Effect of demineralization and remineralization on microhardness of irradiated dentin

The aim of this in vitro study was to evaluate the effects of irradiation and fluoridation on the demineralization and remineralization patterns of root dentin. From the cervical regions of 84 bovine incisors, each of four dentinal blocks were prepared and randomly assigned to four groups: 1) no irradiation; 2) irradiation of specimens up to 60 Gy (2 Gy/d, 5 d/w); 3) no irradiation, but fluoridation of specimens for 5 min/d with Elmex Gelée; and 4) irradiation and daily fluoridation of specimens for 5 min/d. Subsequently, the specimens were demineralized for 2, 4, 6 or 8 days with acidulated hydroxyethylcellulose (n = 21), and Knoop hardness numbers (KHN) were determined before, as well as after the demineralization period. Then seven specimens from each group were fluoridated with one of three fluoride gels (Elmex Gelée [1.25%], STOP [0.4%], Fluoridgel [1.25%]; 2 x 15 min/d, 10 d), and immersed in synthetic saliva at a temperature of 37 degrees C. Finally, KHN for all specimens were determined. Irradiation resulted in a significant decrease in microhardness. There was a reduction in microhardness with increasing demineralization time in all groups. The highest percentage decrease in microhardness could be observed with group 1. Due to the preceding decrease of KHN in the irradiated specimens (group 2), these samples showed less percentage reduction in microhardness during demineralization when compared to group 1 (ANOVA; p < 0.001). The decrease in microhardness was significantly hampered by fluoridation in the non-irradiated, as well as in the irradiated samples (groups 3 and 4; p < 0.001). The remineralization with Fluoridgel resulted in the greatest increase in microhardness. It is concluded that demineralization can be hampered by regular fluoride application in irradiated dentin. However, due to the considerable irradiation effect, this benefit might be negligible.