Scholarly article on topic 'Polishing mechanism of light-initiated dental composite: Geometric optics approach'

Polishing mechanism of light-initiated dental composite: Geometric optics approach Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Yu-Chih Chiang, Eddie Hsiang-Hua Lai, Karl-Heinz Kunzelmann

Background/Purpose For light-initiated dental hybrid composites, reinforcing particles are much stiffer than the matrix, which makes the surface rugged after inadequate polish and favors bacterial adhesion and biofilm redevelopment. The aim of the study was to investigate the polishing mechanism via the geometric optics approach. Methods We defined the polishing abilities of six instruments using the obtained gloss values through the geometric optics approach (micro-Tri-gloss with 20°, 60°, and 85° measurement angles). The surface texture was validated using a field emission scanning electron microscope (FE-SEM). Based on the gloss values, we sorted polishing tools into three abrasive levels, and proposed polishing sequences to test the hypothesis that similar abrasive levels would leave equivalent gloss levels on dental composites. Results The three proposed, tested polishing sequences included: S1, Sof-Lex XT coarse disc, Sof-Lex XT fine disc, and OccluBrush; S2, Sof-Lex XT coarse disc, Prisma Gloss polishing paste, and OccluBrush; and S3, Sof-Lex XT coarse disc, Enhance finishing cups, and OccluBrush. S1 demonstrated significantly higher surface gloss than the other procedures (p < 0.05). The surface textures (FE-SEM micrographs) correlated well with the obtained gloss values. Conclusion Nominally similar abrasive abilities did not result in equivalent polish levels, indicating that the polishing tools must be evaluated and cannot be judged based on their compositions or abrasive sizes. The geometric optic approach is an efficient and nondestructive method to characterize the polished surface of dental composites.

Academic research paper on topic "Polishing mechanism of light-initiated dental composite: Geometric optics approach"

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Journal of the Formosan Medical Association (2015) xx, 1-8

ORIGINAL ARTICLE

Polishing mechanism of light-initiated dental composite: Geometric optics approach

Yu-Chih Chiang a b *, Eddie Hsiang-Hua Laia c, Karl-Heinz Kunzelmann b

a School of Dentistry and Graduate Institute of Clinical Dentistry, National Taiwan University and National Taiwan University Hospital, Taipei, Taiwan

b Department of Operative Dentistry and Periodontology, Dental School of Ludwig-Maximilians-University Munich, Munich, Germany

c Department of Dentistry, National Taiwan University Hospital, Hsin-Chu Branch, Hsin-Chu, Taiwan Received 14 October 2015; received in revised form 26 October 2015; accepted 29 October 2015

KEYWORDS

FE-SEM;

geometric optics; hybrid dental composites; nondestructive

testing; polish

Background/Purpose: For light-initiated dental hybrid composites, reinforcing particles are much stiffer than the matrix, which makes the surface rugged after inadequate polish and favors bacterial adhesion and biofilm redevelopment. The aim of the study was to investigate the polishing mechanism via the geometric optics approach.

Methods: We defined the polishing abilities of six instruments using the obtained gloss values through the geometric optics approach (micro-Tri-gloss with 20°, 60°, and 85° measurement angles). The surface texture was validated using a field emission scanning electron microscope (FE-SEM). Based on the gloss values, we sorted polishing tools into three abrasive levels, and proposed polishing sequences to test the hypothesis that similar abrasive levels would leave equivalent gloss levels on dental composites.

Results: The three proposed, tested polishing sequences included: S1, Sof-Lex XT coarse disc, Sof-Lex XT fine disc, and OccluBrush; S2, Sof-Lex XT coarse disc, Prisma Gloss polishing paste, and OccluBrush; and S3, Sof-Lex XT coarse disc, Enhance finishing cups, and OccluBrush. S1 demonstrated significantly higher surface gloss than the other procedures (p < 0.05). The surface textures (FE-SEM micrographs) correlated well with the obtained gloss values. Conclusion: Nominally similar abrasive abilities did not result in equivalent polish levels, indicating that the polishing tools must be evaluated and cannot be judged based on their compositions or abrasive sizes. The geometric optic approach is an efficient and nondestructive method to characterize the polished surface of dental composites.

Copyright © 2015, Formosan Medical Association. Published by Elsevier Taiwan LLC. All rights reserved.

Conflicts of interest: The authors have no conflicts of interest relevant to this article.

* Corresponding author. No.1, Changde St., Zhongzheng Dist., Taipei City 10048, Taiwan. E-mail address: munichiang@ntu.edu.tw (Y.-C. Chiang).

http://dx.doi.org/10.1016Zj.jfma.2015.10.010

0929-6646/Copyright © 2015, Formosan Medical Association. Published by Elsevier Taiwan LLC. All rights reserved.

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Y.-C. Chiang et al.

Introduction

Because of esthetic demand, hybrid dental composites are popular for many dental applications. Inadequate polishing of composites can increase surface roughness and plaque accumulation.1,2 The disinfected composite surface favors secondary bacterial adhesion and biofilm redevelopment even though the plaque was removed.3 Furthermore, polymerization shrinkage of light-initiated dental composites will cause intrinsic stress within the tooth cavity.4,5 Inadequately polishing the composite surface can damage the margin, and the intrinsic stress can cause marginal gap formation, which, consequently, will increase the risk not only of recurrent caries but also of periodontal disease.2 Therefore, optimal surface polishing of a hybrid dental composite is important for the longevity of the restoration and for healthy adjacent tissues.

To appropriately perform polishing procedures, the characteristics of the polishing instruments that are often used and the mechanisms of rubbing on resin composite surfaces must be explored. Surface quality is an essential parameter that represents the effects of the finishing and polishing procedures of dental restorations.6 The surface quality of dental composites can be managed using various techniques. In the early 1970s, the original geometric optics assessment of gloss surface was developed by Budde at the National Research Council of Canada (NRC),7 and it is currently maintained by the National Institute for Standards and Technology (NIST) and NRC.8,9 According to the standard measuring methods of the American Society for Testing and Materials (ASTM) D523 and the International Organization for Standards (ISO) 2813, this standard gloss measurement defines three illumination angles (i.e., 20°, 60°, and 85°) to measure the surfaces of specimens.10,11

Gloss, similar to surface roughness, is considered to be a useful surface quality parameter and is obtained using the specular reflectance of incident light. Surface gloss is represented by the amount of incident light that is reflected at the specular reflectance angle of the mean of that surface.11 Therefore, specular gloss is proportional to the reflectance of the surface, which can be expressed by the Fresnel equation, as follows12:

' cos i

i — \f\

m2-sin

Vcosi+

\Jm2 —

sin2 A /m2cos i — \/m2 — s in 2 A sin2 if \m2cos i +V m2 — sin2 iJ

where Rs is the specular reflectance; l0 is the intensity of the incident unpolarized light; i is the angle of incidence; lr is the intensity of the specular reflection of the beam of light, and m is the refractive index of the surface specimen.

The main factors that affect gloss are the topography of the specimen surface, the angle of incident light, and the refractive index of the material. To characterize the surface properties of dental composites, current studies commonly employ methods such as atomic force microscopy, scanning electron microscope/field emission scanning electron microscope (SEM/FE-SEM), profilometry, and glossmeters.6,13,14 Atomic force microscopy and

profilometry can detect the surface in three dimensions but at a different resolution level. Using the atomic force microscopy, only surface areas in the order of magnitude of 100 mm2 are possible. With such a small area, important details outside of this area might be missed. In addition, a hydrate film on the sample surface can obscure structures, whereas coarse scratches or holes from filler plucking can introduce measurement errors or even damage the Atomic-force microscopy (AFM) tip in a worst-case scenario. By contrast, the profilometer uses a diamond tip with a tip radius no smaller than 5 mm; sometimes, even larger tip radii are used. The tip of a profilometer works like a longpass filter, which only allows the evaluation of surface details larger than the tip radius. In addition, depending on the applied load, the diamond tip can change the surface during the measurement process.15,16

Many researchers have studied the polishability of different finishing and polishing systems on the surfaces of various commercial dental composites. Usually, the polishing systems have been evaluated according to the manufacturers' recommendations. However, the manufacturers rarely support their recommendations with objective investigations that have proven the suggested protocol to be superior to others. Sometimes, the number of steps in such a sequence is rather complicated and does not satisfy the clinical demands for efficacy and cost effectiveness. Therefore, it would be helpful to propose efficient polish sequences for clinical use and to supply quantitative proof for the suggested procedure. Combining the use of a glossmeter with three measurement angles using the geometric optics approach and FE-SEM micrographs to interpret surface textures on the resin composites could supply the data necessary to perform quantitative evaluations and to identify the abrasive mechanism. The objectives of this study were: (1) to determine the surface quality and explore the polish mechanism that could be achieved by common polishing instruments with the aid of both a specular glossmeter and FE-SEM; and (2) to test the hypothesis that different polishing sequences with similar abrasiveness would achieve the same gloss level on a dental hybrid composite surface.

Methods

Specimen preparation

A light-cured dimethacrylate-based nanohybrid composite (Tetric EvoCeram, Ivoclar Vivadent, GmbH, Schaan, Liechtenstein) was used in this study because this material has been commercially successful in Europe. In addition, the composition of this material has challenged the polishing sequence (Table 1). By mixing the inorganic fillers into the organic matrix, Tetric EvoCeram includes part of the fillers as ground, prepolymerized hybrid composite filler particles. Square blocks of Tetric EvoCeram nanohybrid composite measuring 24 mm in length, 12 mm in width, and 2.5 mm in thickness were prepared. All specimens were irradiated using a light-curing unit (Dentacolor XS, Kulzer & Co., GmbH, Wehrheim, Germany) through Mylar strips for 180 seconds. To obtain an optimally polished surface as a control, the composite block surface was polished with 320-

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Table 1 Study materials.

Nanohybrid composite

Composite Composition (% weight) Batch no. Manufacturer

Tetric EvoCeram (A3) Dimethacrylates (17), barium glass 1 mm (50.6), Ba-Al-F-B-Silicate 1 mm (5), SiO2 40 nm (5), mixed oxide 0.2 mm (5), YbF3 (17), copolymer (prepolymerized filler) (47) JO 9286 JO 7188 Ivoclar Vivadent GmbH

Polishing systems

Code Polishing system Polishing tool Composition Manufacturer

1 Sof-Lex Sof-Lex XT Disc, coarse

("brown")

2 Sof-Lex Sof-Lex XT Disc, fine

("orange")

3 Enhance Enhance finishing cup

4 Enhance Prisma Gloss-composite

polishing paste

5 Enhance Prisma Gloss-extra fine

composite polishing paste

6 OccluBrush OccluBrush

1/2" Pop-on disc, 85/Bx, coarse aluminium oxide, 85/Bx 2382C

1/2" Pop-on disc, 85/Bx, fine aluminium oxide, 85/Bx 1982F

Polymerized urethane dimethacrylate resin, aluminum oxide, silicon dioxide 1-mm aluminum oxide (<65%), glycerine (<50%), hydrophobic amorphous fumed silica (<5%) 0.3-mm aluminum oxide

Special fibers impregnated with abrasive silicon carbide particles

3M ESPE/St. Paul, MN, USA 3M ESPE/St. Paul, MN, USA Dentsply/Caulk/DE, USA Dentsply/Caulk/DE, USA Dentsply/Caulk/DE, USA KerrHawe, SA

grit to 4000-grit Sic abrasive paper under running water using a polishing device (EXAKT, Apparatebau GmbH, Norderstedt, Germany).

The gloss values of the optimally polished surfaces from all specimens were individually measured at baseline using a micro-TRI-gloss specular glossmeter (BYK-Gardener GmbH, Geretsried, Germany) prior to the surface treatment with the polishing instruments. The micro-TRI-gloss consists of three reflectometers differentiated by the angle of incidence of the illuminating light-emitting diode (LED). The geometries are set according to ISO/CEN (European Committee for Standardization) standards at 20°, 60°, and 85°.10 The reflectometer values (RV-20, RV-60, and RV-85; RV = reflectometer value) were obtained to define the arithmetic mean of the surface gloss (Figure 1).

Surface gloss of geometric optics measurement

To define the polished surface characteristics of the nano-hybrid composite, three polishing systems consisting of six individual instruments were evaluated in this study (Table 1): Sof-Lex XT coarse disc; Sof-Lex XT fine disc; Occlu-Brush; Enhance finishing cup; a foam polishing cup with Prisma Gloss composite polishing paste; and a foam polishing cup with Prisma Gloss extra-fine composite polishing paste. These instruments were selected based on recommendations found in the literature or those made during continuing education courses taught by eminent lec-turers.1,17,18 Composite blocks were divided into six groups to receive the six tested polishing instruments, using a slow-speed handpiece (4000 rpm) for 60 seconds. Representative specimens were observed using a field emission scanning electron microscope (FE-SEM, ZEISS GEMINI, SUPRA 55VP, Carl Zeiss SMT AG, Oberkochen, Germany).

Simplicity of polishing procedures

Based on the obtained surface gloss values and the surface textures of the FE-SEM micrographs, we grouped the polishing instruments based on their abrasive ability into three grades: coarse, intermediate, and high gloss. Based on these three grades, three simplified polishing sequences were proposed to test the hypothesis. All specimens were preroughened using a coarse abrasive instrument to obtain baseline measurements. The FE-SEM micrographs were examined to characterize the abraded surfaces of the different polish sequences.

Statistical analysis of the gloss values was performed to evaluate the differences among these sequences using oneway analysis of variance, followed by Tukey test at a = 0.05.

Results

Surface gloss of individual polishing steps

The arithmetic gloss values at 20°, 60°, and 85° (RV-20, RV-60, and RV-85) for the polished surfaces of the nanohybrid composite with various surface treatments are presented in Figure 2. The average coefficient of variation was 9.1%, indicating that the measurement method was quite reproducible. However, the coefficients of variation for the Enhance finishing cup and Sof-Lex XT coarse disc polishing tools were higher, with maximums of 13.4% and 24.7%, respectively. The missing difference (p > 0.05) between the optimal polish and OccluBrush gloss values demonstrated that they had similar polishing abilities. There were no statistically significant differences between the Sof-Lex

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Figure 1 Illustration of polished surface of hybrid dental composite. (A) Light cured hybrid dental composite. (B) Abraded surface of hybrid composite. (C) Geometric optics assessment (20°, 60°, and 85° measurement angles) of polished surface of hydrid composite and surface texture validation.

XT fine disc and Prisma Gloss polishing paste groups at RV-20 or RV-85 (p > 0.05). The abrasive ability of each instrument could be described as follows: Sof-Lex XT coarse disc (coarse) > Enhance finishing cup (intermediate) > Sof-

Lex XT fine disc (intermediate) = Prisma Gloss polishing paste (intermediate) > Prisma Gloss extra-fine polishing paste (high gloss) > OccluBrush (high gloss) = optimal polish.

Figure 2 Gloss levels of the hybrid composite surfaces polished using individual polishing instruments (value = mean ± standard deviation, n = 8/group). The same superscript letter indicates no significant difference (p < 0.05, analysis of variance and Tukey honestly significance difference test).

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Polishing mechanism of dental composite 5

Simplified polishing sequences

Three simplified polishing sequences were proposed based on the defined abrasive ability: Sequence 1 (S1), Sof-LexXT coarse disc, Sof-Lex XT fine disc, and OccluBrush (in that order); Sequence 2 (S2), Sof-Lex XT fine disc, Prisma Gloss polishing paste, and OccluBrush (in that order); and Sequence 3 (S3), Sof-Lex XT fine disc, Enhance finishing cup, and OccluBrush (in that order) (Table 2).

The means and analyses of variance for the arithmetic gloss values of the composite surface (polished with the proposed simplified polishing sequences) are shown in Table 3. The composite surfaces polished with S1 demonstrated the highest gloss values at each illumination angle of the glossmeter (e.g., 50.84 at RV-60), and significant differences with both S2 (e.g., 33.05 at RV-60) and S3 (e.g., 32.58 at RV-60) (p < 0.05) could be calculated. In S1 and S2, the largest gloss change (%) occurred in the intermediate polishing stage, whereas the difference at the final high gloss polishing stage was less pronounced.

FE-SEM examination of simplified polishing sequences

Under FE-SEM evaluation, most of the wide scratches caused by the Sof-Lex XT coarse disc were removed by the Sof-Lex XT fine disc in S1, and a flat composite surface was present following the final stage (Figure 3A—3C). In S2, the Prisma Gloss polishing paste removed some of the scratches caused by the Sof-Lex XT coarse disc, and fewer scratches were still present after the final polish stage (Figure 3D—3F). Irregular scratches and a rougher surface remained after the final polishing stage of S3 (Figure 3G—3I).

Discussion

Silikas et al19 indicated that the 20° angle gloss measurements were more sensitive than the 60° angle measurements in demonstrating gloss differences. ISO 2813 states that 60° geometry is applicable to all semigloss surfaces, however, for very high gloss and near-matte surfaces, 20° or 85° might be more suitable. The 20° geometry is believed to result in improved differentiation between high-gloss surfaces. The 85° geometry should allow for improved differentiation between low-gloss surfaces.10 In our study, the appearance of the baseline surface quality qualified it for the 20° measurement protocol. However, after using the

polishing instruments, the quality of the gloss changed; thus, the 60° geometry would have been more appropriate. The standard specifications do not mention the protocol that should be applied in cases of gloss category changes, which was the main reason why we presented the data from all three evaluation geometries. During our study, we realized that although the glossmeter evaluation was a highly standardized method that allowed for the rapid comparison of different surface qualities, it was not the perfect optical method for dental samples due to the limited measurement angles. A better alternative than gloss measurements at three angles would have been to evaluate all possible angles for incident light rays and also for reflected light, such as that process used in the experimental setup, which was used to determine the bidirectional reflectance distribution function.20 Unfortunately, such a setup was not currently commercially available, nor do measurement standards currently exist.

In our study, we polished the composites in the best possible manner; then, we used the individual polishing instruments (the "top-down" approach). This sequence was used to determine the deepest scratches caused by the given instrument. If we started with a smooth surface, then the instrument under investigation caused the visible scratches. If we had used a "bottom-up" approach and started with a coarse surface, we could not have determined which instrument caused a scratch. Clinically, a bottom-up approach is the only method, however, we wanted to group the instruments with our setup first and then apply our sequences to a rough surface.

The composite surface managed with the S1 final stage showed the highest gloss values and the smoothest surface represented (p < 0.05; Table 3), reflecting the FE-SEM examination (Figure 3C). The gloss value changes (85%, 77%, and 50%) in the intermediate stage of the S1 group (Sof-Lex XT fine disc) also revealed statistically significant differences (p < 0.05) among the proposed simplified groups. The intrinsic elasticity of the disc, based on its diameter and thickness, ensured a constant pressure during its application. Using fresh discs for each sample and gently flattening the waves caused by the Sof-Lex XT coarse disc achieved reproducible, smooth surfaces. This result could be verified using the glossmeter and also with the FE-SEM observations.

The composite surface following the use of the Enhance polishing system, which consists of a foam cup for applying the polishing paste, showed comparable polishing quality. However, in the fine polishing paste group (Prisma Gloss polishing paste), the outline of the harder glass fillers was more exposed than in the other polishing groups. This

Table 2 Proposed modified polishing sequences.

Sequence Optimal polish Preroughening (baseline) Intermediate stage Final stage

No. 1 (S1) OP / Sof-Lex XT coarse disc / Sof-Lex XT fine disc / OccluBrush

No. 2 (S2) OP / Sof-Lex XT coarse disc / Prisma Gloss Polishing Paste / OccluBrush

No. 3 (S3) OP / Sof-Lex XT coarse disc / Enhance Finishing Cup / OccluBrush

Optimal polish was obtained by polishing the resin block surface with 320-grit to 4000-grit silicone carbide abrasive paper under running water at a rotation speed of 500 rpm and a vertical load of 4 N. OP = optimal polish surface.

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Table 3 Changes in gloss values at 20°, 60°, and 85° for the three modified polishing sequences (N = 8).

Reflector Modified Gloss after $Gloss after 1st $1st Gloss UGloss after 2nd U 2nd Gloss

angle ( + ) sequences roughening polish change (%) polish change (%)

Mean (SD) Mean (SD) Mean (SD)

RV-20 S1 0.97 (0.17)a 6.34 (1.19)a 85 15.98 (1.89)a 60

S2 1.06 (0.13)a 3.72 (0.53)b 72 7.02 (0.81)b 47

S3 0.99 (0.13)a 1.68 (0.38)c 41 6.32 (0.64)b 73

RV-60 S1 6.85 (1.57)a 30.01 (4.65)a 77 50.84 (3.57)a 41

S2 8.50 (2.62)a 23.21 (2.60)b 63 33.05 (3.66)b 30

S3 6.73 (1.35)a 13.64 (3.08)c 51 32.58 (3.26)b 58

RV-85 S1 26.46 (7.15)a 52.97 (2.95)a 50 56.88 (3.06)a 7

S2 29.34 (5.72)a 45.73 (4.67)b 35 46.63 (4.51)b 2

S3 26.65 (6.32)a 40.41 (5.49)b 34 45.55 (4.59)b 5

The mean difference was significant at p = 0.05 (Turkey honestly significant difference test). Different superscript letters indicate significant difference in each test reflector angle (RV-20, RV-60 or RV-85).

$ The first polish indicates the polishing stage from the Sof-Lex XT coarse disc to the intermediate step. First gloss change indicates the gloss change from the 1st polishing.

U The second polish indicates the polishing stage from the intermediate to the final step, and the second gloss change indicates the gloss change from the second polishing.

finding could be explained by the softer resin matrix being subjected to preferential reduction between the harder glass fillers during the polishing procedures.21'22 The soft foam was easily deformed during polishing and brought the Prisma Gloss extra-fine polishing paste in close contact with the entire surface, even between larger fillers, which is also why we chose Prisma Gloss polishing paste with larger particles (1 mm aluminum oxide) for the S2 group to abrade both the fillers and resin matrix on a preroughened composite surface (Figure 3D). For the surface gloss evaluation, the S2 final polish step (2nd gloss change) obtained the least amount of gloss change of all three reflectance angles (Table 3), however, this result did not indicate the worst surface quality. Compared to S3, the FE-SEM examinations showed that S2 demonstrated flatter surfaces in both the intermediate and final steps (Figure 3C and 3F) because the intermediate instrument of S2 was more effective than S3; therefore, there was less work for the final instrument. This phenomenon could also be explained by the fact that the first gloss change of S3 demonstrated fewer polish effects than S2.

For the S3 polishing regime, the hybrid composite surface polished with the Enhance finishing cup was difficult to smooth with the OccluBrush due to the resulting deeper and broader scratches. Figure 3H shows that the irregular abrasive particles embedded in the Enhance finishing cup measured approximately 30—50 mm. The size and hardness of these particles (Figure 3H) indicated that the Enhance finishing cup could reduce deeper scratches from the Sof-Lex XT coarse disc, however, it also produced rather deep scratches by cutting both the matrix and particles. On the Enhance finishing cup's abraded composite surfaces, some prepolymerized fillers showed signs of fracture within the filler, and glass fillers were plucked from the resin matrix. The challenge included polishing both the glass and pre-polymerized fillers, which have a slightly different elastic modulus than the embedding composite material. This mismatching of mechanical properties influenced the surface quality after polishing. Enhance finishing cup with

elastic binding deforms on contact with the surface, which is an advantage when polishing complex tooth shapes, such as the occlusal surfaces of the posterior teeth. However, the deformation of the cup was also a disadvantage because harder prepolymers could withstand the polishing process more than the embedding matrix. Therefore, the prepolymers would be noticeable, sometimes fractured, plucked out, or immediately above the level of the embedding matrix when such elastic, soft polishers were used. After the interfaces of the fillers and resin matrix were exposed or even crushed, they became weak points that could accelerate the aging and wear of the composite, and even change the color due to staining and plaque remaining in the material surfaces.23 Therefore, to flatten the composite surface during the S3 procedure, the clinician should add an intermediate polishing instrument, such as the Sof-Lex XT fine disc, in addition to the Enhance finishing cup.

Thus far, we could conclude that the intermediate step should achieve approximately 50—60% of the gloss level to obtain the optimal polish for the next instrument. A clinically important finding was that most of the deep scratches should be removed by the intermediate polishing tools to easily continue to the final polishing. Based on the current results, we found that our polishing sequence S1—Sof-Lex XT coarse disc / Sof-Lex XT fine disc / OccluBrush—could effectively polish the surfaces of the nanohybrid composites. For the dental hybrid composites, we do not recommend using Prisma Gloss extra-fine polishing paste. Of course, we can polish the composite material to a high gloss level, but after being exposed to the oral environment, the surface was worn. The superficial gloss was removed (by food and tooth brushing), and what remained was the "intrinsic roughness", which was determined by the large, glass fillers.24 Therefore, by using the Sof-Lex XT fine disc intermediate polishing step, a dentist could save time in achieving satisfactory surface quality of light-cured resin composites. The S2 and S3 sequences alone were insufficient for this purpose. Dentists could choose appropriate

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Polishing mechanism of dental composite

Figure 3 Scanning electron microsope micrographs of hybrid composite surfaces and the polish instrument following the S1 procedure. (A) Intermediate polishing stage, Sof-Lex XT coarse disc / Sof-Lex XT fine disc. (B) Sof-Lex orange disc; the debris of composite resin remained on the Sof-Lex XT fine disc. (C) Final polishing stage, Sof-Lex XT coarse disc / Sof-Lex XT fine disc / OccluBrush. S2 procedure. (D) Intermediate polishing stage, Sof-Lex XT coarse disc / Prisma Gloss polishing paste. (E) Enhance system, foam with polishing paste. The polishing paste and debris attached to the foam. (F) Final polishing stage, Sof-Lex XT coarse disc / Prisma Gloss polishing paste / OccluBrush. S3 procedure. (G) Intermediate polishing stage, Sof-Lex XT coarse disc / Enhance finishing cup. (H) Enhance finishing cup. 30—50 mm abraded particles at the border of finishing cup appeared during the polishing procedure. (I) Final polishing stage, Sof-Lex XT coarse disc / Enhance Finishing Cup / OccluBrush.

polishing for particular situations. For example, the Enhance finishing cup or the Sof-Lex XT coarse disc might be appropriate for higher hardness hybrid composite materials in cases of irregular surfaces, followed by the Sof-Lex XT fine disc and Prisma Gloss polishing paste. If the hybrid composite materials present lower hardness, which might be the case with anterior or cervical restorations, the Enhance finishing cup or the Sof-Lex XT coarse disc should be avoided as the first instrument. Clinicians could simplify optimize polish quality by using the Sof-Lex XT fine disc and OccluBrush instruments after finishing the contour of the resin composite.

In summary, the gloss values of the geometric optics assessment at 20°, 60°, and 85° were useful and reliable for

determining the surface quality of hybrid dental composites, and they strongly correlated with the subjective interpretation of the surface textures, as evaluated using FE-SEM micrographs. The study represented a clinically relevant minimum polishing sequence, independent of any company suggestions or interests. Nominally similar abrasive abilities did not necessarily result in equivalent polish levels, indicating that the polishing tools must be evaluated and cannot be judged based on their compositions or abrasive sizes. The three illumination angles glossmeter has great potential to provide a quantifiable and nondestructive way to evaluate and optimize finishing and polishing sequences of different kinds of dental composites via various abrasive instruments.

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Acknowledgments

The authors thank Mrs. Gisela Dachs, who works at Tribo-labor at Ludwig Maximilian University of Munich, for her technical assistance during the laboratory work.

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