Correction of some mechanical characteristics of human dentin under compression considering the shape effect

Fracture Strength and Stress Distribution in Premolars Restored with Cast Post-and-Cores or Glass-Fiber Posts Considering the Influence of Ferule

Objectives

The aim of this study was to evaluate the influence of ferule and the post type on the fracture strength and stress distribution in premolars.

Materials and Methods

Forty human mandibular premolars were decoronated, allocated in four groups, and restored as follows: CPC-NF: cast post-and-core and absence of ferule; CPC-F: cast post-and-core and presence of ferule; FPC-NF: glass-fiber posts and absence of ferule; FPC-F: glass-fiber posts and presence of ferule. The fracture strength (FS) and failure patterns were evaluated. Finite element analysis (FEA) evaluated the stress distribution.

Results

FS did not differ between CPCs and FPC either in presence or in absence of ferule. The presence of ferule increased FS with both post types. Mean values of FS for ferule groups were higher than functional or parafunctional loads reported in literature, which was not the case for FPC-NF when compared to parafunctional loads. FEA with a functional load showed slightly higher compressive stresses in dentin in the group CPC-NF, which was much lower than the compressive strength of dentin. Lower percentage of catastrophic failures was observed in nonferule groups irrespective of post type, which was explained by the stress concentration in the cervical root region when FEA with the FS load was simulated.

Conclusion

Ferule effect was shown to be more important than post type in the analysis. Both posts showed potential to withstand functional loads irrespective of presence of ferule. However, the mean FS was lower than parafunctional loads for FPC in the absence of ferule.

Mechanical properties of human enamel under compression: On the feature of calculations

The paper is aimed at determination of the causes of shape effect in human tooth enamel under compression and correction of the relevant mechanical characteristics. For this purpose, six groups of samples with different ratios of the compression surface diagonal to the sample height, which consisted of 10 cuboid samples in each, were prepared from the backside of human enamel. The lateral deformation of a sample was calculated at the maximum compressive stress for correction of the mechanical characteristics. It is shown that the ratio between the lateral and axial deformations decreases with an increase in the ratio of the compression surface diagonal to the sample height. This is caused by the friction between the compression plates and the working surfaces of the enamel sample when the lateral deformation is suppressed. In addition, the slope of enamel sample by about 15° occurred during compression due to the inclination of rigid and low deformable enamel rods. The corrections of the elastic modulus and the compression strength taking into account the lateral deformation and the sample slope are carried out. The mechanical properties of enamel samples with the 2.1 aspect ratio are closer to the intrinsic properties of human enamel samples. The elastic modulus and the compression strength of human enamel under compression are 5.64 GPa and 363 MPa, respectively. The lateral deformation (~10%) may be considered as the critical parameter that indicates the strength of human enamel.

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.