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PubMed2002

Sensitivity of a disc rupture strength test to air bubble pores in phosphate-bonded investment materials at elevated temperatures.

OBJECTIVES: To investigate the sensitivity of a modified disc rupture test to variables affecting the strength of four phosphate-bonded investment materials under conditions closely resembling the in-service environment. This study examined the influence of air bubble pores on the strength of phosphate bonded investment materials at a temperature of 900 degrees C. METHODS: A modified disc rupture configuration was used to test a circular investment diaphragm with clamped edges, in tension, initiated by bending. A sequentially-varying molten metal mass was used to apply the load using an electronic centrifugal casting machine. The staircase method was used to determine the increase or decrease of the applied load with the standardized increment of load being no greater than 1 or 2g. The mean load applied to 30 samples defined the transition of survival to failure of the investment disc and was taken as a measurement of investment strength. Two material handling techniques, one giving virtually no pores and one showing a wide pore size distribution were used. RESULTS: The diaphragm clearly either failed or survived the test with the pattern of fracture indicating brittle failure. A statistically significant difference in strength (P<0.050) of diaphragms with and without air bubble pores was demonstrated for three materials. Fewer pores resulted in increased strength of those materials and produced a much reduced scatter of strength in the fourth. SIGNIFICANCE: The modified disc rupture test is sufficiently sensitive to identify variations in strength of phosphate bonded investment materials caused by differing pore size distributions.

Air↗
PubMed2002

Thermoplastic composites for veneering posterior teeth-a feasibility study.

OBJECTIVES: This pilot study was conducted to explore selected commercially-available thermoplastic composites that potentially had physical properties superior to currently available dental systems for restoring esthetic posterior crowns. METHODS: Polyurethane, polycarbonate, and poly(ethylene/tetrafluoroethylene) (ETFE) composites and unfilled polyurethane specimens were injection molded to produce shapes adaptive to five standardized mechanical tests. The mechanical testing included abrasive wear rate, yield strength, apparent fracture toughness (strength ratio), flexural strength, and compressive strength. RESULTS: Compared to commercially available dental composites, abrasion wear rates were lower for all materials tested, yield strength was greater for the filled polycarbonates and filled polyurethane resins, fracture toughness testing was invalid (strength ratios were calculated for comparison of the pilot test materials), flexural strength was roughly similar except for the filled ETFE which was significantly greater, and compressive strength was lower. SIGNIFICANCE: Commercially available thermoplastic resin composites, such as polyurethane, demonstrate the potential for development of an artificial crown material which exceeds the mechanical properties of currently available esthetic systems, if compressive strength can be improved.

Analysis of Variance↗
PubMed2002

Dynamic mechanical analysis of storage modulus development in light-activated polymer matrix composites.

OBJECTIVES: The goal of this study was to evaluate the potential for using dynamic mechanical analysis of tubular geometry in a three-point flexure fixture for monitoring the storage modulus development of a light-activated polymer matrix composite. METHODS: Composite samples were inserted into PTFE tubes and tested in a three-point bend fixture in a dynamic mechanical analyzer (DMA) at 200 Hz with 20 microm amplitude. Samples were light activated for 60s (385 mW/cm(2) at the composite surface) and storage modulus (E') was measured continuously for the seven light-activated composites studied (one microfill, four hybrids and two unfilled resins). Cores of composite were removed from the PTFE sheath after 13.5 min and evaluated with the same parameters in the DMA. A finite element model of the test configuration was created and used to estimate operating parameters for the DMA. Degree of conversion (DC) was measured using micro-Fourier Transform Infrared (FTIR) spectroscopy for the microfilled composite samples and one hybrid 13.5 and 60 min after light activation. RESULTS: The E' for a generic hybrid and microfilled composite was 13,400+/-1100 and 5900+/-200 MPa, respectively, when cured within the tube and then removed and tested in the DMA. DC was 54.6% for the hybrid and 60.6% for the microfill. A linear regression of E' for the sheath and core vs core alone (r(2)=0.986) indicated a linear scaling of the sheath and core values for E' enabling a correction for estimated E' values of the composite core. SIGNIFICANCE: This method estimates the storage modulus growth during light-activated polymerization of highly filled dimethacrylates. Although the approach is phenomenological in that quantitative measurements of E' are not made directly from the DMA, estimates of early polymerization kinetics appear to be validated by three different approaches.

Barium Compounds↗
PubMed2002

A note on the temperature dependence of the flexural strength of a porcelain.

OBJECTIVES: The purpose of this study is to determine the temperature dependence of the flexural strength of a dental porcelain (IPS-Classic Dentin; manufacturer: Ivoclar, Liechtenstein) with temperature between its glass transition (T(g) = 581.7 degrees C) and room temperature. METHODS: The flexural strength was measured in three-point bending tests on an Instron 4204 testing apparatus. The strength values were determined for the temperatures T = 20, 300, 400, 450, 500, 550, and 600 degrees C. RESULTS: In the temperature interval 20 < or = T < or = 400 degrees C the flexural strength decreased slightly from approximately 80 to approximately 73 MPa (mean values), as temperature increased. That is a decay of less than 10%. At higher temperatures the flexural strength increased to a maximum of approximately 98 MPa at 500 degrees C, probably due to the closure of microcracks in the surface on account of the onset of viscous flow. A further increase of the temperature delivered again decreasing strength values. At its glass transition temperature the porcelain's flexural strength was approximately 76 MPa which is only about 5% less than the value at room temperature. SIGNIFICANCE: In order to be able to evaluate the risk of fracture of ceramometallic crowns and bridges due to high temperature gradients and accompanying large transient thermal stresses in the veneer during the fabrication process, flexural strength values at high temperatures must be known. This study was carried out to fill this knowledge gap because to the authors' knowledge there was little published in the literature.

Aluminum Silicates↗
PubMed2002

Elimination of low-quality ceramic posts by proof testing.

OBJECTIVES: Prosthetic posts made of zirconia ceramics are beneficial because of their esthetic advantages. The mechanical reliability of ceramic posts is limited because of the strength level and the large scatter in strength of ceramic materials. The hypothesis of this study is that the failure of ceramic posts in vivo, which occur on low-quality posts because of critical microscopic defects can be excluded using the so-called proof test technology. METHODS: The theory of the proof test technology is explained in detail in this study. In the experimental part 22 ceramic posts were tested at different proof load levels using a stepwise rotating four-point bending proof test device. RESULTS: The experimental failures deviated from the theoretical failure probabilities by 0.24-12.69%. The deviation would be further reduced in a serial proof test using batches with a greater quantity of posts. The test eliminated those posts, which contained microscopic defects above a critical size of clinical relevance. SIGNIFICANCE: The technology can help to reduce clinical failures of non-vital abutment teeth with ceramic posts, which are caused by critical microscopic defects of the ceramic material as a result of the industrial manufacturing process.

Ceramics↗
PubMed2002

Transformation-toughened zirconia for dental inlays, crowns and bridges: chemical stability and effect of low-temperature aging on flexural strength and surface structure.

OBJECTIVES: One concern, for transformation-toughened zirconia (Y-TZP) is their liability to low-temperature aging with accompanying alterations of properties such as strength. The loss in strength is attributed to the transformation of tetragonal grains to monoclinic. The transformation is related to loading of the dental inlays, crowns and bridges (reconstructions), temperature and time of exposure to surrounding media (aging) and the manufacturing process of Y-TZP. The purpose of this study was to determine chemical stability and effect of aging (4% acetic acid at 80 degrees C for 168 h) on flexural strength, surface and crystalline structures of two shades, P0 and P17, of a Y-TZP ceramic used for dental reconstructions. The hypotheses to be tested were that both shades of the dental Y-TZP ceramic have high flexural strength and chemical stability compared to other dental ceramics, and that the strength, surface and crystal structures of the ceramic were not affected by aging. METHODS: Forty specimens of Y-TZP, 20 of the shade P0 and 20 of the shade P17 were ground and polished. Ten specimens of each shade were exposed to low-temperature aging. The flexural strength of all 40 specimens was registered. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM), X-ray diffractometry and roughness recorder. The chemical solubility in 4% acetic acid was recorded by weight loss, and SEM was used to evaluate the surfaces of Y-TZP and dental feldspathic porcelain samples immersed in 8% SnF. Wilcoxon Signed Rank test was used for statistical analysis. RESULTS: As expected, the two shades, P0 and P17, of the studied dental Y-TZP had high strengths that were not affected by aging, and high chemical stability in the tested solutions. Contrary to what was assumed the crystal and surface structures of P0 and P17 were affected. Transformation from tetragonal to monoclinic structures occurred and small elevations on the ceramic surfaces were observed after aging. SIGNIFICANCE: The transformed-toughened Y-TZP ceramic used for dental inlays, crowns and bridges has high flexural strength compared to other dental ceramics and is chemically stable. The chemical solubility is well below the limit values of ISO 6872:1995 for dental ceramics.

Acetic Acid↗
PubMed2003

Extended creep behavior of dental composites using time-temperature superposition principle.

OBJECTIVES: The purpose of the study was to characterize long-term creep deformation behavior of selected dental composite resins. METHODS: Two hybrid composite resins (one midifill type Clearfill and the other minifill type Charisma) and one microfill type (Durafill) were evaluated. 25 mm x 10 mm x 2 mm rectangular bar specimens were subjected to single cantilever mode of flexural loading and their creep compliance changes determined over 10 min durations in the temperature range of 32-120 degrees C at 5 degrees C intervals. The reference temperature selected was the body temperature of 37 degrees C for the oral environment. All creep compliance curves at other temperatures were shifted along the log time axis to superimpose them on the reference temperature (37 degrees C) curve to generate an extended isothermal master curve over eight decades of time (seconds). The shift factors were determined as a function of temperature and fitted by the least square method to the William-Landel-Ferry model. RESULTS: The data showed excellent fit to the model, indicating the validity of the model to characterize the long-term behavior using the time-temperature superposition principle. The creep compliance changed from initial mean values of 76, 245 and 605 microm(2)/N to 3 year mean values of 217, 880 and 1653 microm(2)/N for Clearfill, Charisma and Durafill systems, respectively. These means were significantly different both as a function of composites and time (p<0.05). SIGNIFICANCE: The results indicate that resin volume fraction and filler parameters may play a critical role in the long-term deformation and energy release processes, and these processes may be an important influence on the long-term clinical durability of composite restorations.

Compliance↗
PubMed2003

An evaluation of the mechanical properties of 'hydrothermal' dental glass after water immersion and surface polishing.

OBJECTIVES: To investigate claims that Duceram low-fusing ceramic (Duceram-LFC), a 'hydrothermal' dental veneer will increase in strength after hydrolytic testing and surface polishing.Methods. Using pre-determined powder/liquid slurry constituents, nominally identical Duceram-LFC disc-shaped specimens were fabricated to obtain mean bi-axial flexure strengths, standard deviations, Weibull modulus (m) and Vickers hardness. Thirty specimens were tested 'dry' (control), after 24 and 48h immersion in a water bath maintained at 37+/-1 degrees C and a further 30 samples polished to a 6 micrometer nap finish. RESULTS: Mean bi-axial flexure strengths of specimens tested dry were found to be 67.04+/-13.87MPa (m=7.23+/-1.32) compared with 61.01+/-15.23MPa (m=4.29+/-0.78) and 68.42+/-23.36MPa (m=3.29+/-0.60) for 24 and 48h immersion, respectively. Following surface polishing, the bi-axial flexure strength was 95.91+/-22.4MPa (m=4.37+/-0.8). Vickers hardness numbers showed no significant difference between the dry control group and after 24 and 48h immersion (P>0.05). SIGNIFICANCE: Results obtained do not support claims for surface flaw and/or defect healing through the formation of an SiOH layer at the surface of Duceram-LFC specimens over time. However, the polishing regime interrupted the defect population associated with flaws present in dry (control) samples of Duceram-LFC which can be attributed to an associated reduction of initial surface flaws and/or defects.

Dental Polishing↗
PubMed2003

Mechanical properties and microstructures of cast Ti-Cu alloys.

OBJECTIVES: This study evaluated the mechanical properties of cast Ti-Cu alloys with the hope of developing an alloy for dental casting with better mechanical properties than unalloyed titanium. METHODS: Ti-Cu alloys with five concentrations of copper (0.5, 1.0, 2.0, 5.0 and 10.0 mass%) were made in an argon-arc melting furnace. The alloys were cast into magnesia-based molds using a centrifugal casting machine. The microstructure, microhardness profile of the specimen cross section, tensile strength, yield strength, and elongation were determined for the castings. Scanning electron microscope fractography was undertaken for the fractured surfaces after tensile testing. XRD was performed on the polished specimens. Results were analyzed using one-way ANOVA and the Student-Newman-Keuls tests. RESULTS: The mean tensile strengths of all the cast Ti-Cu alloys were significantly (p<0.05) higher than for cast commercially pure titanium (CP Ti). Of the Ti-Cu alloys tested, the 5 and 10% Cu alloys had significantly higher strength than the rest. The 10% Cu alloy exhibited the lowest mean elongation. CP Ti and the 0.5 and 1% Cu alloys showed higher ductility. The bulk hardness of all the cast Ti-Cu alloys, except for the 10% Cu alloy, and CP Ti was approximately the same. SIGNIFICANCE: By alloying with copper, the cast titanium became stronger. Increases in the tensile strength (30%) and yield strength (40%) over CP Ti were obtained for the 5% Cu alloy. Elongation was approximately 3%, which was similar to cast Ti-6Al-4V. Ti-Cu alloys, such as the 5% Cu alloy, could be used for prosthetic dental applications if other properties necessary for dental castings are obtained.

Copper↗
PubMed2003

Shear punch strength and flexural strength of model composites with varying filler volume fraction, particle size and silanation.

OBJECTIVE: The effect of filler volume fraction, particle size and silanation on shear punch strength, flexural strength and flexural modulus of model composites has been evaluated. METHODS: Hybrid type glass filled (0-65.2 vol%), composites some without filler silanation (30.7-51.0 vol%) and microfilled type (0-13.0 vol%) resin matrix (UDMA and EGDMA) composites (Shofu Inc., Japan) were used in the study. For the shear punch test, 10 disc specimens, 0.5mm thick and 9 mm diameter, were prepared for each composite and tested with a 3.2mm dia punch at 1.0mm/min. Flexural strength (n=10) was measured by the method outlined in ISO 4049. Data were analyzed using ANOVA and Fisher's multiple-range test. RESULTS: Shear punch strength and flexural strength increased with increasing filler content up to 52.2% for hybrid composites and between 0 and 9.1% for microfilled composites. Shear punch strength and flexural strength decreased with increasing filler volume fraction for un-silanated composites. Flexural modulus for all materials increased with increasing filler volume fraction. Hybrid composites with silanated fillers have significantly higher values of flexural strength, flexural modulus and shear punch strength than equivalent materials with un-silanated fillers. SIGNIFICANCE: The results showed that filler silanation is an important factor for determining material strength. Additions of small quantities of microfillers appeared to have a greater effect on shear strength than equivalent amounts of hybrid filler. The shear punch test may prove beneficial for routine testing of composites as specimen preparation was simple, specimen quality was easy to maintain and the results are meaningful.

Analysis of Variance↗
PubMed2003

Zinc sulfate addition to glass-ionomer-based cements: influence on physical and antibacterial properties, zinc and fluoride release.

OBJECTIVES: The aim of this study is to evaluate the effect of ZnSO(4) addition to a conventional glass ionomer and a resin-modified glass ionomer on solubility, flexural strength, zinc and fluoride (F) release, and Streptococcus mutans growth inhibition. METHODS: 5 or 10% ZnSO(4) was added to Vitremer and Ketac-Fil powders. Solubility test was performed based on ISO 7489. Flexural strength was determined by 3-point bending test based on ISO 4049. Zn release/uptake was determined by atomic emission spectrometry; F release/uptake was measured using a F-specific electrode. Both release measurements were performed for 15 d before and 15 d after recharging. Antibacterial test was conducted according to agar plate methods against S. mutans, by measuring the inhibition halos in 1-h and 15-d specimens. Data were analyzed by ANOVA. RESULTS: Solubility increased with higher ZnSO(4) content, but remained below the ISO 7489 limit. Flexural strength was not affected by ZnSO(4) addition, and Vitremer performed better than Ketac-Fil. The control materials released no zinc. Vitremer with 10% ZnSO(4) released the highest amount of zinc. Fluoride release was similar for Ketac-Fil and Vitremer. In both cases, the highest amounts were released in the first 24 h. The growth inhibition halo of S. mutans was similar for both materials with highest content of ZnSO(4) and occurred only with 1-h specimens. SIGNIFICANCE: Zinc addition decreased microorganisms growth and improved fluoride release, without significantly affecting the materials' flexural strength and solubility.

Analysis of Variance↗
PubMed2003

Static and cyclic loading of fiber-reinforced dental resin.

OBJECTIVE: The aim of this study was to evaluate the flexure strength of unidirectional fiber-reinforced resins under static and cyclic loading with and without thermal cycling. METHODS: The fiber-reinforced resin materials chosen for this project were commercially available endodontic posts and commercially procured bar samples. For all materials, controls for flexure strength were tested in air and in water using three-point loading. Specimens were thermal cycled between 7 and 63 degrees C for 6000 cycles. A staircase approach was used to determine the flexure fatigue limit and scanning microscopy was used to examine the microstructure. RESULTS: The carbon/graphite fiber-reinforced resin posts and the glass FiberKor posts were significantly stronger than the ceramic (zirconia) and the other glass-reinforced resin materials. Thermal cycling caused a significant lowering (11-24%) of the flexure strength for each resin based post system. The ceramic post system decreased only by 2%. Further, for standard size glass fiber-reinforced resin bars, no significant differences between testing in air and water was observed, but a significant difference between static and cyclic loading was noted. SIGNIFICANCE: The decreases in the strength property due to thermal cycling and the cyclic loading of these materials indicates that their utilization in the oral environment enhances their degradation, and potentially shortens their clinical life.

Air↗
PubMed2003

In vitro characterization of two laboratory-processed resin composites.

PURPOSE: To compare various characteristics of two new-generation laboratory-processed resin composites (BelleGlass HP/SDS-Kerr and Sinfony/3M-ESPE). The properties evaluated were degree of C=C conversion, microhardness, roughness, biaxial flexural strength and polymerization shrinkage-strain. MATERIALS AND METHODS: All specimens were subjected to a first and a second polymerization cycle according to the manufacturers' instructions. The degree of C=C conversion (DC) was recorded on rectangular (3 x 2 x 0.5mm(3)) specimens (n=3) by FT-IR micromultiple internal reflectance spectroscopy immediately after each of the two polymerization cycles. Twenty cylindrical specimens (10 x 2mm(2)) of each material were prepared for surface microhardness (n=10, VHN, 200 g load, 20s) and surface roughness (n=10, Ra) measurements. The biaxial flexural strength and stiffness were determined on disk-shaped (n=8, 15 x 0.7 mm(2)) specimens loaded to fracture at 1 mm/min crosshead speed. The polymerization shrinkage-strain was calculated with the bonded-disk method. All values were statistically analyzed by Student's unpaired t-test (p<0.05). RESULTS: The second polymerization cycle significantly increased the degree of C=C conversion for both materials (p<0.05). BelleGlass HP exhibited significantly higher degree of C=C conversion, surface microhardness, surface roughness, biaxial flexural strength and stiffness values compared to Sinfony (p<0.05). SIGNIFICANCE: Several differences exist between the materials although both products are recommended for the same clinical applications.

Composite Resins↗
PubMed2003

Irradiance effects on the mechanical properties of universal hybrid and flowable hybrid resin composites.

OBJECTIVES: A potential problem with high-intensity lights might be failure of polymer chains to grow and cross-link in a desired fashion, thereby affecting the structure and properties of the polymers formed. The purpose of this study was to evaluate mechanical properties of resin composites polymerized using four different light-curing units. METHODS: A conventional quartz-tungsten-halogen (QTH) light, a soft-start light, an argon-ion laser, and a plasma-arc curing light were used to polymerize disk-shaped (9.0mm diameter x 1.0 mm high) and cylinder-shaped (4mm diameter x 8 mm high) specimens of a universal hybrid and a flowable hybrid composite. Biaxial flexure strength, fracture toughness, hardness, compressive strength, and diametral tensile strength were determined for each composite. RESULTS: The use of the plasma-arc curing light, a high-intensity light, resulted in significantly lower hardness for the universal hybrid composite compared with the hardness obtained using the conventional QTH and the soft-start units. Hardness was the only mechanical property that was adversely affected by the use of a high-intensity light. SIGNIFICANCE: High-intensity lights might affect some resin composite mechanical properties, but this effect cannot be generalized to all resin composites and all properties.

Composite Resins↗
PubMed2003

Aspects of water sorption from the air, water and artificial saliva in resin composite restorative materials.

OBJECTIVE: Primarily to establish whether artificial saliva (AS) at 37 degrees C is essential as a clinically relevant environment for testing filled, resin composite restorative materials. The effect of other storage conditions was also investigated for comparison and controls: desiccation, exposure to the laboratory atmosphere, high humidity cabinet, saturated water vapor, and deionized water. METHODS: Two visible light-cured products were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein). Bar specimens (26 x 1.5 x 1.0 mm(3)) were cured at five overlapping spots for 60s per spot and randomly distributed into groups of six. Trial 1: one group of each material was exposed first to atmospheric air at 24 degrees C, approximately 50% RH (24WV(50)), then to water vapor at 37 degrees C, approximately 97% RH (37WV(97)), and then immersed in deionized water at 37 degrees C (37DW). Trial 2 used three groups of each material, one first exposed to 37WV(97) followed by 37DW, the other two were immediately immersed in 37DW or artificial saliva (37AS). Trial 3: two groups of each material were vacuum desiccated at 37 degrees C, then exposed to 37 degrees C, approximately 100% RH (37WV(100)), then immersed in 37DW or 37AS. Trial 4: four groups of HR were treated similarly to Trial 3; one was left under desiccation, and another in 37WV(100) for the remaining period. Three-point bend tests for flexural strength (F), flexural modulus (E), and total energy to failure (W) were performed at the end of Trials 2-4. RESULTS: Environmental moisture absorption was substantial at 24WV(50)(c. 0.2%), at least 40% of that in 37DW (HR: c. 0.7%, TC: c. 0.5%). Saturation was achievable in 37WV(100). Mass loss on desiccation (HR: c. 0.4-0.5%, TC: c. 0.25%) was reversible in 37WV(100). There were some significant effects of exposure conditions on mechanical properties (e.g. F for HR: after desiccation, 85.7+/-1.4MPa; after 37WV(100), 73.2+/-3.6MPa; difference: p<0.0002), but overall the results were unclear. After a rapid gain in mass, there was a gradual loss in both 37DW and 37AS for both materials, slightly more in 37AS than 37DW. SIGNIFICANCE: Water vapor absorption is substantial, hence attention must be paid to the laboratory working environment and conditions of storage and testing, i.e. temperature and RH must be stated to assist interpretation of data and comparisons between studies. Test conditions need to be standardized and with reference to normal oral conditions, immediate immersion in artificial saliva at 37 degrees C is the preferred treatment for these materials, whatever time of testing is chosen.

Absorption↗
PubMed2003

Flexural fatigue behavior of resin composite dental restoratives.

OBJECTIVE: The aim of this study was to evaluate the mechanical properties of resin composite dental restoratives under quasi-static and cyclic loading. METHODS: Four-point-bending bars of 10 different resin composite materials were manufactured according to ISO standard and stored for two weeks in distilled water. The fracture strength (FS) was measured with the four-point-bending test in an universal testing machine. The flexural fatigue limits (FFL) for 10(5) cycles were determined under equivalent loading. All specimens were tested and fatigued in water at 37 degrees C. The data were analyzed using ANOVA, Weibull statistics of FS and the 'staircase' approach of FFL. Fractographic analysis was performed using SEM. RESULTS: The initial flexural strength values for the resin composite materials varied from 55.4 MPa for Solitaire up to 105.2 MPa for Filtek Z250. The mean flexural fatigue limit for 10(5) cycles ranged between 37 and 67% of the initial strength. SEM analysis of the fractured surfaces suggests two kinds of failure mechanisms for initial and fatigue fracture. SIGNIFICANCE: The fatigue behavior of resin composite materials does not correlate with initial strength values. Materials providing high initial strengths do not obviously reveal the best fatigue resistance. Flexural fatigue measurement of resin composite materials should be viewed as a useful tool to evaluate long term mechanical properties.

Analysis of Variance↗
PubMed2003

The effect of light intensity on double bond conversion and flexural strength of a model, unfilled dental resin.

OBJECTIVE: Two visible light sources (tungsten-quartz-halogen and xenon-arc plasma) with vastly different intensities (200 and 1800 mW/cm(2)) but similar spectral outputs, were used to examine the effects of light intensity on conversion and flexural strength of a model dental resin formulation (75/25wt% bis-GMA/TEGDMA). METHODS: The exact same polymer samples were used to correlate double bond conversion (measured with near-IR spectroscopy) to flexural strength, both immediately after light exposure and after storage. RESULTS: In general, polymers which were irradiated with the high light intensity source exhibited greater double bond conversion. However, increasing the light intensity also increased the maximum temperature reached during polymerization. Therefore, the greater double bond conversion was caused by a combination of both photo and thermal effects. Regardless of the light intensity, a single linear relationship existed between conversion and final flexural strength (measured 4 days after cure) over the conversion range analyzed (50-80%). However, deviations from linearity were noted in several samples that were tested immediately after exposure. SIGNIFICANCE: These findings illustrate that light intensity does not affect the final flexural strength of a dental resin as long as the final conversions are similar.

Analysis of Variance↗
PubMed2003

Hydrolytic stability of experimental hydroxyapatite-filled dental composite materials.

OBJECTIVES: The purpose of this study was to analyze the behavior in water, related to mechanical properties, of experimental composites for dental restoration. METHODS: The studied materials were composed of a visible-light-curing monomer mixture (Bis-GMA and TEGDMA or HEMA) and micrometric, nanometric or a mixture of both sizes hydroxyapatite particles as a reinforcing filler. Filler particles were modified with a coupling agent (citric, hydrosuccinic, acrylic or methacrylic acid or silane). The hydrolytic stability of the evaluated materials was studied through total elution and water-uptake tests. Percent net-mass variation was daily monitored and analyzed as a function of time. Mechanical performance was examined through flexural properties and Vickers hardness. Morphological surface changes were observed with scanning electron microscopy. ANOVA statistical analysis was performed (P<0.05). RESULTS: In general, the use of HEMA instead of TEGDMA did not substantially worsen the composite quality. Dental composites containing only nanometric particles of hydroxyapatite as a filler are unsuitable for clinical performance. Midway-filled composite resins loaded with micro-HAP particles, coated with citric, acrylic or methacrylic acid displayed low percent elution and water-uptake values. Mechanical properties were similar or even superior to those measured for silane treated particles. SIGNIFICANCE: More research is needed to further improve the interaction of nano-HAP particles with the polymeric matrix, either as a single filler or, preferentially, mixed with micro-HAP, that will allow to increase the total loading of reinforcing filler and, hence, to improve the mechanical properties.

Analysis of Variance↗