Comparative Evaluation of Microhardness of Two Conventional and Hydrophilic Fissure Sealants: An in vitro Study

Dental caries is highly prevalent and is considered one of the major global concerns affecting both younger and older populations.¹ It is caused mainly by to increased sugar-rich diet, poor oral hygiene, or insufficient dental plaque removal.² If left untreated, it can lead to harmful consequences.^3–5 Hence, it is necessary to treat and restore them at the earliest. Micro-invasive techniques of caries management prevent the progression of decay.^6,7 One of the various micro-invasive methods is sealing the lesion with resin penetration into enamel.⁸

Fissure sealant helps to arrest incipient caries in pit and fissures by treating the enamel surfaces with orthophosphoric acid followed by placement of sealant material.^9,10 It acts as a physical barrier against caries-forming bacteria and dietary carbohydrates. Sealants can be either glass ionomer-based or composite-based.⁸ Recent technology has introduced the formulation of different types of resin materials with the incorporation of biocompatible fluoride particles.¹¹

The clinical success of these sealants also depends on the oral environment, the chemical composition, and physical and mechanical properties. Sealants also differ in the filler content and size, viscosity and even composition which can affect the properties of different pit and fissure sealants.¹² The major concern with pit and fissure sealants is moisture contamination.

Recent advances include the introduction of hydrophilic sealants^13,14 in the market for better ease of usage and to overcome its technique sensitivity while increasing the wear resistance and anticaries behavior by adding fluoride.¹⁵

Micro-hardness is the ability of the material resistant to distortion wherein a recommended load is given to an indenter in proximity to the sample and the square or rhomboidal impression formed is assessed under a microscope or optical imaging.¹⁶ Among the important properties of resin-based sealants, microhardness also plays a vital role with respect to the longevity of the sealant. As there is limited scientific literature related to the microhardness of hydrophilic sealants, this present in vitro study was planned to compare and estimate the microhardness of two conventional and hydrophilic sealants.

Thirty extracted sound molar teeth were used. These teeth were carious-free and were obtained from the Department of Oral Biology, Saveetha Dental College. Teeth with carious lesions and other defects were exempted.

The Clinpro conventional and Ultra seal XT Hydro sealants were allocated to 2 different groups. Group I was 3M ESPE Clinpro which is a resin-based fluoride-releasing sealant and Group II was Ultra-seal XT Hydro which is also a resin-based, fluoride-releasing buthydrophilic sealant. Each of these two groups was further divided into intermediate and aging groups.

Mesiodistal sectioning of the tooth was done and the tooth was sectioned into two halves using a low-speed diamond cutting blade. One-half of the sectioned teeth were subjected to an immediate subgroup and the other part was used for aging. On the buccal surface of the tooth specimen, a slot was made and these slots were subjected to etching and sealant placement. The sealants were applied according to the manufacturer’s instructions. The first half was tested for immediate microhardness evaluation followed by the other half of the tooth which was tested after aging (Figures 1–3).

Figure 1:
Shows the etched samples on prepared rectangular slots of microtome teeth.

Figure 2:
Shows the 3M™ Clinpro™ Sealant application on the prepared rectangular slots of microtome teeth.

Figure 3:
Shows the Ultra-seal XT Hydro sealant application on the prepared rectangular slots of microtome teeth.

Aging subgroup samples were subjected to thermocycling, where the samples were placed in a water bath at a temperature of 5 to 55°C for 15 sec at each degree with a dwell duration of 10 sec for 1500 cycles.

The Microhardness of immediate and aging subgroups for both groups were evaluated using the Vickers hardness instrument. The pyramidal diamond indenter (Figure 4) present in the Vickers hardness tester was used to determine the microhardness of the sealant surface at a 200 g load for 20 sec. All the samples were analyzed and for each point, the Vickers hardness number (VHN) (kg/mm²) was recorded (refer to Figures 5 and 6).

Figure 4:
Shows the Vickers hardness test on the surface of the sample.

Figure 5:
Indenter impression on the tooth surface after treatment with 3M ESPE Clinpro.

Figure 6:
Indenter impression on the tooth surface after treatment with Ultra-seal XT Hydro.

The data was analyzed using SPSS: Statistical Package for Social Sciences(version 23; SPSS Inc., Chicago, Illinois, USA). A p-value of less than 0.05 was considered statistically significant.

Shapiro Wilks test was used to determine the normality of the data, and a non-normal distribution was observed. Hence non-parametric tests were employed. Descriptive statistics were expressed in the form of mean and standard deviation. Mann Whitney U test was used for the comparison of mean microhardness values of Immediate and ageing between Group I and II, and the Wilcoxon signed-rank test was used to compare the mean microhardness values of Immediate and ageing within groups.

Table 1 represents the descriptive data for Immediate and Aging for Groups I and II. The immediate mean microhardness value for Group II (21.06±3.92) was a little higher than Group I (20.08±2.69) and the difference was found to be statistically not significant. A statistically insignificant difference was also observed between the mean microhardness aging values of Group I (28.01±4.02) and Group II (30.04±5.31) using the Man-Whitney U test, but the mean microhardness value for Group II was comparatively higher than the Group I (Table 2). Wilcoxon signed rank test revealed a significant difference within each group for Immediate and Aging time factors (Table 3).

Two different parameters were used for microhardness evaluation. They are the Knoop hardness number and the Vickers hardness number. Vickers hardness tests were used in numerous cases to determine the material hardness within the microhardness test load range. Vickers hardness test generates a square shape residual indentation which is later measured under a microscope.¹⁷

In our study, the microhardness for the sealant Ultra-seal XT Hydro immediately had a mean of 21.06 and after ageing the mean was 30.04 and the difference was statistically significant. Group II ultra seal hydrophilic sealant had a comparatively higher mean aging microhardness value than Group I clinpro conventional sealants. Similar results were observed for hydrophilic sealants in a study done by Sulimany AM et al.,¹⁸ where the embrace wet bond group had the highest mean microhardness value compared to other groups.

The indirect measure of micro-hardness will be the Degree of Conversion (DC). The degree of conversion represents the magnitude of a polymer assembled by the monomer conversion after irradiation.^19,20 Any low value of degree of conversion in the sealant placed upper and lower layers on occlusal tooth surface may result in poor marginal integrity, which in turn increases dissolution, thereby permitting the cariogenic bacteria proliferation and finally resulting in clinical retention failure of the pit and fissure sealant.²¹

The present study results were found to be more compared to Mazaheri et al. study results, where they reported a mean micro-hardness of 15.96±4.27 for Fissurit F fluoride-releasing sealant.²² The reason for this lower value compared to the present study results could be attributed to differences in study procedure and assessment tools used for evaluating the outcome parameter.

In a study conducted by Diener et al.,²³ Embrace WetBond hydrophilic sealant reported a substantial difference between the means provided by the existing literature and the mean value mentioned in this study, where the result mean was much lower. The difference could be due to the immediate testing of the sample.

In a clinical trial comparing the effectiveness of hydrophilic Embrace WetBond and conventional Helioseal F sealant, two sealants exhibited no significant difference with respect to clinical retention, caries formation and marginal adaptation of sealant.²⁴ The limitations which were observed in the present study were the application of the sealant material on the smooth buccal surface. But in the clinical condition, pit and fissure sealants are placed in various morphological types of pit and fissures with different depths. Another drawback could be in vitro study analysis, which does not provide the same environment as a clinical condition. Hence clinical trials need to be conducted to determine the effectiveness of these sealants.

Based on the findings of the present study, Hydrophilic pit and fissure sealants (Group II) had higher micro-hardness mean values compared to conventional sealants (Group I).