The influence of ‘nanocluster’ reinforcement on the mechanical properties of a resin-based composite material

The introduction of innovative filled methacrylate resin composites has revolutionised the field of aesthetic restorative dentistry and provided a clinically viable alternative to amalgam-based restorations. The mechano-physical properties and resultant clinical longevity of these materials was insufficient. To improve these properties the on-going development of resin-based composites (RBCs) has sought to modify the filler size and morphology and to improve the loading and distribution of constituent filler particles. This has resulted in the introduction of so-called ‘nanofills’ which possess a combination of nano- and micro-sized filler to produce a hybrid material. A variation to this approach was the introduction of ‘nanocluster’ particles, which are essentially an agglomeration of nano-sized silica and zirconia particles. Although these materials have demonstrated a degree of clinical and experimental success debate remains as to their specific benefit compared with existing conventionally filled systems. Following placement RBC restorations are exposed to masticatory loading (repeated sub-critical stresses) which are typically detrimental to the clinical longevity of the material. The current study determined that RBCs reinforced with the ‘nanocluster’ particles possessed statistically similar or significantly increased bi-axial flexure strengths and associated Weibull moduli following pre-loading regimes which produced catastrophic failure of conventionally filled RBCs. This was attributed to the unique reinforcement provided by the ‘nanocluster’ particle, which were identified by a novel micromanipulation technique to possess distinctive fracture mechanisms, in addition to possessing an IPC-like structure. These acted in combination to absorb and dissipate loading stresses and to provide enhanced damage tolerance. Near-infra-red spectroscopy was also employed to determine the water sorption and it did not identify any direct correlation between water content and extent of strength reduction. However, immersion of the materials in water and also in sodium hydroxide or ethanol highlighted that the long-term hydrolytic stability of the ‘nanoclusters’ was limited. This suggested that degradation of the interfacial silane layer weakened the ‘nanocluster’ particle causing them to act as defect centres within the resin matrix and to consequently generate a greater loss of strength. Therefore, whilst the ‘nanocluster’ reinforced RBCs have the potential to provide enhanced damage tolerance and improved clinical longevity the limited long-term hydrolytic stability suggests further development of hydrophilic silane coupling agents and resin monomers is required to realize these properties