Evaluation of the effect of home bleaching gel on microleakage of different glass ionomers reinforced with micro-hydroxyapatite.

AIM: This study aimed to evaluate the effect of home bleaching gel on microleakage of glass-ionomer cements reinforced with micro-hydroxyapatite (HAP).
METHODS: Class V cavities prepared on the forty extracted third molars were restored in four groups (n = 10): Group 1, Zirconomer; Group 2, resin-modified glass ionomer (RMGI); Group 3, Zirconomer + micro-HAP (20% WT); and Group 4, RMGI + micro-HAP (20% WT). After thermocycling (1000 cycles at 5 ± 2/55 ± 2°C, a dwell time of 30 s), each group was randomly divided into two groups. The first half was kept in distilled water and the restorations of the second half were bleached with carbamide peroxide 15% (14 days, each time 6 h/day). A uniform thickness of bleaching agent (0.5-1 mm) was applied on the surfaces of the restorations, extending 1 mm beyond the margins, and the bleaching agent was exchanged every 6 h. Microleakage was evaluated using dye penetration technique that is based on the amount of dye penetration (0.5% basic fuchsine solution) from the occlusal/gingival margins up to the axial wall.
RESULTS: In distilled water, no significant difference was found between the occlusal microleakage scores (P > 0.05). The lowest and highest gingival scores in distilled water were observed in Group 4 and Group 3, respectively. In bleaching environment, there was no significant difference between four groups (P > 0.05). Comparing each glass ionomer in two environments revealed statistically significant differences in gingival and occlusal microleakage of Group 4 and in occlusal microleakage of Group 1 (P < 0.05).
CONCLUSIONS: Micro-HAP incorporation did not affect the microleakage of the RMGI and Zirconomer in bleaching environment and their occlusal microleakage in distilled water. The lowest and highest gingival scores in distilled water were observed for the RMGI + micro-HAP and Zirconomer + micro-HAP, respectively. Bleaching procedure negatively affects the microleakage score of RMGI + micro-HAP and occlusal microleakage of Zirconomer.

INTRODUCTION

Several studies have been recently conducted on the effect of tooth bleaching on the properties of teeth and composite restorations.[12] Bleaching agents could adversely affect dental hard tissues and may have an adverse effect on the bond strength to dentin.[34]

Glass-ionomer cements (GICs) are common restorative materials with some unique properties.[5] Compared to GIC, resin-modified glass ionomer (RMGI) provides the highest tensile bond strength for both the enamel and the dentin and represents better esthetic, adhesion, and mechanical properties. It is also more stable in an acidic environment.[678]

A high-strength GIC reinforced with ceramic and zirconia fillers known as Zirconomer has been recently introduced. The results of previous studies indicated that the mechanical properties of hydroxyapatite (HAP)/Zirconomer were better than those of HAP-GICs.[910]

HAP is a bioceramic material which contains calcium and phosphorus, which is the main mineral component of the tooth structure.[11] The apatite formation of HAP can improve the mechanical properties of GICs such as hardness and fracture resistance.[12]

Considering the wide application of RMGI, Zirconomer, and bleaching process in the clinical setup, the present study was conducted to evaluate the effect of home bleaching gel on microleakage of different glass ionomers reinforced with micro-HAP.

METHODS

Forty caries-free human third molars were collected, cleaned with a periodontal curette, and stored in 0.5% chloramine solution at 4°C for 48 h and then stored in distilled water until use. The teeth were mounted in the acryl cylinders from 4 mm under cementoenamel junction (CEJ). Straight fissure burs (Diamond Fissure 330, SS White, USA) were used to prepare standard Class V cavities with gingival margins extending 1 mm under CEJ (5 mm in length, 3 mm in width, and 2 mm in depth) on the buccal and lingual surfaces of each tooth. The teeth were randomly assigned into four equal groups of twenty cavities. To prepare the mixtures of filling materials, RMGI (GC, Tokyo, Japan), Zirconomer (Shofu, Tokyo, Japan), and micro-HAP (Sigma-Aldrich, St. Louis, USA) were weighed carefully using a weighing machine accurate to 0.0001 g (A and D, GR + 360, Tokyo, Japan). Micro-HAP (Sigma-Aldrich, St. Louis, USA) at 20% by weight (8 g powder of RMGI or Zirconomer and 2 g micro-HAP) was added to RMGI and Zirconomer and was mixed in the amalgamator (Ultramat, SDI, Bayswater, Victoria, Australia) using clean amalgam capsules for 20 s to have a uniform mixture.[8] All the cavities were rinsed with water, gently air-dried, and restored in the following groups: Group 1 – the powder and liquid of Zirconomer with the ratio of 3.6/1 by weight were mixed for 30 s, according to the manufacturer's instruction. The cavity was filled with a plastic instrument. A Transparent Mylar Matrix (M-TP, Pulpdent Corporation, USA) was placed on its surface, and the restoration was cured with a light-emitting diode light-curing unit (BlueLex, Monitex, Taiwan) with a light intensity of 1200 Mw/cm2 for 20 s. After 3 min, a layer of varnish was applied on the surface of the restoration and dried. Group 2, RMGI: the cavities were restored with RMGI with the powder-to-liquid ratio of 3.2/1 by weight, and the mixing and restoration were done as described for Group 1. Group 3, Zirconomer + micro-HAP: the cavities were filled with the mixture of Zirconomer + micro-HAP with the powder-to-liquid ratio of 3.6/1 by weight, and the mixing and restoration were done as described for Group 1. Group 4, RMGI + micro-HAP with the powder-to-liquid ratio of 3.2/1 by weight and the mixing and restoration were done as described for Group 1. All the teeth were stored in deionized water for 24 h at room temperature, which was adjusted by the air conditioner (24°C). After finishing and polishing (Shofu Inc, Kyoto, Japan) the surface of all the restorations after 24 h, a thermocycling machine (TC-300, Vafaie Industrial, Tehran, Iran) was used for 1000 cycles at 5 ± 2/55 ± 2°C with a dwell time of 30 s. Ten restorations of each group were kept in distilled water, which was considered as control environment, and the other half were home bleached with carbamide peroxide gel 15% (Opalescence, Ultradent, USA) for 14 days and each time 6 h. At the first, five teeth of each group were bleached buccally for 6 h and then they were bleached lingually (totally ten restorations) for 6 h. During bleaching the restoration on the buccal or the lingual surface, the other side restoration was laid on a moist cotton pad. After 6 h, buccal restorations which had undergone bleaching were rinsed with distilled water. Then, the lingual cavities were bleached. While lingual restorations were under bleaching, the buccal cavities were laid on a moist cotton pad to prevent desiccation. A uniform thickness of bleaching agent (0.5–1 mm) was applied on the surfaces of the restorations, extending 1 mm beyond the margins, and the bleaching agent was exchanged every 6 h.

After 14 days when home bleaching process finished, the surface of all the teeth was covered with two layers of nail varnish, except for 1 mm from the margins of the restoration. The teeth were stored in 2% basic fuchsine solution (Merck, Germany) for 24 h at room temperature (24°C). The superficial dye was washed with tap water and cleaned with the rubber cup and pumice slurry.

To remove acrylic cylinders, the teeth were sectioned horizontally 2 mm below the gingival margins of the restorations to prevent damage to the gingival margins of the restorations. Then, the teeth were sectioned longitudinally in a buccolingual direction from the middle of the restorations using a diamond disk (Diamond disk, Microdont, Brazil) in a nonstop cutting machine (Demco E96, CMP Industries, USA) under a water spray. The sectioned teeth were examined under a stereomicroscope (Estscope BS-3060, Best Scope, China) at ×25 by two blinded examiners to measure the extent of dye penetration at the gingival and occlusal margins using these microleakage scores:[13]0 = no dye penetration; 1 = dye penetration along tooth-restoration margins up to the one-half of the gingival or occlusal wall; 2 = dye penetration extending beyond the one-half of the gingival or occlusal wall but not reaching the axial wall; and 3 = dye penetration extending to the axial wall. Figure 1 illustrates the sectioned teeth which were evaluated at ×25.

Figure 1

Microleakage scoring (×25): (a) 0 = no dye penetration; (b) 1 = dye penetration up the one-half of the occlusal wall; 2 = dye penetration extending beyond 1-s of the distance but not reaching axial wall; and (c) 3 = axial wall dye penetration extending

Statistics

Kruskal–Wallis and Mann–Whitney tests were used to compare the microleakage values between the groups. P <0.05 was considered statistically significant.

RESULTS

The occlusal and gingival microleakage scores and percentage of the experimental groups are summarized in Tables 1 and 2, respectively. Furthermore, means and standard deviations of the occlusal and gingival microleakage are summarized in Tables 3 and 4, respectively. In control environment, there was no significant difference in the occlusal microleakage score between four groups (P = 0.136), and there was no significant difference in microleakage score of Zirconomer and RMGI with or without micro-HAP [Tables 3 and 4]. However, there was a significant difference in gingival score (P = 0.03). Group 4 had the lowest gingival score (mean = 1.2), whereas Group 3 had the highest gingival score (mean = 2.7).

Table 1

Occlusal microleakage score and percent of the experimental groups

The microleakage score	Environment
	Control (%)				Bleached (%)
	Group 1	Group 2	Group 3	Group 4	Group 1	Group 2	Group 3	Group 4
0	1 (10)	2 (20)	3 (30)	3 (30)	0 (0)	1 (10)	0 (0)	0 (0)
1	2 (20)	3 (30)	0 (0)	5 (50)	1 (10)	1 (10)	1 (10)	1 (10)
2	4 (40)	1 (10)	1 (10)	2 (20)	1 (10)	1 (10)	0 (0)	1 (10)
3	3 (30)	4 (40)	6 (60)	0 (0)	8 (80)	7 (70)	9 (90)	8 (80)

Table 2

Gingival microleakage score and percentage of the experimental groups

Microleakage score	Environment
	Control (%)				Bleached (%)
	Group 1	Group 2	Group 3	Group 4	Group 1	Group 2	Group 3	Group 4
0	1 (10)	7 (10)	0 (0)	5 (50)	1 (10)	3 (30)	1 (10)	1 (10)
1	2 (20)	1 (10)	0 (0)	1 (10)	0 (0)	1 (10)	0 (0)	0 (0)
2	0 (0)	0 (0)	2 (20)	1 (10)	1 (10)	2 (20)	0 (0)	1 (10)
3	7 (70)	2 (20)	8 (80)	3 (30)	8 (80)	4 (40)	9 (90)	8 (80)

Table 3

Occlusal microleakage of the experimental groups (means and standard deviations)

Materials	Environment, mean±SD		P
	Control	Bleached
Group 1 (Zirconomer)	1.4±0.99	2.7±0.67	0.036*
Group 2 (RMGI)	1.1±1.05	2.1±1.07	0.183
Group 3 (Zirconomer + micro-HAP)	2±1.41	2.8±0.63	0.111
Group 4 (RMGI + micro-HAP)	0.9±0.73	2.4±0.67	0.000*
Kruskal-Wallis, P	0.136	0.735

*Statistically significant. HAP: Hydroxyapatite, RMGI: Resin-modified glass ionomer, SD: Standard deviations

Table 4

Gingival microleakage of the experimental groups (means and standard deviations)

Materials	Environment, mean±SD		P
	Control	Bleached
Group 1 (Zirconomer)	2.3±1.15	2.6±0.96	0.584
Group 2 (RMGI)	1.3±1.15	2.1±1.33	0.101
Group 3 (Zirconomer + micro-HAP)	2.7±0.42	2.8±0.94	0.626
Group 4 (RMGI + micro-HAP)	1.2±1.09	2.6±0.96	0.023*
Kruskal-Wallis, P	0.03*	0.078

*Statistically significant. HAP: Hydroxyapatite, RMGI: Resin-modified glass ionomer, SD: Standard deviation

In bleaching environment, there was no significant difference between four groups considering occlusal microleakage score and gingival microleakage score (P = 0.078 and P = 0.735, respectively).

Comparing each glass ionomer in two environments (control and bleach) revealed a statistically significant difference in Group 4 in both gingival and occlusal microleakage levels (P = 0.023 and P = 0.000, respectively). There was a significant difference in occlusal microleakage of Group 1 in two environments (P = 0.036).

DISCUSSION

The most employed technique for microleakage evaluation is the dye penetration technique.[1415] As methylene blue has the problem of dissolution in clearing process[16] and silver nitrate particles are smaller than bacterial sizes,[17] basic fuchsine as an economical, easy to manipulate, and popular technique stands out.[13] Hence, we chose basic fuchsine 2% for this study.

The results of the present study revealed that in distilled water, there was no significant difference in occlusal microleakage among the four groups, yet significant difference in microleakage of gingival margins was seen. In all the groups, gingival microleakage values were higher than those of occlusal. This result can be contributed to this common knowledge that because of the higher mineral content of enamel, adhesion to enamel is more effective than dentin.[18] Moreover, due to the lower bond strength of GIC to dentin and cementum, it might be affected by thermocycling more than that of enamel. It can be another reason for higher dye penetration at the gingival margin. Micro-HAP incorporation also did not affect the microleakage of the RMGI and Zirconomer in bleaching environment.

In the current study, no adverse effect on occlusal microleakage score was observed when Zirconomer or RMGI was incorporated with HAP in distilled water or in bleaching environment. Moreover, RMGI with/without HAP exhibited statistically lower gingival microleakage compared with Zirconomer with/without HAP, and Zirconomer incorporated with micro-HAP showed the highest gingival microleakage score in distilled water. We also observed that adding HAP to RMGI resulted in less gingival microleakage in distilled water, which is in accordance with a previous study.[18] After applying GIC on enamel and dentin, a chemical reaction occurs between carboxylic groups of polyacrylic acid and the calcium ions in HAP structure. Adding HAP with its excellent biological activity and crystal structure similar to those of dental apatite to GIC can increase bond strength to dentin and the mechanical properties.[19] Enan andHammad reported that adding nano-HAP to RMGI had positive effects on microleakage around the orthodontic bands.[20] As HAP particles have a role in the interaction between the powder and liquid of GIC, they can release ions participating in the acid–base reaction. The presence of ions in the tooth structure-restorative material interface may lead to the formation of more hydrogen and ionic bonds.[19] The lower microleakage score for RMGI incorporated with micro-HAP in the current study may be related to better bond strength properties, which should be investigated in the future studies. The results of the present study and the mentioned studies are not directly comparable, because, in contrast to the mentioned studies which had incorporated nano-HAP into the GICs, micro-HAP was added into the RMGI and Zirconomer in the present study. We did not assess the effect of micro-HAP incorporation into the GICs on their bond strength properties, and this should be evaluated in the future studies.

Bleaching procedure negatively affects the occlusal and gingival microleakage score of RMGI incorporated with micro-HAP and occlusal microleakage of the Zirconomer. In consistence with the present study, Attin et al.[21] found that GIC's bond strength to dentin and enamel decreased following bleaching with carbamide peroxide. Comparing each group in the control and bleaching environment in the present study, higher microleakage values in the bleaching environment were seen. It may be related to the alterations occurring after the bleaching treatment. Carbamide peroxide breaks down into urea peroxide and hydrogen peroxide. Hydrogen peroxide is in charge of bleaching effects, which may cause HAP dissolution and calcium loss.[22] Moreover, it may result in increased dental structure permeability and dentin organic matrix degradation because of organic components loss during replacement of the hydrocarbon, tertiary amines, and carbon groups by oxygen, calcium, and phosphorus.[2324] It seems that these residual substances may lead to higher microleakage in the bleached group compared to the control group.

Narayana et al.[22] found that regarding the microleakage at enamel and dentin margins, RMGIs performed better when compared to conventional GICs after bleaching cycles. The present study showed less microleakage in groups containing RMGI in comparison with those containing Zirconomer after bleaching. The results may be due to the different qualities of chemical reaction between Zirconomer versus RMGI's carboxylic groups of polyacrylic acid and the calcium ions in HAP structure, which has made RMGI a more stable GIC in acidic environments[5] and has led to less microleakage.

Future in vivo researches using different bleaching agents should be done on a greater population to produce even more exact results.

CONCLUSIONS

Zirconomer incorporated with HAP demonstrated the highest gingival microleakage in distilled water. RMGI incorporated with micro-HAP showed the least gingival microleakage in distilled water. Bleaching procedure showed some adverse effects on the microleakage of glass ionomers used in this study. Therefore, it is suggested to perform GI restoration after completion of bleaching treatment in clinical practice.

Financial support and sponsorship

This study was financially supported by the Shiraz University of Medical Sciences, Shiraz, Iran.

Conflicts of interest

There are no conflicts of interest.