Elsevier

Dental Materials

Volume 21, Issue 1, January 2005, Pages 68-74
Dental Materials

Siloranes in dental composites

https://doi.org/10.1016/j.dental.2004.10.007Get rights and content

Summary

Objective

The purpose of this study was to compare the product profile of a Silorane based composite which polymerizes by a cationic ring opening process with the product profile of different methacrylate based restoratives.

Methods

Four methacrylate based materials Filtek Z250, Filtek P60,Tetric ceram, Spectrum TPH and a Silorane based material were investigated with regard to their compressive strength, flexural strength, E-Modulus and ambient light stability. The data were analyzed by 1-way ANOVA and 2 sample t test (p<0.05). Shrinkage data were determined by the Archimedes method and the bonded disk method. The reactivity of the Silorane and Tetric ceram were derived from the time resolved shrinkage behaviour and the development of the E-Modulus over time.

Results

The Silorane Composite revealed with 0.94 vol% (bonded disk method) and 0.99 vol% (Archimedes method) the lowest polymerization shrinkage among all tested composites. Its reactivity was comparable to the reactivity of Tetric Ceram. However, the ambient light stability of >10 min for Silorane was higher than the ambient light reactivity of the other tested methacrylates (55–90 s).

Significance

The ring opening chemistry of the Siloranes enables at the first time shrinkage values lower than 1 vol% and mechanical parameters as E-Modulus and flexural strength comparable to those of clinically well accepted methacrylate based composites.

Introduction

The history of resins in dental restorative composites saw a continuous development during the last decades. The age of modern composites starts with Bowen's resin, BisGMA [1], that was modified in various ways regarding different properties like viscosity or polarity. These changes were realized by variations of functionality or different backbones. Different comonomers like triethylene glycol dimethacrylate (TEGDMA) or urethane dimethacrylate (UDMA) were developed over time and contributed in very different directions to the product profile of the composites. So far, all commercially available composites have their common basis in the radical polymerization of methacrylates. Even the so called Ormocers (organically modified ceramics) polymerize by their methacrylate functionality. The development of the different resins lead to remarkable improvements in terms of physical strength, wear resistance, and stability in the oral environment in general. Modern composites exhibit very good physical resistance and also beautiful esthetics.

Two major properties of dental composites that still have to be improved are their polymerization shrinkage and the related polymerization stress. Both parameters contribute to different clinical challenges such as reduced marginal integrity and post-operative sensitivity. Imperfect margins result in marginal staining and eventually secondary caries, and represent the most important reason for the replacement of existing insufficient composite fillings. Polymerization shrinkage also leads to cuspal displacement and even up to cracks in healthy tooth structure.

There are two main strategies to reduce polymerization shrinkage:

  • Reduction of reactive sites per volume unit

  • Reduction of shrinkage using different types of resin.

The density of reactive sites per volume unit can be reduced principally in two ways:

  • by increasing the molecular weight per reactive group

  • by increasing the filler load.

The low molecular weight methyl methacrylate (MMA) reveals a polymerization shrinkage of 22.5 vol%. By increasing the molecular weight of MMA from 86.1 g/mole to 514.6 g/mole of Bis-GMA the shrinkage is reduced to 8 vol% in the unfilled resin. Extending this idea, macromolecules should be the ultimate goal by following this pathway of shrinkage reduction. As a counterbalance, the use of high molecular weight monomers is limited by their viscosity, increased stickiness, and undesirable general rheology which compromise the handling characteristics of the resulting restorative composites.

The increased filler load also finds its limitation at a certain level. Very low filled composites like flowable composites (45–67% filler load by wt) exhibit typical shrinkage values of 4.0–5.5% vol. Normal hybrid composites (74–79% filler load) undergo a volume reduction upon curing from 1.9 to 3.5%. Very highly filled systems like packable posterior composites or materials with optimized filler load of up to 82% by addition of nano particles reveal shrinkage values down to 1.7% vol. However, also the filler load cannot be further increased when the consequently reduced amount of resin cannot any longer provide for the chemophysical incorporation of the filler particles, and for the wetting of the increased filler surface.

Recently, several attempts to reduce the shrinkage by changing the nature of the resin were made by universities and manufacturers. One approach was the use of liquid crystalline monomers (Fig. 1) as a resin which was described to shrink less due to the transition of its nematic phase to an isotropic amorphous state when photocured [2].

Moszner et al. published vinyl cyclopropane derivates (Fig. 2) as radical curing ring opening monomers, also suitable to copolymerize with common methacrylate based resins [3a].

A different chemical approach was made by Eick and coworkers focussing on the cationic ring opening spiro ortho carbonates (Fig. 3), especially in combination with epoxy monomers [3b].

In the recent years, 3M ESPE investigated new cationic ring opening monomer systems, with the target profile of a low shrinking, highly reactive, biocompatible composite [4] that withstands the aggressive environment of the oral situation.

Section snippets

Materials and methods

The Silorane material is an experimental composite of 3M ESPE, Seefeld, Germany and is described in detail below. It was compared with the following methacrylates: 3M ESPE Filtek Z250 (3M ESPE, St. Paul MN, USA), 3M ESPE Filtek P60 (3M ESPE, St Paul MN, USA), Tetric Ceram (Vivadent-Ivoclar, Vaduz, Liechtenstein), Spectrum TPH (Kerr Corporation, West Collins, Orange CA, USA), Aelite LS (Bisco Inc., Schaumburg IL, USA), Quixfil (Dentsply DeTrey, Konstanz, Germany), Solitaire 2 (Heraeus-Kulzer,

The Silorane resin

The solution for this target profile was achieved by the development of Siloranes (Fig. 4). The name Silorane derives from the combination of its chemical building blocks siloxanes and oxiranes.

The siloxane backbone was introduced in order to provide a most hydrophobic nature, which is very important since too high water sorption limits the long term intraoral physical strength of the composite [7]. Additionally, hydrophobic materials tend much less to absorb the dyes of the daily nutrition and

The initiating system

The generation of radical species for the methacrylate cure is realized by a two component system consisting of camphor quinone which is the actual photoinitiator and a tertiary amine (Fig. 6), responsible for hydrogen transfer reaction. This system decomposes immediately by exposure to light with a wavelength between 430 and 490 nm and generates the radical species to start the polymerization process.

The development of a photoactivated Silorane composite was realized with a three component

The filler

The use of fine particular quartz with a D50 of below 0.5 μm (Graph 1) is the basis for the composite's esthetic performance and mechanical stability. The silane layer is modified with an epoxy functionality (Fig. 8) and is introduced by a silanization process which is very similar to that used for methacrylate based restoratives. As it is known for the methacrylates, the silane layer increases the hydrophobic character of the surface of the filler. At the same time, the silane layer acts as the

Shrinkage

The polymerization shrinkage can be determined by different methods. In order to generate reliable shrinkage values it is important to apply different types of measurement principles and not to rely on a single protocol. Here, the two independent protocols of the Archimedes method and the bonded disk method are compared. The volumetric shrinkage according to the Archimedes method derives from the different densities of the cured and uncured sample.shrinkage=100%[1δ(paste)δ(polymer)]

Therefore,

Reactivity and physical properties of siloranes

The clinical practitioner appreciates the cure on command and the capability to finish the restoration immediately after its placement. Therefore, it is important to provide a reactive material that develops its mechanical strength directly after light cure. In order to compare the reactivity of a methacrylate based system (Tetric Ceram, Vivadent) and a Silorane composite, the shrinkage values of each composite were monitored over time using the bonded disk method. The data show comparable

Conclusion

The Silorane technology provides restorative composites with the lowest polymerization shrinkage and stress, and the highest ambient light stability. At the same time, the Siloranes reveal high reactivity and mechanical properties comparable to clinically successful methacrylate composites.

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