## 4 Discussion  
Brushing movement can be adopted after glide path to carry tapered files against the outer wall of the coronal curvature in order to obtain straight access to the apical third. It may be especially useful for the mesial canals of mandibular molars where it is intentionally directed against the outer side of  
Figure 1: Micro-CT scan matching of pre- and post-shaping sections, according to previously selected cutting planes. Note the difference between pre-operative (green) and post-shaping with WaveOne Gold Primary (red) specimens at the fraction (F), 1.5 mm apically from the function (1.5F), 3 mm apically from the function (3F), and in correspondence with the point of maximum curvature (M). B = brushing group; NB = no-brushing group. Figure 2: Micro-CT scan matching of pre- and post-shaping volumes from a coronal (**a**) and lingual (**b**) projection. Note the difference between pre-operative (green) and post-shaping with WaveOne Gold Primary (red) volumes. In this representative sample the mesial root was shaped with (ML) and without (MB) brushing motion in the two different root canals. Note the wear at the furcation level “danger zone” especially in the ML canal. the curvature where the dentine is thicker. This procedure should ease the use of shaping instruments up to WL and subsequent root canal filling, when using the continuous wave technique [18,23]. However, the instrument's cutting action is not perfectly selective, and more aggressive shaping may lead to a weakening of the inner dentine walls [23,24]. The dentine in the mesial root of mandibular molars is thinner at the distal surface of the root, 1.5 mm below the furcation, which is generally only 1.2 mm thick [25, 26, 27]. Therefore, preservation of the original canal anatomy, without weakening the radicular structure, may lead to more favorable outcomes, in particular for the mesial root of the lower molars [4,24]. Moreover, the use of carrier-based techniques or single cones with bioceramic sealers could allow three-dimensional (3D) filling, even with low-tapered preparations [28,29]. This study evaluated the cutting efficiency and centering ability of WaveOne Gold Primary reciprocating single files used with or without brushing motion. Intentional brushing movement during shaping with the WaveOne Gold single-file technique led to a higher centroid shift and greater dentine removal from the inner aspect of the root canal, especially in the coronal third. The evaluation of post-instrumentation geometrical variations is a reliable assessment of the ability of a shaping technique to preserve the canal anatomy [21]. An extracted tooth model is generally considered transferable to the clinical situation due to the similarity of the experimental conditions [30]. Homogeneity of pre-operative characteristics among selected samples is essential to ensure adequate standardization [24,31]. Micro-CT scans enable the identification of morphological changes associated with different biomechanical preparations, including canal transportation and dentine removal [20]. Moreover, the overlapping of pre- and post-operative root canal volumes allows for 3D quantitative analysis [1]. This study considered four levels of analysis in relation to the furcation area of the mesial root of mandibular molars where the dentine is thinner [27]. The limited amount of dentine at the furcal side of the mesial root of mandibular first molars means that this can be considered a \"danger zone\" [25,27]. Previous studies have reported that centering ability of the instruments is influenced by their design, the type of alloy, and the root canal anatomy [31,32], but no information is available about the effect of brushing motion on the shaping outcomes of single-file reciprocating instruments. The reciprocating motion helps to keep the instrument centered, promoting equal cutting force on the inner and external sides of the curvature [9,10,33].