The effect of cavity configuration on the mechanical properties of resin composites

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I. INTRODUCTION

Although dental composites has been improved and widely used, there are still some undesirable characteristics, which affect their performance.

One of them is the shrinkage that inevitably occurs during polymerization as monomer molecules are converted into a polymer network, exchanging Van der Waals spaces for shorter covalent bond spaces. This shrinkage produces contraction stress in confined structures such as a tooth cavity. The majority of the contraction stress of composite occurs during the initial polymerization period after gelation, and the stress development rate decreases gradually with time1). Before gelation, most of contraction stresses caused by polymerization shrinkage relieved by flow capacity of the resin composite. The contraction stress in dental composite plays an important role in marginal adaptation2). The internal stress generated in the restricted environment of a tooth cavity can exceed the adhesive bond strength and produce a delamination of the restoration interface3). In cases where higher bond strength is present, this stress may fracture the marginal tooth substrate and/or the composite restoration itself4). Either case results in the formation of a marginal gap, allowing for the possible ingress of oral fluids and bacteria through leakage. It is not easy for leakage around the cavity wall to be clinically detected immediately after placement.

However, the marginal leakage can be caused postoperative sensitivity, and may eventually produce discoloration of the margins and/or recurrent caries, and consequently may reduce the life of a restoration.

Due to the reasons mentioned above, throughout its lifetime, the interface between a dental resin and the restored tooth is subjected to the challenge of stresses, which can potentially cause de-bonding and lead to clinical failure of the restorative treatment. Before the restored tooth is subjected to functional load and thermal strains, an early interfacial stress build-up occurs during the polymerization in situ of resins bonded to tooth tissues. The setting reaction is, in fact, accompanied by volumetric shrinkage of varying magnitude, depending on the resin formulation5). The magnitude of the polymerization contraction stress has been found to be dependent on the ratio of the bonded to free, unbonded surface area of the sample6,7,8), on the material properties of the resin composite1,6,9,10) and on the amount of compliances of the substrate materials6,11,12).

Among the material properties, which are thought to reduce early interfacial stress build-up is the ability to undergo plastic flow during the early phase of polymerization. Plastic flow allows part of the shrinkage to take place without stress build-up at the interface and will determine the proportion of remaining shrinkage that manifests itself as determination rigid contraction7). As a result of the ratedependent character of visco-elastic behavior, slower rates of polymerization reaction have been shown to be associated with lower stress build-up and better interfacial integrity scores1,13,14,15,16).

The elastic modulus of the shrinking material has also been found to be a factor on which shrinkage stresses depend. In vitro studies have shown that the interfacial stress during the setting shrinkage of a resin composite is positively correlated with the rigidity of the setting material17). The elastic modulus also increases as the polymerization reaction proceeds18).

Polymerization shrinkage and elastic modulus are in close relationship with the survival of the interfacial bond. However, so far, little has been done to establish how close that relationship is, starting with task of a systemic characterization of the relevant properties of resin composites.

In this aspect, the present study was designed to evaluate the effect of cavity configuration on the mechanical properties of resin composites by measuring flexural strength, elastic modulus, polymerization shrinkage and examination of the fractured surfaces of resin composites cured in the cavities with different configuration.

II. MATERIALS AND METHODS

Two composite resins(Table 1) and their corresponding dentin bonding agents(Table 2) were used in this study. The 48 specimens of control group and two experimental groups of each resin composite were made according to the configuration factor (Table 3).

Table 1

Resin composites used in this study.

Table 2

Dentin bonding agents used in this study.

Table 3

Code of control and experimental group by C-factor. (n=8)

1. Specimen fabrication

The specimens of control group of each resin composites were made in a split steel mold (25×2×2mm, Fig. 1) with top and bottom surfaces covered by a clear matrix. The specimens(n=8) were made with light irradiation(60s) in the curing unit(LABOLIGHT LV I, GC, Japan) on both the top and bottom surfaces. The specimens were stored in distilled water at 37℃ for 24hours prior to testing. In experimental groups, the specimens of each resin composite with 2.4 configuration factor(C-factor)(AC2 & EC2) were made in a simulated cavity(25×5×3mm) made of glass plates. Before filling the resin into the cavity, the inner surfaces of the glass cavities were sandblasted and treated with silane and corresponding dentin bonding systems. The specimens were made with light irradiation(60s) in the curing unit on both the top and bottom surfaces. Then the specimens were cut with slow speed diamond saw (ISOMET, Buehler, USA) under copious water to make resin bars(25×2×2mm). The storage and testing procedures of specimens were carried as same as control groups.

Fig. 1

Split steel mold used in this study

The specimens of each resin composite with C-factor of 3.4(AC3 & EC3) were made as the same procedures as the specimens with C-factor of 2.4, but the size of the simulated cavity(25×5×5mm) was different.

2. Flexural strength and elastic modulus determination

Flexural strength and elastic modulus were evaluated in three-point bending by International Standards Organization(ISO) 4049 for testing of resin-based filling materials reported previously19). The specimens were tested on a universal testing machine(EZ Test, Shimadzu, Japan, Fig. 2) in bending on a span of 20mm at a crosshead speed of 1mm/min(Fig. 3). The crosshead travel of the universal testing machine was synchronized with the motion of the strip chart. The slope of the linear portion of the load-versus-time curve was used in the calculation of the modulus, according to the followed equation ;

Fig. 2

Universal testing machine

Fig. 3

3-point bending procedure

Flexural strength calculation : 3 P·L / 2 b·W²

• P : Maximum load at fracture

• L : Length of the span(20mm)

• b : Thickness of the specimen

• w : Width of the specimen

Elastic modulus calculation : P·L³ / 4 d·b w³

• P : Change of the load

• L : Length of the span

• d : Crosshead travel

• b : Thickness of the specimen

• w : Width of the specimen

III. RESULTS

4. SEM examination of fractured surface

Fractured surface of the specimens of group ACO(Fig. 7, 8) showed smooth and homogenous appearance, but those of the specimens of the group AC2(Fig. 9) and AC3(Fig. 10) showed irregular and rough features. Especially, the specimen of group AC3 showed crack lines at the fractured surface. It means that there was stress concentration within the resin mass during polymerization.The examination of the groups using micro-hybrid composite(Fig. 11, 14) was similar to that of hybrid composite, but general features of fractured surfaces were smoother than those of hybrid composite.

Fig. 7

The fractured surface of the ACO group(×500)

Fig. 8

The fractured surface of the ACO group(×2,000)

Fig. 9

The fractured surface of the AC2 group(×500)

Fig. 10

The fractured surface of the AC3 group(×500)

Fig. 11

The fractured surface of the ECO group(×500)

Fig. 14

The fractured surface of the EC3 group(×500)

IV. DISCUSSION

The contraction stress of resin composite depends upon the type and level of filler included. Generally, an increased filler loading contribute to reduced polymerization shrinkage, since the overall polymerization shrinkage depends on the amount of polymer matrix presented20,21). The higher stiffness leads to increased stress for a given contraction strain, according to Hooke's law. (E=κσ/ε, E : Elastic modulus, κ: Proportional constant, σ: Stress, ε: Strain). Therefore, the composite stiffness and the amount of contraction both play important roles in the generation of stress in dental composite restoration. In this study, hybrid type resin composite showed higher mechanical properties(flexural strength and elastic modulus) than micro-hybrid composite for all configuration factor(p<0.05). Generally, it is accepted that an increased filler level should contribute to increased mechanical properties and reduced polymerization shrinkage. From the polymerization shrinkage graph in this study, the gel-point of hybrid composite was found to be occurred earlier than micro-hybrid composite. During the period before occurring gel-point(pre-gel stage), the developing contraction stresses can be relieved due to the flow capacity of the material, but after gel-point(post-gel stage), the flow capacity of the material decreased and the developing contraction stress can cause microdefect or cracks in the materials. The specimen being stressed, the defect and/or cracks played a role as stress raiser and the mechanical properties decreased. The lower elastic modulus and higher shrinkage of the micro-hybrid composite is indirect evidence that flow capacity is achieved mainly by increasing the proportion of monomer in the formulation of the composite pastes. So despite of the less polymerization shrinkage, hybrid composite was affected more than micro-hybrid composite by increase of C-factor.

In SEM, the fractured specimens of the groups of Clearfil™ AP-X showed rougher surfaces than those of Esthet-X™. The phenomenon was maybe due to the difference of filler particle size between the resin composites. The mean filler particle size of hybrid and micro-hybrid composite was 3.0µm and 0.6 0.8µm, respectively. Indeed, the specimens with lower configuration factor showed smoother appearance than those with higher configuration factor. Generally the fact is accepted that the higher the configuration factor of the cavity, the more stresses develop. So resin composites filled in the cavity with higher configuration factor experience severe contraction stresses during polymerization and the forces can cause crack in the resin mass. Flexural and/or tensile strength are significantly changed by the presence of defects due to generation of stress concentrations. In brittle materials such as dental resin composites or porcelain, microcracks which exist on the surface or inside the material, decrease the material strength22).

It is supposed that as the configuration factor of the cavity increased, the flow capacity of the resin composites decreased and more stresses occurred that cause detrimental effect on the mechanical properties of the resin composites in the restricted polymerization environment.

There have been many studies about the effect of polymerization shrinkage on dentin bonding23,24,25,26,27,28,29), and it is shown that polymerization shrinkage can cause contraction stress which has detrimental effect on the bond strength between resin composites and tooth structures. But, there are few studies about the effect of contraction stresses on the mechanical properties of the resin composites. In this study, the flexural strength and the elastic modulus of resin composites had a tendency to decrease with increase of the C-factor. In case of hybrid type composite, the flexural strengths of test groups showed significant difference(p<0.05), but those of micro-hybrid composite were not significantly different(p>0.05). It may be due to the filler content of the resin composite. Hybrid type composite used in this study had higher filler content(84.5wt%) than micro-hybrid type resin composite(77.0wt%).

So, hybrid composite showed less polymerization shrinkage than micro-hybrid composite. But despite of the less shrinkage, the higher elastic modulus(10.96±0.94GPa) and earlier emergence of gel-point of hybrid composite makes it more influenced by increase of C-factor than micro-hybrid composite.

In clinical situation, many dentists have faced to problem related to the contraction stress of resin composite. Actually, the contraction stress of resin composites and bonding agents under clinical situations is much more complex than what was previously thought. The high values of contraction stress reported from some studies for specific configurations of restorations may overcome the bond strength of resin to dentin. This might explain a large number of bond failures and gap formation frequently observed in studies with bonding agents. However, an early interfacial stress build-up occurs during the polymerization in situ of resins bonded to tooth structures. Moreover, under the rigid set-up situation(in case that bond strength is higher than contraction stress), the contraction stresses may affect the mechanical properties of resin composites.

There are several solutions to the competition between the bond strength of resins to dentin and the forces of polymerization contraction.

One approach of these solutions is to develop dentin bonding agents that are able to develop immediate bond strengths that are greater than those developed by polymerization contraction and are equal to those obtained to acid-etched enamel. But, this can cause another problem, that is, the restrict polymerization environment decrease the mechanical properties of the resin composites.

A second solution is to cover the bonded surfaces with an elastic cavity liner. This acts as a "shockabsorber" and places a low modulus material between the relatively rigid dentin and resin composite30). The resin interdiffusion zone generated as the main bonding mechanism of most of the current dentin bonding systems may also function as a stress relaxation layer, since it has a lower modulus than the underlying mineralized dentin31).

A third solution is to insert resin composites in increments to reduce the volume of the resin that is shrinking during polymerization. The use of reduced amounts of resin to be polymerized at each increment has been shown to reduce the stresses generated to the cavity walls15). Incremental filling techniques actually lower the C-factor to below 1, since there is usually almost as much free surfaces as bonded surface in any single increment.

A fourth solution is development of the new materials. Several researchers are attempting to develop new resin composites that do not shrink when they polymerize32). For example, Spiro- orthocarbonate monomers have been synthesized that expand during polymerization through a double ring-opening process33). Further improvements are being developed to cause sufficient double ring-opening to generate enough expansion to overcome the shrinkage generated by conventional dimethacrylates.

In this study, hybrid type resin composite was more influenced than micro-hybrid composite by increase of C-factor. This fact is due to the difference of time, which gel-point appeared and filler content between the two resin composites. In case of hybrid composite, high filler content makes it more stiff and the earlier emergence of gel-point decrease the flow capacity of the material. The polymerization shrinkage was proved to have an effect on flexural strength and elastic modulus of composites. The results of this study shown that the higher the C-factor of cavity, the lower the mechanical properties of the resin composites.

Then the contraction stress of composites could be caused by polymerization shrinkage affect both bond strength of resin to tooth structures and mechanical properties of resin composites itself.

V. CONCLUSION

To achieve successful resin composite restoration, many factors such as volumetric shrinkage of resin mass, configuration factor of the cavity and the type of the resin can be considered. This study was designed to evaluate the effect of configuration factor of cavity on the mechanical properties of resin composites such as flexural strength and elastic modulus. From the results of this study, it can be concluded as follows :

This study suggests that the mechanical properties of resin composites can be affected by the configura factor that plays an important role in the confined curing environment.

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