A six-year prospective randomized study of a nano-hybrid and a conventional hybrid resin composite in Class II restorations
Introduction
The use of amalgam has decreased in many countries to negligible levels during the latest decade. Advantages like aesthetics and tooth substance saving adhesive techniques increased the popularity of resin composites (RC) also in the posterior region. However, polymerization shrinkage, manifested as shrinkage stress when monomer molecules are converted into a polymer network is still considered remaining a challenge. Contraction forces exceeding the bond strength at the tooth-restoration interface affect the interfacial adaptation [1], [2]. The following interfacial debonding is often discussed to result in marginal staining, bacterial microleakage, secondary caries or pulpal inflammation [3]. However, neither evidence nor a good correlation between contraction stress and clinical durability of resin composite materials has been presented. Filler particles incorporated in the resin matrix has been continuously in focus for improvements over the years. The last generation hybrid resin composites contain 0.5–1.0 μm sized filler particles of glass or zirconium completed with smaller amounts of colloidal silica particle clusters. Newer generations of hybrid resin composites have been marketed with claims that modified filler-loading and matrix monomers result in lower polymerization shrinkage, improved polish retention and aesthetics [4], [5], [6]. A recent development is the application of nanotechnology in dental materials, incorporating nanofiller particles in RC and bonding systems. Nanotechnology is known as the production and manipulation of materials and structures in the range of about 0.1–100 nanometers by various physical or chemical methods [7]. Nanofillers with sizes ranging from 5 to 200 nm have recently been developed ([8], [9]) although 40 nm particles already were present in the microfilled resin composites [8], [9]. The main difference is the higher filler loading of the recent materials compared to the earlier microfilled RC. Nano-hybrid and nanofilled resin composites are two types of resin composites referred to under the term “nanocomposite”. In vitro, good mechanical properties, improved surface characteristics and esthetics, better gloss retention, reduced polymerization shrinkage and diminished wear have been reported [5], [6], [8], [10], [11], [12]. In recently introduced RC, an approach to wet the larger surface area of the included smaller fillers is the use of prepolymerized filler made of ground RC containing microfine particles. Ergücü et al. showed that this material was the less surface-stained nanocomposite among four nano-hybrid and one nano-filled resin composite materials [5].
Prediction of restorative systems is mostly performed in vitro, but its value is often limited and clinical studies are required to test these materials in the oral cavity. Initial short time studies of nanocomposite materials in posterior cavities have been promising, but no differences in durability have been shown compared to conventional microhybrid resin composites [5], [13], [14], [15], [16]. No long time clinical evaluations of nanocomposites have been reported. The aim of this study was to evaluate the long-term clinical performance of a new nano-hybrid RC (Tetric EvoCeram) containing prepolymerized filler of ground composite microfine particles, in Class II cavities and to compare with an established conventional hybrid RC (Tetric Ceram) in a split mouth study. The reference RC has a widespread use and has been evaluated in several earlier trials which enable a noninferiority/equivalence design of the trial. The null hypothesis tested was that the nano-hybrid resin composite showed different durability as its predecessor. The alternative hypothesis was that they were equally effective.
Section snippets
Experimental design
Fifty-two patients, 27 women and 25 men, with a mean age of 53 years (29–82) participated in the study. During September–December 2003, all adult patients, visiting one of the author's PDHS's clinic, who at the yearly examination needed two or four extensive Class II restorations, were invited to join the study. The subjects were representative for the patients attending Swedish dental practises as in most of our earlier trials [2], [4], [16]. All patients invited participated in the study. No
Results
Two patients (1 female, 1 male) with 2 restorations each could not be evaluated at all recalls. The reasons for drop out were death of the first and moving of the second participant. 118 restorations, 59 pairs, were evaluated after 6 years. One patient reported mild postoperative sensitivity symptoms after cold and hot stimuli during the first weeks after baseline from a premolar tooth (TEC) and one molar tooth (TC) was extracted after 4 years due to pain. Relative frequencies of the scores of
Discussion
This study evaluated the clinical long-term effectiveness of a well-established conventional RC, indicated as the golden standard during the late nineties and early 2000 and its successor introduced in 2004 as a nano-optimized material. During the last years, great effort has been put into improving handling characteristics, and mechanical and adhesive properties of resin based materials. Highly filled hybrid and packable RC, polyacid-modified RC, ormocers, nanohybrid and nanofilled RC have
Acknowledgments
The support from the County Council of Västerbotten and the National Board of Welfare is gratefully acknowledged.
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