Modulation of Periapical Inflammation Following Nano-sized Intracanal Medication. A Randomized Controlled Clinical Trial

The main cause of endodontic disorders is microbial infection. From the infected pulp tissue, microorganisms can penetrate into the deeper layers of root dentine and propagate a periapical tissue through the apical foramen and lateral canals causing apical periodontitis. Apical periodontitis is an inflammatory condition affecting the periapical area of teeth with a global prevalence of 52% among individuals and 5% at tooth level. At the periapical region, microbes and their products encounter the host immune defense. Innate immune cells combat bacteria through phagocytosis and release anti-microbial substances, while adaptive immune cells initiate both cell-mediated and humoral immune responses. This immune activation also stimulates osteoclasts, leading to bone resorption and creating space for the inflammatory cells' infiltration . During periapical inflammation, immune cells migrate to the periapical area, where they release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), along with anti-inflammatory cytokines like interleukin-10 (IL-10). These cytokines play crucial roles in modulating inflammation and tissue repair.

MicroRNAs (miRNAs) are evolutionarily conserved small (18-22 nucleotides) single-stranded, noncoding RNAs. It has been well acknowledged that miRNAs influence gene expression at the posttranscriptional level by targeting mRNA 3'-untranslated regions (3'-UTRs). Accumulating evidence has suggested that miRNA expression is related to the clinical and biological features of diverse diseases, and they can be potential biomarkers for diagnosis, treatment, and prognosis. miRNAs have revealed their critical roles in regulating various cellular functions, including proliferation, apoptosis, differentiation, metabolism, and tumorigenesis, by targeting specific mRNAs.

MiR-146a regulates innate immunity, inflammatory responses, and the antiviral pathway. Changes in miR-146a expression have been noted in many human diseases, including autoimmune disorders and cancers. In addition, miR-146a is an important biomarker of disease diagnosis, prevention, and treatment. It is a negative regulator of inflammatory responses by suppressing target gene expression. However, limited data is available regarding the functional role of miR-146a in modulating the inflammatory response of periapical tissues following root canal disinfection in endodontic retreatment cases.

Eliminating microorganisms is the primary objective of root canal therapy in order to create an environment as free of bacteria as possible. However, not all root canal treatments are completely successful, and in some cases, the infection persists leading to development of secondary apical periodontitis in previously treated teeth.

Disinfecting the root canal during retreatment is significantly more challenging due to the presence of persistent microorganisms that have settled in the root canal system. These microbes are often resistant to standard irrigations and antimicrobial agents leading to the formation of periradicular lesions. Such bacteria can survive for extended periods around the previously filled root canals. Sodium hypochlorite (NaOCl) is the most widely used irrigant in root canal therapy due to its strong antibacterial effect and its ability to dissolve organic substances. Effective canal cleaning is difficult to achieve without the use of NaOCl at a sufficiently high concentration. However, NaOCl has several drawbacks, including its cytotoxicity which can lead to tissue damage and patient symptoms. Additionally, its strong oxidizing nature negatively affects the mechanical properties of dentin such as microhardness and elastic modulus. NaOCl should be used with caution in endodontic procedures to prevent hypochlorite accidents.

Calcium hydroxide (Ca (OH)2) is the most widely utilized intracanal medication. It has the potential to dissolve tissue, acts as a physical barrier and generates hydroxyl ions, creating an extremely alkaline environment. It has been shown to be quite effective in the treatment of teeth with persistent periapical lesions. To provide optimal endodontic treatment, the root canal system should be thoroughly cleaned of soft-tissue debris, smear layer, and bacteria. Nanoparticles (NPs) have unique characteristics such as smaller sizes, increased surface area to volume ratio, and higher chemical reactivity and charge density leading to greater interaction with the environment and negatively charged bacterial cells, compared to their bulk counterparts. These advantages can be used to design highly anti-microbial agents with maximal therapeutic efficacy and minimal side effects.