Effects of heating on the mechanical and chemical properties of human dentin

ObjectivesWe had previously discovered that the flexural and tensile strengths of human dentin were 2–2.4 times greater after being heated to 140 °C, and deduced that the generation of higher-density structures and therefore dehydration probably promoted the increased strength. Our test hypotheses were that intertubular dentin, which constitutes a major part of organic components, was selectively affected by heating, and such changes could happen without critical damages to the basic structure of dentin type I collagen.MethodsMicro-mechanical changes of human dentin by heating at 140 °C were investigated by nano-indentation. Chemical changes in dentin collagen after heating were also investigated by X-ray diffraction study, a microscopic Fourier transform infrared (micro-FTIR) and a laser Raman spectroscopic analyses, and a cross-linking analysis by high-performance liquid chromatography.ResultsThe results of nano-indentation showed that the micro-hardness of intertubular dentin increased after heating at 140 °C to 1.8 times more than unheated dentin; on the other hand, peritubular dentin was unchanged. Results of X-ray diffraction showed that the lateral packing of collagen molecules shrank from 13.6 ± 0.3 to 10.6 ± 0.1 Å after heating, but the shrinkage reversed to the original after rehydration for seven days. After heating, no substantial chemical changes in the collagen molecules were detected in tests by micro-FTIR or Raman analyses, or by cross-linking analysis.SignificanceThese results suggest that intertubular dentin, which contains most of the type I collagen, was selectively affected by heating at 140 °C without critical damage to its collagen.