An In Vitro Evaluation of the Effect of Dimethyl Sulfoxide Pre-Treatment on the Bond Strength of Fiber Post

Introduction

Bonding to dentin has been refined throughout the years with the advances in materials and research. However, it is still under scrutiny because of several inherent problems associated with dentin structure. Bonding to enamel has always been found to be stronger, durable and less technique sensitive than dentin bonding. This difficulty in bonding to dentin was encountered as it has variable composition than enamel such as dentinal tubules with dentinal fluid thus having higher organic content than enamel, the number of dentinal tubules that differs in superficial and deep dentin, the structure and number of dentinal tubules that vary in coronal and radicular dentin. These factors affect the efficacy and durability of dentin bonding. Several factors were brought under light that affect the hybrid layer integrity which is crucial for resin dentin bond through extensive research for the past years. The proposed mechanisms were hydrolysis of hybrid layer by endogenous host enzymes like matrix metalloproteinases (MMPs) and cysteine cathepsins, insufficient penetration of resin into demineralized dentin, excess water within hybrid layer due to insufficient solvent evaporation etc.^1,2 Several strategies were proposed to improve longevity of resin dentin interface which involves application of enzymatic inhibitors, collagen cross linkers on dentin, biomimetic remineralization, wet bonding techniques etc.³ Earlier, water wet bonding was suggested to improve the performance; however, difficulty in controlling the amount of wetness affected the bonding efficacy. Thus, ethanol wet bonding was researched which replaces water in the dentin matrix and supports demineralized collagen fibers, thereby prompting the penetration of adhesive monomers and avoiding collagen collapse.⁴ However, the major drawbacks were that it is time consuming due to high ethanol vapour pressure which deemed its usage in clinical condition impractical. For the past few years, dimethyl sulfoxide has been evaluated for its application in dentin bonding. It is a polar aprotic solvent which dissolves both polar and nonpolar compounds. Research states that dimethyl sulfoxide is miscible in all solvents that were used in adhesive dentistry, improves wettability in dentin, has MMP inhibitory activity, making it an attractive option for dentin pre-treatment during composite restoration.^5,6,7

Loss of a large amount of coronal tooth structure due to dental caries or trauma requires endodontic treatment followed by post and core for crown retention. Fibre posts were advantageous as it provides esthetics, low modulus of elasticity, thus preventing fracture of teeth that were treated endodontically, requiring less time for placement unlike metal posts.

Debonding at the fibre post, resin cement and root dentin interface can occur due to difficulties in bonding, ultimately leading to treatment failure. Thus, establishment of a long-lasting bond between the resin luting cement, glass fibre post and root dentin is a crucial factor to provide adequate coronal seal and retention. This in vitro study aimed to assess the potential of dimethyl sulfoxide on the bond strength of fiber post luted to intra radicular dentin.

Methodology

Sample preparation

Twenty single rooted non-carious mandibular premolars without calcification, previous endodontic treatment, resorption or excessive root curvature that were extracted for orthodontic purpose from patients of age group 15-28 years were selected for this study (Fig 1B). The teeth were collected from the department of Oral and maxillofacial surgery. Patients had been informed about the study and informed consent was obtained. Buccolingual and mesiodistal preoperative radiographs were taken to ensure a single root canal. They were stored in 0.1% thymol for 24 hours and were rinsed in running water and air dried. Teeth were decoronated 1 mm above the cemento-enamel junction using a diamond disc mounted on a dental lathe machine at slow speed with constant water irrigation to obtain a uniform root length of 17 mm (Fig 1C). Patency of root canal was achieved with the help of 10 K file and working length was assessed by introducing 15 K stainless steel file until the tip was visible beyond the apical foramen and 1 mm was deducted from this length. Protaper gold instruments were used till F3 along with 5 mL of 3% sodium hypochlorite irrigation in between using a 30G side vented needle on luer lock 5 mL syringe. Obturation was done using gutta percha master cone and AH plus sealer. The coronal portion of the root was sealed with type 2 GIC and stored in distilled water for 1 week.

Preparation of dimethyl sulfoxide solution

50 mL of the dimethyl sulfoxide solution was mixed in 50 mL of distilled water to prepare 50% v/v dimethyl sulfoxide solution.

Treatment of samples

Post space preparation was done using peeso reamer till size # 4 to a depth of 12 mm with 5 mm of remaining gutta percha in the apical portion of all samples. All the posts were noted at a distance of 12 mm from the apical end and were cut to that size by using diamond disks. The surface of posts was cleaned with 70% ethanol for 60 seconds, and rinsed with distilled water and air dried. The remaining post space was acid-etched with 37% phosphoric acid for a duration of 15 seconds. The acid etchant was rinsed and blot dried by using absorbent paper points. The specimens were then randomly divided into two groups with 10 teeth each based on the treatment of samples.

Group A – 50% v/v Dimethyl sulfoxide (DMSO)

Group B – Distilled water (control) In group A, the root dentin was treated with 1 mL of 50% DMSO solution for 1 min and in group B, the root dentin surface was treated with 1 mL of distilled water for the same duration. The excess solutions in post space were blotted with the help of paper points. Etchand-rinse resin cement (Variolink II, Ivoclar Vivadent) was used following the manufacturer’s instructions. For cementation of the fiber posts, equivalent amounts of luting pastes were mixed and then applied onto the post surfaces and into the root canals with the help of a lentulo spiral instrument. The posts were then positioned into the canal to full depth by using gentle finger pressure and the surplus was removed immediately using a disposable brush. Light polymerization was carried out for 1 min by placing the light tip directly contacting the coronal end of the post at 800 mW/cm2 using a light curing unit (Fig 1D). Teeth were sealed coronally using composite resin and all the specimens were stored in sterile saline for 24 hours.

After 24 hours of storage, each root was sectioned perpendicular to the long axis with the help of diamond disk at low speed under continuous irrigation into 1 mm thick slices at the middle third (Fig 1E). Push out test was performed to a compressive load in a universal testing machine, 0.5 mm/min at the middle of the apical surface of the post in apico-coronal direction without contact on the root dentin till the shear stress caused dislodgement of the post (Fig 1F)

Statistical analysis

Sample size was calculated as 10 per group using confidence level of 95% power of the test 80% and using the mean of push out bond strength value from previous study.⁸ Comparison between groups was done using Mann Whitney U test with the help of SPSS software version 19.0 (SPSS Inc, Chicago, IL). A p value of <0.05 was considered as statistically significant.

Results

Statistical data examination showed that there was no significant difference in bond strength scores between the pre-treated and control samples of the respective group. The mean and standard deviation of push out scores between group A and B are given in table 1.

Discussion

Restoration following endodontic treatment of the teeth frequently requires placing post into the root canal which needs to be luted to radicular dentin by adhesive resin in order to provide retention of the coronal restoration. However, dentin is a complex and dynamic tissue which is composed of more organic than inorganic constituents. There are anatomical variations in dentin structure at various levels of the canal such as the differing diameter of dentinal tubules, variation in the number of dentinal tubules in the coronal and apical portion of the root canal, which ultimately affects the quality of adhesive resin bond formed. Major factors which affect the hybrid layer quality includes hydrolytic degradation of the resin and degradation of collagen scaffold in hybrid layer. Hydrolysis and degradation of the resin that occurs mainly due to hydrophilic resin monomers in adhesive which increases water sorption, presence of residual water in acid etched dentin etc. and collagen fibril degradation occurs due to endogenous enzymes in dentin like matrix metalloproteinases (MMP), cysteine cathepsins. Several strategies have been proposed to prevent degradation of hybrid layer such as inhibition of enzymatic activity by using matrix metalloproteinase inhibitors like chlorhexidine, EDTA, quaternary ammonium compounds, by using crosslinking agents like carbodiimides, genipin, polyphenolic compounds like tannins, proanthocyanidins, curcumin etc, strategies to remove residual water within hybrid layer like ethanol wet bonding, self-etch primers, biomimetic remineralization of hybrid layers, calcium chelation by dry bonding etc.³ The present study evaluated the possible potential of dimethyl sulfoxide solution in enhancing the bond strength of fibre post which was luted to intraradicular dentin. Dimethyl sulfoxide has been included as it is found to possess MMP inhibitory activity, improved wettability, better adhesive penetration.

In the current study, there was no significant difference in bond strength scores between the control and dimethyl sulfoxide pre-treated group when it was subjected to push out test. This is in accordance with study done by Shafiei et al in 2016, who assessed whether using 5% dimethyl sulfoxide, for 60 seconds enhanced the bond strength of fiber posts luted to root dentin using two step etch and rinse resin cements. Push out bond strength test was performed immediately or after one year of storage in water. They found that dimethyl sulfoxide did not affect the immediate bond strength of the resin cements; however, aging significantly reduced push out bond strength values of control groups. They concluded that dimethyl sulfoxide wet bonding might be a beneficial method in conserving the resin dentin bond strength stability over time when posts were cemented with the etch and rinse adhesive cements.⁸

However, Stape et al in 2016 assessed the outcome of wet bonding technique on quality of the hybrid layer and bond strength where flat dentin surfaces of fortyeight permanent human third molar teeth were treated with dimethyl sulfoxide and with distilled water before bonding with the help of an etch and rinse or a self-etch adhesive system. Notable improvement in immediate bond strength values were seen on dentin surface treated with etch and rinse adhesive.⁹

According to Guo et al in 2017 who assessed the effect of 50% (v/v) dimethyl sulfoxide wet bonding on composite dentin bond strength and nano leakage, dimethyl sulfoxide pre-treatment significantly prevented the reduction in bond strength as compared to water wet bonding group, was successful in boosting the wettability of the etched surface of dentin, aiding the penetration of the adhesive monomer, and augmenting the stability of the dentin collagen at the adhesive dentin interface by diminishing the amount of collagen exposure along with limiting the collagenolytic activity. They suggested that dimethyl sulfoxide wet bonding could be a possible substitute in improving the dentin bonding longevity.² According to Tjaderhane et al in 2013, the usage of water storage of micro tensile beams showed a positive effect of dimethyl sulfoxide on bond longevity of two step etch and rinse adhesive to the control dentin, while there was no outcome on immediate bond strength.⁵ These studies showed that dimethyl sulfoxide can prevent the decline in bond strength over a time period without much change in immediate bond strength. Beneficial impact of dimethyl sulfoxide was attributed mainly due to enhancement of collagen wetting, improvement of monomeric conversion, turning the adhesive less prone to water mediated degradation, inactivation of matrix metalloproteinases especially MMP-2 and 9, lowers the surface tension and cohesive forces by breaking down self-associative tendencies of water by forming steady complexes with water molecules, dissociates the collagen fibrils into a looser network thus facilitating the infiltration of monomers, minimizing the number of water molecules that are entrapped within the polymeric chain, thus increasing the degree of conversion in the hybrid layer.

Stape et al in 2021 assessed the efficacy of dimethyl sulfoxide pre-treatment on long term bond strength, quality of hybrid layer, degree of conversion of monomers and collagen structure. They found that that dimethyl sulfoxide pre-treatment produced a surge in bond strength values that did not change remarkably over time, uniformity in the conversion of monomer across the hybrid layer amongst all pre-treated groups, also induced collagen stiffening which were reversible in water with lower peptide solubilization.¹⁰

Hebling et al in 2015 assessed the effect of dimethyl sulfoxide cytotoxicity on the repair related activities of cultured odontoblast-like cells. They found that solutions with differing concentrations such as 0.05, 0.1, 0.3, 0.5 and 1.0 mM which were diluted in culture medium and placed in direct contact with odontoblast like cells developed slight or no cytotoxic effects on the pulp tissue repair related activity of the cells.¹¹

Dimethyl sulfoxide has many applications in the medical field such as penetration enhancer, optical clearing agent, as vehicle/carrier for many drugs etc. Because of its superior characteristics, its application and potential commercial usage in dentistry as an agent to improve resin dentin bond integrity, as solvent for many dental adhesives, wetting agent is preferable. Limitations of this in vitro study were that clinical conditions were not simulated and only immediate bond strength was evaluated. Effect on more than one adhesive and multiple post systems, thermomechanical cycling, long term assessment of bond strength was not considered.

More in vivo and in vitro studies, modification of the dimethyl sulfoxide solution which could potentially improve its activity need to be done to evaluate the efficacy of dimethyl sulfoxide pre-treatment on dentin.

Conclusion

Within the constraints of this in vitro study, there was no remarkable difference in the immediate bond strength values of 50% v/v dimethyl sulfoxide pre-treated group when compared to control group. Further studies are required to validate the effect of long-term bond strength efficacy of fiber post luted to dimethyl sulfoxide pretreated dentin.

Conflict of Interest

None.