Shrinkage of composite resin during polymerisation seems to be the greatest limitation, leading to marginal staining, poor marginal seal, and recurrent caries. For adhesive restorative materials, there is no method that can guarantee a perfectly sealed restoration; therefore clinicians must address the problems of shrinkage leading to destructive shrinkage stress. Due to the fact that recurrent caries are a major cause leading to replacement of restoration, it is crucial for low-shrinkage composite resins to be developed.
Composite resin has four principal components, consisting of an organic matrix, inorganic fillers, a coupling agent (binds the filer to the matrix), and an initiator/accelerator system. It polymerises by free radical polymerisation, generated when a photo-initiator (e.g. camphoroquinone) absorbs light energy emitted from the light-cure, thereby initiating polymerisation by reacting with a photoreducer.
An exothermic reaction is created when the monomer converts to a polymer. This results in a decrease in molecular vibrations and intermolecular distances. During formation of the polymer, the resin matrix converts from a paste (pre-gel state) into a viscous solid, and the composite resin contracts by approximately 1% – 1.5%. The point at which the composite resin changes from a viscous paste to an elastic solid, is known as the gel point. This is the point where the stress is conducted to the surrounding tooth structures from the resin. On the other hand, no stress is transmitted to the surrounding structures when the resin is a paste (or pre-gel state). In order to accommodate for shrinkage during light curing, the material flows from the unbound surfaces. When the composite resin increases in rigidity, due to the increasing modulus of the composite, the flow stops, and shrinkage stresses are transmitted from the bonded composite to the surrounding tooth. The stress produced may exceed the adhesive bond, or cohesive strength of the composite or the tooth, producing a marginal defect.
Shrinkage must be compensated by flow of the composite, adhesive, or tooth, when composite is bonded on all tooth surfaces. Damage occurs within the composite, if the stress is greater than the cohesive strength of composite. If the tensile strength of enamel is exceeded by the stress, the enamel will fracture. If the adhesive is improperly placed, it will fail. Failures can be observed as a white line, which appears during finishing of the restoration, as the debris collects within the defect and alters the index of refraction of light. In teeth with bucco-lingually wide restorations, cracks or fractures can be seen, as the cavity walls are predominantly enamel, which is too thin and too brittle to withstand forces generated by polymerisation shrinkage.
Three factors are dependent on the conversion of the monomer to a polymer; polymerisation shrinkage, modulus development and stress. Whether or not the tooth structure, and or composite resin can flex, is the main factor that affects stress development. The ratio of bonded to un-bonded surfaces in a restored tooth is known as the C-factor. As the composite cannot flow to relieve shrinkage stresses, the stress placed on the tooth increases, and the ratio of bonded to un-bonded surfaces also increases.
Composite is bonded to five walls of a prepared cavity in Class I or Class V restorations (C = 5). This is when the greatest stress occurs. The composite shrinks towards the bonded surfaces, and is restrained by the bonded areas on the opposing surface. In Class IV cavities, the C-factor is at its lowest, as the material has sufficient unbounded surfaces to allow it to flow, thereby providing stress relief. A high C-factor creates a risk for de-bonding of the restoration. However, large and shallow designs reduce the C-factor.
A list of factors affecting polymerization shrinkage stress:
‘ Distance between light cure and surface being cured
‘ Intensity and wavelength of light curing unit
‘ Curing mode of composite (whether chemical-cure or light-cure)
‘ Flow of composite
‘ Water sorption of composite
‘ Composition (low fillers increases shrinkage)
‘ Opacity and shade of composite
‘ Type of composite (highly filled vs. flowable)
In a given composite resin, polymerisation shrinkage is related to the opacity, shade and composition, the irradiance levels or exposure times of light cure used, incompatibility between the spectral output of the light cure and the photo-initiator system, geometry of cavity preparation, and the thickness of the composite layers.
During light curing, as the light passes through the composite, it diminishes; therefore the deeper layers of composite are less cured. Hence, why it is recommended to cure composite in 2mm increments. Any factor that reduces the light intensity will lower conversion rates of the composite. Insufficient levels of conversion during polymerization will reduce mechanical properties as well as wear resistance.
The darker, more opaque shades of composite, and microfills, require longer curing time. The further the distance between the light curing unit, the less the intensity, the longer that particular material needs to be cured. Consequently, curing composite through the tooth decreases light intensity, and is not recommended as an effective method for polymerising composite resin restorations.
Three different settings are available in light curing units. When the output is constant for a specific period of time, a continuous cure is used. The ramp- or step-cure begins at low intensity and switches to higher intensity. The pulse-delay cure is an intermittent curing procedure. Although it has been suggested that composite resin placement technique and the curing mode may affect the integrity of the margin of a composite resin restoration, to date, some in vitro investigations have shown mixed results. Some have shown improvement in resistance to leakage whilst others have been unable to demonstrate efficacy with such techniques.
Many variations of the incremental placement method have been advocated, as incremental placement of composite ensures more complete curing. On the other hand, bulk placement and curing have been recommended to decrease stress at the cavosurface margins, however, it has also been proven that little light reaches the centre of the preparation, therefore reducing the amount of polymerisation that occurs, and hence why there is less shrinkage stress. Studies have also shown that the hardness of composite in bulk-filled restorations is significantly less than in incrementally cured restorations, again demonstrating the limited depth of cure. To conclude, neither of the two methods have shown superior results to the other. Some report less leakage with bulk placement technique, and others reported less with incremental placement.
In conclusion, it has been proved that the factors that largely affect composite resin shrinkage are cavity preparation size and configuration. As the light passes through composite and tooth material, light intensity attenuates. Hence, it is best to place 2mm increments of composite resin to allow for maximum polymerisation. Even though there are different curing modes of light curing units, the selected composite affects shrinkage more that the method of curing. A composite resin that does not allow shrinkage needs to be developed in order to improve marginal integrity.