Ljubo Marion

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 Tukeys 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.

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.

Gingival tissue inflammatory response following treatment with chemical retraction agents in Beagle dogs.

Abstract A normal finding of a clinically healthy gingiva is minimal inflammatory infiltration of subepithelial connective tissue which increases after exposure of gingiva to retraction agents. The study investigates and compares the inflammatory potential of four retraction agents on the gingival connective tissue. The gingiva of Beaagle dogs was exposed for 10 minutes to chemical agents. Byopsies were taken one hour, 24 hours and 7 days after treatment. Morphometric analysis of treated and control gingiva was performed on parasagittal tissue sections to compare the gingival tissue inflammatory potential of four retraction chemicals. Retraction agents produced no significant effect on connective tissue under the junctional epithelium. On the other hand the connective tissue under sulcular epithelium exhibits extensive inflammatory infiltration. The most aggressive retraction agent being Racestyptine and the least Visine, which therefore could be recommended for clinical practice.

The Influence of Crown Ferrule on Fracture Resistance of Endodontically Treated Maxillary Central Incisors

Summary Background: Prefabricated zirconia posts can contribute to increasing the fracture resistance of the endodontically treated teeth. Purpose. This in vitro study compared the fracture resistance of endodontically treated central maxillary incisors prepared with 2 mm ferrule length to the ones without ferrule. Material and methods: Twenty-four caries-free maxillary central incisors were divided into 2 groups of 12. In group A circumferential external dentin shoulders were prepared for 2 mm external dentin ferrule length. There was no ferrule preparation in Group B. Zirconia VALLPOST BO-S (Ø 1,6 mm), Ljubljana, Slovenia were used with retention forms in the coronary part. Core build-up was made of pressed ceramics (IPS e.max Press, Ivoclar, Liechtenstein). Crowns were manufactured from the same ceramic material (IPS e.max Press, Ivoclar). After root canal treatment and post space preparation, all posts were cemented with an adhesive resin cement (Multilink Automix, Ivoclar). The specimens were embedded in acrylic resin blocks (ProBase Polymer/Monomer, Ivoclar) and loaded at an angle of 45° to the long axis in an Instron Testing Machine 4301 (Instron Corp., USA) at a crosshead speed of 1 mm/min until fracture. Fracture patterns and loads were recorded. A significance level of p<0.05 was used for all comparisons. Two-way analysis of variance was used for statistical analysis. Failure patterns were analyzed with the optical microscope Stereo Discovery V.8 (Carl Zeiss, Germany) and compared using the chi-square nonparametric test. Results: The mean values (±SD) of fracture loads (N) for the Groups A and B were 664.63N (±49.14) and 519.36N (±71.65) recpectively. Significantly lower failure loads were recorded for the specimens in the group B. Failure patterns within the groups revealed non-catastrophic failure in 70% of the specimens for group A and 85% for group B. Conclusions: Within the limitations of this in vitro study, it can be concluded that zirconia VALLPOST BO-S (Ø 1,6 mm) with press-ceramic cores and crowns, can be used for restoration of endodontically treated teeth. The teeth prepared with 2 mm external dentin ferrule length were found to be more fracture resistant than teeth without ferrule.

Sterilization effect on structure, thermal and time-dependent properties of polyamides

This article studies the effect of different sterilization techniques on structure, thermal and time-dependent mechanical properties of polyamide 6 materials. We used two different types of polyamide 6 material: conventional polyamide 6 with monomodal molecular mass distribution and modified with bimodal one. Samples were prepared and properly shaped in order to observe the mentioned features before and after sterilization with three different techniques, namely sterilization with autoclave, ethylene oxide, and hydrogen peroxide plasma. Optical microscopy, differential scanning calorimetry and torsional creep testing were used for the properties investigation. As a main criterion to evaluate the effect of sterilization durability of materials was used. There was observed significant difference between monomodal and bimodal PA6 materials regarding their morphology and consequently thermal and mechanical properties: bimodal material exhibits more complex structure than chemically identical monomodal one. The results of analysis did not show considerable influence on both materials structure and thermal properties. However, there was observed strong effect on creep behavior. The most significant effect is detected for monomodal material sterilized by ethylene oxide, while creep compliance of the bimodal material did not change much due to any of applied sterilization techniques. Thus, modification the complexity of the material inherent structure improves sensitivity to sterilization.

The Modification of Time‐Dependent Mechanical Properties of Polyamides due to Sterilization

We examine the effect of sterilization on functionality and durability of Polyamide 6. Nowadays there are several applications of this material in medicine in a form of surgical sewing material, vascular catheters and other implants. Understanding the time‐dependent behavior of PA‐6 is critical in predicting the durability of different medical products made from this polymer. We show that two PA‐6 materials having different initial kinetics and processed with the same technology, when exposed to sterilization, change their time‐dependent mechanical properties, and hence the durability in significantly different ways.