Mechanical behavior of hydroxyapatite-based dental resin composites

In commercial resin composite, the main reason of posterior stress-bearing restoration failure is the brittle behavior of quartz particle filler. For this reason, in recent years, the use of nano powders or different nano crystal morphologies filler has been increasing in dentistry. In fact, nanoparticles can provide larger load transfer and favor toughening mechanisms and thus, increasing flexural modulus and fracture toughness of dental resin composites. In this regard, hydroxyapatite nanocrystals showed the best performances as a novel bioactive and biocompatible reinforcing filler. After a preliminary review about conservative dental composites, the physical, mechanical, and chemical properties of HA-filled composites have been analyzed (i.e., by varying hydroxyapatite filler content and filler morphologies) in a critical overview of the literature. In addition, adequate polymerization of restorative composite is a fundamental factor in order to obtain optimal clinical performances. Low conversion rates, in fact, affect several important parameters, such as flexural strength, fatigue, solubility, discoloration, and biocompatibility, thus limiting the life span of the composites. Furthermore, one of the laboratory tests widely used to evaluate the mechanical behavior of resin composites is the flexural strength test, according the International Standard ISO 4049. However, modifications of test parameters are commonly reported in the literature due to the possibility to increase their reproducibility and clinical relevance in oral environment. These aspects will be investigated within this chapter with the aim to provide the key components on the potential applicative implications of hydroxyapatite-based dental resin composite and to define a systematic experimental selection of different methods in order to analyze the effect on mechanical and physical chemical properties of hybrid ceramic-based composites.