{'fraction_non_alphanumeric': 0.030688136327532672, 'fraction_numerical': 0.01675211206585313, 'mean_word_length': 5.216337522441652, 'pattern_counts': {'":': 0, '<': 2, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Effect of Mechanical Load Cycling on Microleakage of Restorative Glass Ionomers Compared to Flowable Composite Resin in Class V Cavities

Objectives: Microleakage is the most important factor responsible for the destruction of restoration margins. The aim of this study was to assess the effect of mechanical load cycling on microleakage of four types of glass ionomer cement (GIC) in comparison with a flowable composite resin. Materials and Methods: In this in-vitro experimental study, 100 Class V cavities were prepared on the buccal and lingual surfaces of 50 intact premolars. The prepared cavities were divided into five groups of (A) Z350, (B) Equia Forte, (C) encapsulated Fuji II LC, (D) hand-mixed Fuji II LC, and (E) Ketac Molar. All the samples were thermocycled (×2,000, 5–55°C), and half of the samples in each group were load cycled. All the teeth were then immersed in 0.5% basic fuchsine for 24 hours, sectioned, and observed under a stereomicroscope. Data were analyzed with Kruskal-Wallis and Mann-Whitney-U tests. Comparison between the incisal and gingival microleakage was made with Wilcoxon test. P<0.05 was considered statistically significant. Results: Load cycling and type of restorative material had a significant effect on microleakage. Gingival microleakage was significantly higher than occlusal microleakage with Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar in the absence of loading, and with Z350 after loading. Conclusion: The sealing ability of Z350 under load cycling was better than that of Equia Forte, hand-mixed Fuji II LC, and Ketac Molar. The marginal integrity of encapsulated Fuji II LC was not significantly different than that of Z350.


INTRODUCTION
Microleakage is the most important factor responsible for the destruction of restoration margins, which causes postoperative tooth hypersensitivity, secondary caries, pulpal irritation, pulp necrosis, and marginal discoloration of restorations [1,2]. Oral conditions such as occlusal forces, variations in temperature, and the difference between the physical properties of teeth and restorative materials can contribute to microleakage [3].
Accordingly, finding a material with proper bonding properties that decrease marginal microleakage has always been a topic of interest [4]. Glass ionomer cements (GICs), introduced in 1972 [5], have the ability to chemically bond to enamel and dentin [6]. They have many other advantages as a restorative material such as biocompatibility, suitable thermal expansion, lower sensitivity to moisture, and the ability to store and release fluoride [7]. However, conventional GICs have some clinical limitations such as long setting time, dehydration potential during the initial steps of polymerization, and rough surface texture. In order to overcome such disadvantages, light-cure resin-modified GICs (RMGICs) were introduced with longer working time, faster setting time, improved esthetic properties, and higher initial strength in comparison with conventional GICs [8].
RMGICs are available in two forms of handmixed and premeasured unit dose capsules that enable easier application and fewer errors on mixing [9]. Today, high viscosity GICs, such as Ketac Molar, and hybrid ionomers, such as Equia Forte, are also available. Their manufacturers claim that they have lower polymerization shrinkage, optimal marginal sealing, and long-term resistance to microleakage [10]. However, clinical and experimental studies are lacking in this respect. Also, the effect of occlusal loading and buccolingual forces on the degree of microleakage should be studied.
There are some controversies about the effect of mechanical load cycling on microleakage. Some studies have suggested that mechanical loading does not increase microleakage [11], while some others have reported a significant increase in microleakage after mechanical load cycling [12][13][14][15][16].  [12,13], fifty intact premolars, freshly extracted for orthodontic reasons, were used for this study. The teeth were immersed in 0.5% chloramine-T solution for one week after mechanical debridement and were then were kept in saline at 4°C until the experiment. Class V cavities were prepared in the buccal and lingual surfaces of the teeth using a high-speed handpiece under water spray with a straight carbide fissure bur (ISO #012; Jota, Ruthi, Switzerland) according to the following standards: The length, width, and depth of the cavities were 3 mm mesiodistally, 3mm occlusogingivally, and 1 mm in dentin, respectively. The gingival margins of the cavities were prepared 1 mm below the cementoenamel junction (CEJ). All cavities were prepared by one clinician, and burs were replaced after five preparations. Finally, 100 dental cavities were prepared. The teeth were then randomly divided into five groups according to the restorative material used (n=20). All the samples were subjected to thermocycling, and half of them randomly underwent mechanical load cycling. The materials' compositions and manufacturers are listed in Table 1.  Half of the teeth in each group were randomly selected and subjected to 50,000 mechanical load cycles in a chewing simulator (SD Mechatronik GmbH, Feldkirchen-Westerham, Germany). The chewing stroke properties were as follows: stroke length of 0.8 mm, frequency of 2 Hz, and 100 N magnitude of stroke. The total load cycling time was approximately 7 hours with a rate of 2 Hz (50,000 cycles). First, 25,000 cycles were applied in the lingual-buccal direction through the buccal cusp of the tooth. Then, the tooth was rotated by 180°, and 25,000 chewing cycles were applied again through the lingual cusp in the buccolingual direction. The teeth were kept wet during the procedure.

Microleakage assessment:
All the surfaces of the teeth were coated with three layers of nail varnish up to 1 mm around the restoration margins. The apical foramina were sealed with modeling wax. The samples were immersed in 0.5 % basic fuchsine solution for 24 hours. Before sectioning, the teeth were washed with water and dried. Finally, the samples were sectioned in the buccolingual direction by a cutting machine and assessed under a stereo-microscope (S-4160; Hitachi, Japan) at ×40 magnification. Based on the color penetration depth, the microleakage scores were classified as follows [1,14]: 0: No evidence of dye penetration 1: Dye penetration along the interface up to onethird of the occlusal and gingival cavity 2: Dye penetration by more than one-third of the occlusal and gingival cavity but not involving the pulpal wall 3: Dye penetration along the pulpal wall. SPSS version 23 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis. The data were analyzed using Kruskal-Wallis and Mann-Whitney-U tests. Wilcoxon matched pairs test was used to compare the occlusal and gingival margin microleakage in each tooth. The statistical significance was set at P<0.05.

Effect of restoration margins:
Gingival microleakage was significantly higher than occlusal microleakage with Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar in the absence of loading, and with Z350 after loading ( Table 2).

Effect of mechanical load cycling:
The application of mechanical loading led to an increase in microleakage in the occlusal and gingival margins of the restorations. The difference was statistically significant in the occlusal margin of Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar and in the gingival margin of Equia Forte ( Table 3).

Effect of restorative materials:
The results of Kruskal-Wallis test showed that there was a significant difference in occlusal microleakage between the restorative materials under load cycling (

DISCUSSION
In the current in-vitro study, we used eccentric mechanical loading in addition to axial loading because they induce more tensile stress at the tooth-restoration interface in comparison with axial forces alone. Evidence shows an association between eccentric loading and marginal failure of Class V restorations due to the generation of flexural forces in the tooth structure [11,17,18]. In this study, flowable composite (Filtek Z350 XT) was used as the control group. Fruits et al [19] reported that microfilled and flowable composite resins, with a lower modulus of elasticity and higher flexibility during flexural movements of the tooth, exhibit less microleakage and better marginal adaptation in Class V restorations. In addition, Filtek Z350 is a nanocomposite containing both nanoparticle and nanocluster fillers (zirconia/silica fillers) in its composition. Nanoclusters further reinforce the composite structure in comparison with microfilled and microhybrid systems [20].

Effect of restoration margins:
Under load cycling, the gingival microleakage of Z350 was significantly higher than its occlusal microleakage, while in the absence of load cycling, no significant difference was observed in the microleakage at the gingival and occlusal margins. These results are in agreement with those of previous studies [12,13,19]. Considering the fact that Z350 is a composite with low polymerization shrinkage [21], the slight contraction due to polymerization before load cycling was unable to overcome the adhesive bond strength; thus, the gap and microleakage were also minimal, and only when the mechanical load was applied to the restoration, the microleakage of weaker gingival margins significantly increased.
In the absence of mechanical loading, the gingival microleakage scores of Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar were significantly higher than their occlusal microleakage scores. The mechanism of adhesion of GICs to dental structures involves chelation of carboxylic groups of poly-acids with calcium in the apatite of enamel and dentin [22]. Better marginal adaptation of GICs to enamel in comparison with dentin has been confirmed in previous studies [12,13], which is related to the presence of higher amounts of hydroxyapatite ions in the enamel, which create a stronger ionic bond.

Effect of mechanical load cycling:
In the present study, after applying mechanical load cycling, the occlusal and gingival microleakage of Z350 increased (not signifycantly). These findings were in agreement with those of previous studies [12,13,16,[23][24][25]; however, a number of studies have reported that microleakage of composite restorations increases under mechanical load cycling [15,16,26]. These differences in the results of experimental studies could be related to variations in the tested restorative materials, the magnitude of the applied force, method of force application and cavity preparation and evaluation technique. The application of mechanical loading increased the microleakage at the margin of GI restorations; this increase was statistically significant in the occlusal margins of Equia Forte, encapsulated Fuji II LC, hand-mixed Fuji II LC, and Ketac Molar and in the gingival margin of Equia Forte. Davidson and Abdalla [16] showed that following the exertion of a 125-N force, microleakage of Ketac Fil GI significantly increased, while Fuji II LC showed good marginal integrity. Increasing the load up to 250 N resulted in degradation of Fuji II LC marginal adaptation [16]. In the present study, this result was obtained by the application of a 100-N force; this difference could be due to the difference in the direction and the frequency of mechanical loading. In the study by Davidson and Abdalla [16], only axial forces were applied, and the frequency of mechanical cycles was 10 times lower than that in the present study.

Effect of restorative materials:
After load cycling, the occlusal and gingival microleakage scores of Z350 were significantly lower than that of Equia Forte, hand-mixed Fuji II LC, and Ketac Molar. These findings are consistent with those of previous studies [12,13] [22] found the same results for the microleakage of high-viscosity GIs in comparison with Fuji II LC. These findings can be due to the higher modulus of elasticity and a more rigid structure of these materials, allowing less elastic changes during the application of mechanical forces. In addition, the micro-leakage might have been due to the high viscosity of restorative materials, interfering with proper wetting of the tooth surface and preventing a good seal at the tooth-restoration interface. The marginal integrity of encapsulated Fuji II LC in the gingival margin under load cycling was better than that of hand-mixed Fuji II LC. The difference between the microleakage of encapsulated and hand-mixed GIs has not yet been reported in the literature. However, Dowling and Fleming [29] demonstrated that encapsulated GI has better mechanical properties in comparison with hand-mixed GI; this difference could be associated with the more complete mixing of the encapsulated type and the higher stickiness of the hand-mixed type on the instrument during the application, which can lead to inappropriate adaptation of restorative material with cavity walls and increased marginal microleakage.

CONCLUSION
As indicated by the results, the application of mechanical load cycling significantly increases the microleakage at the restoration margins. The sealing ability of Z350 under load cycling was better than that of Equia Forte, hand-mixed Fuji II LC, and Ketac Molar in Class V cavities. The marginal integrity of encapsulated Fuji II LC was not significantly different than that of Z350.

ACKNOWLEDGMENTS
The present study was part of a postgraduate thesis by the corresponding author with grant number 4866. The authors would like to thank the Dental Material Research Center of Babol University of Medical Sciences for the time and material supplements for the laboratory procedures.