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ePaper created 2016-01-20, 13:53:08 | version 1.38.0
I 23 case report _ composite restoration I roots1_2015 _Polymerisation contraction (shrinkage) Manydifferenttypesofcompositesarenowavail- able to the practitioner including microfills, macro- fills, hybrids, and small particle hybrids, nanofills, nanohybrids, or microhybrids.13 Even though the formulations can be adjusted in handling to make these composites ‘packable’, ‘flowable’, or ‘sculpt- able’, polymerisation shrinkage or contraction stress isstillthemostimportantclinicalchallengeorprob- lem associated with their use.14,15 This shrinkage or contraction and the stress created varies from com- posite to composite, and can be affected by: its filler type and loading content, the resin matrix and its molecular weight, the shade and opacity, the cavity preparation shape (C-Factor) width and depth, the composite thickness, the elastic modulus of the composite and tooth, the irradiance level and cur- ing time, the spectral output of the curing light, the curing light placement, bulk or incremental fill, the rate of force development (high irradiance lights), the initiator system used, and the degree of con- version.16-25 In published studies, shrinkage values for various composites have been reported from 2.00 to 5.63 vol. per cent,26 and 1.67 to 5.68 per cent,27 with flowables demonstrating the highest shrinkagewithacontractionsstressmeasurements ranging from 3.3 to 23.5MPa.26 Not all composites advertised as low-shrinkage actually have reduced polymerisationshrinkagemeasurements.Wheneval- uating seven low-shrink BisGMA-based compos- ites, Aelite LS Posterior and N’Durrance presented relatively high shrinkage values.28 The polymerisation contraction of the compos- iteresinandcontractionstresscreated,asdiscussed previously, can produce tensile forces on the tooth structureandthebondingsystemthatmaynotonly disrupt the bond to the cavity walls,29,30 but also fracture enamel along the prisms (white line mar- gins).31 This failure can lead to caries, sensitivity in vitalteeth,andmicroleakage,allowingthepenetra- tion of bacteria, fluids and toxins which can nega- tively affect the success of endodontic treatment (coronal leakage).32 Braga et al state that ‘shrinkage stress development must be considered a multi- factorial phenomenon’ and that ‘the volume of the shrinking composite becomes a variable to be con- sidered’.33 Unterbrink and Liebenberg in their publi- cation state that shrinkage stress increases with in- creasing C-Factor and that the size of the restored cavity is an important factor when bulk filling.34 Their study35 also shows that incremental filling lowers the C-Factor and that it is better than bulk curebecauseofbetteradaptationtothecavitywall, decreasing microleakage and increasing the degree of conversion. In a study looking at microleakage andcavitydimensions,itwasfoundthatmicroleak- age seemed to be related to a restoration’s volume, but not to its C-Factor.36 With bulk filling tech- niques, the hardness or conversion of composites are significantly lower than those of the same ma- terialplacedwiththeincrementaltechnique.37 Watts et al38 recommend that the restorative mass must be equally considered when translating shrinkage science into specific clinical recommendations. Fig. 3_Radiographic presentation of a patient with pain in the lower left second molar, which has been minimally restored. Fig. 4_The clinical presentation of the second molar which would demonstrate sufficient tooth structure remaining after root canal treatment so that a fibre post and core is not required. Fig. 5_Magnified view of the distal ridge of the second molar demonstrating a vertical crack. Fig. 3 Fig. 4 Fig. 5