A finding from an international collaborative study has revealed a fact that has long gone unnoticed by many clinicians. Composite resin dental fillings that have been in the mouth for an extended period are continuously exposed to bacterial layers known as biofilms. This attack does not merely damage the surface of the filling; it can also threaten the success of repair procedures. The study, published in the Journal of Dental Research in 2010, involved drg. Margareta Rinastiti, M.Kes., Sp.KG(K)., Ph.D., from the Department of Conservative Dentistry, Faculty of Dentistry, Universitas Gadjah Mada, together with researchers from University Medical Center Groningen in the Netherlands and the University of Zürich in Switzerland. The findings provide a clear answer to a question that had remained unresolved: what is the most effective method for repairing old, degraded fillings?
Composite resin is one of the most commonly used dental filling materials because its appearance resembles that of natural teeth. However, it is not without limitations. Over time, composite resin surfaces undergo wear due to chewing forces, exposure to acidic foods, temperature changes, and, most importantly, exposure to oral biofilms.
A biofilm is a thin layer of microorganisms that adheres to almost every surface in the mouth, including filling surfaces. Bacteria within biofilms produce organic acids that gradually degrade the polymer matrix of composite resin. As a result, the filling surface becomes rougher, filler particles that should remain protected by the matrix become exposed, and the overall surface quality deteriorates significantly.
Under these circumstances, dentists often choose to repair a damaged filling using a minimally invasive approach, simply applying new material over the worn area rather than removing the entire restoration. This procedure preserves more healthy tooth structure. The problem, however, is that there has been insufficient scientific evidence to answer an important question: how strong is the bond between new filling material and an old filling that has already been "eaten away" by biofilm?
The research team, including drg. Rinastiti, designed an experiment to simulate oral conditions in the laboratory. Four types of composite resin were tested: microhybrid, two different nanohybrid composites, and nanofill. Each specimen was exposed for 14 days to a mixed biofilm consisting of six species of microorganisms commonly found in the oral cavity, including Streptococcus oralis, Streptococcus sobrinus, Actinomyces naeslundii, Fusobacterium nucleatum, Veillonella parvula, and Candida albicans.
Following biofilm exposure, the filling surfaces were conditioned using two different methods before being repaired. The first was the conventional method, using an intermediate adhesive resin (IAR), in which an adhesive layer was applied directly to the old surface. The second was the silica-coating (SC) method, in which the surface was sprayed with silica-coated alumina particles using a specialized abrasive device, followed by the application of a silane agent and then an adhesive resin.
Bond strength was tested using a shear testing machine, while failure patterns were examined under a light microscope. The results were quite surprising. Before biofilm exposure, both methods produced comparable bond strengths. However, after the old fillings had been exposed to biofilm, the results changed.
"The mean bond strength of composites conditioned with intermediate adhesive resin (IAR) decreased significantly after aging due to biofilm exposure, whereas silica-coating was able to maintain bond strength at the same level as before aging."
The difference was not limited to bond strength values; the failure patterns also differed. Fillings treated with silica-coating tended to experience cohesive failure, meaning that the filling material itself fractured rather than the bond at the interface between the old and new fillings. Clinically, this is the desired outcome because it indicates that the bonded interface is not the weakest point. In contrast, the IAR method more frequently resulted in adhesive failure, in which the new filling detached from the old surface.
The researchers also confirmed that laboratory biofilm exposure produced surface changes comparable to actual clinical wear measured from specimens mounted on a palatal appliance for 180 days. This finding confirms that the laboratory biofilm model is a valid and clinically relevant method for simulating the condition of old fillings in the mouth.
These findings have direct implications for everyday dental practice. Until now, simple repair procedures have often relied solely on the application of an intermediate adhesive resin because it is easy and quick and does not require additional equipment. However, this study demonstrates that for old fillings that have deteriorated as a result of biofilm exposure, this method is inadequate.
The reason silica-coating performs better is directly related to the mechanism of biofilm-induced damage. Acids produced by bacteria cause swelling of the resin's surface matrix, which in turn creates more microscopic cavities that allow silica particles to penetrate and form stronger mechanical interlocking with the adhesive. Put simply, the damage caused by biofilm can actually be utilized by the silica-coating technique to create a stronger bond.
The study also found that the type of composite resin affected the amount of biofilm that accumulated. The Grandio nanohybrid composite and Anterior Shine microhybrid composite attracted nearly twice as much biofilm as the other two materials. This difference may be related to the positively charged inorganic elements present on the surface of each material.
Ultimately, this research is not merely about the technical properties of filling materials. It addresses a more fundamental question: how well do we really understand what happens inside a patient's mouth between one dental visit and the next? A filling that appears intact on the surface may actually be undergoing silent degradation, and choosing the appropriate repair method can make the difference between a restoration that lasts and one that fails prematurely.
Author: drg. Achmad Zam Zam Aghasy, M.Kes., Annisa Dwi Noviyanti
Photo: FreePik
Source DOI: https://doi.org/10.1177/0022034510381395