Comparative Evaluation of Microleakage of Zinc Phosphate Cement, Resin-Modified Glass Ionomer, and Two Dual-Cure Resin Cements: In Vitro Study.

Aims: The aim of this study is to evaluate and compare the sealing ability of a new resin cement which was undergoing trial with other three luting cements. Settings and Design: In vitro study. Subjects and
Methods: Forty extracted intact human premolars were selected and standardized class 5 preparations for inlays were done at cementoenamel junction of buccal surfaces and direct composite inlay was fabricated. The samples were randomly grouped into 4: Group I luted with Rely X ARC resin cement, Group II with DCRC-10, a new indigenously developed resin cement, Group III with FujiCEM, and Group IV with zinc phosphate. After 24 h storage, samples were thermocycled and immersed in basic fuchsine dye. The longitudinal sections were assessed under a stereomicroscope for microleakage using graded criteria. Statistical Analysis Used: To analyze leakage scores among four groups and between enamel and dentin/cementum margins, Kruskal-Wallis nonparametric test and the Mann-Whitney test at P < 0.05 were used.
Results: The dye penetration at cementum/dentin margins showed a statistically significant difference among four groups (P < 0.001). Adhesive luting cements produced lesser leakage scores at both margins than those cemented with zinc phosphate. Conclusions: The indigenously developed resin cement (DCRC-10, Group II) is comparable to that of Group I (RelyX ARC) in terms of its luting efficiency. Copyright:

INTRODUCTION

Luting agents constitute a broad spectrum of materials used to bond and seal restorations to tooth.[1] This study was conducted to evaluate the sealing ability of a new cement which may provide a better choice in quality and cost. Identifying luting materials that seal the cavity most effectively reduces the chances of retreatment. Hence, this in vitro study aimed to evaluate and compare the marginal adaptation of four different luting materials in terms of microleakage.

Traditionally, the most popular was zinc phosphate cement.[2] The introduction of resin-modified cements and resin cements was a breakthrough in the field of adhesive dentistry.[34] No studies have been reported in literature comparing zinc phosphate, resin-modified glass ionomer, a commercial dual-cure resin system, and the newly developed dual-cure system.

SUBJECTS AND METHODS

In this in vitro study, forty human intact upper first premolars extracted as a part of orthodontic treatment from patients aged 14–26 years were obtained. The study was conducted at the Department of Conservative Dentistry, Raja's dental college, Vadakangulam, Tamil Nadu. The collected samples were stored in 0.5% chloramine solvent and were used for the study within 2 weeks of storage. Direct visual comparison and the measurements verified that the teeth were similarly structured. The teeth with detectable defects upon transillumination were discarded. Teeth with untreated or treated caries, developmental defects, and trauma were also excluded from the study. Materials used were physiological saline, chloramine t-0.5%, glycerine, 37% phosphoric acid etchant gel, zinc phosphate (Harvard, Germany), FujiCEM-Fuji (GC Corp/Asia), Adper single bond (3M ESPE), RelyX – arc adhesive resin cement (3M ESPE), Chitra bond (SCTIMST*, Trivandrum, India), DCRC-10 – a newly developed resin cement (SCTIMST*, Trivandrum, India), dye 0.5% basic fuchsin, distilled water – 100 ml, and nail varnish [Figure 1].

Figure 1

Armamentarium and materials used

The study was conducted systematically based on the following steps:

Step 1: Tooth preparation

Class V box preparations of dimensions 1.5 mm deep, 2 mm wide, and 3 mm long were done on the buccal surface at the cementoenamel junction using straight fissure diamond (No: 1091) flat end tapered fissure abrasive (No: 4072) and a high-speed handpiece. A metal template was used to standardize the outline form and all preparations were done by the same operator. The cavity preparations were made with butt joint cavosurfaces and were checked using a magnifying lens [Figure 2].

Figure 2

Prepared samples

Step 2: Fabrication of composite inlays

For each preparation, a direct composite inlay was fabricated using light-curing hybrid composite (Solitaire, Heraeus Kulzer) [Figure 3]. Glycerin was used as the separating medium. The composite placed within the cavity was photopolymerized for 30 s (QHL 75, Dentsply). The inlay was further polymerized for 30 s after removal from the preparation. No treatment was done on the intracoronal surface of the composite inlay. All the inlays were tried-in for the fit and marginal adaption. Defective inlays were discarded. The 40 samples were then randomly assigned into four groups [Table 1].

Figure 3

Direct composite resin inlays

Table 1

Grouping of samples

Groups	Cement used	Details of material
Group I (n=10)	RelyX ARC (3M, ESPE, America)	A dual-cure adhesive resin cement system
Group II (n=10)	DCRC-10 (SCTIMST, Thiruvananthapuram, India)	Newly developed, dual-cure resin cement system
Group III (n=10)	FujiCEM (Fuji GC Corp. Asia)	Resin-modified glass ionomer luting cement
Group IV (n=10)	Zinc phosphate (Harvard, Germany)	One of the oldest luting cement

ARC: Adhesive resin cemen, DCRC: Dual cure resin cement, SCTIMST: Sree chitra Tirunal Institute for Medical Sciences and Technology

Step 3: Inlay cementation

Group I

Surface of specimens was etched with 37% phosphoric acid gel (15 s), then rinsed (10 s) and excess water was blotted to keep the tooth moist. After applying Adper single bond, the surface was dried (5 s), later light cured (10 s). Then appropriate amount of RelyX ARC mixed for 10 s, was applied to the bonding surface of the inlay and onto the prepared tooth surface. Excess cement was removed 3–5 min after seating, followed by light curing the margins (40 s).

Group II

Surface of specimens was etched with 37% phosphoric acid gel (15 s), followed by rinsing (10 s) and excess water was blotted leaving tooth moist. Bonding agent (Chitra bond) applied with applicator, dried for 5 s, then light cured for 10 s. Indigenously developed dual-cure resin cement (DCRC-10) was developed at Dental Products Laboratory, SCTIMST, Thiruvananthapuram. It is also a low viscosity two paste system. The paste A contains the initiator benzoyl peroxide and the paste B contains the inhibitor BHT. This cement mixed for 10 s was uniformly applied onto the preparation and the inlay was fixed gently with finger pressure. The margins were light cured (40 s). Dual-cure resin cement undergoes only a portion of their polymerization from chemical curing, and therefore, adequate light exposure is required for complete polymerization.

Group III

Specimens were washed and dried gently. The cartridge was placed into the dispenser (Paste Pak). The two pastes of the resin-modified glass ionomer were dispensed by squeezing the lever of Paste Pak Dispenser on a mixing pad. It was then mixed thoroughly for 10 s and coated the internal surfaces of the preparations. The inlays were seated gently with finger pressure. Excess cement was removed 1 min after seating the inlays.

Group IV

Zinc phosphate cement was mixed as per the manufacture's recommendations and was uniformly painted onto the prepared cavity walls thoroughly. The inlays were seated with finger pressure and cement was allowed to set undisturbed until the final set.

All the inlays were placed manually into the preparation by finger pressure. Though the technique is not uniform or reproducible, it represents the clinical situation. All specimens were stored in distilled water for 24 h (PH 5.63) at 37°C. Then, the inlays were finished and polished using Sof-Lex discs (3M, ESPE). Thermocycling was done with two constant temperature baths (Julabo, India). One was maintained at 5°C ± 2°C. The other at 55°C ± 2°C. The dwell time selected was 30 s in each bath and a transfer time was 7 s.

All 40 samples were manually immersed in the bath at 5°C ± 2°C first and then at 55°C ± 2°C for 300 cycles. When the samples returned to room temperature, they were stored for 24 h in saline bath at 37°C.

Step 4: Immersion in dye solution

Sticky wax was applied at the apices of the samples to avoid dye penetration into the root canals. Each tooth surface was entirely coated with two layers of nail varnish except for 1 mm width around the gingival and enamel margins. The dye solution was prepared by mixing 0.5 g of basic fuchsin powder with 100 ml of distilled water to get 0.5% dye solution.[5] The samples were immersed in the dye solution for 24 h. As an additional precaution to avoid dye penetration into the root canal, samples were kept vertically, and the apical third of them were above the level of dye solution [Figure 4].

Figure 4

Dye penetration

Step 5: Sectioning of samples

All specimens were longitudinally sectioned through the center of the restoration buccopalatally using a double-sided diamond disc in a slow-speed, straight handpiece. Water was used as the coolant throughout the procedure to prevent excess heat generation. From two specimens obtained from sectioning of each specimen, one with more dye penetration was selected for scoring.

Step 6: Scoring

A binocular stereomicroscope was used for scoring the microleakage. A graded criterion was used for scoring the linear penetration of dye through the restoration–tooth interface. Specimens were viewed under ×10, ×15, and ×50 magnification. Scoring was done as follows. Photographs were taken to record the scoring [Figures 5-9].

Figure 5

Stereomicroscopic picture showing Group 1 samples

Figure 9

Graded criteria used for scoring leakageE = Enamel R = Composite InlayD = Dentin L = Luting cement C = Cementum P = Pulp

Stereomicroscopic picture showing Group 2 samples

Stereomicroscopic picture showing Group 3 samples

Stereomicroscopic picture showing Group 4 samples

Score 0 = No leakage

Score 1 = Half of external wall was penetrated by dye

Score 2 = Dye penetrated through the entire external wall, but not up to the axial wall

Score 3 = Dye penetrated up the axial wall.

Statistical analysis

The data were entered into Microsoft Excel Sheet and the analysis was done using Statistical Sofware IBM SPSS version 21.0 (IBM corporation, Chicago, USA). Mann–Whitney U test and Kruskal–Wallis test were used. The P value was considered significant when <0.05.

RESULTS

This study assessed the marginal adaptation of four luting cements – zinc phosphate, resin-modified glass ionomer, a commercial dual-cure resin system, and the newly developed dual cure system at enamel and cementum margins.

The lowest microleakage score (both at enamel and dentin/cementum margins) was observed in the adhesive resin cement groups (Group I and II) followed by resin-modified glass ionomer (Group III FujiCEM). Only scores 0 and 1 were obtained in Groups I and II. Score 3 was obtained in 70% of samples at the cemental margin when zinc phosphate was used (Group IV).

The mean microleakage scores among the four groups at the enamel (P = 0.001) and cementum margins (P < 0.001) showed a statistically significant difference.

When individual cements were compared for their microleakage at both enamel and cementum margins, only zinc phosphate cement showed a statistically significant difference at both sites (P = 0.02). The newly developed cement when compared with zinc phosphate cement showed a statistically significant difference at enamel margins (P = 0.01) [Figure 10] as well as cementum margins (P < 0.001) [Table 2 and Figure 11].

Figure 10

Microleakage scores of the four luting cements at the enamel margin

Table 2

Comparison of microleakage scores of Group II with others

Groups	Enamel margin (P)	Cementum margin (P)
Group II and Group I	0.45	0.45
Group II and Group III	0.13	0.07
Group II and Group IV	0.01	<0.001

Figure 11

Microleakage scores of the four luting cements at the cementum margin

DISCUSSION

Dental luting agents form the bond between an indirect restoration and the prepared tooth structure. Microleakage is defined as the passage of bacteria, fluids, ions or molecules between a cavity wall and the restorative material. The premature loss or failure of restorative materials is often associated with break down at the interface between the restoration and the tooth structure.[67]

The clinical microleakage index is better in assessing effectiveness of modern adhesive systems. Most restorations show leakage shortly after their insertion.[89]

Camps and others studied 317 Class V restorations and concluded that although cytotoxicity of the restorative materials and the release of inflammatory mediators during luting procedure contributed to an adverse pulp response, the greatest contributor was bacterial colonization of the cavity walls micro leakage. A good clinical practice needs materials that create nonleaking restorations.[10]

In the current study which aimed to assess the microleakage of RelyX ARC, FujiCEM, DCRC-10 and zinc phosphate, it was observed that Rely X ARC and DCRC-10 yielded better results whereas zinc phosphate exhibited the highest degree for microleakage. Studies assessing the microleakage after inlay cementation at enamel and cementum margins using these cements is scarce in literature. Hence, comparisons are done wherever possible.

Relyx ARC, was preferred for the study because the manufacture (3M ESPE) recommends separate application of bonding agent prior to cementation which is similar to that of the newly developed resin cement (DCRC-10). FujiCEM is recommended by the manufacturers as a universal luting cement in all clinical situations. The reason for selecting zinc phosphate as a control was its well-documented history as a luting cement. The imported resin cements are costly and rarely available. Indigenously available new materials will offer a better choice at quality and price. The new cement DCRC 10 was the fourth cement selected as no studies have been done with this material so far in this regard.

All samples were collected from young adult patients (age 14–26 years) who underwent orthodontic treatment. This was to avoid the variability of age changes such as physiological dentinal sclerosis or irritational sclerosis due to erosion.

The study was performed on class V inlays with both enamel and cementum/dentin margins so that it was able to investigate the marginal adaptation to both substrates with various luting agents. Composite inlays were fabricated by a direct technique on the assumption that the marginal discrepancy would be less when compared to indirect technique. After initial light curing of 30 s, the inlays were removed from the preparations and the inner surfaces were further light cured for 30 s. This was to achieve the maximum polymerization.

The use of thermal cycling in laboratory studies of dental materials has been considered as one of the potential methods to simulate in vivo challenges.[11] Thermo cycling is defined as the in vitro process of subjecting a restoration and tooth to temperature extremes that conform to those found in the oral cavity.[12] Substantial stress may be imposed on the tooth-restoration interface by rapid temperature fluctuations.

The use of organic dyes to assess micro leakage helps to produce sections showing leakage in contrasting colours without any chemical reaction or hazardous radiation exposure.

Based on site

Highest degree of microleakage was observed at enamel than cementum margin. This result is in line with that reported by other studies which found a greater microleakage at cementum/dentin margin.[131415]

This difference is believed to be due to the different chemical compositions of enamel and dentin/cementum. Acid etching or initial acidity of luting cements remove the smear layers and mineral phase, and increases the microscopic roughness of the enamel surface thereby cements set intimately on the etched enamel.[16] There are no spaces around collagen fibres to initiate microleakage.[17] Another reason for adhesive failure at the cementum/dentin interface may be the fatigue stresses induced by thermal cycling.[18] Even though there was a comparatively higher dye penetration at the cementum/dentin margins in every group, a statistically significant difference (P < 0.05) occurred only in Group IV (zinc phosphate).

Based on material

Group IV (Zinc Phosphate) samples showed a higher leakage at both enamel and cementum/dentin margins compared to other groups. Similar results were obtained in other studies which assessed microleakage after crown placement.[1920] It could probably attributable to the lack of micromechanical and chemical bonding, setting shrinkage, and adhesive failure. It is possible that porosities introduced into the cements during mixing may have contributed. These porosities may have expanded and contracted during pressure cycling, which in turn weakened the cement. Other possibilities include the appearance of microcracks as a result of volumetric contraction, or the presence of internal stresses within the brittle, thin-film, zinc phosphate cements, that, when subjected to the effects of pressure cycling, may have produced stresses that exceeded the cohesive and adhesive strength of the material and resulted in disruption of the cement layer, which allowed microleakage to occur.[21] The crystalline structure of set cement is slightly porous and allows the penetration of dye particles through the cement.[22]

Other studies have also reported significantly higher leakage scores of the cementum margin with acid-base cements than the adhesive luting cements immediately after insertion of inlays.

The highest leakage scores were found for inlays cemented with zinc phosphate, which was in agreement with the present study. They reported that the initial high acidity (pH-1.6) of zinc phosphate could demineralize the apatite mineral phase of both the smear layers and intact dentin. The creamy cement paste did not diffuse entirely through demineralized dentin. Thus demineralized dentin with exposed collagen fibers is left beneath the set cement.[5]

In another in vivo microleakage study of luting cements, it was revealed that under normal clinical conditions, an adhesive luting agent, with or without a dentin bonding agent, can significantly reduce microleakage at the tooth-restoration interface when compared with a zinc phosphate control.[23]

In the present study also, resin-modified glass ionomer (FujiCEM) showed much more microleakage scores at both enamel and cementum margins when compared to resin cements but was not statistically significant. One of the reasons for this might be the inability of cement to infiltrate completely into the demineralized dentin formed as a result of the initial acidity of the cement. As per the recommendation by the manufacturer, FujiCEM was applied directly to the inlay cavity preparation without any preconditioning. Although glass ionomers remain as the only material that are self-adhesive to tooth structure, in principle, pretreatment with a weak polyalkenoic acid conditioner significantly improves bonding efficiency.[24] Lack of preconditioning might be one of the reasons for slightly higher leakage scores in Group III samples.

In a similar study, microleakage of class V composite inlays luted with resin-modified glass ionomer and resin cements were compared and concluded that resin cements were more effective in preventing leakage.[24] This is comparable to the results of the present study.

The present study also demonstrated a significant reduction but not complete elimination of marginal microleakage when resin luting cements are used. It is reported that most microleakage occurs at the weakest link, which is the tooth-cement interface.[25] The less microleakage detected with the resin cements may be the result of obstruction of the dentinal tubules by resin tags, or the resin cements may have been sufficiently flexible to resist microfracture during thermal loading. Inlays cemented with Rely X ARC showed the least microleakage scores. In this group, Single Bond (3M, ESPE) was used as a bonding agent.

CONCLUSION

It can be observed that the values obtained for the indigenously developed resin cement (DCRC-10, Group II) were highly comparable to that of Group I (RelyX ARC). Due to the superior results, the new material can be recommended in clinical practice. The novel dual-cure resin cement can be recommended for bracket bonding, crown cementation, and luting applications in dentistry. However, additional in vitro and in vivo tests such as microleakage and nanoleakage tests after cyclic loading and SEM evaluation have to be done for a better prediction of the newer adhesive luting materials.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.