Effect of Water Storage on Microleakage of Class II MOD Bulk-fill Resin Composite Restorations: In Vitro Evaluation

Abstract

Objective

To evaluate and compare the effect of 6-month water storage on the marginal microleakage of class II MOD cavities restored with two different bulk-fill resin composites (sculptable and no-cap flowable) and a conventional resin composite.

Materials and Methods

A total of 60 sound freshly extracted mandibular molars, each with a standardized class II MOD cavity, were assigned into three main groups according to the restorative material used as follows: bulk-fill resin composite (SimpliShade, Kerr), no-cap flowable bulk-fill resin composite (G-aenial Bulk Injectable, GC), and conventional resin composite (Neo Spectra ST LV, Dentsply Sirona). Each group was further divided into two subgroups (n = 10) based on the water-storage time: 24 hours (immediate) and 6-month water storage (aged). All specimens were thermocycled 5,000 times between 5°C ± 2°C and 55°C ± 2°C, then immersed in 2% basic fuchsin dye for 24 hours. The specimens were longitudinally sectioned, and the amount of dye penetration was evaluated using stereomicroscope under 25× magnification. The results were analyzed by non-parametric Kruskal-Wallis, Mann-Whitney, and Wilcoxon signed-rank tests. The level of significance was set at p <0.05.

Results

Both bulk-fill resin composites exhibited significantly lower microleakage scores than conventional resin composite (p <0.05). However, 6-month water storage significantly increased the microleakage for all tested groups.

Conclusions

Water storage has a negative impact on the marginal microleakage regardless of the type of the restoration.

Keywords

microleakage - class II cavities - resin composite - bulk-fill - flowable bulk-fill

Introduction

One of the most essential factors determining the success of a restoration is its ability to conform to the prepared tooth surface, properly adapt, and effectively seal the cavity walls, thereby ensuring long-term clinical performance.[1] [2] Ideally, a strong bond between the restorative material and the tooth structure should result in a tight, concealed marginal seal.[3] Despite stupendous technological advancements, achieving a perfect marginal seal remains a persistent challenge,[4] since no restorative material could optimally adhere and form such intimate contact to the tooth surface.[5] Consequently, marginal gaps may develop along the interface between the cavity walls and the restorative material, allowing the passage of bacteria, molecules, ions, and fluids through the tooth–restoration interface, which is referred to as microleakage.[6] [7] Clinically, microleakage can result in marginal discoloration, secondary caries, pulpal inflammation, and postoperative sensitivity.[8]

Indirect restorations are considered a highly recommended treatment option for managing large defects in posterior teeth.[9] [10] [11] However, using indirect restorations is more time-consuming and involves additional costs and appointments that may not align with the patient's preferences and budget.[12] Consequently, direct resin composite restorations have been adopted as an easier and more practical alternative, showing comparable clinical longevity to indirect restorations in posterior teeth.[13] Resin composite restorative materials have undergone remarkable improvements since their introduction to the field of restorative dentistry.[14] [15] A major clinical challenge, however, is the selection of an appropriate placement technique particularly when restoring class II cavities.[16] Direct resin composite restorations are conventionally applied in increments, with each layer cured separately in the so-called incremental filling technique. Despite considered the standard technique, it exhibits several drawbacks including prolonged application time, potential internal flexure, and entrapment of voids between layers.[17] [18]

Bulk-fill resin composites were recently introduced to the dental market in an attempt to overcome the limitations of conventional resin composites. These materials can be placed in a single increment of up to 4 to 5 mm and cured in bulk, which simplifies the restorative procedures and reduces the time required for placement.[19] [20] [21] Their ability to undergo bulk polymerization is attributed to modifications in chemical composition by using modified monomers that minimize polymerization shrinkage stresses. Moreover, the increased translucency achieved by modifying the filler systems, along with incorporating alternative photoinitiators, facilitates deeper light penetration and improved depth of cure.[22] [23]

Current bulk-fill resin composites are categorized into sculptable (high-viscosity) and flowable (low-viscosity) based on their filler content.[24] High-viscosity bulk-fill composite can substitute both enamel and dentin in a single increment without any capping layer owing to its high filler load.[25] [26] On the other hand, the flowable bulk-fill composites were only limited to restoring simple proximal cavities or serving as dentin replacement materials, as their low surface hardness and modulus of elasticity necessitate a final coverage with a conventional resin composite.[25] [27] More recently, an enhanced category of flowable bulk-fill composites has been developed, which can be placed in one layer up to 4 mm without requiring a composite capping layer owing to the full-coverage silane coating of its filler nanoparticles, thereby providing high flexural strength and extraordinary wear resistance.[18] [28] [29]

Since microleakage represents a major cause of restoration failure, its evaluation remains an important parameter for assessing the clinical success of bulk-fill restorative materials.[8] Furthermore, another parameter to consider is the water-storage effect, as the tooth–restoration interface is constantly exposed to saliva and consequently subjected to degradation in the oral cavity.[30] Indeed, numerous studies have evaluated the microleakage of bulk-fill compared to conventional resin composites in class II restorations.[8] [31] [32] [33] However, according to the authors' knowledge, no studies have evaluated the microleakage of the recently introduced G-aenial Bulk Injectable, a flowable bulk-fill resin composite without a capping layer, particularly with respect to the effect of water storage. Therefore, the aim of this study was to assess and compare the microleakage of class II MOD cavities restored with bulk-fill, no-cap flowable bulk-fill, and conventional resin composite, both immediately and after 6-month water storage. The null hypothesis tested was that the microleakage would not be influenced by the type of resin composite or the time of storage.

Materials and Methods

Restorative Materials

In the current study, the resin composite restorative materials and their corresponding adhesive systems were: bulk-fill resin composite (SimpliShade/OptiBond Universal, Kerr, Orange, CA, USA), no-cap flowable bulk-fill resin composite (G-aenial Bulk Injectable/G-Premio Bond, GC, Tokyo, Japan), and conventional resin composite (Neo Spectra ST LV/Prime & Bond Universal, Dentsply Sirona GmbH, Konstanz, Germany). A detailed description of each material is presentenced in [Table 1].

Table 1 Materials used in the study

Restorative system	Type	Shade	Manufacturer	Composition	Batch no.
SimpliShade Bulk-Fill	Nanohybrid bulk-fill resin composite	Universal	Kerr	Zirconium silicate, urethane dimethacrylate, ethoxylated bisphenol-A-dimethacrylate, silicon dioxide, silica, triethylene glycol dimethacrylate, butylated hydroxytoluene, oxybenzone ethyl 4-dimethylaminobenzoate	A174727
OptiBond Universal	Universal adhesive			Acetone, ethanol, 2-hydroxyethyl methacrylate, glycerol phosphate dimethacrylate, glycerol dimethacrylate	00317838
G-aenial Bulk Injectable	Flowable nanohybrid bulk-fill resin composite	A2	GC	Bis-EMA, bismethacrylate, dimethacrylate, UV-light absorber, UDMA	2009072
G-Premio Bond	Universal adhesive			MDP, 4-MET, MDTP, BHT, dimethacrylate monomer, acetone, water, photoinitiator, silica fillers	2310064
Neo Spectra ST LV	Nanoceramic resin composite	A2	Dentsply Sirona	Spherical pre-polymerized SphereTEC fillers, non-agglomerated barium glass and ytterbium fluoride, highly dispersed methacrylic polysiloxane nano-particles	2305000755
Prime & Bond Universal	Universal adhesive			Bi- and multifunctional acrylate, phosphoric acid modified acrylate resin, initiator, stabilizer, isopropanol, water, PENTA, MDP	2002000692

Abbreviations: 4-MET, 4-methacryloxyethyl trimellitate; BHT, butylated hydroxytoluene; Bis-EMA, bisphenol-A ethoxylated dimethacrylate; MDP, methacryloyloxydecyl dihydrogen phosphate; MDTP, 10-methacryloyloxydecyl dihydrogen thiophosphate; PENTA, dipentaerythritol pentacrylate phosphate; UDMA, urethane dimethacrylate; UV light, ultraviolet light.

Teeth Selection

A total of 60 sound freshly extracted, non-carious human mandibular molars with comparable dimensions were collected from healthy individuals at the Oral Surgery Department Clinic after gaining informed consent. The ethical approval was granted by the Faculty Research Ethics Committee under protocol number 024121261. All methods were carried out in accordance with relevant guidelines and regulations of Helsinki Declarations. After removing any residual calculus or plaque, teeth were disinfected using 0.5% chloramine-T solution and then were cleansed using a rubber cup and fine pumice water slurry. A binocular stereomicroscope (SZ TP, Olympus, Tokyo, Japan) was used for detecting any microscopic cracks or enamel craze lines. To avoid teeth dehydration, they were stored in distilled water at 37°C ± 1°C using an incubator (BTC, BioTech Company, Cairo, Egypt) and removed only for testing.[34] [35]

Sample Size Calculation

Sample size was calculated using a statistical software program (G*Power, Ver.3.1.9.1, Dusseldorf, Germany) based on a previous study.[36] A two-tailed test with an effect size of 1.37, a significance level (α) of 0.01, and 99% statistical power was considered. Therefore, a representative sample composed of 10 teeth per subgroup was selected for this study.

Specimens' Preparation

Each tooth was fixed in self-curing acrylic blocks up to 2 mm below the cementoenamel junction (CEJ).[22] Standardized class II MOD cavities were prepared using a flat-ended, straight fissure diamond instrument (836.HP.027; Komet, Brasseler, Lemgo, Germany) in a high-speed handpiece (Dentsply Sirona GmbH, Konstanz, Germany) under continuous air–water cooling. The depth of the prepared cavity was 4 mm from the central groove, with a width of two-third of the intercuspal distance. The buccal and lingual walls were prepared parallel to each other, with a 90 degrees cavosurface angle. The width of proximal boxes was also two-third of the buccolingual distance. The axial wall of each box was at 1.5 mm depth. The cervical margins were placed 1 mm coronal to CEJ. All prepared cavities were finished using extra-fine diamond instruments (835KREF.314.012, Komet) in order to obtain smooth walls and rounded internal angles.[18] A magnifying aid with LED light (Perioptix Inc, Central Ave, CA, USA) was used to ensure proper cavity preparation. A digital caliper (Whitworth, CA, USA) was used for measuring the cavity dimensions. The prepared teeth were assigned randomly into three main groups (n = 20) using a random generator software via a specific website (https://www.random.org), based on the resin composite restorative material as follows:

• Group 1–Bulk-fill resin composite, SimpliShade bulk-fill

• Group 2–No-cap flowable bulk-fill resin composite, G-aenial Bulk Injectable

• Group 3–Conventional resin composite, Neo Spectra ST LV

Restorative Procedures

A circumferential metal matrix/band (Omni-Matrix, Ultradent Products, South Jordan, UT, USA) was fixed around each prepared tooth in order to restore the proximal contours.[37] The enamel margins were etched using 37% phosphoric acid gel for 15 to 20 seconds (N-Etch, IvoclarVivadent AG, Schaan, Liechtenstein) following selective enamel etching protocol. The cavity preparation was then thoroughly rinsed by water for 10 seconds and gently dried with oil-free air. Subsequently, each universal adhesive was applied to enamel and dentin using microbrush, scrubbed for 20 seconds, gently air-dried for 5 seconds, and then light-cured for 10 seconds according to the manufacturers' instructions using a light-emitting diode (LED) curing unit (Elipar S10, 3M Oral Care) with a light intensity of 1,200 mW/cm², as calibrated by the built-in radiometer.

For both bulk-fill groups, the prepared cavities were restored in one bulk increment (4 mm) and light-cured for 10 seconds. Meanwhile, conventional resin composite was applied in increments of 2 mm thickness and cured separately for 10 seconds. The proximal surfaces of all restorations were further cured for 10 seconds from buccal and lingual surfaces after matrix removal to ensure optimum polymerization. Finishing and polishing procedures were performed using high-speed diamond finishing instruments (4092.314, Komet) under copious air–water cooling followed by flexible discs (Sof-Lex XT Pop On, 3M Oral Care) and rubber polishing points (OneGloss, Shofu, Kyoto, Japan).

Testing

Half of the specimens of each group (n = 10) were stored in distilled water in an incubator (BTC, BioTech Company, Cairo, Egypt) at 37°C ± 1°C for 24 hours (immediate), then thermocycled for 5,000 cycles between 5°C ± 2°C and 55°C ± 2°C with dwell time of 30 seconds and transfer time of 2 seconds from one bath to the other.[18] The other half of the specimens were stored for 6 months before subjecting to thermocycling (aged).

The teeth were then coated with three coats of nail polish except for the restoration and 1 mm rim of the tooth structure, then immersed in 2% basic fuchsin dye solution in dark room for 24 hours.[35] [38] After washing thoroughly with distilled water, teeth were sectioned in mesio-distal direction into two halves by a cutting saw machine (ISOMET 4000, Buehler, Lake Bluff, IL, USA) at speed of 2,500 rpm under copious water cooling. The resulted cross-sections were examined by stereomicroscope (SZ TP, Olympus, Tokyo, Japan) at 25× magnification.[31] [32]

Microleakage was scored based on the degree of dye penetration using a five-grade scoring system as follows: 0 = no dye penetration; 1 = superficial dye penetration not beyond amelodentinal junction (ADJ); 2 = dye penetration along the gingival wall; 3 = dye penetration along the gingival and axial walls; 4 = dye penetration along the pulpal floor.[32]

Statistical Analysis

Data were collected and statistically analyzed using a statistical software package (IBM-SPSS, version 24, Armonk, NY, USA). Since the results exhibited clearly ordinal structural data, and the data did not follow a normal distribution pattern according to the Shapiro–Wilk test (p < 0.05), a non-parametric analysis of variance, Kruskal-Wallis test, was used to compare between different groups at each storage time. Mann-Whitney U test was used for pairwise comparisons, while Wilcoxon signed-rank test was used to assess the effect of storage time on group. The level of significance was set at p <0.05.

Results

Microleakage results among the tested groups are shown in [Table 2]. Kruskal-Wallis test indicated a significant difference in the microleakage scores among the three tested groups immediately (p = 0.001) and after 6 months (p = 0.002). The microleakage scores in both bulk-fill groups were significantly lower than those of the conventional group (p < 0.05). For all immediately restored groups, score 3 was observed only for conventional resin composite group. Score 4 was not noted for any of the immediate groups, while it was recorded for both no-cap flowable bulk-fill and conventional resin composite groups after 6-month water storage.

Table 2 Comparison of microleakage scores among the tested groups

	SimpliShade Bulk-Fill	G-aenial Bulk Injectable	Neo Spectra ST LV	p-value	p1	p2	p3
Immediate	1.00 (0.00–1.00)	1.00 (0.00–2.00)	1.5 (1.00–3.00)	0.001[*]	0.326	0.002[*]	0.009[*]
Aged	2.00 (1.00–3.00)	2.00 (1.00–4.00)	3.00 (1.00–4.00)	0.002[*]	0.804	0.001[*]	0.005[*]
p -value	0.003[*]	0.006[*]	0.001[*]

Notes: p1: SimpliShade Bulk-Fill vs. G-aenial Bulk Injectable.

p2: SimpliShade Bulk-Fill vs. Neo Spectra ST LV.

p3: G-aenial Bulk Injectable vs. Neo Spectra ST LV.

^* Statistically significant at p <0.05.

For immediately tested groups, Mann-Whitney U test indicated no statistically significant difference between bulk-fill and no-cap flowable bulk-fill restored groups (p = 0.326). Both bulk-fill restored groups exhibited significantly lower microleakage scores compared to the conventional resin composite group (p < 0.05) ([Fig. 1]). For groups tested after 6-month water storage, there was also no statistically significant difference between bulk-fill and no-cap flowable bulk-fill restored groups (p = 0.804). The conventional resin composite group showed significantly higher microleakage scores than both bulk-fill restored groups (p < 0.05) ([Fig. 2]).

Fig. 1 A boxplot showing microleakage scores of the immediate groups.

Fig. 2 A boxplot showing microleakage scores of the aged groups.

Regarding the effect of storage time, the results of the Wilcoxon signed-rank test revealed that each immediately restored group showed significantly lower microleakage scores compared with its corresponding aged group (p < 0.05). Different microleakage patterns and the amount of dye penetration are presented in [Fig. 3].

Fig. 3 Representative stereomicroscopic images showing microleakage scores: (A) Score 0: No dye penetration, SimpliShade Bulk-Fill immediate group. (B) Score 1: Superficial dye penetration not beyond amelodentinal junction (ADJ), G-aenial Bulk Injectable immediate group. (C) Score 2: Dye penetration along the gingival wall, G-aenial Bulk Injectable delayed group. (D) Score 3: Dye penetration along the gingival and axial walls, Neo Spectra ST LV immediate group. (E) Score 4: Dye penetration along the pulpal floor, Neo Spectra ST LV delayed group.

Discussion

One of the major causes of restoration failure is microleakage that occurs due to presence of microscopic openings at the tooth–restoration interface.[39] The current in vitro study evaluated the microleakage of class II MOD cavities restored with two different types of bulk-fill resin composite restorative materials in comparison to conventional resin composite immediately and after 6-month water storage. The study results showed that both types of bulk-fill resin composites exhibited superior resistance to microleakage compared to conventional resin composite. Moreover, water storage had a negative effect on the tested groups. Therefore, the null hypothesis formulated at the beginning of the study was rejected.

Class II MOD cavities were prepared to simulate a clinically challenging scenario involving the loss of both marginal ridges.[18] Unlike conventional resin composites, which require incremental placement, bulk-fill composites can be applied in a single 4-mm increment, simplifying the procedure and reducing chair time.[40] [41] A novel flowable bulk-fill composite was selected in this study, providing the additional advantage of eliminating the need for a final capping layer.[28] The incorporation of densely packed, ultra-fine fillers contributes to high flexural strength and reduces internal shear stress within the polymer matrix. Additionally, the distribution of fillers, represented by the closely packed particles, helps protect the interstitial resin from abrasion by neighboring particles, thereby providing better wear resistance.[29]

Each tested composite material was applied with its corresponding universal adhesive from the same manufacturer to ensure compatibility and avoid potential inaccuracies.[2] A key limitation of universal adhesives when applied in self-etch mode is their inadequate etching of enamel, producing a shallow and poorly defined etching pattern. Therefore, selective enamel etching was employed to ensure reliable bonding to enamel, which is essential for establishing an effective marginal seal and protecting the underlying dentin bond from degradation.[42]

Dye penetration technique is one of the simplest and most commonly used methods for evaluating microleakage, being highly feasible, easily reproducible, and involving no reactive chemicals or radiation.[5] [43] [44] [45] Basic fuchsin dye was utilized for its low cost, wide availability, and ease of use without requiring sophisticated laboratory equipment. Specimens were immersed in the dye for 24 hours, a duration deemed sufficient for leakage assessment without compromising the adhesive interface, as supported by Pushpa and Suresh[38] and Sawani et al.[46]

Evaluating the performance of new restorative materials requires replicating oral challenges, including fluctuations in temperature and humidity. In this study, specimens underwent 5,000 thermal cycles following ISO standard protocol (ISO TR 11450) to simulate thermal stresses from hot and cold extremes intraorally.[18] To further replicate oral conditions and intensify the aging effect, half of the specimens were stored in distilled water for 6 months, promoting water sorption and the release of incompletely polymerized monomers, which may affect the long-term durability of the restorations.[47] [48]

The two bulk-fill resin composites demonstrated superior marginal sealing compared to the conventional resin composite, both immediately and after aging. This favorable performance of bulk-fill resin composites may be attributed to their lower polymerization shrinkage and associated stresses, which may contribute to better adaptation to cavity walls and reduced microleakage.[49] [50] [51] These findings are consistent with several in vitro studies that reported reduced microleakage and improved marginal sealing when bulk-fill resin composites were compared with incrementally placed conventional ones.[52] [53] In contrast, other studies have reported comparable or even inferior marginal sealing performance of bulk-fill resin composites relative to conventional resin composites.[33] [39] [54] Such discrepancies may be attributed to differences in evaluation protocols, cavity design, materials tested, and the dye employed.

The comparable performance of both the bulk-fill resin composites may be attributed to their high translucency and improved depth of cure, which result in a favorable degree of conversion and controlled polymerization stresses, contributing to better marginal sealing irrespective of material consistency.[55] This observation aligns with previous reports indicating no significant influence of bulk-fill resin composite consistency on marginal microleakage.[32] [56] However, some studies reported contradictory outcomes when flowable types were compared with higher-viscosity formulations.[57] [58] These conflicts may be ascribed to the compositional differences between the investigated materials as well as variations in the experimental conditions.

Water storage impaired marginal sealing in all tested materials. This effect may be attributed to resin composite degradation induced by water sorption, which can compromise the integrity of the polymer network and promote hydrolysis at the filler–matrix interface. These findings are in line with those of Scotti et al[59] who reported increased dye penetration in resin composites after 12 months of storage. However, a previous study found no significant effect of water storage on marginal leakage, likely due to the 3-month storage duration being insufficient to induce measurable degradation.[30]

It should be recognized that the current study has certain limitations. One of these limitations was the 6-month storage period, which may not fully capture the effects of long-term aging on the materials' marginal sealing. Another limitation was relying on the basic fuchsin dye penetration technique as the sole method for microleakage assessment. Along with its subjective assessment, the small particle size of basic fuchsin dye allows deeper penetration into dentinal tubules, possibly staining dentin, which could affect the reliability of the test.[45]

Future studies evaluating marginal sealing should consider using alternative dyes, such as silver nitrate, which may provide a more reliable assessment and reduce potential exaggerated results. In addition, advanced evaluation techniques, including confocal laser scanning microscopy (CLSM), optical coherence tomography (OCT), and micro-computed tomography (micro-CT), should be incorporated to allow a more accurate, objective, and comprehensive evaluation of microleakage. Furthermore, studies with longer storage durations are recommended to better assess the durability of marginal sealing under long-term aging conditions.

Conclusion

Within the limitations of the current study, it was concluded that:

• None of the tested resin composites were completely free from microleakage.

• Bulk-fill and no-cap flowable bulk-fill resin composites outperformed conventional resin composite with lower microleakage values in large class II MOD cavities.

• For all tested materials, 6-month water storage had a negative effect on marginal sealing.

Conflict of Interest

None declared.

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Publication History

Article published online:
26 February 2026