| Abstract|| |
Aim: To evaluate the effect of prepolymerized custom-made composite megafiller and configuration factors (C-factor) on marginal adaptation of resin composite restorations.
Materials and Methods: Forty extracted human mandibular molars were divided into four experimental groups consisting of 10 samples each. In groups I and III, class V (configuration factor 5) and in groups II and IV, class II box-shaped cavities (configuration factor 2) were prepared. Groups I and II were restored with a nanofilled composite (Filtek™ Z350 3M ESPE, St Paul, MN, USA) placed in horizontal increments. In groups III and IV, preformed composite megafiller was placed and cavities were restored with nanofilled composite Z-350. After curing, the samples were finished and prepared for examination under low vacuum scanning electron microscope. The data were analyzed with one-way analysis of variance. The significance between the individual groups was calculated by Post hoc test using the Holm-Sidak method ( P≤0.05).
Results: The marginal gap values in groups III and IV were significantly lower than in groups I and II, respectively ( P≤0.05). Minimum gap values were seen in group IV.
Conclusion: Use of prepolymerized composite megafiller and a lower C-factor decreased the marginal gaps between the tooth and composite restorations.
Keywords: Marginal adaptation, megafiller, C-factor
|How to cite this article:|
Bhushan S, Logani A, Shah N. Effect of prepolymerized composite megafiller on the marginal adaptation of composite restorations in cavities with different C-factors: An SEM study. Indian J Dent Res 2010;21:500-5
Composite resins have become one of the most commonly used direct restorative materials for anterior and posterior teeth. But one of the inevitable drawbacks of dental composites is shrinkage during free radical polymerization, which may be as high as 3% by volume. , Polymerization shrinkage stresses have the potential to initiate failure of composite-tooth interface, causing microleakage, secondary caries, cuspal deflection of the tooth, which may result in postoperative sensitivity. The intensity of the developed stresses is associated with three main factors: (1) characteristic of the material; (2) geometry of the cavity, that is, configuration factor (the C factor); and (3) restorative technique.
|How to cite this URL:|
Bhushan S, Logani A, Shah N. Effect of prepolymerized composite megafiller on the marginal adaptation of composite restorations in cavities with different C-factors: An SEM study. Indian J Dent Res [serial online] 2010 [cited 2014 Nov 28];21:500-5. Available from: http://www.ijdr.in/text.asp?2010/21/4/500/74218
The percentage of polymerization shrinkage is a direct function of volume fraction of matrix phase. The increase in the filler content of composites proportionately decreases the matrix phase and improves the material qualities, such as strength, hardness, modulus of elasticity, coefficient of thermal expansion and polymerization shrinkage.  However, there is a limit to which the amount of filler can be incorporated without adversely affecting the working properties.  It is possible to place one or more large fillers into the composite, referred to as "inserts" (0.5-2 mm) or "megafillers" to reduce the bulk of matrix phase. The concept of the megafilled composite resin restoration was put into practice with the introduction of glass-ceramic inserts. ,, β-quartz as inserts was introduced in the 1980s. These were stronger and harder but difficult to finish and polish. Therefore, they were replaced with silica glass and ceramics. These commercially available inserts are designed to be pressed into an uncured composite resin restoration, displacing about 50%-75% (by volume) of the resin that normally occupies the cavity reducing the overall polymerization shrinkage and microleakage. The technique is reported to be beneficial for improving the marginal integrity. ,, However, these ceramic inserts are expensive and need to be pretreated by etching and silanization to improve bonding with composite material.  The bond between ceramic inserts and resin composite seems to be a weak link and prone to fracture.  Also the difference in composition and hardness between the glass-ceramic and the composite material results in unequal polishability.  In order to overcome these disadvantages associated with ceramic inserts, use of prepolymerized composite as megafiller was introduced. They can be custom-fabricated, hence are cost-effective, do not require pretreatment, bond micromechanically, and unlike with ceramic inserts, the insert-composite interface is less critical than that achieved with composite-ceramic inserts. 
Previous studies in the literature analyzed the microleakage and marginal adaptation of composite restorations using ball-shaped prepolymerized composite megafillers. , Studies have shown that adapting insert size as precise to the cavity dimensions produces a quality of margin comparable to that of an inlay.  The maximum advantage is achieved when a resin composite contains the largest volume of insert possible.  The second factor that influences the composite to enamel-dentin bond is the cavity configuration factor or the C-factor. When resin bonds to the walls and floor of the cavity preparation, a competition occurs between the opposing walls, as the restorative material shrinks during polymerization and pulls them closer together.  The magnitude of this phenomenon depends on the configuration of the cavity.  When a curing material is bonded on all sides to rigid structures, bulk contraction cannot occur and the shrinkage increased stress, flexure, or gap formation at the tooth-adhesive interface. It has been hypothesized that a large unbonded area would facilitate plastic deformation of composite resin during polymerization before the gel point is reached, thus reducing the final stress values.  The objective of this ex-vivo study was to evaluate the effect of indigenously made composite megafiller and configuration factors (C-factor) individually and simultaneously on marginal adaptation of resin composite restorations.
| Materials and Methods|| |
Forty freshly extracted caries free, human permanent mandibular molars having approximately same height and width were divided into four experimental groups, consisting of 10 samples each. Class V box-shaped cavities were prepared in groups I and III and class II mesio-occlusal box-shaped cavities in groups II and IV, using diamond points in an air rotor hand piece.
Dimensions of class V cavities prepared were 2.5 mm depth pulpally, 3 mm height occluso-gingivally, and 3.5 mm width mesiodistally. Dimensions of class II box only cavities prepared were 2.5 mm depth axially, 3 mm height occlusogingivally, and 3.5 mm width buccolingually. The gingival floor was kept above the cemento-enamel junction. An acrylic mold was prepared in which the roots of specimen teeth were embedded. Tooth preparation was done by a single operator.
The composite resin used in this study (Filtek™ Z350 3M ESPE, St Paul, MN, USA) was a visible light-activated direct restorative nanocomposite. The filler contains a combination of a nonagglomerated/nonaggregated, 20 nm nanosilica filler, and loosely bound agglomerated zirconia/silica nanocluster, with size of 5-20 nm fillers. The cluster particle size range is 0.6-1.4 μm. The filler loading is 78.5% by weight.
Fabrication of megafiller
A wax rod 25 mm long was made with dimensions of 2.5 mm width and 2 mm height. An impression of this rod was made with an elastomeric impression material (addition silicone). The negative replica was then filled with the same resin composite material and light cured for 20 s. The resin composite rod was then taken out of the impression material and placed in the Lumamat 100 furnace (Ivoclar vivadent, Greece), at 104°C for 20 min, for postcuring. The composite rod was cut into 3 mm long pieces with a cutting disc, resulting in megafillers with dimensions 2×2.5×3 mm.
The prepared cavities were etched with 37% phosphoric acid (3M ESPE St Paul, MN, USA) for 15 s, washed with distilled water for 10 s, and dried with a gentle blast of air. Caution was taken not to overdry the preparation. Two coats of dentin-bonding agent (single bond, 3M ESPE St Paul, MN, USA) were applied at intervals of 10s and cured for 10 s. A clear matrix strip was placed for class II cavities. The cavities were restored as follows.
Groups I and II: The restorative resin (Z 350, 3M ESPE) was placed in horizontal increments of approximately 1.5 mm and cured for 20 s each by light curing unit LED (SmartLite PS, Dentsply, USA) with irradiance power of 900 mW/cm2 for 20 s/mm.
Groups III and IV: A small amount of restorative resin (Z350, 3M ESPE) was placed in the cavity without curing. The surface of prepolymerized composite megafiller was air abraded using air abrasion unit (Prepstart, Danville Engineering, USA), which uses 27-or 50-μm aluminum oxide powder particles under air pressure of 60 psi for 5 s. Megafillers were then coated with dentin-bonding agent, cured for 10 s, and embedded into the cavity containing uncured composite material. Curing was done in all the samples from all directions and subsequently finished and polished with composite polishing kit (Shofu Inc, Kyoto, Japan).
Specimen preparation for evaluation of marginal adaptation under scanning electron microscope
The margins of the restorations were cleaned with 10% ortho-phosphoric acid for 5 s and washed with water for 10 s to remove the debris over the margins collected during polishing procedure. The samples were dried with the help of a blotting paper to remove excess of water. Care was taken not to desiccate them. They were then placed on standard half-inch pin-type aluminum stubs with restored surfaces facing upward. The stubs were then placed in the LEO specimen chamber mounting table of scanning electron microscope LEO VP 435 (Carl-Zeiss NTS GmbH, Oberochen, Germany). Robinson BSE detector was used to evaluate the marginal adaptation of the samples at a magnification of 200×.
Evaluation of marginal adaptation
The gingival margins of both class V cavities (without and with insert, [Figure 1] and [Figure 2] and class II cavities (without and with insert, [Figure 3] and [Figure 4] and one of the axial walls, mesial margins for class V and buccal margins for class II cavities of all the specimens were evaluated at 200× magnification for marginal adaptation. Minimum and maximum gap width was taken at each margin and average of these was calculated.
|Figure 1: Class V composite restorations without insert: tooth-resin composite margins showing maximum marginal gap values >25 μm under 200×|
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|Figure 2: Class V composite restorations with insert: tooth-resin composite margins showing maximum marginal gap values <25 μm under 200×|
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|Figure 3: Class II composite restorations without insert: tooth-resin composite margins showing maximum marginal gap values >25 μm under 200×|
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|Figure 4: Class II composite restorations with insert: tooth-resin composite margins showing no marginal gap under 200×|
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These findings were recorded onto a Microsoft Excel sheet (Microsoft Office Excel).
| Results|| |
Maximum and minimum marginal gap values for all the groups were recorded and mean was calculated as depicted in [Table 1]. The raw data were used for the statistical analysis using SPSS version 11. One-way analysis of variance was used for comparing the variables. Difference in means of all the groups were compared and significance between the individual groups was calculated by post hoc test using the Holm-Sidak method (P<0.05) [Table 2].
To evaluate the effect of C-factor and megafiller on the marginal adaptation of composite restorations, group I was compared with group II, group III with group IV and group I was compared with group III, and group II with group IV, respectively.
It was found that megafiller decreased the marginal gap values in both group III and group IV. Also, there was reduced gap formation in group II as compared with groups I and III as compared with group IV. Minimal gap formation was seen in group IV.
| Discussion|| |
One of the major problems of any restoration is the loss of marginal integrity with the resultant microleakage and pulpal irritation. ,, In composite restorations, the interfacial integrity is closely related to polymerization shrinkage stress which in turn is directly related to composite volume and C-factor.  Two of the several methods suggested to reduce polymerization shrinkage of composites in this study are the use of prepolymerized composite megafiller and creation of lower C-factor.
The concept of megafillers or inserts was introduced by Donly et al.  Both ceramic and composite megafillers have been used in various studies with varying results, some in favor ,,,,, and others in opposition. ,
The results of the present study showed that the presence of the composite megafiller reduced the marginal gap values. This can be explained by the fact that the marginal gap formation depends on the polymerization shrinkage stresses on the composite-tooth interface, which in turn depends on the volume of the composite in the cavity. The incorporation of inserts into the composite restoration increases the overall filler loading within a restoration without increasing its volume and hence reduces the shrinkage. This fact has also been confirmed by others. ,,,
On the other hand, the effectiveness of megafiller in improving the marginal integrity of composite restorations was refuted by few investigators. Tjan et al and Strobel et al, in their in-vitro studies, found no difference in marginal integrity of composite restorations with and without ceramic inserts. , They stated that ceramic materials do not have a chemical bonding with composite materials and hence instead of improving, marginal adaptation may deteriorate it. They stated that although this truly seems to be reasonable from the standpoint that less resin composite volume means less shrinkage, placing a relatively large ceramic body into an uncured bulk of resin composite being cured afterward in more or less one layer enlarges the configuration factor approximately fivefolds.
To overcome the disadvantage discussed in the above studies, an indigenous method of fabrication of megafiller made of the same composite material as was used in the restoration of the remaining cavity was used and air abraded, before bonding to have increased micromechanical bond. Use of prepolymerized megafiller of the same restorative resin composite is expected to have an advantage as being of identical physicochemical characteristics, in the oral cavity. Hence it has the same thermal behavior as that of the matrix composite, diminishing stress development in the restoration.  According to a few authors, incremental technique may produce more interfacial stresses than the conventional bulk placement as the successively bonded layers do not always bond well together. Therefore, using prepolymerized megafillers has an added advantage of reducing interlayer stresses. ,
The role of C-factor and its effect on marginal integrity was studied by several investigators. When the configuration factor is low, apparently the stress relaxation by flow allowed by the unbonded, free surface is sufficient to maintain the resin composite-tooth bond. The remaining shrinkage stress slowly attains values that are lower than or just reach the range of reported bond strengths of dentin-bonding agents. Higher gap formation was reported with increase in C-factor by Da Silva et al,  Wattanawongpitak et al,  and Choi. 
In the studies by Cunha et al, higher C-factor was shown to enhance the amount and rate of stress generated in composite restorations. ,
The results of the present study are in agreement with these studies. It showed that cavities with a low C-factor (class II) had lesser marginal gap values than cavities with higher C-factor (class V).
The incorporation of prepolymerized composite megafiller further improved the results.
From the results of present study, it can be recommended that measures should be taken to reduce the effect of C-factor while performing composite restorations. Also, megafiller, as described, should be evaluated in future carefully planned, prospective clinical studies to determine its effectiveness in improving the marginal integrity and other clinical parameters of direct composite restorations.
| Conclusions|| |
The results of the study indicated that:
- Use of prepolymerized custom-made composite megafiller in composite restorations improved the marginal gap values in both class V and class II cavities.
- The marginal gap values were significantly better in class II cavities with lower C-factor than in class V cavities with higher C-factor.
Innovative technique of fabrication of precured composite inserts merits further studies, as it is highly cost-effective and easy to fabricate as compared with commercially available ceramic and β-quartz inserts. Its performance in clinical situations is needed to evaluate the wear resistance and proximal contour of restorations. Moreover, studies can be done by using two or more composite inserts into the cavities.
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Department of Conservative Dentistry and Endodontics, All India Institute of Medical Sciences, New Delhi
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]