied to dry dentin, in comparison to wet dentin. However, it is very difficult to standardize the dentinal surface moisture, when such adhesives are used clinically. In summary, though the residual solvent content left on a tooth surface after the air-drying step could have a negative effect on the conversion of monomers, this effect could be less pronounced with acetone, when compared to ethanol.  

8. ETHANOL Ethanol is currently the solvent most often used in commercial dental adhesives. It is incorporated either alone or with water as a co-solvent. In self-etch adhesives, ethanol is also able to ionize acidic monomers, although with a lower capacity as compared to water. The vapor pressure of ethanol (40 mmHg), though lower to acetone (184 mmHg), is still much higher than water (17 mmHg). Cadenaro et al. showed that when the ethanol-containing co-monomer blends were evaporated as per manufacturer’s suggestion under ambient atmospheric pressure and temperature, it was not possible to evaporate all the ethanol from that mixture. The same author stated that residual ethanol within certain limits could improve the degree of conversion (DC) of monomers, as the DC of ethanol-containing monomers were superior to the DC of neat monomers without it. In comparison, water-containing monomers did not exhibit this behavior and show reduced DC. The other main property of ethanol is its high H-bonding capacity. Given this, it is able to re-expand the collapsed demineralized collagen due to excessive air drying. Perdigão et al. demonstrated that the ethanol-based adhesive was more sensitive than ethanol-water-based adhesive, but less than acetone-based adhesive to dentin surface moisture. In addition, Reis and Loguercio showed that an etch-and-rinse adhesive with ethanol as its solvent produced superior retention at the non-carious cervical lesions after 36 months of service, when compared to their corresponding acetone-based adhesive.

9. WATER In the wet bonding technique used in contemporary dentin bonding procedure, polar solvent water, H2O, is already present on the dentinal surface and in the entire depth of demineralization. Hence, it is logical to incorporate water as a solvent in dental adhesives. Also, water is usually used as a secondary co-solvent because the “solvent” effect is not the primarily role of water in dental bonding agents. Self-etch adhesives consist of acidic monomers that demineralize dentin and encapsulate the collagen fibrils to form the hybrid layer. For the acidic monomers to be able to demineralize dentin, they need to be ionized, for which water is essential. Hence, water, an inorganic and polar solvent, is almost and always incorporated in self-etch adhesives. Moreover, water-containing self-etch adhesives (both the one-step and two-step adhesives) have shown degradation with time, when stored above room temperature. When incorporated in etch-and-rinse adhesives, water as a solvent could re expand the collapsed demineralized collagen fibrils caused by air drying. In this property, water is much superior to ethanol and acetone with its higher hydrogen bonding capacity . Hence, the water-based adhesives are not affected much by the differences in dentin surface moisture, when compared to ethanol- or acetone-based adhesives. Water retention in the hybrid and adhesive layers of the resin-dentin bonded interface would eventually lead to failure of such bonds from the hydrolytic degradation caused by water sorption.

10. 2-Propanol 2-Propanol is a relatively newly introduced solvent in dentin bonding. It is incorporated along with water as a co-solvent in one commercial one-step self-etch adhesive “ AdheSE One F” ( Ivoclar-Vivadent , Schaan, Liechtenstein). Salz and Bock and El-Deeb et al showed that AdheSE One F could produce stable bonding for up to 6 months and the authors of these studies attributed this stability in resin dentin bonding to the stability of the meth-acrylamide-based monomers and their resistance to hydrolytic degradation, when compared to the conventional methacrylate-based monomers. Propanol has a high vapor pressure, comparable to ethanol. However, lower bond strengths with heavy silver deposits, which indicated nano-leakage along the bonded interfaces, have been reported after dentin was treated with HEMA/propanol primer, when compared to HEMA/water, HEMA/methanol and HEMA/ethanol primer. This could be attributed to the low hydrogen bonding capacity of propanol, which is not sufficient to break the interpeptide H-bonds that collapsed the matrix and shrank the fibrils. Furthermore, stiffening rate of propanol (0.5 MPa/min) was also much less than acetone (0.9 MPa/min) and ethanol (0.8 MPa/min). This may increase shrinkage of demineralized matrix and reduce the spaces between collagen fibrils for resin infiltration.

11. Tert-butanol Blunck et al. tested a commercial adhesive XP Bond (Dentsply, Caulk, Milford, DE, USA), which contained tert-butanol as a solvent incorporated in the dental adhesive for the first time. Tert-butanol has a chemical structure, a C4 body with a hydroxyl group, which makes it totally soluble with water and adhesive resin components. In addition, the vapor pressure of tert-butanol (30.6 mmHg) is lower than ethanol (40 mmHg), which allows it to evaporate more slowly than ethanol, resulting in less shrinkage of the demineralized dentin matrix. Latta confirmed from their study that tert-butanol-based XP bond could produce bond strength to both enamel and dentin, not varying from those produced by ethanol/water-based ( Scotchbond 1 XT) and ethanol-based ( Optibond Solo Plus) adhesives. In another study by Manhart & Trumm, tert-butanol was shown to have produced exceptional marginal adaptation to both enamel and dentin.

12. TETRAHYDROFURAN Tetrahydrofuran (THF) is a heterocyclic organic and polar aprotic solvent that dissolves a broad range of polar and non-polar components. Due to this property, THF is able to mix with the hydrophobic and hydrophilic resin components. THF has high vapor pressure (173 mmHg), very close to that of acetone (184 mmHg), which means that the solvent is easily removed after application. THF is capable of ion chelation and hence, could contribute to chemical bonding to dentin. This is a very unique property of THF, which the other solvents used in dental adhesives do not possess. In regard to bond durability, THF-based dentin bonding systems have shown evidence of stable resin-dentin bonds after 12-month storage in water.

13. DIMETHYL SULFOXIDE Dimethyl sulfoxide (DMSO) is an outstanding infiltration facilitator in the contemporary medical practice. As a potential solvent in resin-dentin bonding, it is completely soluble in all the solvents used in contemporary dental adhesives. DMSO could also solvate many resin monomers used in dental adhesives. It could compete with water for the interpeptide hydrogen bonding of the dried demineralized dentin collagen. DMSO has a very strong affinity to hydrogen atoms. This affinity of DMSO to hydrogen atoms of water is even stronger than the bonds formed between hydrogen atoms of water molecules. Hence, DMSO not only improve the wettability of the demineralized dentin, but could also remove water from the hybrid and adhesive layers during dentin bonding procedure. The resultant bonded interface could resist the hydrolytic degradation and may therefore exhibit increased durability. Since DMSO had shown a very good penetration potential, application of this solvent in resin-dentin bonding could lead to an increase in the potential toxicity of the resin monomers through the pulp tissue.

14. PROBLEMS ASSOCIATED WITH SOLVENTS IN DENTIN BONDING Loss of solvents from Dental Adhesives Solvents are prone to evaporation from the adhesive containers upon storage and from their frequent usage in clinical practice. In particular, organic solvents such as ethanol, and more importantly, acetone, have higher chances for evaporation than water due to their high vapor pressure. The loss of solvent(s) from dental adhesives would lead to their increased viscosity. The resultant viscous resin adhesives would face greater difficulty to diffuse into demineralized dentin and would therefore lead to poor resin-dentin bonds. 2. Residual solvents in the bonded interface Residual solvent within the bonded interface has several deleterious effects. This includes: prevention of resin infiltration to the entire depth of demineralized dentin, and interference with resin polymerization as well as facilitation of resin dentin bond degradation. The biomechanical properties of resin adhesives, such as ultimate tensile strength and modulus of elasticity had an inverse relationship with their solvent content.

15. 3. For two-step etch-and-rinse and one-step self-etch adhesives, the excess solvent (ethanol, acetone, water) has a deleterious effect on resin-dentin bonding. 4.On the other hand, if excess drying is done with an aim of removal of such residual solvents, then there would be an oxygen inhibition of the resin polymerization, which led to decrease in bond strength. 5. The air-drying step is more important for the one-step self-etch adhesives due to presence of solvents and also due to the lack of a hydrophobic adhesive coat as in two-step self etch adhesives. 6. When using commercial dental adhesives, although manufacturers recommend that adhesives should be “gently” air-dried, clinicians should not be “afraid” of drying the solvent. 

16. METHODS TO FACILITATE REMOVAL OF SOLVENTS Prolonged application time of the primer/adhesive Prolonged application time of the primer/adhesive is the most common method recommended for removal of solvents from the hybrid and adhesive layers. An extended application time of primer/adhesives meant a longer waiting time after adhesive application with no further disturbance before light curing. This would facilitate the removal of residual solvents due to their evaporation. This method would also help in the reduction of nanopores and facilitate the formation of a highly cross-linked polymer. The resultant resin-dentin hybrid layer would have an increased resistance to degradation. 2. Multiple adhesive coatings Hashimoto et al. using two 2-step etch-and-rinse adhesives and Ito et al. using two 1-step self-etch adhesives showed that by simply performing multiple adhesive coatings, the bonding performance of adhesive resin to dentin could be greatly improved. Multiple adhesive coatings could have increased the concentration of hydrophobic resin monomers, which in turn improved the quality of the polymerized resin.

17. 3. Vigorous mode of primer/adhesive application When the resin primer/ adhesive were applied with vigorous rubbing motion to demineralized dentin, the resin and solvent could diffuse/penetrate better into demineralized dentin and the solvent could be removed due to the excess time spent, while performing such application. 4. Prolonged air-drying of the primer/adhesive Though a prolonged air-drying of the primer/adhesive should logically improve the dentin bond strength by evaporation of solvents, there are possibilities of several risks by the application of this technique. The risks are: reduced degree of conversion as a result of excessive loss of solvents inhibition by oxygen excessive decrease in the thickness of the adhesive layer 5. Increased duration of the light exposure during resin polymerization The heat generated from the light source could help in the removal of the solvent from the dental adhesives and therefore help to achieve better polymerization. Cadenaro et al. showed that 10 to 20 s of light exposure for polymerization of dental adhesives recommended by the manufacturer usually were insufficient, in particular for those adhesives with 30% ethanol. Cadenaro et al. recommended that by performing longer curing times, the adhesives would become less permeable.

18. 6. Changing the air blowing distance/pressure Increased air-drying pressure has been shown to improve the dentin bonding effectiveness due to the efficient removal of solvents from the dental adhesives during application. However, some evidence from the literature for the effect of changes in the air-blowing distance or air-blowing pressure on resin-dentin bond strength showed mixed results. 7. Proper storage and handling of the adhesive container The operator should mix the ingredients by shaking the bottle well before use; failing to do so would lead to an excess solvent in the applied adhesive, during the initial applications from the bottle and at the later stages, the applied adhesive would lack adequate solvent. Both the excess and shortage of solvent in the applied adhesive would affect bonding to dentin. It is recommended replacing the cap of the container immediately after dispensing the adhesive, to prevent the vaporization of the solvents.

19. CONCLUSION In conclusion, we may state that: An acetone-based water-free etch-and-rinse adhesive should never be applied to dry demineralized dentin. Proper storage and use of dental adhesives is very important to prevent evaporation of solvents, in particular for the acetone-based adhesives. The self-etch adhesives are less sensitive to the dentin surface moisture compared to the etch-and-rinse adhesives. Water-based adhesives can re-expand the collapsed demineralized collagen due to air drying. However, the removal of water from the water-based adhesives is relatively difficult due to its lower vapor pressure. Water retention of dental adhesives depends on their hydrophilicities, more than their solvent types. Inadequate solvent removal from the hybrid/adhesive layers would seriously affect the longevity of the resin-dentin bonds. Air-drying is more important for the simplified two-step etch-and-rinse and all-in-one adhesives, as they lack a hydrophobic adhesive resin layer, unlike the three-step etch-and-rinse and two-step self-etch adhesives.

20. REFERENCES [1] Tay FR, Pashley DH. Dental adhesives of the future. J Adhes Dent 2002; 4: 91 103. [2] Perdigão J, Frankenberger R. Effect of solvent and rewetting time on dentin adhesion. Quintessence Int 2001; 32: 385-90. [3] Reis AF, Oliveira MT, Giannini M, De Goes MF, Rueggeberg FA. The effect of organic solvents on one-bottle adhesives' bond strength to enamel and dentin. Oper Dent 2003; 28:700-6. [4] Hashimoto M, Tay FR, Ito S, Sano H, Kaga M, Pashley DH. Permeability of adhesive resin films. J Biomed Mater Res B: Appl Biomater 2005; 74B: 699-705. [5] Ikeda T, De Munck J, Shirai K, Hikita K, Inoue S, Sano H, et al. Effect of evaporation of primer components on ultimate tensile strengths of primer-adhesive mixture. Dent Mater 2005; 21: 1051-8. [6] Klein-Júnior CA, Zander-Grande C, Amaral R, Stanislawczuk R, Garcia EJ, Baumhardt-Neto R, et al. Evaporating solvents with warm air-stream: effects on adhesive layer properties and resin-dentin bond strengths. J Dent 2008; 36: 618-25. [7] Burke F, McCaughey A. The four generations of dentin bonding. Am J Dent 1995; 8: 88-92.

21. Present study suggests that solvent type and drying methods might have an effect on bond strengths of one-step total etch adhesives to dentin. Tertiary butanol-based adhesive showed higher bond strength values than ethanol or acetone-based adhesives. Blot drying of dentin improved the bond strength values of tertiary butanol-based adhesive. • Solvents play a significant role in the bond strength ability of self-etch adhesives. Thus, comparison of contemporary adhesives revealed that the constituents of the self-etch bonding agent do affect its bonding abilities, out of which type of solvent may be one of the contributing factors • Self-etch adhesives containing ethanol as solvent showed highest tensile bond strength when compared with the other two self-etching bonding agents. Within the limitation of the present study, it can be suggested that the blot-drying method provides better strength to both acetone- and ethanol-based solvents. Acetone-based solvents can be better to use in clinical conditions where moisture control could be a problem.