Sodium carbonate improves adherence of two-layer alginate for preliminary impressions of complete dentures.

PURPOSE: This in vitro study evaluated the effect of sodium carbonate (Na2CO3) concentration on adhesion between two layers of irreversible hydrocolloid wash impression material. METHODS: The first layer of irreversible hydrocolloid was brushed with three concentrations (0.1, 0.7, or 7.0 wt.%) of Na2CO3. Irreversible hydrocolloid not brushed with Na2CO3 was used as a negative control. Adhesion between the two layers (bond strength and adhesion energy) was then evaluated using a traction test, and the fracture modes were identified. The results of traction test were analyzed with a one-way ANOVA followed by a Tukey post hoc test. Chi-square test was used for analyzed failure mode after testing (α= 0.05). RESULTS: The bond strength (0.034 ±0.005 MPa) and adhesion energy (60.240 ±12.817 J.m⁻ ²) of the 0.7% Na2CO3 group were significantly higher than those of the other groups (P< 0.05). The 0.7% Na2CO3 group displayed only cohesive failure (ײ= 16; P< 0.0001). CLINICAL SIGNIFICANCE: Conditioning the surface of the first layer of irreversible hydrocolloid with 0.7% sodium carbonate improved adhesion between the two layers and may help clinicians improve the quality of the preliminary impressions of removable complete dentures.

Effect of build orientation on the manufacturing process and the properties of stereolithographic dental ceramics for crown frameworks.

STATEMENT OF PROBLEM: Stereolithography (SLA) ceramic crown frameworks are suitable for clinical use, but the impact of SLA build orientation has not been identified. PURPOSE: The purpose of this in vitro study was to investigate the effect of 3 build orientations on the physical and mechanical properties and the microstructure of SLA alumina dental ceramics. MATERIAL AND METHODS: The physical and mechanical properties and microstructures of 3 different oriented SLA alumina ceramics (ZX, ZY, and XY) were evaluated by visual observation, hydrostatic weighing (n=10/group), Weibull analyses (n=30/group), scanning electron microscopy, 3-point flexural strength (n=30/group), fracture toughness (indentation, single-edge-V-notched-beam) (n=4/group), and Vickers hardness (n=15/group) testing. The hydrostatic weighing, 3-point flexural strength, fracture toughness, and Vickers hardness testing data were statistically analyzed (α=.05). RESULTS: The minimum resting period of slurries between the polymerization of 2 layers was shorter for the ZY- and ZX-oriented specimens and increased with the layer surface. The density and Vickers hardness of the SLA-manufactured specimens were similar for all groups (P>.05). The 95% confidence intervals of the Weibull moduli of the ZX- and ZY-oriented specimens were higher than that of the XY-oriented specimens, with no overlap fraction. The ZY-oriented specimens displayed significantly higher 3-point flexural strength (P<.05) and fracture toughness as evaluated by the single-edge-V-notched-beam method than the ZX-oriented specimens (P<.05). They also displayed significantly higher 3-point flexural strength than the XY-oriented specimens (P<.05). The microstructural analysis showed that the texturing was heterogeneous and that the major axis of the large grains of alumina ran parallel to the orientation of the layers. CONCLUSIONS: The ZY orientation produced a reliable dental ceramic by SLA, with the shortest general manufacturing time and the highest mechanical strength when the layers were perpendicular to the test load surface.

Influence of polymerization pressure and post-cure treatment on conversion degree and viscoelastic properties of polymer infiltrated ceramic network.

This study aimed at determining an optimum polymerization pressure for Polymer Infiltrated Ceramic Network (PICN) blocks by characterizing the conversion degree (DC) and the viscoelastic properties of experimental PICN blocks polymerized at 90 °C under various high pressures followed or not by post-cure treatment (PC). Near infrared analysis and dynamic mechanical analysis were used to characterize DC and viscoelastic properties of sixteen PICN: one control (thermo-cured) and fifteen experimental groups (one thermo-cured followed by PC and fourteen high pressure polymerized PICN, in the range of 50-350 MPa without and with PC). Conversion degree of high pressure polymerized PICN blocks without post curing displays an optimum between 100 and 150 MPa resulting in an improved E' and Tg. Post curing induces a higher DC with a controversial effect on thermomechanical properties. The results suggested that 100-150 MPa without PC is an optimum polymerization parameter, resulting in PICN blocks with significantly better DC, Tg, E'.

Does Evidence Support a Combined Restorative Surgical Approach for the Treatment of Gingival Recessions Associated With Noncarious Cervical Lesions?

OBJECTIVES: Gingival recessions are frequently associated with noncarious cervical lesions. Combined restorative surgical approaches have been suggested for the management of these combined defects. The aim of this review was to analyze the current evidence on the effectiveness of these combined restorative surgical approaches. METHODS: A systematic review of randomized controlled studies published from January 2006 to June 2016 was performed. RESULTS: Ten articles comparing combined restorative surgical approaches to surgery alone were included. Both the procedures showed similar outcomes in term of root coverage but the combined restorative surgical approach achieved better result in the reduction of dentin hypersensitivity. In combined restorative surgical treatments, resin-modified glass ionomers showed better antimicrobial properties but poorer esthetic outcome than resin composites. CONCLUSION: Data to support or refute definitely the interest of combined approaches for the treatment of gingival recessions associated with noncarious cervical lesions are limited.

Stereolithography: A new method for processing dental ceramics by additive computer-aided manufacturing.

OBJECTIVES: The aim of this study was to compare the physical and mechanical properties of stereolithography (SLA)- manufactured alumina ceramics of different composition to those of subtractive- manufactured ceramics and to produce suitable dental crown frameworks. METHODS: The physical and mechanical properties of a control and six experimental SLA ceramics prepared from slurries with small (S) and large (L) particles (0.46±0.03 and 1.56±0.04µm, respectively) and three dry matter contents (70%, 75%, 80%) were evaluated by dynamic rheometry, hydrostatic weighing, three3-point flexural strength measurements, and Weibull analyses, and by the micrometrics measurement of shrinkage ratio before and after the heat treatments. RESULTS: S75 was the only small particle slurry with a significantly higher viscosity than L70. The viscosity of the S80 slurry made it impossible to take rheological measurements. The viscosities of the S75 and S80 slurries caused deformations in the printed layers during SLA manufacturing and were excluded from further consideration. SLA samples with low dry matter content had significantly lower and densityflexural strengths. Only SLA samples with a large particle size and high dry matter content (L75 and L80) were similar in density and flexural strength to the subtractive- manufactured samples. The 95% confidence intervals of the Weibull modulus of the L80 ceramic were higher (no overlap fraction) than those of the L75 ceramic and were similar to the control (overlap fraction). The Weibull characteristics of L80 ceramic were higher than those of L75 ceramic and the control. SLA can be used to process suitable crown frameworks but shows results in anisotropic shrinkage. SIGNIFICANCE: The hH High particle size and dry matter content of the L80 slurry allowed made it possible to produce a reliable ceramic by SLA manufacturing with an anisotropic shrinkage, and a density, and flexural strength similar to those of a subtractive-manufactured ceramic. SLA allowed made it possible to build up a dense 3D alumina crown framework with controlled shape. Further studies on the marginal adaptation and shrinkage model of alumina crown frameworks will be required to optimize the process.

Optimum pressure for the high-pressure polymerization of urethane dimethacrylate.

OBJECTIVES: The aim of this study, part of our research to improve properties of resin composite blocks suitable for CAD/CAM and to better understand underlying mechanisms associated with high-temperature/high-pressure (HT/HP) polymerization, was to determine an optimum polymerization pressure of urethane dimethacrylate (UDMA) in the presence of an initiator (0.5% benzoyl peroxide) by determining the degree of conversion (DC) and viscoelastic properties of polymers obtained at 90°C under varying HP. METHODS: DC and viscoelastic properties of 16 UDMA polymers, two controls (thermo-cured and thermo-cured followed by post-cure relaxation) and 14 experimental groups (HP polymers, in the range of 50-350 MPa, in 50 MPa increments, without and with post-cure relaxation) were determined via near infrared spectroscopic analysis and dynamic mechanical analysis, respectively. RESULTS: The results have shown that HP UDMA polymers have DC superior to that of the control group. With regards to E' and E″, the results have shown no significant difference between control and HP polymers. The damping factor, tanδ, decreased with increasing pressure, while E'rub and Tg increased. Polymerization at 150 MPa or higher resulted in significantly higher E'rub and Tg. SIGNIFICANCE: The results of this study suggested that HP polymerization at 90°C of UDMA reduced the number of defects and the free volume, leading to a more homogeneous polymer network. The results have also suggested that 200 MPa is an optimum polymerization pressure, resulting in polymers with significantly higher DC, E'rub, and Tg, while maintaining adequate damping capacity (tanδ).

Influence of dentist experience with scan spray systems used in direct CAD/CAM impressions.

STATEMENT OF PROBLEM: Direct optical impression systems requiring powder opacification are widely used in prosthodontics, but little information is available about the influence of the dentist on the quality of scan sprays used for computer-aided design and computer-aided manufacturing (CAD/CAM) impressions. PURPOSE: The purpose of this study was to evaluate the influence of operator experience on scan spray homogeneity and thickness on tooth surfaces before direct optical impressions are made. MATERIAL AND METHODS: A total of 36 crown-prepared acrylic resin teeth were coated (Optispray) for direct optical impressions by 3 experienced dentists and 3 inexperienced dentists (n=18 teeth per group). The teeth were visually scored for coating homogeneity (1, heterogeneous; 2, average; 3, homogeneous). To determine the coating thickness (µm), the teeth were resin-embedded, cut, and polished. A stereomicroscope (×100 magnification) was used to examine 148 areas per tooth. The results were statistically analyzed with the nonparametric Spearman and Mann-Whitney tests (α=.05). RESULTS: Coating homogeneity was significantly higher (P<.001) for the experienced group (92.41 ±6.65%) than for the inexperienced group (66.11 ±12.69%). Coatings were significantly thinner for the experienced group (43.1 ±14.09 µm) than for the inexperienced group (70.19 ±31.26 µm) for all crown areas (P=.007). Coatings were 52 µm thinner for occlusal areas (P<.001) for the experienced group than for the inexperienced group. Coatings were significantly thinner for buccal (P=.015) and lingual surfaces (P=.002) for the experienced group than for the inexperienced group. Coatings on cervical surfaces were significantly thinner for the experienced group (43.52 ±15.76 µm) than for the inexperienced group (63.45 ±29.93 µm) (P=.035). CONCLUSIONS: Experienced dentists achieved greater homogeneity and thinner coatings on crown-preparation surfaces than inexperienced dentists.

Dynamic mechanical analysis of high pressure polymerized urethane dimethacrylate.

OBJECTIVES: The aim of this study was to compare the viscoelastic properties of high pressure (HP) polymerized urethane dimethacrylate (UDMA) with those of control, ambient pressure thermo-polymerized and photo-polymerized, UDMA and to assess the effect of varying polymerization parameters (protocol, temperature, and initiator) on the viscoelastic properties of HP polymerized UDMA. METHODS: The viscoelastic properties of the two control polymers, polymerized under atmospheric pressure, and four experimental polymers, polymerized under HP, were determined via dynamic mechanical analysis (DMA), in three point bending configuration. Atomic force microscopy (AFM) was used to characterize fractured polymer surface morphologies. RESULTS: The results showed that: HP-polymerization lead to a polymer with significantly higher Tg and E'rub, indicative of a higher crosslink density; modifying the polymerization protocol resulted in a significant increase in tanδ; increasing the polymerization temperature lead to a significant decrease in E'rub and Tg; and that the polymer with no initiator had the lowest E', E″, Tg, and E'rub and the highest tanδ, suggesting that under this conditions a polymer with significantly reduced crosslink density had been obtained. A characteristic nodular appearance was seen for the two control polymers under AFM, while a modified surface morphology was present in the case of HP polymerized materials. SIGNIFICANCE: The DMA results suggest that polymerization under HP resulted in polymers with an increased crosslink density and that the higher polymerization temperature or the lack of initiator was detrimental to the viscoelastic properties determined. Changes in polymer network morphology were identified by AFM characterization.