Firing temperature accuracy of four dental furnaces.

In spite of using recommended firing and displayed temperatures, low-fired dental porcelain more often demonstrates unsatisfactory results after firing than porcelain fired at higher temperatures. It could therefore be anticipated that temperatures shown on the display are incorrect, implying that the furnace does not render correct firing programs for low-fired porcelain. The purpose of this study is to investigate deviations from the real temperature during the firing process and also to illustrate the service and maintenance discipline of furnaces at dental laboratories. Totally 20 units of four different types of dental furnaces were selected for testing of temperature accuracy with usage of a digital temperature measurement apparatus, Therma 1. In addition,the staffs at 68 dental laboratories in Sweden were contacted for a telephone interview on furnace brand and on service and maintenance program performed at their laboratories. None of the 20 different dental furnaces in the study could generate the firing temperatures shown on the display, indicating that the hypothesis was correct. Multimat MCII had the least deviation of temperature compared with displayfigures. 62 out of 68 invited dental laboratories chose to participate in the interviews and the result was that very few laboratories had a service and maintenance program living up to quality standards. There is room for improving the precision of dental porcelain furnaces as there are deviations between displayed and read temperatures during the different steps of the firing process.

A finite element analysis of stress distribution in bone tissue surrounding uncoupled or splinted dental implants.

BACKGROUND: Several studies on one-stage surgery in the treatment of the edentulous maxilla with implant-supported fixed prostheses have reported problems with removable provisional prostheses, which can load the implants in an uncontrollable manner during healing, and jeopardize healing. Immediate splinting of the implants with a fixed provisional prosthesis has been proposed to protect the bone-implant interface. PURPOSE: This study used the finite element method (FEM) to simulate stresses induced in bone tissue surrounding uncoupled and splinted implants in the maxilla because of bite force loading, and to determine whether the differences in these stress levels are related to differences in observed bone losses associated with the two healing methods. MATERIALS AND METHODS: Stress levels in the maxilla were studied using the FEM program TRINITAS (Institute of Technology, Linköping University, Linköping, Sweden) in which all phases - preprocessing/modeling, equation solving, and postprocessing/evaluation - were simulated. RESULTS: Stress levels in bone tissue surrounding splinted implants were markedly lower than stress levels surrounding uncoupled implants by a factor of nearly 9. CONCLUSION: From a mechanical viewpoint, FEM simulation supports the hypothesis that splinting reduces damage evolution in bone tissue, which agrees with clinical observations.

Bond strength of composite luting cement to zirconia ceramic surfaces.

OBJECTIVES: To evaluate the bond strength of dental resin agent to zirconia ceramic after surface pre-treatment with different techniques. METHODS: Specimens of hot isostatic pressed yttrium-oxide-partially-stabilized zirconia blocks (ZF) were fabricated (Procera Zircon, Nobel Biocare, Sweden) and compared to glossy dense zirconia blocks (ZG). Four groups of specimens with different surface treatment were prepared. Group I: ZF (n = 5) and ZG (n = 5) without any pre-treatment, Group II: ZF-s (n = 5) and ZG-s (n = 5) treated with silane solution, Group III: ZF-P (n = 10) and ZG-P (n = 10) treated with RF plasma spraying (hexamethyldisiloxane) using a reactor (Plasma Electronic, Germany), Group IV: ZF-p (n = 10) and ZG-p (n = 10) treated with micro pearls of low fusing porcelain (720 degrees C) on the surfaces. Composite cylinders (Charisma, Hereus Kulzer, Dormagen, Germany) were luted with Variolink II (Ivoclar-Vivadent, Schaan, Liechtenstein) to the test specimens. The specimens were then stored in air for 1 h before shear loading in a universal testing machine (LRX, Lloyd Instruments, Farnham, England) until failure. RESULTS: No statistical difference was found between the untreated ZF and ZG specimens (Group I) neither between the specimens treated with silane (Group II). Plasma spraying treatment improved bond strength by a factor of three (p < 0.001). Treatment with low fusing porcelain micro pearls increased the bond strength by a factor of 10 compared to untreated surfaces (p < 0.001). No significant difference was seen between the surfaces treated ZF-p and ZG-p specimens. The thickness of the glass pearls layer did not exceed 5 microm. SEM showed dense grain borders of ZF and a flat glossy texture of ZG. SIGNIFICANCE: Treatment of zirconia ceramic surfaces with plasma spraying or a low fusing porcelain pearl layer significantly increased the bond strength of resin cement to the ceramic surface.

The effect of two configuration factors, time, and thermal cycling on resin to dentin bond strengths.

Most in vitro testing of bonding systems is performed using specimens made in a mold with a low configuration (C) factor (ratio of bonded/unbonded surfaces) whereas clinically the C-factor is usually much greater. This study compared the effect of thermal cycling on the measured shear bond strength of 3M Single Bond dental adhesive bonded to dentin using molds with two different C-factors. The hypothesis was that neither C-factor nor thermal cycling would affect measured bond strengths. Resin composite was bonded to human dentin in cylindrical molds with an internal diameter of 3.2mm and either 1mm or 2.5mm deep. The 1mm deep molds had a C-factor of 2.2 and the 2.5mm deep molds had a C-factor of 4.1. Specimens were debonded either 10min after they had been bonded to dentin, or after they had been stored for 7 days in water at 37+/-1 degrees C, or after thermal cycling 5000 times for 7 days. Two-way ANOVA showed that overall both the C-factor and the storage condition had a significant effect on bond strength (p<0.001). There was a significant interaction (p<0.001) between the C-factor and how the specimens had been stored. The GLM/LSMEANS procedure with Sidak's adjustment for multiple comparisons showed that overall the specimens made in the mold with a high C-factor (4.1) had a lower bond strength than those that had been made in the mold with a lower (2.2) C-factor (p<0.001). Thermal cycling had a negative effect on the bond strength only for specimens made in molds with a C-factor of 4.1 (p<0.001).

Effect of light source and specimen thickness on the surface hardness of resin composite.

PURPOSE: To compare the Knoop hardness of different thicknesses of resin composite irradiated using either a plasma arc curing (PAC) light, or a quartz tungsten halogen (QTH) light. MATERIALS AND METHODS: A 2.5-mm diameter hole was drilled through the center of 2, 3, 4, and 5-mm thick discs of enamel and dentin. This hole was filled with either Filtek P60 or Prodigy Condensable composite and irradiated in bulk using: (a) a PAC light with an 8-mm, 470 nm light guide for 3 seconds, (b) a QTH light with an 8-mm standard light guide for 40 seconds (QTH+S), or (c) a QTH light with a 13/8-mm Turbo light guide for 40 seconds (QTH+T). Immediately after irradiating the composite, the surface hardness was measured at the bottom and then the top. Hardness values were remeasured after 24-hour and 7-day storage in water at 37 degrees C. The hardness values were compared using the Analysis of Variance for Repeated Measures and Least Squares Means test for multiple comparisons (LSM test) at P< 0.05. RESULTS: The power density delivered by each light was: PAC over 1999 mW/cm2, QTH+S 588 mW/cm2, and QTH+T 844 mW/cm2. Both the thickness of the composite and the light source had a significant effect on the hardness. Overall, using the QTH+T produced the highest hardness values and the PAC light the lowest values. For all 2-mm thick specimens, irrespective of the light source, the hardness values at the bottom were not significantly different from their top hardness values measured at the same time. For the 3-mm thick specimens using the QTH+T, the hardness values at the bottom of both composites were not significantly different from their top hardness values at all time intervals. For the 3-mm thick specimens using the QTH+S, the bottom hardness values were not significantly different from those at the top for P60 only, and only at 24 hours and 7 days, but not when measured immediately. For all the 4 and 5-mm thick specimens, the hardness values at the bottom were all significantly different from those at the top measured at the same time interval.

Three-dimensional finite element analyses of all-ceramic posterior fixed partial dentures with different designs.

The aim was to evaluate the influence on the stress distribution patterns in and the magnitude of stresses on fixed partial dentures (FPDs) under different anatomic and design conditions. Three-dimensional finite element models of posterior 3-unit all-ceramic FPDs were created with and without periodontal ligaments, with different radii of curvature at the embrasure area, and in a curve shape with a periodontal ligament. The model with a periodontal ligament showed 40% higher stress values compared to the no-ligament model. A smaller radius of curvature resulted in 20% to 40% higher stress values. The curved model increased stress values at the distal connector by 65% compared to the straight model. Support by teeth, occlusal curvature, and a small radius of curvature at the embrasure area negatively affect stress distribution patterns in the connector area of all-ceramic FPDs.

Bond strength of a composite luting agent to alumina ceramic surfaces.

OBJECTIVE: The purpose of this study was to evaluate the shear bond strength (SBS) of a dental luting agent to alumina ceramics after different surface pretreatment. MATERIAL AND METHODS: Specimens (n=50) of pressed blocks (10 x 0 x 5 mm) of alumina ceramic (Procera AllCeram) were divided into untreated specimens (AF) as provided by the manufacturer and polished specimens (AP). Five groups of specimens (n=5 x 10) with different surface pretreatments were prepared. Groups 1 and 2: AF and AP without any pretreatment; Group 3: AF treated with silane, (AF-s); Group 4: AF treated with RF plasma spray (AF-RF); Group 5: AF treated with low fusing porcelain (AF-p) glass pearls. Composite cylinders (5 x 5 mm) were cemented to the test specimens with a resin luting agent. The specimens were loaded to failure in shear mode using a universal testing machine. Recorded loads were used to calculate SBS in MPa. The results were analyzed using one-way ANOVA and the Tukey HSD multiple comparison test at alpha = 0.05. Scanning electron microscopic micrographs (SEM) were used to characterize surfaces of interest. RESULTS: Polished surfaces had significantly lower SBS (p < 0.05) compared with untreated specimens (AP vs AF). Silanated, non-polished surfaces (AF-s) revealed lower SBS, even though the result was not significantly different from that of AF-s without silane treatment. Plasma treatment improved SBS by a factor of 2 (p<0.05) and treatment with low-fusing porcelain micro pearls increased SBS by a factor of 3 compared to untreated surfaces (p<0.05). The layer of glass pearls did not exceed 5 microm (SEM). CONCLUSIONS: Within the limitation of the conditions of this study, treatment of alumina oxide ceramic surfaces with a plasma spray coating or a low-fusing porcelain pearl layer significantly increased the SBS of a resin luting agent to the ceramic surface.