Polymerization shrinkage strain of interocclusal recording materials

ObjectivesThe aim of this in vitro study was to investigate the polymerization shrinkage behavior and to measure the polymerization shrinkage-strain of interocclusal recording materials.MethodsThe materials investigated in this study were five polyvinylsiloxane (Imprint? Bite, Silagum Automix Bite, O-Bite, Blu-Mousse^® Classic and Exabite II), one polyether (Ramitec) and one dimethacrylatebased (Luxabite) materials. The polymerization shrinkage values of ten specimens for each material were measured by the Bonded-disk method at 1, 3, 5, 7 and 10 min after mixing at 37 °C. The amount of shrinkage-strain (%) was derived and all data were statistically analyzed by one-way ANOVA and the multiple comparison Scheffé test (α = 0.05).ResultsThe representative shrinkage-strain kinetic graphs showed that all specimens shrank immediately, except Luxabite which expanded for the initial few seconds. After that, the shrinkage-strain values increased in the magnitude up to 10 min, but its rate decreased gradually with time. The shrinkage-strain values (0.18 ± 0.03–0.16 ± 0.03%) of O-Bite at 5, 7 and 10 min were significantly lower than the other materials, but Luxabite exhibited the highest values (3.10 ± 0.17–3.30 ± 0.16%).SignificanceThe interocclusal recording materials investigated presented significantly different polymerization shrinkage-strain kinetics and showed dimensional changes even after the setting time indicated by respective manufacturers.     

Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites

ObjectivesThe degree of conversion (DC) and polymerization shrinkage of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and monomer ratios on the DC and the shrinkage-strain of experimental composites based on (BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the shrinkage-strain and the degree of conversion values was also investigated. The radiopacity of these experimental composites has been investigated in a previous paper.MethodsExperimental resin composites were prepared by mixing different monomer ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacrylate (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La₂O₃, BaO, BaSO₄, SrO and ZrO₂ at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm², was measured using the bonded-disk technique at room temperature with respect to time.ResultsThe result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated.SignificanceThe nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites.     

Pre-heating time and exposure duration: Effects on post-irradiation properties of a thermo-viscous resin-composite

ObjectiveTo evaluate the effects of pre-heating time and exposure duration on the degree of conversion (DC), maximum rate of polymerization (RP_max), polymerization shrinkage strain (PS) and surface micro-hardness (VHN) of Viscalor.MethodsViscalor syringes were pre-heated using a Caps Warmer (VOCO, Germany) in T3 mode (at 68 °C) for 30 s (T3-30s) and 3 min (T3-3min) and then the composite paste was extruded into appropriately sized molds. Light irradiation was applied at zero distance from the upper surface with a LED-LCU of mean irradiance 1200 mW/cm² for either 20 or 40 s. The real-time polymerization kinetics and DC at 5 min and 24 h post-irradiation (DC_5min and DC_24h) were measured using ATR-FTIR (n = 3). PS was obtained with the bonded-disk technique (n = 3). Top and bottom Vickers micro-hardness (VHN_top and VHN_bottom) were measured at 5 min post-irradiation and after 24 h dry storage (n = 5). Data were analysed using one-way ANOVA, two-way ANOVA, independent t-test and Tukey post hoc tests (p < 0.05).ResultsPolymerization kinetic curves of Viscalor from 0 to 15 min were similar for different pre-heating times and exposure durations. Pre-heated Viscalor (T3-30s and T3-3min) with 40 s exposure had greater VHN_top and VHN_bottom than for Viscalor (no heat) (p < 0.05). Exposure duration did not significantly affect DC, RP_max and PS (p > 0.05). After 24 h storage, DC and VHN increased. Pre-heating did not increase the DC_24h, relative to no pre-heating (p > 0.05). Two-way ANOVA showed that there was no significant interaction between pre-heating time and exposure duration (p > 0.05).SignificanceIncreasing irradiation time from 20 to 40 s did not affect DC, RP_max or PS, but increased VHN_top. Composite pre-heating had no adverse effect through any premature polymerization. For Viscalor, 3 min pre-heating and 20 s irradiation were sufficient to provide adequate hardness, without increasing PS or compromising polymerization kinetics.     

Mechanical manifestation of the C-factor in relation to photopolymerization of dental resin composites

ObjectiveThis study aims to assess the validity of a recent theory which proposes that (1) the magnitude of the shrinkage stress of resin composites depends on the thickness of the boundary layer under triaxial constraints relative to the total thickness of the specimen and (2) the boundary-layer thickness is proportional to the diameter of the specimen.MethodsCylindrical specimens of three commercially available resin composites, three diameters (4, 5 and 6.3 mm) and four thicknesses (2, 3, 5 and 6.5 mm) were tested. Curing was applied using a LED light for 40 s. Microscopic images (32×) of the specimens before and after curing were analyzed to determine the lateral shrinkage profile along the vertical axis. Boundary-layer thickness was determined from the point where lateral shrinkage displacement first reached the maximum value found at mid-thickness.ResultsLateral shrinkage displacement at mid-thickness was close to the theoretical value based on published shrinkage-strain data, with the ratio between experimental and theoretical values being 1.04 ± 0.06. The boundary-layer thickness was found to be proportional to specimen diameter only, independent of material, C-factor, and specimen thickness. The proportionality constant was 0.64 ± 0.07, which was approximately 3 times that of the effective value indicated by shrinkage strain/stress calculations.SignificanceThis study validates the assumption made in the shrinkage-stress theory recently proposed and provides a more precise and mechanistic interpretation for the C-factor, i.e. the C-factor, as a measure of a specimen's constraint, is the ratio between the boundary-layer thickness and the total thickness of the specimen.     

Maxillary sensory nerve responses induced by different types of dentures

PurposeThe purpose of this study was to investigate whether different types of dentures induced different responses to stimulations in sensory nerve underlying the denture-supporting mucosa using current perception threshold (CPT).Materials and methodsThe study population comprised 45 complete denture wearers with a mean age of 69.7 years (CD), 30 partial denture wearers (PD) with a mean age of 67.1 years, and 40 dentulous participants with a mean age of 69.0 years (Dent). Current perception threshold (CPT) on the greater palatine nerve at 2000 Hz, 250 Hz, and 5 Hz, corresponding to A-beta, A-delta, and C fibers respectively, were measured by the Neurometer^® NS3000 device. The differences CPTs among CD, PD, and Dent groups were analyzed by Kruskal–Wallis test and Mann–Whitney U test with adjusting the multiple comparisons’ inflation of type 1 error rate by a Bonferroni correction.ResultsCPTs of CD, PD, and Dent group at 2000 Hz were 61.5 ± 45.8, 53.5 ± 25.3, 33.0 ± 11.4 (10^?2 mA) respectively. CPTs of CD, PD, and Dent group at 250 Hz were 29.2 ± 28.2, 20.1 ± 13.2, 14.3 ± 5.9 (10^?2 mA) respectively. CPTs of CD, PD, and Dent group at 5 Hz were 28.9 ± 23.4, 17.8 ± 12.2, 12.2 ± 5.6 (10^?2 mA) respectively. The CPTs at all frequencies increased in the following order: Dent < PD < CD wearer. The statistical analyses showed that the different types of dentures significantly affected CPTs at 2000 Hz (p < 0.0001), 250 Hz (p < 0.0001), and 5 Hz (p < 0.0001).ConclusionThe different types of dentures induce different responses to stimulations in the sensory nerve underlying the denture-supporting mucosa.     

Antibacterial response of oral microcosm biofilm to nano-zinc oxide in adhesive resin

ObjectiveVarious nanoparticles are currently under investigation to impart biointeractivity for dental materials. This study aimed to: (1) formulate an experimental dental adhesive containing ZnO nanoparticles; (2) evaluate its chemical and mechanical properties; and (3) assess the antibacterial response against oral microcosm biofilm.MethodsNanosized ZnO was chemically and morphologically evaluated. ZnO was incorporated at 0 (G_CTRL), 2.5 (G_2.5%), 5 (G_5%) and 7.5 (G_5%) wt.% in an experimental dental adhesive. The adhesives were evaluated for the degree of conversion (DC), flexural strength (FS), and elastic modulus (E). The antibacterial activity was evaluated using a 48 h-microcosm biofilm model after the formation of acquired pellicle on samples’ surfaces. Colony-forming units (CFU), metabolic activity, and live/dead staining were assessed.ResultsNanosized ZnO presented characteristic peaks of Zn-O bonds, and the particles were arranged in agglomerates. The DC ranged from 62.21 (±1.05) % for G_Ctrl to 46.15 (±1.23) % for G_7.5% (p < 0.05). G_7.5% showed lower FS compared to all groups (p < 0.05). Despite achieving higher E (p < 0.05), G_2.5% did not show differences for G_Ctrl regarding the FS (p > 0.05). G_7.5% had lower CFU/mL compared to G_Ctrl for mutans streptococci (p < 0.05) and total microorganisms (p < 0.05), besides presenting lower metabolic activity (p < 0.05) and higher dead bacteria via biofilm staining.SignificanceThe dental adhesives' physicochemical properties were similar to commercial adhesives and in compliance with ISO recommendations. G_7.5% restricted the growth of oral microcosm biofilm without impairing the physicochemical performance.     

Conversion kinetics of rapid photo-polymerized resin composites

ObjectiveTo measure the degrees of conversion (DC), conversion kinetics, and the effect of post-irradiation time on rapid photo-polymerized bulk-fill resin composites under conditions equivalent to clinical depths of 1 and 4 mm.Methods36 specimens (n = 3), based on two resin composites incorporating PowerCure rapid-polymerization technology in two consistencies (PFill; PFlow) and two comparators with matching consistencies (Eceram; EFlow), were investigated from the same manufacturer (Ivoclar AG, Liechtenstein). Specimens were prepared within 4 mm diameter cylindrical molds, of either 1 mm or 4 mm depths respectively, to simulate near-surface and deep locations in a bulk-fill restoration. The independent variables in this study were: materials, thickness and time. Two high irradiance polymerization protocols were utilized for PowerCure materials: 2000 and 3050 mW/cm² for 5 and 3 s, respectively. A standard (1200 mW/cm²) polymerization protocol was used with control materials. FTIR was utilized to measure DC in real-time for 24 h post-irradiation. The data were analyzed using Welch’s-ANOVA, Games-Howell post-hoc test, kinetic dual-exponential sum function and independent sample t-tests (p = 0.05).ResultsThe DC of the materials ranged between 44.7–59.0 % after 5 min, which increased after 24 h reaching 55.7–71.0 % (p < 0.05). Specimen thickness did not influence the overall DC. At 5 min, the highest DC was shown in EFlow. But PFlow, irradiated for 3 s and 5 s exhibited comparable results (p > 0.05). PFill composite irradiated with the 3 s and 5 s protocols did not differ from ECeram (p > 0.05). Specimen thickness and material viscosity affected polymerization kinetics and rate of polymerization (RP_max). Faster polymerization occurred in 1 mm specimens (except PFill-5 s and ECeram). PFill and PFlow exhibited faster conversion than the controls. RP_max varied across the specimen groups between 4.3–8.8 %/s with corresponding DC _RPmax between 22.2–45.3 %.SignificancePolymerization kinetics and RP_max were influenced by specimen thickness and material viscosity. PFill and PFlow materials produced an overall comparable conversion at 5 min and 24 h post-irradiation, despite the ultra-short irradiation times, throughout the 4 mm specimen thickness.     

Concentration additivity of coefficients for maxillofacial elastomer pigmented to skin colors

ObjectiveThe purpose of this study is to determine a regression model which performs accurately for predicting Kubelka–Munk (K–M) absorption (K) and scattering (S) coefficients of pigment mixtures in maxillofacial elastomer (MFE).MethodsSpecimens at varying thicknesses of 19 shades of skin-colored MFE were made incorporating tan, red, yellow and black pigments. Using a non-contact 45°/0° spectral reflectance system, reflectance values were obtained and fit by non-linear regression to corrected K–M theory to determine observed K and S values. Predicted values were calculated using: first order linear regression model (M₁), second order square-factor model (M_2S), second order interaction-factor model (M_2I) and complete second order model (M_2C). Errors between observed and predicted values for each model at each wavelength studied are evaluated. Differences in color and the Translucency Parameter (TP) at a thickness of 1.5 mm based on CIE2000 color difference formula between observed and predicted values were also analyzed using repeated-measures ANOVA (α = 0.05).ResultsM_2C shows less absolute differences in color and TP (1.5 mm) than those for the M₁ and M_2S models (P < 0.05).SignificanceWithin limitations of this study, the complete second order regression model performs accurately in concentration additivity of pigmented maxillofacial elastomer mixture. This regression model is recommended as a viable alternative in the application of colorant formulation based on corrected K–M model for improving appearance match of pigmented maxillofacial prosthesis to human skin.     

Development and characterisation of dental composites containing anisotropic fluorapatite bundles and rods

ObjectivesTo develop dental composites incorporating fluorapatite (FA) crystals as a secondary filler and to characterise degree of conversion, key mechanical properties and fluoride release.MethodsFA rod-like crystals and bundles were hydrothermally synthesised and characterised by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and ¹⁹F MAS-NMR. Composites were formulated containing BisGMA/TEGDMA/BisEMA and barium-aluminium-silicate glass (0FA). FA crystals were incorporated at 10 (10FA), 20 (20FA), 30 (30FA) and 40 wt% (40FA) maintaining a filler content of 80 wt% (63–67 vol%). Degree of conversion (DC), flexural strength (FS), flexural modulus (FM), fracture toughness (K_1C), Vickers hardness (HV) and 2-body wear were measured. Fluoride release was measured in neutral and acidic buffers.ResultsXRD and ¹⁹F MAS-NMR confirmed that only FA was formed, whilst SEM revealed the presence of single rods and bundles of nano-rods. DC ranged between 56–60% (p > 0.05). FA composites showed lower FM and lower FS (p < 0.05), but comparable wear resistance and HV (p > 0.05) to 0FA. 30FA and 40FA showed similar K_1C to 0FA (p > 0.05), with SEM showing evidence of toughening mechanisms, whereas 10FA and 20FA showed lower K_1C (p < 0.05). FA containing composites released fluoride that was proportional to the amount of FA incorporated (p < 0.05) but only under acidic conditions.SignificanceThe addition of FA to the experimental composites reduced strength and stiffness but not the DC, hardness or wear rate. 30FA and 40FA had a higher K_1C compared to other FA groups. Fluoride release occurred under an accelerated acidic regime, suggesting potential as a bioactive ‘smart’ composite.     

Evaluation of alternative photoinitiator systems in two-step self-etch adhesive systems

ObjectiveThe aim of this study was to evaluate the effect of different photoinitiator systems on the cell viability and physico-chemical properties of an experimental adhesive resin of a two-step self-etch adhesive system.MethodsEight photoinitiators and coinitiators were evaluated, camphorquinone (CQ); ethyl-dimethylamino benzoate (EDAB); diphenyliodonium hexafluorophosphate (DPIHFP); 1,3-benzodioxole (BDO); piperonyl alcohol (AP); 1,3-diethyl-2-thiobarbituric acid (TBA); bisphenyl phosphinic oxide (BAPO); and diphenyl phosphinic oxide (TPO). Seven experimental adhesive resins (Bis-GMA:TEGDMA – 1:1 by wt%) were formulated by varying the initiation systems: R_CQ+EDAB (control), R_{CQ+EDAB+DPHFP}, R_CQ+BDO, R_CQ+AP, R_CQ+TBA, R_BAPO and R_TPO. The cell viability of the different photoinitiators in their isolated form and after being incorporated into the adhesive resins was evaluated using the MTT assay. The degree of conversion within the hybrid layer (DC in situ) was evaluated by micro-Raman spectroscopy; the polymerization kinetics, by FTIR spectroscopy. The water sorption (Wsp) and solubility (Wsl) were calculated by using percentage of gain and loss of mass. Data were analyzed using One-way ANOVA and Kruskal-Wallis tests (α = 0.05).ResultsWhen evaluated in their isolated form, the DPIHFP was considered cytotoxic in all concentrations evaluated. When incorporated into and adhesive resin, R_CQ+TBA presented higher cell viability values than control and the highest values of DC in situ (p < 0.05). Additionally, the use of CQ + TBA showed a higher cell viability when compared with the conventional CQ + EDAB system.SignificanceThe pair CQ + TBA could be potentially useful in the development of materials with improved biocompatibility.     

Pre-heating effects on extrusion force,stickiness and packability of resin-based composite

ObjectivesTo measure temperature effects on stickiness and packability of representative resin-based composites and the effect of pre-heating time on pre-cure properties of Viscalor, including extrusion force.MethodsFive resin-based composites (RBC) and an additional RBC, Viscalor, used with a Caps Warmer (VOCO, Germany) were studied. The extrusion force (N) and extruded mass (g) were measured from Viscalor compules heated in T3 mode for 30 s (T3-30 s) and 3 min (T3-3 min). For stickiness and packability measurements, RBCs were packed into a brass cylindrical cavity controlled at 22 and 37 °C. A flat-ended probe was lowered into the RBC pastes at constant speed. Stickiness: F_max (N) and W_s (N mm), and packability: F_p (N), were measured. Viscalor was LED photo-cured at 1200 mW/cm² for 40 s. The degrees of conversion at 5 min and 24 h post cure (DC_5min and DC_24h) of Viscalor (no heat, T3-30 s and T3-3 min) were measured by ATR-FTIR. Data were analysed by one-way ANOVA, independent t-test and Tukey post-hoc tests (p < 0.05).ResultsThe maximum temperature of the Caps Warmer, in T3 mode, reached 68 °C in 20 min. Viscalor temperatures of 34.5 °C and 60.6 °C were recorded after 30 s and 3 min pre-heating, respectively. Pre-heating significantly reduced extrusion force and increased extruded mass, especially after 3 min. RBCs varied in F_max, W_s and F_p (p < 0.05). Temperature also affected F_max (p = 0.000), W_s (p = 0.002) and F_p (p = 0.000). Pre-heating Viscalor for either 30 s or 3 min did not increase the post-cure DC at either 5 min or 24 h, relative to no pre-heating (p > 0.05).SignificanceThe composites varied to an extent in stickiness and packability but the overall magnitudes remained within a clinically acceptable range. Pre-heating was beneficial in placement of Viscalor and caused no adverse effects through premature polymerization.     

Bis-GMA co-polymerizations: Influence on conversion,flexural properties,fracture toughness and susceptibility to ethanol degradation of experimental composites

ObjectivesThe aim of this study was to evaluate the influence of monomer content on fracture toughness (K_Ic) before and after ethanol solution storage, flexural properties and degree of conversion (DC) of bisphenol A glycidyl methacrylate (Bis-GMA) co-polymers.MethodsFive formulations were tested, containing Bis-GMA (B) combined with TEGDMA (T), UDMA (U) or Bis-EMA (E), as follows (in mol%): 30B:70T; 30B:35T:35U; 30B:70U; 30B:35T:35E; 30B:70E. Bimodal filler was introduced at 80 wt%. Single-edge notched beams for fracture toughness (FT, 25 mm × 5 mm × 2.5 mm, a/w = 0.5, n = 20) and 10 mm × 2 mm × 1 mm beams for flexural strength (FS) and modulus (FM) determination (10 mm × 2 mm × 1 mm, n = 10) were built and then stored in distilled water for 24 h at 37 °C. All FS/FM beams and half of the FT specimens were immediately submitted to three-point bending test. The remaining FT specimens were stored in a 75%ethanol/25%water (v/v) solution for 3 months prior to testing. DC was determined with FT-Raman spectroscopy in fragments of both FT and FS/FM specimens at 24 h. Data were submitted to one-way ANOVA/Tukey test (α = 5%).ResultsThe 30B:70T composite presented the highest K_Ic value (in MPa m^1/2) at 24 h (1.3 ± 0.4), statistically similar to 30B:35T:35U and 30B:70U, while 30B:70E presented the lowest value (0.5 ± 0.1). After ethanol storage, reductions in K_Ic ranged from 33 to 72%. The 30B:70E material presented the lowest reduction in FT and 30B:70U, the highest. DC was similar among groups (69–73%), except for 30B:70U (52 ± 4%, p < 0.001). 30B:70U and 30B:35T:35U presented the highest FS (125 ± 21 and 122 ± 14 MPa, respectively), statistically different from 30B:70T or 30B:70E (92 ± 20 and 94 ± 16 MPa, respectively). Composites containing UDMA or Bis-EMA associated with Bis-GMA presented similar FM, statistically lower than 30B:35T:35U.SignificanceComposites formulated with Bis-GMA:TEGDMA:UDMA presented the best compromise between conversion and mechanical properties.     

Mechanical and physical properties of silorane and methacrylate-based composites

ObjectivesThis study measured the degree of conversion (DC), sorption, solubility and microhardness of methacrylate (Filtek Z250 and Filtek Z350XT) and silorane-based composites (Filtek P90).MethodsDC was measured using near infrared spectroscopy immediately and 24 h after the photoactivation. Sorption and solubility measurements were performed after 24 h, 4 weeks and 12 weeks of storage in water. Knoop microhardness was measured after 24 h and after thermal cycling. The data were statistically analyzed using ANOVA followed by Tukey's, Tamhane or paired t-tests (α = 0.05).ResultsThe DC for P90 (37.22 ± 1.46) was significantly lower than the Z250 (71.44 ± 1.66) and Z350 (71.76 ± 2.84). Water sorption was highest in the Z250 and lowest in the P90. All the tested composites exhibited similar values after 24 h of immersion, and no significant differences were observed. No significant differences were observed between the solubilities of the P90 composite (12 weeks) and the Z250 or Z350 composites (4 weeks). KHN values were less elevated for the P90 composite and similar for the Z250 and Z350 composites. An effect of thermal cycling on KHN values was observed for all the composites (p < 0.001).ConclusionsSilorane produced the lowest DC and KHN values and exhibited lower water sorption and solubility compared to methacrylate-based composites. These differences suggest that silorane composites exhibit better hydrolytic stability after 3 months of water immersion compared to conventional methacrylate-based composites.Clinical significanceSilorane had higher hydrolytic stability after 3 months of water immersion than the methacrylate-based resins, despite the lower values of DC and KHN recorded.     

Immediate bonding properties of universal adhesives to dentine

ObjectivesTo evaluate the dentine microtensile bond strength (μTBS), nanoleakage (NL), degree of conversion (DC) within the hybrid layer for etch-and-rinse and self-etch strategies of universal simplified adhesive systems.Methodsforty caries free extracted third molars were divided into 8 groups for μTBS (n = 5), according to the adhesive and etching strategy: Clearfil SE Bond [CSE] and Adper Single Bond 2 [SB], as controls; Peak Universal Adhesive System, self-etch [PkSe] and etch-and-rinse [PkEr]; Scotchbond Universal Adhesive, self-etch [ScSe] and etch-and-rinse [ScEr]; All Bond Universal, self-etch [AlSe] and etch-and-rinse [AlEr]. After restorations were constructed, specimens were stored in water (37 °C/24 h) and then resin–dentine sticks were prepared (0.8 mm²). The sticks were tested under tension at 0.5 mm/min. Some sticks from each tooth group were used for DC determination by micro-Raman spectroscopy or nanoleakage evaluation (NL). The pH for each solution was evaluated using a pH metre. Data were analyzed with one-way ANOVA and Tukey's test (α = 0.05).ResultsFor μTBS, only PkSe and PkEr were similar to the respective control groups (p > 0.05). AlSe showed the lowest μTBS mean (p < 0.05). For NL, ScEr, ScSe, AlSe, and AlEr showed the lowest NL similar to control groups (p < 0.05). For DC, only ScSe showed lower DC than the other materials (p < 0.05).ConclusionsPerformance of universal adhesives was shown to be material-dependent. The results indicate that this new category of universal adhesives used on dentine as either etch-and-rinse or self-etch strategies were inferior as regards at least one of the properties evaluated (μTBS, NL and DC) in comparison with the control adhesives (CSE for self-etch and SB for etch-and-rinse).     

Nano-structured hydroxyapatite and titanium dioxide enriching PENTA /UDMA adhesive as aesthetic coating for tooth enamel

ObjectivesThe aim of this study was to investigate the effect of the nanostructured hydroxyapatite (NHAp) and titanium dioxide nanoparticles (NTiO₂) on dispersion in an adhesive containing monomers of Dipenta erythritol penta-acrylate monophosphate (PENTA) and Urethane dimethacrylate (UDMA), as well as evaluating the structural, optical and mechanical behavior of the composite material for dental aesthetic application.MethodsThe NHAp powders were synthesized through the wet chemical methods of hydrothermal and ultrasound-assisted precipitation. The microstructure, morphology and composition analysis of the powder of NHAp and NTiO₂ were performed by scanning and transmission electron microscopy. The optical microscopic identification of the different colors was obtained due to varying the amounts of NHAp and NTiO₂ in the adhesive. On the other hand, the diffuse reflectance spectra of the coatings were evaluated every 2 nm with the wavelength from 400 to 800 nm for combined specular and diffuse reflectance. The nanomechanical properties of the aesthetic coating such as (H), elastic modulus (E) and nanoscratching were evaluated by nanoindentation. The roughness of the composite coatings were evaluated by AFM.ResultsFrom different powders combinations, NHAP 75%Wt-NTiO₂ %25 Wt, at (10Wt %) into a dental adhesive, the resulting mixture manifested the optimum aesthetic white appearance. The scanning and transmission electron microscopy images confirmed that the HAp nanorods and TiO₂ nanoparticles sized were 55 nm and 20 nm respectively prepared by the high-energy ball mixed process. The values of nanomechanical properties of the optimum aesthetic coating were hardness, H = 3.2 ± 0.3 GPa, elastic modulus, E = 78 ± 3 GPa, Yield point, Y = 107 MPa ± 2 and scratching, maximum wear track deformation 3.7 ± 0.12 μm². The percentage of reflectance to optimum aesthetic white appearance was of 46.83% at 423 nm of wavelength.ConclusionsThe nanocomposite PENTA/UDMA with mixtures of Nanohydroxyapatite and titanium dioxide may be considerate as a mechanical toughened, also an option to modify shade qualities for dental aesthetic applications.     

Degree of conversion of Filtek Silorane Adhesive System and Clearfil SE Bond within the hybrid and adhesive layer: An in situ Raman analysis

ObjectivesTo examine the degree of conversion (DC) of the adhesive interfaces created by Filtek Silorane Adhesive and Clearfil SE Bond using micro-Raman spectroscopy.MethodsThe adhesives were applied on human dentin in accordance with manufacturer's instructions. Specimens were cut to expose the bonded interfaces to the micro-Raman beam (Ranishaw InVia; laser wl 785 nm). Raman spectra were collected along the dentin/self-etching primer/adhesive interface at 1 μm intervals. The relative intensities of bands associated with mineral (P–O functional group at 960 cm^?1) and adhesive (C–C–O group at 605 cm^?1) components within the bonded interface were used to detect monomer penetration into the dentin matrix and to calculate the degree of conversion (CC at 1640 cm^?1 as reaction peak, C–C–O at 605 cm^?1 as reference peak). Data were statistically analyzed with two-way ANOVA.ResultsDC of Filtek Silorane Adhesive was 69 ± 7% in the adhesive layer, increasing (p < 0.05) to 93 ± 5% in the primer and 92 ± 9% in the hybrid layer. Clearfil SE Bond showed a DC of 83 ± 3% in the hybrid and 85 ± 3% in the adhesive layer. Thus, Filtek Silorane Adhesive showed a higher DC than Clearfil SE Bond in the hybrid layer (p < 0.05), but a lower DC in the adhesive (p < 0.05).SignificanceAs high DC is a fundamental pre-requisite for the stability of the bond over time, this study supports the hypothesis that optimal stability of Filtek Silorane Adhesive can be obtained. However, further research is needed to investigate the mechanical properties of the hybrid layer created by Filtek Silorane Adhesive and its long-term stability.     

Hydrogen peroxide induces cell proliferation and apoptosis in pulp of rats after dental bleaching in vivo: Effects of the dental bleaching in pulp

ObjectiveThis study provides an in vivo evaluation of the inflammatory response, levels of cell proliferation and apoptosis, and the presence of necrosis after dental bleaching with two concentrations of hydrogen peroxide (H₂O₂).DesignWistar rats were divided into Control (placebo gel), BLUE (20% H₂O₂, 1 × 50 min), and MAXX (35% H₂O₂, 3 × 15 min) groups. At 2 and 30 days, the rats were killed (n = 10). The jaws were processed for histology analysis and PCNA and Caspase-3-cleaved immunohistochemistry, and data were submitted to the Mann-Whitney or ANOVA test (P < 0.05).ResultsAt 2 days, the MAXX group showed necrosis and the BLUE group revealed moderate inflammation on the occlusal third of the crown (P < 0.05). At 30 days, tertiary dentin had formed and there was an absence of inflammation. The level of cell proliferation was higher in the middle third of the BLUE group (P < 0.05), and cervical of MAXX at 2 days (P < 0.05), decreasing at 30 days. The apoptosis was present at 2 days, particularly in the cervical third of the crown in the bleached groups (P < 0.05), with a decrease only at 30 days in the BLUE group (P < 0.05).ConclusionsThe concentration of H₂O₂ influences effects on the pulp tissue, where a higher concentration of H₂O₂ can cause necrosis in the pulp and a prolonged effect within the apoptotic process; lower concentrations of H₂O₂ provide moderate inflammation, cell proliferation and apoptosis with a reduction of these processes over time.     

Effect of residual solvent on performance of acrylamide-containing dental materials

ObjectiveMethacrylamide-based monomers are being pursued as novel, hydrolytically stable materials for use in dental adhesives. The impact of residual solvents, due to the chemical synthesis procedures or the need for solvated adhesives systems, on the kinetics of polymerization and mechanical properties was the aim of the present investigation.MethodsTwo base monomers (70 wt% BisGMA or HEMAM-BDI — newly synthesized secondary methacrylamide) were combined with 30 wt% N,N-dimethylacrylamide. Eethyl acetate (EtOAc), or 75 vol% ethanol/25 vol% water (EtOH/H₂O) were added as solvents in concentrations of 2, 5, 15 and 20 wt%. The resins were made polymerizable by the addition of 0.2 wt% 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and 0.4 wt% diphenyliodonium hexafluorophosphate (DPI-PF6). Specimens (n = 3) were photoactivated with a mercury arc lamp (Acticure 4000, 320–500 nm, 250 mW/cm²) for 5 min. Degree of conversion (DC, %) was tracked in near-IR spectroscopy in real time and yield strength and modulus of elasticity were measured in three-point bending after dry and wet storage (n = 6). The data was subject to one-way ANOVA/Tukey’s Test (p ≤ 0.05), or Student’s t-test (p ≤ 0.001).ResultsIn all groups for both BisGMA and HEMAM-BDI-based materials, DC and DC at Rp_max increased and maximum rate of polymerization decreased as solvent concentration increased. Despite the increased DC, BisGMA mixtures showed a decrease in FS starting at 5 wt% EtOAc or 15 wt% EtOH/H₂O. Yield strength for the HEMAM-BDI groups was overall lower than that of the BisGMA groups, but the modulus of elasticity was significantly higher.SignificanceThe presence of residual solvent, from manufacturing or from practitioner’s handling, affects polymerization kinetics and mechanical properties of resins. Methacrylates appear to be more strongly influenced than methacrylamides.     

The effect of excitation laser power in Raman spectroscopic measurements of the degree of conversion of resin composites

ObjectivesTo evaluate the effect of excitation laser power in Raman spectrometry by comparing the spectra and the degree of conversion (DC) values obtained using excitation powers between 300 and 1000 mW.MethodsFive commercial and three experimental resin composites were light cured at 1200 mW/cm² for 10–20 s from a commercial blue-violet LED dental curing unit. Raman spectra were collected from composite specimens within 9 min after light-curing. The excitation laser (1064 nm) was focused on the spot of 0.4 mm in diameter. The following powers were used for specimen excitation (mW): 300, 400, 600, 800, and 1000. From Raman spectra, the DC values were calculated and compared among different laser powers. Also, vector-normalized Raman spectra collected using the lowest excitation power (300 mW) were compared to those collected using the maximum excitation power (1000 mW).ResultsVarying the excitation laser power between 300 and 1000 mW resulted in statistically significant differences in both the DC values and the intensity of particular spectral features. The effect of varying laser power on Raman spectra and obtained DC values was material-dependent. The DC values measured within an individual material using different laser powers varied between 3.2 and 7.2% (absolute DC difference). The spectral bands affected by variations in laser power were assigned to symmetric and asymmetric stretching of −CH₂ (2900-3100 cm⁻¹), symmetric stretching of aliphatic CC (1640 cm⁻¹) and scissoring of C–H (1458 cm⁻¹).SignificanceThe DC can be artificially elevated through increasing excitation laser power. This effect should be considered in Raman spectroscopic evaluations of DC in specimens during ongoing post-cure polymerization.     

Translucency,flexural strength,fracture toughness,fracture load of 3-unit FDPs,Martens hardness parameter and grain size of 3Y-TZP materials

ObjectiveThis investigation tested pre-shaded 3Y-TZP materials on optical, mechanical and structural properties and calculated correlations between these properties.MethodsSeven A2-shaded 3Y-TZP zirconia materials were investigated on translucency (T) via UV–vis-spectrophotometer, fracture load of 3-unit FDPs (FL), biaxial flexural strength (FS), Chevron-Notch Beam (CNB), fracture toughness (K_IC) and Martens parameter (hardness: HM and indentation modulus: E_IT). FL, FS and K_IC were measured in a universal testing machine. The grain size was evaluated by scanning electron microscopy (SEM). Data was analyzed using one-way ANOVA followed by post hoc Scheffé, Kruskal–Wallis-, Mann–Whitney-U- and Pearson-test (p < 0.05).ResultsFor translucency, negative correlations were found with results of facture load (R = −0.444, p < 0.001) and K_IC (R = −0.503, p < 0.001). While a positive correlation was found between translucency and flexural strength (R = 0.238, p = 0.019), between fracture load and E_IT (R = 0.227, p < 0.029), between fracture load and K_IC (R = 0.362, p < 0.001) as well as between fracture load and the grain size (R = 0.598, p = 0.007). While the grain size positively correlated with E_IT (R = 0.534, p = 0.017) as well as E_IT with HM (R = 0.720, p < 0.001).SignificanceDespite of being based on the same raw material, tested zirconia materials significantly differed regarding optical, mechanical (except biaxial flexural strength and Martens hardness) and structural properties. Materials with highest optical properties were those with lowest mechanical properties (CER, COP).