Linear dimensional change,compressive strength and detail reproduction in type IV dental stone dried at room temperature and in a microwave oven

The type IV dental stone is widely used for the fabrication of dyes and master casts for fixed and removable partial prostheses. It is typically normal to wait at least 24 hours for the casts to dry prior to beginning the laboratory procedures. The waiting time has been shown to be greatly reduced by using microwave drying.

Objective

This study evaluated the influence of drying techniques at room temperature and microwave oven on the linear dimensional change, compressive strength and detail reproduction in type IV dental stones.

Material and Methods

Three type IV dental stone brands were selected; Elite Rock, Shera Premium and Durone IV. Two different drying protocols were tested in 4 groups (n=10); G1 - room temperature (25±4ºC) dried for 2 hours; G2 - room temperature dried for 24 hours; G3 - room temperature dried for 7 days and G4 - microwave oven dried at 800 W for 5 minutes and after 2 hours at room temperature. After drying, the samples were assayed for dimensional charges. The sample surface was submitted to the ImageTool 3.0 software for compressive strength in a universal testing machine with a cell load of 50 KN at a crosshead speed of 0.5 mm/minutes and the detail reproduction was analyzed with a stereomicroscope at 25x magnification. The statistical analysis of the linear dimensional change and compressive strength data were conducted by the ANOVA test followed by the Tukey test (p<0.05). Detailed reproduction values were reported in percentages.

Results

For the compressive strength test, Elite Rock and Durone IV did not present significant differences between G2 and G4, while Shera Premium did not present differences between G3 and G4. The best reproduction levels were observed for G3.

Conclusions

Dental stone microwave oven drying showed a linear dimensional change similar to after room temperature drying for 24 hours and 7 days. The compressive strength of the stone dried in the microwave oven was similar to those dried at room temperature for 24 hours, with the exception of Shera Premium, which had similar results for microwave and room temperature drying for 7 days. For the microwave drying method the detail reproduction levels for samples dried at room temperature for 24 hours and 7 days were similar, except for the Durone IV.     

Comparative Evaluation of Dimensional Accuracy and Tensile Strength of a Type IV Gypsum Using Microwave and Air Drying Methods

To evaluate dimensional accuracy and tensile strength of a type IV gypsum product, at different time intervals, dried in air or a microwave oven. Eighty specimens prepared from a cylindrical mould were used for measuring tensile strength (group A). Twenty specimens from a master die mould were used for determining dimensional accuracy (group B). In group A, 40 specimens were dried in open air at room temperature (A1). The other 40 were removed after 30 min and air dried for 20 min. These were subjected to microwave oven drying for 5 min (A2). Ten specimens each were tested under diametral compression at each of the following time periods: 1, 2, 4 and 24 h after drying. In group B, ten specimens were dried in open air at room temperature (B1). Ten specimens were removed from the mould after 30 min and air dried for 20 min. These were then dried in a microwave oven for 5 min (B2). The data was statistically analyzed using students unpaired “t” test. At all time intervals, diametral tensile strength (DTS) values for specimens dried in microwave oven were significantly higher than for those dried in open air. There were no significant differences between the dimensional accuracy of the two groups. In this study, microwave oven drying had a positive effect on the DTS of a type IV gypsum and the microwave oven dried specimens were as accurate as the air dried specimens over the same time period.     

Tensile strength of type IV dental stones dried in a microwave oven

STATEMENT OF PROBLEM: It is known that drying dental stones in a microwave oven can save time, but the strength of the material may be affected by different drying methods. PURPOSE: This study evaluated the diametral tensile strength (DTS) of 5 type IV gypsum products at different time intervals using microwave and air-drying methods. MATERIAL AND METHOD:. A total of 300 cylinder specimens were prepared from 5 type IV dental stones (Moldano, Amberok, Herastone, Shera-Sockel, and Fujirock; n = 60 per stone) in accordance with the manufacturers' recommendations. Half of the specimens of each stone (n = 30) were dried in open air within a temperature range of 20 +/- 2 degrees C; the other half (n = 30) underwent initial setting in a silicone rubber mold in open air for 10 minutes and then were dried in a microwave oven for 10 minutes. Within these groups, 10 specimens were tested under diametral compression at each of the following time periods: 30, 60, and 120 minutes after drying. Three-way analysis of variance and Scheffe's post hoc test were performed for statistical comparisons at a significance level of P<.05. RESULTS: At all time intervals, the diametral tensile strength values for dental stones dried in a microwave oven (mean 2.99 MPa) were significantly higher (P<.01) than the values for specimens dried in open air (mean 2.53 MPa). CONCLUSION: Within the limitations of this study, microwave oven drying had a positive effect on the diametral tensile strength of 5 type IV dental stones.     

Comparision of diametral tensile strength of microwave and oven-dried investment materials.

STATEMENT OF PROBLEM: Microwave drying technique for investment materials is believed to be timesaving by accelerating the elimination of free water content. PURPOSE: This study compared the diametral tensile strength of 4 investment materials used in removable partial denture framework fabrication. The investment materials were subjected to microwave and conventional oven drying at different time intervals. MATERIAL AND METHODS: Type III partial denture casting investments (Wirovest, Wiroplus, Biosint, PH2) were tested. A total of 160 specimens were prepared in cylindrical form at a height of 40 mm and a diameter of 20 mm, in accordance with the manufacturers' recommendations. Forty specimens were prepared from each product; 20 specimens were dried at 230 degrees C for 1 hour in an electric furnace, the other 20 were dried in a microwave oven for 10 minutes at 600 W. The dried specimens were tested at 2- and 4-hour intervals in diametral compression at a crosshead speed of 0. 5 cm/min. RESULTS: The microwave drying technique resulted in greater diametral tensile strength values for all investment materials. CONCLUSION: Within the limitations of this study, microwave drying of type III dental investment materials at 600 W for 10 minutes was, apart from strengthening the material, timesaving for the dental laboratory.     

Effect of microwave treatments on dimensional accuracy of maxillary acrylic resin denture base

Microwave energy has been used as an alternative method for disinfection and sterilization of dental prostheses. This study evaluated the influence of microwave treatment on dimensional accuracy along the posterior palatal border of maxillary acrylic resin denture bases processed by water-bath curing. Thirty maxillary acrylic bases (3-mm-thick) were made on cast models with Clássico acrylic resin using routine technique. After polymerization and cooling, the sets were deflasked and the bases were stored in water for 30 days. Thereafter, the specimens were assigned to 3 groups (n=10), as follows: group I (control) was not submitted to any disinfection cycle; group II was submitted to microwave disinfection for 3 min at 500 W; and in group III microwaving was done for 10 min at 604 W. The acrylic bases were fixed on their respective casts with instant adhesive (Super Bonder) and the base/cast sets were sectioned transversally in the posterior palatal zone. The existence of gaps between the casts and acrylic bases was assessed using a profile projector at 5 points. No statistically significant differences were observed between the control group and group II. However, group III differed statistically from the others (p<0.05). Treatment in microwave oven at 604 W for 10 min produced the greatest discrepancies in the adaptation of maxillary acrylic resin denture bases to the stone casts.     

Surface detail, compressive strength, and dimensional accuracy of gypsum casts after repeated immersion in hypochlorite solution

STATEMENT OF PROBLEM: One method used to disinfect a dental cast is immersion in a disinfecting solution. Repeated immersion of a cast in disinfectant may affect surface detail as well as other physical properties. PURPOSE: The purpose of this study was to evaluate the changes in surface detail quality, dimensional accuracy, and wet compressive strength of dental casts as a result of repeated disinfection in slurry containing 0.525% sodium hypochlorite solution. MATERIAL AND METHODS: Two different test specimen configurations were used, one for evaluating surface detail quality and dimensional accuracy and the other for compressive strength. Sixty specimens each were made of type III and type IV dental stone (Excalibur). Thirty specimens were immersed in slurry, a supernatant solution of calcium sulfate in distilled water (control casts), and 30 in slurry with sodium hypochlorite (test casts) solutions for 30 minutes and air dried for 24 hours. This process was repeated 7 times before testing. Linear dimensional change, surface detail quality, and wet compressive strength were determined according to American National Standards Institute/American Dental Association (ANSI/ADA) standards. The data were analyzed using 2-way analysis of variance and 2-sample independent t tests (alpha=.05). RESULTS: For specimens prepared with type III and type IV stone, treatment by immersion in slurry (control casts) and in slurry with sodium hypochlorite (test casts) resulted in an increase (P<.0001) in linear dimensional change. The type IV casts treated with slurry with sodium hypochlorite showed significantly less (P<.0001) mean linear dimensional change (0.025%) compared with type III stone casts (0.063%), and the values remained within the ANSI/ADA specified standards. Both test solutions caused some degree of erosion or damage to the surface quality of casts made from type III and type IV stones. However, the difference between the control and test casts was not significant. Slurry with 0.525% sodium hypochlorite solution decreased the compressive strength of both types of stones significantly (P<.001) compared with distilled water slurry. However, the values remained close to ANSI/ADA standards. CONCLUSION: The results demonstrated that repeated immersion of type III and type IV stone specimens in slurry with distilled water and slurry with 0.525% sodium hypochlorite, along with drying in air, caused a significant increase in linear dimension and a significant decrease in wet compressive strength. Although both the solutions caused some degree of damage to surface details for type III and type IV stones, the difference was not significant.     

Effect of Different Exposure Times on Microwave Irradiation on the Disinfection of a Hard Chairside Reline Resin

Purpose: This study evaluated the effectiveness of different exposure times of microwave irradiation on the disinfection of a hard chairside reline resin.
Materials and Methods: Sterile specimens were individually inoculated with one of the tested microorganisms ( Pseudomonas aeruginosa , Staphylococcus aureus , Candida albicans , and Bacillus subtilis ) and incubated for 24 hours at 37°C. For each microorganism, 10 specimens were not microwaved (control), and 50 specimens were microwaved. Control specimens were individually immersed in sterile saline, and replicate aliquots of serial dilutions were plated on selective media appropriate for each organism. Irradiated specimens were immersed in water and microwaved at 650 W for 1, 2, 3, 4, or 5 minutes before serial dilutions and platings. After 48 hours of incubation, colonies on plates were counted. Irradiated specimens were also incubated for 7 days. Some specimens were prepared for scanning electron microscopic (SEM) analysis.
Results: Specimens irradiated for 3, 4, and 5 minutes showed sterilization. After 2 minutes of irradiation, specimens inoculated with C. albicans were sterilized, whereas those inoculated with bacteria were disinfected. One minute of irradiation resulted in growth of all microorganisms. SEM examination indicated alteration in cell morphology of sterilized specimens. The effectiveness of microwave irradiation was improved as the exposure time increased.
Conclusion: This study suggests that 3 minutes of microwave irradiation can be used for acrylic resin sterilization, thus preventing cross-contamination.     

Alterations of surface hardness with gypsum die hardeners

STATEMENT OF PROBLEM: Die stones require abrasion resistance, dimensional stability with time, and high surface wettability material properties. PURPOSE: The purpose of this study was to compare the surface microhardness (Knoop) of 4 contemporary gypsum materials with and without surface die hardener. MATERIAL AND METHODS: Materials used were a Type III stone (Microstone) and 3 die stones (Die-Keen, Silky-Rock, and ResinRock). Die hardener was cyanoacrylate (Permabond 910) or Clear Coat. Specimens of stone were hand mixed with distilled room temperature water and vacuum spatulated according to manufacturer's directions. Five cylinders (15 x 15 mm) per group were poured, using vibration, into phenolic ring molds positioned on top of a glass slide. The face of each specimen was polished with 2400-grit Al2O sandpaper. One face of each of 5 specimens/material was coated with cyanoacrylate; 5 specimens/material were coated with Clear Coat, air thinned and dried; and 5 specimens/material had no treatment (control). Knoop hardness (kg/mm2) readings were made on each face (5 readings/time point) 3, 12, and 24 hours after pouring. An ANOVA procedure with post hoc Tukey tests were performed (alpha=.05). RESULTS: Microhardness did not vary between 3 and 24 hours for any material (P>.05). Microstone had significantly lower surface hardness (P<.0001) than the die stones. Specimens coated with die hardener had lower hardness values (P<.001 in all cases) CONCLUSIONS: The 3 types of die stones evaluated in this study did not differ significantly in surface microhardness. Under these conditions, die hardener coatings reduced the surface hardness of the gypsum material.     

Microwave Disinfection of Complete Dentures Contaminated In Vitro with Selected Bacteria

Purpose: This study evaluated the effectiveness of microwave irradiation for disinfection of simulated complete dentures.
Materials and Methods: Seventy dentures were fabricated in a standardized procedure, subjected to ethylene oxide sterilization, individually inoculated (10⁷ cfu/mL) with Staphylococcus aureus (n = 20), Pseudomona aeruginosa (n = 20), and Bacillus subtilis (n = 30) and incubated for 24 hours at 37°C. After that, 40 dentures were selected for microwaving. For each microorganism, 10 dentures were submitted to microwave irradiation at 650 W for 3 minutes. In addition, 10 dentures contaminated with B. subtilis were irradiated for 5 minutes. Thirty non-microwaved dentures (n = 10 for each bacteria) were used as positive controls. Replicate aliquots (25 μL) of suspensions were plated at dilutions of 10⁻³ to 10⁻⁶ on plates of selective media appropriate for each organism. After incubation (37°C for 48 hours), colonies were counted (cfu/mL). TSB beakers with the microwaved dentures were incubated at 37°C for a further 7 days to verify long-term disinfection. The data were statistically analyzed by the Kruskal-Wallis test (α= 0.05).
Results: No evidence of growth was observed at 48 hours for S. aureus and P. aeruginosa on plates, and no turbidity was visible in the TSB beakers of these specimens after 7 days of incubation. Dentures contaminated with B. subtilis and irradiated for 3 minutes produced microbial growth on six plates and turbidity on all TSB beakers. Microwaving for 5 minutes resulted in survival of B. subtilis in two plates and two beakers.
Conclusion: Microwave irradiation for 3 minutes at 650 W produced sterilization of complete dentures contaminated with S. aureus and P. aeruginosa . Dentures contaminated with B. subtilis were disinfected by microwave irradiation after 3 and 5 minutes at 650 W.     

Effect of processing variables (different compression packing processes and investment material types) and time on the dimensional accuracy of polymethyl methacrylate denture bases

In this study we determined the effect of different compression packing processes, investment materials (a hemihydrate and dental stone) and time on the dimensional accuracy of polymethyl metacrylate denture bases. Square stainless steel plates (15 mm x 15 mm x 5 mm) were prepared to make an acrylic resin specimen. The linear dimensional changes of acrylic resin were determined by measuring the distances of fix points. Measurements were made at 24 hours, 48 hours, 12 days and 30 days after setting with a digital compass. Dimensional changes of test specimens that were obtained with three different flasks and two press techniques were compared by univariate analysis. Measurements of the linear dimensions of specimens cured by different compression packing techniques suggested that differences existed. The time interval differences were not significant. According to the results; flask and investment material types affect the dimensional accuracy of test specimens (p < 0.05). The least dimensional change observed in the specimens was obtained with Type 1 flask-dental stone-manual press combinations.     

Fractography off cast gypsum

A fractographical study of dental cast gypsum was made in order to correlate the mechanical properties with the microstructure. Wet specimens fractured under tensile stress showed intercrystalline fracture and the tensile strength depended on the porosity present. Thus, it was assumed that tensile strength was dependent on the contact area between individual gypsum crystals and changes in porosity approximated to changes in contact area. Strength differences among specimens of a given W/P ratio, therefore, can be related to differences in intercrystalline contact areas. These theoretical considerations suggest that the classification of dental die stone and dental stone into high and low strength types based on strength properties only would be more practical and less confusing than at present.     

Efficacy of high-level microwave disinfection of dental gypsum casts: the effects of number and weight of casts

The aim of this study was to determine if the bactericidal effect of microwaving gypsum casts is maintained at maximum capacity of the oven (16 casts). Batches of 8 and 16 gypsum casts made from in vivo impressions were divided in half. One half of each cast was microwaved at 900 W for 5 minutes. The remaining halves were left untreated. When assessed for bacteriological growth, the median cfu/mL of the untreated casts was between 10(5) and 10(6) [corrected] while the microwaved casts showed a cfu/mL of 0, indicating that microwaving as described will disinfect gypsum casts even at maximum capacity of the oven.     

高温高压消毒对超硬石膏模型三维精度的影响

目的：评价高温高压消毒对超硬石膏模型三维精度有无影响。方法：对30例患者制取藻酸盐印模,再用超硬石膏灌模,1h后脱模,静置24h定标志点。石膏模型扫描成数字模型后用测量软件进行标志点三维测量。测量后进行高温高压消毒。消毒后用同种方法测量标志点距离。最后用SPSS软件分析消毒前后数据。结果：高温高压消毒后对超硬石膏模型三维精度无统计学差异。结论：高温高压消毒对超硬石膏模型三维精度无明显影响。     

The influence of working cast residual moisture and temperature on the fit of vacuum-forming athletic mouth guards

STATEMENT OF PROBLEM: The comfort and effectiveness of athletic mouth guards are believed to depend on their degree of fit to oral tissues. Vacuum-forming machines are simpler and less expensive than pressure-forming machines. However, it is thought that vacuum-formed mouth guards often do not exhibit adequate adaptation. PURPOSE: The purpose of this study was to evaluate the effects of cast residual moisture and temperature on the fit of athletic mouth guards made with a vacuum-forming machine. MATERIAL AND METHODS: A metal master model simulating the cross section of the maxillary molar region was used to form 20 working plaster casts. The casts represented 4 (5 specimens each) conditions before the forming of the mouth guard specimens: storing in a wet environment at room temperature and storing in a dry environment at room temperature, 5 degrees C, and 40 degrees C. Mouth guard specimens were fabricated with ethylene vinyl acetate sheets (3.8-mm thick) with the use of a vacuum-forming machine. Test casts were created by pouring hand-mixed type III dental stone into each of the mouth guard specimens. The differences in the sagittal cross-sectional heights at the line angle area of the test casts and the working casts were compared. This was achieved by superimposing their 3-dimensional images scanned by a laser scanner. The air permeability was also measured for the hand-mixed stone casts under wet and dry conditions, the vacuum-mixed stone cast, and the high-strength stone specimen. This was achieved by measuring the volume of transmitted air passing through the specimen in the testing tube. One-way analysis of variance with the Scheffé post hoc test (P <.05) was applied to determine the conditions of the working cast required to achieve the best fit. RESULTS: With regard to the fit of the mouth guard specimens to the working cast, those with dry and heated working casts showed a significantly better fit than those with wet working casts (P<.05). A significantly larger volume of transmitted air was found in the dry stone specimen (P<.05) followed by the dry high-strength stone cast and then the wet stone cast. CONCLUSION: Within the limitations of this study, residual moisture in the working cast was the most critical factor in determining the fit of the mouth guard made by vacuum-forming machines. The best fit was achieved when the working cast was thoroughly dried and its surface temperature was elevated.     

Effect of a dental unit waterline treatment solution on composite-dentin shear bond strengths

OBJECTIVE: The control of biofilm and effluent contamination of dental unit water lines (DUWL) includes additions of antimicrobial solutions, as well as automatic dosing units. There are, however, varying reports on the effects of such agents on the bond strength of restorative dental materials and, particularly, between these agents and dental hard tissues. METHODOLOGY: The possible effects of an antimicrobial DUWL treatment solution on the adhesion of composite resin to dentin was evaluated by shear bond strength (SBS) testing. A total of 20 caries-free human molar and premolar teeth were used as the test substrates. The teeth were divided into two sets of 10 teeth which, after appropriate cleaning with water and pumice, were embedded horizontally in dental die stone. The buccal surface of each tooth was ground flat to a 17 microns finish using water-lubricated SiC paper. The teeth were then etched for 15 seconds with 37% H3PO4 and rinsed with either water (control) or a proprietary DUWL treatment (ICX) solution. Thereafter, the teeth were lightly blown dry with clean dry air, and the dentin conditioned with Prime & Bond NT for 20 seconds. The excess solvent was then removed by gentle air drying for 5 seconds, and the conditioner cured with visible light for 10 seconds. A cylinder of composite was placed on the conditioned surface and cured. A second group of 20 caries-free human molar and premolar teeth were used as test substrates to evaluate the effect of the ICX DUWL treatment solution on a different dentin priming system (OptiBond Solo Plus). The teeth in the second group were divided into two sets and after a 15 second etch with 37% H3PO4, were rinsed with water (control) or the proprietary ICX DUWL treatment solution. Thereafter, the teeth were lightly blown dry with clean, dry air and the dentin conditioned with OptiBond Solo for 20 seconds. The excess solvent was then removed by gentle air drying for 5 seconds, and the conditioner cured with visible light for 10 seconds. A cylinder of composite was placed on the conditioned surface and cured. Shear bond strength testing was performed with a universal test machine at the default cross-head speed of 0.1 mm/min. A set of teeth, sectioned, mounted and etched as above but rinsed with a 0.01% mineral oil/water mix prior to conditioning and bonding, was used as the negative control. A separate corrosion testing was performed by immersing brass coupons in water and ICX for 31 days and measuring the weight loss. The brass coupons were bright-dipped, electroless nickel-plated and bright nickel electroplated. RESULTS: The bonding studies indicated that the DUWL treatment solution applied to a cut and etched dentin surface prior to conditioning and bonding with an adhesive system has no effect (p > 0.05) on bond strength for either group of specimens, compared to water. Negative control specimens were found to have minimal bond strengths. The corrosion study indicated no difference in the behavior of the test specimens in ICX compared to those in water, although differences were noted between the different surface finishes applied to the brass substrate. CONCLUSION: The findings of this study demonstrate that exposure of an etched dentin surface to a water-based DUWL treatment mixture has no adverse effects on subsequent adhesion strength. Minimal corrosive attack was noted in the ICX solution and water for brass coupons provided with three different surface finishes.     

The Effect of Repeated Microwaving Disinfection on the Dimensional Stability of Acrylic Dentures

Objective

The aim of this study was to evaluate the effect of repeated microwave disinfections on the dimensional stability of acrylic dentures.

Materials and Methods

Three groups of dentures made of a heat polymerized acrylic resin were tested. I: dentures kept in water (control group). II: dentures microwaved daily while being immersed into water (wet disinfection). III: dentures microwaved daily without being immersed into water (dry disinfection).
Measurements were taken across three reference points, on two occasions: after curing and immersion in water for 24 hours, and one week later.
Data obtained were analyzed using one-way analysis of variance (ANOVA) and Scheffe’s multiple range test.

Results

The results showed that the microwave disinfection provokes dimensional changes of the same pattern (shrinkage). The dentures which underwent wet disinfection exhibited the greatest shrinkage (p<0.05).

Conclusions

Disinfection using microwave energy may cause dimensional changes (shrinkage) of complete dentures.
The microwave “dry disinfection” method can be safely applied in everyday practice since the dimensional changes which occurred seem to be of no clinical significance.Key words: Complete Dentures, Acrylic Resins, Disinfection, Microwaves, Dimensional Measurement Accuracy     

Dimensional accuracy of thermoformed polymethyl methacrylate

Thermoforming of polymethyl methacrylate sheet is used to produce a number of different types of dental appliances. The purpose of this study was to determine the dimensional accuracy of thermoformed polymethyl methacrylate specimens. Five blanks of the acrylic resin were thermoformed on stone casts prepared from a silicone mold of a brass master die. The distances between index marks were measured both on the cast and on the thermoformed blanks with an optical comparator. Measurements on the blanks were made again 24 hours after processing and then 1 week, 1 month, and 3 months after immersion in water. Linear shrinkage of less than 1% (range 0.37% to 0.52%) was observed 24 hours after removal of the blanks from the cast. Immersion of the thermoformed specimens in water resulted in an increase in measured dimensions, but after 3 months' immersion these increases were still less than those of the cast (range 0.07% to 0.18%). It was concluded that it is possible to thermoform Perspex polymethyl methacrylate accurately.     

Effects of warm air-drying and spreading on resin bonding.

The purpose of this study was to compare the effect on resin-to-enamel bonding produced by warm air from a hair dryer, and to correlate changes in resin bond strength with resin tag structure. Herculite-XR resin composite and Bondlite bonding resin were used. The three technique variables were the type of air used for drying, air dryer distance, and drying and spreading time. Control samples were dried and the bonding resin spread with a dental air syringe, whereas warm air from a hair dryer was used on the experimental samples. The bond strength (MPa) was determined in shear at a crosshead speed of 1 mm/min. Following bond strength evaluation, the teeth were immersed in 10% HCl for enamel dissolution and the resin tag structure was examined with the SEM. ANOVA analyses of shear bond strengths were performed. Warm air-drying and spreading for 15 seconds at 6 cm and 5 seconds at 6 cm respectively, produced statistically greater shear bond strengths (x = 20.4 +/- 4.4 MPa, P less than 0.05). The other drying time/distance combinations, including the control (x = 14.4 +/- 3.3 MPa), were not statistically different. Differences in resin tag structure were qualitatively evident under the SEM, with sharp tags produced by the warm air-drying and spreading techniques, compared to the blunt tags created by syringe air-drying and spreading. Warm air-drying and spreading significantly improved the bond strength. No apparent correlation exists between bond strength and tag length.     

Microwave-induced fast decalcification of rat bone for electron microscopic analysis: an ultrastructural and cytochemical study

Bone decalcification is a time-consuming process. It takes weeks and preservation of the tissue structure depends on the quality and velocity of the demineralization process. In the present study, a decalcification methodology was adapted using microwaving to accelerate the decalcification of rat bone for electron microscopic analysis. The ultrastructure of the bone decalcified by microwave energy was observed. Wistar rats were perfused with paraformaldehyde and maxillary segments were removed and fixed in glutaraldehyde. Half of specimens were decalcified by conventional treatment with immersion in Warshawsky solution at 4 masculineC during 45 days, and the other half of specimens were placed into the beaker with 20 mL of the Warshawsky solution in ice bath and thereafter submitted to irradiation in a domestic microwave oven (700 maximum power) during 20 s/350 W/+/-37 masculineC. In the first day, the specimens were irradiated 9 times and stored at 40 masculineC overnight. In the second day, the specimens were irradiated 20 times changing the solution and the ice after each bath. After decalcification, some specimens were postfixed in osmium tetroxide and others in osmium tetroxide and potassium pyroantimonate. The specimens were observed under transmission electron microscopy. The results showed an increase in the decalcification rate in the specimens activated by microwaving and a reduction of total experiment time from 45 days in the conventional method to 48 hours in the microwave-aided method.     

Bond strength to primary tooth dentin following disinfection with a chlorhexidine solution: an in vitro study

PURPOSE: The aim of this study was to determine the effect on shear bond strength of chlorhexidine used as a cavity disinfectant on primary tooth dentin. METHODS: Thirty specimens were randomly divided into 3 groups (N = 10) and treated as follows: (1) in Group I, the dentin was acid etched with a 37% phosphoric acid for 15 seconds, washed and dried; (2) in Group II, a 2% commercial chlorhexidine solution (Cav Clean) was applied for 40 seconds, washed and dried following acid etching for 15 seconds; and (3) in Group III, dentin was treated with a 37% phosphoric acid gel containing 2% digluconate of chlorhexidine (Cond AC) for 15 seconds. In all specimens, the adhesive Single Bond (3M) was applied, and composite cylinders (Filtek Z 250) were built. The specimens were sheared with a universal testing machine (Instron) running at a crosshead speed of 0.5 mm/min, and the results calculated in MPa. The specimens were also observed to record the failure mode. RESULTS: ANOVA analysis revealed that the shear bond strength of Group II (17.99+1.15 MPa) was significantly lower than Group I (19.88+1.02 MPa) and Group III (19.57+1.02 MPa). After debonding, 63% of the specimens presented cohesive failure of the material, 24% adhesive failure and 10% cohesive failure of dentin. CONCLUSIONS: The commercial cavity disinfectant containing 2% chlorhexidine had an adverse effect on Single Bond and produced significantly lower shear bond strength than the etch gel with chlorhexidine and the acid etch alone.