Analysis of the performance of a standardized method for the polishing of methacrylic resins.

Adhesion of micro-organisms to resin surface may be caused by inadequate polishing. Most of the studies published in literature are relative to manually prepared samples and do not take into account that test repeatability is not guaranteed a priori since skills may change from one operator to another and the quality of the work done by the same expert operator may depend on "human" factors such as the level of attention, wrist trembling, etc.This paper aims to investigate on the efficiency and reliability of a standardized protocol for polishing methacrylic resins. For that purpose, five different methacrylic resins are considered. For each resin, 20 specimens are realized: 10 are polished by the same expert operator and 10 are polished by means of a mechanical system comprised of a milling tool, a mobile support for samples and a micrometric advance isoparallelometer. Roughness measurements are carried out with a +/-0.01 microm resolution profilometer. An extensive statistical analysis is conducted on a population of 100 specimens. Two-way Analysis of Variance (ANOVA) is carried out taking the type of resin (i) and the polishing technique (ii) as predictors in order to evaluate how those variables will finally affect the roughness of the polished surface. The significance of the variable interaction term is assessed. The null hypothesis Ho where response is independent from individual factors as well as from their interaction is assumed to hold true for p-values>0.05 (interval of confidence of the 95%). Experimental data confirm that mechanical polishing leads to obtaining surfaces of much more uniform quality. In fact, statistical dispersion of roughness parameters can decrease significantly. This behavior is observed for all of the tested resins. Therefore, the new approach can eliminate the influence of "human" factors thus making it possible to assess the inherent features of each resin and compare different dental materials submitted to polishing.