Aspects of water sorption from the air, water and artificial saliva in resin composite restorative materials.
OBJECTIVE
Primarily to establish whether artificial saliva (AS) at 37 degrees C is essential as a clinically relevant environment for testing filled, resin composite restorative materials. The effect of other storage conditions was also investigated for comparison and controls: desiccation, exposure to the laboratory atmosphere, high humidity cabinet, saturated water vapor, and deionized water.


METHODS
Two visible light-cured products were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein). Bar specimens (26 x 1.5 x 1.0 mm(3)) were cured at five overlapping spots for 60s per spot and randomly distributed into groups of six. Trial 1: one group of each material was exposed first to atmospheric air at 24 degrees C, approximately 50% RH (24WV(50)), then to water vapor at 37 degrees C, approximately 97% RH (37WV(97)), and then immersed in deionized water at 37 degrees C (37DW). Trial 2 used three groups of each material, one first exposed to 37WV(97) followed by 37DW, the other two were immediately immersed in 37DW or artificial saliva (37AS). Trial 3: two groups of each material were vacuum desiccated at 37 degrees C, then exposed to 37 degrees C, approximately 100% RH (37WV(100)), then immersed in 37DW or 37AS. Trial 4: four groups of HR were treated similarly to Trial 3; one was left under desiccation, and another in 37WV(100) for the remaining period. Three-point bend tests for flexural strength (F), flexural modulus (E), and total energy to failure (W) were performed at the end of Trials 2-4.


RESULTS
Environmental moisture absorption was substantial at 24WV(50)(c. 0.2%), at least 40% of that in 37DW (HR: c. 0.7%, TC: c. 0.5%). Saturation was achievable in 37WV(100). Mass loss on desiccation (HR: c. 0.4-0.5%, TC: c. 0.25%) was reversible in 37WV(100). There were some significant effects of exposure conditions on mechanical properties (e.g. F for HR: after desiccation, 85.7+/-1.4MPa; after 37WV(100), 73.2+/-3.6MPa; difference: p<0.0002), but overall the results were unclear. After a rapid gain in mass, there was a gradual loss in both 37DW and 37AS for both materials, slightly more in 37AS than 37DW.


CONCLUSIONS
Water vapor absorption is substantial, hence attention must be paid to the laboratory working environment and conditions of storage and testing, i.e. temperature and RH must be stated to assist interpretation of data and comparisons between studies. Test conditions need to be standardized and with reference to normal oral conditions, immediate immersion in artificial saliva at 37 degrees C is the preferred treatment for these materials, whatever time of testing is chosen.
Dental Materials Science, Faculty of Dentistry, Prince Philip Dental Hospital, The University of Hong Kong, 34 Hospital Road, Hong Kong, People's Republic of China.
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Musanje
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Darvell
D000042:Absorption; D000180:Acrylic Resins / Q000737:chemistry; D000388:Air; D003188:Composite Resins / Q000737:chemistry*; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D003890:Desiccation; D004356:Drug Storage; D004548:Elasticity; D006813:Humidity; D008422:Materials Testing / Q000379:methods; D018583:Pliability; D011140:Polyurethanes / Q000737:chemistry; D012464:Saliva, Artificial; D013696:Temperature; D014867:Water / Q000737:chemistry; D017155:Wettability
D000180:Acrylic Resins; C029012:Composite Dental Resin; D003188:Composite Resins; C513708:Heliomolar; D011140:Polyurethanes; D012464:Saliva, Artificial; C121818:Tetric ceram; D014867:Water