Masahiro Ohsawa

Fatigue resistance of composite restorations: effect of filler content.

OBJECTIVES The purpose of this study was to evaluate the effects of filler level on the fatigue impact resistance of resin composite. METHODS A series of experimental composite materials was prepared by incorporating a silanized quartz filler (3-5 microns in size) into a light-cured resin matrix of Bis-GMA/TEGDMA. The filler contents in the experimental composites varied from 40 to 85 wt%. The composites were placed in standardized Class I cavities prepared in bovine teeth. The specimens were stressed with a repetitive impact load (1.6 x 10(2) joule) with loading cycles ranging from 50,000 to 150,000 times. The cracks induced by cyclic loading were observed on the sectioned surfaces of the tested specimens. RESULTS The composites with considerably low or high filler content (< 60% or > 80% by weight) were significantly low in fatigue resistance. The results revealed that an inverse linear relationship tended to exist between filler level and fatigue resistance of the composite materials beyond a certain level of filler content. SIGNIFICANCE Increased filler level does not necessarily improve the fatigue resistance of a resin composite as determined by applying a repetitive impact load.

Quantitative analysis of unreacted hemihydrate in set dental stone by adiabatic calorimetry

The unreacted hemihydrate in set dental stone was estimated quantitatively by means of specific heat measurements. With decreases in the water-to-powder ratio (W/P), increases were observed in the amount of unreacted hemihydrate in the set mass. These increases were marked in set stone with a W/P ratio less than 0.25. Furthermore, the amount of unreacted hemihydrate was higher in the interior of the dental stone than in the set surface. It is very difficult to attain a complete reaction, even with higher W/P ratios such as 0.8 in the dental mixing procedure.