Evaluation of surface roughness of different restorative composites after polishing using atomic force microscopy.

INTRODUCTION: Resin based composites are widely used aesthetic restorative materials in clinical restorative dentistry. The filler size and the percentage of fillers affects smooth surface, clinical durability, aesthetics, better optical properties, compatibility with natural enamel tissue, surface gloss, and preventing the discoloration of the restoration. The finishing and polishing of tooth-coloured restorations are necessary clinical steps for better aesthetics and longevity of restored teeth. AIM: In this study nano composites were chosen, because these contain nano particles which provide better overall composites features, including the quality of polished surface. The aim of this study was to evaluate the surface roughness of different newer posterior composites. MATERIAL AND
METHOD: Five commercially available posterior restorative composite were tested in this study. All the specimens were polished with shofu multi step polishing system. After polishing the samples were all analyzed by atomic force microscopy which is used to study surface topography and surface morphology of materials.
RESULTS: The values of surface roughness of each specimen were statistically analyzed using Kruskal Wallis ANOVA, and Pair wise comparisons by Mann-Whitney U test setting the statistical significance at p ≤ 0.05.
CONCLUSION: Tetric Evo Ceram, Z350 exhibited less surface roughness compared to Ever X, Clearfil Majesty and Sure fil SDR. There was no statistical difference between groups regarding surface rough ness between groups.

INTRODUCTION

Dental composites are the most widely used material in clinical restorative dentistry. Since its introduction from late 1950's to recent nanocomposites, composite materials are constantly considered for research. Composition of dental composite resins comprises of the resin matrix (organic phase) bisphenol A glycidyl methacrylate (Bis-GMA) or urethane dimethacrylate (UDMA), and other resins added for the viscosity correction, such as triethylene glycol dimethacylate,[1] filler matrix coupling agent (interface), filler particles (dispersed phase) consist of silica in the form of quartz, or silicates of various types,[1] and other minor additions including polymerization initiators, stabilizers and coloring agents. Improvements have been made by making the composites more wear resistant, less shrinkage, stronger, and color stable in recent years. Fillers in composites have multiple roles such as they reduce polymerization shrinkage, the coefficient of thermal expansion and water sorption and solubility. They mechanically reinforce the material to enable better initial polishing and polish retention and to reduce wear during the masticatory forces.[234567]

Finishing and polishing of tooth-colored restorations are very important for esthetics and longevity of restored teeth. Finishing and polishing procedures are necessary clinical steps to establish a proper reconstruction of dental crowns and to restore an anatomical and morphological form of the tooth.[8] Dental materials need to be biocompatible materials, with optimal physical, mechanical, chemical, and esthetic properties.[9] The very important properties of dental materials are their polish ability and polish retention, and the surface quality that do not cause undesirable biological interactions and the adhesion of the bacterial plaque on the reconstructive material.[610] Biofilms are the main cause of caries lesions and gingival and periodontal diseases, and its retention can be reduced by decreasing the surface roughness of the restorative biomaterial.[11] Several experimental findings state that dental material's surface roughness lower than 0.2 μm, significantly reduced the possibility of bacterial adhesion.[12] Smooth surface enables clinical durability, good esthetic appearance, better optical compatibility with natural enamel tissue and surface gloss, as well as, preventing the discoloration and staining of the restoration.[1314] There are many dental processing methods for obtaining good surface quality.[15] Routinely, final processing of restorations includes contouring, finishing, and polishing procedures to obtain the adequate anatomical morphology of the restoration[16] and attain “high gloss polishing” or “paste-polishing” with the application of polishing aluminum-dioxide or diamond pastes for the intraoral use at the end of the restorative treatment.[171819] Recently, a study conducted on contemporary nanohybrid composites showed better overall composites features, including the quality of polished surface because this new material contains nanoparticles.[2021]

Perfect finished and polished surface of composite restoration is difficult to obtain, as composites contain resinous matrix and organic fillers showing different degrees of hardness, thereby preventing a homogeneous material abrasion.[22] Finishing and polishing of composite resin restorations are performed with several types of devices, such as carbide finishing burs, finishing burs with diamond particles, abrasive rubber gums, abrasive disks covered with an aluminum oxide layer, abrasive strips, and abrasive pastes, all of them generating different roughness to the restored surfaces. Several polishing protocols are used recently, from the “multiple-step” systems, which require different instruments, to the “one-step” systems, based on the use of unique equipment, e.g., silicon carbide brushes or rubberized cups and points permeated with diamond dust. In the present study, multistep polishing system Super Snap rainbow kit (Shofu, Inc. Kyoto, Japan) was used according to manufacturers instructions. This system has four grits — coarse, medium, fine, superfine (silicon carbide and aluminum oxide) for contouring, finishing, polishing, and super polishing.

Surface roughness can be measured up to nanoscale by qualitative methods, such as scanning electron microscopy, or quantitative methods, such as profilometry.[23] In recent years, atomic force microscopy (AFM) has been largely used in dentistry to study characteristics of different materials.[242526] AFM allows a three-dimensional (3D) imaging at a nanometric resolution and does not need neither to work in a vacuum nor any preparation of the specimen.[24252627] This technique has emerged as the most reliable in the evaluation of surface roughness.[24] On this basis, the purpose of our preliminary in vitro study was to estimate, by AFM, the surface roughness of different nanocomposite resins after polishing procedures performed with different polishing systems currently in use.

MATERIALS AND METHODS

Five commercially available posterior composites were tested in the study and divided into five groups and named as A, B, C, D, and E. Their composition and grouping is depicted in Table 1.

Table 1

Composition of composite resin materials

Specimen size was standardized by preparing them in cylindrical plastic molds (8 mm diameter × 2 mm depth). To obtain a flat surface without any defects and entrapped air, pecimens were prepared on the glass microscope slide, filled with material, and covered with a polyester strip and a glass slide. The composite material was then polymerized for 40 seconds with a Smart Lite LED Light Curing Unit (Dentsply-DeTrey, Konstanz, Germany). After removing glass plate and polyester strip the top of the samples were polished with multi-step polishing system-Super Snap Rainbow (Shofu, Inc., Kyoto, Japan). Polishing procedure involved use of abrasive disk of all four grits in a dry condition, for 30 seconds, using micromotor handpiece speed not exceeding 30,000 rpm. The abrasive disks used for polishing was according to manufacturer's instructions and performed in the following sequence from black to red (coarse to extra fine): Black (coarse), violet (medium), green (fine), and red (extra-fine). One single operator did all of the polishing treatments to standardize condition similar to clinical finishing and polishing procedure in two directions. Care was taken to obtain a flat polished surface. After the polishing, the specimens were cleaned, and specimen topography was evaluated by Veeco di CP-II Atomic Force Microscope at six different points, two points at the center, two points at the periphery and two points at mid distance from the periphery to center. The AFM images obtained were subjected to surface roughness analysis using software provided, and the following parameters were compared among specimens: Average roughness (Ra) and maximum peak-to-valley distance (Rp-v). Data were analyzed using ANOVA and Pairwise comparisons using Mann-Whitney U-test (P ≤ 0.005).

RESULTS

All groups showed variable values of surface roughness after polishing. Variations in values were due to different fillers, sizes and different matrix of composites. Table 2 show surface roughness (Ra) values and Table 4 show R (p-v) [Graph 1] peak to valley values of mean and standard deviation of different composites In the present in vitro study Ever x, Z350, Tetric evo ceram showed least surface roughness (Ra) and (Rp-v) values followed by Clearfil majesty and Surefil SDR. Inter comparison with groups Table 3 (Ra) was analyzed using Kruskal Wallis ANOVA showed Tetric evo ceram with least roughness and sure fil SDR showed the maximum roughness. Inter comparison with groups Table 5 (Rp-v) was analysed using Kruskal Wallis ANOVA showed similar results. Group B and C (CFM and EX) showed almost similar results. Group E (Z350) showed better results both Ra and R (p-v) values.

Table 2

Mean, SD, and SE of Ra values in five groups

Table 4

Mean, SD, and SE of Rp-v values in five groups

Graph 1

R (P-V) peak to valley graph

Table 3

Comparison of five groups with respect to Ra values by Kruskal-Wallis ANOVA

Table 5

Comparison of five groups with respect to Rp-v values by Kruskal-Wallis ANOVA

DISCUSSION

Finishing and polishing procedures are necessary clinical steps to restore an anatomical and morphological form of the tooth after any restorative procedure.[8] Smooth surface enables clinical durability, good esthetic appearance, better optical compatibility with natural enamel tissue and surface gloss, as well as, preventing the discoloration and staining of the restoration.[1314] According to Pereira et al. 2011,[28] polyester strip promotes greater smooth surface to the composite restoration, but clinically restorations require final contouring, which requires removal of excess material and final finishing and polishing. There are many dental finishing and polishing methods for obtaining good surface quality. In this study single polishing system with multi-step Super Snap (Shofu, Inc., Kyoto, Japan) was used for standardizing the uniform method of material used for polishing.

Five types of posterior composites were used in this study: Sure Fil SDR Posterior Composite, Clear Fil Majesty Posterior Composite, Ever X Posterior Composite, Tetric Evo Ceram Posterior Composite and Filtek Z350. All these composites have different fillers and matrix composition and are recommended for restorations in posterior teeth. Their composition is depicted in Table 1. The manufacturers claim the newer materials to have superior properties as they differ in their inorganic fillers from macro to micro to nano fillers, the size of the particles and the extent of the filler loading vary widely among these materials in addition to the difference in the resin matrix. These factors influence their polish ability.[2930]

The average size of the filler particles in a micro filled composite is approximately 0.04 μm, where as in micro hybrid composite the particle sizes may range between 0.01 and 2.0 μm. Recently, new filler materials with sizes between 5 and 100 nm have been developed.[31] Nanotechnology applied to resin composites is aimed toward the production of composites resins with improved mechanical and esthetic characteristics attributed to the reduced size and wide distribution of the fillers.[6] These nano-filled composites also possess differences in their organic formulations, which may lead to distinct mechanical performance.[32] The reduced size and wide distribution of the nano-fillers may increase filler load, consequently, improve the mechanical properties of these new materials, such as their polymerization shrinkage, tensile strength, compressive strength, resistance to fracture, and reduced wear.[33] It has been observed that nano composites promote translucency and polish, and retain that polish similar to microfilled composites but with physical properties and wear resistance equivalent to those of hybrid or universal composites.[32]

Polishing of composites is necessary to finish the restorations with rotating devices, in order to remove any excess of material and reduce possible excess contacts when in occlusion.[33] In this study, Single polishing system with multi-step polishing system-Super Snap (Shofu, Inc., Kyoto, Japan), was used for standardizing the polishing protocol AFM was used to evaluate the surface roughness in this study. This method has recently been proved as the most reliable method to measure surface roughness.[24]

The results of this study showed the existence of some differences in surface roughness with multi-step polishing systems on all composites tested. The 3D images showing the difference in (Ra) and (Rp-v) values may be attributed to the differences in composition among the materials. All specimens were stored in distilled water to simulate the moist condition of the oral environment. In the present in vitro study, Ever X [Figure 1a–c], Z350 [Figure 2a–c], Tetric evo ceram [Figure 3a–c] showed least surface roughness (Ra) and (Rp-v) values followed by Clearfil majesty [Figure 4a–c] and Surefil SDR [Figure 5a–c].

Figure 1

(a-c) Group C - Ever X posterior composite

Figure 2

(a-c) Group E - Filtek Z350 posterior composite

Figure 3

(a-c) Group D - Tetric evo ceram posterior composite

Figure 4

(a-c) Group B - Clearfill majesty posterior composite

Figure 5

(a-c) Group A - Surefill SDR posterior composite

The Filtek Z350 nanocomposite consists of both nanoparticles and nanocluster fillers 82% by wt. Nanoparticles are discrete nonagglomerated and nonaggregated silica and zirconia fillers of 20 nm and 4-11 nm in size. The nanocluster particles increase filler loading, physical properties and polish retention of the nanocomposite.[35] Garcia et al.[34] 2004 in their study stated the reason for less abrasion of Z350 is because of uniform distribution of pre-cured silica particles in the organic matrix in our study Z350 exhibited least roughness [Figure 2a–c] when compared to other posterior composites. In this study, group c and group d exhibited almost similar results.

Tetric Evo Ceram Bulk Fill is nano-hybrid composite, composition[36] as shown in Table 1. Glass fillers present results in low wear and favorable polishing. This results in restorations that are virtually indiscernible from the surrounding tooth structure. Luca Giacomelli Giacomelli et al.[37] 2010 in a similar study on different polishing systems and composites concluded all composites and polishing systems exhibited surface roughness. Lainovic et al.[38] 2012 in a similar study on nano hybrid (FiltekZ550 and Tetric Evo ceram), nano filled (FiltekZ250) and micro hybrid (Gradia direct) concluded all composites exhibited surface roughness Tetric evo ceram exhibited consistent results both in surface roughness (Ra) and (Rp-v) values [Figure 3], similar results were exhibited in our study. Abdurazaq and Al-Khafaji[38] 2013 in their study concluded Tetric evo Ceram exhibited intermediate roughness, which is similar to our study. The GC Ever X posterior composite is a fiber reinforced composite containing Bis-GMA Resin 10-20%, triethyleneglycol di methacrylate 5-10%, silicon dioxide 1-5%, barium glass 60-70%, glass fiber 5-15%, polymethyl metacrylate traces, and photo initiators traces.[39] The glass fillers and glass fibers material may be the reason for less surface roughness [Figure 1a–c] compared to other composites used in this study.

In this in vitro study, Clearfil majesty exhibited surface roughness as other composites. This is a nano hybrid posterior composite resin composed of nano and micro inorganic filler treated with a proprietary new surface coating technology. This breakthrough technology permits a larger quantity of nanofiller (0.02-1.5 μ) to be dispersed in the resin matrix. The resulting resin matrix is reinforced with a filler loading of 92 wt% (82 vol%). The manufacturers claim the composite to have an improved surface hardness close to that of human enamel, high mechanical strength, and a low thermal coefficient.[40] These features assure a durable and reliable posterior restoration. In this study, the increase in filler loading may be the reason for less roughness. Can Say et al. 2014[41] evaluated the surface roughness of composites after polishing with two step polishing system. In their study, chemical force microscopy, showed high roughness Ra [Figure 4a–c]. This is in agreement to our study when compared to Ever X and Z350 values. Hosoya et al.[42] evaluated surface roughness after polishing with different grit silicon carbide paper. They concluded that surface roughness and color changes were greater with Clearfil majesty. This is also in agreement with our study.

The SureFil SDR exhibited greater surface roughness when compared to other composites the reason may be it is a Flowable bulk fill posterior composite having a complex formulation of UDMA resin, Di-Methacrylate resin, Di-functional diluents, barium, and strontium alumio-fluoro-silicate glasses(68% wt, 45% by vol), photo initiating system, and colorant. The possible reason for high surface roughness [Figure 5] could be because of lower filler loading and polymerization modulator that are chemically embedded in the center of the polymerizable resin that is the backbone of SDR.[43] The Surefil SDR is recommended to be used as dentine substitute in large fillings with a replacement layer of enamel composite.

AFM has become an important tool for imaging surfaces and analysis. AFM allows a 3D imaging at nanometric resolution and is emerging as reliable in evaluation of surface roughness features of composites. The AFM offers quantitative data on surface morphology. In this study, AFM analysis indicates that all the posterior composites tested exhibited surface roughness after polishing. To conclude, all composites exhibit roughness after polishing, the filler technology in composites may show variable results after polishing. Ever X, Filtek Z350, Tetric Evo Ceram showed less surface roughness when compared to other posterior composites.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.