Acid neutralizing,mechanical and physical properties of pit and fissure sealants containing melt-derived 45S5 bioactive glass

Objectives

The aim of this study was to examine the effects of 45S5 bioactive glass (BAG) on the acid neutralizing, mechanical and physical properties of pit and fissure sealants.

Methods

45S5BAG (<25 μm) was mixed with the silanized glass (180 ± 30 nm) and added into a resin matrix [Bis-GMA/TEGDMA 50/50 (wt%) containing 1% of DMAEMA/CQ 2:1 (wt%)] with varying filler proportions; 0% 45S5BAG + 50% glass (BAG0); 12.5% 45S5BAG + 37.5% glass (BAG12.5); 25% 45S5BAG + 25% glass (BAG25); 37.5% 45S5BAG + 12.5% glass (BAG37.5); and 50% 45S5BAG + 0% glass (BAG50). To evaluate the acid neutralizing properties, specimens were immersed in lactic acid solution (pH 4.0). Then, the change in pH and the time required to raise the pH from 4.0 to 5.5 were measured. In addition, flexural strength, water sorption and solubility were analyzed.

Results

The acid neutralizing properties of each group exhibited increasing pH values as more 45S5BAG was added, and the time required to raise the pH from 4.0 to 5.5 became shorter as the proportion of 45S5BAG increased (P < 0.05). Additionally, the flexural strength decreased according to the increasing proportions of 45S5BAG added (P < 0.05). Water sorption showed an increasing trend with increasing proportions of 45S5 BAG added (P < 0.05). However, the solubility results were similar among the groups (P > 0.05), except for BAG50.

Significance

The novel pit and fissure sealants neutralized the acid solution (pH 4.0) and exhibited appropriate mechanical and physical properties. Therefore, these compounds are suitable candidates for caries-inhibiting dental materials.     

Synthesis and study of properties of dental resin composites with different nanosilica particles size

Objectives

The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical-mechanical properties of the composites.

Methods

Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C.

Results

As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm² is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed.

Significance

The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.     

Effect of gloss and heat on the mechanical behaviour of a glass carbomer cement

Objectives

The effect of gloss and heat on the mechanical behaviour of a recently launched glass carbomer cement (GCP, GCP dental) was evaluated and compared with resin-modified glass ionomer cements (Fuji II LC, GC and Photac Fil Quick Aplicap, 3M ESPE).

Methods

120 bar-shaped specimens (n = 20) were produced, maintained in distilled water at 37 °C and tested after one week. The GCP specimens were cured with and without heat application and with and without gloss. The flexural strength and modulus of elasticity in flexural test as well as the micro-mechanical properties (Vickers Hardness, indentation modulus, creep) of the top and bottom surface were evaluated. The amount and size of the fillers, voids and cracks were compared using a light and a scanning electron microscope.

Results

In the flexural test, the resin-modified glass ionomer cements performed significantly better than GCP. Fuji II LC and Photac Fil (Weibull parameter: 17.7 and 14.3) proved superior reliability in the flexural test compared to GCP (1.4–2.6). The highest Vickers Hardness and lowest creep were achieved by GCP, whereas Fuji II LC reached the highest indentation modulus. The results of this study proved that relationships exist between the compositions, microstructures and mechanical properties of the cements.

Conclusions

Heat treatment and gloss application did not influence the mechanical properties of GCP. The mechanical properties were basically influenced by the type of cement and its microstructure.

Clinical significance

Considering the measured mechanical properties, there is no need of using gloss or heat when restoring teeth with GCP.     

Mechanical performance of novel bioactive glass containing dental restorative composites

Objectives

Bioactive glass (BAG) is known to possess antimicrobial properties and release ions needed for remineralization of tooth tissue, and therefore may be a strategic additive for dental restorative materials. The objective of this study was to develop BAG containing dental restorative composites with adequate mechanical properties comparable to successful commercially available composites, and to confirm the stability of these materials when exposed to a biologically challenging environment.

Methods

Composites with 72 wt% total filler content were prepared while substituting 0–15% of the filler with ground BAG. Flexural strength, fracture toughness, and fatigue crack growth tests were performed after several different soaking treatments: 24 h in DI water (all experiments), two months in brain–heart infusion (BHI) media + Streptococcus mutans bacteria (all experiments) and two months in BHI media (only for flexural strength). Mechanical properties of new BAG composites were compared along with the commercial composite Heliomolar by two-way ANOVA and Tukey's multiple comparison test (p ≤ 0.05).

Results

Flexural strength, fracture toughness, and fatigue crack growth resistance for the BAG containing composites were unaffected by increasing BAG content up to 15% and were superior to Heliomolar after all post cure treatments. The flexural strength of the BAG composites was unaffected by two months exposure to aqueous media and a bacterial challenge, while some decreases in fracture toughness and fatigue resistance were observed. The favorable mechanical properties compared to Heliomolar were attributed to higher filler content and a microstructure morphology that better promoted the toughening mechanisms of crack deflection and bridging.

Significance

Overall, the BAG containing composites developed in this study demonstrated adequate and stable mechanical properties relative to three successful commercial composites.     

Synthesis and study of physical properties of dental light-cured nanocomposites using different amounts of a urethane dimethacrylate trialkoxysilane coupling agent

Objective

The purpose of this work was the study of the effect of the amount of a urethane dimethacrylate silane (UDMS) coupling agent on physical properties of dental light-cured resin nanocomposites based on Bis-GMA/TEGDMA (50/50 wt/wt) matrix and Aerosil OX50 as filler.

Methods

Silica nanoparticles (Aerosil OX 50) used as filler were silanized with 5 different amounts of UDMS 1.0, 2.5, 5.0, 7.5 and 10 wt% relative to silica. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (60 wt%) were mixed with a Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of light cured composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by DMA analyzer. Measurements were taken in samples stored, immediately after curing, in water at 37 °C for 24 h. Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water of 75 vol% for 30 days. Thermogravimetric analysis of composites was performed in nitrogen atmosphere from 50 to 800 °C.

Results

Almost all of used amount of silane remained chemically bounded on the surface of silica particles, forming a layer around them, which have dense accumulation of methacrylate groups. No significant statistic difference was found to exist between the degree of conversion values of composites with different silane contents. The composite with the lowest amount of UDMS (1.0 wt%) showed the lower flexural strength value, the higher static and dynamic elastic modulus values and the higher sorbed liquid value and solubility.

Significance

The optimum concentration of UDMS seems to be that of 2.5 wt%. Higher concentrations of UDMS did not improve the properties of composites.     

Improved polymerization efficiency of methacrylate-based cements containing an iodonium salt

Objective

This study evaluated the effect of adding diphenyliodonium hexafluorphosphate (DPI) as a third component of the free-radical photoinitiator system of model resin cements on their photopolymerization kinetics/stress and fundamental properties.

Methods

A model resin cement containing a 1:1 mass ratio of 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane and triethyleneglycol dimethacrylate was obtained. Camphorquinone (1 mol%) and 2-(dimethylamino)ethyl methacrylate (2 mol%) were added to monomer blend. Six mixtures were obtained by incorporation of 0 (control), 0.25, 0.5, 1, 2, or 4 mol% of DPI. The cements were loaded with a 60% mass fraction of silanated glass fillers. Polymerization kinetics (using Fourier-transform near-infrared spectroscopy), flexural strength and modulus, water uptake/solubility, and polymerization stress were assessed. Data were statistically analyzed using one-way ANOVA and Student–Newman–Keuls’ test (P ≤ 0.05).

Results

In the DPI-modified materials, the onset of autodeceleration occurred earlier and at higher conversion compared with the control cement. The addition of DPI also led to a more active early-stage polymerization. The flexural modulus was generally higher for DPI-containing materials. Water uptake and solubility were generally lower for materials with DPI fractions ≥0.5%. Significant increase of polymerization stress was observed only in the group with 1 mol% of DPI compared with the control cement.

Significance

The effect of DPI on the photopolymerization reactivity of the cement is concentration-dependent; the use of DPI could result in better polymerization efficiency of resin-based cements.     

Role of filler and functional group conversion in the evolution of properties in polymeric dental restoratives

Objectives

To examine effects of shrinkage and modulus on the dynamic development of shrinkage stress as a function of methacrylate conversion and filler loading in a model photocurable dimethacrylate-based resin with a silanized barium glass filler.

Methods

BisGMA/TEGDMA samples with filler loading levels of 0–70 wt% were evaluated. Irradiation times and intensities were varied to achieve a wide range of conversion. Shrinkage stress measurements were accompanied with real-time conversion monitoring, while shrinkage and modulus measurements were made at different static conversion points.

Results

Shrinkage increased nearly linearly with respect to conversion, while for a given value of conversion, it decreased proportionally with increasing filler content. Modulus advanced in an exponential fashion with conversion and also increased incrementally with filler content; however, modulus values rose disproportionately rapidly for the highest filler loading. At either high or low filler loading levels, stress at limiting conversion, which was inversely proportional to the filler load, was high while at an intermediate filler content, a minimum in stress was observed due to the combined effects of filler based shrinkage reduction, restricted limiting conversion and only moderately enhanced modulus. The level of polymerization stress predicted from the conversion-indexed shrinkage and modulus measurements over-estimated the experimental stress states as modulus evolved due to system compliance that to some degree mimics the clinical situation presented by photocuring bonded composite restorations.Significance Measurement of monomer conversion provides a common basis by which different material properties can be rationally compared.     

Effect of silanized nanosilica addition on remineralizing and mechanical properties of experimental composite materials with amorphous calcium phosphate

Objectives

Experimental composite resins with amorphous calcium phosphate (ACP) have the potential to regenerate demineralized tooth structures. The aim of the study was to investigate the effect of the addition of silanized silica nanofillers to the ACP-based composites on their mechanical properties and the kinetics of calcium and phosphate release.

Materials and methods

The test materials comprised 5 wt% (5-ACP) or 10 wt% (10-ACP) of silanized silica admixed to the 40 wt% ACP and 50 or 55 wt% resin. The ACP control (0-ACP) contained 40 wt% ACP and 60 wt% resin. Additionally, composite material CeramX (Dentsply, Germany) was included as control. Three-point bending test was performed to calculate flexural strength and modulus of elasticity. Inductively coupled plasma atomic emission spectroscopy was used for measurement of ion release. The micromorphology of calcium phosphate depositions on composite samples has been qualitatively evaluated using a scanning electron microscope. The results were analyzed using Mann–Whitney and Wilcoxon rank sum tests (α?<?0.05).

Results

Ion release was enhanced by the silica fillers, when compared to the 0-ACP. Although not statistically significant, flexural strength of 10-ACP was improved by 46 % compared to 0-ACP. Flexural modulus of 5-ACP was significantly higher than 0-ACP.

Conclusions

The admixture of silanized fillers seems to be a promising approach for the improvement of mechanical and remineralizing properties of ACP composite resins.

Clinical relevance

ACP-based composite resins with modified composition could serve as an effective remineralizing aid as base materials in restorative dental medicine.     

Reinforcement of denture base resin with short vegetable fiber

Objectives

Short ramie fibers were selected to investigate the effect of fiber length and volume fraction on the flexural properties of ramie fiber reinforced denture base PMMA. With the aid of measured interfacial shear strength and theoretical prediction values, experimental results were well interpreted.

Methods

Interfacial properties between denture base PMMA and ramie fibers were evaluated by single fiber pull-out test. Then, chopped ramie fibers were pre-stirred with PMMA powder by a mechanical blender and then mixed with MMA liquid to fabricate composites. Two crucial influencing factors, fiber volume fraction and fiber length, were studied to clarify their effects on flexural properties of composites.

Results

With 1.5 mm fibers addition, flexural modulus of denture base PMMA rose from 2.50 to 3.46 GPa with 10 vol.% fibers, while flexural strength declined steadily with increment of fiber content. If fiber length was 3.0 mm, the modulus showed a growth to 3.5 GPa at 4 vol.% fiber content followed by a drop to 3.00 GPa at 10 vol.%, whereas fluctuation in strength was experienced. Experimental results were discussed by comparison with two theoretical models.

Significance

Short ramie fiber reinforced denture base PMMA had higher flexural modulus than neat resin, while strength was lowered due to the weak interfacial adhesion. The potential of vegetable fibers as reinforcing agents for denture base should be further investigated by strengthening the interface between cellulose and denture base PMMA.     

Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers

ObjectivesThe aim of this study was to examine the influence of the addition of glass fillers with different sizes and degrees of silanization percentages to remineralizing composite materials based on amorphous calcium phosphate (ACP).MethodsFour different materials were tested in this study. Three ACP based materials: 0-ACP (40 wt% ACP, 60 wt% resin), Ba-ACP (40 wt% ACP, 50 wt% resin, 10 wt% barium-glass) and Sr-ACP (40 wt% ACP, 50 wt% resin, 10 wt% strontium-glass) were compared to the control material, resin modified glass ionomer (Fuji II LC capsule, GC, Japan). The fillers and composites were characterized using scanning electron microscopy. Flexural strength and modulus were determined using a three-point bending test. Calcium and phosphate ion release from ACP based composites was measured using inductively coupled plasma atomic emission spectroscopy.ResultsThe addition of barium-glass fillers (35.4 (29.1–42.1) MPa) (median (25–75%)) had improved the flexural strength in comparison to the 0-ACP (24.8 (20.8–36.9) MPa) and glass ionomer control (33.1 (29.7–36.2) MPa). The admixture of strontium-glass (20.3 (19.5–22.2) MPa) did not have any effect on flexural strength, but significantly improved its flexural modulus (6.4 (4.8–6.9) GPa) in comparison to 0-ACP (3.9 (3.4–4.1) GPa) and Ba-ACP (4.6 (4.2–6.9) GPa). Ion release kinetics was not affected by the addition of inert fillers to the ACP composites.SignificanceIncorporation of barium-glass fillers to the composition of ACP composites contributed to the improvement of flexural strength and modulus, with no adverse influence on ion release profiles.     

A urethane-based multimethacrylate mixture and its use in dental composites with combined high-performance properties

Objectives

To synthesize and characterize different molar weight urethane multimethacrylates with a single stage (one-pot) procedure. To prepare and characterize the properties of related composites.

Methods

Two methacrylate precursors were initially synthesized. Then, these precursors and the multimethacrylate system formed by their coupling were characterized by FTIR and ¹H NMR. The final product was used as a matrix (with TEGDMA and SiO₂ silanized microparticles) in the preparation of composites and their physical and mechanical properties were compared to those of a bis-GMA-based resin. Water sorption and solubility measurements of the composites were also performed.

Results

FTIR and NMR suggested that the proposed synthesis route yields a mixture of mainly urethane-di, -tri, and tetramethacrylates. The composites presented low polymerization shrinkage (e.g. 1.88 ± 0.08% for a resin with 70% of SiO₂) and high flexural strength (e.g. 124.74 ± 9.68 MPa for a resin with 65% of SiO₂) when compared to the bis-GMA based resin and other composites found to date. Water sorption and solubility results show that the composites were deemed compliant with ISO 4049 requirements.

Significance

The mixture containing different molar weight of urethane multimethacrylates showed to be an excellent substitute for bis-GMA, achieving an equilibrium of properties (unlike reports elsewhere which show the enhancement of some parameters in detriment to others) and composites with low polymerization shrinkage, suitable microhardness and degree of conversion, and up to standard water sorption/solubility and flexural strength.     

Effect of rigid rod polymer filler on mechanical properties of poly-methyl methacrylate denture base material.

OBJECTIVES: The aim of this study was to evaluate the mechanical properties of denture base material with rigid rod polymer (RRP) particulate fillers. METHODS: Specimens were fabricated from autopolymerized polymethylmethacrylate denture base resin (Palapress Heraus-Kulzer) and RRP particles were used as fillers (Parmax Mississippi Polymer Technologies, Inc.). Five groups were tested: 0 wt% RRP, 10 wt% RRP, 20 wt% RRP, 30 wt% RRP, and 100 wt% RRP. Specimens were stored dry at room temperature for 2 days or in water at 37 degrees C for 44 days before testing until failure at a three point bending test (ISO 1567) for measuring flexural properties. The surface microhardness, water sorption, and solubility were also measured. Existence of interpenetrating polymer network (IPN) between filler and denture resin was examined using solvent treatment and scanning electron microscopy (SEM). RESULTS: Specimens with RRP filler revealed higher flexural modulus, but the flexural strength decreased. Specimens with 30% RRP filler showed flexural strength of 67.4 MPa, whereas specimens without fillers gave strength of 93.9 MPa. The 100% RRP group revealed the highest flexural strength (305 MPa). Flexural strength of water-stored test specimens decreased in most groups when compared to dry specimens. Microhardness increased as a function of RRP filler. SEM micrographs revealed no IPN-network on the surface of RRP fillers. Addition of RRP fillers decreased the water sorption, whereas solubility was not affected. SIGNIFICANCE: This study revealed that although RRP polymer has good mechanical properties, addition of RRP to denture base resin as fillers did not increase mechanical properties. This was explained by lack of IPN-formation between RRP fillers and polymer matrix.     

In vitro investigation of coupling-agent-free dental restorative composite based on nano-porous alumina fillers

Objectives

The study aims at demonstrating the feasibility of a novel type of coupling-agent-free resin composite based on nano-porous fillers.

Methods

The fillers were obtained by ball-milling anodic alumina membranes. Composites were prepared with standard resin at maximum loading of 50% by weight. The resin matrix penetration into the pores was verified visually by scanning electron microscopy and mechanically by atomic force microscopy in force modulation mode. The dynamic flexural modulus at 1 Hz was measured by dynamic mechanical analysis. Silver nanoparticles were also synthesized in the pores and their release was investigated with inductive coupled plasma optical emission spectrometry.

Results

A storage modulus of 5 GPa was measured, similar to the ∼6 GPa ones of two coupling-agent-based dental restorative composites used for comparison, which is a promising starting point, additionally showing better one-year equivalent ageing as compared to both commercial materials. Loading the pores with silver nanoparticles was demonstrated as well as their subsequent release in a model system.

Significance

The alumina micro-particles with interconnected nano-pores allow mechanical interlocking between fillers and matrix without the need for chemical bonding. This material is also promising for being made bio-active, after pore filling with different agents.     

The effect of custom adaptation and span-diameter ratio on the flexural properties of fiber-reinforced composite posts

Objectives

To evaluate whether custom modification resulting in an anatomically shaped post and whether the span/diameter ratio (L/D) would affect the mechanical properties of fiber-reinforced composite posts.

Methods

Preformed glass-fiber posts (Group 1) and modified glass-fiber posts (Group 2) and glass-fiber rods (Groups 3 and 4) (n = 20) were loaded to failure in a three-point bending test to determine the maximum load (N), flexural strength (MPa) and flexural modulus (GPa). The span distance tested for Group 3 was 10.0 mm, while for Group 4 was 22.0 mm. Data were subjected to different statistical analysis with significance levels of P < 0.05.

Results

The maximum load recorded for Groups 1 and 2 was 72.5 ± 5.9 N and 73.4 ± 6.4 N respectively, while for Groups 3 and 4 was 215.3 ± 7 N and 156.6 ± 3.6 N respectively. The flexural strength for Groups 1 and 2 was 914.6 ± 53.1 MPa and 1069.2 ± 115.6 MPa, while for Groups 3 and 4 was 685.4 ± 22.2 MPa and 899.6 ± 46.1 MPa. The flexural modulus recorded for Groups 1 and 2 was 32.6 ± 3.2 GPa and 33.4 ± 2.2 GPa respectively, while for Groups 3 and 4 was 13.7 ± 0.3 GPa and 34.4 ± 0.3 GPa respectively.

Conclusions

The flexural properties of an anatomically custom modified fiber post were not affected by the modification procedure and the span-diameter ratio is an important parameter for the interpretation of flexural strength and flexural modulus values.     

The effect of high fiber fraction on some mechanical properties of unidirectional glass fiber-reinforced composite

Objectives

This study was designed to evaluate the effect of an increase of fiber-density on some mechanical properties of higher volume fiber-reinforced composite (FRC).

Methods

Five groups of FRC with increased fiber-density were fabricated and two additional groups were prepared by adding silanated barium-silicate glass fillers (0.7 μm) to the FRC. The unidirectional E-glass fiber rovings were impregnated with light-polymerizable bisGMA-TEGDMA (50-50%) resin. The fibers were pulled through a cylindrical mold with an opening diameter of 4.2 mm, light cured for 40 s and post-cured at elevated temperature. The cylindrical specimens (n = 12) were conditioned at room temperature for 2 days before testing with the three-point bending test (Lloyd Instruments Ltd.) adapted to ISO 10477. Fiber-density was analyzed by combustion and gravimetric analyzes.

Results

ANOVA analysis revealed that by increasing the vol.% fraction of E-glass fibers from 51.7% to 61.7% there was a change of 27% (p < 0.05) in the modulus of elasticity, 34% (p < 0.05) in the toughness, and 15% (p < 0.05) in the load bearing capacity, while there was only 8% (p < 0.05) increase in the flexural strength although it was statistically insignificant. The addition of particulate fillers did not improve the mechanical properties.

Significance

This study showed that the properties of FRC could be improved by increasing fibervolume fraction. Modulus of elasticity, toughness, and load bearing capacity seem to follow the law of ratio of quantity of fibers and volume of the polymer matrix more precisely than flexural strength when high fiber-density is used.     

Crystallization of high-strength nano-scale leucite glass-ceramics

Objectives

Fine-grained, high strength, translucent leucite dental glass-ceramics are synthesized via controlled crystallization of finely milled glass powders. The objectives of this study were to utilize high speed planetary milling of an aluminosilicate glass for controlled surface crystallization of nano-scale leucite glass-ceramics and to test the biaxial flexural strength.

Methods

An aluminosilicate glass was synthesized, attritor or planetary milled and heat-treated. Glasses and glass-ceramics were characterized using particle size analysis, X-ray diffraction and scanning electron microscopy. Experimental (fine and nanoscale) and commercial (Ceramco-3, IPS Empress Esthetic) leucite glass-ceramics were tested using the biaxial flexural strength (BFS) test. Gaussian and Weibull statistics were applied.

Results

Experimental planetary milled glass-ceramics showed an increased leucite crystal number and nano-scale median crystal sizes (0.048–0.055 μm²) as a result of glass particle size reduction and heat treatments. Experimental materials had significantly (p < 0.05) higher mean BFS and characteristic strength values than the commercial materials. Attritor milled and planetary milled (2 h) materials showed no significant (p > 0.05) strength difference. All other groups’ mean BFS and characteristic strengths were found to be significantly different (p < 0.05) to each other. The mean (SD) MPa strengths measured were: Attritor milled: 252.4 (38.7), Planetary milled: 225.4 (41.8) [4 h milling] 255.0 (35.0) [2 h milling], Ceramco-3: 75.7 (6.8) and IPS Empress: 165.5 (30.6).

Significance

Planetary milling enabled synthesis of nano-scale leucite glass-ceramics with high flexural strength. These materials may help to reduce problems associated with brittle fracture of all-ceramic restorations and give reduced enamel wear.     

The effect of clinically relevant thermocycling on the flexural properties of endodontic post materials

Objectives

It is suggested that fibre-reinforced composite (FRC) posts have lower elastic moduli than metal posts and this will reduce the incidence of root fracture. However, the mechanical properties may be altered in the oral environment. The aims of this study were to determine the effect on the flexural properties of FRC and metal post materials produced by: (1) a thermocycling regime which was clinically relevant and representative of that which would occur during 1 year in the mouth and (2) storage for 1 year at body temperature.

Methods

Nine FRC and two metal post material samples were sealed in polythene sleeves and thermocycled between 10 °C and 50 °C for 10,000 cycles. Additional samples were stored dry at 37 °C for 1 year. The flexural strength and moduli were determined by three-point bending and compared with untreated control samples.

Results

Thermocycling and storage at 37 °C for 1 year decreased the mean flexural modulus of all materials. This was statistically significant for 8 of 11 materials after thermocycling, and 4 of 11 materials after storage at 37 °C (p < 0.05). Thermocycling and storage at 37 °C produced a non-significant increase in yield strength for both metal post materials. Thermocycling significantly increased the flexural strength of Postec while it decreased for the other FRC materials. Storage at 37 °C increased the flexural strength of three FRC materials (significantly for Postec) while it was decreased among the other materials.

Conclusions

Although some of the changes noticed in flexural properties were statistically significant, it is doubtful that they are of sufficient magnitude to affect clinical performance.     

Fracture toughness and cyclic fatigue resistance of resin composites with different filler size distributions

Objectives

To verify the influence of filler size distributions on fracture toughness (K_Ic), initial fracture strength (IFS) and cyclic fatigue resistance (CFR) of experimental resin composites.

Methods

Four composites were prepared with same inorganic content (78 wt%), in which 67 wt% was constituted by glass particles with d₅₀ of 0.5; 0.9; 1.2; 1.9 μm K_Ic of the composites was determined by the single-edge notched beam (SENB) method. To evaluate the IFS and the CFR a biaxial bending test configuration was used. The CFR was determined under cyclic loading for 10⁵ cycles using the ‘staircase’ approach. The fracture surfaces of IFS and CFR specimens were analyzed under scanning electron microscope (SEM).

Results

There was a positive linear correlation between d₅₀ vs. K_Ic and statistical difference was found only between C0.5 (1.24 ± 0.10 MPa m^0.5) and C1.9 (1.41 ± 0.17 MPa m^0.5). There were no statistical differences among IFS means, which ranged from 155.4 ± 18.8 MPa (C0.9) to 170.7 ± 23.1 MPa (C1.2). C0.5 (93.0 ± 18.6^a MPa) showed the highest and C0.9 the lowest CFR (82.5 ± 8.0^c MPa). There was no correlation between CFR with d₅₀ values or with K_Ic means. SEM images showed the morphology with brittle fracture patterns for the surfaces of IFS specimens and a more smooth fracture surface for CFR specimens.

Significance

Resin composites showed different failure mechanisms for quasi-static and fatigue loading. For K_Ic and IFS, composites with larger filler size distributions showed better results due to crack deflection; while under cyclic loading, viscous behavior was predominant and composites with smaller particles showed higher fatigue resistance.     

Surface structure and mechanical properties of impaction-modified Y-TZP

Objectives

The objectives of the study were to describe the surface structure and the chemical surface composition of Y-TZP ceramics produced by using the modified additive technique and to evaluate the flexural strength of Y-TZP with or without surface modification and with different pretreatments: etching before or after sintering combined with or without an adhesive cement system.

Methods

Y-TZP discs were used for surface analysis (n = 48) and for biaxial flexural strength testing (n = 200). The specimens were divided into groups depending on the cementation surface of Y-TZP: unmodified, sandblasted or glass-modified Y-TZP surfaces, and according to the production process: etching before or after sintering.

Results

The surface structure and the chemical composition of glass-modified Y-TZP differ; a rougher surface and phase transformation was identified compared to unmodified Y-TZP.The unmodified Y-TZP groups showed significantly higher flexural strength compared to the glass-modified groups (p < 0.001) and showed increased flexural strength after sandblasting (p < 0.001). Furthermore, by adding cement to the surface, the value increased even further in comparison with the sandblasted non-cemented specimens (p < 0.01). After thermocycling, however, the cement layer on the unmodified and the sandblasted surfaces had air pockets and regions with loose cement.

Significance

A rougher surface structure, superficial glass remnants and a higher content of m-phase was present in the cementation surface of glass-modified Y-TZP. The glass modification creates a bondable cementation surface that is durable. By etching the glass-modified Y-TZP before sintering, a more homogenous surface is created compared to one that is etched after sintering.