Water sorption and mechanical behaviour of cosmetic direct restorative materials in artificial saliva

Abstract

Objectives: To evaluate the water sorption and mechanical behaviour of a compomer in comparison with those of its nominal forerunners, a filled resin restorative material and a conventional glass ionomer cement. Method: Compomer (Dyract AP) (D-AP), filled resin (SureFil) (SF), and glass ionomer (ChemFlex) (CF) (all Dentsply, Addlestone, UK) restorative materials were tested. Forty bar specimens (26×1.5×1.0 mm 3) of each material were prepared according to the manufacturer's instructions and randomly distributed into eight groups: dry air (22% RH), saturated water vapour (WV) (100% RH), and five in artificial saliva (AS) at pH6, all at 37°C, as well as untreated control (UC) (23°C, 50% RH). Water sorption was assessed gravimetrically; flexural strength and elastic modulus were determined in three-point bend. The control group was tested at 24 h; AS groups were separately tested after 0.5, 1, 3, 6 and 9 months; the other two at 9 months. Results: Mass gain for SF, D-AP and CF in AS was up to 0.17%, 1.2% and 7.0%, respectively. CF showed a marked decrease of strength in AS compared with other groups, followed by a gradual slight rise to a peak at 3 months. Unlike SF and CF, whose flexural strength remained relatively stable, that of D-AP showed a sharp decline from the 1 month peak ( P=6×10 −7) after 6 months in AS. D-AP also showed a slight decline in flexural modulus from a peak, that of SF was quite stable, while CF showed no peak. The values of flexural strength for both CF and D-AP at 9 months were significantly lower in AS than WV, but SF showed no such difference. Significance: Materials intended for service in the mouth must be stored in a realistic medium if the results of testing are to be interpretable. Dyract AP, a compomer, does not seem suitable for application in stress bearing areas as is currently recommended by its manufacturer. The rapid decline in flexural strength after 1 month of exposure to AS and its progressive fall in flexural modulus suggest a progressive deterioration of the material and this necessitates re-examination of the chemistry of compomers, if the behaviour is typical of the class.