Surface roughness and gloss of dental materials as a function of force and polishing time in vitro

The purpose of this study was (1) to analyze the influence of polishing time and press-on force on the surface gloss and roughness of dental materials by using a three-component rubber-based polishing system and to compare the results with those obtained in conjunction with an optimal polishing procedure and application of the specimens to a metal matrix, (2) to estimate the correlation between surface gloss and surface roughness.

Eight flat specimens were made of each dental material (Tetric Ceram [TC], Tetric EvoCeram [TE], Heliomolar [HM], Compoglass [CO] and Amalcap [AM]) and polished with 4000-grit SiC and a polishing liquid. Subsequently, the mean surface roughness (Ra) was measured with an optical sensor (FRT MicroProf) and the gloss (Gl) with a glossmeter (Novocurve). Prior to polishing the specimens with the polishing system, the surface was pre-roughened with 320-grit SiC paper and polishing was performed under water cooling at 10,000rpm, applying a controlled force of 2N. Each component of the Astropol system (F/P/HP) was used for 30s and Ra and Gl were measured at 5-second intervals. Other specimens of the same materials were polished with a controlled force of 4N or they were applied to a metal matrix and, subsequently, Ra and Gl were measured. To measure the influence of the polishing time on the surface gloss and roughness, repeated ANOVA with post-hoc paired t-tests was carried out in log-transformed Ra and gloss values. To estimate the correlation between Ra and Gl, the Spearman and Pearson correlation was calculated.

(1) Surface gloss and surface roughness were time-dependent, showing the greatest improvement already after 5s of polishing with each of the polishing components, with the exception of AM, for which HP was not effective. The patterns of improvement varied considerably from material to material, but they were more consistent with regard to Gl than to Ra. (2) Ra was statistically significantly higher only in TC, TE and CO when 4N of force was applied instead of 2N. (3) Polishing the surface with a polishing machine resulted in a significantly better surface gloss in all materials. (4) After applying the specimens to a metal matrix, the surface roughness of all dental materials was significantly higher when compared to polished specimens except for TC. (5) Correlations between gloss and roughness were in general negative but they were higher for individual measurements compared to the differences between two consecutive measurements.

Both surface gloss and surface roughness were material-dependent and influenced by the polishing time and applied force. As gloss and roughness proved to be closely associated with each other, gloss assessment may be a sufficient method to screen materials with regard to their polishability.