The effect of NaOCl and heat treatment on static and dynamic mechanical properties and chemical changes of dentine

Objectives: To determine the effect of heat on flexural strength (FS), maximum strain (MS), storage modulus (SM), tan delta (TD)and chemical changes through micro-Raman spectroscopy of dentine exposed to 2.5% NaOCl or saline. Method: ology: Dentine bars were randomly allocated to 8 test groups. Half (groups 2,4,6,8)were treated with NaOCl for 20 min; the rest (groups 1,3,5,7)remained in saline. FS/MS were measured in groups 1–4 (n = 15)(3/4 were also heated to 200 °C & re-hydrated in saline). Micro-Raman spectroscopy was performed on bars from groups 1–4. SM/TD were measured in 5–8: in 5/6 (n = 10), repeated after heating (200 °C), then following re-hydration; in 7/8 (n = 3)after heating to 25–185 °C. Results: Increase in MS on heat and FS/MS on heat + NaOCl was not significant (P > 0.05). SM increased (P = 0.06)after heat treatment but reduced to initial state after rehydration (P = 0.03). TD did not change (P = 0.4)after heat (200 °C)treatment but rehydration increased it compared with pre-treatment state (P = 0.001). For dentine bars pre-treated with NaOCl, SM did not change (P = 0.6)after heat (200 °C)treatment or rehydration but TD significantly increased (P = 0.02)upon re-hydration compared with pre- (P=0.007), or post- (P = 0.03)heat-treatment states. SM and TD varied between 25–185 °C with no consistent trend amongst the NaOCl pre-treated bars. Micro-Raman only detected chemical changes following NaOCl treatment in the mineral phase. Conclusions: Exposure of dentine bars to heat and NaOCl produced only moderate changes to quasi-static but marked changes to viscoelastic properties, which may be explained by chemical alterations.