How UVA-activated Riboflavin Strengthen Human Dentin

Objectives: Type-I collagen, a major organic component of human dentin, plays an important role in regulating the mechanical strength of dentin. The purpose of this study was to investigate how the exposure to riboflavin followed by UVA irradiation (riboflavin/UVA) affects the mechanical strength of human dentin by producing collagen crosslinking.
Methods: Beam-shaped specimens (n=5/group), measuring approximately 1.7 x 0.2 x 8.0mm were obtained from the coronal dentin of human third molars. These specimens were exposed to riboflavin (control, 0.1, 1%) followed by 365nm UVA irradiation (control, 800, 1200, 1600mW/cm²) with various time exposures (control, 5, 10, 15min). Flexural strength was measured to each specimen by a universal testing machine, and then scanning electron microscopy (SEM) analysis was performed. To investigate the changes in collagen molecules caused by riboflavin/UVA, demineralized dentin specimens (n=5/group) before and after the riboflavin/UVA treatment were subjected to laser Raman spectorascopic analyses and SDS-PAGE.
Results: Flexural strength is the highest in the group of 0.1% riboflavin exposure and UVA irradiated with 1600 mW/cm² for 10min (295.3±46.6MPa), which was significantly higher than control group (145.8±29.8MPa) (ANOVA, Scheffe’s F test, p<0.05). Raman spectra showed that a C-C bond in a proline ring (922cm^-1) was amplified, and the ratio between C-N bond (1243cm^-1) and N-H bond (1267cm^-1) was changed by the riboflavin/UVA treatment. SDS-PAGE analysis suggested that the molecular weight of collagen was increased after the riboflavin/UVA treatment due to new collagen crosslinking.
Conclusions: Human dentin was strengthened by the riboflavin/UVA treatment. It was suggested that the treatment induced the change of chemical structures in dentin collagen such as forming new crosslinking. (This study was supported by Grants-in-Aid for Scientific Research (25293387 and 15K15700) from the JSPS.)