Linear coefficient of thermal expansion evaluation of glass ionomer and resin-modified glass ionomer restorative materials

Objective: The purpose of this evaluation was to evaluate the linear coefficient of thermal expansion (LCTE) of 12 conventional glass ionomer (GIC) and four resin-modified glass ionomer (RMGI) restorative materials. Methods: GIC and RMGI specimens (2 mm 3 5 mm 3 5 mm) were fabricated (n=12) following manufacturer instructions and were placed in 0.2M phosphate-buffered saline and stored at 378C and 98% humidity for one week. Specimens had LCTE determined with a thermomechanical analysis (TMA) unit using a 158C-508C heating cycle as well as a 508C-158C cooling cycle at a 58C/min rate, using a 3-mm ball-point probe under 0.02 N probe pressure with all specimens kept saturated with PBS using a specially designed quartz container. Each specimen was tested three times, with the mean representing the specimen LCTE. Mean results between specimen heating and cooling were compared with paired Wilcoxon sign rank test, while results between materials were compared with Kruskal-Wallis/Dunn's (a=0.05). Results: GIC LCTE ranged from approximately 58C to 208C ppm 8K¹, while the RMGI LCTE ranged from approximately 258C to 478C ppm 8K¹. With some exception, the LCTE during cooling displayed a greater trend. Significance: Under moisture conditions similar to the oral cavity, GIC materials overall had LCTE values closer to that reported for tooth structure. RMGI materials displayed higher values, which was thought to be related to the amount of resin in the matrix. A generally greater LCTE trend with cooling for all materials was noted, but the small magnitude of the difference is presently thought to be of minor clinical significance.