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Introduction

Resin composites have been extensively employed in the restorative dentistry for more than four decades. Since 1980, light-activated composite materials are commonly used for tooth-colored restorations. The popularity of light-activated composites is due to their excellent esthetics, biocompatibility, wear resistance and above all to their “clinical friendly” use compared to the fast curing two-paste chemically activated composites. However, light-activated composites are time-consuming and technique sensitive restorative material and placement of the restoration can be stressful experience . The quality of placement is directly related to the retention rate, final quality and life-span of composite restorations.

The primary reason for the clinical failure of composite restoration is secondary caries, followed by fracture . Secondary caries have been proposed to be related to the polymerization shrinkage and shrinkage stress created on the cavo-restoration interfacial bond, as well as durability of this bond. Different clinical methods have been developed in order to reduce the polymerization shrinkage stress, such as altered light-curing mode and speed, flowable resin liner application, and an incremental placement technique . Flowable composites are low viscosity resin-based restorative materials with a reduced filler content, compared to non-flowable composites. On the basis of a low filler content, flowable composites are less rigid and have a modulus of elasticity lower than conventional hybrid composites. Simplified placement of the flowable composite through a syringe needle results in superior adaptation to the cavity walls due to the material flow. Moreover, flowable composites are pseudoplastic materials meaning that they become more fluid during placement with a syringe. However, they could only be applied as an elastic intermediate liner underneath the conventional composites, fissure sealants and restorative materials for very small cavities .

Incremental layering technique has been accepted as a golden standard for placement of resin-composite restoration. However, the latest trend in composite technology is the development of flowable resin composites intended for posterior bulk-filling placement . Several authors have evaluated the properties of the so-called low-shrinkage “bulk-fill” resin composites. These evaluations included adhesion to dentin , creep deformation and nanomechanical properties , polymerization shrinkage stress , and viscoelastic properties of “bulk-fill” composite resins .

According to Ferracane , the viscosity is a property that is most important for flowable composites, while Sarret reported that viscosity is directly related to the handling characteristic of the composite material like ease of placement and anatomical shaping, as well as the final quality of a restoration. Different methods were used in previous works in order to characterize rheological properties of resin composites. Beun et al. and Lee et al. determined viscoelastic properties of resin composites using dynamic oscillatory experiments.

In general, paste type materials such as dental composites are viscoelastic materials. They show non-Newtonian, shear-thinning flow behavior . Watts reported that an understanding of both elastic moduli and viscoelasticity is valuable in the appraisal of clinical performance of biomaterials.

The aim of this study was to develop a fractional derivative model for the assessment of viscoelastic properties related to handling characteristics of three different flowable resin composites. We shall use the generalized Newton (see , p. 351) and generalized Zener model (the so-called three parameter model).