|Year : 2017 | Volume
| Issue : 2 | Page : 56-61
Surface characterization and mechanical behavior of bulk fill versus incremental dental composites
Dalia A Abuelenain1, Ensanya A Abou Neel2, Ayman Al-Dharrab1
1 Department of Restorative Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia
2 Department of Restorative Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia; Department of Dental Biomaterials, Tanta University, Tanta, Egypt; Department of Oral and Maxillofacial Prosthodontics, UCL Eastman Dental Institute, Biomaterials and Tissue Engineering, London, UK
|Date of Submission||22-Nov-2016|
|Date of Acceptance||12-Mar-2017|
|Date of Web Publication||30-May-2017|
Dalia A Abuelenain
Department of Restorative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah
Source of Support: None, Conflict of Interest: None
The aim of this study was to evaluate surface and mechanical properties of bulk fill composite compared to conventional incremental composites.
Materials and methods:
The bulk fill composites were Filtek Bulk Fill, Sonic Fill, SDR Smart Dentin Replacement and Tetric-N-Ceram Bulk Fill while the incremental ones were Filtek Z350 × T and Herculite XRV Ultra. Surface roughness and wettability was measured using profilometer (Bruker) and drop shape analyzer (Kruss), respectively. Surface hardness of the top and bottom surface was measured using Micromet 6040 (Buehler). For mechanical test, the universal testing machine was used under the three-point bending test.
There was no statistical significant difference in wettability and surface roughness between bulk fill and incremental composites, except the SDR that showed statistically significance higher roughness than incremental composites. All composites showed significantly lower hardness than Filtek Z350; the lowest hardness was recorded for SDR. There was no significant difference between bulk fill and incremental composites in flexure strength and modulus. SDR showed the lowest flexure strength and modulus but the highest strain% (P < 0.05) compared to all tested materials. Sonic fill system showed significantly higher flexure strength and modulus when compared to other bulk fill materials (P < 0.05).
The difference between bulk fill and incremental composite is mainly material dependent.
Keywords: bulk fill, hardness, incremental composite, surface roughness, wettability
|How to cite this article:|
Abuelenain DA, Abou Neel EA, Al-Dharrab A. Surface characterization and mechanical behavior of bulk fill versus incremental dental composites. Tanta Dent J 2017;14:56-61
|How to cite this URL:|
Abuelenain DA, Abou Neel EA, Al-Dharrab A. Surface characterization and mechanical behavior of bulk fill versus incremental dental composites. Tanta Dent J [serial online] 2017 [cited 2017 Jun 26];14:56-61. Available from: http://www.tmj.eg.net/text.asp?2017/14/2/56/207304
| Introduction|| |
Dental composite is the material of choice for restoring damaged tooth structures with the advantages of chairside application, reasonable cost and near ideal esthetic and functional results. Bulk fill composite was introduced to the market to overcome the multistep application of the 2 mm incremental conventional composites saving clinicians' time and provide them with a less technique sensitive material, claiming the suitability of bulk fill material for posterior, stress bearing areas. The difference between bulk fill and incremental composite, is that, in bulk fill composite the increased depth of cure was achieved by enhancing materials' translucency through decreasing the amount of fillers and increasing fillers size  or by adding different photoinitiators to the same composite ,.
The performance of bulk fill composite material was investigated in several studies and compared to the conventional hybrid and nanohybrid universal composite available in the market within the last few years ,,,,,,,. The 24 h degree of conversion was significantly lower in some bulk fill composite (Filtek Bulk Fill and X-tra Base Bulk Fill) when compared to other types of bulk fill and conventional composites. This was attributed to variation in the chemistry of resin matrix. Accordingly, the physicomechanical behavior of some bulk fill composites (Tetric Evo Ceram Bulk Fill, Venus Bulk Fill, SDR, X-tra Fil, X-tra Base, Sonic Fill, Filtek Bulk Fill and Xenius) were significantly lower than the conventional high viscosity nanohybrid composite . Furthermore, the annual failure rate of bulk fill composite observed in a clinical evaluation of classes I and II cavities restored with different composites, were slightly higher in bulk fill composite than the conventional composite . On the other hand, bulk fill composites showed lower cuspal deflection  but good marginal integrity  when compared to conventional incremental composites.
The longevity of restorative materials could be influenced by their mechanical  as well as surface properties ,,,,,. For clinicians to be confident in selecting the proper material for specific application and judge the suitability of the recently introduced bulk fill material for stress bearing areas, proper evaluation of physical and mechanical behavior of such materials in comparison to the available well established incremental hybrid and nanohybrid filling materials is needed. So the aim of the present study was therefore to evaluate surface properties (roughness, wettability, hardness and relative hardness) and mechanical (flexure strength, flexure modulus and strain percent) of bulk fill compared to incremental composites.
Null hypothesis is that, there is no difference in surface and mechanical properties between bulk fill composite materials and conventional incremental nanohybrid composite filling material.
| Materials and Methods|| |
Six commercially available dental composites were selected for this study, four bulk fill and two incremental conventional nanohybrid composites. Technical details are presented in [Table 1].
Cylindrical samples (n = 5) of 6 mm diameter were prepared according to the manufacturers' instructions. The thickness was 2 mm for the incremental types but 4 mm for the bulk fill composite. Shade A2 was used for all tested composites. Composite were packed in one increment, covered with celluloid strip and sandwiched between two glass slides to obtain a flat and smooth surface, then light cured from the top surface using light-emitting diode curing unit (3M ESPE Elipar, Seefeld, Germany) delivering 1200 mW/cm 2, 430–480 nm. A pen mark was placed on the top cured surface. Samples were stored dry for 24 h in a plastic container at 22 ± 1°C. Samples were subjected to surface roughness and wettability measurement using profilometer (Bruker) and drop shape analyzer (Kruss), respectively. Surface wetting (contact angle) was performed using distilled water and glycerol. Top Vickers Hardness Number (VHN) and relative hardness (top/bottom ratio) was measured using Micromet 6040 (Buehler).
Composite bars (2 × 2 × 25 mm) were prepared (n = 5) using split Teflon molds. Composite materials were packed in the same way described previously. Curing was done according to manufacturer's instructions in three consecutive cycles to the top surface only using light-emitting diode light. Samples were stored dry for 24 h in plastic container at 22 ± 1°C. The dimensions of each sample were measured using a micrometer (STONEC, Germany) accurate to 0.01 mm, then samples were subjected to three-point bending test using a universal testing machine (Instron 5944, USA) at a crosshead speed of 0.25 mm/min and 2 N loading cell. The flexural strength (FS), modulus (EM), and fracture strain (S%) were calculated using Bluehill 3 software.
The test was don according to ISO 4049 , flexure strength (FS) and flexure modulus (EM) were calculated according to the following equations :
Where L is the applied load (N) at the highest point of load–deflection curve, I is the span length (20 mm), b is the width of test specimens and h is the thickness of test specimens. S is the stiffness (N/m) S = L/d and d is the deflection corresponding to load L at a point in the straight-line portion of the trace. Data were collected and subjected to one-way analysis of variance and Tukey's post-hoc test at 0.05 level of significance using SPSS software (Released 2011, IBM SPSS Statistics for Windows, Version 20.0., IBM Corp., Armonk, New York, USA).
| Results|| |
The mean values of contact angle, surface roughness and top VHN/relative hardness, are presented in [Figure 1],[Figure 2],[Figure 3].
|Figure 1: Contact angle of tested composites with either water or glycerol.|
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|Figure 3: Top Vickers Hardness Number (VHN) and relative hardness of tested composites.|
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Analysis of variance and Tukey's post-hoc test revealed that when using the Filtek Z350 as a control, all materials showed no statistical significant difference in the contact angle using either water or glycerol when compared with Z350. While there was statistical significance difference in contact angle of different bulk fill materials when using water as a test liquid (P < 0.05), there was a statistically significant difference between Sonic Fill system and Filtek Bulk Fill (P < 0.05).
When using Filtek Z350 as a control, all other materials except SDR showed no statistical significance difference in their surface roughness when compared with Z350 (P > 0.05). Comparing different types of bulk fill materials; there is a statistical significance difference between SDR and Filtek Bulk Fill, and between SDR and Tetric-N-Ceram. The highest roughness was observed with SDR followed by Sonic Bulk Fill composite. All composites showed significantly lower hardness than Filtek Z350; the lowest hardness was recorded for SDR (P < 0.05). There was no statistical significance difference between Tetric-N-Ceram and Herculite. All other composites showed statistically significant different VHN from each other.
The relative hardness ranges from 92 to 97% for all tested composites. All composites showed no statistically significance difference from Filtek Z350 (P > 0.05).
The mean values of flexure strength and flexure modulus and strain% are presented in [Figure 4].
|Figure 4: (a) Flexure strength, (b) flexure modulus and (c) strain (%) of tested composites.|
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There was no significant difference between the bulk fill materials and the two control nanohybrid composite Filtek Z350 (P < 0.05) and Herculite XRV Ultra (P < 0.05) in flexure strength and flexure modulus, while SDR and Tetric-N-Ceram were significantly lower than the other bulk fill composite materials (P < 0.05). Sonic Fill System was significantly higher than the other bulk fill materials.
SDR showed the lowest flexure strength, flexure modulus and the highest strain percentage (P < 0.05) compared to all tested materials and Sonic Fill System showed significantly higher flexure strength and modulus when compared to the tested bulk fill materials (P < 0.05).
| Discussion|| |
This study showed that the difference between bulk fill and incremental composite is material dependent and there is a significant difference in some properties while no differences in others. The bulk fill composites investigated in this study (Filtek Bulk Fill, Sonic Fill System, Tetric-N-Ceram, SDR Smart Dentin Replacement) showed variable results when compared to conventional incremental composites (Filtek Z350 and Herculite XRV Ultra). SDR showed the highest surface roughness, lowest surface hardness, lowest flexure modulus and the highest strain% when compared to both incremental and other bulk fill composites. However, Sonic Bulk Fill system exhibited the significantly highest hardness number and flexure modulus compared to Tetric-N-Ceram and SDR Bulk Fill materials. Accordingly, the null hypothesis is partially rejected.
Wettability of dental materials gives an indication of materials' hydrophilicity  and surface roughness of dental composite showed a significant correlation to composite depth of wear . Surface roughness and wettability of esthetic restorative material may influence bacterial accumulation and discoloration of restoration caused by food stains and adhesion of microorganism  which can accelerate material failure under functional conditions. In the present work, the lower water contact angle of Sonic Bulk Fill compared to Filtek Bulk Fill indicates higher hydrophilicity of Sonic Bulk Fill composite . Craig  confirmed that the hydrophobic composite with high water contact angle can reduce or eliminate marginal leakage even in the absence of chemical bonding between composite and dentin. In the present work, the water and glycerol contact angles were both higher than that of bisphenol-A-glycidyl dimethacrylate (65°) , indicating high hydrophobicity of all tested materials. This could be due to the development achieved in composite resin technology and the inclusion of more hydrophobic monomers than bisphenol-A-glycidyl dimethacrylate.
Surface roughness of composite resin materials is influenced by resin matrix, filler type, size, shape and distribution . A maximum surface roughness (Ra) of 200 nm has been suggested in previous work  as a threshold value for bacterial retention. Below this value, no further reductions were observed, while over this value, biofilm accumulation increased with increasing roughness . In the present work, Ra values higher than 200 nm were observed with SDR and Sonic Bulk Fill materials, which indicate the need to cover these filling materials with other veneering composites to reduce the possibility of bacterial accumulation. The surface roughness of most materials will increase over time in the oral cavity. In order to achieve a surface roughness under the threshold value (Ra = 200 nm) on dental polymers, special routines with stepwise polishing and finishing must be performed and repeated at regular intervals .
Surface hardness of composite resin material is influenced by resin matrix, filler type and filler loading and degree of conversion . In previous work  there was no correlation observed between degree of conversion and surface hardness. However, it has been confirmed in other studies ,, that a composite material with a higher surface hardness, is considered to be more wear resistant and a significant relationships were confirmed between depth of wear and hardness or average surface roughness. Therefore, measurement of surface hardness may give an indication of wear resistance and clinical performance of different dental composites. In addition, relative hardness (bottom hardness/top hardness) is used in polymer research to investigate irradiance transmission within the full depth of material indicating the degree of conversion of composite material . Therefore, surface hardness and relative hardness were investigated in the present study to compare the bulk fill material to conventional incremental composites. The results of the present work was in agreement with the previous work in which incremental-fill composite showed higher VHN than bulk-fill composites and SDR exhibited the lowest Vickers hardness when compared to other flowable hybrid composite materials . In addition, another study showed low to very low hardness and elastic modulus of SDR and Filtek Bulk Fill . Several investigations confirmed a positive correlation between filler loading and surface microhardness ,, in which the lower hardness value of SDR can be explained by its lower filler loading (68 wt%) which indicates high resin contents based on urethane dimethacrylate and ethoxylated bisphenol-A-dimethacrylate . The relative hardness of all tested materials (92-97%) were higher than the minimum values (80%) required to insure proper irradiance transmission through the full thickness of the bulk fill or incremental composite . Therefore, proper conversion is expected at the base of bulk fill material.
Mechanical behavior of different resin material is influenced mostly by monomer system, filler type, filler loading and filler-resin interphase . In the present study, the Sonic fill material with the highest filler loading (83 wt%) has the highest hardness number and highest flexure strength and modulus compared to other bulk fill materials. While SDR with the low filler loading (68 wt%) showed the lowest mechanical behavior. Filtek Bulk Fill (64.5 wt%) showed comparable results to Sonic Bulk Fill and Z350 Filtek (78.5 wt%). Several investigations found very low correlation between filler loading and mechanical behavior of composite resin materials ,,. Therefore, the differences observed in the resent study cannot be inferred to filler loading only; other factors such as the use of different resin matrix, different types of fillers and filler size and distribution may contribute to the different results observed. Generally, the mechanical properties of bulk fill composite were lower than conventional microhybrid or nanohybrid composite and this is in accordance with other study  that recommended the need for conventional composite to veneer bulk fill materials.
| Conclusion|| |
Within the limitations of the present work, it can be concluded that surface and mechanical properties of different bulk and incremental-fill composites will vary according to materials composition. During clinical application, clinicians should carefully select the proper material for each application based on the needs of the case and properties of the available material. For example, since SDR is a dentin replacement material and its intended use is to be veneered with resin composite material with higher surface and mechanical properties, its application should adhere to what is recommended by manufacture and it should not be applied routinely with no indication. On the other hand, Sonic Bulk Fill proved to be a suitable material for stress bearing area when indicated due to its improved mechanical properties.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant no. 3-165-35-RG. The authors therefore acknowledge with thanks DSR technical and financial support. The authors would also like to acknowledge the 'Advanced Technology Dental Research Laboratory' at King Abdulaziz University, Faculty of Dentistry.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]