Dental Materials

Editorial Board

2012-06-01

Antimicrobial and physicochemical properties of experimental light curing composites with alkali-substituted calcium phosphate fillers

Wolf Herzlieb, Karolin M. Köhler, Andrea Ewald, Norbert Hofmann, Uwe Gbureck — 2012-06-01

Abstract: Objective: The antimicrobial and physicochemical properties of experimental light curing composites prepared with fillers made of mechanically activated alkali-substituted calcium phosphates like CaKPO4, CaNaPO4 or Ca2KNa(PO4)2 were compared with a commercial silane-modified cristobalite filler.Methods: The antimicrobial properties were tested using Streptococcus mutans, Staphylococcus aureus and a clinically isolated plaque mixture. The potential for reducing bacteria growth on modified composites was determined using the proliferation reagent WST-1, which enables the measurement of metabolic activity and therefore the colonization with living bacteria. Investigated material properties included the degree of conversion and a test of flexural strength.Results: All alkali-substituted composites provide a changed, mainly basic micro-milieu leading to a reduction of bacteria population with respect to the non-modified composite of about 25–70% with a flexural strength of cured composites in the range of 55–77MPa complying with the clinical standard and a degree of conversion of 44–66%.Significance: This study suggests that the modified composites increase antimicrobial properties while basic composite characteristics are not influenced by the filler.

Surface fluorination of zirconia: Adhesive bond strength comparison to commercial primers

Jeffrey R. Piascik, Edward J. Swift, Krista Braswell, Brian R. Stoner — 2012-06-01

Abstract: Objective: This study evaluated contact angle and shear bond strength of three commercial zirconia primers and compared them to a recently developed fluorination pre-treatment. Earlier investigations reported that plasma fluorinated zirconia modifies the chemical bonding structure creating a more reactive surface.Materials and methods: Yttria-stabilized zirconia (LAVA, 3M ESPE) plates were highly polished using 3μm diamond paste (Ra ∼200nm) prior to pretreatments. After primer and fluorination treatment, contact angles were measured to quantify surface hydrophobicity before and after ethanol clean. Additionally, simple shear bond tests were performed to measure the adhesion strength to a composite resin.Results: Plasma fluorination produced the lowest contact angle (7.8°) and the highest shear bond strength (37.3MPa) suggesting this pretreatment facilitates a more “chemically” active surface for adhesive bonding.Conclusions: It is hypothesized that plasma fluorination increase hydroxylation at the surface, making it more reactive, thus allowing for covalent bonding between zirconia surface and resin cement. A strong correlation was observed between contact angle and adhesion strength for all specimens; a relationship which may help understand the frequency and modes of failures, clinically. It is also believed that this surface treatment can increase long-term viability of zirconia restorations over other adhesive techniques.

Effect of resin-composite filler particle size and shape on shrinkage-stress

Julian D. Satterthwaite, Amit Maisuria, Karin Vogel, David C. Watts — 2012-06-01

Abstract: Objectives: The aim of this study was to investigate the effect of variations in filler particle size and shape on the polymerization shrinkage-stress kinetics of resin-composites.Methods: A model series of 12 VLC resin-composites were studied. The particulate dispersed phase volume fraction was 56.7%: these filler particles were systematically graded in size, and further were either spherical or irregular. A Bioman instrument (cantilever beam method) was employed to determine the shrinkage-stress kinetics following 40s irradiation (600mW/cm2) at 23°C (n=3). All data were captured for 60min and the final shrinkage-stress calculated.Results: Shrinkage-stress varied between 3.86MPa (SD 0.14) for S3 (spherical filler particles of 500nm) and 8.44MPa (SD 0.41) for I1 (irregular filler particles of 450nm). The shrinkage-stress values were generally lower for those composites with spherical filler particles than those with irregular filler particles. The differences in shrinkage-stress with filler particle size and shape were statistically significant (p<0.001).Significance: Composites with spherical filler particles exhibit lower shrinkage-stress values compared to those with irregular filler particles. Shrinkage-stress and shrinkage-stress rate vary in a complex manner with variations in the size of the dispersed phase particles: a hypothesized explanation for the effect of filler particle size and shape is presented.

Three years in vivo wear: Core-ceramic, veneers, and enamel antagonists

2012-06-01

Abstract: Objectives: Test the hypotheses that there are equivalent wear rates for enamel-versus-enamel and ceramic-versus-enamel, analyzing the in vivo wear of crown ceramics, their natural enamel antagonists, and the corresponding two contralateral teeth; and, that bite force does not correlate with the wear.Methods: A controlled, clinical trial was conducted involving patients needing full coverage crowns opposing enamel antagonists. Bite forces were measured using a bilateral gnathodynamometer. Single-unit restorations of metal/ceramic (Argedent 62, Argen Corp/IPS d.SIGN veneer); or, core-ceramic/veneer from either, Empress2/Eris, or e.max Press core/e.max Ceram glaze (ceramics: Ivoclar Vivadent, USA) were randomly assigned, fabricated and cemented. Impressions were made of the ceramic crowns, as well as each maxillary and mandibular quadrant at one week (baseline) and one, two and three years. Resulting models were scanned (3D laser scanner). Maximum wear was calculated by superimposing baseline with annual images.Results: There were a total of thirty-six crowns required for thirty-one patients. Each restoration had three associated enamel teeth: crown, (1) antagonist, (2) contralateral and (3) contralateral–antagonist. SAS PROC MIXED (α=0.05) indicated no statistical significance for mean maximum wear among crown ceramics, enamel antagonists and contralaterals. However, enamel wear was statistically significant in relation to intraoral location (p=0.04) and among years (p<0.02). Analyzed alone, the enamel contralateral–antagonist exhibited significantly greater wear (p<0.001). Considering all wear sites, there was no correlation with bite force (p=0.15).Significance: The ceramics and their antagonists exhibited in vivo wear rates within the range of normal enamel. Future studies should examine the wear implications of the contralateral–antagonist enamel.

Assessment of the quality of resin–dentin bonded interfaces: An AFM nano-indentation, μTBS and confocal ultramorphology study

Salvatore Sauro, Raquel Osorio, Timothy F. Watson, Manuel Toledano — 2012-06-01

Abstract: Objective: The aim of this study was to assess by using confocal microscopy (CLSM), AFM nano-indentation and microtensile bond strength test (μTBS) the quality of the resin–dentin interfaces created with selected bonding parameters.Methods: Dentin conditioned with H3PO4 or EDTA was bonded in ethanol- or water-wet condition using a HEMA-free or HEMA-containing adhesive. The resin-bonded teeth were stored in distilled water (24h) and sectioned as match-sticks (0.9mm2) for μTBS. Further resin-bonded teeth were sectioned and analyzed using CLSM, and AFM nano-indentation. The AFM imaging and nano-indentation processes were undertaken using a Berkovich diamond indenter. The modulus of elasticity (Ei) and hardness (Hi) across the interface were evaluated with the specimens in a fully hydrated status. The AFM imaging was performed both in dry and wet conditions for evaluating the shrinkage of the hybrid layer on dehydration.Results: The HEMA-containing adhesive applied onto H3PO4-etched ethanol or water-wet dentin created hybrid layers with the lowest biomechanical nano-properties (p<0.05); no significant differences in μTBS were found between the two wet-bonding techniques (p>0.05). However, the ethanol-wet bonding reduced the dye penetration into the adhesive layer created with the HEMA-containing adhesive. Hybrid layers with high biomechanical properties, low micropermeability and no shrinkage were only possible when using HEMA-free adhesive applied in ethanol wet-dentin. In particular, a significant increase in Ei and Hi was achieved at the hybrid layer and underneath the resin–dentin interface of ethanol-wet EDTA-treated dentin.Significance: The use of HEMA-free adhesives applied onto ethanol-wet dentin may be considered as an alternative and suitable bonding strategy to achieve high quality resin–dentin interfaces.

A comparative study between crack analysis and a mechanical test for assessing the polymerization stress of restorative composites

2012-06-01

Abstract: Objectives: To verify the hypothesis that crack analysis and a mechanical test would rank a series of composites in a similar order with respect to polymerization stress. Also, both tests would show similar relationships between stress and composite elastic modulus and/or shrinkage.Methods: Soda-lime glass discs (2-mm thick) with a central perforation (3.5-mm diameter) received four Vickers indentations 500μm from the cavity margin. The indent cracks were measured (500×) prior and 10min after the cavity was restored with one of six materials (Kalore/KL, Gradia/GR, Ice/IC, Wave/WV, Majesty Flow/MF, and Majesty Posterior/MP). Stresses at the indent site were calculated based on glass fracture toughness and increase in crack length. Stress at the bonded interface was calculated using the equation for an internally pressurized cylinder. The mechanical test used a universal testing machine and glass rods (5-mm diameter) as substrate. An extensometer monitored specimen height (2mm). Nominal stress was calculated dividing the maximum shrinkage force by the specimen cross-sectional area. Composite elastic modulus was determined by nanoindentation and post-gel shrinkage was measured using strain gages. Data were subjected to one-way ANOVA/Tukey or Kruskal–Wallis/Mann–Whitney tests (alpha: 5%).Results: Both tests grouped the composites in three statistical subsets, with small differences in overlapping between the intermediate subset (MF, WV) and the highest (MP, IC) or the lowest stress materials (KL, GR). Higher stresses were developed by composites with high modulus and/or high shrinkage.Significance: Crack analysis demonstrated to be as effective as the mechanical test to rank composites regarding polymerization stress.

Nanocomposite containing CaF2 nanoparticles: Thermal cycling, wear and long-term water-aging

2012-06-01

Abstract: Objectives: Fluoride (F) releasing dental restoratives are promising to promote remineralization and combat caries. The objectives of this study were to develop nanocomposite containing calcium fluoride nanoparticles (nCaF2), and to investigate the long-term mechanical durability including wear, thermal-cycling and long-term water-aging behavior.Methods: Two types of fillers were used: nCaF2 with a diameter of 53nm, and glass particles of 1.4μm. Four composites were fabricated with fillers of: (1) 0% nCaF2+65% glass; (2) 10% nCaF2+55% glass; (3) 20% nCaF2+45% glass; (4) 30% nCaF2+35% glass. Three commercial materials were also tested. Specimens were subjected to thermal-cycling between 5°C and 60°C for 105 cycles, three-body wear for 4×105 cycles, and water-aging for 2 years.Results: After thermal-cycling, the nCaF2 nanocomposites had flexural strengths in the range of 100–150MPa, five times higher than the 20–30MPa for resin-modified glass ionomer (RMGI). The wear scar depth showed an increasing trend with increasing nCaF2 filler level. Wear of nCaF2 nanocomposites was within the range of wear for commercial controls. Water-aging decreased the strength of all materials. At 2 years, flexural strength was 94MPa for nanocomposite with 10% nCaF2, 60MPa with 20% nCaF2, and 48MPa with 30% nCaF2. They are 3–6 fold higher than the 15MPa for RMGI (p<0.05). SEM revealed air bubbles and cracks in a RMGI, while composite control and nCaF2 nanocomposites appeared dense and solid.Significance: Combining nCaF2 with glass particles yielded nanocomposites with long-term mechanical properties that were comparable to those of a commercial composite with little F release, and much better than those of RMGI controls. These strong long-term properties, together with their F release being comparable to RMGI as previously reported, indicate that the nCaF2 nanocomposites are promising for load-bearing and caries-inhibiting restorations.

Influence of veneer application on fracture behavior of lithium-disilicate-based ceramic crowns

Ke Zhao, Yu Pan, Petra C. Guess, Xin-Ping Zhang, Michael V. Swain — 2012-06-01

Abstract: Objectives: To assess the influence of veneer application on fracture behavior, namely failure load and failure mode, of standardized lithium-disilicate-based crowns.Methods: Forty molar crowns (IPS e.max Press, IvoclarVivadent) were fabricated in full anatomic (without veneer, 1.5–2.0mm at occlusal surface) and bi-layer (the occlusal surface is 0.7mm of veneer and 0.8–1.3mm core) contour representing two groups. Crown specimens were seated and adhered on composite resin dies. All specimens were loaded with a 6mm diameter steatite sphere over the central fissure to failure. Failure modes and fractographic patterns were analyzed by optical stereo and scanning electron microscopy (SEM). Fracture loads of the two groups were compared by the t-test, while the failure modes were analyzed by Pearson Chi-square test.Results: There was a statistically significant difference in mean fracture load values (N±S.D.) between full anatomic [(2665.4±759.2)N] and veneered crowns [(1431.1±404.3)N] (p<0.001) and also in failure modes (χ2=6.465, p=0.011). Full anatomic crowns mainly showed bulk fracture, whereas veneered specimens predominately showed cohesive veneer and ceramic interface failure (75%); solely cohesive veneer failure (20%); and bulk fracture (5%).Significance: Within the limitations of this study, veneer application resulted in significant lower fracture load values compared to full anatomic crowns. Fracture initiated from occlusal fissures near the load application site. A combination of cohesive veneer and ceramic interface failure represents the main failure mode of lithium-disilicate-based bi-layered crowns, whereas full anatomic crowns failed mainly from ceramic bulk fracture at the occlusal fissures.

Confocal Raman microscopic analysis of the zirconia/feldspathic ceramic interface

2012-06-01

Abstract: Objectives: Esthetic demands and biocompatibility have prompted the development of all-ceramic dental crowns. Yttria tetragonal zirconia polycrystalline (Y-TZP) framework material has the best mechanical properties compared to other all-ceramic systems, but the interface is the weakest component of core veneered restorations. Confocal Raman microscopy possibilities are used to ensure the understanding of the zirconia-feldspathic ceramic relationship, which is not well known.Methods: Bilayered zirconia (Vita In-Ceram® YZ) veneer (Vita VM®9) blocks were manufactured. Raman analyses were performed using two protocols: (1) single spectra, line scans and images on a sectioned and polished specimen and (2) in depth line scans on unprepared specimen. Single spectra, images and line scans provide information about the crystalline phases, their distribution and the existence of a possible diffusion at the Y-TZP/VM®9 interface, respectively. The elemental distribution of zirconium (Zr) and silicon (Si) around this interface were investigated using energy dispersive X-ray spectroscopy (EDS).Results: Raman single spectra embodied a unique spectrum (crystalline) on Y-TZP and two spectra (crystalline and amorphous) on VM®9; these spectra were clearly distinguished. Raman line scans showed a series of transition spectra across the interface from VM®9 to Y-TZP. It emphasized an interdiffusion zone, which was estimated at a maximum of 2 microns, found on 2d Raman images and confirmed by EDS. The elemental distribution with EDS showed a mutual diffusion of Zr and Si and was mainly dominated by Si diffusion in Y-TZP.Significance: Confocal Raman microscopy highlights an interdiffusion zone at the zirconia-feldspathic ceramic interface. The elemental transition layer is estimated and is supported by EDS analysis as a coupling technique.

pH neutralization and influence on mechanical strength in self-adhesive resin luting agents

Jose Zorzin, Anselm Petschelt, Johannes Ebert, Ulrich Lohbauer — 2012-06-01

Abstract: Objective: The aim of this study was to evaluate if pH-neutralization behavior of self-adhesive resin cements has an influence on their mechanical properties.Methods: pH-neutralization, water sorption, solubility and flexural strength of G-Cem Automix (GCA), G CEM Capsules (GCC), Maxcem Elite (MCE), RelyX Unicem Clicker (RXC), RelyX Unicem Maxicap (RXM), RelyX Unicem 2 (RX2), and Speed-Cem (SPC) were tested in self-curing mode. Specimen's pH-values were recorded up to 24h with a pH-electrode. Water sorption (WS) and solubility (SO) were measured. Flexural strength (FS) was evaluated before and after thermocycling (TC) and fracture surfaces analyzed under SEM.Results: RX2 (pH24 5.89), RXC (pH24 5.49) and SPC (pH24 5.42) showed highest 24h pH-values, followed by GCA (pH24 5.34). Capsulated products and MCE (pH24 3.90) reached lower pH-values. GCC (75.78μg/mm3) showed the highest WS followed by MX (69.64μg/mm3), RXM (64.76μg/mm3), GCA (25.86μg/mm3) and SPC (30.42μg/mm3). Capsulated products RXM (56.58μg/mm3) and GCC (30.94μg/mm3) presented the highest SO, GCA (4.06μg/mm3) and SPC (4.26μg/mm3) revealed the lowest. GCA (114.03MPa) had the highest initial FS followed by SPC (79.81MPa), RXM (41.61MPa) the lowest. FS of all materials decreased significantly after TC except for RXC (44.65MPa) and RX2 (65.92MPa). FS of GCC (16.47MPa) and MCE (28.21MPa) decayed the most. A highly linear correlation was observed between percentage decrease of FS after TC and 24h pH-values (R2=0.861).Significance: Within the limits of this study pH-neutralization behavior has an influence on mechanical stability. When using self-adhesive resin luting agents, automix-syringe products with improved pH-neutralization behavior should be favored.

A comparative evaluation of polymerization stress data obtained with four different mechanical testing systems

Flávia Gonçalves, Leticia C. Boaro, Jack L. Ferracane, Roberto R. Braga — 2012-06-01

Abstract: Objectives: The null hypothesis was that mechanical testing systems used to determine polymerization stress (σpol) would rank a series of composites similarly.Methods: Two series of composites were tested in the following systems: universal testing machine (UTM) using glass rods as bonding substrate, UTM/acrylic rods, “low compliance device”, and single cantilever device (“Bioman”). One series had five experimental composites containing BisGMA:TEGDMA in equimolar concentrations and 60, 65, 70, 75 or 80wt% of filler. The other series had five commercial composites: Filtek Z250 (3M ESPE), Filtek A110 (3M ESPE), Tetric Ceram (Ivoclar), Heliomolar (Ivoclar) and Point 4 (Kerr). Specimen geometry, dimensions and curing conditions were similar in all systems. σpol was monitored for 10min. Volumetric shrinkage (VS) was measured in a mercury dilatometer and elastic modulus (E) was determined by three-point bending. Shrinkage rate was used as a measure of reaction kinetics. ANOVA/Tukey test was performed for each variable, separately for each series.Results: For the experimental composites, σpol decreased with filler content in all systems, following the variation in VS. For commercial materials, σpol did not vary in the UTM/acrylic system and showed very few similarities in rankings in the others tests system. Also, no clear relationships were observed between σpol and VS or E.Significance: The testing systems showed a good agreement for the experimental composites, but very few similarities for the commercial composites. Therefore, comparison of polymerization stress results from different devices must be done carefully.

Mechanical properties and characteristics of developmental copolymers incorporating catechin or chlorhexidine

Smiley Pallan, Michel Victor Furtado Araujo, Renato Cilli, Anuradha Prakki — 2012-06-01

Abstract: Objectives: This study evaluated catechin and chlorhexidine release from copolymers based on bis-GMA diluted with TEGDMA (R1), propoxylated bis-GMA–CH3bis-GMA (R2) or fluorinated bis-GMA–CF3bis-GMA (R3). Mechanical properties, degree of conversion, water sorption and solubility were also tested.Methods: Experimental comonomers (R1, R2, R3) were prepared combining bis-GMA with the diluents (70/30mol%). Each comonomer had either catechin or chlorhexidine incorporated in the following ratios: 0, 0.2, 1.0, 2.0wt%. The degree of conversion (%DC) of resins was evaluated by FT-IR spectrophotometry (n=3). Flexural strength and elastic modulus were tested on a universal testing machine at a crosshead speed of 0.5mm/min (n=6). Resin disks (7mm in diameter×2mm high) were immersed in deionized water and mass changes recorded daily. Spectral measurements were made to follow any changes in optical densities of deionized water in order to examine catechin and chlorhexidine release rates (n=6). After a 28-day period, water sorption and solubility were calculated using appropriate equations (n=6). Data within the tests were evaluated by one- and three-way ANOVA and Tukey–Kramer's test (α=0.05).Results: Except for R1 with 2.0% drug none of the groups had changes in degree of conversion and water sorption. Solubility increased with increased drug ratios. Changes in flexural strength and elastic modulus were shown to be variable and material-dependent. All resins presented a ∼24h burst of drug release. Release rates were drug-ratio dependent and significantly higher for chlorhexidine than for catechin.Significance: Monomer type, %DC, drug ratio and type lead to different interactions between resins and chlorhexidine/catechin drug particles. It should be possible to develop different resins based on clinical needs.

Fractal analysis of heterogeneous polymer networks formed by photopolymerization of dental dimethacrylates

Izabela M. Barszczewska-Rybarek, Monika Krasowska — 2012-06-01

Abstract: Objectives: In this work the influence of the dimethacrylate monomer chemical structure on structural heterogeneity and physico-mechanical properties of the resulting polymer networks was investigated. Rigid aromatic dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA) and flexible aliphatic urethane-dimethacrylate (UDMA) were chosen for room-temperature homopolymerizations and copolymerizations induced by camphorquinone/N,N-dimethylaminoethyl methacrylate photoinitiating system.Methods: Atomic force microscopy (AFM) was used for visualizing the morphology of poly(dimethacrylate)s, which was described by: the fractal dimension (DF), the generalized fractal dimensions (Dq and ΔD) as well as the modified fractal dimension (Dβ). Estimated fractal characteristics were correlated with polymer density, hardness and impact strength.Results: AFM images of fractured surfaces revealed the highly complex morphology of dimethacrylate polymer networks. They were found to possess the fractal character. The fractal parameters were observed to be proportional to the density, hardness and impact resistance of investigated polymers. ΔD appeared to be a good indicator of the structural heterogeneity of dimethacrylate networks. The results suggest that the fracture behavior of poly(dimethacrylate) matrix of dental materials can be controlled by the fractal morphology.Significance: Correlating the morphological studies with the mechanical tests would be beneficial in defining the role of morphology in the mechanical behavior of dimethacrylate networks and consequently, lead to the development of a reliable method for identifying the cause of dental material failures under stress. Thus, fractal analysis could become one of the key elements in designing and developing dental materials.

Water sorption characteristics of restorative dental composites immersed in acidic drinks

2012-06-01

Abstract: Objectives: To determine the diffusion coefficient, water sorption and solubility of various types of restorative dental composites and to evaluate the effect of acidic media (orange juice and coke) on their characteristics.Methods: Resin composite specimens (Filtek™ Z350, Spectrum® TPH®3 and Durafill® VS) were prepared in a stainless steel mold of 1mm thickness and 10mm diameter (n=5) and light-cured. All samples were dried at 37°C, immersed in media (distilled water, orange and coke) at 37°C and weighed at suitable time intervals (15, 35, 155, 320, 785, etc.min) until 40 days of immersion and then were dried again for 40 days. Diffusion coefficient (m2s−1) was determined according to Fick's second law while water sorption and solubility (μg/mm3) were calculated based on BS EN ISO 4049:2000. Data of water sorption and solubility were analyzed with One-Way ANOVA and post hoc Scheffe test at p=0.05.Results: The experimental data obtained were obeyed and nearly fitted to the diffusion theoretical data plot. The highest values of diffusion coefficients were presented by Durafill® VS (32.23–45.25×10−13m2s−1). Diffusion coefficients of Filtek™ Z350 and Spectrum® TPH®3 were the highest when immersed in coke media followed by distilled water and orange juice. The water sorption of most composites was significantly increased after immersion in coke and orange (p<0.05). Meanwhile only Spectrum® TPH®3 showed an increase in solubility when immersed in coke media. Z350 presented the highest water sorption after immersion in distilled water and coke (16.13 and 18.22μg/mm3) while Durafill® VS presented the highest solubility (7.20–9.27μg/mm3).Significance: The exposure of restorative dental composites to acidic drinks can cause an increase in diffusion coefficient, water sorption and solubility parameters which may accelerate the degradation process and thus reduce the life span of composite restoration.

Letter to the Editor: Response to Dr Simonsen

Jan Kühnisch — 2012-06-01

Thank you very much for reading and analyzing our paper very carefully. We enjoyed to read your comments. Your main concerns were addressed (1) to our practice-based statement that light-polymerizing sealants can be applied “faster and less error-prone” in comparison to auto-polymerizing materials and (2) that includes obviously miss-calculated data.