Viscoelastic properties, creep behavior and degree of conversion of bulk fill composite resins.

OBJECTIVES: The aim of this study was to investigate the viscoelastic properties and creep behavior of bulk fill composites under different conditions and evaluate their degree of conversion. METHODS: Seven bulk fill composites were examined: everX Posterior (EV), SDR (SD), SonicFill (SF), Tetric EvoCeram Bulk Fill (TE), Venus Bulk Fill (VE), x-tra base (XB) and x-tra fil (XF). Each material was tested at 21°C, 37°C and 50°C under dry and wet conditions by applying a constant torque for static and creep testing and dynamic torsional loading for dynamic testing. Degree of conversion (%DC) was measured on the top and bottom surfaces of composites with ATR-FTIR spectroscopy. Statistical analysis was performed with two-way ANOVA, Bonferroni's post hoc test and Pearson's correlation coefficient. RESULTS: Shear modulus G ranged from 2.17GPa (VE) to 8.03GPa (XF) and flexural modulus E from 6.16GPa (VE) to 23GPa (XF) when the materials were tested dry at 21°C. The increase of temperature and the presence of water lead to a decline of these properties. Flowable materials used as base composites in restorations showed significantly lower values (p<0.05) than non-base composites, while being more prone to creep deformation. %DC ranged from 47.25% (XF) to 66.67% (SD) at the top material surface and 36.06% (XF) to 63.20% (SD) at the bottom. SIGNIFICANCE: Bulk fill composites exhibited significant differences between them with base flowable materials showing in most cases inferior mechanical properties and higher degree of conversion than restorative bulk fill materials.

Mechanical viscoelastic behavior of dental adhesives.

OBJECTIVES: The purpose of the study was to evaluate the mechanical properties of dental adhesive materials at different testing temperatures after dry and wet storage. METHODS: Specimens (d=1 mm, l=18 mm) from six materials were tested: Silorane Adhesive System (SL), Heliobond (HE), One-Step Plus (OS), Optibond Solo Plus (OP), cmf Adhesive System (CF) and Protobond (PR). Static and creep testing was performed by applying a constant torque below the proportional limit of the materials, while dynamic testing consisted of dynamic torsional loading. Experiments were performed after 24h of dry and wet storage under temperatures from 21°C to 50°C and various viscoelastic parameters were calculated. RESULTS: Shear modulus ranged from 0.19 to 1.99 GPa, while flexural modulus from 0.67 to 5.69 GPa. Most of the materials were affected by the presence of water and increase of temperature. OP showed the highest recovery after creep, while SL exhibited the highest permanent deformation. SIGNIFICANCE: Contact with water after polymerization and increase of temperature resulted in a decline of the mechanical properties, especially for the HEMA-containing adhesives.

Dynamic and static mechanical analysis of resin luting cements.

Various types of indirect restorations are available for dental treatment and resin cements are commonly used as a luting medium. The aim of this study was to evaluate the mechanical properties of contemporary resin luting agents under different testing conditions and temperatures. The materials tested were Choice 2 (CH), Clearfil Esthetic Cement (EC), Resicem (RC) and RelyX Unicem (RX). Each material was examined after 24 h of storage at 21 °C dry and wet at 21, 37 and 50 °C under dynamic and static testing and parameters such as shear and flexural modulus, loss tangent, dynamic viscosity and Poisson's ratio were calculated. The resin cements were also subjected to creep testing under different constant loads for 3 h and a recovery time of 50 h. The material with the highest modulus was CH, while RX had the lowest. All resin cements were affected by the presence of water with RX being the least affected and by the increase of temperature, with RC being the least susceptible. None of the materials exhibited full recovery after creep testing and permanent deformation ranged from 0.43% to 5.53%. The resin cements tested in this study showed no major transitions under the different testing conditions. Their behavior was satisfactory for restorations that do not require increased mechanical properties. However, in the case of stress-bearing restorations the conditions in the oral cavity may affect the performance of these materials.

Creep and dynamic viscoelastic behavior of endodontic fiber-reinforced composite posts.

PURPOSE: Fiber-reinforced composite (FRC) posts have gained much interest recently and understanding of their viscoelastic properties is important as they can be used in stress-bearing posterior restorations. The aim of this study was to evaluate the creep behavior and the viscoelastic properties of four commercial FRC posts under different temperatures and different storage conditions. METHODS: The FRC posts tested were Glassix, C-Post, Carbonite and Snowlight. For the creep measurements a constant load below the proportional limit of the posts was applied and the angular deformation of the specimens was recorded. The viscoelastic parameters were determined by using dynamic torsional loading under four different conditions. RESULTS: All materials were susceptible to creep and exhibited linear viscoelastic behavior. Residual strain was observed in all FRC posts. The viscoelastic properties were affected by the increase of temperature and water storage (p<0.001) resulting in their decline. Carbon fiber posts exhibited better performance than glass fiber posts. CONCLUSIONS: FRC posts exhibit permanent strains under regular masticatory stresses that can be generated in the oral cavity. Their properties are susceptible to changes in temperature, while direct contact with water also affects them deleteriously.

The effect of temperature on the viscoelastic properties of nano-hybrid composites.

OBJECTIVES: The purpose of this study was to examine the viscoelastic properties of nanofilled dental composites under both static and dynamic testing and to determine the influence of temperature, medium of storage and storage time. METHODS: Three nanofilled composites, one packable and one ormocer were tested. The specimens were examined dry at 21 degrees C and wet at 21, 37 and 50 degrees C after being stored for 24h and 1 month under both static and dynamic testing. Shear modulus, elastic modulus, loss tangent, Poisson's ratio and other viscoelastic parameters were calculated. Data were analyzed with one-way and two-way analysis of variance (ANOVA) (p=0.05). RESULTS: All materials tested showed a significant decrease in their moduli with the increase of temperature, while the effect of water storage was different among the composites. Grandio was the composite with the highest Young's modulus followed by Filtek P60. SIGNIFICANCE: Most of the materials tested did not have elastic moduli near to that of dentin, making them less satisfactory in posterior restorations. The materials possessing nanosized filler particles had different elastic properties among them and this implies that filler size is not the only factor that affects the elastic behavior of dental composites.

The effect of temperature on viscoelastic properties of glass ionomer cements and compomers.

The objective of this study was to determine the viscoelastic properties of different types of glass ionomer cements (GICs) and compomers under varying temperature conditions found in the mouth. The materials tested were a conventional GIC (Aqua Ionofil U), a resin modified GIC (Fuji II LC), a highly viscous GIC (Voco Ionofil Molar), and two polyacid modified composite resins/compomers (Glasiosite and Dyract Flow). Six groups of four specimens were prepared from each material. One group was stored dry for 24 h and was subsequently tested dry at 21 degrees C. Each of the remaining five groups was stored for 24 h in distilled water at the temperatures 21, 30.5, 37, 43.5, and 50 degrees C, respectively, and was subsequently tested at that temperature. Shear storage modulus and loss tangent were determined by conducting dynamic torsional loading. Static shear moduli were determined by applying a constant torque (below the proportional limit of the materials) for 10 s and recording the angular deformation of the specimens. Data were analyzed by ANOVA and Duncan's test (alpha= 0.05). It was found that the viscoelastic properties varied significantly (p < 0.05) across the different materials. The compomer Glasiosite, with the highest filler content, and the highly viscous GIC Voco Ionofil Molar exhibited the highest elastic moduli and lowest loss tangents. Viscoelastic properties varied also significantly (p < 0.05) with temperature levels, but changes in the tested region were not indicative of a glass transition. Dynamic shear storage moduli were highly correlated to the static ones. Storage in water lowered the values of elastic moduli.

Fatigue of packable dental composites.

OBJECTIVE: The purpose of the study was to measure the fatigue properties of four dental resin composites using a dynamic mechanical analysis and to relate the results with viscoelastic properties. METHODS: Dynamic torsional loading was conducted at resonance at 30-50Hz. Specimens were thoroughly cured and tested dry at 21 degrees C. RESULTS: All of the specimens showed a loss of strength following repeated stress, due to material fatigue. The material with the highest shear modulus had the lowest damping and the highest fatigue strength. SIGNIFICANCE: Dental composites exhibit a modest loss of strength due to fatigue. Since mastication involves many cycles of stress during the life of a restoration, fatigue properties should be taken into account in restoration design.

Dynamic and static elastic moduli of packable and flowable composite resins and their development after initial photo curing.

OBJECTIVE: The purpose of this study was to evaluate the dynamic (storage) shear modulus and the static shear modulus of elasticity of packable and flowable composite resins and to investigate their development after initial photo-curing. METHODS: Three pairs of a packable versus a flowable composite and a microfill composite resin were tested (Alert/Flow It, Filtek P60/Filtek Flow, Admira/Admira Flow, A 110). Cylindrical specimens (0.85 mm x 18 mm) were made for each material. All specimens were conditioned and tested dry at 21 degrees C. The specimens were tested at 30 min, 24h and 1 week after the end of photo curing. Storage shear modulus and loss tangent were determined by conducting dynamic torsional loading in the frequency range from 1 to 150 Hz. Static shear modulus measurements were made by applying a constant load (below the proportional limit of the materials) for 10s and recording the angular deformation of the specimens. Data were analyzed by ANOVA and Duncan's Post hoc test (alpha=0.05). RESULTS: Storage shear moduli (at 1 week measurement) ranged from 3.39 to 9.67 GPa, and loss tangents from 0.0735 to 0.0235; static shear moduli ranged between 2.66 and 9.80 GPa. High values of elastic moduli and low tandelta values were obtained with packable composites, while low moduli values were obtained with flowable composites. Statistically significant (alpha=0.05) differences were recorded between materials of the same category. Storage time, 24h and 1 week after initial polymerization, resulted in significant increases in both moduli of elasticity. Dynamic shear storage moduli were highly correlated to the static ones (r(2)=0.92; P<0.05). SIGNIFICANCE: The results of the aging studies showed that the rigidity of these materials increases significantly even 1 week after the clinician turns off the curing unit.

The effect of pulse activation on microleakage of a 'packable' composite resin and two 'ormocers'.

The purpose of the study was to examine the effect of pulse polymerization on microleakage of one packable composite resin and two organically modified ceramics (ormocers), within a high C-factor preparation. Class-V cavity preparations with occlusal margins in enamel and gingival margins in dentin were prepared on 60 freshly extracted premolars or molars. Teeth were randomly assigned to 12 experimental groups (five teeth each) representing the different restorative systems, mode of polymerization and thermal cycling test. Three groups of 20 teeth each were restored with one of the following restorative systems: Scotch bond 1/Filtek P 60, Admira bond/Admira, Etch&Prime 3.0/Definite. 10 restorations of each restorative system (two experimental groups) were conventionally polymerized, whereas the other 10 restorations were polymerized with a pulse program. The specimens were stored in double-distilled water at 37 degrees C for 24 h. Ten restorations from each restorative system (30 restorations, half of the specimens) were thermocycled, the other 30 restorations were kept in double-distilled water at 37 degrees C for 24 h. Then all 60 restorations stained with dye, sectioned and scored for microleakage. Results showed that there were no significant differences (P > 0.05) among the materials with respect to microleakage scores. The experimental groups that were conventionally polymerized revealed significantly more microleakage (P < 0.05) after thermal cycling than all the other groups. The gingival microleakage was significantly more (P < 0.05) than occlusal in conventionally polymerized, thermocycled experimental groups.

Dynamic mechanical analysis of viscoelastic functions in packable composite resins measured by torsional resonance.

The objective of this study was to evaluate the viscoelastic functions of packable composite resins with the use of a resonant dynamic mechanical analysis technique in torsion. The materials tested were: Alert (Jeneric Pentron), Prodigy Condensable (Kerr Corporation), Surefil (Dentsply DeTrey), and Filtek P60 (3M Dental Products). Dynamic torsional loading was conducted in the frequency range from 1 to 150 Hz. Composite specimens were tested after storage in water at 37 degrees C for 24 h. One group was thermal cycled for 3000 cycles with temperatures of 5-37-50 degrees C. Measurements were taken at 21 degrees C dry, and at 37 and 50 degrees C wet. Storage modulus, loss tangent, and other viscoelastic parameters were determined from the amplitude/frequency curves. Data for storage modulus and loss tangent of the materials were analyzed by means of ANOVA and Student-Newman-Keuls test (alpha = 0.05). It was found that there were significant differences (P < 0.001) in storage modulus and loss tangent among the packable composites tested. The highest value of storage modulus, in measurements at 21 degrees C, was for Alert (10.3 GPa), followed by Filtek P60 (9.31 GPa), Surefil (7.29 GPa), and Prodigy Condensable (6.74 GPa). There were significant differences (P < 0.001) in storage modulus and loss tangent among the four different conditions tested. Storage modulus decreased at higher temperatures, whereas the loss tangent increased. Thermal cycling increased storage modulus and decreased loss tangent. The results showed that both monomer and filler composition and filler loading of the materials significantly affect their viscoelastic functions, and the mechanical properties of the products cannot be characterized from the packability alone.

Dynamic viscoelastic behavior of resin cements measured by torsional resonance.

OBJECTIVE: The purpose of the study was to measure the viscoelastic properties of four dental resin composite cements using a dynamic mechanical analysis technique. METHODS: Dynamic torsional loading was conducted in the frequency range from 1 to 80 Hz. Cement specimens were tested after storage in 37 degrees C water for 24 h. One group was thermal cycled prior to testing. Measurements were taken at 21, 37, and 50 degrees C. Storage modulus, loss tangent and other viscoelastic parameters were determined from the amplitude/frequency curves. RESULTS: Storage moduli of the cements ranged from 2.9 to 4.1 GPa at 37 degrees C. Loss tangents ranged from 0.054 to 0.084. Storage moduli decreased in a regular way with increasing temperature, whereas, loss tangents increased. Thermal cycling caused small decreases in storage moduli. SIGNIFICANCE: Resin cements with higher filler loading were found to have higher storage moduli and lower loss tangents. Since these properties have been associated with better clinical performance in the areas of retention and prevention of fracture of porcelain and resin restorations, the more highly filled cements may be recommended. Temperature variations influenced viscoelastic behavior of the cements. However, within the temperature range studied no sharp drop in modulus was seen, so the materials should function satisfactorily in the oral cavity.

In vitro cytotoxicity of six dentin bonding agents.

The cytotoxicity of six dentin bonding agents (Syntac, Solobond, Bond 1, Scotchbond 1, Heliobond and F-2000) was tested against an established cell line, L929. Under aseptic conditions 3, 5 and 10 microL dentin bonding agents were placed in the centre of Petri dishes. Each dish was covered with a 5-mL suspension of fibroblasts at a concentration of 40 000 cells mL(-1). The cultures were incubated at 37 degrees C and cytotoxicity was assessed by a quantitative technique at 24 and 72 h. All the dentin bonding agents were found to be cytotoxic. Scotchbond 1 and F-2000 showed the highest cytotoxicity followed by Solobond and Bond 1. Heliobond and Syntac were the least toxic materials.

Fluoride release from restorative materials and a luting cement.

STATEMENT OF PROBLEM: In addition to conventional glass ionomers, a considerable number of different types of materials have been formulated to release fluoride. Variation in composition results in quantitative differences in the amount of fluoride release by these materials. PURPOSE: This study evaluated and compared fluoride release in distilled water from different types of restorative materials and a luting cement. MATERIAL AND METHODS: Fluoride release from 4 glass ionomer formulation restorative materials (Miracle-Mix, Fuji ionomer type III, Fuji II LC improved, and Ketac-Silver), a luting cement (Ketac Cem), a compomer (Compoglass Flow), 2 sealants (Fissurit F, Helioseal F), and a composite resin (Tetric) was evaluated at time intervals of 4, 8, 12, and 24 hours and 2, 3, 7, 14, 28, 56, and 112 days. Seven disks of each material were made and stored for equilibration in double distilled water at 37 degrees C for the time of each measurement. The equilibrated solution was analyzed for fluoride with a TISAB and an ion-specific combination electrode (ORION 960900) connected to an expandable ion analyzer (Crison micropH 2002). Data were analyzed by means of univariate analysis of variance, the Dunnett C post hoc test, and repeated measures analysis. RESULTS: Fluoride was released from all the evaluated materials, with considerable variation in the rate of release but a similar pattern. Among the materials tested, fluoride release from glass ionomer formulations was greater than that from composite resin formulations; the rank of decreasing order was as follows: Miracle Mix > Fuji III, Ketac Cem > Fuji II LC > Ketac Silver, Compoglass F > Fissurit F, Helioseal F > Tetric (> indicates statistical significance; P< .05). CONCLUSION: Under the conditions of this study, glass ionomer formulations and the compomer released more fluoride than the sealants and the composite resin tested.

Fluoride release from three glass ionomers, a compomer, and a composite resin in water, artificial saliva, and lactic acid.

The amounts and the pattern of fluoride release from one metal-reinforced glass ionomer cement, two resin-modified glass ionomer cements, one compomer, and one composite resin placed in double-distilled water, artificial saliva, and lactic acid were evaluated in this study. Measurements of fluoride ion release were made for a total of 105 cylindrical specimens (10 mm in diameter and 1.5 mm in height). They were taken over a period of 16 weeks at the intervals of 4, 8, 12, and 24 hours, as well as 2, 3, 7, 14, 28, 56, and 112 days. The pattern of fluoride release was similar for all of the examined materials. The greatest amount of fluoride was released from the metal-reinforced glass ionomer Argion. The resin-modified glass ionomers Vitremer, Fuji II LC; the compomer Dyract; and the composite resin Tetric followed in ranking order. The pH of the environment strongly affected the fluoride release from the materials. There was a significant difference (P < 0.001) in the amounts of fluoride released in lactic acid vs water and artificial saliva, whereas, there was no significant difference (P > 0.05) in the amounts of fluoride released in water vs artificial saliva.

Microleakage of bonded amalgam restorations: effect of thermal cycling.

This study examined the effect of thermal cycling on the microleakage of bonded amalgam restorations. Three dental amalgam alloys and a gallium alloy were tested with two adhesive resin systems and copal varnish as a control. Class V cavity preparations were prepared on 168 freshly extracted premolars or molars. The preparations were placed parallel to and 1.0 mm occlusal to the cementoenamel junction (CEJ). Four groups of 42 teeth each were treated with one of the following adhesive dentin systems: Bond-It, All-Bond 2/Resinomer or a copal varnish (Copalite). The four groups of 42 teeth each were then restored with one of three dental amalgams: Orosphere Plus, Indiloy, Oralloy or a Gallium alloy (Galloy), resulting in 12 test groups of 14 teeth each. The specimens were stored in double distilled water at 37 degrees C for 24 hours. Final contouring and polishing of the restorations were performed under water spray. Half of the restorations in each group were thermocycled for 3000 cycles (5 degrees C-37 degrees C-55 degrees C-37 degrees C) with a dwell time of 15 sec at each temperature. The other half were stored in double distilled water at 37 degrees C for 24 hours. Then all 168 restorations were stained with dye, sectioned and scored for microleakage. Results showed that the adhesive dentin systems reduced microleakage in amalgam restorations compared to copal varnish only in non-thermocycled specimens. Statistical analysis of the results showed that there was an extremely significant difference (p < 0.001) in microleakage between the non-thermocycled and the thermocycled specimens in all test groups, whereas, there was no significant difference (p > 0.05) among thermocycled specimens. The reduction of microleakage was not significantly different between Bond-It and All-Bond 2/Resinomer in non-thermocycled specimens. Oralloy showed the most microleakage in the non-thermocycled groups when compared to the other alloys using the same adhesive liner.

Development of a computer-assisted learning software package on dental traumatology.

The development of computer-assisted learning software packages is a relatively new field of computer application. The progress made in personal computer technology toward more user-friendly operating systems has stimulated the academic community to develop computer-assisted learning for pre- and postgraduate students. The ability of computers to combine audio and visual data in an interactive form provides a powerful educational tool. The purpose of this study was to develop and evaluate a computer-assisted learning package on dental traumatology. This program contains background information on the diagnosis, classification, and management of dental injuries in both the permanent and the deciduous dentitions. It is structured into chapters according to the nature of the injury and whether injury has occurred in the primary or permanent dentition. At the end of each chapter there is a self-assessment questionnaire as well as references to relevant literature. Extensive use of pictures and video provides a comprehensive overview of the subject.

Artificial caries formation around fluoride-releasing restorations in roots.

Secondary caries is one of the most important factors leading to replacement of dental restorations. This investigation assessed the capacity of fluoride-releasing restorative materials to resist caries in vitro when used in roots. Class 5 cavities were prepared in the buccal and lingual surfaces of 30 extracted premolars. The six materials used were: glass-ionomer cement (Fuji), glass-ionomer cement with silver particles added (Ketac-silver), fluoride-containing composite resin (Tetric), composite resin (Silux plus), fluoride-containing amalgam (Fluor-Alloy) and high-copper amalgam (Dispersalloy). After 5 weeks in an acid gel for caries-like lesion formation, the teeth were sectioned longitudinally and examined with polarized light. The results showed that repair with glass-ionomer materials of a carious lesion may be of great importance in the prevention of secondary caries around the restorations in roots.

Viscoelasticity and aging of dental composites.

Effects of aging on several dental composites were studied using a torsion creep apparatus. A constant torque was applied from 1 second to 3 hours, and recovery was observed from 10 seconds to 2 days, for specimens aged from 3 hours to 8 weeks following polymerization. Specimens aged for shorter times exhibited more creep and less complete recovery than specimens aged for longer times. Modulus and creep results were qualitatively consistent with a composite model for particulate inclusions.

Artificial secondary caries around two new F-containing restoratives.

Replacement of restorations due to secondary caries is a continuing problem in restorative dentistry. This investigation evaluated the ability of two new light-cured fluoride-containing restorative materials to inhibit caries in vitro. Class 5 cavities were prepared in buccal and lingual surfaces of 20 extracted premolars. The occlusal cavosurface margin of each preparation was on enamel and the gingival cavosurface margin was on root surface. The four materials used were: glass-ionomer cement (Fuji II), composite resin (Silux Plus), light-cured glass ionomer (Vitremer), and compomer (Dyract). After 5 weeks in an acid gel for caries-like lesion formation, the teeth were sectioned longitudinally and examined with polarized light. The results showed that use of a light-cured glass ionomer and/or compomer may prevent both secondary caries around restorations and primary caries in surface enamel adjacent to the restorations.

Degree of microleakage of some pit and fissure sealants: an in vitro study.

The purpose of this study was to evaluate and compare the degree of microleakage exhibited by five resin sealants (Concise, Helioseal, Durafil. Fissurit, Sci-Pharm) and a glass-ionomer sealant (Fuji Glass lonomer Type III). The fissure systems of the occlusal surfaces of freshly extracted teeth were opened using a pear-shaped bur; the preparations were confined to enamel. For each material, 14 teeth were prepared. From each group, seven teeth were thermocycled through 5 degrees, 37 degrees, 60 degrees and 37 degrees for 2000 cycles, and the other seven teeth were not thermocycled. All teeth were immersed in 0.5% solution of basic fuchsin dye for 48 hours. Each tooth was sectioned longitudinally in the buccolingual direction and three 1 mm-thick sections (distal, central and proximal) were obtained from each tooth. Each section was cleaned, examined, and photographed in a stereomicroscope. All the materials presented microleakage in both the thermocycled and non-thermocycled specimens; the degree of microleakage was greater in the thermocycled specimens. The results support the practice of opening up questionably carious fissures and removing caries (if present) before sealing.