Water sorption and solubility of resin-based composites

Interaction of resin-based composites with water is a continuous process from the early stages of composite placement. Water plays an important role in the long-term stability of composite fillings and may induce hygroscopic expansion of the material, hydrolytic degradation of intra- and intermolecular bonds within the resin matrix and at the resin-filler interface, plasticization of polymer chains, elution of leachable substances and reduction in mechanical properties.

The importance of composite-water interaction has been acknowledged in the ISO standard 4049 which states the maximum values for water sorption and concurrent solubility for resin-based materials (composites and cements). In order to comply with this ISO standard, resin-based materials must have  water sorption and solubility values equal or lower than 40 micrograms per cubic milimetre (sorption) and 7.5 micrograms per cubic milimetre (solubility) for specimens 15 mm in diameter and 2 mm thick.

Water sorption and solubility values are based on mass changes of the samples before (m1) and after immersion (m2) in water and after dessication (m3) until constant mass is achieved.  Mass changes m2-m3 are divided by sample volume to calculate water sorption and m1-m3 divided by sample volume give solubility values. 

Recent papers* published in Dental Materials investigated water sorption and solubility and hygroscopic dimensional changes of several resin-based composites:

low-shrinkage Filtek Silorane

universal Gradia Kalore

micro-hybrid Gradia Direct Anterior and Posterior and

self-adhering flowable Vertise Flow.

After 150 days of storage in de-ionized water, the lowest sorption of about 13 μg/mm³ was found for Filtek Silorane and the greatest of about 72 μg/mm³ was found for Vertise Flow. Vertise Flow also showed the greatest solubility of about 16 μg/mm³ whereas other materials showed either negative values (Filtek Silorane and Gradia Kalore) or values below 4 μg/mm³. The authors suggested that the negative solubility values for Filtek Silorane and Gradia Kalore meant that the dessication was not sufficient or that some water was irreversibly bound to the resin matrix.

Hygroscopic dimensional expansion as the result of water sorption over the 150-day period was lowest for Filtek Silorane (about 0.7%) and highest for Vertise Flow (about 4.8%) whereas the values for Gradia composites were between 1.5 and 2%. Hygroscopic expansion may compensate to a certain extent polymerization shrinkage which was found to be 0.99% for  Filtek Silorane, 1.7-2.4% for Gradia composites and  4.4% for Vertise Flow. However, this expansion occurs over a much slower time scale than shrinkage and its effect on the clinical performance of resin-based composite is yet to be determined.

The greatest stability in the aqueous environment found for Filtek Silorane may be explained by the hydrophobic siloxane and low-shrinkage ring-opening oxirane units of the silorane monomer. Furthermore, cationic polymerization is relatively oxigen-insensitive with the potential of reaching higher degree of conversion than methacrylate-based composites.

On the other hand, aqueous instability of Vertise Flow was attributed to the hydrophilic monomer, GPDM, which is responsible for the self-adhesive property of Vertise Flow but also seems to attract more water uptake by the resin matrix compared to other resin-based composites.


* Wei YJ, Silikas N, Zhang ZT, Watts DC. Hygroscopic dimensional changes of self-adhering and new resin-matrix composites during water sorption/desorption cycles. Dent Mater. 2011 Mar;27(3):259-66.

Wei YJ, Silikas N, Zhang ZT, Watts DC. Diffusion and concurrent solubility of self-adhering and new resin-matrix composites during water sorption/desorption cycles.Dent Mater. 2011 Feb;27(2):197-205.

[Reprints of the cited papers may be obtained from the corresponding authors]

Click here for more on Vertise Flow.
 

Vertise Flow: the first self-adhering composite (flowable, though)

A long time ago, Michael Buonocore, one of the pioneers of adhesive dentistry, suggested four approaches to overcome the lack of adhesion between filling materials and dental tissues:
"(1) the development of new resin materials with adhesive properties;
(2) modification of present materials to make them adhesive;
(3) the use of coatings as adhesive interface materials between filling and tooth and
(4) the alteration of the tooth surface by chemical treatment to produce a new surface to which present materials might adhere." (Buonocore 1955)

In many respects, this was not only a suggestion but a visionary prediction for modern adhesive dentistry. We now know that all 4 of Buonocore's suggestions have been addressed by dental science which has led to the development of composite resins, adhesive systems and glass ionomer cements. These are three major groups of materials in adhesive dentistry today but there is a number of modifications and subgroups within each of them.

The latest news in adhesive dentistry is the development of self-adhering flowable composite, Vertise Flow by Kerr. Vertise Flow comes as a result of ongoing efforts to rationalize clinical treatment, currently including the use of adhesive systems and resin-based composites to create popular "white" fillings. Although a flowable composite, Vertise Flow clearly indicates the direction of current research by Kerr - the creation of the ultimate self-adhering composite for posterior teeth.

The manufacturer claims that Vertise Flow is based on Optibond technology which utilizes GPDM (glycero-phosphate dimethacrylate), a functional monomer, to obtain etching of enamel and dentine and HEMA, another functional monomer, most commonly used in dental adhesives to enhance wetting and resin penetration in dentine. It has been stated in many scientific papers that BisGMA is the main resin component of Optibond adhesives, though not clearly stated in manufacturer's safety data sheet. It can be expected that Vertise Flow contains BisGMA as the main cross-linking monomer as well.

One of the main questions that a dental material scientist would ask is: How does this material overcome the hydrophobic-hydrophilic mismatch between composite resins and human dentine to produce an interface that would ensure optimal bonding for long-term clinical success? This is currently achieved by the use of adhesive systems as an intermediary layer that is supposed to bridge hydrophobic composite and hydrophilic dentine.

Manufacturer's data suggest that the shear bond strength of Vertise Flow to enamel and dentine is comparable to self-etch adhesive systems. Furthermore, it is suggested that the tooth-restoration interface prevents microleakage, the passage of fluids, bacteria, molecules and ions between the restoration and cavity walls. This phenomenon has been proved to exist for all current resin-based materials due to polymerization contraction of composite resins.

Undoubtedly, Vertise Flow will soon be subjected to a vast array of studies by independent researchers that will address various properties of this material and compare it with other materials on the market. Independent evidence-based results, if in favor of this material, will be the best marketing for Vertise Flow. As always, the last word lies upon the dental practice.

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