Comparative evaluation of sealing ability of two self-adhesive flowable composites following various restorative techniques in Class V lesions: An in vitro study.

Background: Self- adhesive flowable composite (SAFC) has been introduced to eliminate etching and bonding procedures. However, they have shown increased microleakage and poor bonding ability when used as per the manufacturer's instructions. It is unclear if prerestorative techniques can improve the same. Aims: This study aimed to evaluate the effects of acid etching, bevel placement and air abrasion on the marginal sealing ability of SAFCs when used in Class V restorations. Materials and
Methods: 56 human mandibular premolars were taken and standard Class V cavities were prepared on the buccal aspect. They were divided into 4 groups comprising of control and three restorative techniques. Group A: SAFC used alone formed the control group, Group B: Acid-etched using 37.5% phosphoric acid gel, Group C: 1 mm bevel placed on the occlusal margin, Group D: Subjected to air abrasion. Each group was further subdivided into two, with one half being restored with Dyad flow (Kerr) while the remaining with Constic (DMG). The samples were thermocycled and the dye extraction method was used to assess microleakage using ultraviolet spectrophotometer. Statistical Analysis: Microleakage values were subjected to analysis of variance and post hoc analysis.
Results: Statistically significant differences in the absorbance values were seen between the two SAFCs. Results with P < 0.05 were considered statistically significant. For Dyad flow, there was a statistically significant difference between Groups B and D (P = 0.028), while for Constic, it was between Groups A and B (P = 0.031) and Groups B and D (P = 0.025).
CONCLUSIONS: Among restorative techniques, air abrasion showed the least microleakage, while acid etching showed the highest values. Among the two SAFCs, Constic presented lower microleakage than Dyad Flow. Copyright:

INTRODUCTION

Selfadhering flowable composites (SAFC) were introduced as a class of “stepless” system to bridge the gap between adhesive and restorative material technologies to form a single product.[12] Thus, they permit fewer steps, less handling errors and ensures reduced clinical time in patients requiring multiple restorations in the same visit.[345]

At present, they are used in a limited number of clinical procedures such as for pits and fissures, small Class I, Class III, and Class V restorations.[6] However, a study evaluating its clinical performance in Class V lesions found that 66% of restorations failed due to poor retention despite adhering to the manufacturer's guidelines.[7]

There are various SAFCs commercially available, and their clinical performance is product-dependent. Therefore, this study aimed to evaluate the in vitro microleakage of two commercially available SAFCs with additional restorative techniques like acid etching, bevel placement, or air abrasion to improve its sealing ability at the tooth restorative interface.

The null hypothesis was that the additional restorative techniques used would not affect the microleakage of SAFCs.

MATERIALS AND METHODS

The protocol of the present in vitro study was approved by the Institutional Ethics Committee (GDCH/IEC/IV 6). 56 human mandibular premolars were used in the study, with the inclusion criteria being teeth that were extracted for orthodontic purpose or due to periodontitis. They were stored in distilled water with 0.2% thymol for <3 months until use. The exclusion criteria were teeth with decay, cracks, restoration, attrition, abrasion, fluorosis or other enamel defects. The extracted teeth were ultrasonically scaled and cleaned with a slurry of pumice and water to remove any remaining soft tissue tags, plaque, calculus, or stains.

A standardized Class V preparation was done on the buccal surface of each premolar tooth [Figure 1a and b]. The mesiodistal width of each preparation was 4 mm, the occlusogingival width was 3 mm, and the axial depth was approximately 2 mm. The preparations were made parallel to the cementoenamel junctions, with the gingival half of the preparations extending 0.5 mm apical to the cementoenamel junction, and the occlusal margins were placed in enamel. The dimensions were measured using a periodontal probe. All preparations were accomplished using a carbide fissure bur (009; Dentsply Maillefer Instruments, Ballaigues, Switzerland) in a high-speed handpiece with water spray. New burs were used after every five preparations. The samples were then randomly divided into four groups, 14 specimens (n = 14). The study flow chart is given in Figure 2.

Figure 1

(a and b) Schematic representation of Class V cavity preparation, orange dashed line represents bevel placement. (c) Samples prepared and grouped into four categories. (d) Restored samples placed in methylene blue solution, K1 and D1 represent Group 1 sample restored with Kerr Dyad Flow and DMG Constic, respectively. (e) Supernatant extracted after centrifugation before ultraviolet spectrophotometric analysis

Figure 2

Samples were divided into four groups. Group A – control, Group B – acid etching using 37.5% phosphoric acid, Group C – bevel placement and Group D – air abrasion. Each group was further subdivided into two based on the self-adhesive flowable composites used

Group A: 14 specimens were restored by direct application of SAFC as per the manufacturer's guidelines. A small amount of SAFC was dispensed onto the cavity and massaged on all the cavity walls using the brush supplied by the manufacturer for 25 s to obtain a thin layer (0.5 mm), the excess was removed and cured. Thereafter, the cavity was filled completely. This formed the control group.

Group B: Acid etching was done using 37.5% phosphoric acid gel (Kerr, Sybron dental specialities, USA). The enamel was etched for 20 s and dentin for 10 s. The acid was rinsed thoroughly for 15 s and blot dried to remove excess water, leaving the dentin visibly moist.

Group C: A coarse tapered diamond (Piranha Diamond SE8F, SS White) was used to bevel the enamel on the occlusal margin of the lesion. Bevelling was standardized to 1 mm.

Group D: Air abrasion of cavities was performed with 50 μm silica particles at an angle of 45° with 60 psi air pressure at a distance of 2 mm for 5 s. Aluminum oxide air abrasion was done using a sandblasting unit (Microetcher ERC, Danville materials, San Ramon, CA).

Each group was further subdivided [Figure 1c] and restored with Dyad Flow (Kerr, Sybron dental specialities, USA) (n = 7) and Constic (DMG, Germany) (n = 7). All tested materials were used according to the manufacturer's instructions and were light cured with an LED curing unit (3M ESPE Elipar Deep Cure LED Curing Unit; Seefeld, Germany) set at a standard power of 1000 mW/cm2. The same light-curing unit was used throughout the study, maintaining the tip not more than 1 mm from the surface of the specimen. All restorations were finished using extra fine grade Diamond burs at high speed and disks (Super Snap; Shofu Inc., Kyoto, Japan) after completing the restoration to flush the margins. Test specimen were then stored in distilled water for 24 h at 37°C.

Samples were subjected to a thermocycling process, consisting of 1500 cycles alternating between 5 ± 2°C and 55 ± 2°C with a dwelling time of 30 s as an artificial aging procedure.

Microleakage analysis was then done using the dye extraction method. The apices of all the teeth were sealed with wax to prevent the ingression of the testing dye. They were then painted with two coats of fast setting nail varnish on the entire tooth surface except for a window that included the restoration and 1 mm margin around it. The specimens were left undisturbed for 24 h to allow the complete set of the varnish. Each group was then immersed in a neutral buffered 2% methylene blue solution for 24 h at 37°C [Figure 1d]. Teeth were then gently rinsed under tap water for 10 min to remove the excess surface dye without the running water applying pressure on the window part of the stained specimen. The varnish was then removed with polishing disks mounted on a handpiece.

Each tooth was stored in a hermetically sealed vial containing 600 μl of concentrated (65 wt%) nitric acid for 3 days. The vials were then centrifuged at 14,000 rpm for 5 mins to separate composite or debris from the extracted dye [Figure 1e]. 200 μl of the supernatant was taken from each vial and analyzed in an automatic spectrophotometer (ultraviolet [UV]-1800 Spectrophotometer, Shimadzu, Kyoto, Japan) at 670 nm using concentrated nitric acid as the blank and readings were recorded as absorbance units.

Statistical analysis

Statistical analysis was carried out using SPSS version 20.00 software (IBM Corp. Released 2011. IBM SPSS Statistics for Windows, Version 20.0. Armonk, NY: IBM Corp). For all tests, the probability level for statistical significance was set at α = 0.05. Shapiro–Wilk test was used to check the normality. Since P > 0.05, the data was found to be normally distributed. One-way analysis of variance (ANOVA) was used to compare the absorbance level for each type of SAFC between the four different restorative technique groups. Tukey's honestly significant difference multiple comparisons test was used for pairwise comparison of the groups.

RESULTS

By comparing the mean absorbance values Table 1, the microleakage was the least for air abrasion and highest for acid etching while no treatment and bevelling showed similar results for both SAFCs.

Table 1

Mean absorbance levels of individual groups

Groups	n	Mean±SD	95% CI for mean		Minimum	Maximum
			Lower bound	Upper bound
Dyad flow
No treatment	7	0.03400±0.009883	0.02486	0.04314	0.015	0.043
Acid etching	7	0.04886±0.017874	0.03233	0.06539	0.029	0.086
Bevel	7	0.03586±0.013284	0.02357	0.04814	0.021	0.064
Air abrasion	7	0.02843±0.006604	0.02232	0.03454	0.016	0.035
Total	28	0.03679±0.014133	0.03131	0.04227	0.015	0.086
Constic
No treatment	7	0.02300±0.012124	0.01179	0.03421	0.003	0.033
Acid etching	7	0.04586±0.017639	0.02954	0.06217	0.020	0.068
Bevel	7	0.02529±0.011644	0.01452	0.03605	0.013	0.046
Air abrasion	7	0.02229±0.015141	0.00828	0.03629	0.004	0.037
Total	28	0.02911±0.016767	0.02261	0.03561	0.003	0.068

CI: Confidence interval, SD: Standard deviation

A statistically significant difference (P < 0.05) was observed between the four main groups for both the SAFCs. As determined by one-way ANOVA [Table 2], dyad flow (P = 0.038) and Constic (P = 0.015) showed a significant difference during the intragroup comparison. On pairwise comparison [Table 2], statistically significant differences in the microleakage were observed between Groups B and D (P = 0.028) for dyad flow and Groups A and B (P = 0.031) and Groups B and D (P = 0.025) for Constic.

Table 2

One-way analysis of variance and post hoc multiple comparisons between individual groups

Groups	Sum of squares	df	Mean square	F	Significance	Multiple comparisons	P
Dyad flow
Between groups	0.002	3	0.001	3.284	0.038	No treatment versus acid etching	0.151
						No treatment versus bevel	0.993
						No treatment versus air abrasion	0.842
						Acid etching versus bevel	0.244
						Acid etching versus air abrasion	0.028
						Bevel versus air abrasion	0.692
Within groups	0.004	24	0.000
Total	0.005	27
Constic
Between groups	0.003	3	0.001	4.298	0.015	No treatment versus acid etching	0.031
						No treatment versus bevel	0.991
						No treatment versus air abrasion	1.000
						Acid etching versus bevel	0.059
						Acid etching versus air abrasion	0.025
						Bevel versus air abrasion	0.979
Within groups	0.005	24	0.000
Total	0.008	27

DISCUSSION

SAFC is a newly introduced material claiming to simplify the restorative procedure as it combines an all-in-one adhesive system and flowable composite.[1] Poitevin et al.[6] deduced that the clinical use of SAFC should be carefully considered in the absence of macro retention as it has demonstrated poor bonding effectiveness. The inferior bonding efficiency was attributed to insufficient removal of the smear layer and inadequate micromechanical retention between the restoration and tooth surfaces as a result of its lower etching ability. They have mild acidity which was confirmed by transmission electron microscopy indicating relatively superficial interaction of SAFC with the tooth structure.[6]

Cavity modifications such as the placement of bevels, retention grooves or surface pretreatments such as micro air abrasion, acid etching, and application of bonding agent improve the bonding and retention, thereby reducing the microleakage of composite resins in Class V cavities. Pretreatments that enhance tooth roughness may affect bond strength by improving interfacial contact between dentin and the adhesive surface.[89]

Insight into literature does not indicate any data on the comparative evaluation of preliminary restorative techniques to improve the bonding ability of SAFCs in Class V cavities.

Class V cavity preparation was done in this study for several reasons. First, these cervical lesions have been a restorative challenge for any kind of restorative material due to their complex morphology where the margins are partly in enamel and partly in dentin/cementum.[1011] Second, they simulate the clinical situation of higher stress due to the higher C-factor. Finally, the corresponding restorative procedure concerning Class V lesions is minimal and comparatively simple, thus lowering operator variability.[1213]

Thermocycling was used to test the performance of adhesive materials. It aims at thermally stressing the adhesive joint at the tooth/restoration interface by subjecting the restored teeth to extreme temperatures that are encountered intraorally. This process highlights the mismatch in thermal expansion between the restoration and tooth structure, resulting in different volumetric changes during temperature alterations and causing fatigue of the adhesive joint with subsequent microleakage.[14]

Dye-extraction technique (quantitative method) was preferred over dye-penetration (qualitative method) as it involves recovering all of the dye that has penetrated, thereby avoiding the questionable results of the sectioning procedure as the axis of cutting is randomly chosen and the probability that the section occurs through the deepest dye penetration is very low.[15]

The concentration of the methylene blue dye absorbed in the micro gap at the tooth restorative interface is proportional to the light absorption by the UV spectrophotometer. The values are obtained based on the formula A = EC (A: absorption, E: molar absorption coefficient, C: concentration).[16] Thus, a higher absorbance level implies a higher microleakage suggestive of poor sealing ability at the interface.

In Group A (control group), the resin was applied as per the manufacturer's guidelines. While in Group C, bevelling the enamel on the occlusal margin of the prepared cavity was done as an additional step and was followed by the placement of resin as mentioned before. Both these restorative techniques did not improve the marginal leakage scores probably due to the inadequate etching efficiency of the functional acidic monomers. They have a pH of mild aggressiveness ~1.9.[11] A butt joint enamel margin was selected for Groups A, B, and D to comply with the traditional enamel margin designs advocated for most preparations for posterior composite restorations.[14]

Adhesion to enamel is increased by etching with phosphoric acid. Etching the surface of enamel improved the bond strength of self-adhesive luting agents by enhancing the surface energy, thus providing significantly more micro-retention.[56] The presence of hyper mineralized layer, bacteria, and the tubular mineral casts in sclerotic dentine are analogous to the presence of smear layer and smear plugs in sound dentine and act as potential obstacles for primer and resin infiltration.[1718] Hence, phosphoric acid etching of enamel and dentin was tried to evaluate the sealing ability of SAFC.

Some authors have reported that the wettability of SAFC resin on the restorative surface is less due to its higher viscosity compared to an adhesive solution leading to inadequate hybridization of the collagen mesh. However, the etchant gel could have led to the depletion of the hydroxyapatite (HAp) content of dentine (surface as well as deeper) and collapsed collagen fibrils resulting in inadequate SAFC resin infiltration, thereby forming a defective interfacial seal and increased the susceptibility to hydrolytic degradation with time impairing the chemical bond.[519] Hence, it could be speculated that preliminary phosphoric acid etching (Group B) of dentine deteriorated the quality of the seal of the SAFC leading to higher microleakage.

Air abrasion, also known as “micro air abrasion,” can be described as a pseudo-mechanical, nonrotary method of cutting and removing dental hard tissue. It involves using a stream of aluminum oxide particles generated from compressed air or bottled carbon dioxide or nitrogen gas. The abrasive particles strike the tooth with high velocity and remove small amounts of tooth structure depending on the hardness of the tissue being removed and the operating parameters of the air abrasion device.[20]

The lower values of microleakage were noted when air abrasion (Group D) was used. This could have resulted due to better bonding of enamel and dentin surfaces when prepared with air abrasion than with conventional carbide burs or acid etching.[20] It also helps in the removal of the smear layer, thereby improving the infiltration of adhesive systems into demineralized dentin, which may result in significantly higher bond strengths.[21] This is suggestive that air abrasion increases surface area for bonding without excessively depleting HAp in comparison to acid etching. Thus, based on the results of the present study, we can reject the formulated null hypothesis.

SAFC has a dual bonding mechanism; chemical bonding between the functional monomer and the HAp and micromechanical bonding between the polymerized SAFC and collagen fibers and smear layer of dentin.[1] Chemical bonding significantly improves bond strength and bond durability.[22] A lower microleakage level was obtained for Constic when compared to Dyad Flow in the study. Such findings can be interpreted with reference to the specific functional monomers that are product-dependent. Dyad flow and Constic are based on glycerol phosphate methacrylate (GPDM) and 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) monomers, respectively.

The “Adhesion-Decalcification concept” models the way specific functional monomers interact with HAp-based tissues. GPDM follows the decalcification route, while 10-MDP follows the adhesion route.[23] Hence, 10-MDP having a long and a hydrophobic spacer chain binds strongly to HAp by ionic bonds, thereby forming stable complexes of MDP-calcium phosphate salts with enhanced hydrolytic stability over time preventing biological degradation.[222324] In contrast, the shorter spacer chain and the higher hydrophilicity of GPDM may not favor stable monomer–calcium formation resulting in significantly more HAp demineralization. It produces and deposits an unstable complex of dicalcium phosphate dihydrate on HAp surface and when in contact with an aqueous environment will lead to gradual dissolution, thus deteriorating the interfacial integrity.[2223]

Further clinical trials are required to evaluate the reproducibility of the results of this in vitro study as it involved a static environment. The restorations in the oral cavity are subjected to a dynamic environment, where they are in constant contact with oral fluids and stresses from the masticatory loads. These conditions may act on the teeth-composite interface leading to deterioration of the bond and thereby the durability. Hence, shear bond strength test and cyclic loading under wet conditions would additionally enhance the assessment of composite adhesion to the teeth.

CONCLUSIONS

Under the limitations of the current microleakage study, it can be concluded that preliminary phosphoric acid etching increased the microleakage suggesting a negative effect on the quality of the seal while air abrasion reduced the microleakage and improved sealing ability in Dyad Flow as well as Constic.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.