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J Am Dent Assoc, Vol 131, No 4, 497-503.
© 2000 American Dental Association |
BIOMATERIALS/RESTORATIVE DENTISTRY |
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Methods. Eleven commercially available composites and a series of experimental composites were evaluated in a mechanical testing machine to measure the maximum stress generated during placement in a confined setting.
Results. A significant relationship between higher filler volume and increased polymerization stress was found among the commercial materials. Introduction of MSAA produced a 30 percent reduction in polymerization stress in an experimental composite material.
Conclusions. Composites that contain lower levels of inorganic filler particles are less likely to produce high levels of polymerization stress during placement. Modifications to traditional composite chemistry can result in materials that produce lower polymerization stress levels.
Clinical Implications. The polymerization stress produced by dental composite materials during light-curing is a leading reason for bond failures in adhesive restorations, resulting in postoperative sensitivity, marginal staining and recurrent caries.
When dental composite is placed and light-cured, the polymerization reaction is accompanied by shrinkage. In a nonconfined setting, such as the restoration of an incisal edge, most of the shrinkage is transmitted to the relatively large free-surface area, and, thus, it does not cause any significant problems. However, when composite is placed in a confined setting, such as a Class I preparation, less of the polymerization shrinkage can be expressed at the free surfaces. Because it is constrained by its adhesion to the wall of the cavity, this unresolved polymerization shrinkage leads to internal stress, which can exceed the strength of the bond with the surrounding tooth structure and cause the interface to fail.1 The resulting marginal gap can lead to postoperative sensitivity and may provide a site for recurrent caries to develop.
Secondary caries has been cited as the most common cause of failure for dental composite restorations.2–5 A survey of 22 dental practitioners in Great Britain reported the reasons for replacement of 876 composite restorations.6 Secondary caries was cited in 25 percent of the cases, while poor margins (15 percent) and postoperative sensitivity (6 percent) were also significant factors.
The relationship between polymerization stress and marginal debonding has been explored to a limited extent. Ferracane and colleagues7 found a significant correlation between the magnitude of polymerization stress and marginal staining in ex vivo restored specimens for three light-cured composites. The influence of some factors associated with polymerization stress also has been examined in the mouth. Opdam and colleagues8 compared bulk placement with incremental placement of one composite with a technique in which composite was placed in 48 teeth that were designated for extraction for orthodontic reasons.
After a few weeks, the Class I restorations were tested for sensitivity and the teeth were extracted and subjected to scanning electron microscopic analysis and staining to test for microleakage. These investigators found higher sensitivity on loading and a higher incidence of marginal gap formation for the bulk-placed composite. In addition, nearly 40 percent of these Class I restorations demonstrated leakage at the enamel margins because of the high polymerization stresses. Other studies examining the effect of different light-curing and placement regimens have used this technique and demonstrated significant leakage or marginal gap formation at dentin or enamel margins.9,10
At least two studies have shown greater marginal leakage in in vivo restorations compared with in vitro restorations,11,12 suggesting that the effects of polymerization stress may be detrimental even when restorations are placed under controlled clinical conditions. Although the direct evaluation of clinical performance is important, establishing a relationship between the composition of the materials used and the mechanical properties of interest should not be overlooked.
With concerns diminishing about composites’ insufficient resistance to intraoral wear, polymerization stress has become the leading area of composite research in dentistry today. As dental composites have gained wider acceptance for a growing number of applications, the need to control polymerization shrinkage stress has become a pressing concern. Placement of the composite in increments gained early widespread acceptance, despite only limited evidence of a beneficial effect in controlled studies.13,14 Also, although many proposals that involve controlling the direction, intensity and duration of light-curing have been explored, no clear consensus about a truly advantageous approach has been reached.15–18 Recently, researchers examining dental materials have devoted greater attention to the effect of the composite formulation itself. What are the compositional factors to look for in a composite that indicate it will not generate high levels of stress when placed? How can these factors be manipulated to produce the next generation of improved composites?
A few efforts to control polymerization stress by changing composite formulation have been reported. Reduced levels of photoinitiator were found to cause lower shrinkage rates, which, by implication, could lead to lower polymerization stress levels.19 Nonbonded microfiller particles have been found to produce significant decreases in polymerization stress by acting as stress-relieving sites within the composite.20 Similarly, self-cured composites contain a greater amount of porosities than light-cured composites and have a slower polymerization rate, both of which tend to reduce polymerization stress.21
Gap formation at the interface between the composite and the surrounding tooth structure occurs because the stress caused by polymerization exceeds the strength of the adhesive bond between the composite and the tooth.1 Although polymerization shrinkage is relatively easy to measure, the tendency to produce polymerization stress is the factor that more fully predicts the vital clinical aspects of composite performance. In this study, we evaluated an assortment of commercial materials to determine their polymerization stress in a confined setting. We also tested a series of experimental composites containing a novel monomer to evaluate the potential for reducing polymerization stress levels. As dental composites have gained wider acceptance for a growing number of applications, the need to control polymerization shrinkage stress has become a pressing concern.
MATERIALS AND METHODS
TOP
ABSTRACT
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION
REFERENCES
One of us (J.C.) used a mechanical testing machine to perform polymerization stress tests. Sections of a 5-millimeter–diameter clear glass rod were sandblasted and treated with a ceramic primer and light-cured adhesive (Scotchbond MP, 3M Dental Products). Two of these glass stubs were bonded in the testing machine, one to a fixture attached to the movable actuator and one to a fixture attached to the force measuring load cell (Figure 1
).
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The degree of confinement in a polymerization stress test has been rated in terms of the configuration factor, or C-factor, which is defined as the ratio of the bonded area to the unbonded area of a volume of shrinking material.22 For this test, the C-factor was equal to 1.0. A small amount of compliance was still present because of the composite coupling between the glass stubs and the steel posts; however, we estimated its effect to be insignificant. The force generated by the composite as it cured was monitored for 10 minutes (except for Bisfil 2, Bisco, the self-cured composite, which was monitored for 30 minutes), and the maximum force was divided by the area of the stub to provide a mean amount of stress. Three specimens for each of the commercial materials were tested.
The composites chosen represented a cross-section of the broad spectrum of composite types (Table). Included were four microfills (Durafill VS, Heraeus Kulzer; Epic TMPT, Parkell; Litefil IIA, Shofu; and Heliomolar, Ivoclar-Vivadent), three minifills containing submicron fillers (Tetric, Ivoclar-Vivadent; Charisma, Heraeus Kulzer; and Herculite, SDS Kerr) and three midifills containing larger-sized fillers (Fulfil, DENTSPLY/L.D. Caulk; Estelite, Tokuyama; and Prisma TPH, DENTSPLY/L.D. Caulk). The self-cured composite Bisfil 2 was also tested to examine the effect of its different curing mode.
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In this experiment, six composites were formulated by replacing MSAA with Bis-GMA at the level of 0, 20, 40, 60, 80 and 100 percent in a light-cured resin. The Bis-GMA/MSAA combination amounted to 50 percent by weight of the resin present. These thick monomers were diluted with triethylene glycol dimethacrylate, or TEGDMA, which made up the remaining 50 percent by weight of the resin phase. Silane-treated filler particles (78 percent by weight [62 percent by volume]) were added to form a composite.
Five specimens of each of these composites were tested. For these materials, we developed an improved stress-testing method, which allows testing of more highly confined specimens. The fixture attached to the actuator contained a slot into which the light-curing gun was situated so that it illuminated the composite through the glass stub. The stub was given a polished end so that it could transmit an adequate amount of irradiance, which was measured at 300 milliwatts per centimeter squared. The space between the stubs was set to 0.83 mm (C-factor = 3.0). The force generated by the composite as it cured was monitored for 10 minutes, and the maximum force was divided by the area of the stub to provide a mean amount of stress. We used analysis of variance/Tukey’s test to compare the results (P < .05).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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). The microfills (Epic TMPT, Litefil IIA, Durafill VS and Heliomolar) produced significantly less stress than the minifills (Herculite and Tetric). In general, we observed no significant differences in polymerization stress between the minifill and midifill composites.
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, the polymerization stress values for the experimental material demonstrate that a significant change occurs with the first increment of MSAA, amounting to a 20 percent reduction in stress. Higher levels of MSAA also produced a decrease in stress over the control material (that is, 100 percent Bis-GMA). The stress levels are higher than those recorded for the commercial materials because of the more highly confined test configuration to which these materials were subjected.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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While few significant differences were found in the polymerization stress levels of the commercial materials, we observed a trend toward higher stress among materials with higher inorganic content. Indeed, a linear regression analysis between stress and filler volume (as reported by the manufacturers) yielded a strong correlation (r2 = .82) (Figure 4). This is somewhat surprising in light of the many differences in filler size, resin type and photoinitiator systems present among these materials. A strong correlation between filler volume of commercial composites and their elastic modulus or stiffness has been demonstrated.24 It appears that the greater stiffness of the more heavily filled materials plays a major role in determining the amount of polymerization stress produced.
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On the basis of our test results, it appears that less heavily filled composites might be preferable because they tend to produce lower levels of polymerization stress. However, lower inorganic filler content has been found to result in lower fracture toughness26 and wear-resistance,27 which are critical to the performance of restored occlusal surfaces. The lower stress levels of the less heavily filled materials can be advantageous when restoring nonocclusal surfaces. This is especially true for Class V restorations, which have only a thin layer of enamel available for adhesion. In addition, the lower elastic modulus resulting from the lower filler volume may be desirable for Class V restorations, because these composites will strain more with the teeth under load compared with the stiffer, more heavily filled composites.28 At least one clinical study exhibited data that support this hypothesis.29 For occlusal surfaces, material selection must involve improvements that do not compromise the wear-resistance of the material.
The use of a novel monomer such as MSAA appears to be one method of reducing the polymerization stress of the composite, while providing improved compressive strength and degree of conversion of methacrylate groups.23 The mobility of the methacrylate groups seems to be able to resolve internal stresses, possibly by producing a structure that is more prone to changes in molecular conformation or by redirecting the shrinkage toward the free surfaces. The MSAA appears to lose its effectiveness at higher concentrations because it seems to operate as a stress-relieving agent. It is possible that once the stress has been reduced to a certain level, further additions of MSAA provide little benefit. Completely replacing Bis-GMA with MSAA might not be beneficial, and its specialized nature may cause it to be non–cost-effective. However, adding low levels of MSAA could lead to significant reductions in polymerization stress in various composites, including heavily filled ones.
Evaluating polymerization stress is, in general, a more complex procedure than measuring polymerization shrinkage. A computer-controlled testing machine is needed to provide a configuration with minimal compliance. We used testing configurations that could compensate for the major sources of compliance in the load cell and the fixtures. We estimated that the remaining source of compliance in the system—the thin layer of composite bonding the glass stubs to the steel post—had only a minimal effect on the measured stress values.
The development of novel monomers and modified photo-initiation systems as well as the addition of nonbonded microfiller will lead to a variety of new composites whose advantages are based on reduced polymerization stress.
Using a system without compliance compensation, Bouschlicher and colleagues30 found polymerization force values for three light-cured composites that amounted to stresses in the range of 1 to 5 MPa. The magnitude of the stresses measured typically is much less than estimates based on final stiffness and shrinkage values. This is because of a stress-relieving mechanism termed the flow capacity of the composite. Although the mechanical nature of the flow capacity, as either a form of plastic deformation or anisotropic shrinkage, has not been determined, it has been found to relieve approximately 80 percent of the predicted stress for a chemical-cured composite in a setting similar to that used in this research.31
When comparing the results of this study with those of studies in a clinical setting, we are hampered by the fact that often only a limited number of commercial restorative materials are included in a given clinical study. In addition, any prediction of long-term marginal integrity also would depend on the adhesive bond strength, water uptake and thermal expansion properties of the material, as well as its resistance to aging. However, the magnitude of composite polymerization stress is an essential, if not dominant, factor in composite restorative performance, and it may provide an important predictive measure of clinical success.
The development of novel monomers and modified photo-initiation systems as well as the addition of nonbonded microfiller will lead to a variety of new composites whose advantages are based, not on the reduction of shrinkage, but on reduced polymerization stress.
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CONCLUSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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This article has been cited by other articles:
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  C.S. Pfeifer, J.L. Ferracane, R.L. Sakaguchi, and R.R. Braga Factors Affecting Photopolymerization Stress in Dental Composites Journal of Dental Research, November 1, 2008; 87(11): 1043 - 1047. [Abstract] [Full Text] [PDF]
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F. Goncalves, C.S. Pfeifer, J.L. Ferracane, and R.R. Braga Contraction Stress Determinants in Dimethacrylate Composites Journal of Dental Research, April 1, 2008; 87(4): 367 - 371. [Abstract] [Full Text] [PDF]
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 R.R. Braga and J.L. Ferracane ALTERNATIVES IN POLYMERIZATION CONTRACTION STRESS MANAGEMENT Critical Reviews in Oral Biology & Medicine, May 1, 2004; 15(3): 176 - 184. [Abstract] [Full Text] [PDF]
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 Z. C. Cuehrel[UNKNOWN], F. Tasman, M. A.[U. Onur, and A. Gumrukcuoglu Influence of a Self-Etching Primer on Compound Nerve Action Potentials J Biomater Appl, January 1, 2004; 18(3): 153 - 161. [Abstract] [PDF]
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R.R. Braga and J.L. Ferracane Contraction Stress Related to Degree of Conversion and Reaction Kinetics Journal of Dental Research, February 1, 2002; 81(2): 114 - 118. [Abstract] [Full Text] [PDF]
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