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J Am Dent Assoc, Vol 132, No 5, 639-645.
© 2001 American Dental Association
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COSMETIC & RESTORATIVE CARE

JADA Continuing Education

The suitability of packable resin-based composites for posterior restorations



JUERGEN MANHART, Dr.Med.Dent., HONG Y. CHEN, Dr.Med.Dent. and REINHARD HICKEL, Prof.Dr.Med.Dent.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Background. Packable composites, promoted for the restoration of stress-bearing posterior teeth, have captured clinicians’ interest.

Methods. The authors tested three packable composites (Alert, Jeneric/Pentron; Solitaire, Heraeus Kulzer, Wehrheim, Germany; SureFil, Dentsply De Trey, Konstanz, Germany); a new packable organically modified ceramic, or ormocer (Definite, Degussa AG, Hanau, Germany); a hybrid composite (Tetric Ceram, Ivoclar Vivadent, Schaan, Liechtenstein) and an ion-releasing composite (Ariston pHc, Ivoclar Vivadent, Schaan, Liechtenstein). They determined modulus of elasticity according to EN 24049:1993 of the European Committee for Standardization. They measured Vickers hardness using a 200-gram load for 40 seconds. To determine the materials’ depth of cure, they used both a scraping method (International Standards Organization standard CD 4049:1997) and a hardness profiling method.

Results. The authors calculated means and standard deviations from 10 replications of each test and used one-way analysis of variance and post hoc Tukey tests ({alpha} = .05). The materials had significant differences (P < .001) in all characteristics. Solitaire had the significantly lowest elastic modulus and microhardness; Alert had the highest values for these characteristics. Ariston pHc exhibited the significantly lowest depth of cure. There was a significant correlation between the two methods of measuring depth of cure (r2 = 0.9945; P = .021).

Conclusions. The material group of packable composites is rather inhomogeneous in terms of mechanical and physical data. Our data suggest that bulk curing of packable composites in deep cavities still is not recommendable.

Clinical Implications. The clinician needs to select packable composites carefully, as it seems that not all of these materials qualify for stress-loaded posterior restorations.

Light-cured resin-based composites have shown increased potential for use as a viable alternative to amalgam in restoring cavities in stress-bearing posterior teeth. Besides the ability to bond to enamel and dentin, they feature the advantage of good esthetics and are less costly than ceramic inlays and cast gold inlays. The filler content, filler particle size and distribution of the filler particles all highly influence the physical and mechanical properties of the composite materials. Filler volume fraction and filler load level correlate with the material strength and modulus of elasticity.1–6 Manufacturers have increased the filler content and reduced the average filler particle size to produce resin-based composite materials for Class II posterior restorations, which need adequate strength and wear resistance to withstand the mastication forces to which they are subjected. In recent studies, advanced hybrid composites have presented good material properties and a good clinical performance.7–12 However, these posterior resin-based composites still are not as easy to handle as dental amalgam, and they are associated with the problems of technique sensitivity and the need for an incremental placement technique.13

Not all packable resin-based composites appear to qualify for stress-loaded posterior restorations.

Bulk placement of direct resin-based composite restorations or fewer large increments of composite are possible if an adequate depth of cure can be achieved. Since this property is associated with the degree of polymerization,14,15 various methods based on hardness measurements have been used for the in vitro evaluation of curing depth.16–18 Factors affecting resin-based composites’ depth of cure have been identified mainly as curing source intensity and light exposure duration,19–23 filler size and content,2,24 interactions at the filler-matrix interface25 and shade and translucency.26 In previous studies, a positive correlation has been established between the Knoop hardness/curing depth and the inorganic filler level of composites.1,4,27,28

Packable composites have been introduced in the market with high expectations as an amalgam alternative. Compared with hybrid composites, they are characterized by a higher filler load, an improved filler technology, modifications in the organic matrices and improved handling properties. An application technique similar to that used with amalgam can be used for the placement. The use of metal matrix bands and wooden wedges allows for easier establishing of interproximal contacts. Their ability to be bulk-cured results in a more cost-effective treatment by reducing the time needed to place a restoration. These high-viscosity resin-based composites are indicated for use primarily in cavities in load-bearing surfaces of permanent posterior teeth. Besides an improvement in handling, packable composites were expected to exhibit excellent mechanical and physical properties owing to their high filler load.

Packable composites’ ability to be bulk-cured results in a more cost-effective treatment by reducing the time needed to place a restoration.

The resin matrix also has an important influence on the properties of resin-based composites.29,30 However, no fundamental changes have been achieved in the monomer systems since the introduction of dimethacrylate monomers, in form of bisphenol glycidyl dimethacrylate, by Bowen in 1962. Meanwhile, a packable restorative material based on the new organically modified ceramic, or ormocer, technology (Definite, Degussa AG, Hanau, Germany) has been developed. The ormocer composite consists of inorganic-organic copolymers and inorganic silanated filler particles. The newly designed inorganic-organic copolymers are synthesized in a solution-and-gelation, or "sol-gel," process from multifunctional urethane and thioether(meth)acrylate alkoxy-silanes.31–34 The alkoxysilyl groups of the silane allow the formation of an inorganic Si-O-Si network by hydrolysis and polycondensation reactions, and the (meth)acrylate groups are available for photochemically induced organic polymerization.33,34 After incorporation of filler particles, the ormocer composites can be manipulated by the dentist as is conventional resin-based composite material. This novel incorporation of inorganic-organic copolymers in the formulation of ormocers allows for the modification of mechanical parameters over a wide range.32 Packable composites and the packable ormocer both are claimed to achieve less polymerization shrinkage and to have the ability to be bulk-cured, but these claims have not yet been extensively evaluated.

An ion-releasing composite (Ariston pHc, Ivoclar Vivadent, Schaan, Liechtenstein) was introduced in 1998. This composite material releases fluoride, hydroxyl and calcium ions in dependence on the pH value immediately adjacent to the restorative material. With a decreasing pH value, due to active microorganisms in dental plaque, the release rate of the functional ions increases and vice versa. This phenomenon is based on a newly developed alkaline glass filler and is expected to reduce the formation of secondary caries at restoration margins owing to an inhibition of bacterial growth, a reduction in demineralization and a buffering of acids produced by cariogenic microorganisms.

We undertook a study to determine mechanical characteristics of three highly filled packable composites (Alert, Jeneric/Pentron; Solitaire, Heraeus Kulzer, Wehrheim, Germany; SureFil, Dentsply De Trey, Konstanz, Germany) and a packable ormocer (Definite) in comparison with those of an advanced hybrid composite (Tetric Ceram, Ivoclar Vivadent, Schaan, Liechtenstein) and an ion-releasing composite (Ariston pHc).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We studied four packable resin-based composites, a hybrid composite and an ion-releasing composite (Table 1Go) to identify three mechanical characteristics of each: modulus of elasticity, Vickers hardness and curing depth.


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  		TABLE 1 RESTORATIVE MATERIALS INVESTIGATED.*

 
Modulus of elasticity. We determined modulus of elasticity according to EN 24049:1993 of the European Committee for Standardization. We made 10 rectangular specimens of each material by inserting the composite material into a brass mold (25 x 2 x 2 millimeters). To avoid the effects of scattering light and, therefore, uncontrolled initiation of polymerization introduced by using only one light-curing unit, we placed three curing lights (Elipar Highlight, ESPE, Seefeld, Germany) close to each other without a gap between them, as described by Mehl and colleagues.35 This setup allowed for a controlled initiation of the reaction over the entire length of the specimens at the same time. The intensity of each curing light was 700 milliwatts per square centimeter, verified before the polymerization using a radiometer (Curing Radiometer Model 100, Demetron Research). After 40 seconds of standard light curing, the specimens were stored in physiological sodium chloride solution at 37 C for 24 hours. We determined the materials’ elastic modulus using a universal testing machine (QTS 2000, Quicktest, Langenfeld, Germany) at a crosshead speed of 0.5 mm/minute. We placed the specimens on a three-point bending test device, which was constructed according to the guidelines of National Institute of Standards and Technology standard 4877 with 20 mm between supports and ensuring an equally distributed load.35 During testing, the specimens were immersed in a physiological sodium chloride solution at 37 C to simulate clinical conditions. We calculated the elastic modulus of each material according to EN 24049:1993.

Vickers hardness. To determine Vickers hardness, we made 10 specimens of each material by inserting the resin-based composites in cylindrical aluminum molds with a diameter of 9 mm and a depth of 2 mm. We covered the surfaces of the specimens with transparent plastic matrix strips before light-curing the specimens for 40 seconds (Elipar Highlight) to avoid an oxygen-inhibited superficial layer with lower hardness. The intensity of the curing light was 700 mW/cm2, verified by use of a radiometer. After polymerization, the specimens were stored in physiological sodium chloride solution at 37 C for 24 hours. We then metallographically polished the surfaces of the specimens using a fine grinder/polisher (VP100, LECO, Kirchheim, Germany) with silicon carbide paper up to 1,200 grit under copious water cooling to minimize heat buildup. We took microhardness measurements on each specimen with a Vickers indenter (Durimet, Leitz, Wetzlar, Germany) with a load of 200 grams for 40 seconds (Vickers hardness, or HV, 0.2/40). We made 10 indentations for hardness measurement on each specimen.

Depth of cure. We measured the depth of cure of the composite materials using two methods: scraping and producing a hardness profile.

Scraping. According to the guidelines of International Standards Organization standard ISO CD4049:1997, we used two-piece aluminum molds with a diameter of 5 mm and a height of 8 mm. We placed a transparent Mylar matrix band on a flat glass slide on top of a white background. We then placed the mold over this and slightly overfilled it in one increment with the composite materials to produce 10 specimens. We placed a second plastic matrix strip on top of the mold and overlaid it with a glass slide, then applied finger pressure to the slide to extrude excess material.

After removing the slide, we placed the exit window of the curing light (Elipar Highlight) against the Mylar strip and irradiated the composites for 40 seconds. The intensity of the curing light was 700 mW/cm2, which we verified with a radiometer. Immediately after completing irradiation, we took the specimens from the molds and removed uncured material by scraping with a plastic spatula. We measured the height of the cylinder of cured material with a micrometer to an accuracy of 0.1 mm. This value was divided by two (in compliance with ISO CD4049:1997), and we recorded it as the depth of cure. (Morrow and colleagues36 used the same procedure.)

Producing a hardness profile. We compared the scraping results with those obtained through an indirect method of measuring depth of cure, based on hardness measurements of the cured composite materials at specific depths.18 We followed the same procedure as in the first method through the step of irradiation for 40 seconds. After irradiation, we took the specimens from the molds and stored them in physiological sodium chloride solution at 37 C for 24 hours in a dark environment. We mounted the specimens in slow-curing epoxy resin (Epoxy-Die, Ivoclar, Schaan, Liechtenstein) and left them in the dark for another 24 hours at 37 C. We longitudinally sectioned each specimen under water coolant on a slow-speed diamond saw (Varicut, LECO, Kirchheim, Germany), producing two equal halves. The halves were polished (under copious water cooling, to minimize heat buildup) with silicon carbide paper up to 1,200 grit on a metallographic polishing machine. We measured the Vickers hardness, as described previously, parallel to the longitudinal axis of the sectioned surfaces in 0.5-mm increments at depths from 1 mm to 6 mm from the light-cured surface. We made three indentations at different sites at each depth and calculated the average value. Depth of cure was defined as the level at which the hardness value was equivalent to at least 90 percent of the hardness the composite had at 1 mm from the light-cured surface.18

Statistical analysis. We calculated means and standard deviations. We conducted a statistical analysis of all data using one-way analysis of variance, or ANOVA, and post hoc Tukey test at a significance level of P = .05. In addition, we calculated Pearson’s correlation coefficient and corresponding level of significance to analyze the level of correlation between the two methods of measuring curing depth.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Table 2Go provides the results of the determination of elastic modulus, Vickers hardness and depth of cure. One-way ANOVA revealed highly significant differences (P < .001) among the tested materials for elastic modulus, Vickers hardness and curing depth (as measured by both methods). Pairwise multiple comparisons with the Tukey test ({alpha} = .05) revealed that Solitaire’s elastic modulus and Vickers hardness were significantly lower than those of all other materials. Alert had the highest elastic modulus in terms of statistical significance, and its hardness values were significantly higher than those of all other materials except SureFil. Tetric Ceram had significantly lower hardness values than Definite, SureFil, Ariston pHc and Alert. According to both methods of measuring depth of cure, Ariston pHc’s was lower and Alert’s was higher than those of all other materials tested.


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  		TABLE 2 MEAN VALUES AND STANDARD DEVIATIONS.*

 
The figureGo shows the results of measuring depth of cure of the different composite materials by scraping vs. producing a hardness profile. A statistically significant correlation between the two methods was apparent (r2 = 0.9945; P = .021).



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  		Figure. Determination of curing depth of the tested materials by scraping vs. producing a hardness profile (r2= 0.9945). mm: Millimeters. Manufacturers are as follows: Ariston pHc and Tetric Ceram, Ivoclar Vivadent, Schaan, Liechtenstein; Definite, Degussa AG, Hanau, Germany; Solitaire, Heraeus Kulzer, Wehrheim, Germany; SureFil, Dentsply De Trey, Konstanz, Germany; Alert, Jeneric/Pentron.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Elastic modulus. Failure of posterior composite restorations often is associated with fractures within the body and at the margins of the restorations.37 Fracture-related material properties, such as fracture resistance and elasticity, have been evaluated by the determination of the material parameters of fracture toughness, flexural strength and modulus of elasticity.38 In agreement with the findings of other authors, in this study the glass-fiber reinforced composite Alert exhibited the significantly highest modulus of elasticity.39,40 Alert and SureFil demonstrated the highest elastic modulus values, which seems to be a result of the high filler load levels (Table 1Go).3,41,42 Solitaire—which contains a high filler fraction volume of 90 percent (66 weight percent) and is composed of a porous SiO2 filler (30 weight percent) with a particle size of 3 to 22 µm, Ba-Al-B-F-Si-glass (26 weight percent), Al-F-Si-glass (5 weight percent), and Sr-F (5 weight percent)—had a significantly lower modulus of elasticity than the other packable composites and Tetric Ceram and Ariston pHc.39,40 Draughn43,44 reported superior mechanical properties for a lower (45 percent) rather than a higher (55 percent) filler volume fraction. This negative effect seems to be related to inherent flaws, especially air porosities included in the composite material.45 The porous filler particles of Solitaire integrate parts of the matrix, resulting in novel matrix-filler interactions, which need to be further investigated. It is assumed that crack propagation results to some extent in a cracking of the matrix-integrating fillers themselves. These particles might exhibit less potential for crack pinning and crack deflection, resulting in inferior physical and mechanical properties.

Besides the filler system, the monomer structure of the resin matrix also has an impact on fracture mechanics.29,30 In our study, we found no significant differences in modulus of elasticity among the composite materials Tetric Ceram and Ariston pHc and the ormocer Definite, which is based on a more rigid matrix of inorganic-organic copolymers. This suggests that the filler particles themselves, the filler load level and filler-matrix interactions probably have a greater effect on fracture mechanical parameters than does the composition of the organic matrix.

Vickers hardness. Hardness is a surface property related to the resistance of a material to local deformation.46 The pyramidal shape of the Vickers indenter results in an elastoplastic or plastic stress at comparatively low load levels on the surface. The value of hardness is indicative of a structure’s ease of finishing and its resistance to in-service scratching.38 In general terms, the mechanical properties show a significant correlation with the filler fraction.2 A positive correlation has been established between the hardness and the inorganic filler content of composite materials.27,28

In our study, the Vickers hardness numbers of the tested materials showed a positive trend that correlated to the filler load level (weight percent), which is in agreement with previous findings.2,47 Increased filler levels result in trends toward increased hardness values.1,4 This phenomenon is quite complex, resulting from an interaction of multiple factors associated with the optical properties of the resin matrix and the filler particles, including particle size and distribution. Alert and SureFil, which are the most highly filled by weight (Table 1Go), were the hardest materials.48 Solitaire had the significantly lowest hardness number despite the high filler volume fraction (90 percent). However, this equals only 66 percent filler loading by weight, the least of all the materials we tested. The majority of large (3- to 22-µm) and porous SiO2 fillers (30 weight percent) in the filler system of Solitaire likely contribute to its softer surface. Definite and Ariston have a filler load similar to that of, but surfaces significantly harder than those of, Tetric Ceram (Table 1Go). This phenomenon is assumed to be the result of Definite’s more rigid matrix of inorganic-organic copolymer and Ariston pHc’s alkaline glass fillers.

Depth of cure. As already described, depth of cure of light-cured resin-based composites is a function of the material’s filler composition and resin chemistry, its shade and translucency, the intensity of the light source, and the length of radiation exposure.2,18–26 In our study, the light-curing unit and light intensity, as well as the shade of the composite materials and irradiation time, were defined to compare the curing depth of different resin-based materials.

The extent of polymerization is reduced at greater depths below the material’s surface because of the lower intensities of light penetrating to these depths. Thus, the hardness at different depths reflects the resin’s extent of cure coupled with the amount and type of the inorganic fillers at this level.49 Depth of cure also is related to the size of the incorporated fillers.15,18 The filler particles in the resin-based composites scatter light. This scattering effect is increased as the particle size of the fillers in the composite approaches the wavelength of the activating light and will reduce the amount of light that is transmitted through the composite.18 Alert, containing large rod-shaped particles (60–80 µm in length, 6–10 µm in diameter), demonstrated a significantly higher curing depth than all other materials. This agrees with other researchers’ findings that larger particle composites had the greatest depth of cure since they were least affected by light-scattering.2,18 As concluded by the authors of previous studies, a high filler concentration also increases the depth of cure of composite materials.1,2


   CONCLUSIONS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Packable composites that are promoted for the restoration of stress-bearing posterior carious lesions are quite different in their mechanical and physical properties. These interesting materials make up a rather inhomogeneous group. Alert and SureFil exhibited promising mechanical and physical data for this use. The elastic modulus and Vickers hardness we found for Solitaire make the decision to restore large carious lesions in stressed posterior teeth with this material rather questionable unless the results of clinical trials show more promising data. Curing depth measurements showed that bulk curing of packable composites in deep carious lesions still is not recommendable. The clinician needs to be aware of the high differences among the packable composites, in terms of both hard facts (such as mechanical data) and subjective handling properties.


   FOOTNOTES
 

Dr. Manhart is an associate professor, Department of Restorative Dentistry, Dental School of the Ludwig Maximilians University, Goethestrasse 70, D-80336, Munich, Germany. Address reprint requests to Dr. Manhart.


Dr. Chen is an assistant professor, Department of Restorative Dentistry, Dental School of the Ludwig Maximilians University, Munich, Germany.


Dr. Hickel is a professor and the chairman, Department of Restorative Dentistry, Dental School of the Ludwig Maximilians University, Munich, Germany.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 

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