Effect of surface coating on water migration into resin-modified glass ionomer cements: a magnetic resonance micro-imaging study.

Magnetic resonance micro-imaging was applied to study water diffusion into resin-modified glass ionomer cement restoration and to evaluate the effect of surface coating over restoration. Two cavities were prepared on the labial surface of extracted teeth and restored with resin-modified glass ionomer cement; one was protected with surface coating and the other was not. Immediately after restoration, the teeth were immersed in water. Progress of water diffusion into restorations was monitored by T(1) weighted spin-echo MRI at one-day intervals after the start of immersion. To quantify the water diffusion, a model was developed and compared with imaging data. Best fit yielded an effective water diffusion coefficient D = (2.3 +/- 0.4) 10(-12) m(2)/sec. Experimental results demonstrated that surface coating protects the dental cement against water intrusion from the surface of the restoration which faces the oral cavity. Such coating, however, does not prevent water penetration from the dentine side.

Strength and reliability of surface treated Y-TZP dental ceramics.

This work was undertaken to evaluate the effects of dental grinding and sandblasting on the biaxial flexural strength and Weibull modulus of various Y-TZP ceramics containing 3 mol% yttria. In addition, the susceptibility of pristine and mechanically treated materials to low-temperature degradation under the conditions adopted for testing the chemical solubility of dental ceramics was investigated. The results revealed that surface grinding and sandblasting exhibit a counteracting effect on the strength of Y-TZP ceramics. Dental grinding lowered the mean strength and Weibull modulus, whereas sandblasting provided a powerful method for strengthening, but at the expense of somewhat lower reliability. The finest-grained material exhibited the highest strength after sintering, but it was less damage tolerant than tougher, coarse-grained materials. Upon extraction with the acetic acid solution and the ammonia solution, a significant amount of tetragonal zirconia had transformed to monoclinic, but extensive microcracking and attendant strength degradation had not yet occurred. Standard grade Y-TZP ceramics are more resistant in an alkaline than in an acidic environment, and there was a strong grain-size dependence of the diffusion-controlled transformation. Since a special Y-TZP grade containing a small amount of alumina exhibited the highest damage tolerance and superior stability in an acidic environment, this material shows considerable promise for dental applications.

The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic.

OBJECTIVES: This study was conducted to evaluate the effect of grinding and sandblasting on the microstructure, biaxial flexural strength and reliability of two yttria stabilized tetragonal zirconia (Y-TZP) ceramics. METHODS: Two Y-TZP powders were used to produce fine grained and coarse grained microstructures. Sixty discs from each material were randomly divided into six groups of ten. For each group, a different surface treatment was applied: dry grinding, wet grinding, sandblasting, dry grinding + sandblasting, sandblasting + dry grinding and a control group. Biaxial flexural strength was determined and data were analyzed using one-way ANOVA, followed by Tukey's HSD test (p < 0.05). In addition, Weibull statistics was used to analyze the variability of flexural strength. The relative amount of transformed monoclinic zirconia, corresponding transformed zone depth (TZD) and the mean critical defect size Ccr were calculated. RESULTS: There was no difference in mean strength between the as sintered fine and coarse grained Y-TZP. Significant differences (p < 0.05) were found between the control group and ground fine grained material for both wet and dry grinding. Sandblasting significantly increased the strength in fine and coarse grained materials. All surface treatment procedures reduced the Weibull modulus of Y-TZP. For both materials, the highest amount of the monoclinic phase and the largest TZD was found after sandblasting. Lower amounts of the monoclinic phase were obtained after both grinding procedures, where the highest mean critical defect size Ccr was also calculated. SIGNIFICANCE: Our results indicate that sandblasting may provide a powerful technique for strengthening Y-TZP in clinical practice. In contrast, grinding may lead to substantial strength degradation and reduced reliability of prefabricated zirconia elements, therefore, sandblasting of ground surfaces is suggested.

Micro magnetic resonance imaging of water uptake by glass ionomer cements.

OBJECTIVES: The purpose of the present study was: 1) to visualize the water penetration into glass ionomer cement samples prepared in two different setting modes as a function of time, and 2) to assess the potential use of micro magnetic resonance imaging by studying penetration processes. METHODS: An encapsulated form of resin-modified glass ionomer cement (Fuji II LC, GC) was used in this study. The mixed cement was syringed into quartz tubes (4 mm ID x 10 mm long). Half of the samples were radially exposed to a light source for 120 s; the other half were allowed to set chemically in a photographic darkroom. One hour after the start of mixing, samples were extruded from the quartz tubes, immediately immersed in distilled water, and stored at 37 degrees C. Eight specimens were prepared with each setting mode and imaged at different times. Micro magnetic resonance imaging was performed on a Bruker Biospec System equipped with micro-imaging utilities. A spin echo technique was used. A small tube containing a mixture of normal and deuterated water was added as a standard to which the signals from the samples were normalized. The average signal, as calculated by the image processing software from each region, was divided by the signal from the standard sample to obtain the normalized intensity. The results were analyzed by a Student's t-test. RESULTS: After 24 h of immersion, water diffused 1 mm into the chemical-cured material and approximately 0.5 mm in the light-cured samples. After 96 h, the water had reached the center of all chemical-cured samples but not of the light-cured samples. After 192 h, water had reached the center of the cylinders of both groups of samples. SIGNIFICANCE: MRI microscopy is a good method for monitoring the water permeability of glass ionomer cements. The technique is nondestructive thus, the process can be followed on the same sample without destroying it. By using some special imaging techniques, refinement of the method will be possible.