The effect of canal geometry on endodontic irrigation and Enterococcus faecalis retention

Abstract

Introduction: Necrotic teeth are considered a harbour for a wide and diverse microbial population. This bacterial population thrives and multiplies within the infected root canal system, which can lead to persistent infection and complications. The current clinical approach of minimally invasive root canal preparation prioritizes tooth structure preservation while trying to manage the infection as efficiently as possible with new advancements in materials, equipment, and techniques. Residual bacteria left behind after the root canal preparation can hinder the healing process. Addressing residual bacteria remains a critical aspect of achieving successful root canal treatment outcome. Aim: to assess the effect of different root canal preparation geometries and their impact on intraradicular bacterial reduction. Objectives: − Conduct a literature review to establish an understanding of the current evidence in regard to root canal preparation apical size and taper and to evaluate the methodological differences between studies. − Assess the correlation between root canal geometry and bacterial load during needle irrigation. − Identify a suitable, achievable, and consistent experimental model for microbiological assessment of the desired outcome. − Evaluate the feasibility of using confocal laser scanning microscopy (CLSM) to evaluate and quantify the amount of live/dead bacteria with the selected experimental model. − Assess potential confounding factors that might impact the irrigation process efficacy in small apical preparations ISO 20 and larger canal geometries. − Evaluate the effect of multiple root canal geometries produced by different apical size preparations with different standard tapers on microbial load reduction. Methods: The first part of this research project involved conducting a review of the existing literature relevant to the subject matter. The second part involved a pilot study to assess any potential trend or correlation between the root canal geometry and the reduction of Enterococcus faecalis bacterial load during needle irrigation. The remainder of the second part focuses on optimizing the experimental protocol of this research project and identifying a feasible incubation period that yields optimal and consistent results. Additionally, this part assesses the suitability of the samples used in this research project (simulated canals in resin block and 3D printed teeth) for confocal microscopy analysis to quantify Live/Dead bacteria in addition to culturing techniques. The third part involves assessing multiple confounding factors that can potentially enhance the disinfection of small and large canals during the irrigation process. The methodology was designed to evaluate a single confounding factor at a time and statistically assess its significance on the irrigation process in small and large geometries produced by WaveOne® Gold reciprocating files (Dentsply Sirona, USA). 3 The last part of this research project aims to assess the effect of two key aspects of the canal geometry —the canal apical size and taper— on the retention of E. faecalis during needle irrigation. This part evaluates nine different canal geometries, each with a standard taper produced by K3 XF rotary files (Kerr, USA), which are relevant to the current clinical practice. Results: Within the limitations of this research project conducted in simulated root canals in resin blocks and 3D printed teeth, a correlation was observed between larger canal geometries and greater log reduction of the microbial load during needle irrigation. In the same geometry, irrigation activation with either manual dynamic activation or passive ultrasonic irrigation showed a trend toward greater log reduction of E. faecalis load compared with a needle irrigation technique, but it wasn’t statistically significant. Irrigation activation statistically significantly improved the disinfection in large canals compared to small canals. Increasing the irrigation volume and time and increasing the needle insertion depth during needle irrigation showed a trend toward more log reduction, but they weren’t statistically significant. Needle shape and gauge had no significant impact on the microbial log reduction when maintained at the same insertion distance from the canal's full working length. Among the nine geometries tested in this study, canal apical size was statistically significant in causing greater microbial load reduction during needle irrigation. The apical sizes of ISO 25 and 30 were statistically significant in reducing the microbial load compared to an ISO 20 apical size. There was no statistical difference between ISO 25 and 30 apical preparation sizes. Additionally, there was no statistically significant difference between 4%, 6%, and 8% canal preparation taper in reducing the microbial load during needle irrigation. Conclusions: The literature shows a correlation between bacterial load reduction and apical periodontitis healing. Also, it has been shown that apical healing can occur even when bacteria are present at the time of the obturation. Healing in these cases seems to depend on the number and virulence of bacteria, as well as the ability of the residual bacteria to communicate with the periradicular tissues. It is crucial to reduce these factors to subcritical levels that hinder the intraradicular microbial biofilm's ability to sustain inflammation. It is important to enlarge the apical segment of the root canal enough to allow for better cleaning and disinfection while taking into consideration the structural integrity of the compromised teeth being treated, and the anatomy of each canal to avoid over enlargement and procedural errors. The results of this thesis showed that apical preparation to a minimum of ISO size 25 is sufficient to reduce the microbial load using needle irrigation. Clinicians should aim to enhance the irrigation process by employing techniques and equipment that enhance the irrigant penetration and flow, such as irrigation activation, especially in canals with smaller apical size preparation.