In vitro comparison of color stability and surface roughness of bulk fill flowable composite and universal packable composite

Specimen size and preparation

A total of 120 disc-shaped specimens were fabricated from four different types of composites materials (30 from each material) with standardized shade (A1): One universal packable composite: Filtek Z250 XT Nano Hybrid Universal Restorative “UNI” (3M ESPE, St. Paul, MN, USA), and three bulk filled flowable composites: G-aenial Flo X GC Flowable Composite “GAN” (GC, Tokyo, Japan), Filtek Bulk Fill Flowable Restorative “FIL” (3M ESPE, St. Paul, MN, USA), and SDR Plus Bulk Fill Flowable “SDR” (Dentsply Sirona, Konstanz, Germany).

Sample size was determined according to a previous study¹⁷ that used five different resin composites material, where the sample size was 25 specimens for each composite material that was tested for color stability after immersion in different drinks. In the present study, four different resin composite materials were used, accordingly the sample size was increased to 30 from each composite material.

All specimens are disc-shaped, with 10 mm in diameter and 2 mm thickness. A metal frame with inner plastic circular openings of the desired dimensions was used to fabricate the specimens. The process of fabricating the specimens included a glass slab of 2 mm thickness covered by a Mylar strip (SNA, Universal Strips, Germany). The metal mold was then placed over, and the composite material then added in each circular opening of the mold. Another Mylar strip and glass slab of 2 mm thickness were placed above the mold. Afterwards, composite was polymerized by using LED curing device (Mini LED, Acteon Satelec, Merignac, France) for 40 seconds. After curing, the Mylar strip and glass slab are removed and the samples were taken out from the mold.

Finishing and polishing

A custom-made holder of rubber-base material was fabricated and used to hold the specimens during the finishing and polishing process. Composite specimens were divided into two main groups in this study, each group was finished and polished by a either a Sof-Lex discs system (3M™ ESPE, St. Paul, MN, USA), or a Rubber stone system “RS” (Astropol, Ivoclar Vivadent AG, Schaan, Liechtenstein), starting from the most coarse to the extra fine bur. The SL system included using four discs (Course, Medium, Fine, Ultra-fine) and attached to mandrel on low-speed handpiece (Gentle-Power Lux 20LP; KaVo, Biberach, Germany). The RS system consisted of three burs (Gray, Green, Pink) in flat wheel shape. When using SL disc and mandrel engage in a low-speed handpiece with water spray at 15000 rpm in intermittent brushing movement with gentle pressure. Whereas, when using RS, a low-speed handpiece is used at 25000-30000 rpm in circular movement with gentle pressure. Both speed and pressure were increased gradually while moving toward finer burs in both systems.

Color stability and surface roughness assessment

Specimen measurements were performed after 24 hours from storing the specimens in room temperature (25-35°C). The color of the specimens was measured with a spectrophotometer (X-Rite, Grand Rapids, MI, USA) for digital color measurements. The machine is calibrated with black and white ceramic tiles as it compares colors by measuring the amount of light reflected from the specimen.²⁵ The color measurements were recorded by means of the CIELAB (Commission International de L’Eclairage L*a*b) color notation system.²⁵ ΔE was calculated using the following formula:

Where:

Δ represents the difference between the object being measured and its reference

L* represents lightness

a* and b* are the chromaticity coordinates

Also, a Profilometer (3D profilometer. Bruker, AZ. USA) machine was used as a 3D optical non-contact surface scan that is accompanied with Vision64 software to test surface roughness.²⁶ Measurements were taken from three random points in each specimen then the average was calculated.

The baseline measurements of all specimens were made (immediately after the finishing and polishing), and then after immersing in different staining solutions.

Immersion in solutions

The specimens were divided equally to three groups, each group was immersed in different solutions: Coffee “CF” (Nescafé^® Classic Instant Coffee), Cola “CL” (regular Pepsi^®), and Distilled Water “W”. Composite specimens were fixed to the container using utility wax covering the bottom surface of the specimens, each sample identified by a code that is written on the container from outside. W and CL were used directly for specimens’ immersion, while coffee required 150 ml of boiled water mixed with 1 full tablespoon (3 grams) of instant coffee to make 1 cup of coffee according to the manufacture instructions, then the solution was left to cool until it reaches room temperature (25-35°C) then used for specimens’ immersion. The acidity of solutions was measured with pH litmus paper strips, the pH for CL was 2, 6 for the CF, and 7 for W. All specimens were kept in an incubator (Thermo Scientific Heraeus Incubator) at (37°C) in the dark, and solutions were changed every day to avoid bacteria or yeast contamination. After 3 days, and 10 days of immersion, the samples were removed from the solutions, rinsed for 60 seconds with a high-pressure water airbrush (Minivapor 93, Effegi Brega, Piacenza, Italy), and air dried. The specimens of all groups were then re-evaluated for surface roughness and color stability after the immersion.

Statistical analysis

Results were presented as mean and standard deviations. Kruskal-Wallis H test was used to calculate the statistical significance between the groups. In post hoc analyses, Mann-Whitney tests was performed to ascertain which pairs of groups differ significantly from one another. Bonferroni correction was applied to the significances obtained from the Mann-Whitney test results. The significance was considered at p≤0.05. For the pairwise comparisons, adjusted significance levels were given by multiplying the unadjusted significance values by the number of comparisons. Statistical analysis was carried out using SPSS version 23.0 (SPSS Inc., Chicago, II, USA).

Color stability

Means and standard deviations (SD) of ΔE values are presented in Table 1.³⁷ Kruskal-Wallis H test was used to evaluate the color change among the tested materials, comparing the polishing systems and the immersing solutions, which revealed statistically significant differences (p<0.05). A Mann Whitney post-hoc test with Bonferroni correction manifested that Delta E was significantly less in FIL specimens when polished with SL and immersed in CF (p<0.017). Likewise, the specimens that were polished with RS system and immersed in CL have presented statistically significant difference between (UNI) and (GAN) (p<0.017) where the (UNI) showing less color change (Table 2). In addition, CF exhibits the greatest staining ability on materials, whereas CL exhibits the least staining ability on materials (Table 2).

Surface roughness

Among the three materials, (FIL) have manifested the least surface roughness (Table 3). Kruskal-Wallis H test showed statistically significant difference in the roughness between the materials, when polished using Sof-lex disc system immersed in water and coffee, and when polished with Rubber stone system immersed in coffee and Pepsi (p<0.05) with less roughness values in (FIL). Furthermore, statistical significance was found between G-aenial flowable and Filtek bulk fill when polished using Rubber stone system and immersed in coffee (p<0.017) (Table 4).

Color stability

Discoloration of tooth colored resin-based composite has been one of the most common reasons for restorations’ replacements, which consumes time and money.²⁷ There have been several studies that have discussed the effect of different beverages on the color stability on resin composites. However, not enough information was introduced regarding the color stability of bulk fill flowable composites.²⁸ In this study, four types of composites have been used, one universal packable and three bulk fill flowable composites (GAN, SDR, and FIL). Each group have been finished and polished with one of the two systems either SL and RS, then specimens were divided according to solutions (CF, CL and W) into three different groups. The solutions have been chosen as they are most commonly used. The pH of the solutions is constant and changed daily. Readings were taken in three different time intervals at baseline, 3 days and 10 days, using spectrophotometer which is a device used to measure the reflected and transmitted light from materials quantitatively, to measure the color stability.²⁹

Specimens that were immersed in CF have had the highest ∆E value compared to the other specimens with the same group, which was in line with previous investigations, where CF has stronger chromatogen than CL, this is due to the less polar yellow colorants in CF than the yellow colorants of CL. The absorption and adsorption of CF pigments into the organic phase of resin based composites is probably due to the compatibility of the polymer phase with yellow colorants of CF. In addition to that, CF has large amounts of staining agents like Gallic acid which facilitate the staining.^21,28 Also, a slight increase in ∆E was noticed in two of the groups that were immersed in water which have been noticed in previous studies due to degree of water absorption and the hydrophilic/hydrophobic nature of the resin matrix. The absorption of water mainly happens as a direct absorption of resin matrix.²⁷ Kruskal Wallis H test manifested no statistically significant difference in the color between the materials when polished using SL system or RS system immersed in W, CF, or CL at 3 or 10 days (p>0.05). While using the same test in between the polishing systems and the immersing solutions regardless of time displayed, a statistically significant difference between the materials was noticed when polished with SL system and immersed in CF, whereas the specimens that are polished with RS system have exhibited a color change when immersed in CL regardless of time (p<0.05). According to the readings of Table 2, CF exhibits the greatest color change on materials whereas, CL exhibits the least color change on materials.

Previous studies showed that when ∆E vales are between 1.0-3.7 color change is visually detectable by the naked eyes.^30,31 Also, it was agreed by multiple former studies that if ∆E ≥ 3.3, it considered clinically unacceptable and that was applied to this study’s results.^31,32 In the current study all four materials ∆E where ≥ 3.3 except U that was polished with SL system at 3 days of immersion, which lead to clinically unacceptable color stability outcomes regarding the tested materials to be used for direct restorations especially in anterior teeth. However, the difference was statistically not significant (p>0.05).

Surface roughness

Throughout years companies have been working on improving the surface roughness of composites as studies have shown how it affects the final results aesthetically and biologically. Surface roughness has been related to filler particle size, as the smaller the filler size will result in less porosity, and better polishability, hence less surface roughness.³³ Increased surface roughness leads to food accumulation, inflammation of the surrounding gingiva and staining of the composite material itself.³³ In our study a profilometer 3D device was used to measure surface roughness with four types of composite materials and two types of the most commonly used polishing systems RS system and SL system. A Kruskal Wallis H test had revealed that (FIL) material had the least surface roughness among the tested materials in spite of other factors. Differing from Karadaş et al. study,³⁴ where (FIL) wasn’t the material with the least surface roughness and that may be due to many factors such as different polishing systems, immersing solutions, and bulk fill materials from other companies that were included. Moreover, surface roughness is believed to be mainly contributed by the material composition, for instance the filler particle size and the filler weight content as many studies have shown.³³

In our study, the 1^st null hypothesis “there is no significant difference between bulk fill flowable composites (GAN, SDR and FIL) and (UNI) in color stability” was approved, as there was no significant statistical difference between the materials when other factors are excluded. This is in agreement with Bahbishi N et al.’s³⁵ study where there was no major difference between the Universal composite and bulk fill composite tested. On the other hand, Silva MF et al.’s study³⁶ found that Universal composite had better color stability than bulk fill composites. Our 2^nd null hypothesis “there is no significant difference between bulk fill flowable composites (GAN, SDR and FIL) and (UNI) in surface roughness” has been disproved, as there was statistically significant difference between the materials with (FIL) displaying the least surface roughness. In addition to that, our study has approved that there was no significant correlation between polishing systems and color stability.

There are certain limitations of the present study that are important to mention. One of the main limitation was that it was performed in laboratory conditions, which differs from the oral environment. Furthermore, the composite specimens were prepared in the form of a discs, however, the restoration shapes and surfaces differ in clinical conditions.

The manufacturer instructions of all four materials used in this study claimed that the materials are indicated to be used as direct dental restorations in posterior teeth, and anterior teeth in cases like Class III and V that commonly demands higher aesthetic features in the dental restorative material. Within the limitations of this study, our results showed no statistically significant difference between the tested materials in terms of color stability and surface roughness, except of FIL. However, it is recommended to have a more well-controlled laboratory environment for further testing the aesthetic and mechanical properties in order to suggest the use of the bulk fill flowable composite materials as final restorative treatment in the anterior teeth.