Transparent dentin region in the tooth root.

Transparent dentin in the tooth root forms during the aging process. The transparent dentin is weaker than normal opaque dentin, which may explain the occurrence of root dentin fractures in aged individuals. Tooth fractures are caused by the brittleness of the transparent dentin. Clinical procedures in aged persons require modification to accommodate the reduced strength of the dentin tissue.

Anisotropic ultimate strength and microscopic fracture patterns during tensile testing in the dentin-enamel junction region.

The dentin-enamel junction (DEJ), which is the interface between the outer tooth enamel and the underlying dentin, has unique biomechanical properties that allow the two different materials to function together without fracture. The aim of this study was to investigate the anisotropic ultimate tensile strength of the DEJ. Twenty dumbbell-shaped samples were prepared from 10 extracted human molar teeth for tensile strength testing. The maximum tensile forces at fracture were measured in the direction parallel (10 samples) and perpendicular (10 samples) to the DEJ. The mean tensile strength of the samples was significantly higher under parallel forces (30.9±3.3 MPa) than under perpendicular forces (20.6±4.8 MPa; p<0.05). Our findings confirm the structural anisotropy of the DEJ.

Three-dimensional representation of microdontia of the maxillary third molar.

Dentists and maxillofacial surgeons may occasionally encounter various dental anomalies in number, shape, size, eruption, etc. In particular, microdontia is relatively rare. Computed tomography during clinical dental examination is essential for early detection of these anomalies.

Relation between incremental lines and tensile strength of coronal dentin.

In one aspect, this study examined the tensile strength of coronal dentin, as a function of the location of incremental lines, in two types of teeth: human molar versus bovine incisor. In another aspect, tensile strength in coronal dentin was examined with tensile loading in two different orientations to the incremental lines: parallel versus perpendicular. There were four experimental groups in this study: HPa, human molar dentin with tensile orientation parallel to the incremental lines; HPe, human molar dentin with tensile orientation perpendicular to the incremental lines; BPa, bovine incisor dentin with tensile orientation parallel to the incremental lines; BPe, bovine incisor dentin with tensile orientation perpendicular to the incremental lines. Tensile strengths of the parallel group (HPa and BPa) were significantly higher (p<0.05) than those of the perpendicular group (HPe and BPe). Effect of structural anisotropy, contributed by the incremental lines, was thus confirmed in coronal dentin. However, there were no differences in anisotropy effect between the two tooth types.

Fatigue and tensile properties of radicular dentin substrate.

The purpose of this study was to compare the fatigue and tensile strengths of radicular dentin. Forty bovine lower central incisors were used, twenty teeth for the fatigue test and twenty teeth for the tensile test. Bovine teeth were each sectioned into coronal and radicular portions. Dentin slabs of 1mm thickness were prepared along the radicular tooth using a low-speed cutting machine and trimmed into dumbbell-shaped specimens. A dentin slab was harvested from each tooth. Subsequently, fatigue and tensile tests were performed in Hank's balanced saline solution at 37 °C. The staircase method was employed to determine fatigue strength and its standard deviation. Fracture surfaces were observed by scanning electron microscopy. Mean fatigue strength and tensile strength were 44.3±5.0 and 84.4±8.3 MPa, respectively. The fatigue strength of radicular dentin was significantly lower than the tensile strength. The fatigue strength of radicular dentin was only approximately one half of the tensile strength.

Comparison of nanohardness between coronal and radicular intertubular dentin.

This study investigated the hardness and Young's modulus of coronal and radicular intertubular dentin. Ten bovine teeth were each divided into coronal and radicular groups, and the flat surfaces of the coronal and radicular dentin were subsequently processed along the tooth axis. The hardness and Young's modulus of the coronal and radicular intertubular dentin were evaluated using nanoindentation tests, at two locations per tooth. Mean hardness and Young's modulus values were statistically compared by one-way ANOVA and Fisher's PLSD test. The hardness of coronal intertubular dentin was 0.81 +/- 0.05 GPa and that of radicular dentin was 0.55 +/- 0.02 GPa. Additionally, the Young's modulus of coronal intertubular dentin was 26.60 +/- 2.19 GPa and that of radicular dentin was 20.89 +/- 1.10 GPa. Findings of this study revealed that the hardness and Young's modulus of coronal intertubular dentin were greater than those of radicular intertubular dentin.

Tensile strength and durability of bovine dentin.

This study investigated the effects of thermal cycling on the tensile strength of dentin. Bovine dentin were divided into 10 groups, which were then subjected to various conditions: intact after preparation, thereby serving as a control; heating in boiling water for 45 minutes; 10,000 thermal cycles in water; 10,000 thermal cycles in PBS; storage in water at 5, 23, or 55 degrees C for two weeks; and storage in PBS at 5, 23, or 55 degrees C for two weeks. Subsequently, bovine dentin were trimmed into dumbbell-shaped specimens and the tensile test performed in distilled water at 37 degrees C. Mean tensile strengths were compared statistically by one-way ANOVA and Fisher's PLTD test (p<0.05). Fracture surfaces were observed by scanning electron microscopy, and reliability of the results was analyzed with Weibull distribution. Tensile strength did not significantly change after thermal cycling or storage in water and PBS at all temperatures tested (71.2-77.0 MPa) but decreased after treatment with boiling water (65.5 MPa).

Effects of laser irradiation on tensile strength of bovine dentin.

OBJECTIVE: The purpose of the present study was to investigate the tensile strengths of dentin after laser irradiation using three kinds of dental lasers to elucidate the laser-irradiation effect on dentin properties. BACKGROUND DATA: Different kinds of laser devices have been developed in dentistry. The characteristics of each laser are determined by its original wavelength; however, one common feature is to generate heat in irradiated tissues, and such heat possibly affects dentin collagen, which contributes to tensile strength of the tissues. MATERIALS AND METHODS: Er:YAG, CO2, and diode (GaAlAs) lasers were used to irradiate bovine dentin. Subsequently, tensile test specimens were made from the irradiated dentin and tensile tests were conducted. The tensile strengths were analyzed using the paired-t test and Weibull analysis. Irradiated dentin was also observed transversally using light microscopy. RESULTS: The tensile strengths of the lased dentin and the control group for the Er:YAG, CO2, and diode lasers were 73.1 and 78.5, 70.3 and 74.3, and 64.3 and 71.0 MPa, respectively. The tensile strength of the dentin had a tendency to decrease with laser irradiation. Weibull analysis indicated that the laser influence was different among the three kinds of laser apparatuses and seemed to correspond to the depths the laser beam reached, which were suggested by light microscopy observation. CONCLUSION: Laser irradiation could possibly decrease dentin tensile strength, which suggests the importance of careful use of laser for hard tissue treatment, considering its energy-transforming characteristics.

Dimensional accuracy and surface property of titanium casting using gypsum-bonded alumina investment.

The aim of the present study was to evaluate the dimensional accuracy and surface property of titanium casting obtained using a gypsum-bonded alumina investment. The experimental gypsum-bonded alumina investment with 20 mass% gypsum content mixed with 2 mass% potassium sulfate was used for five cp titanium castings and three Cu-Zn alloy castings. The accuracy, surface roughness (Ra), and reaction layer thickness of these castings were investigated. The accuracy of the castings obtained from the experimental investment ranged from -0.04 to 0.23%, while surface roughness (Ra) ranged from 7.6 to 10.3microm. A reaction layer of about 150 microm thickness under the titanium casting surface was observed. These results suggested that the titanium casting obtained using the experimental investment was acceptable. Although the reaction layer was thin, surface roughness should be improved.

Anisotropy of tensile strengths of bovine dentin regarding dentinal tubule orientation and location.

The purpose of this study was to investigate the effects of the location and orientation of dentinal tubules in the tooth on tensile strengths of the dentin. Dumbbell-shaped specimens of 12 groups from various locations and dentinal tubule orientations were prepared. The tensile test was performed in distilled water at a temperature of 37 degrees C. The tensile strengths of the parallel to the orientation were significantly greater than those of the perpendicular to the orientation; the tensile strengths of the radicular dentin were significantly greater than those of the coronal dentin. Nevertheless, in the radicular dentin, the tensile strengths of the perpendicular to dentinal tubules differ with respect to tensile forces. These results suggest that tensile strength of the dentin varies according to the location and orientation of dentinal tubules in the tooth.