INTRODUCTION:

In dentistry, resin-based composites have been the prime materials used for direct restorations for both anterior and posterior teeth due to their aesthetics, superior physical properties, better handling characteristics, and bonding systems, curing refinements, environmental concerns, and better clinical performance^1,2,3.These properties are mainly influenced by the wide range of organic and inorganic constituents and their microstructure. The main constituents of the composite materials include organic polymeric matrix, inorganic fillers, and a silane coupling agent that links these two components.

Over the years, major changes have been witnessed in the filler particles rather than the monomer system which was developed by Bowen in 1962⁴. Fillers in the composites significantly reduce the polymerization shrinkage, coefficient of thermal expansion, water sorption, and solubility. Also, mechanically reinforce the material to enable better initial polishing and polish retention and to resist wear during the masticatory forces⁵.

Ferracane chronologically classified resin composites as macro-fill (10 to 50µm), micro-fill (40 to 50nm), and hybrid (10 to 50µm+40nm). Later on, hybrid composites were distinguished as midi-fill resin composites with an average particle size slightly greater than 1µm and a portion of the 40nm fillers. Then refinement of the fillers resulted in micro-hybrids with particle sizes 0.6 to 1µm and 40nm and further improvement resulted in micro-hybrids with 0.6µm-1µm and 40nm⁴.More recently, nano-filled and nano-hybrid composites have been developed which are claimed to be possessing high initial polishing combined with superior polish and gloss retention apart from excellent wear resistance and mechanical strength suitable for use in high-stress-bearing areas⁴.Nano-sized fillers can be categorized as either isolated discrete particles with 5-100nm or as fused aggregates as clusters exceeding100nm. The less inter-particle space created with the fine filler particles in the composite resins provides more protection to the resin matrix resulting in the reduced removal of filler particles from the surface of the restoration. Newer nano-fillers along with various methacrylate resins have been developed by Mitra et al., that are being marketed as the Filtek range of restorative materials (3M ESPE, St Paul, MN, USA)⁴.

For composites, smooth surfaces can be obtained when the restoration is polymerized against a matrix strip. Despite the careful placement of the matrix, removal of excess material and re-contouring of the restoration is often clinically necessary that requires some degree of finishing and polishing⁶and the eventual smoothness of the composite surface is significantly influenced by the quality of the finishing and polishing. The smooth surface is clinically important as it determines the aesthetic appearance and longevity because the rough and poorly polished surfaces contribute to staining, plaque accumulation, gingival irritation, recurrent caries, and the discoloration of the restoration⁷.Decreasing the surface roughness also reduces the biofilm formation and as mentioned in the previous studies, surface roughness less than 0.2μm significantly reduces the possibility of bacterial adhesion on the surface of the restoration⁵.However, regardless of the polishing technique employed, the resin matrix and variations in the filler particle type, size, shape, filler content optimization play a significant role and it can be assumed that large ﬁllers on the polished surface cause rough texture.

The surface roughness of dental materials can be measured at Nanoscale both qualitatively and quantitatively using different methods like the optical electron microscope, scanning electron microscope, profilometry, etc.., and recently atomic force microscopy (AFM) has also been used widely as it allows a three-dimensional (3D) imaging⁸.Hence, the present study is undertaken to evaluate the surface roughness and colour stability of different composites that are polished with different polishing systems.

MATERIALS AND METHODS:

The present in vitro study was carried out in the Department of Pedodontics and Preventive Dentistry, GITAM Dental College and Hospital in collaboration with the Centre for Nanotechnology, Andhra University after obtaining Ethical clearance. Cylindrical Teflon molds of 6mm diameter and 2mm depth were procured and the molds were filled with resin composite using a Teflon-coated plastic instrument taking care to prevent air entrapment and voids. The molds were then placed between two glass slabs stuck with Mylar strips and constant pressure was applied to expel the excess material. Then, curing was done with LED source (3M ESPE, St. Paul, MN, USA) for 40seconds. The distance between the composite material and the light source was standardized by placing the curing tip perpendicular to the glass slab on the mold surface. After curing, the specimens were removed from the Teflon molds. This way 20 specimens were prepared using Solare Sculpt composite, 10 specimens were prepared using Filtek Bulk-fill composite, and 10 specimens were prepared using Tetric N-Ceram Bulk-fill composite.

Division of Samples:

The 40 specimens were divided into 4 groups with 10 specimens of Solare Sculpt in group 1, 10 specimens of Solare Sculpt in group 2, 10 specimens of Filtek Bulk-fill in group 3, and 10 specimens of Tetric N-Ceram Bulk-fill in group 4. The specimens in group 1 were left unpolished whereas the specimens in all other groups were polished either with Sof-Lex discs (subgroups 2a, 3a, 4a) or Diacomp® Plus Twist (subgroups 2b, 3b, 4b).

Polishing procedure:

For the polishing procedure to be carried out, the specimens were stabilized in a Teflon mold mounted on an acrylic base.

Polishing with Sof-Lex discs:

Using a contra-angled micro motor handpiece at 10,000 rpm, sequential application of Coarse, Medium, Fine, and Superfine aluminum oxide discs was done following the manufacturer’s instructions. Each disc was applied to the specimen for 15sec with light pressure in constant planar motion. Irrigation was performed with water spray for 5sec between each application.

Polishing with Diacomp® Plus Twist system:

The sequential application of pink and grey diamond-impregnated flexible spiral wheels was done using a contra-angled micro motor handpiece at 5000rpm. Each spiral wheel was applied for 15sec and Irrigation was performed with water spray for 5sec between each application.

After polishing, the specimens were thoroughly rinsed with air/water spray for 10sec and stored in distilled water at 37°C for 24hr to allow for complete polymerization of samples and then subjected to analysis of surface roughness followed by colour stability assessment.

Surface roughness Analysis:

To measure the surface roughness of the composite specimens, an Atomic Force Microscope (Park XE7 systems) that was calibrated before obtaining the measurements was used in the following manner. Firstly, the sample to be measured is placed on an XY scanner and the silicon carbide cantilever of 0.5µm diameter was allowed to access the 1µm×1µm scan area in non-contact mode and the images of the scanned area were obtained. This procedure was randomly carried out over all the samples by a single examiner. All the surface features were obtained at the Nanoscale and any roughness detected is shown as small grains or particles. The 3D images thus obtained were evaluated both visually and numerically and were analyzed using Park XEP data acquisition software. The 3D surface analysis provides the phase-type data as well as numerical data of surface properties or histogram analysis data. After completing the surface roughness analysis, the same specimens were used for the measurement of colour stability.

Colour assessment:

Using a digital spectrophotometer (Vita Easy Shade Advance 4.0, Vita Zahnfabrik, Bad Säckingen, Germany) initial baseline colour measurements of all the specimens were obtained. While recording Colour measurements, the active point of the spectrophotometer is placed in the center of the specimen and each specimen is placed on a white background to prevent potential absorption effects. The procedure is performed three times and a mean value for “L” which represents the value (lightness or darkness), “a” which is a measure of redness (positive a) or greenness (negative a), and “b” which is a measure of yellowness (positive b) or blueness (negative b) were obtained for each specimen facilitating the determination of colour in the three-dimensional space. All the procedure was carried out by a single examiner to avoid bias. After obtaining the baseline colour measurements, the specimens were subjected to immersion in carbonated apple juice.

Immersion procedure:

Carbonated apple juice was taken and p^H was measured using a digital p^Hmeter which was found to be 3.34. The specimens were then subjected to immersion in it for 30mins. Then the specimens were taken out and were immersed for the rest of the day in artificial saliva.

Artificial saliva was prepared in the laboratory by adding 0.4gm of Sodium chloride (NaCl), 1.21gm of Potassium chloride (KCl), 0.78gm of Sodium dihydrogen phosphate dehydrate (NaH₂Po₄.2H₂O), 0.005gm of Hydrated sodium sulfide (Na₂S.9H₂O), and 1gm of Urea (CO(NH₂)₂) to 1000ml of distilled water. To this mixture, 10N Sodium hydroxide (NaOH) was added to adjust the p^H of the solution to 6.75±0.15. The solutions were renewed every day and this procedure was repeated for 7days. After this period the specimens were removed from the immersion media and rinsed with distilled water for 5mins and blot dried with tissue paper for15 sec and final colour assessment was done using a spectrophotometer.

The colour difference (ΔE) was then calculated Using Hunter’s equation⁹ as follows,

ΔE= [(L₂ – L₁)²+(a₂– a₁)²+(b₂– b₁)²]^1/2

Where, L_1,a₁, and b₁ are the initial values and L₂, a₂, and b₂ are the final values.

Statistical Analysis:

The surface roughness and colour change (ΔE) measurements were subjected to statistical analysis using SPSS version 20. One-way analysis of variance (ANOVA) was done to compare the surface roughness and colour stability and Tukey’s Posthoc tests were done for pair-wise comparison of surface roughness and colour stability between four groups and their subgroups. The Probability value was set at 0.05.

RESULTS:

Surface roughness:

Table 1: Mean and Standard Deviation values of the surface roughness (Ra) of analyzed composites.

Material	Sample No (n)	Min (nm)	Max (nm)	Mean (nm)	S.D
Group 1	10	2.56	7.08	3.98	1.33
Group 2a	5	7.99	19.45	12.96	5.17
Group 2b	5	6.83	13.99	9.80	3.10
Group 3a	5	3.76	7.54	5.80	1.67
Group 3b	5	4.78	17.20	8.43	5.23
Group 4a	5	3.58	12.66	7.72	3.56
Group 4b	5	5.08	14.50	7.91	3.84

The mean and the SD values of surface roughness for the composites after polishing were shown in table-1. The mean Ra values of Solare Sculpt without polishing, and after polishing with Sof-Lex and Diacomp® discs were found to be 3.98nm, 12.96nm, and 9.80nm respectively. The mean Ra of Filtek Bulk-Fill and Tetric N-Ceram after polishing with Sof-Lex and Diacomp® discs were found to be 5.80nm and 8.43nm and 7.72nm and 7.91nm respectively. The lowest Ra was observed in unpolished Solare Sculpt specimens and a significant difference was found only between Solare Sculpt and Filtek Bulk-Fill after polishing with Sof-Lex discs with a P-value of 0.0349 and no significant difference was found among any other groups regardless of the polishing method.

Colour stability:

Table 2. Mean and Standard Deviation values of colour stability (ΔE) of analyzed composites.

Material	Sample No (n)	Min (nm)	Max (nm)	Mean (nm)	S.D
Group 1	10	1.02	6.67	2.79	1.58
Group 2a	5	2.41	8.64	4.16	2.58
Group 2b	5	1.88	8.67	4.04	2.77
Group 3a	5	2.81	6.73	4.85	1.47
Group 3b	5	1.69	4.61	3.19	1.29
Group 4a	5	3.11	7.02	5.11	1.58
Group 4b	5	2.40	9.39	4.32	2.92

The mean ΔE and the SD after polishing with Sof-Lex and Diacomp systems were shown in table-2. For Solare Sculpt, the mean ΔE value without polishing was found to be 2.79. The ΔE values after subjecting to polishing were found to be 5.11 and 4.32 respectively for Tetric N-Ceram. The lowest ΔE was obtained for unpolished Solare Sculpt. Among the polishing systems, Sof-Lex showed the least ΔE values for Solare Sculpt followed by Filtek Bulk-Fill followed by Tetric N-Ceram and for Diacomp, least values were obtained by Filtek Bulk-Fill followed by Solare Sculpt followed by Tetric N-Ceram. The ΔE values when polishing with Sof-Lex and Diacomp® discs were found to be 4.16 and 4.04 and for Solare Sculpt, 4.85 and 3.19 respectively for Filtek Bulk-Fill. No Statistical difference was found in ΔE among the tested composites and polishing systems with P-value > 0.05.

Figure 1-3-D image showing of Unpolished specimen of Solare Sculpt composite

Figure 2: 3-D image showing of Solare Sculpt after polishing with a) Sof-Lex discs b) Diacomp discs

Figure 3: 3-D image showing of Filtek Bulk-fill composite after polishing with a)Sof-Lex discs b)Diacomp discs

Figure 4: 3-D image showing of Tetric N-Ceram Bulk-fill composite after polishing with a)Sof-Lex discs b)Diacomp discs

DISCUSSION:

The term surface quality reﬂects a wide range of properties such as morphology, colour, roughness, gloss, and polarity,among which surface roughness plays a major role as the increase in surface roughness of resin-based composite restorations causes discomfort to the patients in terms of tactile perception. On the other hand, the smooth surface prevents plaque retention and formation of discoloring bio-ﬁlms thereby enhancing aesthetics^10,11. Ereifej NS, Oweis YG, and Eliades G (2012) stated that the surface roughness of resin-based composite restoration depends on several factors that include filler content, size, shape, and inter-particle spacing, monomer type, degree of cure, and efficient matrix-filler bonding⁸.

Recently introduced Nano-filled composites have strontium fillers that strengthen the adhesion between Nano-ceramic and resin matrix for high flexural strength and a beautiful high gloss finish with low surface roughness. The other type of composites are Nano-hybrids that combine nanometer-sized particles with more conventional filler technology. The small size of the filler particles improves the optical properties of resin-based composites because their diameter is a fraction of the wavelength of visible light (0.4–0.8μm) resulting in the human eye’s inability to detect the particles⁶.The shape of the fillers present in the Nano-hybrids is also found to be different when compared to Nano-fill composites. The properties of Nano-particles and resulting Nano-composites are size and shape-dependent and very irregularly-shaped particles tend to decrease the retention of smoothness³.Both the Nano-fill and Nano-hybrid composites presenting clusters showed lower surface roughness values than all the composites indicating that the presence of small fillers forming clusters is beneficial.³According to the previous studies, a maximum surface roughness value of 200 nm has been suggested as a threshold value for bacterial retention. Below this value, no further reductions were observed and above this value, the incidence of biofilm accumulation increases with an increase in roughness. To achieve the surface roughness values under the threshold value, step-wise finishing and polishing of resin-based composites must be performed and repeated at regular intervals with special attention¹².Weitman RT and Eames WB (1975) conducted a clinical study on the plaque accumulation after various finishing procedures and stated that mean surface roughness of 0.7-1.44µm leads to plaque accumulation on the surfaces of polished composites¹³. Quirynen M, Marechal M, Busscher HJ, Weerkamp AH, Darius PL, and van Steenberghe D (1990) demonstrated that an increase in the surface roughness Ra value above 2 µm leads to a steep increase in biofilm formation in vivo¹⁴.

The surface roughness of the composites was analyzed by AFM equipped with a silicon carbide cantilever of 0.5µm diameter and a scanner of the maximum range of 100µm×100µm×100µm, in x, y, and z directions respectively and an optical microscope to locate the area of interest by monitoring the sample on the screen of a computer system connected to it. The 3-dimensional images of each polished surface obtained in non-contact mode at a range of 1µm×1µm in the center of each sample with no visual defects were analyzed to avoid the bias of sensitivity readings of AFM cantilever. The 3D images thus obtained were evaluated both visually and numerically and analyzed using the AFM roughness analysis software, surface roughness parameter Ra obtained is equivalent to the line profilometric parameter of the profilometers. The 3D surface analysis provides the phase -type data as well as numerical data of surface properties or histogram analysis data.

The AFM images obtained for unpolished Solare Sculpt specimens showed low-profile surface randomly interrupted by projections and scratch lines indicating low surface roughness. The polished specimens of Solare Sculpt showed a non-uniform surface with distinct sharp projections with dotted pores with deep heights and valleys indicating high surface roughness. The images of Filtek Bulk-fill composites after polishing showed moderately irregular surfaces with slight relief showing heights and valleys with sharp projections at some points indicating low surface roughness. The AFM images of Tetric N-Ceram bulk-fill showed narrow deep scratch lines causing an irregular surface showing moderate roughness. The scratches observed on polished composites may be attributed to the grinding effect of the dislodged ﬁllers during the polishing procedure. Thus, the resin composites composed of smaller size ﬁllers exhibited narrower and shallower lines. The differences found in the surface roughness and texture among the composites tested by AFM may reﬂect variations in their composition and size distribution of the ﬁllers.

Filtek Bulk-fill showed the lowest Ra values when compared to Solare Sculpt and Tetric N-Ceram Bulk fill composites when polishing was done with Sof-Lex discs. Yap AU, Yap SH, Teo CK, and Ng JJ (2004) conducted various studies on surface roughness of different composites and found that the predominance of zirconia-silica nanoclusters in Filtek Nano-fill composites is the reason for lower surface roughness values¹⁵. Followed by polishing with Sof-lex discs, Ra values recorded for Tetric N-Ceram Bulk fill composites were slightly higher than Solare Sculpt. The reason could be due to disruption of the filler matrix interface from the loss of pre-polymerized fillers. Similar findings were obtained in the study conducted by Senawongse and Pongprueksa (2007)¹⁶.

Solare Sculpt showed higher surface roughness values after polishing when compared to other composites tested in the study. A small percentage of larger strontium glass ﬁllers of 300nm in size are incorporated in the reinforcing phase and these strontium filler particles are more likely to be responsible for the rounded projections found on its surface resulting in higher roughness values. But, followed by polishing with Diacomp® Plus Twist polishing system, Filtek Bulk-fill composites showed higher Ra values than Tetric N-Ceram Bulk-fill composites. These results could be attributed to the rotary motion of the Diacomp® Plus Twist polishing system, where the axis of rotation is parallel to the surface is smoothened. On the whole, Filtek Bulk-fill composites which contain smaller filler particles showed overall superior surface quality after finishing and polishing when compared to other composites. Thus, the highly exposed large-sized filler particles on the surface of the restoration will produce higher surface roughness values.

When the application time was considered, each polishing procedure was performed for a different amount of time. The sequential use of four different Sof-Lex discs caused the longest time for the polishing procedure. This was considered as an important factor for obtaining smoother surfaces particularly seen in Filtek Bulk-fill and Tetric N-Ceram Bulk-fill composites after polishing with Sof-Lex contouring and polishing system. Stoddard and Johnson (1991) suggested that because of the variations in filler particles and types of resin, it is important to pair a resin composite with a matching polishing system¹⁷.

Among all the groups, the smoothest surface was obtained in the specimens that were polymerized against Mylar strips in accordance with many previous studies^18,19. The use of Mylar matrix on the top of the surface of a resin composite prevents the formation of an oxygen-inhibited layer during the polymerization resulting in a smooth, non-sticky surface thereby giving it a smoother appearance. However, despite the careful placement of the matrix, removal of excess material and re-contouring of the restoration is often clinically required and some degree of finishing and polishing is always deemed necessary⁶.However, even after following appropriate finishing and polishing procedures, the surface of the composites exhibits micro-irregularities that invariably leads to the material wear, deterioration, and marginal infiltration of the restoration in the oral environment^1,20 because pH varies in the oral cavity as a result of the bacterial metabolism leading to an adverse effect on the wear resistance of resin composites²¹.

When the colour changes of the composites were measured after immersion in carbonated apple juice for one week, lower ΔE values were obtained for Filtek followed by Tetric N-Ceram composites followed by Solare Sculpt composites. Periods as low as 7 days are sufficient to produce staining and colour changes and with the increase in time, colour changes become more pronounced.²²Hence, One-week is considered as an immersion period in the present study as it allows for the composite post-cure and elution of all leachable components from the materials tested.The presence of UDMA in the Tetric® N-Ceram Bulk Fill composite might have played a role in the discoloration. The traditional di-meth-acrylates form cross-linked networks with unreacted pendant meth-acrylates that serve as plasticizers and the resultant plasticization impart a more open structure to the polymers, which facilitate the absorption of the staining agent. In addition, an increase in the concentration of the staining agent attributes to the porosity of some glass particles of the filler.²³ Next to Tetric N-Ceram, polished specimens of Solare Sculpt have shown higher ΔE values, and these changes are due to TEGMA in its composition. The ethoxy groups in TEGMA acts as a hydrophilic component that tends to show more affinity with water molecules by bonding hydrogen to oxygen²⁰.

Carbonated apple juice (Appy Fizz) was used as an immersion medium due to its frequent consumption in daily life. Although there is a significant improvement in the development of the filler technology regarding the filler particle size, low wear, and high resistance to the degradation of resin-based composites, the present study showed perceptible colour changes after immersion in carbonated apple juice. The lower pH of apple juice likely affected the surfaces of resin-based composites increasing the pigment absorption. Previous studies²⁴ have evaluated the association between pH variations and staining and concluded that the acidic pH can cause dissolution or surface erosion of the restoration and the resultant high temperature can interfere with the surface properties¹. Though the difference was not found to be significant among the test groups, all the specimens showed perceptible colour changes after immersion for one week. But no correlation was established between the surface roughness and the colour changes, signifying the fact that the surface colour changes of the composite restorations might not solely depend on the surface roughness and there may be many other factors involved which have been proved by many studies previously. Yildiz E, Karaarslan Es, Simsek M, Ozsevik As, and Usumez A (2012) conducted a study on colour stability and surface roughness of polished anterior restorative materials and showed no relationship between colour stability and surface roughness in composite resins and compomer groups²⁵.A study conducted by Patel SB, Gordan VV, Barrett AA, and Shen C (2004) concluded that the degree of colour change does not depend on the type of composite resin, rather it is influenced by the type of staining solution used²⁶.It is known from previous studies that resin-based composites allow stain penetration into the matrix or filler-matrix interface through the micro-cracks that are formed due to hydrolyzing the silane component of the composites²³.

To determine the true colours of translucent specimens and avoid the inaccuracies of edge loss, it is recommended that a spectrophotometer be used. Hence, to evaluate the colour changes, a digital spectrophotometer, VITA Easy Shade Advance 4.0 system (Vita Zahnfabrik, Bad Säckingen, Germany) was used to overcome the low reproducibility of the visual assessment and also to eliminate the potential subjective errors caused by colorimeters. It translates the wavelength reflected by a specific body into values expressed as ΔE. Spectrophotometer measures more precise sections of the visible light spectrum within the range of 400 to 700nm²⁷.During colour measurement, both the actual color of the surface and the lighting condition under which the surface is measured will affect the measured colour, and so a standard illuminant against a white background was used.

It is important that there is a uniform distribution of filler particles in the matrix of the resin-based composites to minimize the formation of filler-rich and filler-depleted areas within the composites. This is especially important concerning the performance of composites in aqueous environments²⁸.Though different factors like components of the composite system, uniformity of the distribution of the filler particles, immersion medium, time of exposure to the staining agents, etc.., contribute to the amount of discoloration, special attention should be paid to obtain a perfect surface finish as it seems to be the single most important factor in surface staining.The results have shown that the outcome and the longevity of the composite restorations depend on the composition of the materials, the finishing and polishing systems used and the environment they are subjected to in the oral cavity.

CONCLUSION:

The following conclusions can be drawn from the present study:

a) The smoothest surface was obtained for specimens finished with Mylar strips.

b) Following polishing with Sof-Lex discs, the smoothest surface was obtained for Filtek Bulk-Fill, a nano fill composite and with Diacomp® Plus Twist, the smoothest surface was obtained for Tetric N-Ceram Bulk-Fill, a nanohybrid composite. Hence, compatibility between the composite and the polishing system also plays a major role in defining the surface characteristics.

c) In addition to the composition of the composite and the type of polishing system, surface smoothness and eventual deterioration depend on many factors.

d) Maximum colour stability was shown by Filtek Bulk-Fill though it was not the smoothest of the surfaces evaluated signifying the fact that in addition to the surface texture, the surrounding environment is also a major determinant of colour stability.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 05.02.2021 Modified on 07.08.2021

Research J. Pharm. and Tech 2022; 15(9):3854-3860.

DOI: 10.52711/0974-360X.2022.00646