Profilometric analysis of two composite resins' surface repolished after tooth brush abrasion with three polishing systems.

AIM: To evaluate the effect of three polishing protocols that could be implemented at recall on the surface roughness of two direct esthetic restorative materials.
MATERIALS AND METHODS: Specimens (n = 40) measuring 8 mm (length) × 5 mm (width) × 4 mm (height) were fabricated in an acrylic mold using two light-cured resin-based materials (microfilled composite and microhybrid composite). After photopolymerization, all specimens were finished and polished with one of three polishing protocols (Enhance, One Gloss, and Sof-Lex polishing systems). The average surface roughness of each treated specimen was determined using 3D optical profilometer. Next all specimens were brushed 60,000 times with nylon bristles at 7200 rpm using crosshead brushing device with equal parts of toothpaste and water used as abrasive medium. The surface roughness of each specimen was measured after brushing followed by repolishing with one of three polishing protocols, and then, the final surface roughness values were determined.
RESULTS: The data were analyzed using one-way and two-factor analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD). Significant difference (P < 0.05) in surface roughness was observed. Simulated brushing following initial polishing procedure significantly roughened the surface of restorative material (P < 0.05).
CONCLUSION: Polishing protocols can be used to restore a smooth surface on esthetic restorative materials following simulated tooth brushing.

INTRODUCTION

The ongoing search for biologically and esthetically acceptable adhesive restorative materials has brought varieties of tooth-colored materials to the market. Currently, the clinician has resin-modified glass-ionomer cements, polyacid-modified composite resins, and heavily filled composite resins as options for direct restorations. The longevity and esthetic appearance of these tooth-colored restoratives, particularly of the composite, is strongly influenced by the final surface polish.[1]

A highly polished and smooth surface slows the rate of plaque and calculus buildup and reduces superficial discoloration.[2] It has been reported that plaque accumulates on composite samples with a surface roughness of 0.7–1.44 mm.[3]

The smoothest surface can be obtained after simply polymerizing the composite resin against a clear matrix during curing. In matrix finish, the surface layer of restoration is found to be rich in resin organic binder. Removal of outermost surface layer by finishing procedure would tend to produce a harder, more wear-resistant, and hence, a more esthetically stable surface.[4]

Roughness can be measured in a number of ways, but the most commonly used both in dentistry and engineering is the surface roughness (Ra) value. The surface roughness (Ra) value is described as the arithmetic mean value of movement of profile above and below the center line of the surface. Mechanical profilometer, Scanning Electron Microscopy (SEM), optical 3-D profilometer, etc. are some of the methods used for measuring the surface roughness (Ra) value.[5]

Extensive research has been conducted to investigate protocols for initial finishing and polishing of composite materials. However, little has been done to investigate the subsequent maintenance of these finishes at prophylaxis recall. All esthetic restorations require ongoing maintenance, including periodic repolish to enhance their esthetics and increase their longevity.[67]

Different finishing and polishing systems are available in today's market. There is no general agreement in the dental literature on the best method for finishing and polishing of composite restorative materials. New products are steadily entering the market, making a continuous appraisal of their effect necessary. Hence, the present in vitro study was undertaken to evaluate the effect of three polishing systems on the surface finish of a microfilled and a microhybrid composite resin.

MATERIALS AND METHODS

Preparation of samples

A total of 40 samples were prepared in two groups (group I, microfilled composite resin – Durafill VS and group II, microhybrid composite resin – Charisma). The samples were made by placing the composite resin materials into a rectangular acrylic mold of 8 mm (length) × 5 mm (width) × 4 mm (height). The molds were slightly overfilled with the material, covered on each side with matrix strip (unident), and placed between two glass slides. Each side of the two-sided sample was irradiated with halogen light curing unit for 30 sec. After initial two-way light curing steps, samples were irradiated for additional 60 sec from both sides without the matrices in place. Then, the samples were removed from the mold.

Grouping of samples

Groups I and II were subdivided into subgroup A – matrix strip, subgroup B – Enhance polishing system, subgroup C – One Gloss polishing system, and subgroup D – Sof-Lex polishing system, and each of these subgroups had five samples.

Finishing of samples

The samples in both groups, except subgroup A (baseline), were finished using 30-fluted tungsten carbide bur (S. S. White) for 3 sec. The finishing procedure was carried out in one direction by one operator. Care was taken to maintain the parallelism during preparation of samples.

Polishing of samples

The four subgroups of groups I and II were polished according to their respective manufacture's directions as follows:

Subgroup A or baseline subgroup: No further treatment was carried out after polymerization against the matrix strip.

Subgroup B: Polishing was carried out using Enhance polishing system (Dentsply caulk, Dentsply International Inc., Milford, PA, USA; lot no. 624075).

Subgroup C: Polishing was carried out using One Gloss polishing system (Shofu, Kyoto, Japan; lot no. 070822)

Subgroup D: Polishing was carried out using Sof-Lex polishing system (3M-ESPE, St. Paul, MN, USA; lot no. 1980).

The samples were rinsed in tap water and stored at 100% relative humidity at 37°C in climate control chamber. Surface roughness of each sample in both groups was measured using 3D optical profilometer.

Simulated tooth brushing of samples

Following surface roughness analysis, each sample of the experimental subgroups (B, C, and D) of groups I and II was further treated with simulated tooth brushing technique. The tooth brushing device consisted of a tooth brush with a movable head that was mounted on a fixed stand and a sample holder. The nylon bristles were fitted into movable head and samples were mounted on the sample holder. Care was taken to see that the tooth brush bristles were perpendicular to the surface of each sample and touched evenly. Equal parts of toothpaste and water were used as abrasive medium. Experimental surface (top surface) in the subgroup B, C, and D in both groups was brushed for 60,000 times with nylon bristles moving at 7200 rpm set at a load of 1 N. After simulated brushing, samples were rinsed with tap water and stored in 100% humidity in climate control chamber until the roughness values were obtained.

Repolishing of samples

After analyzing the brushed surface of each sample, repolishing of samples was carried out using the initial polishing protocols, respectively. Final surface roughness analysis for all samples was done.

RESULTS

The mean values were compared by one-way and two-factor analysis of variance (ANOVA). Tukey's honestly significant difference (HSD) test was used to identify significant differences among the subgroups. In the present study, P < 0.05 was considered as the level of significance. Statistical analysis revealed significant difference between polishing systems and restorative materials (P<0.05). No significant difference (P > 0.05) was found in the surface roughness between group I (Durafill VS.) and group II (Charisma) in subgroup A (baseline). Surface roughness was least in subgroup A in both group I and group II. Among the experimental subgroups, subgroup B showed the least surface roughness values for both group I and group II (P < 0.05). There was a statistically significant difference between subgroup C and subgroup D of both group I and group II (P < 0.05). Surface roughness was maximum in subgroup C of both group I and group II (P < 0.05). In both group I and group II, simulated brushing following initial polishing procedure significantly roughened the surface of restorative material (P < 0.05). Repolishing after simulated brushing showed significant decrease in surface roughness values (P < 0.05).

DISCUSSION

A highly smooth and polished surface finish is said to contribute to patient comfort, enhances the appearance of restorations, slows the rate of plaque retention, and reduces superficial surface discoloration.[234567891011] Finishing refers to the gross contouring or reducing of the restoration to obtain the desired anatomy. Polishing refers to the reduction of the roughness and scratches created by the finishing instruments.[78910111213]

In order to combine the desirable properties of polishability and strength and to be used as an anterior and posterior restoration, microhybrid came into existence. Microhybrid composite resin has average particle size of 0.06 μm, which is very close to the particle size of microfilled composite (0.04 μm)[6] [Table 1]. Various studies have compared different composite resins of different manufacturers with different polishing systems in order to obtain surface roughness.[89101112] However, in this study, the composite resins of the same company have been utilized, i.e. microfilled composite resin – Durafill and microhybrid composite resin – Charisma, in order to avoid variation that may occur in the particle size of filler and the resin filler ratio. These composites were used to test the various polishing systems to find which one gives the smoothest surface. Various finishing and polishing devices are available including silicon carbide–coated or aluminum oxide–coated abrasive disks and wheels, multifluted carbide finishing burs, fine diamond finishing burs, impregnated rubber or silicon disks and wheel, polishing pastes and abrasive embedded in resin polishing points, etc. that are commonly used to finish dental restoratives.[14]

Table 1

Mean Ra values (μm) and standard deviation for the various restorative materials and polishing systems evaluated

This study was conducted in three parts which included initial polishing followed by simulated brushing and then repolishing of the composite resins. Every effort was made to standardize the different aspects of the methodology in this study. To avoid any procedural error, the study was carried out by a single operator.

The smoothest surface was produced when the material was allowed to cure against the cellophane matrix strip[151617] [Table 1]. Among the experimental subgroups in both groups, the Enhance polishing system (subgroup B of both the groups) gave the smoothest surface [Table 1]. The reasons could be attributed to the following factors. This polishing system comprises the following: aluminum oxide impregnated polishing disks and points, prisma gloss, and prisma gloss extra fine aluminum oxide pastes. For ideal results, it is recommended by the manufacturer to use the points or disk in combination with the aluminum oxide pastes. In doing so, one could achieve a three-body abrasion that, apart from polishing, also aids in dissipation of any heat generated. The aluminium oxide paste causes finer abrasion in comparision to impregnated discs or points used alone. Compared to other experimental subgroups, Enhance offered advantages because of its various shapes. There are certain areas where disks alone will be difficult to use and the availability of the abrasive points serves to a greater extent in overcoming this difficulty. One Gloss polishing system (subgroup C of both the groups) among the experimental subgroups in both the groups produced the roughest surface [Table 1]. In 2001, Marigo et al. reported that the final glossy surface obtained by polishing depends on the flexibility of the backing material in which abrasives are embedded, the hardness of the abrasives, geometry of the instruments, and the instrument employed. It is, therefore, possible that the poor result obtained by this system might be attributed to the inflexibility of the cups.[11] Surface roughness (Ra) value of the composite polished with Sof-Lex polishing system was higher than that o Enhance polishing system and lesser than that of One Gloss polishing system in both the groups [Table 1]. The disadvantage of aluminum oxide disks lies in their geometry. Using the disks, it is often difficult to efficiently create, finish, and anatomically polish contoured surface. Another disadvantage of this system is the projection of metal head in the center of the disk that will produce scratches and later cause discoloration. Studies have also shown that aluminum oxide disks generate heat during finishing and polishing procedures, which tends to cause micro-cracks in the composites.[23456789101115]

Following simulated tooth brushing, there was a statistically significant increase in surface roughness compared to the initial polished surface of the composite in both groups [Table 1]. The reason could be the resin matrix supporting inorganic filler particles in composite materials that wear away and leave particulate matter or irregularities projecting from the surface.[1014171819]

In the third part of this study, application of polishing protocols for repolishing following simulated tooth brushing in both the groups showed significant decrease in their overall surface roughness [Table 1]. Composites are biphasic, with filler embedded in a resin/polymer matrix. During hygiene procedures, the matrix phase is preferentially removed as the abrasives employed in prophylaxis agents are harder than the resin matrix; thus filler particles are exposed, resulting in a rough surface. Therefore, composite restorations may require repolishing after exposure to some hygiene maintenance procedures, as Ra values exceed the critical threshold surface roughness for bacterial adhesion (0.2 μm). Repolishing of the restoration might be necessary to maintain periodontal health, longevity, and esthetics in the clinical setting.[202122]

The final polish obtained on composite restoration is determined by two factors: composition of composites with relation to matrix and filler particles and the type of polishing system used. The degree of polymerization of matrix and the size, composition, and volume of filler particle affect the surface finish obtained on composite as the resin matrix and filler particles do not abrade to the same degree.[232425]

Within the limitations of this study, lowest surface roughness values were obtained with Enhance polishing system and there was a significant decrease in roughness values following repolishing after hygiene maintenance procedure. Wear and tear due to brushing was considered along with toothpaste, which acts as an abrasive as well as a lubricant. But in real life, the nature of food consumed, occlusal contact, etc. can also contribute to wear of composite resin materials. Further series of studies have to be carried out utilizing different composite materials and polishing systems till we are able to pair a specific composite resin material with matching polishing system in order to produce smoothest surface, thereby reproducing surface similar to matrix strip that is considered as gold standard as far as smoothest polish is concerned.

CONCLUSION

Microfilled composite resin showed better polishability as compared to microhybrid composite resin.

Enhance polishing system gave the least surface roughness (Ra) value.

3-D profilometer has provided a very reliable quantitative measurement.

Oral hygiene maintenance measures such as brushing definitely increase the surface roughness of composite resins.

Routine recall polishing of restoration is mandatory.