Curing potential and color stability of different resin-based luting materials

PurposeTo determine the curing potential and color stability of resin-based luting materials for aesthetic restorations.Material and MethodsFour resin-based luting agents were tested: traditional dual-activated resin cement (RelyX ARC, ARC), amine-free dual-activated resin cement (RelyX Ultimate, ULT), light-activated resin cement (RelyX Veneer, VEN), and pre-heated restorative resin composite (Filtek Supreme, PHC). Degree of C = C conversion was determined by infrared spectroscopy (n = 3) with direct light exposure or with interposition of 1.5-mm-thick ceramic (e.max Press HT) between the luting material and light. The curing potential considered the ratio between these two scenarios. Color difference (n = 6) was determined by CIELAB (ΔE_ab) and CIEDE2000 (ΔE₀₀) methods, by spectrophotometer measurements made 24 h after photoactivation and 90 days after storage in water. Data was submitted to ANOVA and Tukey’s test (α = 0.05).ResultsThe luting agents affected both conversion and color stability. With ceramic, ARC produced the highest conversion among the tested groups (75 ± 1%) and the pre-heated composite (PHC) the lowest one (51 ± 3%), but the curing potential was similar for all materials. ULT produced lower ΔE_ab than ARC. PHC presented the lowest color difference when considered both CIELAB and CIE2000 methods (ΔE_ab 2.1 ± 0.4; ΔE₀₀ 1.6 ± 0.2).SignificanceAll luting strategies presented high curing potential. Amine-free dual-activated material was able to reduce color difference than that formulated with the amine component. Pre-heated composite produced the least color variation after storage.     

Effect of types of luting agent on push-out bond strength of zirconium oxide posts

ObjectiveThe aim of this study was to investigate the effect of different luting agents on the bond strength of zirconium oxide posts in root canals after artificial ageing.Material and methodsThirty single-rooted extracted teeth were collected. Post spaces were prepared. Custom milled zirconium oxide posts (Cercon, Degudent) were fabricated. Specimens were divided into 3 groups (n = 10), according to the luting agents used: group RA, conventional resin luting agent (RelyX ARC); group RU, self-adhesive resin luting agent (RelyX Unicem); and group Z, zinc phosphate luting agent (DeTrey). Specimens were subjected to thermocycling and water storage at 37 °C. Specimens were horizontally sectioned into three sections and subjected to a push-out test with 0.5 mm/min crosshead speed. The failure mode was assessed by scanning electron microscopy. Data were analysed by using 2-way ANOVA.ResultsThe following bond strength values were obtained: group RA – 8.89 MPa, group RU – 10.30 MPa and group Z – 9.31 MPa. There was no significant difference in bond strength among the groups (P = 0.500). Adhesive failure mode at the cement/post bonded interface was seen in 100%, 66.67% and 83.3% of examined sections in groups RA, RU and Z, respectively. There was no significant difference in bond strength among different root regions (P = 0.367).ConclusionThe type of luting agent had no significant effect on the push-out bond strength of zirconium oxide posts after artificial ageing.Clinical significanceConventional luting agents, such as zinc phosphate cement, seem to provide comparable retention to resin luting agents for cementing custom milled zirconium oxide posts.     

Operator vs. material influence on clinical outcome of bonded ceramic inlays

ObjectiveThe aim of the present study was to clinically evaluate the suitability of Definite Multibond and Definite ormocer resin composite for luting of Cergogold glass ceramic inlays in a two-center trial involving two dentists.MethodsThirty-nine patients received 98 Cergogold inlays with at least one inlay luted with Definite Multibond/Definite (n = 45) and at least one inlay luted with Syntac/Variolink Ultra (n = 53) in a split mouth design. Treatments were carried out in two private practices by two operators (Operator A: n = 38; Operator B: n = 60). Forty-four cavities required caries profunda treatment, 23 cavities exhibited no enamel at the cervical margin. At baseline (2 months), and after 6, 14, 27, and 51 months of clinical service, the restorations were investigated according to modified USPHS criteria.ResultsThe drop-out rate was 3% after 4 years. After 48 months of clinical service, 21 restorations in 16 patients (9 luted with Definite, 12 with Variolink; 2 placed by operator A and 19 by operator B) had to be replaced due to inlay fracture (n = 11), tooth fracture (n = 4), hypersensitivities (n = 3), or marginal gap formation (n = 3). Seventy-seven inlays were in good condition (survival rate 89.9%, median survival time 4.2 years (95% confidence interval ±0.25; survival analysis by Kaplan–Meier algorithm). Survival rate after 4 years was 97.4% for operator A, and 75.4% for operator B (p = 0.002; Log Rank/Mantel-Cox) resulting in annual failure rates of 0.6% and 6.2%, respectively. The operators did not differently judge the clinical behaviour of the luting procedures (Mann–Whitney U-test, p > 0.05). Independent of the operator and the used luting system, the following criteria significantly changed over time: color match, marginal integrity, tooth integrity, inlay integrity, sensitivity, hypersensitivity, and X-ray control (p < 0.05; Friedman test). Significant differences between operators over the whole period were computed for the criteria marginal integrity, tooth integrity, and inlay integrity (p < 0.05, Mann–Whitney test). Differences between luting materials were only present during single recalls.SignificancesFor luting of ceramic inlays, only slight differences between the two luting systems were detectable. The operator influence on clinical outcome was clearly proven.     

Mathematical model for assessing true irradiance received by luting materials while curing through modern CAD/CAM resin composites

Statement of problemMeasurement of irradiance passing through a dental restoration for properly curing a dual- or light-polymerized luting composite is imprecise due to surface reflection.ObjectiveTo provide a mathematical correction of measured transmitted irradiance for predicting true transmitted light intensity through CAD/CAM restorations.MethodsA total of 432 specimens were fabricated. Seven modern CAD/CAM resin-based composites (RBCs) and one CAD/CAM glass-ceramic (control group) were sectioned and polished into specimens of 0.5–5 mm thickness (in 0.5 mm steps, n = 6). Irradiance of a violet-blue LED light curing unit (LCU) (power modes: Standard, High and Plasma) was measured after passing through each specimen with a spectrometer. Data was compared based on 95% confidence intervals and using univariate ANOVA followed by Tukey HSD (α = 0.05).ResultsThe measured transmitted irradiance passing through the specimens decreased exponentially. Significantly highest values of transmitted irradiance were measured for 0.5 mm thick specimens for all materials (p < 0.05). The decadic absorption coefficient for CAD/CAM-RBCs ranged from 0.292 mm⁻¹ to 0.387 mm⁻¹ while the control group (glass-ceramic) reached a significantly lower value of 0.283 mm⁻¹. The reflection ratio for all materials ranged from 12.6% to 18.5%.SignificanceA correction can be implemented to predict the true transmitted irradiance after passing through a dental restoration as function of initial irradiance, specimen thickness and material specific parameters. For a practitioner, this model may be applied depending on the specific treatment conditions, the individual LCU's radiant emittance and restoration thickness for the tested materials.     

Influence of various irradiation processes on the mechanical properties and polymerisation kinetics of bulk-fill resin based composites

ObjectivesTo assess the effect of irradiation time and distance of the light tip on the micro-mechanical properties and polymerisation kinetics of two bulk-fill resin-based composites at simulated clinically relevant filling depth.MethodsMicro-mechanical properties (Vickers hardness (HV), depth of cure (DOC) and indentation modulus (E)) and polymerisation kinetics (real-time increase of degree of cure (DC)) of two bulk-fill resin-based composites (Tetric EvoCeram^® Bulk Fill, Ivoclar Vivadent and x-tra base, Voco) were assessed at varying depth (0.1–6 mm in 100 μm steps for E and HV and 0.1, 2, 4 and 6 mm for DC), irradiation time (10, 20 or 40 s, Elipar Freelight2) and distances from the light tip (0 and 7 mm). Curing unit's irradiance was monitored in 1 mm steps at distances up to 10 mm away from the light tip on a laboratory-grade spectrometer.ResultsMultivariate analysis (α = 0.05), Student's t-test and Pearson correlation analysis were considered. The influence of material on the measured mechanical properties was significant (η² = 0.080 for E and 0.256 for HV), while the parameters irradiation time, distance from the light tip and depth emphasise a stronger influence on Tetric EvoCeram^® Bulk Fill. The polymerisation kinetics could be described by an exponential sum function, distinguishing between the gel and the glass phase. The above mentioned parameters strongly influenced the start of polymerisation (gel phase), and were of less importance for the glass phase.ConclusionsBoth materials enable at least 4 mm thick increments to be cured in one step under clinically relevant curing conditions.Clinical significanceThe susceptibility to variation in irradiance was material dependent, thus properties measured under clinically simulated curing conditions might vary to a different extent from those measured under ideal curing conditions.     

The effects of light curing units and environmental temperatures on CC conversion of commercial and experimental bonding agents

Background and purposePolymerization of bonding agents (BA) is a critical factor in determining the success of bonded restorations. We aimed to assess the effects of two light curing units and two temperatures on the extent of polymerization (EP) of a commercial BA and an experimental BA.MethodsForty BA specimens were randomly divided into 8 subgroups of n = 5 to compare the polymerization of two BAs (experimental/Scotchbond) based on the variables: temperature (23/37 °C) and light-curing unit (quartz-tungsten-halogen/light-emitting diode). The EP (%) was measured using differential scanning calorimetry, and analyzed using the t-test, two- and three-way analyses of variance (ANOVA), and the Bonferroni test (α = 0.05).ResultsThere were significant differences between the EP results between the two BAs (P = 0.012) and due to the different temperatures (P = 0.001), but not between the different light-curing units (P = 0.548). The interaction between BA and temperature was significant (P < 0.001). The other interactions were nonsignificant.ConclusionsThe two light-curing units had similar effects on the EP. The EP values were better when curing was performed at human body temperature.     

Influence of curing protocol on selected properties of light-curing polymers: Degree of conversion, volume contraction, elastic modulus, and glass transition temperature

ObjectivesThe purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T_g) as measured on a model polymer. It was a further aim to correlate the measured values with each other.MethodsDifferent light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T_g was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses.ResultsED, PD, and mode of cure influenced DC, C, E, and T_g of the polymer. A significant positive correlation was found between ED and DC (r = 0.58), ED and E (r = 0.51), and ED and T_g (r = 0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r = 0.54), DC and E (r = 0.61), and DC and T_g (r = 0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T_g. Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous.SignificanceA complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.     

Curing efficiency of four self-etching,self-adhesive resin cements

ObjectivesTo evaluate the degree of cure (%DC) of four self-etching, self-adhesive resin cements, and one conventional resin cement, in their self- and dual-curing mode.MethodsThe self-etching, self-adhesive resin cements studied were RelyX? Unicem (3M? ESPE? AG), Maxcem? (Kerr Corporation), Biscem? (Bisco, Inc.) and Multilink^® Sprint (Ivoclar Vivadent^® AG) and the classic resin cement was Multilink^® Automix (Ivoclar Vivadent^® AG). Twelve specimens of each material (1.8 mm × 4 mm × 4 mm) were prepared in room temperature (23 ± 1) °C following the manufacturers’ instructions. Six of them were treated as dual-cured, thus irradiated for 20 s with a halogen light curing unit and left undisturbed for 5 min. The other six were treated as self-cured and were not irradiated, but left in dark and dry conditions for 10 min. The assessment of the %DC was made using micro-ATR FTIR spectrometry.ResultsThe %DC in their self-curing mode was very low (10.82–24.93%), with Multilink Sprint exhibiting the highest values among the five. In the dual-curing mode the values obtained were also low (26.40–41.52%), with the exception of Multilink Automix (61.36%). Maxcem was found to have the lowest DC.SignificanceThe low %DC found raises questions as to whether these materials can be successfully used in clinical applications, where light attenuation takes place. Increased irradiation times could potentially lead to higher %DC, in applications where light is not completely blocked by the overlying restoration.     

Retrospective analysis on the repair vs. replacement of composite restorations

ObjectiveOur retrospective study aimed to assess the impact of repairs on the longevity of anterior and posterior direct composite restorations and to compare longevity of repaired and replacement restorations.MethodsPatient records were retrospectively screened for anterior and/or posterior composite restorations with 2+ surfaces placed in permanent teeth between 2000 and 2015. During follow-up, repaired and replaced restorations were assessed and mean annual failure rates (mAFR) calculated. Outcome was considered in three levels: Success (no further intervention), survival 1 (first repair = survival, second repair = failure) and survival 2 (more than one repair = survival, all surfaces repaired = failure). Statistical analysis was performed using Kaplan–Meier statistics, log-rank tests, and multi-variate Cox-regression analyses with shared frailty (p < 0.05).Results8542 initial restorations placed in 3239 patients were included (4.7 ± 4.4 years follow-up, 2.5 ± 0.7 surfaces). Longevity of initial restorations was prolonged by repair(s) (after 10 years: success: 68.3%, mAFR: 3.7%; survival 1: 77.3%, mAFR: 2.5%; survival 2: 80.4%, mAFR: 2.2%; p < 0.001). 616 repaired restorations (4.9 ± 4.2 years follow-up, 3.5 ± 1.0 surfaces) and 264 replacement restorations (5.6 ± 4.1 years follow-up, 3.0 ± 0.8 surfaces) were analyzed. Success of repaired restorations amounted to 43.4% after 10 years (mAFR: 8.0%), further repair(s) prolonged survival (survival 1: 65.7%, mAFR: 4.1%; survival 2: 74.8%, mAFR: 2.9%; p < 0.001). Success of replacement restorations amounted to 48.6% after 10 years (mAFR: 7.0%), repair(s) prolonged survival (survival 1: 67.4%, mAFR: 3.9%, p = 0.044; survival 2: 74.1%, mAFR: 3.0%, p = 0.003).SignificanceRepairs are suitable to increase the survival of restorations; repaired restorations last as long as replacements.     

Clinical performance and SEM marginal quality of extended posterior resin composite restorations after 12 years

ObjectiveTo investigate the clinical behavior of two different resin-based restorative systems in extended Class II cavities in a controlled prospective split-mouth study over 12 years and to assess marginal quality under a SEM using epoxy replicas.MethodsThirty patients received 68 resin composite restorations (Solobond M + Grandio: n = 36; Syntac + Tetric Ceram: n = 32) by one dentist in a private practice. 35% of cavities revealed no enamel at the bottom of the proximal box, 48% of cavities provided <0.5 mm remaining proximal enamel. Restorations were examined according to modified USPHS criteria at baseline, and after six months, one, two, four, six, eight, 10, and 12 years. Expoxy replicas of 21 restoration pairs were analysed under a SEM at 200× magnification regarding marginal quality.ResultsAt the 12-years recall, 59 of the original 68 restorations in 27 of 30 patients were available (drop out 13%). Two restorations failed due to cusp fracture (Tetric Ceram) and marginal fracture (Grandio). The overall success rate of all restorations was 97.1% (Kaplan–Meier survival algorithm) with no differences between the two materials (p = 0.923). After 144 months of clinical service, restorations in molars performed worse than in premolars regarding the integrity of restoration and tooth (p < 0.05) being detected as wear, chippings, and cracks. Beyond the 4-year recall, marginal staining significantly increased. SEM evaluation of replicas revealed that perfect margin (49% baseline vs. 10% after 12 years), overhang (13% at baseline vs. 3% after 12 years), negative step formation (34% at baseline vs. 75% after 12 years), and marginal fractures (0% at baseline vs. 6% after 12 years) significantly changed during the evaluation period (p < 0.001).SignificanceExtended direct resin composite restorations performed satisfactorily over 12 years of clinical service. SEM analysis delivered qualitative data regarding marginal deterioration over time.     

Nanohybrid vs. fine hybrid composite in Class II cavities: Clinical results and margin analysis after four years

ObjectivesThis controlled prospective split-mouth study evaluated the clinical behavior of two different resin composites in extended Class II cavities over a period of four years.MethodsThirty patients received 68 direct resin composite restorations (Grandio bonded with Solobond M: n = 36, Tetric Ceram bonded with Syntac: n = 32) by one dentist in a private practice. All restorations were replacement fillings, 24 cavities (35%) revealed no enamel at the bottom of the proximal box, in 33 cavities (48%) the proximal enamel width was less than 0.5 mm. The restorations were examined according to modified USPHS criteria at baseline, and after six months, one, two, and four years. At each recall, impressions were taken for replica preparation. Replicas of 44 select subjects were assessed for marginal quality under a stereo light microscope (SLM) at 130× and 22 replicas were assessed under a scanning electron microscope (SEM) at 200×.ResultsBoth recall rate and survival rate were 100% after four years of clinical service. No significant difference was found between the restorative materials (p > 0.05; Mann–Whitney U-test). Hypersensitivities were significantly reduced over time (p < 0.05; Friedman test). A significant deterioration over time was found for the criteria marginal integrity (66% bravo after four years), tooth integrity (15% bravo), filling integrity (73% bravo) and proximal contact (p < 0.05; Friedman test). SLM and SEM analysis of restoration margins revealed differences in the amount of perfect margins, in favor of Tetric Ceram (p < 0.05).SignificancesBoth materials performed satisfactorily over the four-year observation period. Due to the extension of the restorations, wear was clearly visible after four years of clinical service with 50% bravo ratings.     

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).     

Curing potential of experimental resin composites filled with bioactive glass: A comparison between Bis-EMA and UDMA based resin systems

ObjectivesTo evaluate the degree of conversion, light transmittance, and depth of cure of two experimental light-curable bioactive glass (BG)-containing composite series based on different resin systems.MethodsExperimental composite series based on either Bis-EMA or UDMA resin were prepared. Each series contained 0, 5, 10, 20, and 40 wt% of BG 45S5. Reinforcing fillers were added up to a total filler load of 70 wt%. The degree of conversion was evaluated using Raman spectroscopy, while light transmittance was measured using visible light spectroscopy. The depth of cure was estimated from the degree of conversion data and using the ISO 4049 scraping test.ResultsReplacement of reinforcing fillers with BG can diminish the degree of conversion, light transmittance, and depth of cure. The effect of BG on the aforementioned properties was highly variable between the experimental series. While in the Bis-EMA series, the degree of conversion was significantly impaired by BG, all of the composites in the UDMA series attained clinically acceptable degree of conversion values. The reduction of the degree of conversion in the Bis-EMA series occurred independently of the changes in light transmittance. The UDMA series showed better light transmittance and consequently higher depth of cure than the Bis-EMA series. The depth of cure for all composites in the UDMA series was above 2 mm.SignificanceWhile the Bis-EMA series demonstrated clinically acceptable curing potential only for 0–10 wt% of BG loading, an excellent curing potential in the UDMA series was observed for a wide range (0–40 wt%) of BG loadings.     

Color change of some aesthetic dental materials: Effect of immersion solutions and finishing of their surfaces

ObjectivesThe purpose of this research was to evaluate the color change of five aesthetic dental materials, before and after immersion in distilled water and blue food color solution for 7 and 21 days, and to study the effect of finishing the surfaces on any color change.MethodsDisc shaped samples of five types of light curing composite (A2) (n = 10 samples/composite) were prepared and all samples were light-cured with a Plasma Arc light cure unit for ten seconds. One side of each sample disc was finished and polished with a Super-Snap system all samples. After 24 h, color measurements of each sample were conducted using a digital spectrophotometer. Five sample discs from each composite group were immersed in 30 ml of food color solution for 7 and 21 days, while the remaining five sample discs were immersed in 30 ml of distilled water as a control. Color measurements were repeated for all samples at 7 and 21 days after immersion. The color changes were statistically analyzed using t-tests within the same group. A result was considered statistically significant at α = 0.05.ResultsThe color differences (ΔE) ranged from 0.4 to 4.66 and statistically significant differences on the finished and unfinished surfaces were observed after immersion in the food color solution for 7 days. No significant differences were found in any group after immersion in the food color solution for 21 days. The Tetric EvoCeram and Arabesk groups showed less color differences after 7 and 21 days than other composites.ConclusionFinished composite surfaces showed less coloration than unfinished surfaces after 7 days, but all surfaces (finished and unfinished) were highly colored for all composite types after 21 days.     

Thermo and mechanical cycling and veneering method do not influence Y-TZP core/veneer interface bond strength

ObjectivesThe purpose of this study was to evaluate the influence of thermal and mechanical cycling and veneering technique on the shear bond strength of Y-TZP (yttrium oxide partially stabilized tetragonal zirconia polycrystal) core–veneer interfaces.Materials and methodsCylindrical Y-TZP specimens were veneered either by layering (n = 20) or by pressing technique (n = 20). A metal ceramic group (CoCr) was used as control (n = 20). Ten specimens for each group were thermal and mechanical cycled and then all samples were subjected to shear bond strength in a universal testing machine with a 0.5 mm/min crosshead speed. Mean shear bond strength (MPa) was analysed with a 2-way analysis of variance and Tukey's test (p < 0.05). Failure mode was determined using stereomicroscopy and scanning electron microscopy (SEM).ResultsThermal and mechanical cycling had no influence on the shear bond strength for all groups. The CoCr group presented the highest bond strength value (p < 0.05) (34.72 ± 7.05 MPa). There was no significant difference between Y-TZP veneered by layering (22.46 ± 2.08 MPa) or pressing (23.58 ± 2.1 MPa) technique. Failure modes were predominantly adhesive for CoCr group, and cohesive within veneer for Y-TZP groups.ConclusionsThermal and mechanical cycling, as well as the veneering technique does not affect Y-TZP core–veneer bond strength.Clinical significanceDifferent methods of veneering Y-TZP restorations would not influence the clinical performance of the core/veneer interfaces.     

The influence of light-curing time on the bacterial colonization of resin composite surfaces

ObjectivesBacterial colonization of composite surfaces represents the main factor in the etiology of secondary caries around adhesive restorations. The authors’ aim was to evaluate the influence of light-curing time on mutans streptococci colonization (MS) of a resin composite material.MethodsSpecimens obtained from a dental resin composite were divided into 12 groups and light-cured with the same light source respectively for 10, 20, 30, 40, 60 or 80 s using two different curing-power levels: 400 and 800 mW/cm². A wild strain of MS was isolated and a 24-h-monospecific biofilm, adherent to the surfaces of the samples, was obtained. A colorimetric technique (MTT assay), based on the reduction of a yellow tetrazolium salt to a purple formazan, was used to evaluate the biomass adherent to the specimen surfaces. ANOVA and Scheffé’s tests were used to statistically analyze the results.ResultsTwo-way ANOVA demonstrated there was no interaction between curing-time factor and curing-power factor (p = 0.970); one-way ANOVA was used to analyze separately the data obtained from each curing-power level. Both levels showed highly significant differences (p < 0.0001) among the different curing time groups. The non-parametric test for trend showed in both levels the existence of a highly significant trend (p < 0.0001) for bacterial colonization reduction as curing time increases.SignificanceA reduced curing time seems to be responsible for increased in vitro colonization of composite surfaces by MS; this phenomenon is likely to be related to the presence of unpolymerized monomers on the material surface.     

Tensile bond strength of cast commercially pure titanium dowel and cores cemented with three luting agents

PurposeThe purpose of this study was to compare the tensile strength of commercially pure titanium dowels and cores cemented with zinc phosphate or resin cements.MethodsTwenty-one extracted human canines were endodontically treated. The root preparations were accomplished using Largo reamers (10 mm in depth and 1.7 mm in diameter). Acrylic resin patterns for the dowel and cores were made, and specimens were cast in commercially pure titanium (n = 7) and divided in three groups: TZ–CP Ti dowels luted with zinc phosphate luting agent, TP–CP Ti dowels luted with Panavia F and TR–CP Ti dowels luted with RelyX U100. Tensile strengths were measured in a universal testing machine at a crosshead speed of 0.5 mm/min. The results (N) were statistically analyzed by ANOVA and Tukey tests (α = 0.01).ResultsThe ANOVA indicated that there were significant differences among the groups tested. A Tukey multiple comparison procedure was performed and revealed statistically significant higher retention values for the dowel luted with RelyX U100 when compared with zinc phosphate or Panavia F.ConclusionCast commercially pure titanium dowels and cores fixed with RelyX U100 cement presented superior bond strength retention when compared to zinc phosphate and Panavia F.     

Spatial and cure-time distribution of dynamic-mechanical properties of a dimethacrylate nano-composite

ObjectiveThe purpose of this study was to evaluate a nano-filled dental composite, with varying cure irradiation-time, in terms of the spatial distribution of dynamic-mechanical properties determined at nanometre scale and the resultant distinction between filler, matrix and inter-phase regions.Materials and methodsSpecimen groups (n = 5) of the composite Filtek Supreme XT were cured in 2 mm deep molds for 5, 10, 20 and 40 s, and stored for 24 h in distilled water at 37 °C. Properties were measured at 2 mm depth, on the lower specimen surfaces. Nano-dynamic-mechanical parameters (complex, storage and loss modulus, tan δ) were determined at an array of 65,000 locations in a 5 μm × 5 μm area. Micro-mechanical properties (hardness, modulus of elasticity, creep and elastic/plastic deformation) were also measured and additionally the real-time degree of cure, by ATR-FTIR, for 10 min after photo-initiation and after storage.ResultsThe spatial distribution of nano-dynamic-mechanical properties varied significantly enabling four distinguishable matrix, filler-cluster and inter-phase regions to be identified. Proceeding from matrix to filler-cluster locations, complex-moduli increased linearly and loss-factors decreased linearly, consistent with visco-elastic composite theory. Curing time strongly affected all measured properties at 2 mm depth. The organic matrix was shown to be inhomogeneous for all curing times. By increasing cure-time, the proportion of less well polymerized area decreased from 37.7 to 1.1%, resulting in a more homogeneous organic matrix.SignificanceThe experimentally observed graduated transition, in complex modulus and related dynamic-mechanical properties, across the matrix – inter-phases – filler-cluster regions is conducive to low internal stresses, in contrast to the abrupt modulus transitions anticipated or observed in many other particulate composite structures. The identification of these phase-regions provides a realistic basis for accurate nano- and micro-mechanical computational modelling.     

Polymerization shrinkage kinetics of dimethacrylate resin-cements

ObjectivesTo determine polymerization shrinkage-strain (S_Y) and shrinkage-stress (S_Z) of six resin-cements and to compare their performance with the aid of degree of conversion (DC) data.MethodsVariolink 2 (VL2), Multilink Automix (MA), Multilink Sprint (MS, all Ivoclar-Vivadent), Nexus 2 (NX2), Maxcem (MX, both Kerr) and RelyX Unicem (RX, 3M-Espe) were investigated. MS, MX and RX were self-adhesive; others require a bonding-agent. All measurements were conducted at 23 °C for 60 min (n = 5), except 80 min for RX, with materials self-cured only (sc) and dual-cured (dc); NX2 and VL2 were additionally light-cured only (lc). S_Y was measured by the bonded-disk method [Watts DC, Cash AJ. Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 1991;7(4):281–7; Watts DC, Marouf AS. Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials. Dent Mater 2000;16(6):447–51]; S_Z by the Bioman instrument [Watts DC, Satterthwaite JD. Axial shrinkage-stress depends upon both C-factor and composite mass. Dent Mater 2008;24(1):1–8 [Epub October 24, 2007]; Watts DC, Marouf AS, Al-Hindi AM. Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development. Dent Mater 2003;19(1):1–11]. Light-cure was achieved by QTH at 500 mW/cm². The respective DCs were measured under the same conditions by FTIR–ATR spectroscopy. Data were analyzed by One-Way ANOVA plus Bonferroni test, and by t-test, at p < 0.05.ResultsDC by self-curing was less than the DC by dual-curing, for all cements. Shrinkage-strain ranged from 1.77 to 5.29% and shrinkage-stress from 3.36 to 10.37 MPa. NX2 and VL2 were not significantly different, when light-cured only. Except for RX, sc and dc shrinkage-strain varied maximally by 0.4%. MX showed the highest S_Y, RX the lowest. When sc, RX initially expanded by <0.5% (t ～ 5 min). For most materials, S_Y correlated with their filler loading. The highest stress with sc was exerted by MX, and when dc by MS, which was not statistically different from MX.SignificanceShrinkage data of resin-cements are of intrinsic clinical importance. Self-cure, despite a lower DC, did not necessarily result in a lower S_Y compared to dual-cure. S_Y-rate and S_Z development depend upon cure mode and S_Y upon filler fraction.     

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.