Hydraulic Calcium Silicate Cements with Incorporations: Modification of Physical, Chemical, and Biological Properties across Diverse Environmental Conditions

Abstract

Hydraulic calcium silicate cements (HCSCs) find application in various clinical procedures within the field of endodontics. Although initially introduced for endodontic surgery, certain aspects of their use in these applications have not been thoroughly explored. Moreover, considering the interactions of these materials with the surrounding environment, it becomes crucial to understand how these might influence their properties. It is possible that specific modifications of the HCSC composition could improve some target biological properties, provided that changes do not compromise any of their current characteristics. This thesis delves into the impact of material modifications on the physical, chemical and biological properties, including antibacterial activity and cytotoxicity, of HCSCs. It also explores the influence of environmental conditions, namely the immersion medium, aging period, and the role of the dentin substrate in this context. The primary aim was to evaluate the effect of incorporating silver nanoparticles or bioactive glass into a basic experimental formulation, which comprised 80% w/w tricalcium silicate cement and 20% w/w zirconium oxide radiopacifier, with the rationale to enhance its biological properties. Commercial materials were also assessed for comparison purposes. An additional aim was to investigate the effect of environmental factors on the material properties. In Study I, the surface characteristics alongside the bacterial adhesion of endodontic cements were assessed. Incorporation of silver nanoparticles (0.5, 1, 2 mg/ml) or bioactive glass (10, 20% w/w) increased the surface roughness of the experimental HCSCs over time but had no impact on the antibacterial effect. Immersion of all tested HCSCs to a serum-containing environment triggered surface reactions compared to immersion in water, but this did not affect the bacterial adhesion. While the physical parameters measured did not appear to be related to the degree of bacterial adhesion in experimental HCSCs, the commercial HCSC Biodentine (Septodont, Saint Maur-des-Fosses, France) displayed improved physical properties but compromised antibacterial efficacy, with significant bacterial adhesion from day one compared to the experimental HCSCs. Study II tested leachates from the cements. The results showed that both modifications of HCSCs and different environmental conditions significantly influenced the leaching properties of the materials. More specifically, immersing the materials in fetal bovine serum resulted in reduced alkalinity, decreased bactericidal properties, and lower cytotoxicity of both experimental and commercial HCSCs compared to water immersion. The 20% bioactive glass-containing cement and Biodentine exhibited overall lower alkalinity, calcium release, and antibacterial activity than the basic experimental HCSC formulation. Mixing the experimental HCSC with a 2 mg/ml silver nanoparticle solution enhanced the antibacterial efficacy of water leachates but had no such effect in the serum leachates. In Study III, the antibacterial characteristics of the dentin/material interface and dentin surfaces exposed to the test materials were examined. All tested HCSCs conferred antibacterial activity onto the adjacent dentin. However, this effect was lower for the experimental HCSC with inclusion of 40% w/w bioactive glass and for Biodentine compared to the basic experimental formulation. Conversely, a 2 mg/ml silver nanoparticle solution increased the antibacterial potential of the basic experimental formulation in an assay with a longer interaction period (three-day bacterial exposure instead of one-day). Taken together, findings of the current thesis suggest that modifications of the composition of HCSCs led to notable changes in their physical and chemical properties, which subsequently impacted their biological activity. The addition of 2 mg/ml silver nanoparticles did not compromise the physico-chemical characteristics of the cement but only led to a modest improvement in antibacterial properties. Incorporation of 20% bioactive glass into tricalcium silicate cement reduced the calcium hydroxide release, thereby compromising the indirect contact antibacterial efficacy of the cement in both the immersion medium and dentin surfaces, with an even greater impact at 40% incorporation for the conferred effect in dentin. Biodentine displayed enhanced physical properties and lower cytotoxicity, but weaker antibacterial efficacy compared to the basic experimental formulation. These differences are attributed to the chemical additives in the manufacturing of Biodentine. Lastly, the immersion medium and aging period had a notable influence on HCSCs. These factors should be thus carefully assessed in the context of the cements’ clinical application. Aging of materials in a clinically relevant medium had a detrimental effect on their leachate antibacterial properties, while achieving long-lasting antimicrobial activity remains a distant goal.