Challenges in long-oval canal cleaning: effects of instrumentation kinematics and ultrasonic irrigation.

The present study aimed to evaluate, using micro-CT imaging, the preparation of long-oval root canals performed by Twisted File Adaptive (TFA) and Reciproc (RC) instruments and the cleaning efficacy of final ultrasonic irrigation (UI) using a correlative scanning electron microscopic (SEM) approach. Distal long-oval canals of thirty-eight lower molars were divided into two groups (n=19): TFA and RC groups. Each specimen underwent three scans: pre- and post-instrumentation and post-UI, and changes in volume, non-prepared surfaces, and accumulated hard-tissue debris (AHTD) were measured. Then, root canals were halved at the apical third and analyzed under SEM for smear layer (SL) evaluation. Mann-Whitney and Kruskal-Wallis tests were used for statistical analysis (p<0.05), and the correlation between the AHTD and the SL was evaluated with Pearson's correlation coefficient. The TFA system exhibited greater efficiency in dentin removal compared to the RC system when considering the entire canal (p<0.05), with approximately 26 to 29% of surface area unprepared for the whole canal (p>0.05). Both systems produced the same volume of AHTD (p>0.05). Final UI led to a notable reduction in AHTD (60-70%), except in the apical third. Qualitatively, SEM analysis revealed non-uniform dentin remnants covering the canal walls, particularly prominent in the RC group. The data analysis determined a strong correlation between AHTD and SEM evaluation. In conclusion, neither system achieved complete preparation of long-oval root canal walls, and the final UI reduced AHTD. The AHTD observed by micro-CT was related to the presence of SL evaluated by SEM.