Mitsunori Uno

Effects of adding silver nanoparticles on the toughening of dental porcelain

STATEMENT OF PROBLEM Dental porcelains currently used for ceramic restorations are brittle, and it is sometimes necessary to replace fractured or chipped restorations. Porcelain is fragile and exhibits elastic deformation rather than plastic deformation, leading to fracture or chipping of restorations. PURPOSE The purpose of this study was to investigate the toughening of porcelain through the addition of silver nanoparticles. MATERIAL AND METHODS Noritake Super (NS) Porcelain AAA modified with the addition of silver nanoparticles was used. The concentration of silver in the solution was adjusted to 100, 200, 500, and 1000 ppm (Ag100, Ag200, Ag500, and Ag1000). The Vickers hardness (Hv) and median crack length extending from the corner of each indent were measured. The fracture toughness (KIC) was calculated by the indentation method. Optical reflectance spectra were recorded by using a spectrometer in the wavelength range of 200 to 700 nm. X-ray diffraction spectroscopy, color measurement, Fourier transform infrared spectroscopy, and electron probe microanalysis were also performed. The observed values of Hv, 2a, E, and KIC were compared and evaluated with a 1-way ANOVA, followed by the Bonferroni method (α=.05). RESULTS The addition of silver nanoparticles significantly increased the Hv of all specimens with the exception of Ag100. The median crack length was significantly smaller in Ag500 (104.5 μm, SD: 11.9) and Ag1000 (100.0 µm, SD: 5.5). Significantly higher toughness values were observed for Ag500 (1.54 MPa·m(1/2), SD: 0.05) and Ag1000 (1.51 MPa·m(1/2), SD: 0.08) than for the control (1.36 MPa·m(1/2), SD: 0.03). In terms of color difference, Ag500 (5.08, SD:1.32) and Ag1000 (5.47, SD:1.05) had values significantly greater than ΔE*=2.69. CONCLUSIONS The addition of silver nanoparticles significantly increased the fracture toughness and Vickers hardness of the NS porcelain. A residual compressive stress was generated due to the ion exchange reaction and differential thermal expansion of the silver metal nanoparticles. However, the addition of Ag500 and Ag1000 nanoparticles led to a color change.

The Effects of Muscle Fatigue Caused by Clenching on Articulatory and Masticatory Movements.

An investigation on the effects of fatigue in the masticatory muscles on the articulatory and the masticatory movements was made from the aspect of movement changes in the perioral soft tissues. Ten healthy males were used as the subjects. They were asked to pronounce the five Japanese vowels through articulatory and masticatory movements before and after clenching for 30 sec at the maximum strength correlated between the two functional movements in each subject. The results suggested that articulatory movement could be utilized for the diagnosis of jaw function.

Correlation between Chewing Path and Muscle Activity in Chewing Movement. Premature Contact.

This study was performed to determine whether or not malocclusion decreases the interdependent relationship between the chewing path and muscle activity. For experimental premature contact attached to the lower 1st molar on the habitual chewing side, two thicknesses (50 μm and 100μm) of metal foil were used. Chewing movements using a stick of gum were recorded simultaneously as the path and electromyograms. The parameters for the path were the amount of anterior-posterior and right-left movements at 1 mm, 7 mm and lowest point vertically below the intercuspation in the opening-closing path and the maximum amount of mouth opening per stroke, and those for electromyograms were the interval of EMG bursts and the integrated in the central portion of the bilateral masseter muscles and the anterior portion of the bilateral temporal muscles. The presence or absence of premature contact, and changes in the relationship between the chewing path and muscle activity according to the thickness of interference were analyzed in terms of the first canonical correlation coefficient in canonical correlation analysis. Premature contact could be a factor that decreases the first canonical correlation coefficient, but no differences were observed between the two thicknesses.