Potential and limitations of low-shrinking and bulk-fill dental composites

Polymerization of a composite restoration is accompanied by volumetric shrinkage, due to closer packing of the molecules when the polymer network is formed. This shrinkage can generate stress within the material, the surrounding tooth structure and at their mutual interface; as a consequence it may result in micro-cracking, interfacial de-bonding, gap formation and eventually secondary caries. The magnitude of the stress depends on the properties of the composite (volumetric shrinkage and rate, degree of conversion, dynamic and final elastic modulus, etc.) as well as several clinical aspects (cavity configuration, cavity size, application technique, etc.). The complex, multifactorial nature of the stress is still poorly understood and despite the lack of paramount evidence, several approaches have been proposed and applied to resolve this issue. The most widely accepted clinical filling technique to limit polymerization-shrinkage effects is the incremental layering technique: by layering, the volume and constraint per increment is reduced, thus reducing the shrinkage stress. Changes in the monomer matrix composition of the composite have been made to reduce the volumetric shrinkage and/or the shrinkage stress within the composite; nevertheless, the developed ‘low-shrinking’ composites were not that enthusiastically adopted in general restorative practice. Newly developed ‘bulk-fill’ composites with increased translucency and/or more potent photo-initiator systems, purporting to be placed in a single increment as thick as 4 or even 5 mm, have next been marketed. However, concerns about polymerization shrinkage stress and sufficient cure at depth still remain. While there are a lot of studies experimentally analyzing composite shrinkage, its heterogeneous nature is often neglected. Starting in this PhD dissertation with assessing the effect of different ‘low-shrinking’ and ‘bulk-fill’ composites on the micro-tensile bond strength to cavity-bottom dentin as a common and established technique within the BIOMAT research group, the PhD research focus was moved in a next phase towards non-destructive testing in order to mechanistically explore newly applied restorative composite techniques. An automated non-rigid image registration was used to generate strain maps from high-resolution micro-CT images before and after polymerization of the restoration. Strain was found to be non-uniformly distributed and the pattern shifted upon cavity depth, with often de-bonding being observed at the cavity bottom of deep cavities. Acoustic emission demonstrated an increase in events upon cavity depth, also indicating an increase in interface de-bonding. Materials with an increased depth of cure performed better in deeper cavities. This may relate to decreased shrinkage stress as well as more uniform mechanical properties throughout the whole depth of the restoration. Finally, the clinical performance of extensive posterior restorations was assessed in the short term in a randomized controlled trial. After one year, acceptable results were obtained for both the incremental and bulk-filling technique. The evaluation period was relatively short, yet considered sufficient to detect major early failures. In conclusion, bulk-filling composites have been shown to possess beneficial qualities regarding shrinkage stress and interfacial quality. Long-term clinical studies remain necessary to evaluate whether this bulk-filling approach also provides advantages in their practical implication.status: publishe