Influence of polymerization conditions on monomer elution and microhardness of autopolymerized polymethyl methacrylate resin

Residual monomer contents and surface hardness are important factors in determining the serviceability of provisional restorations. The intent of this study was to systemically evaluate the effects of curing conditions on provisional polymethyl methacrylate (PMMA) resins which utilize a free-radical polymerization reaction. Combinations of the three curing factors of temperature, pressure, curing environment (water/air) were adjusted during the fabrication of autopolymerized specimen disks. The initial hardness of tested materials was measured with a microhardness tester 1 h after disc fabrication, and the amounts of residual methyl methacrylate (MMA) released into water were analyzed by reverse-phase HPLC after 7 d of water immersion. Results from multiple regressions showed that curing temperature was the dominant factor in improving resin surface hardness, whereas curing in water was the key factor for reducing the quantity of residual monomer. The pressure factor, which was thought to be critical for managing autopolymerized resins, showed no significant influences on the properties tested. ANOVA results showed that provisional PMMA resins cured in hot water, with or without pressure, significantly reduced the amount of residual MMA elution (up to 80%) and increased the microhardness values (up to 50%).     

Atomic analysis and knoop hardness measurement of the cavity floor prepared by Er,Cr:YSGG laser irradiation in vitro

In the present study, the compositional changes and knoop hardness of the cavity floor prepared by Er,Cr:YSGG laser irradiation was compared with that of the conventional bur cavity. Fifteen laser and 15 bur cavities were cross-sectioned, and subjected to atomic analysis by SEM-EDX and knoop hardness test. Statistical analyses were performed using the Mann-Whitney U-test; a value of P < 0.01 was considered significant. Surface characteristics of the prepared cavities were also investigated by light microscopy and scanning electron microscopy (SEM). The results showed that the quantities of Ca (Ca weight %) and P (P weight %) were increased significantly in the laser cavity floor but no significant differences were found between the Ca/P ratio and knoop hardness number of laser and bur cavities. The SEM observation revealed that the lased cavity surface was irregular and there was also the absence of a smear layer; the orifice of dentinal tubules was exposed. Er,Cr:YSGG laser device is considered as one of the most effective and safe devices for cavity preparation because of its many advantages. This includes easy delivery system, minimal thermal damage to the surrounding tissues, minimal thermal-induced changes of dental hard tissue compositions, and favourable surface characteristic.     

The Effect of Water Absorption on Acrylic Surface Properties

PURPOSE: The aim of the present study was to determine whether an increased water content during thermal cycling of hot water-treated acrylic was associated with a reduction in surface hardness and an increased opacity or whitening of the surface. MATERIALS AND METHODS: Ten acrylic samples were treated with 30 soak cycles (cycle duration, 24 hours), using warm water (40 degrees C) and an alkaline peroxide tablet (Efferdent control group); a further ten samples were treated with boiling water (100 degrees C) and one Efferdent tablet (experimental group). Indentation hardness of the acrylic specimens was measured prior to and immediately following the completion of the warm and hot water treatments, using an automated micro-indentation system. The hydrated acrylic specimens were then allowed to air dry at room temperature (20 degrees C) and were weighed weekly until they had obtained a constant dry weight. The loss in weight of the acrylic specimens represented the maximum water absorption. RESULTS: The hot water-treated specimens were much whiter than the warm water-treated specimens. The mean reduction in hardness (H(IT)) of the acrylic specimens following the treatment with hot water and alkaline peroxide tablet was 12.9%. Treatment with warm water and alkaline peroxide resulted in a slight increase in mean hardness (2.63%). There was a significant correlation between the water content of the acrylic specimens after treatment and the percentage of change in indentation hardness (r= 0.495, p= 0.026). CONCLUSIONS: The hot water treatment of the acrylic was associated with a significant reduction in hardness. We attribute the whitening and reduction in the hardness of the hot water-treated specimens to absorption of water and a disruption of the acrylic surface structure.     

Properties of a new photo-activated composite polymerized with three different laboratory photo-curing units

This study determined the hardness, solubility and curing depth of a new photo-activated composite polymerized with three different laboratory photo-curing units for the purpose of evaluating the post-curing properties of the material. A new photo-activated composite material for both direct and indirect applications (DiamondCrown) was polymerized with three photo-curing units equipped with the following light sources: (i) two halogen lamps (DiamondLite-VL. Halogen Light Curing Booth); (ii) two metal halide lamps (Hyper LII) and (iii) two xenon stroboscopic tubes (UniXS II). Knoop hardness, water solubility and curing depth were determined for groups of five specimens according to standardized testing methods. All data were compared using analysis of variance (anova) and Scheffe's S intervals (P < 0.05). The Knoop hardness number (KHN) generated with the metal halide unit (63.3 +/- 2.4 KHN) was statistically (P < 0.05) greater than those produced by the other two curing units. Water solubility values for both the halogen unit (2.5 +/- 0.5 microg mm(-3)) and the metal halide unit (2.5 +/- 0.5 microg mm(-3)) were significantly (P < 0.05) lower than for the xenon unit (3.8 +/- 0.5 microg mm(-3)). Of the three photo-curing units, the metal halide curing-unit consistently exhibited the greatest depth of cure. The composite material appears to be reliable, although its post-curing properties were found to be influenced by the type of curing unit.     

Effect of Disinfection Treatments on the Hardness of Soft Denture Liner Materials

PURPOSE: To evaluate the effects of disinfection treatments with chemical solutions (2% glutaraldehyde, 5% sodium hypochlorite, and 5% chlorhexidine) and microwave energy on the hardness of four long-term soft denture liners. MATERIALS AND METHODS: Forty rectangular specimens of four soft lining materials (Molloplast-B, Ufi Gel P, Eversoft, and Mucopren soft) were made for each material. Ten samples of each material were immersed in different disinfectant solutions for 10 minutes or placed in a microwave oven for 3 minutes at 500 W. The hardness values were obtained with a Shore A durometer, before the first disinfection cycle (control), and also after two cycles of disinfection. Data were submitted to analysis of variance and Tukey's test (p < 0.01). RESULTS: The highest value of hardness was obtained for Molloplast-B, independent of the disinfection technique. Mucopren soft demonstrated intermediate values and Ufi Gel P and Eversoft the lowest values of Shore A hardness. For Molloplast-B, the disinfection using glutaraldehyde demonstrated the highest value of hardness. The number of disinfections had no effect on the hardness values for all the materials studied and disinfection techniques. CONCLUSIONS: The application of two disinfection cycles did not change the Shore A hardness values for all the materials. The glutaraldehyde solution demonstrated the highest values of Shore A hardness for the Molloplast-B, Mucopren soft, and Ufi Gel P materials, while Eversoft did not present any differences in hardness when submitted to different disinfection treatments.     

Influence of hydration and mechanical characterization of carious primary dentine using an ultra-micro indentation system (UMIS)

The conditions under which mechanical properties of dentine are tested influence the values recorded. The aims of this study were to examine the effect of hydration on the mechanical properties of primary carious dentine and to provide information on changes in hardness and modulus of elasticity change caused by the demineralizing caries process in dentine. Three primary molar teeth with untreated carious dentine were prepared for nano-indentation tests under both wet and dry conditions. Further tests were conducted on eight primary molars with untreated carious dentine under hydrated conditions. The mechanical properties of dehydrated carious dentine increased approximately 10-fold for hardness and 100-fold for the modulus of elasticity compared with hydrated dentine. The hardness and elastic modulus of the carious primary dentine deteriorated progressively toward the lesion cavity floor, ranging from 0.001 to 0.52 GPa and from 0.015 to 14.55 GPa, respectively, and could be fitted to a simple linear relationship when plotted in logarithmic scale vs. distance. The total depth of dentine affected was around 1100 microm parallel to the tubule direction. This depth was significantly greater than observed subjectively, implying that the demineralization process is more advanced than might be suspected on simple clinical examination.     

Development of dental Pd-Ti alloys

Alloys, in which the main constituents are palladium and titanium, have the following potential advantages for dental application: corrosion resistance, biocompatibility and acceptable casting temperatures for porcelainfused-to-metal dental prostheses. Alloy compositions chosen for investigation were as follows: 30 w/o Ti which lies in the 5% single-phase region near TiPd, 50 w/o Ti which lies near the 1120°C melting temperature minimum of the Pd?Ti system, and 70% Ti which minimizes Pd content while still in a relatively low melting range (1300°C). Analysis of the X-ray diffraction of 70%Pd30%Ti showed mostly PdTi with some Pd₅Ti₃; in 50%Pd50%Ti mostly PdTi₂ was observed with some PdTi; in 30%Pd70%Ti mostly PdTi₂ was found with some alpha Ti and PdTi₄. The amounts of the minor phases in each of the 3 alloys depended on heat treatment. The Knoop hardness of the alloys ranged from 300–500 KHN for the as-melted condition; after heat treatment the maximum hardness values were 400 KHN. After centrifugal casting, hardness values increased to a range of 350 to 560 KHN, depending on composition. These values exceed those obtained for the pure metals which were in the neighborhood of 100 KHN. From anodic potentiodynamic scans the breakdown potentials in Ringer's solution for 70%Pd30%Ti, 50%Pd50%Ti and 30%Pd70%Ti were 600, 650 and 500 (SCE); the repassivation potentials for the same alloys are 450, 300 and 350 (SCE), respectively. These are all above oral potential. The Pd?Ti system investigated was, thus, found to have adequate corrosion resistance and hardness. Therefore, it can be considered of good potential as a dental prosthetic alloy.     

Hardness and compressive strength of indirect composite resins: effects of immersion in distilled water

The aim of this study was to evaluate the effect of ageing in distilled water on the hardness and compressive strength of a direct composite resin Z100, a feldspatic porcelain (Noritake) and three indirect composites (Artglass, Solidex and Targis). For the Vickers hardness tests, five disk-shaped specimens (2 x 4 mm) of each material were prepared according to the manufacturers' instructions. The hardness tests were conducted using a Vickers diamond indentor. Compressive strength measurements were recorded on cylindrical specimens with a diameter of 6 mm and a length of 12 mm. The compression tests were carried out with a constant cross-head speed of 0.5 mm min(-1) on a mechanical test machine. For each material, 10 specimens were tested after 7 days of dry storage at 37 +/- 1 degrees C and 10 specimens were tested after water storage at 37 +/- 1 degrees C for 180 days. Noritake porcelain specimens showed higher hardness values than the composites. Among the composite materials, Z100 promoted the highest VHN values, regardless of the ageing periods. The results showed that Solidex and Z100 had the highest compressive strength values. Ageing in water reduced the hardness for all composites, but had no long-term effect on the compressive strength.     

Evaluation of wear behavior of dental restorative materials against zirconia in vitro

ObjectivesTo evaluate two-body wear (2BW) and three-body wear (3BW) of different CAD/CAM and direct restorative materials against zirconia using a dual-axis chewing simulator and an ACTA wear machine.Methods3 CAD-CAM resin-based composite or polymer infiltrated ceramic network blocs, 1 lithium disilicate CAD-CAM ceramic (LS₂), 3 direct resin composites, amalgam and bovine enamel were tested. For 2BW, 8 flat specimens per material were produced, grinded, polished, stored wet (37 °C, 28d) and tested (49 N, 37 °C, 1,200,000 cycles) against zirconia. For 3BW, specimens (n = 10) were stored accordingly, and tested against a zirconia antagonist wheel (3Y-TZP, d = 20 mm, h = 6 mm; 200,000 cycles, F = 15 N, f = 1 Hz, 15% slip) in millet seed suspension. Wear resistance was analysed in a 3D optical non-contact profilometer, measuring vertical wear depth and volume loss for 2BW and mean wear depth and roughness (R_a) for 3BW. Vickers hardness (15 s, HV2) was measured. Statistical analysis was performed using non-parametric tests (Mann-Whitney-U test, p < 0.05).Results2BW and 3BW have a different impact on material surfaces. Similar wear resistance was observed for direct and indirect resin based materials with analogous filler configurations in both methods. Bovine enamel exhibited the best wear resistance in 2BW, but the least wear resistance in 3BW against zirconia. Regarding 2BW, a direct/indirect composite material pair of the same manufacturer showed the significantly highest mean volume losses (2.72/2.85 mm³), followed by LS₂ (1.41 mm³). LS₂ presented the best wear resistance in 3BW (mean wear depth 2.85 µm), combined with the highest mean Vickers hardness (598 MPa). No linear correlation was found between Vickers hardness and both wear testing procedures. The zirconia antagonists showed no recordable signs of wear.SignificanceDental restorative materials behave differently in 2BW and 3BW laboratory testing. Vickers hardness testing alone cannot hold for a correlation with wear behavior of materials. Micromorphological investigation of material composition can reveal insights in wear mechanisms related to variations in filler technologies.     

Effect of CAD/CAM aesthetic material thickness and translucency on the polymerisation of light- and dual-cured resin cements

ObjectivesThis study investigated potential variations in polymerisation of light- and dual-cured (LC and DC) resin cements photoactivated through four CAD/CAM restorative materials as a function of substrate thickness.MethodsFour CAD/CAM materials [two resin composites CeraSmart (CS) and Grandio Blocs (GB); a polymer infiltrated ceramic Vita Enamic (VE) and a feldspathic ceramic Vita Mark II (VM)], with five thicknesses (0.5, 1, 1.5, 2, and 2.5 mm) were prepared and their optical characteristics measured. 1 mm discs of LC and DC resin cement (Variolink® Esthetic, Ivoclar AG) were photoactivated through each specimen thickness. After 1 h post-cure, polymerisation efficiency was determined by degree of conversion (DC%) and Martens hardness (H_M). Interactions between materials, thicknesses and properties were analysed by linear regressions, two-way ANOVA and one-way ANOVA followed by post hoc multiple comparisons (α = 0.05).ResultsAll substrates of 0.5- and 1.0-mm thickness transmitted sufficiently high peak irradiances at around 455 nm: (I_t = 588–819 mW/cm²) with translucency parameter TP = 21.14 – 10.7; ranked: CS> GB = VM> VE. However, increasing the substrate thickness (1.5–2.5 mm) reduced energy delivery to the luting cements (4 – 2.8 J/cm²). Consequently, as their thicknesses increased beyond 1.5 mm, H_M of the cement discs differed significantly between the substrates. But there were only slight reduction of DC% in LC cements and DC cement discs were not affected.Significance: Photoactivating light-cured Ivocerin? containing cement through feldspathic ceramics and polymer-infiltrated ceramics achieved greater early hardness results than dual-cured type, irrespective of substrate thickness (0.5 – 2.5 mm). However, only 0.5 and 1 mm-thick resin composites showed similar outcome (LC>DC). Therefore, for cases requiring early hardness development, appropriate cement selection for each substrate material is recommended.