Fraction-exponential representation of the viscoelastic properties of dentin.

We propose the fraction-exponential description of the viscoelastic properties of dentin. Creep tests are performed on specimens cut from the molar coronal part. Four parameters determining instantaneous and long term Young's moduli as well as the relaxation time are extracted from the experimental data. The same procedure is repeated using the experimental measurements of Jantarat et al (2002) for the specimens cut from the root part of incisor. Physical meaning of the parameters and the difference between them for different sets of specimens are discussed.

Transmission electron microscopical study of teenage crown dentin on the nanometer scale.

This is the first transmission electron microscopic study of teenage crown dentin on the nanometer scale. Samples for TEM were prepared by mechanical thinning and chemical polishing that allowed obtaining the electron transparent foils. It was firstly shown that human dentin possesses the layered morphology: the layers are oriented normally to the main axis of a tooth and have the thickness of ~50nm. HA inorganic phase of teenage crown dentin is in the amorphous state. The cellular structure, which was formed from collagen fibers (diameter is ~5nm), are observed near DEJ region in teenage dentin, whereas bioorganic phase of teenage crown dentin near the pulp camera does not contain the collagen fibers. Cracks in dentin thin foils have sharp tips, but big angles of opening (~30°) with plastic zone ahead crack tip. It means that young crown human dentin exhibits ductile or viscous-elastic fracture behavior on the nanometer scale.

The Difference of Structural State and Deformation Behavior between Teenage and Mature Human Dentin.

Objective. The cause of considerable elasticity and plasticity of human dentin is discussed in the relationship with its microstructure. Methods. Structural state of teenage and mature human dentin is examined by using XRD and TEM techniques, and their deformation behavior under compression is studied as well. Result. XRD study has shown that crystallographic type of calcium hydroxyapatite in human dentin (calcium hydrogen phosphate hydroxide Ca9HPO4(PO4)5OH; Space Group P63/m (176); a = 9,441 A; c = 6,881 A; c/a = 0,729; Crystallite (Scherrer) 200 A) is the same for these age groups. In both cases, dentin matrix is X-ray amorphous. According to TEM examination, there are amorphous and ultrafine grain phases in teenage and mature dentin. Mature dentin is stronger on about 20% than teenage dentin, while teenage dentin is more elastic on about 20% but is less plastic on about 15% than mature dentin. Conclusion. The amorphous phase is dominant in teenage dentin, whereas the ultrafine grain phase becomes dominant in mature dentin. Mechanical properties of human dentin under compression depend on its structural state, too.

On some features of the shape effect in human dentin under compression.

Contribution of inorganic and organic phases of human dentin in the shape effect under uniaxial compression is discussed. Comparison of the deformation behavior under compression of the samples with the different ratios between the diagonal of the compression surface and the height of quartz glass, aluminum oxide and PMMA with dentin samples having similar aspect ratios is carried out. In addition, the comparison of the deformation behavior of these materials under tensile stress is carried out. It has been shown that the shape effect of human dentin under compression is caused by the inorganic phase. The organic phase of dentin is responsible for the lowering of the Young's modulus and the compression strength and the increasing of its plasticity. Plasticity of the dentin can be additionally provided by its porosity, when the d/h ratio of the samples exceeds 1.5.

Influences of the sample shape and compression temperature on the deformation behavior and mechanical properties of human dentin.

Deformation behavior and mechanical properties of samples of human dentin having different geometries were studied under compression in liquid nitrogen. In this case, the plastic response of the collagen fibers in dentin was excluded. The findings were compared with the mechanical properties of dentin at room temperature. Such a comparison allows the plastic contribution of collagen in human dentin to be estimated for samples of different shapes. It was shown that the deformation behavior of human dentin under compression is similar at 77K and 300 K. The dentin samples with low aspect ratio exhibited almost brittle behavior, whereas those with high aspect ratio were prone to considerable deformation. SEM study of the fracture surfaces of samples tested at room and liquid nitrogen temperatures has shown that they are similar. Examination of cracks on the compression surface of samples agrees with this conclusion. However, the mechanical characteristics of dentin depended on the temperature of testing. The compression strength and elastic deformation of dentin at 77K are higher than these parameters at room temperature, while the plasticity of dentin at 77K is lower. The plastic contribution of collagen fibers at room temperature was estimated on the basis of this comparison. The total plasticity of dentin is the sum of the contributions of both collagen and the geometry of the sample. The plasticity of dentin samples having a low aspect ratio is provided by collagen fibers only, while geometric factors are dominant for samples with a high aspect ratio. The contribution of collagen fibers to the plasticity of dentin depends on the geometry of samples with an intermediate aspect ratio.

Deformation behavior of human dentin in liquid nitrogen: a diametral compression test.

Contribution of the collagen fibers into the plasticity of human dentin is considered. Mechanical testing of dentin at low temperature allows excluding the plastic response of its organic matrix. Therefore, deformation and fracture behavior of the dentin samples under diametral compression at room temperature and liquid nitrogen temperature are compared. At 77K dentin behaves like almost brittle material: it is deformed exclusively in the elastic regime and it fails due to growth of the sole crack. On the contrary, dentin demonstrates the ductile response at 300K. There are both elastic and plastic contributions in the deformation of dentin samples. Multiple cracking and crack tip blunting precede the failure of samples. Organic phase plays an important role in fracture of dentin: plasticity of the collagen fibers could inhibit the crack growth.

On the deformation behavior of human dentin under compression and bending.

The cause of difference in deformation behavior of human dentin under compression and bending is discussed. Mechanical properties of dentin under these deformation schemes are compared. Microstructural study of fracture surfaces of samples and cracks in dentin is carried out, too. Dentin behaves like a brittle solid under bending, whereas it exhibits various types of response from brittle to highly deformable under compression that depended on the geometry of sample (d/h ratio of a cubic sample). It is shown that the quantity of cracks on the compressed sample increases when its elasticity and plasticity grow up, whereas under bending the failure of sample occurs due to the advancement of dominant crack. Deformation and crack growth are the channels for the accommodation of applied stress in dentin. Crack growth is the leading one when the level of tensile stress in sample is dominant, whereas deformation becomes the leading channel when compression stress is dominant. However, in both cases contribution of the concurrent channel cannot be ignored. This feature is caused by the ductile fracture mode of dentin on the mesoscopic level.

Deformation behavior of human enamel and dentin-enamel junction under compression.

Deformation behavior under uniaxial compression of human enamel and dentin-enamel junction (DEJ) is considered in comparison with human dentin. This deformation scheme allows estimating the total response from all levels of the hierarchical composite material in contrast with the indentation, which are limited by the mesoscopic and microscopic scales. It was shown for the first time that dental enamel is the strength (up to 1850MPa) hard tissue, which is able to consider some elastic (up to 8%) and plastic (up to 5%) deformation under compression. In so doing, it is almost undeformable substance under the creep condition. Mechanical properties of human enamel depend on the geometry of sample. Human dentin exhibits the similar deformation behavior under compression, but the values of its elasticity (up to 40%) and plasticity (up to 18%) are much more, while its strength (up to 800MPa) is less in two times. Despite the difference in mechanical properties, human enamel is able to suppress the cracking alike dentin. Deformation behavior under the compression of the samples contained DEJ as the same to dentin. This feature allows a tooth to be elastic-plastic (as dentin) and wear resistible (as enamel), simultaneously.

Deformation Behavior of Human Dentin under Uniaxial Compression.

Deformation behavior of a human dentin under compression including size and rate effects is studied. No difference between mechanical properties of crown and root dentin is found. It is mechanically isotropic high elastic and strong hard tissue, which demonstrates considerable plasticity and ability to suppress a crack growth. Mechanical properties of dentin depend on a shape of samples and a deformation rate.