A new dual cured resin cement (Unicem, 3M ESPE) has been introduced which bonds to sandblasted metal and tooth structure without an etching or bonding step. This simplified adhesive cement may improve crown retention, however some adhesive cements have expanded when placed into water.  Objective:  This study measured and compared linear expansion of 5 cements stored in phosphate buffered saline for 4 time periods.  Methods:  Five cements were mixed following the manufactures directions and placed into a split Teflon mold.  The light cured cements were cured with a Spectrum curing light (550 mW/cm2 output) for 60 sec, removed from the mold, the length measured and stored in an incubator in phosphate buffered saline for the time intervals listed below and re-measured.  The differences in length were expressed as % change in length.  The data were analyzed with 2 factor ANOVA and Tukey B post-hoc test. Results:

CEMENT	24 hrs	1 Week	1 Month	2 Month	3 Months
Principle	.00	.05	.15	.22	.31
RelyX ARC	-.09	.01	.17	.18	.18
Fuji Plus	.33	.61	.71	.81	.89
Fuji Cem	.18	.36	.42	.48	.51
Unicem	.2	.3	.35	.38	.4

The resin modified glass ionomer Fuji Plus had the greatest expansion (p=.01), Unicem was significantly lower and equal to Principle and Fuji Plus.  RelyX ARC had significantly less expansion compared to all other materials (p<.05).  Conclusions: Low expansion cements should be used.