Caries protective effect of two Self-etch dentin bonding agents on denuded root dentin surface: In Vitro Study

Introduction

Advances in medicine and dentistry have led to significant improvement of people’s general and oral health. Studies have shown that people are not only living longer, but are retaining more of their natural permanent teeth.^[1] The retention of teeth into their older age has concomitantly resulted in increased prevalence of gingival recession, thereby exposing root surfaces. This could results esthetic impairment, fear of tooth loss, increase the chances of developing root caries along with dentinal hypersensitivity.

In the cases of denuded roots due to gingival recession, normally the cementum is lost soon as it has very low abrasive resistance leading. This leads to hypersensitivity due to open dentinal tubles and biofilm formation contributing in the development of root surface caries.²

Root caries is defined as soft, progressive lesion that is found at or gingival to the cement-enamel junction on the root surface that has lost its connective tissue attachment and is exposed to the oral environment.¹ The rate at which the demineralization of root surface occurs, is much faster than that of enamel since the cementum has less mineral content than enamel.

Several methods and materials have been put forward to prevent root caries initiation and to promote remineralization prior to contemplating invasive treatments. Some of the preventive methods are daily use of fluoridated tooth pastes and mouth rinses, professional application of fluoridated gel, ozone therapy and antimicrobial varnishes.³ However, simpler techniques to protect the exposed root surface from long term caries attack is needed. In this pursuit of a simple preventive treatment option against root caries, coating the root surface with a dental adhesive agent has been studied. These dental adhesives provide a thin uniform film coating which creates a barrier on the exposed root surface against root caries formation.

Recently self-etching dental adhesives with and without a solvent have gained popularity because of a myriad of advantages; simplified bonding procedure that require only one application step thereby reducing chairside time and technique sensitivity with reliable bonding to dentin.⁴

Thus the aim and objective of this study was to evaluate the efficiency of two newly introduced self-etching dental adhesive agents in preventing the progression of demineralization after its application on the root dentin.

Materials and Methods

A total of 30 extracted caries free human premolars were selected, cleaned of all the debris from the root surface using a ultrasonic scaler and stored in 0.9% sodium chloride solution until they were utilized for the study. The selected samples were then decornated at the level of cement-enamel junction. The apical foramen and the coronal aperture were then sealed with sticky wax. The root surface 1 mm apical to the CEJ was abraded with sof lex disc to denuded it of cementum (to simulate clinical situation). The whole root surface was then coated in two layers of nail varnish exposing two rectangular windows (one on buccal surface and one of lingual surface) each measuring 4mm x 4mm. The buccal window served as the experimental window and the window on the lingual aspect served as the un-treated control group.

Artificial root lesions were produced by immersing the samples in demineralizing solution of acidified hydroxyethylcellulose (HEC) with a pH of4.5 to 4.8 at 37◦C for 6 days. The sapecimens were then washed thoroughly to clear all the acidifying gel and blotted onn sterile paper to dry them. They were randomly divided into two experimental groups; Group A(Optibond all in one), Group B (Bond-1 SF) each having 15 samples. Application of bonding agent was done following manufactures instructions. Table 1.

Group C, out of the 30 specimens in which the lingual windows were left un-treated, 15 of them were selected as the control group.

Specimen were then placed in separate glass vials (for each group) containing 10ml solution of saline for 14 days. Each sample was sliced in planoparallel sections perpendicular to the experimental surface using a diamond discs. All the slices were then ground with sand paper until a thickness of 80 µm (±20 µm) was obtainee. The sliced tooth sections were washed thoroughly with water and were observed under polarized microscope. The depth of lesion and penetration of the material was measured quantitatively using zen software. The values obtained were statistical analysed using one-way ANOVA followed by Tukey’s Post hoc Analysis

Results

The lesion depth results are show in Table 1. Comparison of lesion depth among the 3 groups were statistically significant. The depth of lesion of control group showed maximum with a mean depth of 80.08 µm. Group A (Optibond all in one) had the lowest mean depth lesion of 50.18 µm. Group B (Bond-1SF) showed a mean depth of lesion of 60.10 µm.

One way ANOVA test depicted in Table 1 revealed that the lesion depths among the 2 experimental groups proved to be statistically significantat p value < 0.001. The depth of lesion in solvent-free adhesive group was significantly deeper when compared to solvent containing adhesive group.

Tukey’s Post hoc Analysis shown in Table 2 revealed that the maximum difference was found between Group A and Group C (29.90µm) and minimum difference was seen between Group A and Group B (12.82 µm) showing statistical significant results at p <0.001. Comparison among Group B and Group C also showed high statistical significance.

The lesion depth was greater in the control group (Group C) followed by Group B(Bond-1 SF) and the least in Group A (Optibond all in one) as depicted in Graph 1.

Discussion

In the present study, cementum was removed prior to starting the experiment to mimic the risk factors leading to root caries. Studies by Melberg and Sanchez, 1995) have stated that the low abrasive resistance of cementum leads to its loss, leaving the exposed dentinal structure susceptible to root caries.⁵

Root caries is a complex decay process initiated at or apical to the CEJ typically following gingival recession due to physiological reasons, periodontal disease or trauma.⁴ It is of particular concern in the geriatric population, afflicted with gingival recession and poor oral hygiene maintenance, affecting multiple teeth at a time. It poses a serious riskdue to weakened tooth structure which is likely to increase sensitivity, cause pain and in extreme situations, cause fracture of the whole tooth. Due to the high incidence of root caries in the middle aged and elderly population, it is important to identify the causative agents for root caries and implement disease prevention and management strategies

. The risk factors for root caries among such individuals include reduced salivary flow, changes in cleaning effectiveness and various systemic conditions.⁶ Among the various non-invasive treatment modalities available, sealing the exposed root dentin with an adhesive material is expected to be promising approach for preventing root caries.⁴ Studies have shown that these adhesivesseal the exposed dentinal tubules by formation of an acid resistant hybrid layer, thereby providing a durable and long-lasting protection against demineralization of exposed root dentin.^1,4,7,8,9

Newer simplified adhesive systems are available, that combine etching, priming and bonding into one bottle, (seventh generation) which is then directly applied on the root surface, saving steps and clinical time involved.^4,10

These adhesives are most promising as the hydroxyapatite crystals are retained in the interactionzone. The calcium is available for interaction with specific adhesive monomers forming a stable bond.⁴ “Optibond all in one” is one such adhesive which contains GPDM monomer (glycerol phosphate dimethacrylate) having low viscosity and improved water solubility. It also contains a solvent such as acetone which is hydrophobic and helps in removing excess water.¹¹ The solvent can also carry resin monomer into the tubular dentin. The residual solvent must be eliminated by air drying after application asits presence might hinder the resin–dentine interface by lowering the degree of monomer conversion and cross linking whilst causing a porous layer which is detrimental to adhesive performance. Solvent evaporation leads to increased viscosity, restricted mobility of monomers but excess solvent would lead to dilution of reactants.¹⁰

To counter this problem, some manufactures have formulated a novel single step adhesive system which is free of water and solvents, referred to as solvent-free adhesives.“Bond-1SF”contains HEMA(Hydroxyethyl Methacrylate), a low molecular weight monomer as a solvent and diluents in place of water thereby reducing phase separation and droplet entrapment, along with 4-MET monomer, which forms ionic bond with hydroxyapatite.^7,8

Studies indicate that the concentration of individual monomers, the pH and their bonding ability determine the extent of infiltration, ionization, and cross-linking obtained during polymerization andare responsible for the overall efficacy of the adhesive system.^8,9

The results of this study showed reduced demineralization depthscompared tothe experimental groups. Minimal lesion depth wasobserved in Group A (Optibond all in one) followed by Group B (Bond-1 SF). This could be attributed to the presence of the monomer, which can penetrate the smear layer forming covalent bonds with dentin collagen and ionic bonds with hydroxyapatite, hence preventing demineralization. The results observed agree with those in other studies conducted using different caries models and different self-etch adhesive agents, which also showed reduction in lesion depth after their application.^4,7,8

In this study, self-etch adhesive with solvent (Optibond) showed lesser lesion depth when compared to solvent free self-etch adhesive. Solvents modify pre-polymer viscosities, helping adhesive spreading and wetting of the substrate.The introduction of acetone in Optibond all in one seems to help residual solvent and excess water removal following air-drying. These solvents serve to introduce monomers into the collagen network which may also be responsible for long resin tag formation in the dentinal tubulesthereby aiding formation of a uniform hybrid layer.

On the contrary, the lack of solvent and higher pH of Bond-1 SF (3-4) when compared to Optibond all in one (2.5) may be the reasonsfor decreased depth of penetration and infiltration capacity of this bonding agent over the latter.

According to studies of Farinaz Shirban et al, solvent free adhesives serve as semi permeable membranes allowing passage of fluid through the adhesive dentin interface. The authors also assumed that the presence of higher concentration of HEMA in Bond-1SF resulted in lower polymerization conversion rate due to increased osmosis and formation of large number of droplets.¹ This could be one of the reasons for Bond-1 SF to show greater lesion depth values than Optibond all in one.

SEM studies by Eugenia Kolinotou-Koumpia et al have shown a partially formed hybrid layer with gaps between the material and the dentin substrate in the solvent free adhesive(Bond-1 SF) system, whereas SEM images of the dentinadhesive system in Optibond all in one bonding agent showed a uniform hybrid layer with prominent resin tag formation.⁹

However, the long-term stability of adhesive restorations depends not merely on the thickness, but also on the quality of the hybrid layer.

Further studies are needed to explore the efficacy of dentin bonding agents for caries prevention in clinical situation Additional studies are required to test the long term effects of these agents when subjected to mechanical, chemical, and physical challenges of the oral environment.

Conclusion

Within the limitation of this in vitro study, it can be concluded that self-etch dentin bonding agents with and without a solvent, arean effective method to prevent demineralization of root dentin for a longer duration. The effectiveness depends on the composition of dentin bonding agent used. In the present study, Optibond all in one selfetch adhesive with solvent showed better results owing to better penetration depths resulting from prominent, durable resin tags and the formation of a uniform hybrid layer.