This in vitro study was conducted in the Department of Conservative Dentistry and Endodontics, K. M. Shah Dental College and Hospital, Piparia, Vadodara. Prior permission from the institutional ethics committee was taken before the commencement of the study (Ethical Approval number - SVIEC/ON/Dent/SRP/15045). The study duration was six months from the date of obtaining ethical approval till the completion of the same.

Sample description: The sample sizes of 40 (10 per group) was decided for the present study. The total sample of 40 tooth specimens achieved 80% power to detect differences among the means versus the alternative of equal means using an F test with a 0.05000 significance level. The size of the variation in the means was represented by their standard deviation which was 3.00. The common standard deviation within a group was assumed to be 0.50.

Formula – (Zα+Zβ)²*√(n*delta²/2kS²)

Z alpha and Z beta are valued from normal tables; Delta is mean difference between groups; S is standard deviation and K is number of groups.

Where Z alpha=1.96

Z beta=0.84

Delta= 3.00; Mean Difference

S = Common Standard deviation= 0.50

K= Degree of Freedom = n-1=3

n= Total Number of Groups=4

Methodology Proper: Forty recently extracted intact human molars indicated for extraction were collected and cleaned using ultrasonic scaler and disinfected by immersing them in 0.5% chloramine T solution for 15 days. Thereafter the teeth were stored in distilled water until use. Class II cavities (3mm wide, 6mm high and 2mm deep) were prepared on the proximal surfaces with the help of straight fissure bur with gingival margin located 1mm below cemento-enamel junction. Dimensions of class II cavities were calibrated using digital Vernier’s calliper. The prepared teeth were mounted between two dummy teeth using silicone impression putty to reproduce proximal contact. Pre-contoured transparent matrix bands and light transmitting wedges were used to establish proper proximal contact and contour of the restoration. Teeth were then randomly divided into four groups (n=10) and restored as per group protocol [Table/Fig-1].

Restored teeth were finished, polished and stored in distilled water at 37°C for seven days. The teeth were then subjected to thermo cycling for 500 cycles (5°C & 55°C, dwell time 15 seconds). Each tooth was covered with two coats of nail varnish except for the area 1mm away from gingival margin of the restoration. Thereafter, all the samples were immersed in 0.5% methylene blue dye solution for 24 hours. The samples were then cleaned and sectioned in mesio-distal direction using diamond disc. Both sections were evaluated at the gingival margin and to stay on the conservative side only section with worst score (greater amount of microleakage) was recorded. The evaluation was done using stereomicroscope (Motic, Causeway Bay, Hong kong) at 40X magnification [Table/Fig-2].

[Table/Fig-2]:

Stereomicroscopic images of representative specimen from each group at 40X magnification. A: Group A; B: Group B; C: Group C; D: Group D.

The severity of dye penetration was evaluated using the scoring criteria suggested by Loguercio et al., [14] as follow: Score 0- No dye penetration; Score 1- Dye penetration up to one half of the gingival floor; Score 2-Dye penetration up to more than one half of the gingival floor; and Score 3- Dye penetration up to the axial wall [Table/Fig-3]. Obtained scores were statistically analysed using one way ANOVA test and Post Hoc test.

One way ANOVA test was used to compare the means of the group (p=0.817, df - 9). [Table/Fig-4].

[Table/Fig-4]:

One way analysis of variance.

Post–Hoc Tukey test was used to find interrelationship between different groups. [Table/Fig-5].

Amongst all, the group B (Resin Modified Glass Ionomer Cement) exhibited least microleakage and group A (Resin Modified Glass Ionomer Cement with ultrasonic activation) demonstrated maximum microleakage with statistically insignificant difference between all the groups. Group C (Giomer) and group D (Giomer with ultrasonic activation) performed better than group A, but were statistically insignificant. Group B performed better than group C and group D which was again statistically insignificant. Group D showed better results as compared to group C but was statistically insignificant.

In the present research, group B (RMGIC with ultrasonic activation) depicted the best results, RMGIC owing to its adhesive properties like GIC and good wear resistance like composite along with ultrasonic activation may be the reason for the better performance and hence was selected as control group for the study. In Group B (RMGIC with ultrasonic) and Group D (Giomer with ultrasonic) ultrasonic activation, accelerated curing process increased powder surface area due to breaking down of glass particles by ultrasonic energy causes increased reactivity and improves physical properties. Previous experiments by Gorseta K, et al., reveals that ultrasonic energy has been successfully used for accelerating the curing process with GIC which was in accordance to the results of this study which reveals that the ultrasonic groups revealed less microleakage [13]. Group C (Giomer) performed better than group A (RMGIC) – low modulus of elasticity and increased flexibility of flowable resin may be helpful in distribution of stresses of polymerisation shrinkage and preserve integrity of bond to tooth structure. Results of this study correlate to the previous research by Payne et al., which demonstrated better marginal sealing ability with flowable composite resin [14]. In giomer glass particles are already pre-reacted and incorporated in the restorative, whereas in RMGIC glass particles are not pre-reacted and hence when ultrasonically activated glass particles are broken down increasing the surface area, providing significant improvement as compared to inactivated group. 0.5% Chloramine T was used because it has no adverse effect on organic phase (collagen) of dentin [15].

Class II cavities were selected for the study because branching of tubules is numerous in the root dentin area and acid etching of this heterogeneous dentin surface is quite difficult. In cases of deep cavities with cervical margin in cementum, bonding itself to cementum is poor; also the formation of hybrid layer is difficult in this critical zone [16]. Beautifil Flow Plus (Giomer) was used as a base because it combines delivery of flowable and strength of hybrid composites, Beautifil II (Giomer) was used for coronal restoration owing to its unique properties like flouride recharge/release, wear resistance, chameleon effect making it an ideal material for coronal restoration.

Thermo cycling (5°c and 55 °c, dwell time – 15 seconds) was done to mimic the temperature extremes found in oral cavity. Methylene blue dye was used as it exhibits better penetration results [17]. Molecular weight of methylene blue dye is lower than bacterial toxins (1.2 mm²) thus exhibiting better results as compared to eosin or other radioisotope tracers [18]. Dye penetration method was used to check microleakage because it demonstrate sections showing leakage in contrasting colours without the need for further chemical reaction or exposure to potentially hazardous chemicals. It has good sensitivity, convenience and ease of use [19].

Stereomicroscope was used for evaluation of prepared samples as it provides well magnified two dimensional view showing areas of microleakage.

Clinical implications: Ultrasonic activation of various recently introduced hybrid ionomers like giomers may provide better adaptation of lining material while restoring subgingival class II defects, thus resulting in long term restorative treatment success.

The present laboratory research needs to be further validated by clinical researches before deriving the final conclusion. The present experiment incorporated 500 cycles of thermocycling to mimic the clinical scenario. However, 10,000 cycles of thermo cycling corresponds to one year of clinical performance whereas 500 cycles used in the present research approximately will correspond to 15 days of clinical performance, which again is a very small period to simulate the clinical situation [20]. From research perspective, it would be desirable to develop an accelerated aging model to challenge the durability of resin dentin bonds in a relatively short period of time. Some approaches that should be taken into consideration include masticatory stresses, pH cycling, temperature cycling, wet environment and various aging conditions [21]. This study partially incorporated the above mentioned parameters, however; strict adherence to aforesaid would have better mimicked the clinical environment.

Ultrasonic activation of the lining material seems to be good alternative to reduce microleakage. Also, recently introduced hybrid ionomers with good anti-cariogenic effect alike giomer may prove to be an effective alternative for the long term success of the highly technique sensitive class II composite restorations.