Effects of rehydration on dentin strengthened by heating or UV irradiation.

Type I collagen, the major organic component of human dentin, plays an important role in regulating the mechanical strength of dentin. Collagen in dentin can be strengthened by heating. We hypothesized that UV irradiation could produce similar strengthening effects and might maintain the strength of dentin after rehydration. Beam-shaped dentin specimens from the crowns of human third molars were subjected to flexural testing. Flexural strengths were two and three times greater than those in the control group after 5 minutes' UV irradiation and heating to 140 degrees C, respectively. After 30 days of rehydration, the heated specimens reverted to their original strength, whereas the UV specimens were 69% stronger than the original. Raman spectra of dental collagen were unchanged after heating, whereas several peaks, including a C-C bond in a proline ring, were amplified by UV irradiation. It is concluded that dentin strengthened by UV irradiation retains strength after rehydration because of chemical changes in collagen.