Comparison of canal transportation and centering ability of manual K-files and reciprocating files in glide path preparation: a micro-computed tomography study of constricted canals

The term “glide path” has been addressed in endodontics since the early 2000s, referring to a smooth tunnel pathway from the canal orifice toward the physiologic terminus [1]. Glide path establishment has been considered a crucial and initial step for safer and efficient biomechanical root canal preparation [2‐4]. It could secure an open pathway to the canal terminus that subsequent instruments can follow [1]. Establishing a glide path may significantly reduce the risk of canal modifications like canal transportation, ledging, and zipping, also prevent rotary instrument separation by decreasing torsional stress to a certain extent [4‐6]. In narrow, tight, and calcified canals, canal negotiation and glide path preparation may face a significant challenge; meanwhile, the previously mentioned procedural errors are more prone to occur [7]. Cvek et al. [8] not surprisingly found that irretrievable instrument separation mostly happens in constricted and calcified root canals.

Instruments like stainless steel #10–#20 K-files have been recommended to establish a glide path, with a manual watching-winding or balanced force motion [5, 6]. However, establishing a glide path with stainless steel files in such manner could be very time consuming, especially in advanced cases; also, it has many limitations and drawbacks, including an increased incidence of canal transportation [9, 10]. To overcome some of the problems with manual motion, for faster preparation and superior anatomy preservation, NiTi rotary glide path files with continuous rotation have been introduced [11, 12]. Yet in calcified and constricted canals, the screw-in effect of continuous rotation motion may cause instantaneous torsional overload, leading to file breakage [3].

A slow-speed handpiece combining alternate reciprocation and rotating reciprocation, the “Optimum Glide Path (OGP)”, was introduced by J. Morita (Osaka, Japan). The OGP kinematics mainly aims to mimic the watch-winding and balanced force motion [13]. Briefly, file motion starts with alternate movement, with a same angle followed by reciprocating movement in which the counterclockwise angle is larger than the clockwise (90°/120°, 180°/270°, and 240°/330°). Then OGP kinematics continuously alternates between the alternate and rotating reciprocation motions. OGP with the smallest angular increment has shown the highest mean time-to-fracture when compared with continuous rotation or other rotation angles, indicating a much safer use in difficult cases [14]. However, the performance of OGP motion regarding canal transportation and centering ability in constricted canals still remains unclear. Therefore, the purpose of this study was to investigate and compare manual and OGP movement in terms of canal transportation and centering ability in glide path preparation of constricted canals.

Nine MB and six ML canals were included in group 2, and group 1 contained six MB and nine ML canals. Group 2 showed a significant lower transportation value than group 1 at both 1-mm and 3-mm cross-section levels (P < 0.05), while the difference at 5-mm level was not significant (Fig. 2). Regarding to the ability of maintaining within the central axis of the root canal, the differences between the OGP reciprocating motion and manual preparation at all three cross-sections studied were not statistically significant (Fig. 3).

A total of six #10 K-files were severely distorted in group 1, at the apical 1 to 3-mm of the files; whereas no file distortion or separation was found in group 2.

The present study revealed that OGP motion has a significant advantage and positive impact on the preservation of apical canal anatomy during glide path establishment in constricted canals, with significant less canal transportation and file distortion.

It is essential to address the differences between a glide path establishment and maintaining apical patency, in terms of their major goals and procedures. Glide path development is an enlargement to a size approaching the subsequent rotaries’ tips, at least larger than the file’s core diameter, therefore to ensure a safe and smooth passage [1]. The concept of apical patency can be defined as ‘the repeated penetration of the apical foramen with a small file during instrumentation to prevent the accumulation of debris and leave the foramen unblocked, i.e., patent’ [21]. The patency file should be smaller than the instrument that binds to the foramen, to be effective in offering a lesser risk of extruding toxic products and dentin fragments into periapical space [21]. In most of the cases, a size #8 file taken 0.5–1 mm long to establish patency contacts the desired endpoint of the preparation with a diameter approaching the tip size of a #10 file [1].

American Association of Endodontists defines transportation as ‘removal of canal wall structure on the outside curve in the apical half of the canal due to the tendency of files to restore themselves to their original linear shape during canal preparation’ [22]. Our study showed that MGP super-files together with OGP 90° motion created significantly less canal transportation in the apical 3 mm of constricted canals than traditional K-flies. Wu et al. [23] stated that apical transportation of more than 300 μm may have a negative effect on the seal of root fillings. Canal transportation can lead to inappropriately dentin removal, with a higher risk of canal curvature modification and ledge formation. Many factors may contribute to certain degrees of canal transportation, such as motion type, canal morphology, design, and alloy of the instrument [24]. According to a new classification scheme proposed by Gambarini et al. [14], OGP is a combined motion of alternate and rotating reciprocation. Alternate reciprocation can be considered the safest motion, as it features clockwise = counterclockwise (usually less than 90°) and does not result in a full rotation. Whereas one angle of motion is larger than the other in rotating reciprocation, like Reciproc (150°/30°, Dentsply VDW, Munich, Germany) and WaveOne (170°/50°; Dentsply Maillefer) [25], and a series cycles ultimately form a full rotation inside the canal. It’s worth noting that the rotating effect given by this difference between clockwise and counterclockwise movements is important since it maintains the cutting efficiency and apical progression; moreover, this asymmetry in rotating reciprocation has the advantage of limiting the angel of rotation in the cutting verse under the endurance limit of the instrument [25]. Our finding indicates that the combination of watch-winding, and balanced force motion of OGP certainly has easier and better control during glide path preparation; whereas manual motion requires much more effort to maintain the original anatomy.

The mean centering ratio is a measure of the ability of the instrument to stay centered in the canal. A previous study revealed that reciprocating motion may promote the canal centering ability, and reduce the risk of root canal deformity [26]. Our data demonstrated similar findings, as OGP has a better centering ratio in apical 5 mm, however the results were not statistically significant.

Endodontic instrument separation may be created as a result of two distinct modes or their combination. While cyclic flexural fatigue may happen when continuous tension–compression cycles generate in curved canals overtime, torsional failure occurs when the file tip gets locked in the calcified canal; however, file shank continues to rotate [27]. Our study demonstrated a notable difference between the two experimental groups. As each instrument investigated was used to prepare only 2 canals (as in one mesial root), not a single MGP super-file under OGP 90° motion distorted or broke, while there were 6 K-files severely distorted in group 1. Several underlying mechanisms might contribute to this phenomenon. The combined alternate + rotating reciprocation movement acts to reduce the stresses of taper lock and screw-in effect, minimizes torsional and flexural stresses on the instrument, thus decreases the chance of instrument breakage [28, 29]; moreover, studies have shown that rotation angle and angular increment are related to the distribution of stress [30, 31]. OGP 90° has better cyclic fatigue and torsional resistance than OGP 180°, OGP 240°, and continuous rotation motion [14]. The up-and-down pecking motion of the handpiece with 1 mm short pecking depth may also lower the torsional stress accumulation and overload during glide path establishment since the contact time, and binding area between the instrument and dentin walls become short and small [32, 33].

Further research is still in need to evaluate other aspects of OGP motion using NiTi instruments, with comparison not just to manual motion but more importantly to continuous rotation and other single rotating reciprocation motions, so that we can have a complete understanding and clinical approach regarding the glide path establishment in constricted canals.

It can be concluded that OGP reciprocating movement performed significantly less canal transportation (apical 3 mm) and file distortion during glide path establishment in constricted canals comparing to manual motion, while the centering ability between the two was similar. These favorable results of OGP movement indicate its potential application as a viable alternative to manual motion in glide path preparation of constricted canals, yet further research is needed to compare this novel motion with other rotary movements and on other parameters.