The influence of nanoscale inorganic content over optical and surface properties of model composites
Introduction
Color and appearance of teeth and restorative materials are a complex phenomenon influenced by several factors such as lighting conditions, translucency, opacity, scattering and light transmission, brightness, fluorescence and opalescence. The surface texture and the size of a restoration may also produce an effect on the perception of light by the observer.1, 2, 3, 4 It is known that the surface of the restoration can be polished to a high gloss.5, 6 Being an attribute of visual appearance, gloss is directly influenced by the surface roughness7 as originates from a geometrical distribution of light reflected by the surface.8 Changes in surface gloss in the oral environment due to mechanical wear and/or chemical degradation of composite restorations may result in unfavourable aesthetics over time.
When incident light strikes a materials’ surface, part of the light is reflected at the boundary between the material and air. The other part of the light is absorbed and interactions such as: reflection, refraction, absorption, scattering and/or transmission occur throughout the restorative composite material. This happens because of changes in the refractive index, once the ratio of the speed of light in a vacuum is different from the speed of light inside.9 The back of the mouth receives little light and appears dark. As a consequence, it is not uncommon to observe in Class III or IV restorations a grey effect in the composite restoration compared with the color of the remaining tooth tissue. This occurs if translucent materials are placed in a cavity with no opaque structure backing, affected by the dark background the restoration will appears dark or grey.10 Thus, translucency and opacity are fundamental characteristics in order to achieve aesthetic restorations with composites, since they are indicators of the quality and quantity of reflected light.11 The main component of a dental composite that significantly affects the translucency is its inorganic content.12 Moreover, the quantity, the size and the shape of the fillers affect the light scattering in dental composites.13
In the last decade several dental resin–composites were introduced utilising nano-filler technology and thus classified as nanocomposites or nanohybrids. The nano-fillers can be dispersed in high concentrations within the organic matrix, improving physical and mechanical properties of the composites after setting.11, 14, 15, 16, 17 Defined as “nanocomposites” these materials appear to bring benefits such as improved surface smoothness,15, 18 excellent resistance to mechanical wear, high aesthetics due to the higher volume fraction of loads and ease of maintenance and polishing.19, 20 Thus, strongly indicated for restorations in areas where aesthetics are indispensable.
Surface texture and optical properties can be modified by several factors such as wear of fillers, degradation of the organic matrix or weakening of organic matrix-filler bonding.21 Some studies have investigated the influence of the type, size and volume of fillers in the optical properties of experimental dental composites.17, 22, 23, 24 Nevertheless, there is no data in the literature about the influence of size and volume of fillers in nanostructured composites on optical and surface properties of the materials when they are submitted to ageing procedures. In this study, a series of model resin-composites with systematically varied filler sizes was examined. The aim was to investigate the effect of filler-size on (i) surface properties such as roughness (SR), surface gloss (SG) and hardness (SH) and (ii) optical properties such as color measurements (CIE L*a*b* parameters), color difference (ΔE*) and translucency parameter (TP) before and after ageing procedures. The hypotheses tested were that filler-size and ageing procedure would affect: (i) optical properties (ii) surface properties.
Section snippets
Materials formulation
The monomers 2,2 bis[4-2(2-hydroxy-3-methacroyloxypropoxy)phenyl] propane (Bis-GMA, Esstech, Essington, PA,USA) and triethyleneglycol dimethacrylate (TEGDMA, Esstech, Essington PA, USA) were mixed in equal parts by weight. Camphorquinone (0.5% mol; ESSTECH, Essington, PA, USA) and ethyl 4-dimethylaminobenzoate (1% mol; Sigma–Aldrich, Chemie, Steinheim, Germany) were added as photoinitiators. The photoinitiator/co-initiator ratio used was based on a previous paper.25 After this, three groups were
Results
Fig. 1 presents values of CIE L*, CIE a* and CIE b* of each group, over time on water storage and after the toothbrush abrasion. For all groups immersion in water for 60 days significantly decreased CIE L* values (p < 0.05). On the other hand, after toothbrush abrasion CIE L* values increased (p < 0.05) compared with immersion for 60 days in water. Materials formulated with larger filler size (G3) revealed higher CIE L* values (p < 0.05) than medium and smaller filler sizes (G2 and G1). Although,
Discussion
Nano filler size composites may have optical features that promote aesthetic matching of restorative materials with surrounding dental hard tissues. In comparison with hybrid resin-composites, nanofillers are believed to offer improved mechanical and optical properties.34 However, there is no consensus on the benefits of these materials, especially in relation with their long term clinical performance.
Optical properties were influenced by ageing procedures and filler size, thus first hypothesis
Conclusions
Within the limitations of the current study, the authors conclude that:
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Filler sizes influenced the optical and surface properties of the nanostructured composites evaluated in this study.
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Distinct advantages of smaller and medium up to larger fillers were shown regarding to color difference, gloss retention and surface roughness;
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All materials evaluated were affected by ageing procedures. Optical and surface properties were adversely affected by immersion in water and toothbrush abrasion.
Acknowledgements
There are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcomes.
This work was supported by FAPERJ/Brazil grant E-26/111.658/2010. The authors thank Esstech Inc. and Degussa Evonik for donation of the materials.
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