"Comparative evaluation of effect of toothbrush-dentifrice abrasion on surface roughness of resin composites with different filler loading:" An in vitro study.

OBJECTIVES: This in vitro study evaluated the effect of toothbrush-dentifrice abrasion on the surface roughness of two restorative posterior resin composites, Filtek Z250 and Z350 after simulated toothbrushing twice daily for a period of 3 months.
METHODS: All the specimens were polished and cleaned and surface topography was evaluated by Veeco di CP-II Atomic Force Microscope (AFM) at six different points; similarly, these specimens were again subjected to evaluation after simulated toothbrushing using dentifrice. The surface roughness evaluation was done for AFM images using software made available and the factor measured were average roughness (Ra) and maximum peak to valley distance R(p v). Data were normally distributed as tested using the Shapiro-Wilk W-test (P > 0.05). Therefore, analysis was performed using the parametric tests, i.e., independent "t"-test (for comparing two groups). The level of statistical significance was set at P < 0.05.
RESULTS: The mean change in Ra and the mean change in Rp-v for Z350 were less as compared to Z250, and this difference was statistically significant.
CONCLUSIONS: Within the limitation of the present study, it can be concluded that toothbrushing increased the roughness in Z250 in comparison to Z350. Copyright:

INTRODUCTION

The rising demands for esthetics united with marvellous improvement in adhesive dentistry have resulted in a progressively more extensive use of resin composite as restorative materials.[1] Restorative resin composites used for restoring all cavities in anterior and posterior teeth require smooth surface finish which is clinically important for long-lasting restoration in the oral cavity.[2] Surface quality of dental restorations is a vital factor in determining the success of the restorations. The surface roughness of restorative resin composite is mainly dependent by the size, hardness, and quantity of filler particles, which in turn affects the mechanical properties of the material. Surface roughness is one of the contributing factors for external discoloration of the composite restoration.[3]

The smooth surface of the restoration provides better esthetics and little debris accumulation. A rougher surface can lead to decreased gloss and discoloration of the material surface which further affects the physical and mechanical properties of restorative material.[4] Smooth surface adds to the patient's comfort as already the difference in surface roughness of 0.3 μm can be detected by the tip of the patient's tongue. Increasing surface roughness is correlated with amplified accumulation of plaque. The mean roughness of 0.2 μm is the crucial threshold assessment for bacterial retention.[5]

Restorative resin composites differ from each other in filler types, filler loading, and matrix monomer composition. Filler grain size, volume, distribution, and polymerization quality of the resins are presumably the chief wear confrontation factor to toothbrushing of composite resins.[6] Toothbrushing causes roughness on the surface of restorative resin composite by roughening the softer polymer matrix, leading to the harder reinforcing particles standing high in relief. This roughening effect from toothbrushing is also to a certain level due to the fact that the bristles do not abrade the surfaces as evenly as flat disks or rubber cups in the polishing procedure.[3]

The function of abrasive particles in dentifrices is to promote the whitening effect by eradicating extrinsic stains. Abrasiveness of the dentifrice can cause changes in the external roughness of some restorative resin composite and may consequently affect the long-term esthetic steadiness of the restorations.[3] Abrasivity measurements are obtained by Relative Dentin Abrasitivity (RDA). Different formulas of dentifrices present different RDA values. The RDA values may vary from 40 to 200, Less than 250 being the American dental association suggested limit.[4]

The aim of this in vitro study was to evaluate the effect of toothbrush-dentifrice abrasion on surface roughness of resin composites with different filler particles.

MATERIALS AND METHODS

Two posterior composites were used in this study, their composition is depicted in Table 1. The simulated brushing of the specimen was done using Colgate Total Plus Whitening Dentifrice [Table 2].

Table 1

Composition of the composites used in the study

Material	Shade	Type	Polymer	Filler content	Filler
FILTEK Z250 XT (3M ESPE, USA)	A2	Nanohybrid	Bis-GMA, Bis-EMA, UDMA	82% by weight (68% by volume)	Surface-modified zirconia/silica with a median particle size of approximately≤3 µ, nonagglomerated/nonaggregated 20 nm surface-modified silica particles
FILTEK Z350 XT (3M ESPE, USA)	A2	Nanofilled	Bis-GMA, Bis-EMA, UDMA TEGDMA, PEGDMA	72.5% by weight (55.6% by volume)	combination of nonagglomerated/nonaggregated 20 nm silica filler, nonagglomerated/nonaggregated 4-11 nm zirconia filler, and aggregated zirconia/silica cluster filler (comprised of 20 nm silica and 4-11 nm zirconia particles)

Bis-GMA: Bis phenol-A-diglycidylether dimethacrylate, Bis-EMA: Ethoxylated bisphenol -A-dimethacrylate, TEGDMA: Triethylene glycal dimethacylate, UDMA: Urethane dimethacrylate, PEGDMA: Polyethylene glycol dimethacrylate

Table 2

Composition of dentifrice used

Dentifrice	RDA	Constituents
Colgate Total Plus Whitening	180	Water, sorbitol, hydrated silica, sodium lauryl sulphate, copolymer, aroma, carrageenan, sodium hydroxide, sodium fluoride, sodium saccharin, and triclosan

RDA: Radioactive relative abrasion

Preparation of samples

Specimen size was standardized by preparing them in customized cylindrical plastic molds (5 mm diameter × 2 mm depth). Materials were manipulated according to the manufacturer's recommendations. The mold with specimen material covered with a Mylar strip on both sides was placed over the glass slab then pressed between two glass slides to remove excess flash. The glass slides were pressed steadily during the set to avoid the occurrence of air bubbles and to get a smooth surface. Specimens were primed at room temperature (23.5°C ± 2°C). The materials were polymerized by a light-emission diode light-curing unit VALO Ultradent using 40 s exposure to each specimen's top and bottom surface. The space between the light source and specimen was standardized by placing the curing light openly over the specimen. The light intensity of the curing light was checked regularly with the radiometer during specimen preparation which was constant at 700 mw/cm2.

The top surfaces of all the specimens were then sequentially polished with coarse followed by medium, fine, and super-fine Shofu Super-Snap polishing discs (San Marcos, USA) with slow speed handpiece under dry conditions.

Simulated toothbrushing

Mechanical brushing was carried with cross-action power (P&G's oral B Ohio, USA) soft toothbrush of samples using Colgate Total Plus Whitening Toothpaste (USA).

Testing of the samples

The specimens were cleaned after polishing and the specimen topography was evaluated by Veeco di CP-II Atomic Force Microscope (AFM) at six different points: two points at the center, two points at the periphery, and two points at mid-distance from the periphery to the center. Similarly, these specimens were again subjected to evaluation after simulated toothbrushing using dentifrice. In the AFM images, surface roughness analysis was done using software provided, and the following parameters were compared among specimens: average roughness (Ra) and maximum peak-to-valley distance (Rp-v) [Figures 1 and 2].

Figure 1

Atomic force microscope images of Z250 XT before brushing (a and c) and after brushing (b and d)

Figure 2

Atomic force microscope image of Z350 XT before brushing (a and c) and after brushing (b and d)

Statistical analysis

Data were normally distributed as tested using the Shapiro–Wilk W-test (P > 0.05). Therefore, the analysis was performed using the parametric tests, i.e., independent “t”-test (for comparing two groups). The level of statistical significance was set at P < 0.05 using Software version SPSS 21(IBN Corp,Armonk, NY,USA).

RESULTS

The mean (standard deviation [SD]) changes in Ra were compared in the two groups using independent Student's “t”-test. For Z350, the mean changes in Ra were less as compared to Z250, and this difference was statistically significant. Hence, toothbrushing on restorative resin composite Z350 results in significantly less change in Ra as compared to toothbrushing done on Z250 [Tables 3 and 4].

Table 3

Mean and standard deviation of Ra values of composites before and after brushing

	Mean±SD
	Presurface roughness reading	Postsurface roughness reading
Z350	28.67±6.15	143±37.84
Z250	27.84±5.52	180±22.11

SD: Standard deviation

Table 4

Comparison of groups with respect to Ra values using independent Student’s t-test

	Postsurface roughness reading - presurface roughness reading
	Mean±SD	t	P a
Z 350	114.33±42.39	1.838	0.00*
Z 250	152.16±72.3

aIndependent Student’s t-test, *Significance of relationship at P<0.05. SD: Standard deviation

The mean (SD) changes in Rp-v were compared in the two groups using independent Student's “t”-test. For Z350, the mean change in Rp-v was less as compared to Z250, and this difference was statistically significant. Hence, toothbrushing on composite Z350 results in significantly less change in roughness as compared to toothbrushing done on Z250 [Tables 5 and 6].

Table 5

Mean and standard deviation of Rp-v values of composites before and after brushing

	Mean±SD
	Presurface roughness reading	Postsurface roughness reading
Z 350	28.67±6.15	161.5±37.37
Z 250	27.84±5.52	214.26±56.02

SD: Standard deviation

Table 6

Comparison of groups with respect to Rp-v values using independent Student’s t-test

	Postsurface roughness reading - presurface roughness reading
	Mean±SD	t	P a
Z 350	132.83±65.56	4.356	0.02*
Z 250	186.42±88.34

aIndependent Student’s t-test, *Significance of relationship at P<0.05. SD: Standard deviation

DISCUSSION

Restorative materials used for restoration in teeth are exposed to various factors affecting their quality in the oral cavity. Among all the other factors, the most significant role is played by oral hygiene. The prophylactic home procedures produce effects like surface roughness, thus enhancing the bacterial growth and staining.[5]

In this present study, Filtek z350 showed better resistance to abrasivity of toothpaste. Filtek Z250 showed more surface roughness and discoloration. Mechanical brushing is suitable for simulating normal oral hygiene procedures to standardize brushing application, distance, and frequency of force on the specimens.[7] While in an estimate, a person brushes their teeth for 2 min, twice a day, each tooth experiences only a fraction of seconds. As an approximation, each tooth may be brushed for 8 s/day that is equivalent to each individual tooth for 4 s twice a day.[4] In the present study, the composite samples were brushed for 720 s equivalent to the revelation of restorative resin composite to the prophylactic home measures for a period of 3 months.

The relative dentin abrasivity (RDA) of the toothpaste is an additional variable that persuades the surface roughness of the materials. McCabe JF et al. showed that the RDA of the toothpaste is the reason for both the surface roughness and wear of dental materials.[8] In the current study, dentifrice used had a RDA value of 180, thus explaining the roughness formed in composites. According to Amaral et al.,[9] whitening dentifrices, such as the RDA 180, contain explicit chemical mechanisms in their formulation, which reduce staining irrespective of the corporal possessions on the material. According to Joiner et al.,[10] whitening dentifrices were planned to optimize cleaning and to diminish wear.

Recent restorative resin composite differs much in filler concepts and filler loading in addition to in matrix monomer composition. The resin matrix of Z250 have Bis GMA-Bisphenol A- glycidyl methacrylate, UDMA- urethane dimethacrylate, Bis EMA and the resin matrix of Z350 has Bis-EMA-ethoxylated bis phenol-A- Glycol dimethacrylate, UDMA and in addition TEGDMA-tetraethylene glycol dimethacrylate, PEGDMA-Polyethylene glycol dimethacrylate to adjust viscosity in its resin matrix. The Z250 is a nanohybrid posterior composite with surface-modified zirconia/silica with a median particle size of approximately ≤3 μ, nonagglomerated 20 nm surface-modified silica. Z350 is a nanofilled posterior composite with a combination of nonagglomerated 20 nm silica filler, nonagglomerated 4 to 11 nm zirconia filler, and aggregated zirconia/silica cluster filler (comprised 20 nm silica and 4 to 11 nm zirconia particles). Both filler grain volume and distribution and polymerization excellence of the resins are presumably of major importance for the resistance to toothbrushing of restorative resin composite. Therefore, it is important to illustrate the restorative resin composite characteristics along with toothbrushing effects to estimate possible relationships.[11]

The color change in Z250 specimens after brushing was due to matrix wear when there was outward removal of filler particles,[4] which was a physical action rather than a chemical reaction. According to Hu et al.,[12] subjects are vulnerable to shear forces on the worn surfaces. Due to the advanced modulus of elasticity of the particles in comparison with the resin matrix, particles are able to bear outsized loads. However, the bond of particles to the matrix in Z250 is not as strong as the stress produced by the filler particle rigidity, resulting in particle disintegration or disarticulation and material loss. Therefore, the restorative resin composite behavior was dissimilar during the replicated procedures.

Suzuki et al.[13] evaluated nanocomposite and nanohybrid resin for surface roughness by simulated toothbrushing over 50,000 cycles and concluded that there was significant differentiation in rates of coarseness of composite resins The nanocomposites showed lower Ra than nanohybrid restorative resin composite, which is similar to our study where Z250 shows more wear, less color stability than Z350.

These nanofilled composites also acquire differences in their organic formulations, which may lead to dissimilar mechanical performance.[14] The reduction in size and broad allocation of the nanofillers may amplify filler load, as a result, progress the mechanical properties of these novel materials, such as their polymerization shrinkage, tensile strength, compressive strength, resistance to fracture, and reduced wear.[15]

According to the manufacturer, the restorative resin composite Z350 XT is nanofilled contrary to Z250 which is nanohybrid. Z350 XT consists of an amalgamation of zirconia and silica nanoparticles in nanocluster plummeting the interstitial space sandwiched between the particles and increasing the quantity of load. The nanoparticles result in greater wear resistance and improved physical properties, which means that during the abrasion, these nanoclusters wear at a speed analogous to the immediate resin matrix. The result is a smooth, glossy, and durable polished surface.[16]

Garcia et al. in their study concluded that the grounds for less abrasion of Z350 is because of homogeneous distribution of precured silica particles in the organic matrix. In our study, Z350 exhibited the least roughness. This study also showed similar results when Filtek Z350 was compared to Filtek Z25017.

The Ra parameter and Rp-v was used in this study. Ra is a true amplitude measurement. Ra is the commonly employed parameter for roughness measurement of a flat surface.[3] AFM has become an important tool for imaging surfaces and analysis. AFM allows a three-dimension imaging at nanometric resolution and is budding as consistent in evaluation of surface roughness features of composites.[17] AFM can measure quantitative surface roughness with extremely high resolution (horizontal resolution of 0.2–1.0nm and vertical resolution of 0.02 nm); the Ra value of a specimen was defined as the arithmetic average height of roughness component irregularities from the mean line measured within the sampling length;[18] however, the only limitation in using AFM is surface roughness measurement using AFM which is restricted due to small scanning area.[3]

CONCLUSIONS

All restorative resin composites exhibit roughness after simulated toothbrushing, the filler technology in restorative resin composite may show variable results before and after simulated toothbrushing. The surface roughness of the restorative resin composite increased over a period of time when subjected to toothbrush-dentifrice abrasion and varied depending on the filler particle. Thus, it is concluded that Z350 has better wear resistance and smoother surface even after being subjected to toothbrushing as observed under AFM.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.