In vitro comparison of the surface roughness of polymethyl methacrylate and bis-acrylic resins for interim restorations before and after polishing

Statement of problem. Polymethyl methacrylate and bis-acrylic based resins are widely used for interim restorations. Their initial surface roughness is important because it determines their aesthetic properties and the potential for biofilm adhesion. Purpose. The purpose of this in vitro study was to assess the surface roughness and morphology of 6 bis-acrylic and 2 polymethyl methacrylate resins widely used for interim dental restorations, both before and after polishing. Material and methods. Specimens made of different bis-acrylic resins (Protemp 4, Luxatemp Star, Systemp, Telio, Structur Premium, Structur 3) or of polymethyl methacrylate (Unifast Trad, Unifast 3) were polished using a 2-step polishing system (Diatech). The average surface roughness before and after polishing (10 seconds at low speed in dry conditions) was measured by optical profilometry. Atomic force microscopy and scanning electron microscopy were used to analyze surface morphology. The Mann-Whitney and Kruskal-Wallis tests were used to evaluate the differences in roughness among specimens (alpha=.05), and the Pearson r correlation was computed to assess the relationship between fillers and average surface roughness. Results. In the 8 groups evaluated, the roughness significantly increased (P<.001) for Protemp 4 (from 0.12 to 0.50 mu m), Luxatemp Star (0.17 to 1.19 mu m), Unifast 3 (0.40 to 1.00 gm), Systemp (0.46 to 1.51 mu m), Structur 3 (0.85 to 1.06 mu m), Structur Premium (1.00 to 1.74 mu m), and Telio (1.13 to 1.21 mu m), except for Unifast Trad (9.20 to 2.59 mu m). Pairwise multiple comparisons identified Protemp 4 as having the smoothest surface before and after polishing, while Unifast Trad was the roughest in both. Atomic force microscopy and scanning electron microscopy observations showed that the surface roughness of bis-acrylic resins was related to their surface morphology and average filler sizes. A positive relation between fillers and roughness was assessed (r=0.345, P<.001). Conclusions. For the bis-acrylic interim resins, the surface roughness after polishing was correlated to the material used and its filler sizes. Nanofiller-based resins showed the smoothest surfaces. For the polymethyl methacrylate-based resins, the recently marketed Unifast 3 had the lowest overall roughness values.