Fracture toughness of water-aged resin composite restorative materials

Which Three-Dimensional Printing Technology Can Replace Conventional Manual Method of Manufacturing Oral Appliance? A Preliminary Comparative Study of Physical and Mechanical Properties

Three-dimensional printing technology is widely being adopted in the manufacturing of oral appliances. The purpose of this study was to determine the most suitable method of manufacturing oral appliances by comparing the physical and mechanical properties of various 3D printing methods with the conventional method. Experimental groups consisted of six 3D-printed specimens via FDM, two polyjets, SLS, SLA, and DLP, and the milling methods. The control group consisted of an acrylic resin specimen made by the conventional manual method. The water absorption and solubility, color stability, flexural strength, and surface hardness were tested and statistically analyzed. The FDM, SLS, and DLP methods exhibited comparable water absorption and solubility with the control group, and only the SLA method exhibited significantly higher water solubility than the control group. In terms of the color stability, only the milling method met the requirements of the allowable clinical range. The FDM, SLA, and DLP methods exhibited comparable flexural strength with the control group. The surface hardness of the PJ-2, DLP, and milling methods was acceptable for replacing conventional manual method. Therefore, the most suitable method of manufacturing oral appliances among the experimental groups was the DLP method in terms of its water absorption and solubility, flexural strength, and surface hardness.

Influence of material type, thickness and storage on fracture resistance of CAD/CAM occlusal veneers

OBJECTIVES

The present study aimed to evaluate the effect of restoration thickness, CAD/CAM material, and 6 months of artificial saliva storage on the fracture resistance of occlusal veneers.

MATERIALS AND METHODS

A total of 84 intact maxillary molars were sectioned 4.0 mm occlusal to the cementoenamel junction to expose the dentine. The teeth were assigned into 3 main groups according to the type of restorative material (e.max CAD, Vita Enamic, and Lava Ultimate). In each group, the teeth were allocated into 2 subgroups (n = 14) according to restoration thickness (1.0 and 1.5 mm). The veneers were adhesively bonded using dual-cure self-adhesive luting agent. A total of 42 specimens comprising half the tested subgroups were stored in distilled water for 24-h before the test. The remaining half was stored in artificial saliva at 37 ± 1 °C in an incubator for 6 months. All specimens (n = 84) were subjected to 5000 thermal cycles between 5 and 55 °C ± 2 before the fracture resistance test. The maximum force at fracture was recorded in Newton. Failure mode was analyzed using a stereomicroscope. The results were analyzed using a parametric Three-way ANOVA test.

RESULTS

The results of the Three-way ANOVA test revealed that material type and restoration thickness significantly affected fracture resistance values (p < 0.5), while 6 months of storage in artificial saliva had no significant effect on mean fracture resistance values (p˃0.5). The most common failure patterns in CAD/CAM resin composite and polymer-infiltrated ceramics were scores I and score II. For glass ceramic groups, score IV and III were more dominant.

CONCLUSIONS

All the tested CAD/CAM restorations in both thicknesses exhibited fracture resistance values exceeding normal and parafunctional bite forces. Polymer-infiltrated ceramics and CAD/CAM resin composite veneers showed more favorable fracture patterns.

Effect of Two Different Surface Treatments on Retention of Cosmopost with Two Different Core Materials

Objective

The objective of this study was to determine the effect of Cosmopost’s two different surface treatments (sandblasting and silica coating) on persistence to various core materials using push-out test set-up.

Material and Methods

A total of 30 Cosmoposts was used in this study. Cosmoposts were divided into three groups (10 samples each) according to the post-surface treatment received. Every category was additionally subdivided into two subgroups according to the type of core material (n = 5 samples). A specially designed copper mold was used for construction of different core materials with standardized dimensions around the posts, in such a way to ensure that the posts will be centralized. Surface roughness was estimated for all Cosmoposts, following different surface treatments using SEM. Cylindrical cores were fabricated of either composite resin or heat-pressed zirconia-containing glass-ceramic (IPS Empress Cosmo, Ivoclar Vivadent). Following the construction of different Core materials, samples were subjected to push-out test set-up to Evaluate the impact of various treatments on post/core bond strength. Data were collected, tabulated and statistically analyzed. SEM was performed on Cosmoposts following debonding of different post/core samples to determine their mode of failure.

Results

Results of push-out bond strength revealed that core material, surface treatment and the interaction between the two variables using Two-way ANOVA had a statistically significant effect on mean push-out bond strength. Regarding the effect of type of core material on Push-out bond strength, results showed that IPS Empress Cores showed statistically significant higher mean push-out bond strength to Cosmopost (36.4±9.7MPa) than composite cores (15.8±2.5 MPa).

Conclusion

Within the limitations of this study, direct heat-pressed ceramic core was more beneficial for zirconia post buildups, than Composite Cores, since they provided higher bond strength. Thanks to a double improvement: increase in fracture resistance and retentive capacity to post. Also, Tribochemical Silicacoating technique was proved to be more effective in Cosmopost treatments than sandblasting technique.

Effect of Type and Concentration of Nanoclay on the Mechanical and Physicochemical Properties of Bis-GMA/TTEGDMA Dental Resins

Bis-GMA/TTEGDMA-based resin composites were prepared with two different types of nanoclays: an organically modified laminar clay (Cloisite® 30B, montmorillonite, MMT) and a microfibrous clay (palygorskite, PLG). Their physicochemical and mechanical properties were then determined. Both MMT and PLG nanoclays were added into monomer mixture (1:1 ratio) at different loading levels (0, 2, 4, 6, 8 and 10 wt.%), and the resulting composites were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and mechanical testing (bending and compressive properties). Thermal properties, depth of cure and water absorption were not greatly affected by the type of nanoclay, while the mechanical properties of dental resin composites depended on both the variety and concentration of nanoclay. In this regard, composites containing MMT displayed higher mechanical strength (both flexural and compression) than those resins prepared with PLG due to a poor nanoclay dispersion as revealed by SEM. Solubility of the composites was dependent not only on nanoclay-type but also the mineral concentration. Dental composites fulfilled the minimum depth cure and solubility criteria set by the ISO 4049 standard. In contrast, the minimum bending strength (50 MPa) established by the international standard was only satisfied by the dental resins containing MMT. Based on these results, composites containing either MMT or PLG (at low filler contents) are potentially suitable for use in dental restorative resins, although those prepared with MMT displayed better results.

A Review of Dental Cements

This review provides an in-depth comparison of advantages and disadvantages of different types of dental cements as they are used for cementing base metal alloy crowns in dogs.

Accuracy of marginal fit and axial wall contour for lithium disilicate crowns fabricated using three digital workflows

STATEMENT OF PROBLEM

Comparative assessment of the effectiveness of computer-aided design and computer-aided manufacturing (CAD-CAM) technologies used to fabricate complete-coverage restorations is needed. A quantitative assessment requires precise documentation of the marginal adaptation and external surface contour of fabricated restorations. Limited information is currently available regarding the effects of milling mode on marginal adaptation and reproduction of the external surface contour for CAD-CAM-fabricated restorations.

PURPOSE

The purpose of this in vitro study was to evaluate the outcomes for 3 different digital workflows on the marginal gap and the external surface contour reproducibility of CAD-CAM-fabricated lithium disilicate complete-coverage restorations.

MATERIAL AND METHODS

Twelve Ivorine molars were prepared to receive lithium disilicate crowns. The preparations were digitally recorded using 2 intraoral scanners (TRIOS 3; 3Shape A/S and Planmeca PlanScan; E4D Technologies), and the restorations were designed using their associated design software with reference to the anatomy of an unprepared tooth. The designed restorations were then manufactured from lithium disilicate blocks using a 3-axis milling machine. Twelve restorations were manufactured using the detailed mode (Planmeca PlanScan detailed mode [PPD-D]), and 12 using the standard mode for the Planmeca system (Planmeca PlanScan standard mode [PPD-S]). Restorations from the 3Shape system were fabricated using the detailed mode (TRIOS 3Shape detailed mode [T3S-D]). The restorations were secured on their associated preparation with an elastomeric material. The marginal gap of each restoration was then measured in the ImageJ software using images captured by a stereo microscope at ×20 magnification. External surface reproducibility was evaluated by measuring undercut at 4-line angles using a dental surveyor. Differences in the marginal gaps of restorations fabricated using the 3 different workflows were compared by Brown-Forsythe robust ANOVA, followed by a post hoc test (α=.05). Chi-square analysis (α=.05) was used to evaluate differences in the contours of the external surface of the restorations, resistance form, and marginal integrity produced using the 3 workflows.

RESULTS

The mean marginal gap for restorations fabricated using the T3S-D workflow was 60 μm, a distance significantly lower (P<.05) than that of PPD-D and PPD-S workflows, which yielded a marginal gap of 95 μm for the detailed mode and 124 μm for the standard mode of milling. Restorations fabricated using PPD-D and PPD-S workflows produced a significantly more reproducible external surface contour than those fabricated using the T3S-D workflow.

CONCLUSIONS

Restorations fabricated using the T3S-D workflow produced the smallest marginal gap. However, reproducibility of the external surface contour for this workflow was the worst of the three workflows analyzed.

Impact of Occlusal Intercuspal Angulation on the Quality of CAD/CAM Lithium Disilicate Crowns

PURPOSE

Modification of intercuspal angulation (ICA) influences the amount of tooth structure removal, which may impact the retention and resistance form of the preparation. This study evaluated the impact of ICA on the marginal gap of CAD/CAM crowns and the influence that tooth structure removal, caused by variation of ICA, has on the resistance and retention form of the preparation.

MATERIALS AND METHODS

Sixty ivorine molars were manufactured with various ICAs (100°, 110°, 120°, 140°, 160°, and 180°; 10 per group). The preparations were digitized using an intraoral scanner, and the crowns were designed using a design software. The designed crowns were then manufactured from lithium disilicate using a 3-axis milling machine, with the "detailed mode" selected for the manufacturing. The marginal gap of each crown was evaluated using a stereomicroscope at 20× magnification. Then, the marginal integrity and the resistance form of the preparation were assessed by tactile-visual evaluation, and they were given a categorical score. Crowns were then secured on their associated preparations using a temporary luting agent, and retention force was measured on a universal testing machine under tension with a 0.5 mm/min crosshead speed. Wilcoxon test followed by post-hoc tests (α = 0.05) were used to evaluate the impact of the ICA on the marginal gap and the retention form of the preparation. Fisher's exact test followed by post-hoc tests (α = 0.05) were used to assess the impact of the occlusal preparation design on the marginal integrity and the resistance form of the preparation.

RESULTS

The marginal gap was significantly larger for ICA-180 preparations (72 μm), compared to the other groups (ICA-180 vs ICA-100, ICA-110, ICA-120, and ICA-160 p = 0.0001; ICA-180 vs. ICA-140 p = 0.0017). None of the crowns for ICA-180 preparations had clinically acceptable resistance form. Preparations with ICAs of 100°, 110°, and 120° had a significantly higher value of retention than the other groups (ICA-100 vs. ICA-120 p = 0.0119; ICA-100 vs. ICA-140, ICA-160, and ICA-180 p < 0.0001; ICA-110 vs. ICA-140, ICA-160, and ICA-180 p = 0.0001; ICA-120 vs. ICA-180 p = 0.0017).

CONCLUSIONS

Crowns fabricated for preparations with various ICAs had clinically acceptable marginal adaptation. Variation in ICA impacts the loss of tooth structure. This loss of tooth structure may influence the resistance and retention form of the preparation.

Interfacial degradation of adhesive composite restorations mediated by oral biofilms and mechanical challenge in an extracted tooth model of secondary caries

OBJECTIVE

To study the combined effect of simulated occlusal loading and plaque-derived biofilm on the interfacial integrity of dental composite restorations, and to explore whether the effects are modulated by the incorporation of sucrose.

METHODS

MOD-class-II restorations were prepared in third molars. Half of the specimens (n=27) were subjected to 200,000 cycles of mechanical loading using an artificial oral environment (ART). Then, both groups of specimens (fatigued and non-fatigued) were divided into three subgroups for testing in CDC-reactors under the following conditions: no biofilm (Control), biofilm with no sucrose (BNS) and biofilm pulsed with sucrose (BWS). BNS and BWS reactors were incubated with a multispecies inoculum from a single plaque donor whereas the control reactor was not. The BWS reactor was pulsed with sucrose five times a day. The biofilm challenges were repeated sequentially for 12 weeks. pH was recorded for each run. Specimens were examined for demineralization with micro-CT and load capacity by fast fracture test.

RESULTS

Demineralization next to the restorations was only detectable in BWS teeth. Fracture loads were significantly reduced by the concomitant presence of biofilm and sucrose, regardless of whether cyclic mechanical loading was applied. Cyclic loading reduced fracture loads under all reactor conditions, but the reduction was not statistically significant.

CONCLUSIONS

Sucrose pulsing was required to induce biofilm-mediated degradation of the adhesive interface. We have presented a comprehensive and clinically relevant model to study the effects of mechanical loading and microbial challenge on the interfacial integrity of dental restorations.

Effect of Short-Term Water Exposure on the Mechanical Properties of Halloysite Nanotube-Multi Layer Graphene Reinforced Polyester Nanocomposites

The influence of short-term water absorption on the mechanical properties of halloysite nanotubes-multi layer graphene reinforced polyester hybrid nanocomposites has been investigated. The addition of nano-fillers significantly increased the flexural strength, tensile strength, and impact strength in dry and wet conditions. After short-term water exposure, the maximum microhardness, tensile, flexural and impact toughness values were observed at 0.1 wt % multi-layer graphene (MLG). The microhardness increased up to 50.3%, tensile strength increased up to 40% and flexural strength increased up to 44%. Compared to dry samples, the fracture toughness and surface roughness of all types of produced nanocomposites were increased that may be attributed to the plasticization effect. Scanning electron microscopy revealed that the main failure mechanism is caused by the weakening of the nano-filler-matrix interface induced by water absorption. It was further observed that synergistic effects were not effective at a concentration of 0.1 wt % to produce considerable improvement in the mechanical properties of the produced hybrid nanocomposites.

Influence of surface treatment of yttrium-stabilized tetragonal zirconium oxides and cement type on crown retention after artificial aging

STATEMENT OF PROBLEM

Information about the influence of zirconia crown surface treatment and cement type on the retention of zirconia crowns is limited. It is unclear whether zirconia crowns require surface treatment to enhance their retention.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of surface treatment on the retention of zirconia crowns cemented with 3 different adhesive resin cements after artificial aging.

MATERIAL AND METHODS

Ninety extracted human molars were prepared for ceramic crowns (approximately 20-degree taper, approximately 4-mm axial length) and were divided into 3 groups (n=30). Computer-aided design and computer-aided manufacturing zirconia copings were fabricated. Three surface treatments were applied to the intaglio surface of the copings. The control group received no treatment, the second group was airborne-particle abraded with 50 μm Al2O3, and the third group was treated with 30 μm silica-modified Al2O3, The copings were luted with a self-etch (RelyX Unicem 2), a total-etch (Duo-Link), or a self-etch primer (Panavia F 2.0) adhesive cement. They were stored for 24 hours at 37°C before being artificially aged with 5000 (5°C-55°C) thermal cycles and 100,000 cycles of 70 N dynamic loading. Retention was measured on a universal testing machine under tension, with a crosshead speed of 0.5 mm/min. Statistical analysis was performed with 1-way and 2-way ANOVA.

RESULTS

Mean retention values ranged from 0.72 to 3.7 MPa. Surface treatment increased crown retention, but the difference was not statistically significant (P>.05), except for the Duo-Link cement group (P<.05). Analysis of the adhesives revealed that the Duo-Link cement resulted in significantly lower crown retention (P<.05) than the other 2 cements.

CONCLUSION

For zirconia crowns, retention seems to be dependent on cement type rather than surface treatment.

Effect of storage media and time on the fracture toughness of resin-based luting cements

BACKGROUND

Resin-based cements are brittle materials and the major shortcomings of these materials are manifested in their sensitivity to flaws and defects. Although various mechanical properties of resin luting cements have been described, few fracture toughness test data for resin-based luting cements using the short rod design have been published.

METHODS

Specimens were prepared from five resin luting cements. For each material a total of 36 disc-shaped specimens were prepared using a custom-made mould. Specimens were randomly divided into six groups of six, immersed in two solutions: distilled water and 0.01 mol/L lactic acid at 37 °C for 24 hours, 1 month or 3 months. The specimens were loaded using a universal testing machine. The maximum load at specimen failure was recorded and the K(Ic) (MPa. M 0.5) was calculated.

RESULTS

There was a relationship between material, storage solution and time (p < 0.05). Nexus 3 showed the highest K(Ic) followed by Panavia F, Calibra, Smart Cem2 and seT.

CONCLUSIONS

The fracture toughness of the resin luting cements was affected by both time and storage solution. Comparable fracture toughness of conventional resin cement and self-adhesive resin cements was observed.

In vitro investigation of the performance of different restorative materials under cast circumferential clasps for removable dental prostheses

OBJECTIVES

The objective of this in vitro study was to investigate the behavior of different composite restorative materials under the load of cast circumferential clasps for removable dental prostheses (RDPs).

METHODS

In 60 human molars, standardized mesial-occlusal-distal cavities were prepared. The cavities were restored with the following materials: Definite, Tetric Ceram, SureFil, Heliomolar RO, Ariston pHc, and Oralloy, and provided with a rest seat. The rest seats were subjected to 5,000 cycles of thermal cycling and 1,200,000 masticatory cycles in a mastication simulator via cobalt-chromium circumferential clasps cast to standardized frameworks in a laboratory model designed to simulate the biomechanics of a free-end denture base. Fracture analysis of the restorations was performed by light microscopy. Before and after loading, material wear was measured with a 3D-laser scanner, and an analysis of the marginal quality was performed in an SEM at ×200 applying the replica technique.

RESULTS

No significant differences in the fracture behavior among the composite materials were found; the amalgam control group showed a significantly higher fracture resistance. Regarding the wear of the materials, the composites Definite and SureFil exhibited a behavior similar to that of amalgam. The other composites demonstrated higher wear rates. The initial marginal quality was significantly worse for Ariston pHc. The marginal adaptation decreased significantly after thermal and mechanical loading for Definite and Ariston pHc.

CONCLUSIONS

In terms of the investigated aspects of mechanical performance, the tested composites seemed to be inferior to amalgam. Further clinical studies are needed to evaluate the ability of composite restorations to provide support for RDP clasps.

CLINICAL RELEVANCE

The use of composites as direct restoration materials should be avoided in teeth, which serve as abutments for clasp-retained RDPs.

A new approach to influence contact angle and surface free energy of resin-based dental restorative materials

The purpose of the present study was to identify novel delivery systems and active agents which increase the water contact angle and reduce the surface free energy when added to resin-based dental restorative materials. Two delivery systems based on zeolite or novel polymeric hollow beads (Poly-Pore), loaded with two low surface tension active agents (hydroxy functional polydimethylsiloxane and polydimethylsiloxane) or a polymerizable active agent (silicone polyether acrylate) were used to modify commonly formulated experimental dental resin composites. The non-modified resin was used as a standard (ST). Flexural strength, flexural modulus, water sorption, solubility, polymerization shrinkage, surface roughness Ra, contact angle θ, total surface free energy γS, and the apolar γSLW, polar γSAB, Lewis acid γS+ and base γS- components, and the active agents surface tensions γL were determined (P<0.05). The active agents did not differ in γL. The modified materials had significantly higher θ but significantly lower γS, γSAB and γS- than the ST. A Poly-Pore/polydimethyl siloxane delivery system yielded the highest θ (110.9±3.5°) acceptable physical properties and the lowest values for γSLW and γS-. Among the modified materials the polymerizable materials containing active agents had the lowest γAB and the highest γS+ and γS-. Although not significant, both of the zeolite delivery systems yielded higher γSLW, γS+ and γS- but lower γSAB than the Poly-Pore delivery systems. Poly-Pore based delivery systems highly loaded with low surface tension active agents were found not to influence the physical properties but to significantly increase the water contact angle and thus reduce surface free energy of dental resin composites.

Strong nanocomposites with Ca, PO(4), and F release for caries inhibition

This article reviews recent studies on: (1) the synthesis of novel calcium phosphate and calcium fluoride nanoparticles and their incorporation into dental resins to develop nanocomposites; (2) the effects of key microstructural parameters on Ca, PO(4), and F ion release from nanocomposites, including the effects of nanofiller volume fraction, particle size, and silanization; and (3) mechanical properties of nanocomposites, including water-aging effects, flexural strength, fracture toughness, and three-body wear. This article demonstrates that a major advantage of using the new nanoparticles is that high levels of Ca, PO(4), and F release can be achieved at low filler levels in the resin, because of the high surface areas of the nanoparticles. This leaves room in the resin for substantial reinforcement fillers. The combination of releasing nanofillers with stable and strong reinforcing fillers is promising to yield a nanocomposite with both stress-bearing and caries-inhibiting capabilities, a combination not yet available in current materials.

Degradation, fatigue, and failure of resin dental composite materials

The intent of this article is to review the numerous factors that affect the mechanical properties of particle- or fiber-filler-containing indirect dental resin composite materials. The focus will be on the effects of degradation due to aging in different media, mainly water and water and ethanol, cyclic loading, and mixed-mode loading on flexure strength and fracture toughness. Several selected papers will be examined in detail with respect to mixed and cyclic loading, and 3D tomography with multi-axial compression specimens. The main cause of failure, for most dental resin composites, is the breakdown of the resin matrix and/or the interface between the filler and the resin matrix. In clinical studies, it appears that failure in the first 5 years is a restoration issue (technique or material selection); after that time period, failure most often results from secondary decay.

Water sorption of resin-modified glass-ionomer cements photoactivated with LED

The Light Emitting Diodes (LED) technology has been used to photoactivate composite resins and there is a great number of published studies in this area. However, there are no studies regarding resin-modified glass-ionomer cements (RMGIC), which also need photoactivation. Therefore, the aim of this study was to evaluate water sorption of two RMGIC photoactivated with LED and to compare this property to that obtained with a halogen light curing unit. A resin composite was used as control. Five specimens of 15.0 mm in diameter x 1.0 mm in height were prepared for each combination of material (Fuji II LC Improved, Vitremer, and Filtek Z250) and curing unit (Radii and Optilight Plus) and transferred to desiccators until a constant mass was obtained. Then the specimens were immersed into deionized water for 7 days, weighed and reconditioned to a constant mass in desiccators. Water sorption was calculated based on weight and volume of specimens. The data were analyzed by two-way ANOVA and Tukey test (p < 0.05). Specimens photocured with LED presented significantly more water sorption than those photocured with halogen light. The RMGIC absorbed statistically significant more water than the resin composite. The type of light curing unit affected water sorption characteristics of the RMGIC.

Influence of Tg, viscosity and chemical structure of monomers on shrinkage stress in light-cured dimethacrylate-based dental resins

OBJECTIVES

The aim of this study was to investigate the influence of the molecular mobility and the chemical structure of dimethacrylates most commonly used in dental composites on shrinkage stress from experimental matrices.

METHODS

Three established neat monomers BisGMA (B), UEDMA (U) and TEGDMA (T), two experimental comonomers BisGMA-based (B-T(70/30) and B-T(50/50)) and two comonomers UEDMA-based (U-T(88/11) and U-T(66/33)) in weight%, were elaborated. Camphorquinone (CQ) and N,N-cyanoethylmethylaniline (CEMA), as photoinitiator and reducing agent, were added. Then the matrices were mixed by centrifugal force at room temperature. The viscosity (eta), the glass transition temperature of the monomers and comonomers systems (T(g(monomer)) and the maximum shrinkage stress (MSS) of each material (five replications) were statistically analysed by one-way ANOVA/Tuckey's test and Pearson's correlation procedure (p = 0.05).

RESULTS

All formulations exhibited a newtonian rheological behavior. The viscosity of the comonomers systems can be divided in two groups: the pair B-T(70/30)/U-T(88/11) with the viscosity 3.5+/-3.10(-3)Pa.s and the pair B-T(50/50)/U-T(66/33) with the viscosity 0.28+/-3.10(-3)Pa.s. This pairs constituted samples allowing to compare the shrinkage stress of the BisGMA and UEDMA-based matrices with each other. The T(g(monomer)) of each group showed equivalent statistically values: -37.1 +/- 0.02 degrees C (U-T(88/11)) with -39.3 +/- 0.02 degrees C (B-T(70/30)) for the 3.5 Pa.s pair, and -53.1 +/- 0.03 degrees C (U-T(66/33)) with -58.5 +/- 0.01 degrees C (B-T(50/50)) for the 0.28 Pa.s one. There was a correlation between eta and T(g(monomer)) (r < 0.45 and p < 0.01). In decreasing order, the shrinkage stress was 14.11 +/- 0.3 MPa (T), 10.64 +/- 0.6 MPa (U-T(66/33)), 8.16 +/- 0.25 MPa (B-T(50/50)) without a significant difference compared to 8.04 +/- 0.5 MPa (U-T(88/11)), 6.83 +/- 0.52 MPa (U), 4.44 +/- 0.25 MPa (B-T(70/30)) and 0.33 +/- 0.3 MPa (B). There was a negative correlation between eta (r < -0.42 and p < 0.01), T(g(monomer)) (r < -0.41 and p < 0.01) and MSS. Whatever the viscosity, the UEDMA-based matrices developed higher shrinkage stresses than the BisGMA homologues.

SIGNIFICANCE

The shrinkage stress development increase with the molecular mobility of the reacting medium. For the same molecular mobility, the large differences in stress values of the matrices studied are correlated to the structure and particularly the functionality of the monomers used.

A review of the management of endodontically treated teeth

BACKGROUND

The clinical decision as to whether an endodontically treated (ET) tooth requires a post and a crown poses a challenge to dental practitioners. The author conducted a review of the principles for the use of post and core and the newer materials such as ceramic and fiber-reinforced posts.

TYPES OF STUDIES REVIEWED

Using a MEDLINE search and resulting cross-references, the author selected original research articles and previous review articles on the topic of ET teeth, as well as that of post and core.

RESULTS

The author reviewed the principles for the use of posts in terms of when it is necessary to use a post, different types of posts, various post materials and designs. He also reviewed the criteria and technique for post space preparation and post cementation. Finally, he discussed the principles of core buildup, as well as options for the final restorations.

CLINICAL IMPLICATIONS

The author provides a review of the principles for the use of post and core, crowns and the different materials available today to help clinicians make a clinical decision based on sound evidence.

Cuspal movement and microleakage in premolar teeth restored with posterior filling materials of varying reported volumetric shrinkage values

OBJECTIVES

To investigate the effect of polymerisation shrinkage stress of various aesthetic posterior filling materials on cuspal movement and cervical gingival microleakage of mesio-occlusal-distal (MOD) restorations placed in increments in extracted maxillary premolar teeth.

METHODS

Forty sound extracted upper premolar teeth were subjected to standardised preparation of a large MOD cavity. One curing regimen was used and each posterior filling material was placed in eight increments with the appropriate bonding system. A twin channel deflection-measuring gauge allowed a measurement of individual cusp deflections at each stage of polymerisation. Restored teeth were thermocycled before immersion in 0.2% basic fuchsin dye for 24h. After sagittal sectioning of the restored teeth in a mesio-distal plane, the sectioned restorations were examined to assess cervical gingival microleakage.

RESULTS

In general, cuspal deflection measurements were dependent upon the constituent monomers and the associated shrinkage on curing, with significantly increased cuspal movement (P<0.05) being recorded for Z100 (20.03+/-2.92 microm) compared with Filtek Z250 (12.34+/-2.18 microm), P60 (13.41+/-4.43 microm) and Admira (11.2+/-2.58 microm). No significant differences were identified between the posterior filling materials when the cervical gingival microleakage scores were examined.

CONCLUSIONS

It would appear that a reduction in the manufacturers' reported volumetric polymerisation shrinkage for Z100 (4.0%) to below 3% for Filtek Z250, P60 and Admira, resulted in a significant reduction in the associated cuspal strain on the MOD cavity. The diluent triethyleneglycol dimethacrylate (TEGDMA) increases the polymerisation shrinkage of Z100 resin-based composite due to an increased concentration of carbon-to-carbon double bonds (CC). The replacement of TEGDMA with urethane dimethacrylate (UDMA) and Bis-EMA in Filtek Z250 and P60, decreases the polymerisation shrinkage stress by increasing the cross-linking of polymer networks. However, no group was identified as producing less gingival microleakage at the cervical dentine cavosurface margin when the cavities were sectioned and examined, regardless of the reported variations in cuspal strain and the associated volumetric polymerisation shrinkage values.

Change of properties during storage of a UDMA/TEGDMA dental resin

The aim of this study was to evaluate the changes in viscoelastic properties of a UDMA-based dental resin as a function of time after initial light exposure. Specimens of a UDMA/TEGDMA (70:30 wt%) resin were irradiated by a visible-light-curing unit. Immediately after the irradiation, the light-cured specimen was stored in the dark for different times from 1 to 120 h at 37 degrees C, and characterized by means of DMA, DSC, and FTIR spectroscopy. The irradiated specimen exhibited a bimodal shape in the form of two rapid declines in log E' corresponding to glass transition with a plateau between the two declines. Two distinct peaks were seen in tan delta versus temperature. The thermal reaction of the incompletely cured sample with residual groups trapped by the fast reaction during irradiation is responsible for the plateau. After storage, significant changes were observed in dynamic mechanical parameters, DSC exotherm, and degree of conversion. Storage modulus continued to increase during the 4 h of storage and leveled off thereafter. Peak heights of tan delta versus temperature were also influenced by storage. Degree of conversion increased from 75 +/- 2% immediately after irradiation to 87 +/- 3% after 120 h storage. The changes of the properties of this dental resin system when stored at 37 degrees C after irradiation are clinically important in terms of stability, durability, and performance after initial polymerization.

In vitro contact wear of dental composites

OBJECTIVE

The aim of this study is to determine the in vitro two-body contact wear mechanisms of three medium filled composites and compare these with a highly filled composite previously investigated.

MATERIALS AND METHODS

Three commercial dental composites with filler mass fraction loading of 75-76% were evaluated. Two of the composites contained Ba-B-Al-silicate glass fillers and fumed silica with different particle sizes and distributions. One of these composites contained a fairly uniform distribution of filler particles ranging in size from 1 to 5 microm, whereas the particle size distribution in the second composite was bimodal consisting of small (less than 1 microm) and large (about 10 microm) particles. The third composite contained Ba-Al-silicate glass and silica with a filler particle size of approximately 1 microm. The composite disks were tested for wear against harder alumina counterfaces. Wear tests were conducted in distilled water using a pin-on-disk tribometer under conditions that represented typical oral conditions (sliding speed of 2.5 mm/s and contact loads ranging from 1 to 20 N). The wear tracks were analyzed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy to elucidate the wear mechanisms. The chemical composition of the water solution collected after the tests was determined using an inductively coupled plasma-mass spectrometer (ICP-MS) to detect possible chemical changes, e.g. dissolution of trace elements due to submersion or wear. The wear results were compared with those reported in an earlier study on a highly filled composite containing predominately alumino-silicate glass fillers and alumina at a filler loading of 92%.

RESULTS

The differences in two-body wear rates between the three medium filled composites were not statistically significant (p<0.05) indicating that the variations in filler particle size and slight differences in chemical composition of the glass fillers do not affect the in vitro wear rates of these composites. Wear rates of these medium filled composites, however, were significantly lower than the highly filled composite (p<0.05). SEM, FTIR and ICP-MS analyses suggested that wear in the medium filled composites occurs by a complex set of processes involving tribochemical reactions between filler particles and water, formation of surface films containing a mixture of filler fragments and reaction products, and film delamination, as well as dissolution of the reaction products.

SIGNIFICANCE

This study reveals that subtle changes in the filler particle size and small differences in filler composition do not significantly affect the two-body wear behavior of medium filled composites. However, the chemistry of filler particles plays an important role in altering the wear performance of composites when significant changes are made in the chemical composition of the fillers and when the filler loading is increased.

Monomer conversion versus flexure strength of a novel dental composite

OBJECTIVES

To quantify the monomer conversion and flexural strength of an experimental oxirane-based composite material (EXL596) compared with two, commercially available, dimethacrylate based restoratives (Z250 and Z100).

METHODS

Fourier-transform infra-red spectroscopy (FTIR) was utilised to evaluate the degree of conversion (DC) (n=5) and biaxial flexure strength (BFS) testing (n=20) was used to analyse flexural strength and associated Weibull moduli (m) of each material following 0.1, 0.5, 1, 4, 24 and 48 h immersion in a lightproof waterbath maintained at 37+/-1 degrees C.

RESULTS

The DC of Z250 and Z100 following 0.1, 0.5 and 1 h post-irradiation was significantly greater than the DC of EXL596 for the same immersion periods. This was manifested as a significant decrease in BFS and associated m of EXL596 compared with Z250 and Z100 for the 0.1, 0.5 and 1 h post-irradiation periods. The DC and BFS of EXL596 were significantly greater than Z250 and Z100 following 24 h immersion.

CONCLUSIONS

Assessment of FTIR spectra, BFS and associated m has provided a useful method in the quantitative analysis of resin-based composite conversion. Identification of the decreased DC of EXL596 compared with Z250 and Z100 was achieved using FTIR. However, decreased conversion rates within the first hour following irradiation of EXL596 may compromise flexural strength properties (associated with a decrease in BFS and m) which may be inadequate under masticatory loading.

Bond durability of the resin-bonded and silane treated ceramic surface

OBJECTIVES

The aim of this study was to explore methods for improving the ceramic-silane-resin bond strength of silica-based ceramics, such that it may be possible to eliminate the hazardous process of acid etching these dental ceramics with hydrofluoric acid (HF).

METHODS

Ni/Cr rods were prepared with ceramic facings, which were polished to a 1 microm finish. A silane coupling agent was applied to the ceramic surface using seven different procedures. Specimens were bonded using a luting resin and the tensile bond strength was measured at a crosshead speed of 1mm/min. One bonding procedure was selected and used for the comparison of four ceramic surface preparations consisting of; 1 microm polish, gritblasted (50 microm alumina), etched with 10% HF, and gritblasted and etched. The durability of the bond was assessed by (1) storing the specimens in water at 37 degrees C for different time periods up to 3 months, (2) thermal cycling, and (3) storing in water at 100 degrees C for 24h.

RESULTS

The results showed that one bonding procedure to a polished ceramic surface gave better results for the tensile bond strength of the resin to the smooth ceramic surface and that the tensile bond strength was not significantly different from the gritblasted, etched or gritblasted and etched groups (P>0.05). There was no deterioration in the tensile bond strength for any of the groups after water storage for up to 3 months or after thermal cycling (P>0.05). The silane bond was also capable of resisting hydrolytic attack in boiling water.

SIGNIFICANCE

The results indicate that a durable resin-ceramic tensile bond can be obtained by appropriate silane application without the need for HF acid etching the ceramic surface.

Influence of shade and storage time on the flexural strength, flexural modulus, and hardness of composites used for indirect restorations

STATEMENT OF PROBLEM

Fracture resistance, elastic modulus, and hydrolytic degradation resistance are important properties of indirect composite restorations. Composite systems developed specifically for indirect application are said to have enhanced mechanical properties due to their elevated monomer conversion.

PURPOSE

This study evaluated the influence of shade and the effect of 30-day water storage on the flexural strength, flexural modulus, and hardness of 4 commercially available indirect composite systems and 1 composite used with the direct technique.

MATERIAL AND METHODS

A variety of commercially available indirect resin composites (Artglass, Belleglass, Sculpture, and Targis) and 1 directly placed composite (Z100, control) were used. Specimens made with either incisal or dentin shade (n = 10) were fractured with a 3-point bending test. Pre-failure loads corresponding to specific displacements of the crosshead were used for flexural modulus calculation. Knoop hardness was measured on fragments (n = 3) obtained after the flexural test. Tests were performed after 24 hours and after a 30-day water storage at 37 degrees C. Flexural strength data were analyzed with the Weibull distribution. Flexural modulus and Knoop hardness data were analyzed with 3-way ANOVA and Tukey's post-hoc test (alpha=0.05).

RESULTS

In general, the directly placed composite (Z100) demonstrated flexural strength similar to that of Artglass, Targis, and Sculpture. Belleglass presented the highest flexural strength (221.7 MPa for incisal shade after 24 hours storage; 95% confidence interval: 208.3-235.4). Z100 demonstrated the highest flexural modulus (range: 10.9 +/- 0.6 to 12.0 +/- 0.9 GPa) and Targis the lowest (range: 5.1 +/- 0.5 to 5.9 +/- 0.9 GPa). Sculpture was the only material that showed differences in flexural strength with respect to shade (incisal-24 hours: 149.8 MPa; incisal-30 days: 148.7 MPa; dentin-24 hours: 200.0 MPa; dentin-30 days: 177.9 MPa). The flexural modulus and hardness of the dentin shade of Sculpture were higher than those of the incisal shade after 30 days. Belleglass also showed a significant difference in flexural modulus (dentin-24 hours: 11.1 GPa; incisal-24 hours: 9.6 GPa). The effect of water storage was more evident on hardness since all composite systems softened after 30 days. Prolonged water storage decreased flexural strength only for Artglass-dentin and Z100, both incisal and dentin shades. Water aging did not affect the flexural modulus of any composite tested.

CONCLUSION

In general, indirect composites did not show enhanced mechanical properties compared to the directly placed composite. Property differences due to shade were more evident for Sculpture. Prolonged water storage had a deleterious effect on the hardness of all composites tested. However, water storage did not affect the flexural strength of most of the indirect composites or the flexural modulus of any composite tested.

Fracture toughness of aged dental composites in combined mode I and mode II loading

Resin-based laboratory dental composites for prosthetic restorations have been developed in the past years as a cost-effective alternative to conventional porcelain-fused-to-metal or full ceramic restorations. The fracture toughness at different stress states (K(Ic), K(IIc), and mixed-modes K(I), K(II) ) was assessed for three laboratory dental composite resins used for prosthetic restorations that were aged up to 12 months in a food simulating fluid (10% ethanol) at 37 degrees C. The materials were mainly di- methacrylate based resins reinforced with submicron glass filler particles. The Brazilian disk test was used on precracked chevron-notched specimens, and different stress states were obtained by angulating the precracked chevron notch relative to the diametral compressive loading direction. The stress intensity factors were calculated using Atkinson et al.'s relation. For all three materials, mode I fracture toughness values ranged between 0.48-0.64 MPa. m(0.5) and mode II values ranged between 0.93-1.2 MPa. m(0.5). Overall, aging time and storage media had little effect on toughness. Considering the inherently low toughness of these restorative materials, their use should be limited to low stress masticatory areas.

Foundation restorations in fixed prosthodontics: current knowledge and future needs

PURPOSE

The Ad Hoc Committee on Research in Fixed Prosthodontics established by the Academy of Fixed Prosthodontics publishes a yearly comprehensive literature review on a selected topic. The subject for this year is foundation restorations.

METHODS

Literature of various in vitro and in vivo investigations that included technical and clinical articles was reviewed to provide clinical guidelines for the dentist when selecting methods and materials for restoration of structurally compromised teeth. Topics discussed and critically reviewed include: (1) desirable features of foundation restorations, (2) foundations for pulpless teeth, (3) historic perspectives, (4) cast posts and cores, (5) role of the ferrule effect, (6) prefabricated posts, (7) direct cores, (8) foundation restorations for severely compromised teeth, (9) problems and limitations, (10) future needs, and (11) directions for future research.

CONCLUSION

This comprehensive review brings together literature from a variety of in vitro and in vivo studies, along with technique articles and clinical reports to provide meaningful guidelines for the dentist when selecting methods and materials for the restoration of structurally compromised teeth.

Dental luting agents: A review of the current literature

STATEMENT OF PROBLEM

The practice of fixed prosthodontic has changed dramatically with the introduction of innovative techniques and materials. Adhesive resin systems are examples of these changes that have led to the popularity of bonded ceramics and resin-retained fixed partial dentures. Today's dentist has the choice of a water-based luting agent (zinc phosphate, zinc polycarboxylate, glass ionomer, or reinforced zinc oxide-eugenol) or a resin system with or without an adhesive. Recent formulations of glass ionomer luting agents include resin components (resin-modified glass ionomers), which are increasingly popular in clinical practice.

PURPOSE

This review summarizes the research on these systems with the goal of providing information that will help the reader choose the most suitable material.

MATERIAL

The scientific studies have been evaluated in relation to the following categories: (1) biocompatibility, (2) caries or plaque inhibition, (3) microleakage, (4) strength and other mechanical properties, (5) solubility, (6) water sorption, (7) adhesion, (8) setting stresses, (9) wear resistance, (10) color stability, (11) radiopacity, (12) film thickness or viscosity, and (13) working and setting times. In addition, guidelines on luting-agent manipulation are related to available literature and include: (1) temporary cement removal, (2) smear layer removal, (3) powder/liquid ratio, (4) mixing temperature and speed, (5) seating force and vibration, and (6) moisture control. Tables of available products and their properties are also presented together with current recommendations by the authors with a rationale.

Bonding to zirconia ceramic: adhesion methods and their durability

OBJECTIVES

Resin bonding to yttrium-oxide--partially-stabilized zirconia ceramic (YPSZ) cannot be established by standard methods that are utilized for conventional silica-based dental ceramics. It was our hypothesis that adhesive bonding methods suitable for glass-infiltrated alumina ceramic can also be used to bond successfully to YPSZ. To prove this hypothesis, bonding methods suitable for alumina ceramic were used on YPSZ and the tensile bond strength and their durability evaluated in vitro.

METHODS

Plexiglass tubes filled with resin composite were bonded to YPSZ discs following various adhesion protocols. Groups of 16 samples were bonded using seven different bonding methods. Subgroups of eight bonded samples were tested for tensile strength following storage in distilled water (37 degrees C) for either 3 or 150 days. In addition, the 150 day samples were thermal cycled 37,500 times as a method to stress the bond interface. The statistical analysis was made with the Kruskal-Wallis test followed by multiple pair-wise comparisons of the groups using the Wilcoxon rank sum test.

RESULTS

Sandblasting alone, the additional use of a silane or acrylizing resulted in an initial bond of a conventional BisGMA resin composite to YPSZ which failed spontaneously over storage time. The use of the BisGMA resin composite after tribochemical silica coating of YPSZ and the use of a polyacid-modified resin composite after sandblasting of YPSZ resulted in an initial bond which decreased significantly over storage time (p = 0.05). A durable resin bond to YPSZ was achieved only after sandblasting the ceramic and using one of two resin composites containing a special phosphate monomer.

SIGNIFICANCE

A durable bond to YPSZ was achieved only by using resin composites with a special adhesive monomer. Therefore, the hypothesis of the study was partially proved as a durable bond to alumina ceramic is achieved with the same resin composites. However, it was partially disproved as tribochemical silica coating of YPSZ did not result in a durable resin bond as it does on glass-infiltrated alumina ceramic.

Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites

OBJECTIVES

This study was conducted to determine the effect of UEDMA, BisGMA and TEGDMA on selected mechanical properties of experimental resin composites.

METHODS

Thirty monomer mixtures of TEGDMA and BisGMA and/or UEDMA were produced. Five base monomer mixtures had the following molar relationships between TEGDMA and BisGMA: 30:70, 40:60, 50:50, 60:40 and 70:30. Monomer mixtures were then produced in which BisGMA was successively substituted by UEDMA, 10 mol% at a time. The resins were made light-curing and loaded with filler. Diametral tensile strength, flexural strength and modulus of elasticity were determined on 1 week old specimens. The results were analyzed by ANOVA and by response surface methodology.

RESULTS

The diametral tensile strength of the resin composites varied between 52 and 59 MPa, the flexural strength between 137 and 167 MPa, and the modulus of elasticity between 8.0 and 11.1 GPa. The statistical analyses showed that substitution of BisGMA or TEGDMA by UEDMA resulted in an increase in tensile and flexural strength, and that substitution of BisGMA by TEGDMA increased tensile, but reduced flexural strength. Further, it was found that, for a given content of UEDMA, variations in the ratio BisGMA/TEGDMA gave rise to a maximum in modulus of elasticity. The size of this maximum in modulus decreased with increasing content of UEDMA.

SIGNIFICANCE

Varying the relative amounts of UEDMA, BisGMA and TEGDMA has a significant effect on the mechanical properties of the resin composition. Thus, by selecting specific combinations of these components, it may be possible to design composites with properties that are tailor made to specific applications.

Resin composites in dentistry: the monomer systems

The present review outlines the history of monomers used in resin composites, motivates further development, and highlights recent and ongoing research reported in the field of dental monomer systems. The monomer systems of most present-day resin composites are based on BisGMA, developed some 40 years ago, or derivatives of BisGMA. In the remaining resin composites, urethane monomers or oligomers are used as the basis of the monomer system. The main deficiencies of current resin composites are polymerization shrinkage and insufficient wear resistance under high masticatory forces. Both factors are highly influenced by the monomer system, and considerable efforts are being made around the world to reduce or eliminate these undesirable properties. The use of fluoride-releasing monomer systems, some of which are under investigation, has been suggested to mitigate the negative effects of marginal gaps formed in consequence of polymerization shrinkage. The very crux of the problem has also been approached with the synthesis of potentially low-shrinking/non-shrinking resin composites involving ring opening or cyclopolymerizable monomers. By the use of additives with a supposed chain transfer agent function, monomer systems have been formulated that improve the degree of conversion of methacrylate double bonds and mechanical properties. Many promising monomer systems have been devised, the implementation of which may be expected to improve the longevity of resin composite fillings and expand the indications for resin composites.