Structural Variables and Outcomes in the Design of Resin-Composites

Dental resin-bonded composites (RBCs) are formulated with substantial volume-fractions of reinforcing particles of hybrid size-ranges. The particles are coated by a coupling agent, and dispersed in an organic resinous matrix. The matrix monomers contain dissolved sensitizer molecules that are activated by visible light to initiate the photo-curing process. During the past 50 years, RBCs have undergone intense development, mostly reduction of the maximum particle size-ranges, and recently by the incorporation of nano-sized particles. During the past decade attention has partly switched to innovations in the resin-monomer matrix due to the polymerization shrinkage inherent in most resin-monomers, notably bisGMA/TEGDMA. Concurrently, resin-composites have been marketed with a wider range of viscosities, especially with the flowable RBCs. Consequently, it is essential for developers and end users to be clear about the key functional requirements of RBCs. In sequence of application, these are: 1. User-friendly handling properties. 2. Rapid property development: ‘kinetic functionality'. 3. Customized space filling: dimensionally stable materials. 4. Distribution of stress / load transmission: strong and retentive /adhesive materials. 5. X-ray contrast: radiopacity. 6. Aesthetic appearance: appropriate optical properties: translucency, gloss, opalescence, etc. It follows that the RBC structural variables can be fine-tuned to achieve quite varied performance outcomes. Certain products, especially some with innovative large-size monomers intended to reduce shrinkage, exhibit greater viscoelastic strain and so are virtually restricted in application to ‘smart dentin replacement' and require coverage by materials that exhibit high strength and good aesthetics. Some users will prefer materials that can achieve the latter properties in a single formulation, possibly placed by a ‘bulk-fill' technique. Irrespective of specific formulation, many bisGMA/TEGDMA composites are rather sensitive to the mode and time-scale of light-curing. Sub-optimal irradiation of such RBCs can generate inferior matrix structures and performance. But when used appropriately, the best of current RBCs have proven to deliver excellent clinical outcomes of adequate longevity.