Effect of Semi-Automated Polishing on Nanoscale Topography of Resin Composites

The atomic force microscope (AFM) is a useful instrument for measuring 3-dimensional topographical parameters of comparably-polished dental biomaterials. Objectives: To identify parameters that can determine the effect of clinically-relevant mechanical polishing protocols on the nanoscale topography of resin-composites. Methods: Polymerized cylindrical specimens of four experimental light-activated resins differing only in barium-glass filler particle size (1.5, 1.0, 0.7 and 0.4μm) were wet-polished in a semi-automated MultiPrep™ grinder-polisher (Allied High Tech, Inc.) using 180, 320, 600 and 800grit SiC disks. Clinically-relevant surfaces were produced using 6, 3, 1, 0.50 and 0.25μm polycrystalline-diamond suspensions (final specimen dimension d=6mm, h=3mm). Control group specimens were prepared against Mylar^™ strips and left unpolished. A MultiMode™ AFM and NanoScope V controller (Veeco Metrology, Inc.) were used to scan the specimens (contact mode; air; 5 locations/specimen). After plane-fitting the scans (n=25/treatment/resin), 21 roughness and depth parameters were calculated using NanoScope software. Parameters capable of discerning topographical differences among the 24 experimental groups were identified using two-factor ANOVA and Linear Regression tests. For each resin, data for these parameters were compared using post hoc Tukey-HSD tests (α=0.05).  Results: Mean surface roughness (R_a), root mean square roughness (R_q), average maximum height (R_pm) and peak-count were found to differentiate among the significantly different nanoscale topographies tested. Mean values for the parameters ranged from 9.9-77.2nm, 13.2-103.2nm, 5.6-55.7nm, and 344-1698, respectively. Specimens polished using 3, 1 and 0.50μm diamond suspensions had lower mean R_a and R_q values than control group specimens.  Conclusions: Four parameters (R_a, R_q, R_pm, peak-count) that accurately characterized the differing resin-composite topographies were identified. Resins containing decreasing filler particle sizes polished with decreasing abrasive particle sizes generally resulted in smoother surfaces, except when polished with 0.25μm diamond suspension. Resins polished using 3μm, 1μm and 0.50μm diamond suspensions had smoother surfaces than those fabricated against Mylar.  Funding: NIH/NCRR grant P20RR018741.