Surface morphology and mechanical properties of new-generation flowable resin composites for dental restoration
Introduction
The development of flowable composites appeared in the 1990s as an important advancement in restorative dental materials [1]. Flowables are low viscosity resin composites obtained from formulations with 20–25% lower filler loading than conventional composites [2]. The lower viscosity of flowables makes their placement by injection syringes possible and it limits stickiness. First-generation flowables were used only as liners due to their low elastic modulus. The second-generation flowables developed since 2000 promise increased mechanical properties and are proposed for use in bulk restorations. However, recently the increasingly available clinical reports have not been as successful as expected. For example, post-operative sensitivity did not improved as claimed [3]. Also, sometimes the use of flowables in non-carious cervical lesions did not improve clinical performance, neither when used alone [4] nor as a liner [5], [6]. Therefore, it emerged that the use of flowables in critical stress applications is not recommended in the absence of appropriate preliminary characterization of their mechanical properties and their relationship to surface morphology [7], [8].
For an evaluation of overall mechanical performance of dental composites no general consensus exists about the most important properties. Commonly, fracture toughness and strength (flexural, uniaxial compressive and tensile) are evaluated [9]. The authors’ focused on Young's modulus and hardness, which were measured by nanoindentation. Hardness, in particular, is often considered the most effective quantity for both assessing and predicting the performance of dental composites [10]. These elastic properties were determined for four flowables currently used (VDF, VF, FF, SF, full names in Table 1) as well as for one positive and one negative control, consisting of a conventional composite for universal restorations and an adhesive system (VD and AO, respectively). Along with the mechanical measurements a morphological analysis of the surface was performed, and the relation between filler distribution and elastic properties discussed.
Section snippets
Specimen preparation
Samples of the six materials, all in A2 shades, were obtained from the respective companies on request. Slabs of ∼10 mm × 10 mm × 2 mm were prepared by filling a Teflon mold and curing the paste through a Mylar sheet. After SEM imaging the samples were polished using a 656 Dimple Grinder (Gatan, USA) with a felt ring loaded with 0.25 μm diameter diamond particles. After polishing the specimens were repeatedly wiped clean with lens paper impregnated with ethanol. All specimens were stored in air before
Morphology
In Fig. 1 electron micrographs of the materials are presented. Different brightness and contrast results from gray level equalization were performed independently in each image. To have an independent means of evaluating the SEM results, AFM was also carried out after specimen polishing. In fact, AFM imaging of as-cured composites does not permit a clear distinction between fillers and matrix [14], even when the signals of lateral force in contact-mode or phase shift in tapping-mode are
Discussion
The BSE signal used for SEM contains compositional information, which helps to distinguish between fillers and resin. One can assign the brightest particles to fillers closer to the surface and the darker ones to fillers deep inside the composite. At the considered acceleration voltage the mean depth of BSE signal is ∼600 nm, in reasonable agreement with the largest fillers size, as both expected and observed from grain analysis. Therefore SEM image depth should correspond to a single filler
Conclusion
Laboratory testing of commercial dental restorative composites is often focused on checking the standards for safety more than the material efficacy. Therefore, the clinical success is only discovered after long periods of use by general practitioners. However, measuring the mechanical properties of novel composites in the laboratory is important in assessing the expected performance without costly clinical trials or experimentation with unaware patients. Recently a new trend of the widespread
Acknowledgement
The authors thank IIT for institutional support.
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