Hardening of dual-cured cements under composite resin inlays

Comparative analysis of self-cure and dual cure-dental composites on their physico-mechanical behaviour

BACKGROUND

Clinical practitioners may have become familiar with the rapid transformation of dental composites. However, they may not scientifically understand the factors influencing the mechanical and physical properties. Scientific knowledge of filler-resin interaction can significantly improve clinical understanding of resin composites. Several independent studies have examined the mechanical and physico-mechanical properties of dental resin composites; however, no comprehensive study has examined the influence of fillers and resin materials on the physico-mechanical properties of both self-cure and dual-cure composites.

METHODS

This study performed investigations on the physico-mechanical behaviour of four commercially available dual-cure dental composites (Bioactive, Fill Up!, Surefil One, Cention N) and two commercially available self-cure dental composites (Stela Capsule and Stela Automix). Test specimens for flexural and compressive strength, microhardness, fracture toughness, and hydrolytic behaviour were prepared and tested as per respective standards. The data sets were statistically analysed using one-way ANOVA and Tukey's post-hoc comparison.

RESULTS

There was a substantial variation in flexural strength and modulus values in this study, ranging from 32.0 to 113.4 MPa and 2.36 to 12.07 GPa, respectively. Similarly, there were significant differences in compressive strength between the materials in this study, ranging from 119.3 to 223.5 MPa. The highest fracture toughness value was found to be 1.41 MPa.m0.5, while the lowest value was 0.43 MPa.m0.5. Variations in surface microhardness were significant (24.11-68.0 N/mm2), which correlated with the filler content. Water sorption and solubility demonstrated high variations among materials, with Surefil One exceeding ISO 4049 thresholds significantly.

CONCLUSIONS

A linear correlation can be established between surface microhardness (HV) and flexural and compressive moduli, as well as filler content (wt.%). However, both flexural and compressive strengths are impacted by the resin's constituent monomers and the resin-filler matrix's cross-linking capability. Additionally, factors such as filler size, shape, and the cross-linking ability of the resin-filler matrix play a crucial role in fracture toughness and the propagation of cracks within the restoration. Also, resin monomers and filler particle size affect the hydrolytic degradation characteristics of composites, which can also affect their mechanical properties. © 2023 Australian Dental Association.

Effect of Photo-polymerization Delay on the Bond Strength and Microhardness of Dual-polymerizing Resin Cements

STATEMENT OF PROBLEM

To fully maximize the potential of dual-polymerizing resin cements, a thorough understanding of how the light- and chemical-polymerizing components interact in a resin system is required. Disorder in the polymerization process between the two components may hurt one of the components versus the other, affecting the overall properties and performance of the resin cements.

PURPOSE

Evaluate photo-polymerization delay time on dentin shear-bond strength and Vickers microhardness of dual-polymerizing resin cements.

METHODS AND MATERIALS

Shear bond strength (SBS) of self-adhesive (RelyX Unicem 2, 3M ESPE) and adhesive (RelyX Ultimate, 3M ESPE) dual-polymerizing resin cements were evaluated. Dentin specimens (n=80) were prepared for the SBS test according to ISO standard 29022:2013. Teeth were randomly allocated into eight groups based on the type of cement, and photo-polymerization delay times (0, 2, 5, and 10 minutes). Vickers microhardness test (HV) was performed following ASTM E384-17 (n=32) prepared based on cement type and photo-polymerization delay times; specimens were tested after 24 hours of storage. Statistical analysis was performed using two-way ANOVA to determine the individual and combined effects of resin cement type and photo-polymerization delay time on SBS and HV.

RESULTS

Resin cement and photo-polymerization delay times for the adhesive cement at 0- and 2-minute pairings had significantly higher SBS means than all other combinations (p<0.0001). Resin cement type was also statistically significant (p<0.0001). Resin cement type and photo-polymerization delay times were not significant (p=0.3550) for HV.

CONCLUSIONS

Photo-polymerization delay time affected dentin SBS with higher bond strength when photo-polymerization delay time was performed between 2 and 5 minutes with a self-adhesive resin cement, and between 0 and 2 minutes with an adhesive resin cement. Delaying photo-polymerization time to 10 minutes led to inferior dentin SBS and HV for both self-adhesive and adhesive dual-polymerizing resin cements.

Could light-curing time, post-space region and cyclic fatigue affect the nanomechanical behavior of a dual-curing cement for fiber post luting?

OBJECTIVE

To evaluate the effects of curing time, post-space region and cyclic fatigue on the micromechanical properties of a fiber-post luting cement. The null hypotheses were that (1) curing time, (2) fatigue and (3) post-space region does not affect the nanoindentation modulus and hardness of the dual-curing cement.

MATERIALS AND METHODS

48 premolars were endodontically treated and a class I cavity and 8 mm deep post space was prepared. Fiber posts were luted with a universal, dualized adhesive system and a dual-curing cement following manufacturer's instructions. Specimens were divided into three groups (16 specimens for each group) according to light-curing time (no light-curing, 20 s light-curing and 120 s light-curing), which was performed with a LED lamp at 1000 mW/cm 2. The coronal part of the cavity was restored using a nano-filled resin composite. After 24 h, 8 specimens for each group were randomly extract in order to undergo to fatigue test in wet condition through a chewing simulator, while the other specimens were kept in distilled water as benchmark. All the restored teeth were then sectioned in 1 mm thick slices perpendicularly to the fiber post axis. Specimen slices were classified in coronal and apical to be tested through a nanoindenter. Data were analyzed through Kruskal-Wallis test with a significance level of 1%, in order to evaluate the influence of treatments (i.e., curing time and cyclic loading) on the micromechanical properties of the tested luting cement.

RESULTS

Both fatigue and curing time significantly influenced nanoindentation modulus and hardness of dual-curing cement (p < 0.01). No significant differences were reported for post space region. A significant interaction was found among the analyzed factors (p < 0.01).

SIGNIFICANCE

120 s light-curing time is recommended in order to achieve optimal mechanical proprieties, independently from post space region and cyclic fatigue. As matter of fact, 120 s light-curing allowed to prevent strain hardening induced by the fatigue simulation.

Time-dependent Microhardness Gradients of Self-adhesive Resin Cements Under Dual- and Self-curing Modes

CLINICAL RELEVANCE

Acid-functional monomers in self-adhesive resin cements may decrease their self-curing polymerization ability. Light irradiation optimizes polymerization performance.

SUMMARY

Purpose: The aim of this study was to investigate Knoop microhardness of self-adhesive resin cements under dual- and self-curing modes in simulated canals for describing the polymerization behavior.Methods and Materials: Slots in lightproof silicone cylinders with one open end were filled with the following eight materials: a traditional resin cement (Duolink), a core build-up resin material (MultiCore Flow), and six self-adhesive resin cements (RelyX Unicem 2, G-Cem Automix, Maxcem, Biscem, Multilink Speed, and PermaCem 2.0). The resins were exposed to light through the open end and then stored in a lightproof box. The Knoop hardness gradient for each resin was measured after 1 hour and 120 hours. Surface readings were obtained at 1-mm intervals from 1 mm to 10 mm away from the open ends. The data were analyzed by two-way analysis of variance and the Student-Newman-Keuls test (α=0.05).Results: All the resin materials had stable Knoop hardness numbers (KHNs) at a certain depth; their KHNs in the self-curing mode did not change (p>0.05). The region above this certain depth was regarded as having undergone the dual-curing mode, and the KHN decreased gradually with depth (p<0.05). Between 1 and 120 hours postexposure, the ratio of the KHN at a 5-mm depth (self-cured) to that at a 1-mm depth (dual-cured) increased in Duolink and MultiCore Flow. However, the ratios of the six adhesive resin cements varied.Conclusion: Without light, most self-adhesive resin cements differed from traditional dual-cured resin materials in terms of Knoop micro-hardness, and they had a lesser capacity for chemical-induced curing.

Depth-dependence of Degree of Conversion and Microhardness for Dual-cure and Light-cure Composites

Clinical Relevance

New dual-cure bulk-fill composites show promise for uniform degree of conversion and microhardness throughout the entire depth of direct restorations.

SUMMARY

Objective:

The aim of this study was to evaluate the degree of conversion (DC) through micro-Raman spectroscopy and surface microhardness in Vickers hardness (VHN) of three new dual-cure bulk-fill resin-based composites (RBCs) compared with light-cure bulk-fill and incremental RBCs at two clinically relevant depths and for two light irradiation times.

Methods:

Three commercially available restorative dual-cure bulk-fill RBCs (BulkEZ, HyperFIL, and Injectafil) were evaluated and compared with three light-cure RBCs (Filtek Bulk Fill Flowable, Filtek One Bulk Fill, and incremental Filtek Z250) as controls. Specimens were prepared in two different depths (0.5 mm and 5 mm) and were light irradiated for 20 seconds or 40 seconds. Self-cure was also evaluated for the three dual-cure bulk-fill RBCs. Micro-Raman spectroscopic measurements and VHN tests (n=5) were made after 24 hours of dry storage in the dark at room temperature for all test conditions. Data were analyzed using one-way and two-way analyses of variance (α=0.05).

Results:

All tested RBCs showed significantly higher DC and VHN values at 0.5-mm depth than at 5-mm depth, with the exception of BulkEZ, which showed similar DC and VHN values at two depths. The three dual-cure bulk-fill RBCs showed significantly higher DC than the three light-cure RBCs under the same curing condition. The three dual-cure RBCs showed much smaller differences in VHN values between the two depths than the three light-cure RBCs. Twenty seconds and 40 seconds of light irradiation did not generate significant difference in DC and VHN values for the three dual-cure bulk-fill RBCs at either depth or for the three light-cure RBCs at the 0.5-mm depth; however, 40 seconds of light irradiation generated significantly higher DC and VHN values for One Bulk Fill and Z250 at the 5-mm depth compared with 20 seconds of light irradiation.

The effect of elapsed time following alumina blasting on adhesion of CAD/CAM resin block to dentin

The aim of this study was to evaluate the time elapsed bond strength of the CAD/CAM resin block (CRB) to bovine dentin after alumina blasting. CRB (KATANA AVENCIA BLOCK) slices were ground with #600-SiC paper and divided into three groups according to alumina blasting pressure -0.1 MPa, 0.2 MPa or untreated- and then divided into two subgroups according to the time elapsed after alumina blasting -same-day or one-week "dry-storage" at controlled laboratory conditions before cementation. The CRB slices were then cemented to bovine dentin with Panavia V5 (Kuraray Noritake Dental), and divided into two subgroups -light curing or chemical curing. After 24 h storage in distilled water at 37°C, the specimens were then subjected to micro-tensile bond strength (µTBS) testing. One-week group showed a significant decrease in µTBS. The µTBS values showed that CRBs must be cemented with light curing immediately after alumina blasting at 0.1 or 0.2 MPa to obtain a stable adhesion.

Effects of coloring procedures on shear bond strength between resin cement and colored zirconia

PURPOSE

Debonding is expected as a frequent failure type in zirconia restorations. Therefore the aim of the current study is to evaluate the shear bond strength between colored zirconia and resin cement.

MATERIALS AND METHODS

There were 11 groups evaluated each containing 12 zirconia discs (15 mm x 12 mm x 1.6 mm). Groups were colored with the colors A3, B1, C4, D2, and D4 of the VITA classical shade scale. Coloring procedure was carried out for either 3 second or 60 seconds for the study groups and the control group was left untreated. Specimens were then bonded to translucent resin cement having a thickness of 3 mm and width of 3 mm. The shear bond strength of the samples was measured in a universal testing machine with a crosshead speed of 1 mm per minute. Two-way analysis of variance and Tukey's HSD test were used for pairwise comparisons. Also paired t-test was used for comparing groups with the same color but having different shading times.

RESULTS

Any significant difference was not found between the shear bond strengths of samples depending on whether color or shading times. Among the groups, B1 (60 seconds of coloring) had the highest bond strength (10.05 MPa), while A3 (60 seconds of coloring) showed the lowest bond strength (6.72 MPa). However, these differences were not statistically significant.

CONCLUSION

Coloring zirconia did not affect the shear bond strength between zirconia and resin cement.

An in vitro study to compare the influence of different all-ceramic systems on the polymerization of dual-cure resin cement

Aim:

The aim of the study is to compare the effect of composition of three different all-ceramic systems on the polymerization of dual-cure resin cement, using different curing cycles and evaluated immediately within 15 min and after 24 h.

Materials and Methods:

Resin cement disc samples were fabricated by polymerization through three different all-ceramic disc, namely: lithium disilicate discs – IPS e.max (Group B), leucitereinforced discs – IPS Empress (Group C), zirconia discs – Cercon (Group D), and without an intervening ceramic disc, as control (Group A). A total of 80 resin cement disc samples were fabricated for fur groups (n = 20). Each group further consisted of two subgroups (n = 10), t10 and t20 according to two different exposure times of 10 and 20 s, respectively. Each of the 80 resin disc samples was evaluated for their degree of polymerization achieved, by measuring the microhardness(Vickers hardness number) of the samples immediately within 15 min and after 24 h, giving us a total of 160 readings. Oneway analysis of variance test, ttest, and paired ttest were used for multiple group comparisons followed by Tukey's post hoc for groupwise comparison.

Results:

Direct activation of the resin cement samples of control (Group A) showed statistically significant higher mean microhardness values followed by Groups C then B and D, both immediately and after 24 h. The mean microhardness for immediate post-activation was always inferior to the 24 h post-activation test. For both 10 and 20 s curing cycle, there was a significant increase in the microhardness of the resin cement discs cured for 20 s through the different ceramics.

Conclusion:

Ceramic composition affected the polymerization of dual cured resin cement. Doubling the light irradiation time or curing cycle significantly increased mean microhardness value. Greater degree of conversion leading to an increase in hardness was observed when the resin cement discs were evaluated after 24 h.

Influence of polymerization time on properties of dual-curing cements in combination with high translucency monolithic zirconia

PURPOSE

The aim of this in vitro study was to assess conversion degree (DC), micro-hardness (MH) and bond strength of two dual-curing resin cements employed under translucent monolithic zirconia irradiated with different time protocols.

METHODS

84 square shaped samples of 1mm thickness were prepared from high translucency zirconia blocks and divided into two groups (n=24) according to the cement employed: (1) Rely-X Ultimate; (2) Panavia SA. Each group was further divided into 3 subgroups (n=8) according to the irradiation time: (a) no light; (b) 20s; (c) 120s. Light curing was performed 60s after the sample was placed on the diamond support of a FT-IR spectrophotometer with a high power multiLED lamp. Final DC% were calculated after 10min. After 24h, Vickers Test on the cement layer was performed. The same protocol was used to lute composite cylinders in order to evaluate microshear bond-strength test. ANOVA and Bonferroni tests were performed to find differences between MH and bond-strength to zirconia, while for DC% the Scheirer-Ray-Hare two-way test was used.

RESULTS

The two cements reached higher DC% in subgroup (b) and (c). As concern MH, statistics showed an increase in curing time was able to improve MH significantly. Bond-strength was not affected by irradiation time only for Panavia SA.

CONCLUSIONS

The first null hypothesis has to be rejected since DC% and MH of the dual-cements tested were influenced by the curing time. The second null hypothesis is partially rejected since the bond strength was influenced by the curing time only for Rely-X Ultimate.

Universal cements: dual activated and chemically activated

Objective: The aim of the present study was to assess the bond strength of universal cements cured either dually or chemically only.

Methods: Three cements were assessed using different types of application: dual activated (DA) or chemically activated (CA). In total 80 dentin blocks were used, obtained through the enamel wear of the lingual and buccal surfaces of bovine incisors. Standard cone-shaped cavity preparations were created using diamond burs. Subsequently, indirect restoration blocks were designed with Filtek Z350 (3M ESPE) composite resin. The teeth were divided into two groups (DA and CA) and then subdivided into four subgroups (n = 10) prior to cementation with the respective products: Duo-Link (Bisco); RelyX Ultimate (3M ESPE); Nexus 3 (Kerr) and conventional RelyX ARC (3M ESPE) as the control. The cementation in the PA group was applied following the manufacturer’s instructions. The CA group was cemented in a darkroom to avoid exposure to light. They were stored in distilled water at 37 °C for 24 h and submitted to the push-out test. Data were analyzed by two-way ANOVA and Tukey’s post-hoc test (p < .05).

Results: The greatest bond strength results were obtained for photoactivated universal cements.

Conclusion: Chemical activation is not sufficient to ensure acceptable bond strength.

Compressive strength and the effect of duration after photo-activation among dual-cure bulk fill composite core materials

Objectives:

To assess compressive strength and effect of duration after photoactivation on the compressive strength of different dual cure bulk fill composites.

Methods:

Seventy-two disc shaped (4x10mm) specimens were prepared from three dual cure bulk fill materials, ZirconCore (ZC) (n=24), MulticCore Flow (MC) (n=24) and Luxacore Dual (LC) (n=24). Half of the specimens in each material were tested for failure loads after one hour [MC1 (n=12), LC1 (n=12) & ZC1 (n=12)] and the other half in 7 days [MC7 (n=12), LC7 (n=12), ZC7 (n=12)] from photo-polymerization using the universal testing machine at a cross-head speed of 0.5 cm/minutes. Compressive strength was calculated using the formula UCS=4f/πd². Compressive strengths among different groups were compared using analysis of variance (ANOVA) and Tukey’s multiple comparisons test.

Results:

Maximum and minimum compressive strengths were observed in ZC7 (344.14±19.22) and LC1 (202.80±15.52) groups. Specimens in LC1 [202.80 (15.52)] showed significantly lower compressive strength as compared to MC1 [287.06 (15.03)] (p<0.01) and ZC1 [276.82 (11.51)] (p<0.01). ZC7 [344.14 (19.22)] specimens showed significantly higher (p<0.01) compressive strengths compared to LC7 [324.56 (19.47)] and MC7 [315.26 (12.36)]. Compressive strengths among all three materials were significantly higher (p<0.01) at 7 days as compared to one hour.

Conclusions:

Bulk fill material with Zr nano-hybrid filler (ZC) showed high compressive strength compared to MC and LC. Increasing the post photo-activation duration (from one hour to 7 days) significantly improves the compressive strengths of dual cure bulk fill material.

Additional chemical polymerization of dual resin cements: reality or a goal to be achieved?

Abstract Introduction This study serves as a warning to dentists and researchers that dual-cured resin cements may not polymerize completely under some prosthetic crowns. Objective The aim of this study was to analyse the polymerization degree of dual-cured resin cements under prosthetic barrier, by microhardness test. Material and method Three cements (Bistite II, RelyX ARC and Variolink II) were light-cured through different barriers, placed between the cement and the light source: G1: without barrier; G2: composite resin (Cesead); G3: Inceram alumina; G4: IPS Empress; G5: Inceram zirconia; G6: tooth fragment. Photopolymerization was carried out using a halogen light unit (650 mW/cm2); microhardness was evaluated using the Microhardness Tester FM 700, under a load of 50gf with a dwell time of 15s, at two evaluation times (30min and 24h). Result The results were submitted to ANOVA and Tukey tests (5%). Both Inceram alumina and Inceram zirconia ceramic barriers hindered polymerization. Bistite, followed by RelyX and Variolink, exhibited the highest microhardness values (p<0.05). As the highest values were obtained without a barrier, it was determined that the barrier, followed by the tooth, influenced microhardness. Both Empress and Cesead had the smallest microhardness values but with no statistically significant difference between them. Conclusion The barrier negatively affected the microhardness of dual-cured resin cements; evaluation time did not affect microhardness values for most of the conditions tested. There is a limited effect of the chemical activator on the polymerization of some dual-cured cements, and their performance is product specific.

Effect of Curing Mode on Shear Bond Strength of Self-Adhesive Cement to Composite Blocks

To overcome the disadvantages of computer-aided design/computer-aided manufacturing (CAD/CAM) processed indirect restorations using glass-ceramics and other ceramics, resin nano ceramic, which has high strength and wear resistance with improved polish retention and optical properties, was introduced. The purpose of this study was to evaluate the shear bond strength and fracture pattern of indirect CAD/CAM composite blocks cemented with two self-etch adhesive cements with different curing modes. Sand-blasted CAD/CAM composite blocks were cemented using conventional resin cement, Rely X Ultimate Clicker (RXC, 3M ESPE, St. Paul, MN, USA) with Single Bond Universal (SB, 3M ESPE, St. Paul, MN, USA) for the control group or two self-adhesive resin cements: Rely X U200 (RXU, 3M ESPE, St. Paul, MN, USA) and G-CEM Cerasmart (GC, GC corporation, Tokyo, Japan). RXU and GC groups included different curing modes (light-curing (L) and auto-curing (A)). Shear bond strength (SBS) analyses were performed on all the specimens. The RXC group revealed the highest SBS and the GC A group revealed the lowest SBS. According to Tukey's post hoc test, the RXC group showed a significant difference compared to the GC A group (p < 0.05). For the curing mode, RXU A and RXU L did not show any significant difference between groups and GC A and GC L did not show any significant difference either. Most of the groups except RXC and RXU L revealed adhesive failure patterns predominantly. The RXC group showed a predominant cohesive failure pattern in their CAD/CAM composite, Lava^TM Ultimate (LU, 3M ESPE, St. Paul, MN, USA). Within the limitations of this study, no significant difference was found regarding curing modes but more mixed fracture patterns were showed when using the light-curing mode than when using the self-curing mode.

Correlation between clinical performance and degree of conversion of resin cements: a literature review

Resin-based cements have been frequently employed in clinical practice to lute indirect restorations. However, there are numerous factors that may compromise the clinical performance of those cements. The aim of this literature review is to present and discuss some of the clinical factors that may affect the performance of current resin-based luting systems. Resin cements may have three different curing mechanisms: chemical curing, photo curing or a combination of both. Chemically cured systems are recommended to be used under opaque or thick restorations, due to the reduced access of the light. Photo-cured cements are mainly indicated for translucent veneers, due to the possibility of light transmission through the restoration. Dual-cured are more versatile systems and, theoretically, can be used in either situation, since the presence of both curing mechanisms might guarantee a high degree of conversion (DC) under every condition. However, it has been demonstrated that clinical procedures and characteristics of the materials may have many different implications in the DC of currently available resin cements, affecting their mechanical properties, bond strength to the substrate and the esthetic results of the restoration. Factors such as curing mechanism, choice of adhesive system, indirect restorative material and light-curing device may affect the degree of conversion of the cement and, therefore, have an effect on the clinical performance of resin-based cements. Specific measures are to be taken to ensure a higher DC of the luting system to be used.

Effect of Different Thicknesses of Pressable Ceramic Veneers on Polymerization of Light-cured and Dual-cured Resin Cements

AIM

This study evaluated the effects of ceramic veneer thicknesses on the polymerization of two different resin cements.

MATERIALS AND METHODS

A total of 80 ceramic veneer disks were fabricated by using a pressable ceramic material (e.max Press; Ivoclar Vivadent) from a Low Translucency (LT) ingot (A1 shade). These disks were divided into light-cured (LC; NX3 Nexus LC; Kerr) and dual-cured (DC; NX3 Nexus DC; Kerr) and each group was further divided into four subgroups, based on ceramic disk thickness (0.3, 0.6, 0.9, and 1.2 mm). The values of Vickers microhardness (MH) and degree of conversion (DOC) were obtained for each specimen after a 24-hour storage period. Association between ceramic thickness, resin cement type, and light intensity readings (mW/cm(2)) with respect to microhardness and degree of conversion was statistically evaluated by using analysis of variance (ANOVA).

RESULTS

For the DOC values, there was no significant difference observed among the LC resin cement subgroups, except in the 1.2 mm subgroup; only the DOC value (14.0 ± 7.4%) of 1.2 mm DC resin cement had significantly difference from that value (28.9 ± 7.5%) of 1.2 mm LC resin cement (p < 0.05). For the MH values between LC and DC resin cement groups, there was statistically significant difference (p < 0.05); overall, the MH values of LC resin cement groups demonstrated higher values than DC resin cement groups. On the other hands, among the DC resin cement subgroups, the MH values of 1.2 mm DC subgroup was significantly lower than the 0.3 mm and 0.6 mm subgroups (p < 0.05). However, among the LC subgroups, there was no statistically significant difference among them (p > 0.05).

CONCLUSION

The degree of conversion and hardness of the resin cement was unaffected with veneering thicknesses between 0.3 and 0.9 mm. However, the DC resin cement group resulted in a significantly lower DOC and MH values for the 1.2 mm subgroup.

CLINICAL SIGNIFICANCE

While clinically adequate polymerization of LC resin cement can be achieved with a maximum 1.2 mm of porcelain veneer restoration, the increase of curing time or light intensity is clinically needed for DC resin cements at the thickness of more than 0.9 mm.

Effects of layering technique on the shade of resin overlays and the microhardness of dual cure resin cement

The purpose of this study was to assess the color of layered resin overlays and to test the early microhardness of dual cure resin cement (DCRC) light cured through the layered resin overlays. Resin overlays of 1.5 mm thickness were fabricated with the A3 shade of Z350 (Group 1L), the A3B and A3E shades of Supreme XT (Group 2L), and the A3, E3, and T1 shades of Sinfony (Group 3L) using one, two, and three layers, respectively (n=7). Each layer of the resin overlays was set in equal thickness. The color of the resin overlays was measured with a colorimeter and compared with an A3 shade resin denture tooth. DCRC was light cured through the resin overlays, and the early microhardness of the DCRC was measured. The ΔE value between the denture tooth and the resin overlays and the Vickers hardness number (VHN) of the DCRC were analyzed with one-way ANOVA and Tukey's HSD test. The color differences were 8.9±0.5, 5.3±1.0, and 7.3±0.5 and the VHNs were 19.4±1.1, 21.1±0.9, and 29.3±0.6 for Groups 1L, 2L, and 3L, respectively. Therefore, to match the designated tooth color of resin inlays and to increase the early microhardness of DCRC, layered resin inlays are more appropriate than single-dentin-layer resin inlays. However, the translucent layer should be used cautiously because the color difference of resin inlays with a translucent layer was affected more than those without a translucent layer.

Influence of light-exposure methods and depths of cavity on the microhardness of dual-cured core build-up resin composites

OBJECTIVE

The purpose of this study was to evaluate the Knoop hardness number (KHN) of dual-cured core build-up resin composites (DCBRCs) at 6 depths of cavity after 3 post-irradiation times by 4 light-exposure methods.

MATERIAL AND METHODS

Five specimens each of DCBRCs (Clearfil DC Core Plus [DCP] and Unifil Core EM [UCE]) were filled in acrylic resin blocks with a semi-cylindrical cavity and light-cured using an LED light unit (power density: 1,000 mW/cm2)at the top surface by irradiation for 20 seconds (20 s), 40 seconds (40 s), bonding agent plus 20 seconds (B+20 s), or 40 seconds plus light irradiation of both sides of each acrylic resin block for 40 seconds each (120 s). KHN was measured at depths of 0.5, 2.0, 4.0, 6.0, 8.0, and 10.0 mm at 0.5 hours, 24 hours, and 7 days post-irradiation. Statistical analysis was performed using repeated measures ANOVA and Tukey's compromise post-hoc test with a significance level of p<0.05.

RESULTS

For both DCBRCs, at 0.5 hours post-irradiation, the 20 s and 40 s methods showed the highest KHN at depth of 0.5 mm. The 40 s method showed significantly higher KHN than the 20 s method at all depths of cavity and post-irradiation times, except UCE at depth of 0.5 mm (p<0.05). The 120 s method did not result in significantly different KHN at all depths of cavity and post-irradiation times (p>0.05). In DCP, and not UCE, at 24 hours and 7 days post-irradiation, the B+20 s method showed significantly higher KHN at all depths of cavity, except the depth of 0.5 mm (p<0.05).

CONCLUSION

KHN depends on the light-exposure method, use of bonding agent, depth of cavity, post-irradiation time, and material brand. Based on the microhardness behavior, DCBRCs are preferably prepared by the effective exposure method, when used for a greater depth of cavity.

Effect of ceramic thickness and shade on mechanical properties of a resin luting agent

PURPOSE

This study aimed to investigate the influence of ceramic thickness and shade on the Knoop hardness and dynamic elastic modulus of a dual-cured resin cement.

MATERIALS AND METHODS

Six ceramic shades (Bleaching, A1, A2, A3, A3.5, B3) and two ceramic thicknesses (1 mm, 3 mm) were evaluated. Disk specimens (diameter: 7 mm; thickness: 2 mm) of the resin cement were light cured under a ceramic block. Light-cured specimens without the ceramic block at distances of 1 and 3 mm were also produced. The Knoop hardness number (KHN), density, and dynamic Young's moduli were determined. Statistical analysis was conducted using ANOVA and a Tukey B rank order test (p = 0.05).

RESULTS

The bleaching 1-mm-thick group exhibited significantly higher dynamic Young's modulus. Lower dynamic Young's moduli were observed for the 3-mm-thick ceramic groups compared to bleaching 3-mm-thick group, and no difference was found among the other 3-mm groups. For the KHN, when A3.5 3-mm-thick was used, the KHN was significantly lower than bleaching and A1 1-mm-thick ceramic; however, no difference was exhibited between the thicknesses of the same shade.

CONCLUSIONS

The dual-cured resin cement studied irradiated through the 1-mm-thick ceramic with the lightest shade (bleaching ceramic) exhibited a better elastic modulus, and there was no effect in KHN of the resin cement when light cured under different ceramic shades and thicknesses (1 and 3 mm), except when the A3.5 3-mm-thick ceramic was used.

CLINICAL SIGNIFICANCE

Variolink II irradiated through ceramic with the lowest chroma exhibited the highest elastic modulus; therefore, the light activation method might not be the same for all clinical situations.

Microhardness of dual-polymerizing resin cements and foundation composite resins for luting fiber-reinforced posts

STATEMENT OF PROBLEM

The optimal luting material for fiber-reinforced posts to ensure the longevity of foundation restorations remains undetermined.

PURPOSE

The purpose of this study was to evaluate the suitability of 3 dual-polymerizing resin cements and 2 dual-polymerizing foundation composite resins for luting fiber-reinforced posts by assessing their Knoop hardness number.

MATERIAL AND METHODS

Five specimens of dual-polymerizing resin cements (SA Cement Automix, G-Cem LincAce, and Panavia F2.0) and 5 specimens of dual-polymerizing foundation composite resins (Clearfil DC Core Plus and Unifil Core EM) were polymerized from the top by irradiation for 40 seconds. Knoop hardness numbers were measured at depths of 0.5, 2.0, 4.0, 6.0, 8.0, and 10.0 mm at 0.5 hours and 7 days after irradiation. Data were statistically analyzed by repeated measures ANOVA, 1-way ANOVA, and the Tukey compromise post hoc test (α=.05).

RESULTS

At both times after irradiation, the 5 resins materials showed the highest Knoop hardness numbers at the 0.5-mm depth. At 7 days after irradiation, the Knoop hardness numbers of the resin materials did not differ significantly between the 8.0-mm and 10.0-mm depths (P>.05). For all materials, the Knoop hardness numbers at 7 days after irradiation were significantly higher than those at 0.5 hours after irradiation at all depths (P<.05). At 7 days after irradiation, the Knoop hardness numbers of the 5 resin materials were found to decrease in the following order: DC Core Plus, Unifil Core EM, Panavia F2.0, SA Cement Automix, and G-Cem LincAce (P<.05).

CONCLUSIONS

The Knoop hardness number depends on the depth of the cavity, the length of time after irradiation, and the material brand. Although the Knoop hardness numbers of the 2 dual-polymerizing foundation composite resins were higher than those of the 3 dual-polymerizing resin cements, notable differences were seen among the 5 materials at all depths and at both times after irradiation.

Early hardness and shear bond strength of dual-cure resin cement light cured through resin overlays with different dentin-layer thicknesses

The purpose of this study was to investigate whether dentin-layer thickness of resin overlays could affect the early hardness and shear bond strength of dual-cure resin cement (DCRC, RelyX ARC) after light curing with light curing units (LCUs) of various power densities: Optilux 360 (360), Elipar Freelight 2 (FL2), and Elipar S10 (S10). Resin overlays were fabricated using an indirect composite resin (Sinfony) with a dentin layer, an enamel layer, and a translucent layer of 0.5 mm thickness each (0.5-0.5-0.5) or of 0.2 mm, 0.5 mm, and 0.8 mm thickness (0.2-0.5-0.8), respectively. The DCRC was light cured for 40 seconds through the overlays, and surface hardness and shear bond strength to bovine dentin were tested 10 minutes after the start of light curing. Surface hardness was higher when the DCRC was light cured through the 0.2-0.5-0.8 combination than when the DCRC was light cured through the 0.5-0.5-0.5 combination with all LCUs. The ratio of upper surface hardness of DCRC light cured through resin overlays relative to the upper surface hardness of DCRC light cured directly was more than 90% only when the DCRC was light cured with S10 through the 0.2-0.5-0.8 combination. The shear bond strength value was higher when the DCRC was light cured with S10 through the 0.2-0.5-0.8 combination than when light cured with S10 through the 0.5-0.5-0.5 combination. This study indicates that reducing the dentin-layer thickness while increasing the translucent-layer thickness of resin inlays can increase the photopolymerization of DCRC, thereby increasing the early bond strength of resin inlays to dentin.

Considerations for ceramic inlays in posterior teeth: a review

This review of ceramic inlays in posterior teeth includes a review of the history of ceramic restorations, followed by common indications and contraindications for their use. A discussion on the potential for tooth wear is followed by a review of recommended preparation design considerations, fabrication methods, and material choices. Despite the improved materials available for fabrication of porcelain inlays, fracture remains a primary mode of inlay failure. Therefore, a brief discussion on strengthening methods for ceramics is included. The review concludes with a section on luting considerations, and offers the clinician specific recommendations for luting procedures. In conclusion, inlay success rates and longevity, as reported in the literature, are summarized.

Power density of various light curing units through resin inlays with modified layer thickness

OBJECTIVES

The purpose of this study was to enhance curing light penetration through resin inlays by modifying the thicknesses of the dentin, enamel, and translucent layers.

MATERIALS AND METHODS

To investigate the layer dominantly affecting the power density of light curing units, resin wafers of each layer with 0.5 mm thickness were prepared and power density through resin wafers was measured with a dental radiometer (Cure Rite, Kerr). The dentin layer, which had the dominant effect on power density reduction, was decreased in thickness from 0.5 to 0.1 mm while thickness of the enamel layer was kept unchanged at 0.5 mm and thickness of the translucent layer was increased from 0.5 to 0.9 mm and vice versa, in order to maintain the total thickness of 1.5 mm of the resin inlay. Power density of various light curing units through resin inlays was measured.

RESULTS

Power density measured through 0.5 mm resin wafers decreased more significantly with the dentin layer than with the enamel and translucent layers (p < 0.05). Power density through 1.5 mm resin inlays increased when the dentin layer thickness was reduced and the enamel or translucent layer thickness was increased. The highest power density was recorded with dentin layer thickness of 0.1 mm and increased translucent layer thickness in all light curing units.

CONCLUSIONS

To enhance the power density through resin inlays, reducing the dentin layer thickness and increasing the translucent layer thickness would be recommendable when fabricating resin inlays.

Influence of zirconia base and shade difference on polymerization efficiency of dual-cure resin cement

PURPOSE

The aim of this study was to investigate the polymerization efficiency of dual-cured resin cement beneath different shades of zirconia-based feldsphathic ceramic restorations.

MATERIALS AND METHODS

Five translucent zirconia (Zirkonzahn) discs (4.0-mm diameter, 1.2-mm height) were prepared. Feldsphathic ceramic (1.2 mm) (Noritake Cerabien Zr) in 5 shades (1M2, 2M2, 3M2, 4M2, 5M2) was applied on the zirconia discs. Twelve dual-cure resin cement specimens were prepared for each shade, using Panavia F 2.0 (Kuraray) in Teflon molds (4.0-mm diameter, 6.0-mm height), following the manufacturer's instructions. Light activation was performed through the zirconia-based ceramic discs for 20 seconds, using a quartz tungsten halogen curing device (Hilux 200) with irradiance of 600 mW/cm(2) . Immediately following light curing, specimens were stored for 24 hours in dry, light-proof containers. Vickers hardness measurements were conducted using a microhardness tester with a 50-g load applied for 15 seconds. The indentations were made in the cross sectional area at four depths, and the mean values were recorded as Vickers hardness number (VHN). Results were statistically analyzed with one-way ANOVA and Tukey HSD test (p < 0.05).

RESULTS

A statistically significant decrease in VHN of the resin cement was noted with increasing depth and darkness of the shade (p < 0.05).

CONCLUSION

Curing efficiency of dual-cure resin cement is mainly influenced by the lightness of the shades selected.

Cements and adhesives for all-ceramic restorations

Dental cements are designed to retain restorations, prefabricated or cast posts and cores, and appliances in a stable, and long-lasting position in the oral environment. Resin-based cements were developed to overcome drawbacks of nonresinous materials, including low strength, high solubility, and opacity. Successful cementation of esthetic restorations depends on appropriate treatment to the tooth substrate and intaglio surface of the restoration, which in turn, depends on the ceramic characteristics. A reliable resin cementation procedure can only be achieved if the operator is aware of the mechanisms involved to perform the cementation and material properties. This article addresses current knowledge of resin cementation concepts, exploring the bonding mechanisms that influence long-term clinical success of all-ceramic systems.

Light-activation through indirect ceramic restorations: does the overexposure compensate for the attenuation in light intensity during resin cement polymerization?

Objectives

This study evaluated the effects of light exposure through simulated indirect ceramic restorations (SICR) on hardness (KHN) of dual-cured resin cements (RCs), immediately after light-activation and 24 h later.

Material and Methods

Three dual-cured RCs were evaluated: eco-Link (Ivoclar Vivadent), Rely X ARC (3M eSPe), and Panavia F (Kuraray Medical Inc.). The RCs were manipulated in accordance to the manufacturers’ instructions and were placed into cylindrical acrylic matrixes (1-mm-thick and 4-mm diameter). The RC light-activation (Optilux 501; Demetron Kerr) was performed through a glass slide for 120 s (control group), or through 2-mm or 4-mm thick SICRs (IPS empress II; Ivoclar Vivadent). The specimens were submitted to KHN analysis immediately and 24 h after light-activation. The data obtained at the 2 evaluation intervals were submitted to 2-way ANOVA repeated measures and post-hoc Tukey’s test (pre-set alpha of 5%).

Results

Lower KHN was observed when light-activation was performed through SICRs for eco-Link at all evaluation intervals and for Rely X ARC 24 h later. For Panavia F, no significant difference in KHN was observed between control and experimental groups, regardless of evaluation interval. Most groups exhibited higher KHN after 24 h than immediately after light-activation, with the exception of Rely X ARC light-activated through SICR, as no significant difference in KHN was found between evaluation intervals.

Conclusion

Light overexposure did not compensate for light intensity attenuation due to the presence of SICR when Rely X and eco-Link were used. Although hardness of such RCs increased over a 24-h interval, the RCs subjected to light overexposure did not reach the hardness values exhibited after direct light exposure.

Influence of the interaction of light- and self-polymerization on subsurface hardening of a dual-cured core build-up resin composite

OBJECTIVE

To investigate the influence of time delay and duration of photo-activation on subsurface microhardness of a dual-cured resin composite.

MATERIAL AND METHODS

A commercially available dual-cured core build-up resin composite (Rebilda DC) was filled in cavities (diameter: 4.0 mm; height: 6.0 mm) of polystyrene molds and light-cured for 20 or 60 s either immediately after the filling procedure (time delay 0 s) or after a time delay of 30, 90, 180 or 300 s. Non-irradiated self-cured specimens served as a control group (n = 15). Specimens were stored in complete darkness and at 100% relative humidity at 37°C for 2 weeks and cross-sectioned. Knoop Hardness Numbers (KHNs) were measured six times per depth and averaged at distances of 0.25, 0.50, 1.00, 2.00, 3.50 and 5.50 mm from the light-exposed surface. Data were statistically analyzed using one- and two-way ANOVA followed by Scheffé's post-hoc test at a level of significance of 0.05.

RESULTS

Mean hardness values in all experimental groups ranged between 54.3 ± 2.1 and 58.1 ± 2.3 KHN. Light-curing did not significantly increase composite KHN at any depth measured. Delaying light exposure had no influence on KHN, irrespective of depth. A longer light-exposure time (60 versus 20 s) resulted in significantly higher KHN only at depths of 3.50 and 5.50 mm.

CONCLUSION

Photo-activation of the tested dual-cured resin composite provided no clinically relevant benefit compared to self-curing regarding the degree of hardening.

Influence of curing mode and time on degree of conversion of one conventional and two self-adhesive resin cements

This study evaluated the effect of curing mode (auto- and dual-polymerizing mode) and time interval (5, 10 and 15 minutes) on the degree of conversion of resin cements. One conventional dual-cured resin cement (Panavia F 2.0 [Kuraray Medical Inc]) and two self-adhesive cements (RelyX Unicem [3M ESPE] and BisCem [BISCO, Inc]) were evaluated. The products (n = 5) were manipulated according to the manufacturer's instructions and applied to the surface of a horizontal attenuated reflectance unit attached to an infrared spectrometer. The materials were either light-cured for 40 seconds (dual-polymerizing mode) or allowed to auto-polymerize. The degree of conversion was calculated according to changes in the aliphatic-to-aromatic peak ratios prior to and 5, 10 and 15 minutes after light-activation or after mixing when the specimens were allowed to auto-polymerize. Data (%) were analyzed by two-way repeated measure ANOVA (curing mode and time interval) and Tukey's post-hoc test (alpha = 0.05%). The light-activating mode led to a higher degree of conversion values than the self-curing mode in self-adhesive cements (RelyX Unicem and BisCem), while there was no difference in the degree of conversion between the self- and light-cured groups of Panavia F 2.0 resin cement. All products showed a higher degree of conversion at 15 minutes postcuring than any other evaluation interval. The self-adhesive cements provide a higher degree of conversion values when light-activated. After 15 minutes of polymerization initiation, the degree of conversion was higher in all resin cements, regardless of the curing mode.

Hardening of a dual-cure resin cement using QTH and LED curing units

OBJECTIVE

This study evaluated the surface hardness of a resin cement (RelyX ARC) photoactivated through indirect composite resin (Cristobal) disks of different thicknesses using either a light-emitting diode (LED) or quartz tungsten halogen (QTH) light source.

MATERIAL AND METHODS

Eighteen resin cement specimens were prepared and divided into 6 groups according to the type of curing unit and the thickness of resin disks interposed between the cement surface and light source. Three indentations (50 g for 15 s) were performed on the top and bottom surface of each specimen and a mean Vickers hardness number (VHN) was calculated for each specimen. The data were analyzed using two-way ANOVA and Tukey-Kramer test was used for post-hoc pairwise comparisons.

RESULTS

Increased indirect resin disk thickness resulted in decreased mean VHN values. Mean VHN values for the top surfaces of the resin cement specimens ranged from 23.2 to 46.1 (QTH) and 32.3 to 41.7 (LED). The LED curing light source produced higher hardness values compared to the QTH light source for 2- and 3-mm-thick indirect resin disks. The differences were clinically, but not statistically significant. Increased indirect resin disk thickness also resulted in decreased mean VHN values for the bottom surfaces of the resin cement: 5.8 to 19.1 (QTH) and 7.5 to 32.0 (LED). For the bottom surfaces, a statistically significant interaction was also found between the type of curing light source and the indirect resin disk thickness.

CONCLUSION

Mean surface hardness values of resin cement specimens decreased with the increase of indirect resin disk thickness. The LED curing light source generally produced higher surface hardness values.

Physical properties of dual-cured luting-agents correlated to early no interfacial-gap incidence with composite inlay restorations

OBJECTIVES

The aims of this investigation were to investigate dual-cured luting-agents as to whether their bond strength, dimensional change and flexural modulus influence no interfacial-gap incidence parameters of composite inlay restorations during the early stages. The correlations of interest were between: (a) their shear bond strength to dentin, (b) their dimensional change on setting, (c) their flexural modulus, and (d) no interfacial-gap incidence with indirect restorations.

METHODS

Seven dual-cured luting-agents, one self-adhesive resin cement, six resin cements and one resin-modified glass-ionomer for luting, were investigated with specimen sub-groups (n=10) for each property measured. The principal series of experiments were conducted in dentin cavities with interfacial polishing either immediately (3 min) after setting or after 1-day water-storage. After the finishing procedure, each tooth was sectioned in a buccolingual direction through the center of the restoration, and the presence or absence of interfacial-gaps was measured (and then summed for each cavity) at 14 points (each 0.5mm apart) along the composite inlay restoration interface (n=10 per group; total points measured=140), and was expressed by percentage of measured total points. The shear bond strengths to dentin, setting shrinkage-strain and flexural modulus were measured. To estimate the dimensional change of luting-agents, the maximum marginal gap-width and the opposing-width that occurred with luting-agents in a Teflon mold were measured. Moduli were measured in 3-point bending.

RESULTS

For all composite inlay restorations, polished immediately after setting, an incidence of summed no-gaps of 69-88% was observed. For specimens polished after 1 day, a significantly (p<0.05) decreased number of 91-96% summed no-gaps occurred. After 1-day storage, shear bond strengths to dentin and flexural modulus increased highly significantly (p<0.001) for many materials, whereas dimensional changes in the Teflon mold were non-significantly different (p>0.05). There was a highly significant correlation between no interfacial-gap incidence and shear bond strength (r=0.702, p=0.002, n=16). As the dimensional change (shrinkage) within Teflon molds increased, the no interfacial-gap incidence of dentin/inlay interfaces with 'no-gaps' decreased (r=-0.574, p=0.02, n=16). Flexural moduli significantly correlated with no interfacial-gap incidence in the composite inlay restorations (r=0.695, p=0.003, n=16).

SIGNIFICANCE

For three classes of luting-agents during the early stage of setting (<1 day), the shear bond strength to dentin, the dimensional change measured by marginal gaps in Teflon mold and flexural moduli correlated with no interfacial-gap incidence in the composite inlay restoration.

Effect of Light Energy Density on Conversion Degree and Hardness of Dual-cured Resin Cement

Light energy density can influence the curing of dual-cured resin cement. The ultimate physical properties of dual-cured resin cement depend on light energy delivered from the light-curing unit. It can guide the clinicians to select the appropriate curing unit for curing dual cement.

Effect of thickness of indirect restoration and distance from the light-curing unit tip on the hardness of a dual-cured resin cement

This study evaluated the Knoop hardness and polymerization depth of a dual-cured resin cement, light-activated at different distances through different thicknesses of composite resin. One bovine incisor was embedded in resin and its buccal surface was flattened. Dentin was covered with PVC film where a mold (0.8-mm-thick and 5 mm diameter) was filled with cement and covered with another PVC film. Light curing (40 s) was carried out through resin discs (2, 3, 4 or 5 mm) with a halogen light positioned 0, 1, 2 or 3 mm from the resin surface. After storage, specimens were sectioned for hardness measurements (top, center, and bottom). Data were subjected to split-plot ANOVA and Tukey's test (a=0.05). The increase in resin disc thickness decreased cement hardness. The increase in the distance of the light-curing tip decreased hardness at the top region. Specimens showed the lowest hardness values at the bottom, and the highest at the center. Resin cement hardness was influenced by the thickness of the indirect restoration and by the distance between the light-curing unit tip and the resin cement surface.

Influence of energy density of different light sources on Knoop hardness of a dual-cured resin cement

The purpose of this study was to evaluate the Knoop hardness of a dual-cured resin-based luting cement irradiated with different light sources as well energy density through a ceramic sample. Three light-curing unit (LCUs) were tested: tungsten halogen light (HAL), light-emitting diode (LED) and xenon plasma-arc (PAC) lamp. Disc-shaped specimens were fabricated from a resin-based cement (Enforce). Three energy doses were used by modifying the irradiance (I) of each LCU and the irradiation time (T): 24 Jcm^-2 (I/2x2T), 24 Jcm^-2 (IxT) and 48 Jcm^-2 (Ix2T). Energy doses were applied through a 2.0-mm-thick ceramic sample (Duceram Plus). Three groups underwent direct irradiation over the resin cement with the different LCUs and a chemically-activated group served as a control. Thirteen groups were tested (n=10). Knoop hardness number (KHN) means were obtained from cross-sectional areas. Two-way ANOVA and the Holm-Sidak method were used for statistical comparisons of activation mode and energy doses (α=5%). Application of 48 J.cm^-2 energy dose through the ceramic using LED (50.5±2.8) and HAL (50.9±3.7) produced significantly higher KHN means (p<0.05) than the control (44.7±3.8). LED showed statistically similar performance to HAL. Only HAL showed a relationship between the increase of LCU energy dose and hardness increase.

Influence of different ceramics on resin cement Knoop Hardness Number

This study evaluated: (1) the effect of different ceramics on light attenuation that could affect microhardness, measured as the Knoop Hardness Number (KHN), of a resin cement immediately and 24 hours after polymerization and (2) the effect of different activation modes (direct light-activation, light activation through ceramics and chemical activation) on the KHN of a resin cement. Resin cement Rely XARC (3M ESPE) specimens 5.0 mm in diameter and 1.0 mm thickwere made in a Teflon mold covered with a polyester film. The cement was directly light activated for 40 seconds with an XL 2500 curing unit (3M ESPE) with 650 mW/cm2, light activated through ceramic discs of Duceram Plus (DeguDent), Cergogold (DeguDent), IPS Empress (Ivoclar), IPS Empress 2 (Ivoclar), Procera (NobelBiocare), In Ceram Alumina (Vita) and Cercon (DeguDent), having a 1.2 mm thickness or chemically activatedwith-out light application. The resin cement specimens were flattened, and KHN wasobtained using an HMV 2 microhardnesstester (Shimadzu) with a load of 50 g applied for 15 seconds 100 microm from the irradiated surface immediately and after storage at 37 degrees C for 24 hours. Ten measurements were made for each specimen, with three specimens for each group at each time. The data were submitted to ANOVA and Tukey's test (p = 0.05). The KHN of the resin cement was not only affected by the mode of activation, but also by the post-activation testing time. The mean KHN of the resin cementfor chemical activation and through all ceramics showed statistically significant lower values compared to direct activation immediately and at 24 hours. The KHN for 24 hourspost-activation was always superior to the immediate post-activation test except with direct activation. The most opaque ceramics resulted in the lowest KHN values.

Influence of ceramic thickness on mechanical properties and polymer structure of dual-cured resin luting agents

OBJECTIVE

To investigate the influence of ceramic thickness on the mechanical properties and polymer structure (degree conversion and cross-linking density) of three dual-cured resin luting agents.

METHODS

Three dual-cured resin luting agents [Linkmax HV (GC), Nexus 2 (Kerr), and Variolink IIHV (Ivoclar-Vivadent)] were polymerized with or without 800 mW/cm2 irradiation through 0-3-mm-thick GN-I (GC) machinable ceramic. Bar-shape specimens were subjected to three-point bending to determine flexural strength (FS) and elastic modulus (EM) after dry storage at 37 degrees C for 24 h. Knoop hardness was measured on the irradiated surface of disk-shaped specimens before (KHN1) and after (KHN2) storage of 100% ethanol solution at 37 degrees C for 24 h. KHN1 and KHN2 were estimated as indirect indicators of degree of conversion (DC) and cross-linking density, respectively. Data were analyzed by one-way ANOVA and Student-Newman-Keuls test for each luting agent, and four mechanical properties were subjected to regression analysis.

RESULTS

For three resin luting agents with dual-cured mode, FS, EM, KHN1, and KHN2 decreased with the increase of ceramic thickness. FS except for Nexus 2 and EM for three resin luting agents had a positive linear relationship with both KHN1 and KHN2.

SIGNIFICANCE

The variables tested behaved differently. When the ceramic thickness increased, the chemical cured components of dual-cured resin luting agents did not produce significant compensation for all variables. Mechanical properties and polymer structure of dual-cured resin luting agents was dependent on the intensity of light irradiation.

Influence of ceramic thickness and curing mode on the polymerization shrinkage kinetics of dual-cured resin cements

OBJECTIVES

The purpose of this study was to assess how ceramic disc thickness and curing mode (light or chemical) affects the polymerization shrinkage of dual-cured resin cements and to evaluate the effect of the ceramic discs on the curing speed of the cements during light exposure.

METHODS

Six commercial resin cements, RelyX ARC, Bistite II, Duolink, Panavia F, Variolink II and Choice were used. Filler weight contents were determined by the ash method. Four ceramic discs with thicknesses of 0.5, 1, 2 and 4mm, respectively, were made. The attenuation of light intensity due to the ceramic discs was measured using a radiometer. The polymerization shrinkage kinetics of the resin cements by chemical or light cure through the different ceramic discs was measured using a bonded-disc method.

RESULTS

There were differences in filler content among brands of resin cement. The polymerization shrinkage without ceramic disc was 2.61-4.59% by chemical cure and 2.93-4.66% by light cure. The polymerization shrinkage of RelyX ARC and Panavia F by chemical cure was statistically lower than by light cure (p<0.05). Polymerization shrinkage and filler weight were inversely related (R=-0.965). Both the transmitted light intensity and polymerization shrinkage decreased with increasing thickness of ceramic discs (p<0.05). The time to reach the maximum shrinkage rate of the resin cements increased with increasing ceramic thickness. The cure speed by light cure was 15-322 times faster than by chemical cure.

SIGNIFICANCE

The polymerization shrinkage kinetics of dual-cured resin cements significantly differed between brands under various curing conditions. Clinicians should be aware of the setting characteristics of the cements, so they can choose the optimal materials for different clinical situations.

Effect of Light-Curing Method and Cement Activation Mode on Resin Cement Knoop Hardness

PURPOSE

To evaluate the Knoop hardness (KHN) of the resin cement Enforce activated by chemical/physical mode or physical mode solely; light-cured directly or through a 1.5 mm thick ceramic disc (HeraCeram) on shade DD2.

MATERIALS AND METHODS

Light-curing was carried out using a conventional quartz tungsten halogen light (QTH) (XL2500) for 40 seconds at 700 mW/cm(2); light-emitting diodes (LED) (Ultrablue Is) for 40 seconds at 440 mW/cm(2); and Xenon plasma arc (PAC) (Apollo 95E) for 3 seconds at 1600 mW/cm(2). Bovine incisors had their buccal faces flattened and hybridized. A mold was seated on these surfaces and filled with cement. A disc of the acid-etched and silanized veneering material was seated over this set for light-curing. After dry storage (24 hours at 37 degrees C), specimens (n= 10) were sectioned for KHN measurements performed in a microhardness tester (50 gf load for 15 seconds). Data were submitted to ANOVA and Tukey's test (alpha= 0.05).

RESULTS

The highest KHN values were obtained with LED, for both dual-cured and light-cured cement. The lowest KHN value was obtained with light-cured PAC. Light-curing with QTH resulted in hardness values similar to PAC in dual-cured groups.

CONCLUSIONS

Light-curing through HeraCeram can influence resin cement hardness.