Correlation between fracture properties and clinical performance of composite resins in Class IV cavities

Effect of post-irradiation polymerization on selected mechanical properties of six direct resins

This study evaluated the post-irradiation mechanical property development of six resin composite-based restorative materials from the same manufacturer starting at 1 h post irradiation, followed by 24 h, 1 week, and 1 month after fabrication. Samples were stored in 0.2M phosphate buffered saline until testing. Flexural strength, flexural modulus, flexural toughness, modulus of resiliency, fracture toughness, and surface microhardness were performed at each time interval. Mean data was analyzed by Kruskal Wallis and Dunn's post hoc testing at a 95% level of confidence (α=0.05). Results were material specific but overall, all resin composite material mechanical properties were found to be immature at 1 h after polymerization as compared to that observed at 24 h. It may be prudent that clinicians advise patients, especially those receiving complex posterior composite restorations, to guard against overly stressing these restorations during the first 24 h.

Physical property investigation of contemporary glass ionomer and resin-modified glass ionomer restorative materials

OBJECTIVES

The objective of this study was to investigate selected physical properties of nine contemporary and recently marketed glass ionomer cement (GIC) and four resin-modified glass ionomer cement (RMGI) dental restorative materials.

MATERIALS AND METHODS

Specimens (n = 12) were fabricated for fracture toughness and flexure strength using standardized, stainless steel molds. Testing was completed on a universal testing machine until failure. Knoop hardness was obtained using failed fracture toughness specimens on a microhardness tester, while both flexural modulus and flexural toughness was obtained by analysis of the flexure strength results data. Testing was completed at 1 h, 24 h, 1 week, and then at 1, 3, 6, and 12 months. Mean data was analyzed with Kruskal-Wallis and Mann-Whitney (p = 0.05).

RESULTS

Physical properties results were material dependent. Physical properties of the GIC and RMGI products were inferior at 1 h compared to that at 24 h. Some improvement in selected physical properties were noted over time, but development processes were basically concluded by 24 h. A few materials demonstrated improved physical properties over the course of the evaluation.

CONCLUSIONS

Under the conditions of this study: 1. GIC and RMGI physical property performance over time was material dependent; 2. Polyalkenoate maturation processes are essentially complete by 24 h; 3. Although differences in GIC physical properties were noted, the small magnitude of the divergences may render such to be unlikely of clinical significance; 4. Modest increases in some GIC physical properties were noted especially flexural modulus and hardness, which lends support to reports of a maturing hydrogel matrix; 5. Overall, GIC product physical properties were more stable than RMGI; 6. A similar modulus reduction at 6 months for both RMGI and GIC produced may suggest a polyalkenoate matrix change; and 7. Globally, RMGI products demonstrated higher values of flexure strength, flexural toughness, and fracture toughness than GIC materials.

CLINICAL RELEVANCE

As compared to RMGI materials, conventional glass ionomer restorative materials demonstrate more stability in physical properties.

Nanofilled Resin Composite Properties and Clinical Performance: A Review

The aim of this review was to compile recent evidence related to nanofilled resin composite materials regarding the properties and clinical performance. Special attention was given to mechanical properties, such as strength, hardness, abrasive wear, water sorption, and solubility. The clinical performance of nanocomposite materials compared with hybrid resin composites was also addressed in terms of retention and success rates, marginal adaptation, color match, and surface roughness. A search of English peer-reviewed dental literature (2003-2017) from PubMed and MEDLINE databases was conducted using the terms "nanocomposites" or "nanofilled resin composite" and "clinical evaluation." The list was screened, and 82 papers that were relevant to the objectives of this work were included in the review. Mechanical properties of nanocomposites are generally comparable to those of hybrid composites but higher than microfilled composites. Nanocomposites presented lower abrasive wear than hybrids but higher sorption values. Their clinical performance was comparable to that of hybrid composites.

Laboratory mechanical parameters of composite resins and their relation to fractures and wear in clinical trials-A systematic review

OBJECTIVE

To evaluate a range of mechanical parameters of composite resins and compare the data to the frequency of fractures and wear in clinical studies.

METHODS

Based on a search of PubMed and SCOPUS, clinical studies on posterior composite restorations were investigated with regard to bias by two independent reviewers using Cochrane Collaboration's tool for assessing risk of bias in randomized trials. The target variables were chipping and/or fracture, loss of anatomical form (wear) and a combination of both (summary clinical index). These outcomes were modelled by time and material in a linear mixed effect model including random study and experiment effects. The laboratory data from one test institute were used: flexural strength, flexural modulus, compressive strength, and fracture toughness (all after 24-h storage in distilled water). For some materials flexural strength data after aging in water/saliva/ethanol were available. Besides calculating correlations between clinical and laboratory outcomes, we explored whether a model including a laboratory predictor dichotomized at a cut-off value better predicted a clinical outcome than a linear model.

RESULTS

A total of 74 clinical experiments from 45 studies were included involving 31 materials for which laboratory data were also available. A weak positive correlation between fracture toughness and clinical fractures was found (Spearman rho=0.34, p=0.11) in addition to a moderate and statistically significant correlation between flexural strength and clinical wear (Spearman rho=0.46, p=0.01). When excluding those studies with "high" risk of bias (n=18), the correlations were generally weaker with no statistically significant correlation. For aging in ethanol, a very strong correlation was found between flexural strength decrease and clinical index, but this finding was based on only 7 materials (Spearman rho=0.96, p=0.0001). Prediction was not consistently improved with cutoff values.

SIGNIFICANCE

Correlations between clinical and laboratory outcomes were moderately positive with few significant results, fracture toughness being correlated with clinical fractures and flexural strength with clinical wear. Whether artificial aging enhances the prognostic value needs further investigations.

Wear and Mechanical Properties of Nano-silica-fused Whisker Composites

Resin composites must be improved if they are to overcome the high failure rates in large stress-bearing posterior restorations. This study aimed to improve wear resistance via nano-silica-fused whiskers. It was hypothesized that nano-silica-fused whiskers would significantly improve composite mechanical properties and wear resistance. Nano-silicas were fused onto whiskers and incorporated into a resin at mass fractions of 0%-74%. Fracture toughness (mean ± SD; n = 6) was 2.92 ± 0.14 MPa•m½ for whisker composite with 74% fillers, higher than 1.13 ± 0.19 MPa•m½ for a prosthetic control, and 0.95 ± 0.11 MPa•m½ for an inlay/onlay control (Tukey’s at 0.95). A whisker composite with 74% fillers had a wear depth of 77.7 ± 6.9 μm, less than 118.0 ± 23.8 μm of an inlay/onlay control, and 172.5 ± 15.4 μm of a prosthetic control (p < 0.05). Linear correlations were established between wear and hardness, modulus, strength, and toughness, with R = 0.95–0.97. Novel nano-silica-fused whisker composites possessed high toughness and wear resistance with smooth worn surfaces, and may be useful in large stress-bearing restorations.

Selected physical properties of new resin-modified glass ionomer luting cements

STATEMENT OF PROBLEM

Two resin-modified glass ionomer (RMGI)-based luting agents have been recently marketed without independent reports of their physical properties.

PURPOSE

The purpose of this in vitro study was to evaluate selected physical properties of 2 newly marketed RMGI luting agents and compare the findings with traditional materials.

MATERIAL AND METHODS

Specimens (N=12) of Nexus RMGI, UltraCem, GC Fuji Cem 2, and RelyX Luting Plus were fabricated using standardized molds for flexural strength and fracture toughness according to manufacturer recommendations and stored in physiologic phosphate-buffered saline solution at 37°C until testing. Specimens were tested at 1 and 24 hours, 1 week, and 1 month. Mean values for flexural strength, flexural modulus, flexural toughness, and fracture toughness were determined. Additionally, film thickness (N=12) for each material was determined following Amerian National Standards Association/American Dental Association (ANSI/ADA) specifications. Mean results were analyzed with Kruskal-Wallis and Mann-Whitney U tests (α=.05).

RESULTS

All luting agents exhibited a similar film thickness that met ANSI/ADA requirements for aqueous-based luting agents. Nexus RMGI surprisingly demonstrated significantly greater flexural strength and fracture toughness at 1 hour, which decreased significantly at 24 hours, making it similar to the other materials evaluated. All materials had similar flexural strength values at 7 days.

CONCLUSIONS

Physical property performance was material dependent. Nexus RMGI demonstrated greater early physical properties that were significantly less at 24 hours. UltraCem, GC Fuji Cem 2, and RelyX Luting Plus demonstrated the increasing physical property development that is normally associated with polyalkenoate-based systems.

Mechanical properties, fracture resistance, and fatigue limits of short fiber reinforced dental composite resin

STATEMENT OF PROBLEM

Cycling masticatory loads decrease the strength of particulate filler composites (PFCs) and initiate the failure process by fatigue. The life expectancy of a composite resin restoration under stress remains difficult to predict.

PURPOSE

The purpose of this study was to determine the fracture resistance and the compressive fatigue limits (CFL) of anterior crown restorations made of a short-fiber reinforced composite resin (SFC), to investigate selected mechanical properties of the material following standard test methods, and to observe their correlation with the CFL.

MATERIAL AND METHODS

Specimens (n=10) were fabricated either from SFC (everX Posterior, GC Corp) or PFC (G-ænial anterior, GC Corp). The properties investigated were flexural strength (FS), compression strength (CS), diametral-tensile strength (DTS), and single-edge-notched-bend fracture toughness (FT) following ISO standards. Fracture resistance was determined by static load (n=10) and the CFL at 10000 cycles was determined using a staircase approach (n=20), both on anterior composite resin crowns. The results were analyzed with 1-way ANOVA (α=.05) or 2-way ANOVA (α=.05) followed by a Tukey B post hoc test and the Pearson-correlation analysis.

RESULTS

The SFC crowns had higher fracture resistance (954 ±121 N) than the PFC crowns (415 ±75 N) (P<.001) and higher CFL (267 ±23 N) than the PFC crowns (135 ±64 N) (P<.001). SFC revealed also higher FT (2.6 ±0.6 MPa·m(1/2)) than the PFC (1.0 ±0.2 MPa·m(1/2)) (F=69.313, P<.001). A significant correlation was observed only between the FT and the CFL (r(2)=0.899; P<.001).

CONCLUSIONS

SFC crowns showed good performance under static and fatigue loading. FT was the only in vitro test method that filtered as a clinically relevant parameter.

Investigation of the correlation between the different mechanical properties of resin composites

The aim of this study was to investigate the relationship between the different mechanical properties with the filler fraction of various resin composites. Mechanical properties of eighteen different resin composites were investigated in this study; flexural strength (FS), flexural modulus (FM), fracture toughness (FT), compressive strength (CS), diametral tensile strength (DTS), Barcol hardness (BH), Vickers hardness (HV), and Knoop hardness (HK). The mean values of mechanical properties and the filler fractions (V(f )) obtained from the literature and the manufacturer were analyzed using Pearson's correlation test at p<0.01. The relationships were compared with the data retrieved from previous studies. Strong correlations between Vf and BH/HV/HK and V(f) and FM were evident in the results of the present study and these results were supported by the retrieved data from previous studies. The other relationships between mechanical properties, such as that between FS and FM and between CS and HV were not significant.

Resin-based composite performance: are there some things we can't predict?

OBJECTIVE

The objective of this manuscript is to address the following questions: Why do direct dental composite restorative materials fail clinically? What tests may be appropriate for predicting clinical performance? Does in vitro testing correlate with clinical performance?

METHODS

The literature relating to the clinical and laboratory performance of dental composite restorative materials was reviewed. The main reasons for failure and replacement of dental composite restorations provided the guidance for identifying specific material's properties that were likely to have the greatest impact on clinical outcomes.

RESULTS

There are few examples of studies showing correlation between laboratory tests of physical or mechanical properties and clinical performance of dental composites. Evidence does exist to relate clinical wear to flexure strength, fracture toughness and degree of conversion of the polymer matrix. There is evidence relating marginal breakdown to fracture toughness. Despite the fact that little confirmatory evidence exists, there is the expectation that clinical fracture and wear relates to resistance to fatigue. Only minimal evidence exists to correlate marginal quality and bond strength in the laboratory with clinical performance of bonded dental composites.

SIGNIFICANCE

The use of clinical trials to evaluate new dental composite formulations for their performance is expensive and time consuming, and it would be ideal to be able to predict clinical outcomes based on a single or multiple laboratory tests. However, though certain correlations exist, the overall clinical success of dental composites is multi-factorial and therefore is unlikely to be predicted accurately by even a battery of in vitro test methods.

Dental glass-reinforced composite for caries inhibition: calcium phosphate ion release and mechanical properties

The two main challenges facing dental composite restorations are secondary caries and bulk fracture. Previous studies developed whisker-reinforced Ca-PO(4) composites that were relatively opaque. The objective of this study was to develop an esthetic glass particle-reinforced, photo-cured calcium phosphate composite. Tetracalcium phosphate (TTCP) particles were incorporated into a resin for Ca and PO(4) release, while glass particles provided reinforcement. Ion release and mechanical properties were measured after immersion in solutions with pH of 7, 5.5, and 4. For the composite containing 40% mass fraction of TTCP, incorporating glass fillers increased the strength (p < 0.05). Flexural strength (Mean +/- SD; n = 6) at 30% glass was 99 +/- 18 MPa, higher than 54 +/- 20 MPa at 0% glass (p < 0.05). Elastic modulus was 11 GPa at 30% glass, compared to 2 GPa without glass. At 28 days, the released Ca ion concentration was 4.61 +/- 0.18 mmol/L at pH of 4, much higher than 1.14 +/- 0.07 at pH of 5.5, and 0.27 +/- 0.01 at pH of 7 (p < 0.05). PO(4) release was also dramatically increased at cariogenic, acidic pH. The TTCP-glass composite had strength 2-3 fold that of a resin-modified glass ionomer control. In conclusion, the photo-cured TTCP-glass composite was "smart" and substantially increased the Ca and PO(4) release when the pH was reduced from neutral to a cariogenic pH of 4, when these ions are most needed to inhibit tooth caries. Its mechanical properties were significantly higher than previous Ca, PO(4), and fluoride releasing restoratives. Hence, the photo-cured TTCP-glass composite may have potential to provide the necessary combination of load-bearing and caries-inhibiting capabilities.

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.

Direct composite restorative materials

Composite dental restorative materials have advanced considerably over the past 10 years. Although composites have not totally replaced amalgam, they have become a viable substitute in many situations. Problems still exist with polymerization contraction stress, large differences in the coefficient of thermal expansion (CTE) of composites compared with tooth structure, and with some technique sensitivity; however, new expanding resins, nanofiller technology, and improved bonding systems have the potential to reduce these problems. With increased patient demands for esthetic restorations, the use of direct filling composite materials will continue to grow. The one major caveat to this prediction is that clinicians must continue to use sound judgment on when, where, and how to use composite restoratives in their practices.

Edge strength of resin-composite margins

OBJECTIVES

Marginal integrity is a major clinical problem in restorative dentistry. The aim of this study was to evaluate the applicability of an edge strength measurement device in an in vitro test to determine the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material.

METHODS

Five representative resin-composites were investigated. Fourteen disks of specimens (12mm diameter x 2.5mm thick) were prepared for each material. These were divided into seven sub-groups corresponding to different edge-distances (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0mm). An edge strength measurement device (CK10) (Engineering Systems, Nottingham, UK) was used. The mode of the failure of each specimen was examined under the integral microscope of the CK10.

RESULTS

The force (N)-to-fracture at a distance of 0.5mm from the edge was defined as the edge strength. The highest failure force (edge strength) was observed for Tetric Ceram (174.2N) and the lowest for Filtek Supreme (enamel) (87.0N). Correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p=0.02) to 0.99 (p=0.01). Two modes of failure were observed: chipping and--generally at greater distances--cracking.

SIGNIFICANCE

Edge strength is a definable and potentially useful parameter to characterize this aspect of clinically related behavior. A standardized distance of 0.5mm from the specimen's edge, when chipping failure prevails, is suitable and convenient as a reference point.

Viscoelastic behavior and fracture toughness of six glass-ionomer cements

STATEMENT OF PROBLEM

Viscoelastic behavior can influence the fracture properties of glass ionomers, which is of clinical relevance. Glass-ionomer cements can display viscoelastic behavior, defined as having displacement rate- or strain rate-dependent mechanical properties. Understanding and describing the viscoelastic behavior of glass ionomers is important to understanding their clinical behavior.

PURPOSE

The purpose of this study was to evaluate the viscoelastic behavior of 6 glass-ionomer cements and determine whether there was a correlation to fracture toughness.

MATERIAL AND METHODS

Three conventional glass-ionomer cements (alpha-Silver, alpha-Fil, and Ketac-Molar) and 3 resin-modified glass-ionomer cements (Vitremer, Fuji II LC, and Photac-Fil Quick) were evaluated using measurements of compressive strength (CS), flexural strength (FS), and diametral tensile strength (DTS) at displacement rates of 0.5, 1.0, 1.5, and 2.0 mm/min. The CS and DTS specimens were cured in glass tubes and cut to 4 x 6-mm and 4 x 2-mm disk-shaped specimens, respectively. The FS specimens were cured in bar molds (2 x 2 x 15 mm). The fracture toughness (FT) specimens were cured in a minicompact mold to obtain precracked specimens. The mechanical testing results were compared statistically using generalized linear model/analysis of covariance and the Ryan-Einot-Gabriel-Welsch multiple range test at the alpha=.05 level.

RESULTS

For all 3 mechanical properties, there was a displacement-rate dependence on the mechanical property. However, there were no differences in the displacement-rate dependence based on the type of material-conventional glass ionomer or resin-modified glass ionomer-for any of the mechanical properties. Only for FS test was there a significant difference based on the brand of material. There was no statistical difference in FT among the glass-ionomer cements tested, although the resin-modified glass ionomers tended to display higher FT.

CONCLUSION

A larger sample size and a much wider range of crosshead speeds are necessary to support a correlation between viscoelastic behavior and FT.

Evidence-based decision making: Guide to reading the dental materials literature

Although potential links between materials data and clinical behavior are often implied, the status of such linkage is often left obscure. This paper provides clinicians a context within which to view materials information as evidence for clinical indications and to broaden readers' appreciation for the subject. Hierarchies of both clinical and nonclinical data are presented and discussed from the point of view of their predictive potential regarding clinical performance. Excellent sources of information are identified for the clinician making treatment decisions, and perspectives are offered on the value of other published materials data.

Static and fatigue compression test for particulate filler composite resin with fiber-reinforced composite substructure

OBJECTIVES

The aim of this study was to determine static load-bearing capacity and compressive fatigue limits (CFL) of laboratory particulate filler composite resin (PFC) with three different types of fiber-reinforced composite (FRC) substructures.

METHODS

A total of 420 test specimens were prepared having 1.0mm of FRC layer as substructure (short random, continuous unidirectional and bidirectional fiber orientations), and a 2.0-mm thick surface layer of PFC. Control specimens were prepared from plain FRC or PFC. The specimens (n=15) were either dry stored or water stored (37 degrees C for 2 weeks) before they were loaded with a steel ball (Ø 3.0mm) under static load until fracture and cyclic load with maximum controlled regimen following a staircase approach with maximum 10(3) cycles. The decrease in CFL compared to static load was calculated and data were analyzed using ANOVA and Weibull statistics.

RESULTS

The highest static loads were registered for plain FRC specimens [short random 1842 N(205), continuous bidirectional 2258 N(233) and unidirectional fiber orientation 538 N(254)]. The specimens with FRC substructure and PFC coverage gave load values of 1517 N(249), 1670 N(241) and 677 N(240), respectively. The specimens made of PFC only, failed with 1047 N(230) load. The CFL for 10(3) cycles ranged between 19 and 39% of the static load values. ANOVA revealed that all factors significantly affected the load bearing capacity (p<0.001).

SIGNIFICANCE

The results suggested that the material combination of continuous bidirectional or random FRC and PFC, gave higher CFL and static load-bearing capacity than that obtained with plain particulate filler composite resin.

Can a single composite resin serve all purposes?

The consensus view less than a decade ago was that direct posterior composites should be restricted to small restorations, preferably in premolar teeth with little, if any, occlusal function. Major advances in adhesive systems, materials and restorative techniques have combined to allow us to question this view and our increased clinical evidence base makes it appropriate to reconsider this viewpoint.

Placement and replacement of restorations by selected practitioners

BACKGROUND

There are few Australian data on the reasons for placement and replacement of restorations, and the extent to which these are carried out in general practice.

METHODS

A survey was carried out of approximately 100 consecutive restorations placed by each of 28 general dental practitioners. The data were coded and statistically analyzed for various associations.

RESULTS

Resin composite was used twice as frequently as amalgam as a restorative material, and nearly four times as often as glass-ionomer cement. Secondary caries was the principal reason for replacing restorations, affecting predominantly amalgam restorations in Class I and Class V cavities. Teeth restored with amalgam fractured nearly twice as often as teeth restored with resin composite. The average ages of amalgam, resin composite and glass-ionomers at replacement were 13.6, 7.1 and 5.7 years respectively.

CONCLUSIONS

Amalgam has the longest clinical service life, but is associated with more tooth fracture. Secondary caries is the main reason for replacing restorations. The anti-cariogenic effect of glass-ionomer cement is equivocal.

Evaluation of sub-critical fatigue crack propagation in a restorative composite

OBJECTIVES

Fracture is a major reason for clinical failure of dental restorations. Many clinical fractures are likely to be preceded by a slow sub-critical crack propagation. The purpose of this study was to determine slow crack propagation in a restorative composite.

METHODS

Notched composite (Z100, 3M ESPE) specimens were fatigued in a four-point bending test. The notch (1 mm) was created by embedding a sharpened razor blade in the specimen mold. The specimens were load-cycled at 5 Hz between -5 and -30 N until failure. Displacement and load were recorded during the fatigue tests and used to derive crack propagation based on beam-compliance.

RESULTS

The number of cycles until failure ranged between 34 and 82,481. In the last 1500 cycles prior to final fracture, the beam compliance increased consistently, indicating sub-critical crack propagation. It was calculated from the compliance change that the crack length increased 8% (77 +/- 14 microm) before final failure. The crack growth rate during sub-critical crack propagation was determined as a function of the stress intensity for the last 1500 cycles before fracture.

SIGNIFICANCE

A method is presented to determine slow crack propagation using a four-point bending test. Although fatigue lifetime varied widely, stable crack growth prior to fracture was consistent. This consistency allowed formulation of stress-based crack propagation relationships that can be used in concert with numerical simulations to predict composite restoration performance. The large variation found for specimen lifetime was attributed to the initiation process that precedes sub-critical crack propagation.

Fracture toughness and microhardness of a composite: do they correlate?

OBJECTIVES

Chipping and bulk fracture are major contributors in clinical failures of composite restorations. Fracture toughness (K(Ic)) quantifies susceptibility for fracture, but experimental determination is complicated. It would be beneficial for the dental community if a relatively simple experiment, such as microhardness (HK), could be used to screen composites for fracture resistance. This study explores a possible correlation between K(Ic) and HK.

METHODS

Composite cylinders (4mm diameter and approximately 7 mm long) were cured for five combinations of light intensity (I, microm W/cm(2)) and curing time (T, s) to achieve a range of different total light energy densities (I x T=100 x 10, 100 x 20, 300 x 20, 300 x 40, and 700 x 60 microm W s/cm(2)). A chevron-notch was cut in the median plane of the cylinders for the fracture toughness test, which was executed in a displacement control mode at 6 micro m/s cross-head speed (sample size 4). Knoop hardness was determined at the median plane of the cylinders (sample size 6). The tests were performed 15 min and 24h after curing.

RESULTS

Both the K(Ic) and HK increased with increased light energy density and storage time. Linear regression analysis indicated a strong correlation between HK and K(Ic) tested at the same time period (R(2)=0.97 and 0.90 for 15 min and 24h, respectively). The correlation became weaker between the different storage times (R(2)=0.71), indicating a change in fracture toughness and/or microhardness mechanisms.

CONCLUSION

Fracture toughness of a composite cannot be simply extrapolated from microhardness.

Long-term water-aging of whisker-reinforced polymer-matrix composites

Long-term water exposure may degrade polymer-matrix composites. This study investigated the water-aging of whisker composites. It was hypothesized that whiskers would provide stable and substantial reinforcement, and that whisker type would affect water-aging resistance. Silica-fused Si(3)N(4) and SiC whiskers were incorporated into a resin. The specimens were tested by three-point flexure and nano-indentation vs. water-aging for 1 to 730 days. After 730 days, SiC composite had a strength (mean +/- SD; n = 6) of 185 +/- 33 MPa, similar to 146 +/- 44 MPa for Si(3)N(4) composite (p = 0.064); both were significantly higher than 67 +/- 23 MPa for an inlay/onlay control (p < 0.001). Compared with 1 day, the strength of the SiC composite showed no decrease, while that of the Si(3)N(4) composite decreased. The decrease was due to whisker weakening rather than to resin degradation or interface breakdown. Whisker composites also had higher moduli than the controls. In conclusion, silica-fused whiskers bonded to polymer matrix and resisted long-term water attack, resulting in much stronger composites than the controls after water-aging.

Fracture toughness of packable and conventional composite materials

STATEMENT OF PROBLEM

The introduction of packable composite has expanded the choices of materials for the restoration of posterior teeth. Few independent studies are available on the fracture toughness (K(IC)) of the presently available packable composites compared with more conventional composite alternatives.

PURPOSE

This investigation evaluated the relative fracture toughness of 3 packable composites, 2 conventional composites, and 1 laboratory-processed composite.

MATERIALS AND METHODS

Six composite materials were tested in this study. These included: 3 packable composites (Alert, SureFil and Solitaire), 2 conventional composites (Herculite and Heliomolar), and 1 laboratory-processed composite (Belleglass). K(IC) was determined by preparing 8 mini-compact test specimens (8.2 mm diameter x 2 mm thickness) for each composite in a polytetrafluoroethylene split-mold with introduced precracks created with a razor blade. Specimens were stored in distilled water at 37 degrees +/- 2 degrees C for 7 days. Testing was performed on a universal testing machine at a displacement rate of 0.5 mm/min until fracture. Analysis of variance (P<.0001) and Ryan-Einot-Gabriel-Welsch multiple range tests (P<.05) were performed on all data.

RESULTS

The mean fracture toughness of Alert (1.57 Mpa x m(1/2)) was significantly greater than any of the other composites tested. Solitaire, a packable composite, exhibited a mean fracture toughness (0.67 MPa x m(1/2)) that was significantly lower than any of the other materials tested. No significant difference was noted between Belleglass (1.27 MPa x m(1/2)), SureFil (1.25 MPa x m(1/2)) and Herculite (1.16 MPa x m(1/2)).

CONCLUSION

Within the limitations of this study, the glass fiber-reinforced packable composite exhibited improved fracture toughness when compared with the other composite materials tested.

Novel fracture toughness test using a notchless triangular prism (NTP) specimen

The aim of this study was to develop and validate a new method for determining the fracture toughness of materials and adhesive interfaces. The new test specimen is a notchless triangular prism (NTP) which, when placed in the testing holder, achieves a configuration similar to that of the standard chevron-notched short rod (CNSR) specimen. It can be cast, ground, or simply machined easily and reproducibly without cutting an initial notch. Finite element analysis of a modeled NTP specimen loaded in tension showed a stress distribution similar to a CNSR specimen. A very good correlation was obtained between the NTP and CNSR fracture toughness values of poly(methyl methacrylate) during a calibration study. Fracture toughness values similar to those reported in the literature were obtained for several dental materials and one adhesive interface using the NTP test. The fracture patterns were indicative of plane strain conditions during testing. All bulk specimens and most of the adhesive specimens showed crack arrest, which suggested a stable, well-controlled testing procedure. These results suggest that the NTP fracture toughness test can be used to determine the fracture mechanics of bulk materials and adhesive interfaces.

Current trends in dental composites

The clinical performance of dental composites has been significantly improved over the past decade through modifications in formulation that include: using more stable polymerization promoters for greater color stability; incorporating high concentrations of finely ground fillers to produce adequate strength and excellent wear resistance while retaining translucency; adding radiopacifying agents for improved diagnostics; and utilizing dentin adhesives. However, there are problems which limit the use of composites, especially in posterior teeth. The materials remain very technique-sensitive, due to the extensive contraction which accompanies polymerization and negatively influences marginal sealing. In addition, the materials are generally considered to have inadequate mechanical properties and wear resistance in contact areas to serve as total replacements for amalgams. Current efforts are focusing on several areas, including the development of non- or minimally-shrinking dental composites containing spiro-orthocarbonates as additives to dimethacrylates or epoxy-base resins, and the production of alternative filler materials for ideal wear resistance and esthetics. This paper reviews the composition and characteristics of current dental composites, as well as recent areas of study.

Fracture toughness of water-aged resin composite restorative materials

OBJECTIVES

The purpose of this study was to assess the effects of aging experimental dimethacrylate resin composites in water at 37 degrees C for periods up to 6 wk by measuring the variations in fracture toughness (K(c)), elastic modulus (E), fracture energy (G(c)), and water sorption.

METHODS

Six experimental resins were formulated from dimethacrylate resins, and were filled to 86 wt% (ca. 70 vol%) with treated inorganic filler to form six experimental composites. The fracture toughness was determined using a double torsion technique, the elastic modulus was measured in flexure, and the fracture energy was calculated from the fracture toughness and elastic modulus.

RESULTS

As a result of aging in water, K(c) and the G(c) increased, and the elastic modulus decreased, but all values approached a plateau near 6 wk. Water sorption also occurred during this period, mainly during the first 2 wk.

SIGNIFICANCE

Variations in the mechanical properties are interpreted as being due to plasticization of the resin matrix by water, which appears to lower the yield stress and increase in the size of the plastic zone ahead of the crack, thereby causing the observed increase in G(c) and K(c). After approximately 6 wk, no further changes in properties occurred.

In vitro fatigue behavior of restorative composites and glass ionomers

OBJECTIVES

This in vitro study was conducted to investigate the fatigue behavior of several dental restoratives, including composites, glass ionomers and a resin-reinforced glass ionomer.

METHODS

Fatigue was imposed under a reverse stress-controlled regimen, following a staircase approach. Samples were stored and tested under both dry and wet conditions. The following parameters were measured and analyzed: Young's modulus, restrained fracture strength, and flexural fatigue limit.

RESULTS

As a general trend, all products showed a decrease in Young's modulus following water sorption. For all products except the resin-reinforced glass ionomer, the same trend was seen in the restrained fracture strength. This is, however, no longer valid for the flexural fatigue limit: the trend is steady-state for the glass ionomers, status quo for the resin-reinforced glass ionomer, and all composites tested show a decrease.

SIGNIFICANCE

The diversity in structure of both composites and glass ionomers does not allow findings for one product to be extrapolated to other similar products.

In vitro aging of a heat/pressure-cured composite

Specimens of a heat/pressure-cured microfill composite were aged in saline or distilled water at 37 degrees C up to 12 mon to determine if the flexure strength was affected by the aging media. No significant difference in the flexure strength was observed between the controls and the 12 mon aged specimens. Cyclic testing in distilled water resulted in a lower flexure strength at fracture than that observed for the aged specimens. This lower strength is attributed to fatigue of the silane bond, the resin matrix, and/or the filler particles.

Post-cure heat treatments for composites: properties and fractography

Two commercial and four experimental composites were subjected to post-cure heat treatments of 10 min and 3 h duration immediately after light-curing. Fracture toughness, flexural modulus, microhardness and degree of conversion (FTIR) were evaluated 24 h later. The results showed that post-cure heat treatments at 120 degrees C of short or long duration can be used to produce significant improvements in the degree of cure and the mechanical properties of dental composites used as inlays. A 10 min heat treatment was as effective as a 3 h treatment in enhancing properties and degree of cure. In addition, a 3 h heat treatment carried out 7 days after the initial light-curing was capable of improving properties and cure to almost the same extent as the immediate heat treatments. The improvement in properties, in conjunction with the fractography, indicate a toughening of the filled resin matrix and possibly an improved filler/matrix adhesion in the microfills. The changes appear to be predominantly the result of an increase in degree of cure.

Clinical evaluation of four anterior composite resins over five years

Four hundred and thirty nine chemical-cured composite resin restorations were placed in the anterior teeth of 86 patients treated in a private dental practice. Four anterior resins were used, and placement was performed with acid etching and appropriate enamel bonding resins. Assessments were made of the handling characteristics, condition of the gingiva, surface staining, marginal staining, color deterioration, and of the longevity of the four materials. Clinical deterioration rates and failures of the different types of composites were evaluated over periods of up to five years. Although all the assessed clinical factors deteriorated with time, there were very few unsatisfactory rating scores. Most of the composite restorations performed well over the study. Eight per cent of the restorations failed during the study. Class IV preparations showed the highest restoration failure rates.

Modulus of resilience as predictor for clinical wear of restorative resins

Eight posterior restorative resins were tested with respect to flexural strength, modulus of elasticity, and modulus of resilience. The mechanical properties were correlated to the two-year results of clinical wear tests. Linear relationships were found between flexural strength and clinical wear and between modulus of resilience and clinical wear. It was concluded that modulus of resilience be used in research and quality control for the prediction of clinical wear.

Solvent degradation and reduced fracture toughness in aged composites

Qartz- and barium-glass-filled composites aged for more than one year in ethanol experienced a significant reduction in fracture toughness (K1c), essentially identical to that experienced after two months of aging. This reduction is mainly attributed to a softening of the resin matrix, but cracking within the resin and at the filler/matrix interface, as revealed by SEM microscopy, may also have contributed. No significant cracking could be seen in the composites aged in water. Composites post-cured at temperatures approaching their glass-transition temperature also experienced a reduction in K1c after alcohol storage. Storage in water for one year had little effect on the K1c of composites cured at oral temperatures, but a significant increase was observed for those post-cured at elevated temperatures. This increase is difficult to explain, but appears to involve a filler/matrix interfacial phenomenon, because it was not observed in the unfilled resin. The results of this study demonstrate that an alteration in the fracture resistance and some degradation of the filler/matrix interface, as has been observed clinically, occur after long-term exposure of dental composites to certain solvents used as food-simulating liquids.

Report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry

Subjects of the past decade in the dental literature are reflected in this year's Committee report. We note the decrease in the prevalence of caries, the influence of dental implants, the advancements in dental materials, and the continued efforts to control adhesive events in the oral cavity. This year we included comments from and about many significant review articles published this past year. The Committee continues to be concerned about the quality of some of the work reported and the quality of the reporting. We have attempted to select the distinguished work, that which provides new information to our profession. The subjects covered include pulp biology, caries prevention, periodontics, implants, craniomandibular function and dysfunction, occlusion, and dental materials.