L. BORCHERS*, F. TAVASSOL, H, TSCHERNITSCHEK (Medizinische Hochschule Hannover)
Different techniques for minimizing surface roughness with temporary restorations, namely polishing and surface coating, were investigated in this study with regard to surface quality, strength and stiffness.
Cylindrical specimens were fabricated from ten temporary crown and bridge resins and machined flat at one end surface. One set of specimens was finished by means of rubber polishers, another one was coated with eight varnishes of different type. One set of specimens was used as control, a second one was aged by thermocycling (5 °C / 55 °C, 2000 cycles) and by immersion in an artificial saliva for 28 days. Surface roughness was determined by means of an electrical contact stylus instrument. Additionally, microphotographs were taken in a scanning electron microscope. As mechanical parameters, bending strength and modulus of elasticity were determined in three point bending tests. One-way factorial analyses of variance were performed using the statistical program SPSS. Each statistical group comprised 8 specimens.
After polishing, arithmetic mean surface roughness values (Ra) were in the range between 0.6 and 0.9 µm. There was no significant difference between materials. With respect to their effect on surface quality as compared with polishing, the coatings could be divided into three groups: group 1 caused Ra to increase up to sevenfold (Cyano Veneer, Liquicoat, Polibond); group 2 gave better results on a bis-acryl resin, but worse on a higher methacrylate (Palaseal, Solobond, Visioseal); group 3 generated smoother surfaces on both materials (Fissurit, Pertac). Six coatings, however, tended to island formation because of surface tension effects and herewith proved not to be applicable clinically. - The mechanical parameters show a considerable variation between different materials (bending strengths were between 20 and 80 MPa, moduli of elasticity between 300 MPa and 3300 MPa) and are in general not significantly affected neither by the storage conditions tested nor by the application of surface coats. The strongest and stiffest materials being two BisGMA resins.
Polishing is the most reliable method of minimizing surface roughness with temporary restorations. Coatings can only be recommended in certain material combinations.