Cariogenicity of composite tooth restorative materials

To restore carious or traumatized teeth, a dental practitioner can choose between two direct (plastic) restorative materials, namely amalgam (silver-colored) or composite (tooth-colored). It is obvious that the aesthetic properties of composites are important for patients, but composites also have the advantage of a minimally invasive preparation technique. Moreover, dental amalgams have been alleged of environmental pollution and even negative health effects due to release of mercury.

As a result, composite restorations have gained important popularity, and in many countries they have already become the standard restorative material. However, composite restorations clinically seem more prone to secondary caries (new caries lesion at the margin of an existing restoration). Several clinical trials reported that secondary caries is the most common reason for premature failure of composites. The short lifetime of composites has become a topic of scientific debate, as premature renewal of restorations is a burden on health care expenditure. In addition, secondary caries is always resulting in further tooth structure loss and may weaken the remaining tooth. Especially in case of extensive secondary caries lesions and repetitive restorative interventions, this may eventually lead to premature loss of the tooth.

Caries is in essence a bacteriological disease caused by acidogenic bacteria that produce acidic metabolites, which demineralize enamel and dentin, and enzymes that degrade the dentin matrix. Research indicates that resin-based materials indeed may exhibit properties that stimulate bacterial growth and adhesion. Composites are not inert in the oral environment, and may release components, initially due to incomplete polymerization, and later due to degradation. Monomers leached from composites have been shown to be a good substrate for cariogenic bacteria, thereby explaining the higher cariogenicity of composites compared to amalgam. In contrast, amalgam restorations seal the cavity margins by forming metal oxides that exhibit antibacterial properties. Moreover, bacterial colonization of composites and bacterial metabolites may alter the surface topography and properties of composites boosting even more biofilm adhesion. Nevertheless, more research is necessary, as there is only limited scientific knowledge available regarding the bacteriologic interactions with composites. Therefore, the general hypothesis of this PhD project is that the substances that are released by dental composites do not stimulate the formation of caries.

In Subproject 1 of the PhD thesis, different cariogenic bacteria (Streptococcus mutans and sobrinus, Lactobacillus acidophilus, Actinomyces naeslundii) and non-cariogenic bacteria (Streptococcus sanguinis) will be exposed to different concentrations of monomers (BisGMA, TEGDMA, UDMA, HEMA, ...). The bacterial growth will be evaluated using microbial techniques such as absorbance measurements (spectrophotometry), CFU-counting and qPCR. In Subproject 2, the influence of different restorative materials, including conventional composite, dental amalgam, glass-ionomer cement (GIC) and the composite with surface pre-reacted glass-ionomer fillers (giomer), as well as of the tooth hydroxyapatite (HAp), on the local pH and the composition of dental biofilm regarding the percentage of cariogenic and non-cariogenic bacteria, will be assessed using pH meter, spectrophotometry and viability qPCR techniques. In Subproject 3, surface of different composites will be exposed to single- (S. mutans) and multi-species biofilms (S. mutans, S. sanguinis, A. naeslundii and Fusobacterium nucleatum), after which the surface changes will be characterized by atomic force microscopy (AFM) and contact angle analysis. In addition, the number of bacteria accumulated on composite surface will be assessed by qPCR. In Subproject 4, an epidemiologic study will be set up to determine the incidence and prevalence of secondary caries in patients visiting the university dental clinic, and to characterize these lesions in more detail. Patients visiting the university dental clinic for a yearly check-up will be examined by calibrated clinicians and registered for the study. In addition, presence of secondary caries and patient related factors such as oral hygiene, sugar eating habits and possible parafunctional activities, will be noted. In case of diagnosis of secondary caries, the following parameters will be described: type and class of restoration, number of tooth, precise location of lesion and the age of the restoration.

This multidisciplinary research will help to better understand the factors involved in the formation and development of secondary caries around resin-based dental materials. This represents the first step in the process of making strategies to overcome the problem of relatively early failures of composite restorations.