Atomic force microscopy

Background

Surface roughness is the main factor determining bacterial adhesion, biofilm growth and plaque formation on the dental surfaces in vivo. Air-polishing of dental surfaces removes biofilm but can also damage the surface by increasing its roughness. The purpose of this study was to investigate the surface damage of different conditions of air-polishing performed in vitro on a recently introduced dental restorative composite.

Methods

Abrasive powders of sodium bicarbonate and glycine, combined at different treatment times (5, 10 and 30 s) and distances (2 and 7 mm), have been tested. The resulting root mean square roughness of the surfaces has been measured by means of atomic force microscopy, and the data have been analyzed statistically to assess the significance. Additionally, a fractal analysis of the samples surfaces has been carried out.

Results

The minimum surface roughening was obtained by air-polishing with glycine powder for 5 s, at either of the considered distances, which resulted in a mean roughness of ~300 nm on a 30 × 30 μm² surface area, whereas in the other cases it was in the range of 400-750 nm. Both untreated surfaces and surfaces treated with the maximum roughening conditions exhibited a fractal character, with comparable dimension in the 2.4-2.7 range, whereas this was not the case for the surfaces treated with the minimum roughening conditions.

Conclusions

For the dental practitioner it is of interest to learn that use of glycine in air polishing generates the least surface roughening on the considered restorative material, and thus is expected to provide the lowest rate of bacterial biofilm growth and dental plaque formation. Furthermore, the least roughening behaviour identified has been correlated with the disappearance of the surface fractal character, which could represent an integrative method for screening the air polishing treatment efficacy.

Background

Dental caries is the most widespread disease, since it affects about 95% of the world population at some point during their lives [1]. Caries follow bacterial plaque formation, which arises after the increase in surface area accessible for bacterial adhesion due to the surface roughness associated with defects or damage of the dental structures [2-5]. In fact, the predominant role of surface roughness for bacterial adhesion with respect to other cofactors such as surface energy has already been clarified in the literature [6].
Traditional hand instruments or oscillating scalers used to remove dental plaque usually cause a significant increase in roughness of the underlying dental surfaces [3,7] made of either pristine or restorative material, causing in turn a faster re-growth of plaque in the time period following the treatment. Therefore, air-polishing (AP) with simultaneously ejected water and pressurized air containing abrasive powders has been introduced in dental cleaning, and is now routinely applied [8-10]. Sodium bicarbonate powder is largely used for AP [10]. Recently, glycine powder has also been tested in several in vitro, ex vivo and in vivo studies, demonstrating a good clinical efficacy and low abrasive effect [7,9,11-14].
Despite being the least invasive technique for the dental surfaces, even AP may result in surface damage [3,15], when the working parameters of type of abrasive powder, spraying time and distance are not correctly set. To date, AP surface effects have been studied by means of laser scanners or profilometers. One recognized advantage of these techniques lies in their ability to allow large areas characterization containing both untreated and AP treated regions. This makes it possible to measure the resulting defect depth and the absolute loss of material, and thus evaluate the integrity of the dental structures [12]. However, laser scanners and profilometers do not permit high-resolution measurement of the surface roughness. In this work we have performed an in vitro analysis on the effect of AP on the surface of a commercial material used in dental restoration using atomic force microscopy (AFM), which allows for a high resolution, direct quantitative characterization of the surface roughness [16,17]. Firstly, the AP treatment conditions resulting in the lowest dental structure damage - i.e. surface roughening - have been identified. Secondly, the effect of the different AP treatment conditions on the possible fractal character of the surface roughness has been analyzed. Surface feature patterns exhibit a fractal character when they are self-affine, meaning that similar patterns can be found when zooming in or out to different orders of magnitude of the lateral field of view. The fractal analysis has already been applied to dental surfaces for classification of dental patterns of different species in zoology [18] and for characterization of the wear patterns of bruxism [19], but to our knowledge has never been used in the analysis of AP of dental composite surfaces. This mathematical tool can provide a new way to account for the complexity of the topographical pattern of the treated material surface, which can in turn depend on the AP conditions. In fact, it is generally accepted that the measures of roughness from the distribution of heights z alone without any information on their spatial localization on the (x, y) plane is insufficient to completely describe the surface roughness.