Our preliminary data indicated that aging a restorative dental composite in an esterase enzyme in artificial saliva (AS) resulted in a significant decrease in diametral strength relative to specimens aged water with or without an acid. Building on this data, this research will utilize a dentin ring containing a composite specimen to the structural integrity of dentin-adhesive and adhesive-composite interfaces subjected to simulated oral environments with two model oral bacteria (cariogenic S. mutans and S. sanguinis) and incubation in the presence of esterase enzyme in AS media. In addition, the specimens will be subjected to axial and radial cyclic loading within this dentin-adhesive-composite ring to simulate a class I restoration. Experiments with the two Streptococcus strains will allow us to determine whether cariogenic incubation conditions and/or biofilm growth and expression of glucsoyltransferases (gtfB, a key enzyme in the production of extracellular matrix formation in cariogenic biofilms) significantly impact the structural integrity of the test specimens. Inclusion of esterases will allow us to test whether these enzymes degrade and reduce the strength of dentin-adhesive-composite bonds, as we have shown previously with composites. Static tests will determine the role aqueous geochemical conditions (either abiotic or biotically-induced) play in attacking the structural integrity of the test specimens, and dynamically cycled compression will allow us to determine the impact of simulated forces from mastication to more realistically model stress in vivo. Analysis of the degradation products leaching from the interfaces and bulk composite will be quantitatively characterized by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) for inorganic components of the composite (e.g. Zr, Si, and Ti), and by liquid chromatography/ tandem MS (LC-MS/MS) for polymer degradation products. Micro- and nano-imaging analysis of the dentin-adhesive-composite volumes (interfaces, pores, and cracks) should aid in clarification of the sequence of events leading to clinical failure of dental composite restorations. It is anticipated that significant differences will be observed between and among the bacteria incubations versus the esterase enzyme in AS environments with respect to control specimens (uncycled and aged 120 d in AS). It appears that behavior of dental composites subjected to multiaxial loading in these two environments has not been reported on in the literature to date. Dental composites are subjected to extreme chemical and mechanical conditions in the oral environment, which contribute to the degradation and ultimate failure of the material in vivo.
This multidisciplinary project encompasses the disciplines of environmental microbiology, image analysis, and material testing to study the impact of aging on the interfaces and material properties of a dentin-adhesive-composite specimen. This interrelated-interfacial approach to dental composites degradation due to bacterial and mechanical insults will provide new information on the breakdown of restorative dental composite in the oral cavity using state-of-the-science imaging capabilities at the Advanced Photon Source at Argonne National Laboratory. This research will lead to improved understanding of the role of oral biofilms on dental composite polymer?s degradation and assist dentists and dental manufacturers in improving the material properties of dental composites as an esthetic and functional restorative material.