Surface roughness and gloss retention of bioactive resin composite after simulated toothbrush abrasion

Background. Newly introduced bioactive resin composites are being used more often. Few studies have evaluated the influence of toothbrush abrasion on their surface characteristics. Methods. This study aims to assess the surface roughness (Ra) and gloss values (GU) of 3 bioactive composites and a conventional nanohybrid composite before and after simulated toothbrush abrasion. Five samples each of Filtek Z250 XT, Beautifil Flow Plus, Activa Presto and Predicta Bioactive Bulk were subjected to 10,000 cycles of simulated toothbrush abrasion. Ra and GU values were evaluated at baseline and after each 2,500 cycles. Results. Significant difference was found in both Ra and GU (P < 0.001). For each material, GU gradually decreased as the brushing cycles increased. GU values of Beautifil Flow Plus were significantly higher at the baseline, 2,500- and 5,000-cycles stages (P < 0.05). The GU values were 78.20 (7.20), 69.67 (6.17) and 63.30 (5.52) respectively. Activa Presto showed the lowest mean GU after 10,000 cycles. A significant increase in Ra at each of the measuring stages was observed in all materials compared to the baseline measurement (P < 0.001). No significant difference in Ra values of all four materials at the baseline and up to 5,000-cycles (P > 0.05). After 7,500 cycles, the mean Ra value of Activa Presto at 0.64 (0.14) was significantly higher than that of Z250 XT at 0.49 (0.03) (P < 0.001). At 10,000 cycles, Activa Presto had significantly higher Ra at 0.70 (0.10) when compared to Z250 XT at 0.52 (0.08), Beautifil Flow Plus at 0.56 (0.07) and Predicta Bioactive Bulk at 0.59 (0.10) (P < 0.001). Conclusion. All materials showed an increase in Ra and decrease in GU after simulated toothbrushing.


Introduction
Recent advances in technology and formulation of dental materials have promoted the development of advanced resin composites with advantageous properties.One of the greatest challenges in resin composite restorative materials is the leakage at the interface with the tooth surface.This is initiated by the inherent property of polymerization shrinkage, which is the primary cause of secondary caries and restoration failure [1].Increasing the demand for a resin composite restorative material that could replace missing tooth structure and possess a caries-inhibition property has led to the development of bioactive resin composites [2].Compared with the traditional resin composites, a significant characteristic of bioactive resin composites is the release of bioactive ions or molecules that could in theory suppress cariogenic bacteria, inhibit demineralization, or enhance remineralization [3,4].
In addition to the prevention of secondary caries, the long-term success of a restoration relies on several other factors such as mechanical properties, diminished wear, and esthetic properties such as color stability and gloss retention [5].However, oral environmental factors are known to affect the integrity and longevity of restorative materials [5][6][7].For instance, mechanical abrasion caused by toothbrushing is considered a critical factor affecting the restoration's surface roughness and gloss, which compromises the clinical performance of the restorations [7].
Achieving and maintaining a smooth surface with high gloss is essential for a successful esthetic restoration.Composite restorations with high gloss tend to have a more natural appearance and are indistinguishable from adjacent teeth structure.Gloss of the resin composite materials is directly related to a smooth surface texture of the restoration [6].In addition to its influence on the gloss, a smooth surface enhances the color stability and reduces the formation and accumulation of plaque, and therefore gingivitis and caries, as surface roughness (Ra) value of 0.2 μm is considered a threshold for bacterial retention [8].Furthermore, a smooth restoration surface is essential for the patient's comfort as alteration of 0.3 μm in the surface roughness is reported to be detectable by the patient [9].
Recently, injectable bioactive restorative materials with improved handling properties that allow homogeneous spread and better adaptation to the prepared cavity have been introduced.Beautifil Flow Plus is a fluoride-releasing flowable resin composite that can be used as restorative material for anterior and posterior cavities.This dental material is classified by its manufacturer as a giomer.Giomers are the product of combining traditional resin composite with glass ionomers, i.e., by incorporating pre-reacted glass filler particles into the bisphenol A-glydicyl methacrylate (bis-GMA) matrix of the resin composite [10].The resulting material offers caries protection via multiple-ion release along with the esthetic properties found in traditional dental resin composites [11].Beautifil Flow Plus's manufacturer claims that it has a low wear rate compared to hybrid composite, while maintaining a high luster and long-term esthetics over the life of the restoration [12].
Activa Presto is a universal nanohybrid bioactive composite manufactured by Pulpdent.Presto has a patented 'Crysta MCP technology' which is what makes it bioactive.According to the manufacturer, 'Crysta is the trade name for a powerful methacrylate-functionalized calcium phosphate (MCP) molecule that can be added to composites to spark mineralization of damaged tooth structure'.The manufacturer also states that Activa Presto has a continuous release and recharge of calcium, phosphate and fluoride ions.These minerals naturally occur in the saliva and are supplemented by dietary sources.Furthermore, the formulation of a patented rubberized resin is claimed to make this composite durable, and wear, chipping and fracture resistant, even in thin areas on bevel margins or high load bearing areas [13].
Predicta Bioactive Bulk is a dual cured, universal composite that is indicated for any class of restoration.The manufacturer claims that Predicta Bioactive Bulk releases both calcium and phosphate ions, while also releasing and recharging fluoride ions, aiming to stimulate mineral apatite formation and remineralization at the material-tooth interface.This nano-filled composite is claimed to have optimized optical characteristics, polishability and durability.
Despite these bioactive advantages, limited information is available in the literature about the mechanical and esthetic properties of these recently introduced restorative materials compared to conventional composite.Therefore, it is of clinical significance to evaluate the effect of abrasive wear on the attributes of surface roughness and gloss of these materials.

Method
This study evaluated the effect of simulated toothbrush abrasive wear on the gloss and surface roughness of three bioactive restorative materials and a conventional nanohybrid resin composite.The null hypotheses were as follows (1) there were no changes in the gloss and surface roughness of each material after toothbrush abrasion; (2) there was no difference in the gloss and surface roughness between the different materials at baseline or after brushing for different cycle lengths.A schematic illustration of the study procedure is presented in figure 1.

Sample preparation
A custom mold (8 × 2 mm) was used to create discs from four different restorative materials (n = 5), Filtek Z250 XT (3 M ESPE, St. Paul, MN, USA), Beautifil Flow Plus F00 (Shofu Inc., Kyoto, Japan), Activa Presto (Pulpdent corp., MA, USA), Predicta Bioactive Bulk (Parkell, Edgewood, NY, USA).After spraying mold release, a mylar strip with a glass slide were placed at the bottom of the mold and each restorative material was placed.The top surface was covered with another mylar strip and glass slide, and pressure was applied to extrude excess material and eliminate voids formation.Samples were then cured using a light emitting diode (LED) curing unit at 1,200 mW cm −2 (Mini LED Satelec, Satelec Acteon Group, Mérignac, France) according to the manufacturer's instructions, as presented in table 1. Sof-Lex Diamond Polishing System (3 M ESPE) attached to a straight handpiece rotating at a maximum of 12,000 rpm was used to polish the top surface of each sample.All sample preparation was performed by a single operator.

Gloss measurement
The gloss of the top surface of each sample was recorded at the baseline, after 2,500, 5,000, 7,500 and 10,000 brushing cycles using a gloss meter (IG-331 Gloss checker, Horiba, Kyoto, Japan) with an oval measurement area of 6 × 3 mm and at light incidence and reflection angles of 60°.Prior to measurement of each group, the device was calibrated using a provided calibration jig with a reference value of 90 gloss units (GU).At each measurement, the sample was positioned on a black cover to eliminate the effect of external light.For each sample, six GU values were obtained by rotating the sample at 90°and averaged to obtain a single value [6,7].

Surface roughness measurement
Surface roughness of the samples was measured with a contact roughness tester (Surftest SJ-210, Mitutoyo, Japan) according to the ISO 12179:2021 standard at the baseline and at four measuring stages, each of 2,500 cycles of toothbrushing simulation with a total of 10,000 cycles.At each measurement, three subsequent surface roughness values (Ra) were obtained from the top surface of each sample using a diamond tip stylus with tip radius of 2 μm and 60°angle at measuring force of 0.75 mN, a speed of 0.5 mm s −1 and a cut off value of 0.8 mm [14,15].The three values were averaged to obtain a single value.

Toothbrushing simulation
Following the baseline gloss and roughness measurements, samples were subjected to brushing abrasion using a brushing simulator device with eight test stations (SD Mechatronik, Feldkirchen-Westerham, Germany).Cylindrical acrylic blocks with a circular central well (Eco-cryl cold, Protechno, Vilamalla, Spain) were produced using a silicone mold.Samples were secured to the central wells of the acrylic blocks using vinyl polysiloxane impression material (Express XT Putty Soft, 3 M ESPE), and mounted to the test stations.Toothpaste slurry prepared from Colgate Fresh Confidence Gel toothpaste (Colgate-Palmolive, NY, USA) and deionized water with a volume ratio of 1:3 was placed over the samples.Toothbrushes (Tara medium toothbrush, Dammam, Saudi Arabia) were attached with 1 N load, and height and inclination of each toothbrush was adjusted so that head is in contact with sample surface.The machine was operated at a speed of 40 mm s −1 in a linear movement pattern for 10,000 cycles.After each 2,500 cycles, acrylic blocks were removed from the test station and rinsed thoroughly under running water.The samples were detached and allowed to air-dry and then subjected to gloss and roughness measurements.The toothbrushes were replaced with new ones every 2,500 cycles.

Scanning electron microscopy (SEM)
Upon completion of 10,000 brushing cycles as well as the gloss and roughness measurements for all material groups, selected samples from each group were carefully prepared for SEM analysis.Prior to imaging, the samples underwent gold-sputter-coating of 12-15 nm thickness to ensure surface conductivity and enhance imaging quality.Subsequently, the samples were inspected under a scanning electron microscope (SEM) (AURA100, Seron Technologies Inc., Uiwang-si, Korea) at ×100, ×500, and ×1,000 magnifications.The SEM was operated at a voltage of 18 kV with a current working distance of approximately 10 mm.

Statistical analysis
The data were subjected to statistical analysis using a statistics software (SPSS version 20, Chicago, IL, USA) with significance level set at P < 0.05.Shapiro-Wilk test was used to assess the normality of all quantitative variables for further choice of appropriate tests, and the variables were found to be normally distributed.Repeated measure mixed design ANOVA test was applied for both gloss and surface roughness as independent variables with number of cycles as within subjects' factor and type of material as between subjects' factor.For each material, one way ANOVA was applied for both gloss and surface roughness measurements to test the difference between each measuring stage.Bonferroni method was used as post hoc test for multiple comparisons.

Results
Mean and standard deviation of gloss of each composite material are shown in table 2 and figure 2. The mean and standard deviation of the surface roughness of each composite material are listed in table 3 and presented as a graph in figure 3. ANOVA test demonstrated a significant effect of toothbrushing on surface gloss and surface roughness values (P < 0.001).
For the surface gloss, toothbrushing significantly decreased the mean GU for all materials (P < 0.001).There was a significant difference at 2,500 cycles for all materials tested (P < 0.05).The decrease was not significant between 5,000 and 7,500 cycles measuring stages of Z250 XT, and between 7,500 and 10,000 ones in Predicta Bioactive Bulk (P > 0.05).
When the GU values of the groups were compared to each other, Beautifil Flow Plus showed statistically significant higher values at the baseline, 2,500-and 5,000-cycles stages (P < 0.05).Other material groups were not statistically different from each other at each of these measuring stages (P > 0.05).After 7,500 cycles, the mean gloss value of Predicta Bioactive Bulk was significantly lower than other groups (P < 0.001).However, after 10,000 cycles the mean value of Activa Presto was the lowest compared to other groups and the difference was significant when compared to Z250 XT (P < 0.001).
For the surface roughness, toothbrushing significantly changed the Ra value of all four materials (P < 0.001).When compared to the baseline, a significant increase in the surface roughness at each of the measuring stage was observed in all materials (P < 0.001).In Z250 XT, the difference between the brushing stages was nonsignificant, except between 2,500 and 7,500 cycles (P < 0.05).For Beautifil Flow Plus, 2,500 cycles of toothbrushing resulted in significantly higher Ra value compared to 5,000, 7,500 and 10,000 measuring stages (P < 0.001), and the difference between the latter groups was non-significant (P > 0.05).The difference between the four stages of brushing was non-significant in both Activa Presto and Predicta Bioactive Bulk groups (P > 0.05).
When the materials were compared to each other, their mean Ra values did not show a statistically significant difference at the baseline, 2,500-and 5,000-cycles measuring stages (P > 0.05).After 2,500 brushing cycles, the values of Predicta Bioactive Bulk and Activa Presto were the same (Ra = 0.665) while Beautifil Flow Plus had the highest mean Ra value (P > 0.05).Activa Presto and Z250 XT showed the same highest value after 5,000 cycles (P > 0.05).After 7,500 cycles, however, Activa Presto showed the highest mean roughness value, and the difference was significant to Z250 XT (P < 0.001).The mean Ra of Activa Presto continued to increase after 10,000 cycles to a value that was significantly higher than Z250 XT, Beautifil Flow Plus and Predicta Bioactive Bulk (P < 0.001).SEM images of each material after 10,000 brushing cycles at ×100, ×500 and ×1,000 magnifications are shown in figure 4. The Filtek Z250 XT sample (a, a 1 , a 2 ) showed a smooth surface and regular structure with some minor light/shallow scratch lines.Other materials samples presented increased amount of scratch lines and deep crevices on the surface (b, c, d).On higher magnifications, their respective surface appeared rough and filler particles of different sized bulging outwards were observed, especially on the surface of Activa Presto group (c 2 ).These findings were consistent with the results of surface roughness and gloss for the materials tested.

Discussion
In the ongoing quest for a dental material which not only restores existing tooth structure but enhances its strength and prevents caries, manufacturers have developed 'bioactive' dental resin composites.These materials aim to actively improve the caries susceptibility of the tooth structure after the filling is placed.The dental composites tested in this work are three newly introduced bioactive dental resin composites and Filtek Z250 XT, a commonly used universal nanohybrid direct restorative composite with no bioactive properties.
The smoothest restored surface has been reported to be the one in direct contact with a matrix strip [16].However, nearly all restorations require some surface adjustment after the final cure of a dental resin composite restoration due to the presence of high points, sharp areas, etc.As a result, surface irregularities appear, and may increase due to toothbrushing along with other patient factors.These irregularities may affect the clinical performance and longevity of the final restoration due to the presence of areas which cause gingival irritation, plaque retention, staining and/or recurrent caries [17,18].Thus, in attempt to simulate the clinical environment, the samples in this work were finished and polished prior to testing.All the samples were subjected to the same finishing protocol.Also, the force used in the toothbrush machine was standardized at 1 N to simulate toothbrushing force [19].The number of toothbrushing cycles was set to 10,000 to simulate one year of brushing [20].
Gloss and surface roughness are dependent on intrinsic and extrinsic material factors.Intrinsic factors are related to the material itself such as the matrix used, particle size, and filler loading.Extrinsic factors are related to the light curing mode as well as the finishing and polishing methods used [21].In this study, the extrinsic factors were standardized to allow for the intrinsic factors to be the only ones determining the difference in results.
In this work, simulated toothbrushing increased the roughness and decreased the gloss of all the composites tested.This is in agreement with several previous studies [21][22][23][24][25].The American Society for Testing and Materials defines gloss as 'angular selectivity of reflectance, involving surface-reflected light, responsible for the degree to which reflected highlights or images of objects may be seen as superimposed on a surface' [26].Gloss characterizes the evenness of the restored surface and is a feature which is easily seen and recognized by patients and dentists alike [27].The typically desired gloss ranges from 40 to 60 GU [28].All the materials tested had reading of 70 GU at baseline.Thus, they were all in the desired gloss range as defined by the American Dental Association.However, the gloss of all the materials significantly reduced with simulated toothbrushing.This aligns with the findings of other researchers [21,29].Even though there was a significant decrease in each material from baseline to 2,500, 5,000, 7,500 and 10,000 cycles, all the materials showed gloss readings in excess of 40 GU at each timepoint.Thus, from a clinical viewpoint, the restorative surfaces of all the materials tested would appear acceptable in terms of gloss up to a year from placement.
Beautifil Flow Plus showed significantly higher gloss than the other materials at baseline.This is most likely due to its smaller, more uniform average particles on the surface compared to the other materials tested.This trend continued until 5,000 toothbrushing cycles.At 10,000 cycles, Z250 XT had the highest gloss.This is most likely due to the larger size of bioactive materials' clusters.Moreover, pre-polymerized fillers do not salinize easily nor integrate fully into the resin matrix; thus, they detach from the matrix causing concavities and increased roughness after toothbrushing.Consequently, the abrasive force of the simulated toothbrushing would displace these particles, which would increase the surface roughness and decrease gloss [23].
Surface roughness was determined using a profilometer which calculated the numerical average of the surface roughness in micrometers (μm).The literature does not have an absolute consensus regarding the ideal Ra value for the surface of a direct composite restoration.A threshold Ra value of 0.2 μm was found to have an effect on plaque accumulation and staining in vitro in titanium implants [30].When discussing restorations, authors found that most patients do not detect roughness if the Ra falls between 0.25-0.5 μm, with some patients detecting it at 0.3 μm [9].Interestingly, the visual appearance of a smooth restorative surface is not necessarily linked to the aforementioned Ra values.It has been reported that a restoration appears visually smooth when the Ra is less than 1 μm [7].
In this study, all the materials showed acceptable Ra values (between 0.2-0.3μm) immediately after polishing with no significant differences between material groups.Once brushing began, however, the roughness of all the materials increased.This is due to the fact that there are both microscopic and macroscopic irregularities which are created on the brushed surface of the material due to the abrasive action of the toothbrush.These irregularities cause an increase in surface roughness [21].Nevertheless, the difference between the materials was non-significant at 2,500 and 5,000 cycles.Thus, all the materials showed a similar roughness for what equates to approximately six months of clinical service.On the other hand, after 7,500 cycles, Activa Presto was significantly rougher than the other materials tested.This trend continued at 10,000 cycles, with both Activa Presto and Predicta Bioactive Bulk being significantly rougher than Z250 XT and Beautifil Flow Plus.This is likely due to the difference in chemistry and filler particle sizes in the materials.Z250 XT has small filler particles, ranging between 0.02 and 3 μm, which may be more easily exfoliated during the abrasive wear caused by the toothbrush and slurry when compared to the larger, possibly more irregular shapes of the particles in Activa Presto or Predicta Bioactive Bulk [22].Another reason for the difference found in material roughness may be the difference in matrix formulation.Z250 XT and Beautifil Flow Plus utilize Bis-GMA as its base, characterized by its large molecular size and chemical structure, resulting in the formation of a rigid resin resilient to removal with abrasive forces [21].As bioactive materials are made to facilitate the diffusion of fluoride, calcium and phosphate, it stands to reason that their matrix would allow the influx and release of these materials while maintaining the overall material integrity, rather than allowing overall wear as was the case in Z250 XT [2].All the materials tested showed Ra 0.5 microns, which should be felt by the patient's tongue [9].
Within each material, there was a significant increase in roughness from the baseline to 2,500, 5,000, 7,500 and 10,000 cycles with the largest change seen at 2,500 cycles.This is in agreement with the work done by other researchers [7,31].All the materials had a decrease in Ra factors at 5,000 cycles compared to the Ra at 2,500 cycles, which indicates that the surface of the material appears smoother despite being subjected to more simulated brushing cycles.While this may appear paradoxical, it is likely due to the fact the entire surface was worn by 5,000 cycles compared to only certain parts of it being worn at 2,500.Thus, the depth of the irregularities has decreased.
SEM images are often used to envision the surface morphology of materials or restorations [7,16,25].The SEM analysis of the dental restorative materials subjected to 10,000 brushing cycles at varying magnifications provided valuable insights into their surface integrity and resistance to mechanical wear.Notably, the Filtek Z250 XT sample exhibited a smoother surface with a regular structure, albeit displaying minor light scratch lines.This can clearly be seen in figure 4 (a, a 2 ).This suggests a relatively high level of resistance to abrasion.Conversely, the other material samples displayed increased scratch lines and deeper crevices on their surfaces, indicating a greater susceptibility to wear and degradation under mechanical stress as seen in figures 4(b)-(d).The presence of filler particles bulging outwards underscores the importance of filler particle size and distribution in determining material performance.
Moreover, the observations made at higher magnifications further elucidate the surface characteristics of the tested materials.The rough appearance and protruding filler particles observed, especially in the Activa Presto sample, highlight potential areas of concern regarding long-term durability and wear resistance of this recently introduced bioactive restorative material.These findings emphasize the importance of filler particle characteristics, morphology, and distribution for structural integrity to ensure longevity and clinical success of dental restorative materials.
The clinical significance of the change in both surface roughness and gloss values is that the restoration will be felt by a patient's tongue and appear less lustrous.These changes were significantly apparent after 2,500 cycles, which is the approximate equivalent of three months in clinical service.What that translates to for a practitioner is that these restorations need to be polished to restore their lustre and smoothness.This is especially important in anterior restorations where both esthetics and comfort are of paramount important to patients.
As with any research done, this work has limitations.The first is that it is an in vitro study of dental materials.While in vitro studies standardize protocols and allow a controlled working environment, the oral cavity is a far more complex environment with multiple factors affecting the materials at once.Another limitation of this study is that it tested the materials using only toothbrushing with toothpaste at a set weight.In real-world situations, restorative dental materials are exposed to brushing along with a variety of different food, drinks, and masticatory forces.A third limitation of this study is that SEM images were taken at the end of the study, and they were the only alternative method of visualizing the roughness.Further work is required to determine exactly when the Ra of the surface begins to change and how much change occurs during toothbrushing.Such a work would include more SEM images along with alternative imaging techniques such as atomic force microscopy (AFM) or confocal microscopy.It would also incorporate a second restorative material, such as glass ionomer to serve as a positive control.Additional parameters, such as thermocycling and water storage, could be investigated to provide a more comprehensive understanding of surface roughness and gloss.

Conclusions
Within the limitations of this work, it can be concluded that after brushing, the nanohybrid resin composite Z250 XT presented the lowest Ra values and highest GU values when compared to the bioactive materials tested.All the materials showed clinically acceptable GU values after testing.

Figure 1 .
Figure 1.Schematic view of study methodology.

Figure 2 .
Figure 2. Bar graph representing mean gloss value (GU) with standard deviation of each material at each measuring stage of toothbrush abrasion; baseline, 2,500 cycles, 5,000 cycles, 7,500 cycles and 10,000 cycles.Horizontal lines through the bars indicate significant difference between denoted groups at each measuring stage.

Figure 3 .
Figure 3. Bar graph representing mean roughness value (Ra) with standard deviation of each material at each measuring stage of toothbrush abrasion; baseline, 2,500 cycles, 5,000 cycles, 7,500 cycles and 10,000 cycles.Horizontal lines through bars indicate significant difference between denoted groups at each measuring stage.

Table 3 .
Mean (and standard deviation) of surface roughness values (Ra) of each material at each measuring stage.In each column, values with the same superscript lower-case letters are not statistically significant.In each row, values marked by similar superscript uppercase letters are not significantly different.
aBTable2.Mean (and standard deviation) of surface gloss (gloss units, GU) of each material at each measuring stage.In each column, values with the same lower-case superscript letters are not statistically significant.In each row, values marked by similar superscript uppercase letters are not significantly different.