Surface Roughness of Methacrylate- and Silorane-Based Composites After Finishing and Polishing Procedures

Resin composite materials became available to dentistry in the 1960s. Since then, their use has been extended to restore both anterior and posterior teeth, and they are the most common alternatives to amalgam. Resin composites present excellent features, such as adhesion to dental structure, easy repair, long working time/command cure, and tooth-colored appearance.¹ Furthermore, due to its bonding ability this material helped make conservative preparations with reduced tooth structure removal possible.²

Despite the advantages exhibited by resin composite materials, restorations placed in areas of high function are prone to excessive wear and marginal fractures.³ Additionally, resin composite materials have no antibacterial properties, and previous studies have reported that levels of cariogenic bacteria present in the plaque of resin composite restoration surfaces are significantly higher than the levels of bacteria present on either amalgam or glass-ionomer materials.^4,5 The longevity of restorations depends on different factors. The presence of irregularities on the surface of resin composite restorations may result in plaque retention, surface discoloration, gingival inflammation, recurrent caries, and accelerated wear.⁶ Adhered plaque can also contribute to the loss of the composite’s organic matrix by acidic bacteria by-products, resulting in increased roughness.⁷ Finishing and polishing procedures are used to shape and smooth the surface of resin composite restorations, resulting in superior esthetic results and extended longevity.^8,9

In general, the final stages of a resin composite restoration involve contouring, finishing, and polishing. Typically, diamond or multi-fluted carbide burs and/or coarse sandpaper discs are used for bulk reduction and to re-establish shape, size, contour, and texture of the newly placed restorative material. Finishing removes the scratches created by the contouring instruments and provides a smooth surface. Polishing, the final step, provides an enamel-like luster to the restoration and reduces the surface energy.¹⁰

A wide variety of finishing and polishing systems with dissimilar compositions, abrasiveness, and shapes are commercially available. They are typically classified into six major categories: burs (diamond and multi-fluted carbide); silicon-impregnated, aluminum oxide-impregnated, and diamond-impregnated rubber-based cups, discs, and points; coated abrasive discs and strips; polishing pastes; diamond-impregnated brushes; and surface sealers. Among all polishing systems available on the market, the effectiveness of discs on the surface of resin composites has been the most extensively evaluated. Although discs have been proven to be efficient in finishing and polishing procedures, their use on the occlusal surfaces of posterior restorations is limited and they are used mostly on the broad flat areas of anterior teeth that are compatible with the discs’ shape or to contour the interproximal, occlusal, or incisal embrasures.^11-13

When working with posterior resin composite restorations, the limited access and complex anatomic features of the occlusal surfaces restrict the possibility of using an extensive armamentarium for finishing and polishing. Generally, this procedure is accomplished by the use of burs and rubber points, which are available in different shapes and sizes, facilitating the contouring and finishing of the occlusal surface, which should be refined but not flattened.¹³

Current resin composites present variations in their composition, including both organic matrices and/or inorganic fillers, which may influence their polishability. The average size filler particles in current resin composite materials has been reduced in order to obtain better color stability, greater wear resistance, smoothness, and strength. Hybrid composites contain a mixture of different particle sizes, with the microhybrids and nanohybrids being the most widely used in posterior teeth because they provide optimal mechanical and physical properties combined with good polishability.¹¹

Because the use of resin composites in posterior teeth has become a common procedure in daily practice, this study compared different finishing and polishing tools used in posterior teeth applications, aiming to address the surface roughness (Ra) of resin composite restorative materials. The null hypotheses of this study are: (1) there would be no significant differences in the Ra between the different surface finishing and polishing tools; and (2) there would be no significant differences in the Ra among the different types of resin composites tested in each condition.

Four resin composite materials (Table 1), including three methacrylate-based composites and one silorane-based composite, were used to prepare a total of 200 disc-shaped specimens (50 specimens of each resin composite). Specimens were prepared by inserting the composite materials into a stainless steel split mold (5-mm diameter x 2-mm height) and covering the surface with a polyester Mylar strip and a clear glass cover slip (Gold Seal™ Microscope Slides, Thermo Scientific, www.thermoscientific.com), using constant pressure to extrude material excess and minimize the formation of an oxygen-inhibited layer on the surface. Specimens were light-cured for 40 seconds using a LED curing-light unit (VALO^®, Ultradent Products, Inc., ultradent.com) with light intensity of 1200 mW/cm² and then immediately stored in distilled water at 37°C for 24 hours. Before starting the finishing and polishing procedures, the specimens were dried using an air spray for 30 seconds and re-positioned on a stainless-steel split mold 1 mm above the base of the ring to facilitate the finishing and polishing procedures.

The top surface of each specimen was finished with a 15-µm fine diamond bur (FDB) (Diamond Finishing Bur #863, Brasseler USA, brasselerusa.com) to remove the shiny surface left by the Mylar strip and to simulate the initial contouring step. Specimens were randomly allocated into five groups (n = 10), according to the finishing and polishing protocols investigated:

• G1: 15-µm FDB (Diamond Finishing Bur #863) (control) (FDB). Type of abrasive: diamond
• G2: 15-µm FDB followed by a 20-fluted tungsten carbide bur (Carbide Bur #H48LF, Brasseler USA) (multi-fluted carbide bur [MCB]). Type of abrasive: tungsten
• G3: 15-µm FDB followed by a diamond-impregnated micropolishing point (D-FINE, Double Diamond™ Polishing System, Clinician’s Choice Dental Products, Inc, www.clinicianschoice.com) (R1). Type of abrasive: diamond-impregnated silicone
• G4: 15-µm FDB followed by a diamond-impregnated micropolishing point (Flame Point Pre-polisher and Shine, Brasseler USA) (R2). Type of abrasive: diamond-impregnated silicone
• G5: 15-µm FDB followed by the application of a surface sealer (PermaSeal, Ultradent Products, Inc.) (SS)

Each resin composite material investigated was submitted to the five finishing and polishing treatments listed (n = 10). To calibrate the operator’s average hand pressure during the finishing and polishing procedures, a preliminary pilot study was conducted. Both high-speed and low-speed handpieces were used under constant pressure of 20 Kg/cm². To reduce variability, the same operator performed all the finishing and polishing procedures. 

A device was used to stabilize the handpiece to keep the bur parallel to the specimen’s top surface. The fine diamond and tungsten multi-fluted carbide burs were used in a high-speed handpiece under water-cooling for 20 seconds with light and uniform intermittent pressure. Burs were discarded after preparation of five specimens. The diamond-impregnated micropolishing points were applied using a slow-speed handpiece for 20 seconds each. Rubber points were discarded after preparation of five specimens.

Specimens were thoroughly rinsed with water after the finishing and polishing steps and allowed to dry for 24 hours at 37°C before starting the average Ra measurements. The specimens from the group to receive the surface sealant were etched with 35% phosphoric acid (Ultra-Etch^®, Ultradent) for 5 seconds, rinsed with distilled water until the acid was completely removed, and dried with air spray. A thin layer of sealant (PermaSeal) was applied to the surface of the material with a microbrush and a gentle air spray was used for 5 seconds to allow the sealant to spread evenly. The surface sealer was light-cured for 20 seconds, according to the manufacturer’s instructions. 

Ra was measured using a surface profilometer (Mitutoyo SJ-210 Surftest, Mitutoyo America, mitutoyo.com). The cut-off value for the profilometer was set at 0.25 µm, and the stylus speed was set to 0.5 mm/s. Track length was set to three times. A measuring force of 4 mN with a stylus tip angle of 90 degrees was used. After preparation, each disc was securely affixed onto a glass slab and subjected to assessment by the profilometer. The profilometer’s stylus was placed on the sample, and a path was selected to ensure that the diamond stylus traversed centrally through the sample, excluding the areas located at 1 mm from the edge, which were not representative of the finishing and polishing procedures. Three passes were made, each at an angle of 120 degrees to the previous path, to ensure a representative sample size. Profilometer data were analyzed taking the Ra value into consideration. Ra is related to the arithmetical average height of surface-component irregularities from the mean line within the measuring length.

Multiple comparisons were obtained using a one-way ANOVA (20 groups, overall P < .001 among 20 groups), based on square root transformed angles with a Tukey adjustment for multiple comparisons. (Pairs with the same letter are not statistically significant at the .05 level.) 

Three different regions were analyzed to determine the Ra, and an average roughness for each group was calculated. The differences in Ra between the four resin composite materials and the five finishing and polishing protocols were analyzed using multiple comparisons obtained using a one-way ANOVA (20 groups, overall P < .001 among 20 groups) based on square root transformed angles with a Tukey adjustment for multiple comparisons. Results are presented in Table 2. The effect of these procedures on the surface morphology of the resin composite materials tested was visualized under a field emission scanning electron microscope (SEM). One illustrative sample of each group is displayed in Figure 1 through Figure 5.

No significant differences in Ra were observed among the resin composites tested in the same condition. All four resin composites showed similar responses to each one of the five finishing and polishing protocols tested. The use of a fine diamond finishing bur generated the highest roughness values, while the use of a surface sealer resulted in the lowest roughness values for all resin composites tested (P < .05). 

No significant difference in Ra was observed between the use of a multi-fluted carbide bur and the rubber point system R1 for all resin composites tested. When evaluating the different rubber systems, Filtek™ LS Low Shrink (3M ESPE, 3m.com) and Filtek™ Supreme Ultra (3M ESPE) presented significantly higher Ra with the use of the rubber point system R2, while Tetric EvoCeram^® (Ivoclar Vivadent, ivoclarvivadent.com) and Tetric Ceram™ HB (Ivoclar Vivadent) presented similar Ra values with the use of a multi-fluted carbide bur and the two rubber point systems, R1 and R2.

Ideally, a restorative material should present surface smoothness and glossiness comparable to human enamel. It has been reported that a surface roughness of 0.3 µm can be detected by the tip of the patient’s tongue.¹⁴ Along with causing patient discomfort, increased Ra may heighten plaque build-up as well as restoration discoloration and wear.^8,15,16 A Ra threshold of 0.2 µm has been suggested for bacterial retention; below this level no further reduction in bacterial accumulation should be expected.^15,17

It is well known that no anticariogenic properties are present in resin composite materials.^4,5 Smooth restoration surfaces are important to avoid increased plaque accumulation and reduce secondary caries. In the present study, both silorane-based composites (SBCs) and methacrylate-based composites (MBCs) presented comparable Ra when submitted to similar finishing and polishing protocols. Based on these data, the null hypothesis that there would be no significant differences in the Ra of different types of resin composites tested in the same condition should be accepted. Similar results were previously reported by Kameyama (2008) ⁹ and Antonson (2011).¹⁸ In their studies, resin composites with different compositions (hybrid/submicron/microhybrid/nanofill) were submitted to several finishing and polishing protocols, and no significant differences in Ra were observed among the composite restorative materials. 

SBCs were developed in an attempt to reduce polymerization shrinkage. The polymerization mechanism is based on open rings on cationic radical oxiranes. SBCs are biocompatible and resistant to conditions of the oral environment, with mechanical properties similar to methacrylates but with lower solubility in water.^6,19-21 MBCs utilize bis-GMA as the main organic matrix component, which is usually combined with other methacrylates, such as, triethylene glycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), and bisphenol glycidyl dimethacrylate ethoxylate (bis-EMA). MBCs are characterized by the presence of free radicals that are involved in the redox polymerization reaction between methacrylate groups.^19,21 Despite presenting different chemical compositions, no significant differences in polishability could be detected between these two resin-based materials. 

Although Ra may be influenced by the differences in the composition of a restorative material, the techniques and tools employed on the finishing and polishing procedures play an important role in the final surface features.^7,13,22,23 In the present study, the fine diamond bur generated the highest Ra values (0.368 to 0.460) among all resin composites tested. It was previously reported that Ra values above 0.2 µm result in plaque accumulation, which can lead to increased risk of caries and periodontal inflammation.^15,22 The depressions and risks imprinted by the use of a diamond bur resulted in a rough surface, which can be visualized on the SEM micrograph (Figure 1). The surface values generated with the use of fine diamond burs were significantly greater than Ra = 0.2 µm, which predispose the surface to increased plaque accumulation. This finding is in agreement with previous studies^18,24,25 that reported similar roughness values when fine diamond burs were used for finishing. Despite the fact that diamond burs are important tools to reshape and contour the surfaces of restorations with high cut efficiency, their use should be followed by less abrasive polishing tools in order to restore smoothness and shine.^18,26

On the basis of these findings, the second null hypothesis that there would be no significant differences in the Ra between the different finishing and polishing protocols should be rejected. The use of a fine diamond bur generated the highest roughness values, while the use of a multi-fluted carbide bur resulted in significantly lower roughness values for all resin composites tested (P < .05).

When comparing the use of a multi-fluted carbide bur with the rubber point system R1, no significant differences in Ra were observed for all resin composites tested. Additionally, no significant differences in Ra were found between the use of a multi-fluted carbide bur and the two rubber point systems, R1 and R2, for Tetric EvoCeram and Tetric Ceram HB composites. This finding is relevant, because reduced chairtime may be achieved when employing multi-fluted carbide rather than finishing diamond burs. In the present study, similar surface aspects can be visualized on the SEM micrographs when either a multi-fluted carbide bur or a rubber point system was used (Figure 2 through Figure 4). Based on these results, it is possible to affirm that the use of multi-fluted carbide burs promoted excellent surface polishing. In the present study 20-fluted carbide burs were used. It is well known that the more blades on a carbide bur, the greater its ability to polish, rather then cut. The cut effectiveness of a multi-fluted carbide bur must be considered when great excess removal and contouring are needed. A previous study reported that multi-fluted carbide burs cut effectively in the first 3 minutes of use; however, cut effectiveness decreases after this period, resulting in a slow and less effective action.²⁷ Finishing and polishing procedures include restorative excess removal, contouring, shaping, and smoothing. Fine diamond or multi-fluted carbide burs are commonly used for bulk reduction and anatomical contouring, while less abrasive tools are used for final polishing.¹⁸

The quest for ideal smooth surfaces on composite resin restorations has resulted in significant improvements in both finishing and polishing systems as well as techniques used.²⁸ The use of rubber polishing systems remains essential as a second-stage procedure if fine diamond burs are employed on the finishing protocol. Rubber polishing systems are available in different shapes, sizes, and dimensions, which facilitates the contact with most surfaces of posterior teeth, overcoming the access limitations associated with aluminum-oxide discs.¹³ The different rubber point systems tested in the present study provided slightly different results depending on the resin composite used. Tetric EvoCeram and Tetric Ceram HB showed no statistical difference in Ra between the two rubber point systems tested (R1 and R2), while Filtek LS Low Shrink and Filtek Supreme Ultra showed significantly higher Ra with the use of the rubber point system R2. The differences in Ra generated from the different finishing and polishing systems on each of the four resin composites tested may be related to the characteristics and composition of each composite. Although no statistical differences were observed among the resin composite materials tested in the same conditions, it was possible to verify that different composite materials showed slightly better or worse results depending on the polishing system used. This result is in agreement with previous reports that have verified that different polishing systems can display different results, depending on the features of the restorative materials.^29,30 Furthermore, non-homogeneous abrasion of the composite material depends on the degree of hardness between the filler particles and organic matrix. To achieve a good polish, the abrasive particles of the finishing and polishing tools must have higher hardness than the filler particles present on the composite material.^6,31

The Ra may be also be influenced by the number of steps involved in each finishing and polishing system. In the present study, both rubber polishing systems (R1 and R2) used two-sequence rubber points as part of the polishing protocol. Systems with more steps have a greater opportunity to provide superior surface refinement due to the longer amount of surface contact time generated by the use of two rubber points when compared to one step. The longer the contact of the instrument with the restoration surface, the more the irregularities and flaws on the surface of the restoration may be accurately diminished. However, the Ra may be similar between one- and two-step systems if the same amount of time is employed.¹¹

The use of a surface sealer resulted in the lowest Ra values in the present study. These findings are in agreement with previous studies.^32,33 Several studies have emphasized the importance of using a low-viscosity surface-modifying agent on the resin composite surfaces to eliminate microcracks and irregularities generated during the finishing and polishing procedures and stop crack propagation.³³ The use of surface sealers also has been proposed to optimize the marginal seal and improve wear resistance, color stability, and smoothness of posterior composite restorations.^34,35

Within the limitations of this in vitro study, the following conclusions may be drawn: The use of a single-stage finishing and polishing protocol utilizing a fine diamond bur yielded the roughest surface; the use of only a 20-fluted carbide bur generated Ra comparable to that of a dual-stage finishing and polishing procedure using a rubber system for two of the resin composites tested; and, lastly, the sealant surface yielded the lowest Ra and its use should be encouraged to enhance finishing and polishing results.

M. Jacinta Santos, DDS, MSc, PhD
Associate Professor
Schulich School of Medicine & Dentistry
Western University
London, Ontario, Canada

Heleine Maria Chagas Rêgo, DDS, MSc
PhD degree candidate
School of Dentistry
Institute of Science and Technology of São José dos Campos, Brazil

Leticia Linares, DDS
Masters degree candidate
Bauru School of Dentistry
University of São Paulo
Bauru, SP, Brazil

Amin S. Rizkalla, BSc, MEng, PhD
Assistant Professor
Chair of the Division of Biomaterials Science
Schulich School of Medicine & Dentistr
University of Western Ontario
London, ON

Gildo C. Santos Jr., DDS, MSc, PhD
Associate Professor
Restorative Dentistry
Schulich School of Medicine & Dentistry
Western University
London, Ontario, Canada