The present micro-CT study evaluated and compared the shaping ability of 3 recent NiTi instruments with or without lateral brushing motion in oval canals. As expected, the canal volume, surface area, and SMI were significantly increased after canal preparation with the 3 tested systems, with or without brushing. The effects of brushing on oval canal shaping were observed only in a few parameters evaluated and depending on the instrument tested.
One of the most important parameters evaluated in micro-CT studies is the canal preparation performance in terms of the amount of prepared surface areas. This is because unprepared areas can exhibit residual bacterial biofilm and/or pulp tissue remnants [22, 23], which may negatively affect the treatment outcome [24]. Regardless of the different instrument types, alloys, geometrical designs, operation modes and the use of brushing strokes, no specimen showed all surfaces prepared. At full canal length, the amount of unprepared areas ranged from 26.9% (RaCe EVO) to 43% (Reciproc Blue), while in the apical canal the range was from 27% (VDW.Rotate/brushing) to 55.9% (Reciproc Blue). These figures are within the range previously reported in the literature [25]. Although the lateral brushing motion was proposed to improve root canal shaping by incorporating more areas in the final preparation shape, the present findings in oval canals revealed that it had no significant impact on the tested systems, either in the full canal or in the isolated analysis of the apical canal. The only exception was the Reciproc Blue instrument in the apical canal segment, for which lateral brushing strokes significantly increased the amount of prepared areas.
The Reciproc Blue R25 instrument is geometrically similar to the previous Reciproc instrument but comprises advanced technology, with increased flexibility and cyclic fatigue resistance, resulting from its manufacturing from a new heat treatment approach with the formation of a blue titanium oxide layer on its surface. The instrument has an S-shaped cross-section and studies have reported satisfactory results in its shaping ability [23, 26, 27]. In addition to improving apical preparation, the brushing motion affected other shaping parameters when Reciproc Blue was used. For instance, centroid shift evaluation revealed that brushing with Reciproc Blue caused more canal deviation than no brushing. On the other hand, when the remaining dentin thickness in the pericervical area was examined, Reciproc with no brushing led to more dentin removal on the mesial side. This may possibly be explained by the brushing motion possibly balancing the distribution of the cutting effect in the coronal part of the canal.
The VDW.Rotate instrument (VDW) has manufacturing similarities to Reciproc Blue. However, contrary to what was observed for Reciproc Blue, the use of brushing strokes resulted in no significant shaping effects in comparison with no brushing for the VDW.Rotate instruments. The main differences between VDW.Rotate and Reciproc Blue that may help explain the different results are related to the number of instruments used for preparation (Reciproc Blue is a single-file system), the direction of the cutting blades, and operation mode (VDW.Rotate is used in continuous clockwise rotation and Reciproc Blue in reciprocation). However, when compared to the other instruments, VDW.Rotate with brushing prepared significantly more surface areas in the apical 4 mm segment than Reciproc Blue with no brushing. Another possible reason for differences between VDW.Rotate and Reciproc Blue performances may be the different instrument dimensions along the master apical instrument shaft, that is, 30/.04 with constant taper for VDW.Rotate versus 25/08 with variable taper for Reciproc Blue.
The RaCe EVO instrument is made from a heat-treated Max-Wire NiTi alloy, with a triangular cross-section design, electropolished surface, and alternate cutting edges. It is recommended for use in continuous clockwise rotation. Application of the brushing motion significantly increased the canal surface area in the full canal length compared with no brushing. Brushing with RaCe EVO also resulted in significantly less remaining dentin at the region of pericervical dentin when compared with no brushing. Compared with the other instruments for unprepared surfaces, RaCe EVO with no brushing performed significantly better than Reciproc Blue with no brushing in both the full canal length and the apical canal segment. RaCe EVO with brushing was also significantly better than Reciproc Blue with no brushing in the apical canal. These findings may be possibly explained by the triangular cross-section with alternating cutting edges, the instrument dimensions, and the higher rotation speed applied to RaCe EVO.
All instruments seemed to produce conservative and safe preparations. An increase in total canal surface area from only 8–15.5% was observed for the 3 instruments tested, with or without brushing. Moreover, no instrument fracture was observed in this study, which is in agreement with other studies [16–18] that showed that brushing may be a safe procedure in terms of risk of instrument breakage. The brushing motion may allow stresses to be distributed along the instrument axis, preventing stress concentration on the same areas.
Another factor related to the safety of canal preparation is the remaining dentinal thickness, especially at the pericervical area, which is the area located between 4 mm coronal and 4 mm apical to the bone crest and believed to play a crucial role in transferring occlusal forces across the root [28]. As expected, the dentin thickness in the mesial and distal walls was significantly reduced in both the CEJ and 4 mm apically in all groups. It has been recommended that, ideally, 1 mm of coronal root dentin thickness be preserved around the prepared canal [29–32]. However, the literature also reports an arbitrary value of 0.3 mm as the minimum dentin thickness that should remain after instrumentation to avoid perforation and prevent root fracture [33, 34]. In the present study, even in specimens showing preoperative pericervical dentin thickness less than 1-mm thick in the mesial or distal aspects, the mean postinstrumentation figure was always greater than 0.8 mm, with only one specimen showing 0.3 mm (minimum value). It was also found that brushing did not substantially affect the thickness of the pericervical dentin. These findings suggest that preparations with the test instruments and sizes, with or without brushing, did not significantly compromise the root structure and possibly the fracture resistance.
An important strength of this study was that the tooth specimens were paired based on their anatomic features as evaluated by micro-CT imaging. The absence of statistically significant differences in the canal volume, surface area, and SMI between groups confirmed that pairing effectively provided a homogeneous sample, helping to reduce potential anatomical biases that might interfere with the results [35]. Limitations of this study include its ex vivo nature and the fact that it was limited to only oval canals of mandibular incisors.
In conclusion, the present study revealed that the brushing motion had minor effects on the overall shaping performance of the 3 instruments tested. One exception was the increase in prepared surface area in the apical canal segment when applied to the Reciproc Blue instrument. Further studies should evaluate the impact of the lateral brushing motion in enhancing root canal disinfection, especially in recess and isthmus areas of oval/flattened root canals.