###### Abstract  
Aim of the study: to evaluate root filling remnants after secondary root canal treatments (SRCTs) of oval-shaped canals with X-rays and cone beam computed tomography (CBCT). The SRCTs were performed using reciprocating NiTi instruments. Methods: Single-rooted teeth (N = 64) were randomly treated with Reciproc Blue (RB) and filled with AH Plus/single cone (SC group) or AH Plus/Guttafusion (GF group). After seven days of storage in HBSS (Hanks balanced salt solution), Gates Glidden burs #2/3 and RB #25 and #40 were used for the SRCTs. The time to complete the procedure was measured. X-rays and CBCT were used to calculate, respectively, the area and the volume occupied by the remnants in the coronal, middle, and apical thirds of each canal. Environmental scanning electron microscopy (ESEM) and energy dispersive X-ray spectroscopy (EDX) were used for qualitative evaluation and morphology composition of the remnants in sectioned roots. A statistical analysis was performed using Sigma Plot (version 13, IBM, Armonk, NY, USA). The study was designed according to PRILE guidelines. Results: After the SRCTs, the middle thirds of the root canals showed the presence of remnants in both groups, as demonstrated by X-rays and CBCT. The GF group showed a statistically significant higher volume of remnants than the SC Group only in the middle third. The ESEM supported by the EDX revealed the remnant composition by the detection of trace elements of sealer and gutta-percha in all root canals. Conclusion: The study demonstrated that the middle third of root canals is a critical region where remnants were packed and spread in the buccal-lingual sides of canals. ESEM-EDX detected a fine layer of filling remnants in all root thirds, suggesting a larger canal contamination than the X-rays and CBCT examinations revealed. CBCT; endodontic retreatment; root filling remnants; Guttafusion; NiTi endodontic instruments; periapical X-ray; Reciproc Blue; Reciprocating NiTi instruments; root canal retreatment; single cone technique +
Footnote †: journal: Medical Imaging  
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Reciprocating System for Secondary Root Canal Treatment of Oval Canals: CBCT, X-rays for Remnant Detection and Their Identification with ESEM and EDX  
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Reciprocating System for Secondary Root Canal Treatment of Oval Canals: CBCT, X-rays for Remnant Detection and Their Identification with ESEM and EDX  
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Reciprocating System for Secondary Root Canal Treatment of Oval Canals: CBCT, X-rays for Remnant Detection and Their Identification with ESEM and EDX  
Acwith flexible NiTi instruments in continuous rotation. An ineffective instrumentation leads to the incomplete removal of the smear layer and root canal remnants of previous treatments. As a consequence, insufficient root canal disinfection and a low-quality obturation are obtained, hindering long-term root canal success [11]. Reciprocating motion was introduced some years ago for root canal treatments. This movement allows us to overcome some of the limits of traditional NiTi instruments and improve their mechanical properties [14]. The clinical criteria to determine the quality of SRCTs are the absence of materials along the visible root canal walls and along the spires of the instrument, the absence of remnants during the irrigation phases, and the tactile perception of smooth canal walls. Periapical X-rays are the most common method to assess the presence of remnants [15], but they potentially might not identify all remnants in retreatment procedures [16]. More recently, CBCT has also been used to allow an accurate 3D evaluation of the remnants [17]. Environmental scanning electron microscopy connected to energy dispersive X-ray spectroscopy (ESEM-EDX) allows us to observe wet samples without any preparation or metallic coating [10]. New carrier-based systems are more frequently used to fill the root canal in association with epoxy-based sealers [18]. These systems are faster and easier to use for filling root canals [19].