|Year : 2019 | Volume
| Issue : 5 | Page : 244-248
Shrinkage in composites: An enigma
Dhakshinamoorthy Malarvizhi, Arumugam Karthick, NewBegin Selvakumar Gold Pearlin Mary, Alagarsamy Venkatesh
Department of Conservative Dentistry and Endodontics, Sree Balaji Dental College and Hospital, Bharath Institute of Higher Education and Research, Chennai, Tamil Nadu, India
|Date of Web Publication||24-Sep-2019|
Dr. Dhakshinamoorthy Malarvizhi
Department of Conservative Dentistry and Endodontics, Sree Balaji Dental College and Hospital, Bharath Institute of Higher Education and Research, Narayanapuram, Pallikaranai, Chennai 600100, Tamil Nadu.
Source of Support: None, Conflict of Interest: None
In recent years, increased demand for perfectly aesthetic restoration coupled with improved performance of composite resin made clinicians to select composite resins over amalgam restoration. Patients prefer composite restoration not only for restoring anteriors but also to replace their unaesthetic amalgam restoration of their posteriors. The greatest limitation of composite resin as posterior restoration is its polymerization shrinkage leading to marginal leakage, tooth or restoration fracture, postoperative sensitivity, ultimately leading to the reduction of the long-term success of the restoration. There are several factors associated with the polymerization process that have its impact on the integrity of the tooth restoration complex. So the astute clinician should have thorough knowledge regarding the mechanism involved in polymerization shrinkage and the techniques of overcoming it. The objective of this article was to make the clinician understand the factors and the problems related to the polymerization shrinkage with the methods to overcome the same for the clinical longevity of the restoration.
Keywords: Curing, Fillers, Marginal Leakage, Polymerization Shrinkage
|How to cite this article:|
Malarvizhi D, Karthick A, Gold Pearlin Mary N, Venkatesh A. Shrinkage in composites: An enigma. J Int Oral Health 2019;11:244-8
|How to cite this URL:|
Malarvizhi D, Karthick A, Gold Pearlin Mary N, Venkatesh A. Shrinkage in composites: An enigma. J Int Oral Health [serial online] 2019 [cited 2019 Nov 19];11:244-8. Available from: http://www.jioh.org/text.asp?2019/11/5/244/267713
| Introduction|| |
In dentistry, restoring posterior teeth has always been an elusive task for the operator, as the posterior teeth are under constant loading. Amalgam and resin-based restorations are commonly preferred for the stress-bearing areas. Although, longevity of amalgam restorations are found to be superior to composite restorations, nowadays, the latter is more preferred than amalgam because of the mercury toxicity and the unaesthetic appearance associated with it.
One of the major concerns in dental practice is the failure of dental restorations. According to Mjör, replacement of failed restorations contributes to approximately 60% of all operative work. In another study, it was found that failure rate of composite restorations due to secondary caries was approximately 3.5 times higher than that of amalgam. The major cause for secondary caries under composite restoration is microleakage, which is a consequence of marginal gaps that can occur due to polymerization shrinkage. When the monomers in the resin system get activated, the intermolecular distance of monomer molecules shorten, which ultimately leads to shrinkage of the resin.
Therefore, the consequence of polymerization shrinkage is due to the resultant marginal failure, which, in turn, leads to secondary caries, marginal staining, restoration displacement, tooth fracture, and postoperative sensitivity. Hence, polymerization shrinkage directly or indirectly affects the clinical outcome of resin-based restorations.
The purpose of this review article was that it provides an insight into the causes of polymerization shrinkage, its effects on the longevity of the restorations, and the strategies to overcome it effectively, thus improving the predictable success.
| Polymerization Reaction|| |
A dental composite comprises a system of mono-, di- or trifunctional monomers. BIS-GMA (bisphenol A–diglycidyl ether methacrylate) and urethane dimethacrylate are the most popular and widely used dimethacrylates. Camphorquinone, the photoinitiator system when activated by blue light, transforms chemically into an excited triplet state. This reacts with the tertiary amine to produce free radicals and also activates the accelerators, which, in turn, produce greater amount of free radicals. These free radicals react with monomer molecules, forming active centers for polymerization. In the propagation stage, monomers are sequentially added to the active centers and form long cross-linking polymer chains, which bring them closer to form covalent bonds.
| Polymerization Shrinkage|| |
Before a polymerization reaction, the monomer molecules are held together by Van der Waals forces. They are separated from one another by a distance of approximately 0.3–0.4nm. During polymerization, the “mer” units move closer together by forming covalent bonds. Consequently, the units move closer to within covalent radius, which is approximately one-third of Van der Waals radius (0.15nm). This results in shrinkage and is dependent on the number of monomer units per unit volume that is converted to polymer.
The reduction in intermolecular distance results in volumetric polymerization shrinkage. At the molecular level, there is a decrease in molecular vibration with increased cross-linking and there is a resultant shrinkage of the polymerized structure. The reaction also results in a change in entropy and also a change in relative free volumes of monomer and polymer. The packing efficiency of macromolecules is the primary determinant of free volume.
| Factors Affecting Polymerization Shrinkage|| |
Filler volume fraction
The dispersed phase of composite is formed by inorganic fillers. Filler volume fraction has an inverse relation to volumetric shrinkage., As the volume of filler content increases, the volume of resin matrix decreases and hence volumetric shrinkage reduces proportionately. The shrinkage values for BIS-GMA and TEGDMA (triethylene glycol dimethacrylate) are 5.2% and 12.5%, respectively, but the shrinkage value for composites is only 2%–3% because of the filler content.
In flowable composites, the shrinkage is high because of the low filler content, whereas in packable composites, there is lesser shrinkage. In homogenous micro-filled composites, the volume percentage of filler is below 50 vol% resulting in shrinkage of 5 vol%, whereas in heterogeneous micro-filled composite, the shrinkage is similar to hybrid because the incorporation of prepolymerized organic fillers increases their total filler volume. The shrinkage is 1–3 vol% in hybrid composites because the filler content is approximately 60 vol%.
The polymerization shrinkage is high in traditional micro-filled composites because of the smaller, spherical filler particles produced by pyrogenic and precipitation methods, which have a very high surface area resulting in agglomeration of filler particles and hence reduced filler loading.
Degree of conversion of resin matrix
The degree of conversion of composite signifies the number of C=C double bonds converted to C–C single bonds, which result in long-chain polymers. The volumetric shrinkage is directly proportional to the degree of conversion., Modulus of elasticity and flow of composites describe the viscoelastic behavior of the composite resin, which in turn determines the polymerization shrinkage [Figure 1].
|Figure 1: Relationship between degree of conversion and shrinkage stress|
Click here to view
The degree of conversion varies with different monomers based on its high molecular weight and initial concentration of C=C double bonds. TEGDMA, which is used as diluent, has high degree of conversion than BIS-GMA. In commercial composites, the final degree of conversion is between 55% and 75%. As the degree of conversion increases, the modulus of elasticity of resin increases, which ultimately increases the shrinkage stress. In some situations, the reduction of contraction stress may be attributed to partial polymerization of the composite resin.
In one study, it is stated that the degree of conversion is the major parameter, which influences polymerization shrinkage and stress development.
Composition of resin matrix
In case of methacrylate monomers, it is impossible to avoid shrinkage, which ranges up to 10%–16% by volume. BIS-GMA is the main part of organic matrix in composite and it is a high viscosity monomer. The high viscosity results in less degree of freedom, which in turn, results in kinetically low degree of conversion. This would result in lower polymerization shrinkage than other monomers.
The extent of contraction depends on the molecular degree of freedom and on the functionality of monomers of resin complex. For monomers of similar viscosity, polymerization shrinkage increases with their functionality and in case of monomers of similar functionality, the shrinkage increases as viscosity reduces. This implies that the shrinkage is influenced by monomer functionalities, molecular structure, molecular mass, and size. Silorane-based composites have lower polymerization shrinkage than methacrylate-based composites.
Feilzer et al. (1987) introduced the term “C factor” or “Configuration factor,” which is defined as the ratio of bonded to unbonded surfaces of the composite restorations. According to Versluis et al., shrinkage of the composite resin was determined by the bonding of the composite resin to the tooth structure and by the free surfaces rather than by the orientation of direction of the curing light as believed earlier [Figure 2].
If the C-factor is high, the shrinkage stress is high and it depends on the number of unbonded surfaces. As the number of unbonded surface increases, there will be lesser stress generation. This is due to the fact that the increased surface area will help to relieve the generated stresses. Cavities with C factor less than one generate least stress and it increases as the C factor increases.
The factors that might influence the C factor and the stress generation includes the extent of caries removed, the amount of remaining tooth structure, type and location of the tooth in the arch, type of the curing light, and photoinitiator and composite resin used. Shallower and wider cavity preparation generates lower stress compared to deeper and narrower cavity.
Intensity of curing light
There exists a linear relationship between polymerization shrinkage and light intensity, which means that higher light intensity produced greater polymerization shrinkage when exposure time is constant. The reason for higher shrinkage with higher intensity is due to greater degree of conversion. The slower polymerization delays the gel point, which provides for stress relaxation in the resin and the interface. Polymerization shrinkage is highest with ramp curing modes and high intensity modes, whereas it is lesser with step-curing and low intensity modes.
Thickness of composite resin, shade, and opacity of composite
It has been proved that incremental curing induces lesser polymerization shrinkage stress than bulk curing., According to Donly and Jensen, bulk placement of composite induces more strain because the buccal and lingual walls are pulled together. Gingivo-occlusal incremental layering technique induces intermediate strain. In this technique, though the buccal and lingual walls are pulled together, the volume of composite increment is small, resulting in lesser strain than bulk placement. Buccolingual incremental layering induces least strain as only one wall is pulled at a time.
Other factors that may influence polymerization shrinkage are exposure time, mode of curing, compatibility between spectral output of curing light, and photoinitiator system.
| Methods to Minimize Polymerization Shrinkage|| |
There are various methods to reduce polymerization shrinkage, which can be classified under the following headings:
Modification of placement technique
Previous research has proved that bulk technique of composite placement results in more polymerization shrinkage than incremental layering technique. Incremental technique reduces the bonded/unbonded ratio, which results in less configuration factor, ultimately lesser shrinkage stress.
Horizontal layering technique increases the C-factor, and hence the stress is greater. Buccolingual incremental technique induces least strain because cuspal tension is minimized in this technique as composite is applied to a single dentin surface without touching the opposing cavity wall. In centripetal buildup technique, as an initial vertical composite increment is placed in contact with the matrix band, class II cavities are converted to class I cavities.
To reduce the shrinkage stress, various modifications have been carried out to the material aspect of composite. The introduction of ring opening silorane molecules has resulted in low shrinkage composite resins. This new monomer system was synthesized by Weinmann et al. It is formed by a reaction between oxirane and siloxane molecules. The advantage of these composites is due to ring opening polymerization of oxirane molecules and increased hydrophobicity of siloxane molecules. These monomers produce local volumetric expansion because of the opening of ring structure, which compensate for the volumetric shrinkage from C=C polymerization.
The cycloaliphatic oxirane moieties undergo cationic ring opening polymerization, which results in low polymerization shrinkage and stress. During the cationic polymerization, an acidic cation starts the initiation, which opens the oxirane ring generating carbocation, which is the new acidic center. The addition of an oxirane monomer results in opening of the epoxy ring to form a chain. A network is formed in case of multifunctional monomers. Studies have been conducted to modify the photoinitiator and inhibitor system, which could influence the polymerization reaction and shrinkage. A study by Braga and Ferracane, in 2002, has shown that the rate of polymerization and shrinkage stress was reduced by increasing the concentration of inhibitor. In another study by Schneider et al., camphorquinone content was substituted partly by phenylpropanedione, which reduced the rate of polymerization stress. Modification of the composite material by the addition of thiourethane oligomers has proven to significantly improve mechanical properties and reduce polymerization stress. In another study, it was found that addition of up to 20% phene into BIS-GMA/TEGDMA resin resulted in the reduction of polymerization shrinkage.
In a systematic review by Meereis et al., it was concluded that the recent advancements in the modification of resin matrix lead to the development of low shrinkage and bulk fill materials, which shows promising improvement in the reduction of polymerization shrinkage stress.
Inadequate cure of resin-based restorations results in inadequate degree of conversion, which leads to polymerization shrinkage. Thus, the types of curing light and modes of curing have shown to affect the degree of polymerization. Of all the techniques, pulse delay technique and soft-start polymerization have been found to reduce the polymerization shrinkage and stress. It has been found that polymerization shrinkage is greatly reduced in pulse delay curing with a delay of 3–5min. Soft-start polymerization is also found to reduce the polymerization shrinkage because it allows for slow initial rate of polymerization.
Use of stress absorbing liners
Flowable resin can be used as an intermediate stress absorbing layer as it has a lower elastic modulus as compared to composite resin, which reduces the stress at the tooth-restoration interface, ultimately reducing the cuspal deflection. This is called the “elastic wall concept.”
Preheating resin composites have found to reduce the polymerization shrinkage because the increased temperature reduces the viscosity of the material and increases radical mobility. This would result in increased polymerization and higher degree of conversion.
If shrinkage stress in composite resin is minimized, the success and survival rates of the restorations can be improved. So a prudent practitioner should be aware of all the updates taking place in the field of composite resin to minimize the shrinkage stress.
| Conclusion|| |
Though shrinkage cannot be eliminated completely, there are numerous methods to reduce it. Therefore, the clinician should implement any of these methods to improve the success rate and longevity of the composite resin restorations.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]