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Introduction

In spite of 150 years’ worth of good clinical performance, the use of amalgam as a tooth filling material remains controversial. The most common allegations against amalgam are environmental pollution and possible hazardous health effects due to release and systemic uptake of mercury . The ongoing discussion about the safety of amalgam has also led to an increased focus on the safety of resin-based restorative materials . The use of resin-based materials in dentistry is nowadays ubiquitous, and during the past decades composite restorations have proved to be a satisfying alternative for amalgam to restore traumatized and decayed teeth .

Resin-based dental materials generally consist of a polymer matrix and inorganic filler particles that are attached to the resin matrix through a siloxane coupling . The most common resins used in dentistry are (meth)acrylates , but recently, new resin systems, such as ormocers (polysiloxane backbone with methacrylate sidebranches) and siloranes (silorane ringopening system) have been introduced .

Despite their growing popularity, there are concerns that resin-based materials may be toxic based on the fact that they may release components . Three main routes of systemic intake of chemical substances released by resin-based restorations have been postulated: the first through ingestion of released compounds in the gastro-intestinal tract, the second through diffusion to the pulp through the dentinal tubules , and the third via uptake of volatile components in the lungs . The last route is of special importance for the dental practitioner and the dental personnel, while the first and second route are more relevant for the patient.

Resin-based materials may release unpolymerized monomers, additives and filler components in the oral environment after placement of the restoration. Even though the patient may come into contact with large amounts of uncured monomers during the placement of the composite restoration, the release of unpolymerized monomers after polymerization causes most concerns in literature. Under clinical circumstances with a short curing time of usually not more than 40 s, and a temperature around 37 °C in the oral cavity, composites are never polymerized to a full extent as the propagation of the crosslinking reaction drastically reduces the mobility of the monomers . As a result, not only unbound substances, like additives, but also uncured monomers can leach out. Depending on the resin-based material, the degree of conversion can vary between 50 and 70% . The maximum degree of conversion is reached only after 24 h due to a post-cure process (‘in-the-dark’ polymerization), which signifies that the polymerization rate immediately after light-curing may be even lower (30–40%) . Filler leachability encompasses both release of complete filler particles after hydrolysis of the filler-matrix siloxane bond, and the release of filler components, such as SiO ₂ , Ba, Sr, Na due to hydrolysis and ion-exchange mechanisms . Release of filler components has mainly been associated with progressive wear of composites; however little is known regarding possible health effects.

Intra-oral degradation processes may induce additional release of components from resin-based restorations . First, mechanical , hydrolytic and enzymatic degradation may result in chain scission and release of polymeric breakdown products in the form of monomeric or oligomeric molecules. Most of these degradation products have probably not yet been identified . Second, aging of composite materials may also lead to more porosities due to an interplay of mechanical swelling and water sorption and chemical/enzymatic degradation , and thus result in increased release of unpolymerized monomers that were initially trapped in the polymer network .

In literature, there are many indications that release of monomers and of some additives are potentially dangerous and might have compromising local or even systemic effects . Apart from the well-documented allergenicity of monomers , several ingredients have been shown to be cytotoxic , genotoxic and mutagenic , and toxic to the reproductive system . Besides the identification of hazards, risk assessment requires an accurate knowledge of the amounts of released compounds .

In spite of many analytical studies, the lack of standardized methodologies for quantification and of uniformity in presenting the results hinders correct interpretation of the quantities of released eluates. In other words, it is still unclear to what amount of specific components a patient may be exposed. This makes risk assessment of possible health hazards due to resin-based dental materials problematic. The objective of this study was to review peer-reviewed international literature on the unintended release of ingredients in the oral environment. Since there have been very few in vivo studies , only in vitro studies were included in this review. The quantities measured in studies that quantified the amount of released ingredients, were converted to a common unit. The main purpose was to gain knowledge on the total quantity of compounds that can be released by resin-based dental materials in the oral cavity.

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Materials and methods