Effects of heating on the flexural strength of human dentin

Objectives: This study sought to investigate the effects of dehydration, heating and rehydration on the flexural strength of human dentin.

Methods: Beam-shaped specimens, approximately 1.7 x 0.9 x 7.0mm, were obtained from the coronal central portions of human third molars. Dentinal tubule orientations were organized to run parallel to the loading surfaces along their length. The specimens were divided for testing under different environmental conditions: wet (stored in HBSS at 4°C); dry (desiccated for 7 consecutive days at humidity < 20%) and heated (in an oven at temperatures from 70 to 170°C for 0 to 60 min). A flexural load was applied to each specimen by a universal testing machine with a crosshead speed of 0.05 mm/min. To investigate the effects of rehydration, some heated specimens were immersed in HBSS for one, three and seven days and only then subjected to fracture testing. Another set of specimens heated and then UV irradiated with 3200 mW/cm2 for 15 min were rehydrated before their fracture strength was measured.

Results: Heating the specimens at temperatures between 90 to 140°C for 10 min produced significantly greater fracture strength (221.9±23.8~253.5± 68.6 MPa, ANOVA and Scheffe's F test, p<0.05), and three times greater than that in the wet condition. This superior strength by heating from 90 to 110°C was completely reversed to the initial levels after rehydration in HBSS for one day. However, combining heating precisely to 140°C and UV irradiation led to 66% of the strengthening effect being retained even after the rehydration for seven days.

Conclusions: Human dentin can be strengthened by a combination of heating and UV irradiation. This may be because heat generates higher-density structures and UV caused chemical changes such as generating new collagen cross-linking.