Differences in Niti and Glide Path Rotary System: Preparation of Canal Centering and Transportation in Double-curved Root Canals

1. INTRODUCTION

Root canal preparation is fundamentally designed to remove infected pulp and necrotic tissue in the root canal, thereby promoting the healing of pre-existing periapical lesions or preventing infection of periradicular tissues [1, 2]. Both conventional and modern endodontic instrumentation aim to thoroughly eliminate microorganisms and tissue debris by enlarging the canal to an adequate size, facilitating a root canal shape that allows a three-dimensional seal [3, 4]. The process of shaping and cleaning contributes to the success of endodontic treatment and is closely linked to disinfection and filling procedures [4].

Straight and uncurved root canals are rarely observed, as the majority of teeth display some degree of curvature in their root canals, with many showing multiple curvatures [5]. Zhang defined a double-curved root canal as one with more than one curvature, each exceeding 10 degrees. Yan et al. reported that 2.7% of double-curved root canals were found in the Western Chinese population [6]. The intricate anatomy of root canals increases the difficulty of shaping, potentially resulting in inadequate disinfection and procedural errors [7]. Dentin removal during root canal treatment poses the risk of straightening the root canal, eliminating curvature and ledge formation [8].

The primary goal of root canal instrumentation is to achieve preparation that maintains the root canal anatomy, ensuring that the foramen remains as small as possible without deviating from the natural canal curvature [9]. Various techniques and instruments have been developed to minimize canal straightening and root canal transportation [10]. Canal centering is an instrument’s ability to remain in the canal’s center during preparation, indicating that dentin removal in the area that has been prepared by the instrument is evenly distributed [11, 12]. Apical root canal transportation refers to the removal of the root canal wall structure on the outer curve of the apical half of the root canal due to the file’s tendency to return to its original shape during canal preparation [13]. This can result in the accumulation of debris and microorganism residues due to inadequate root canal cleaning and potential procedural errors such as zipping, ledging, or perforation. Wu et al. suggested that apical transportation exceeding 0.3 mm can reduce the sealing ability of filling materials and adversely impact treatment outcomes [14].

A glide path is defined as an unimpeded pathway from the canal orifice to its physiological ends, serving as the pathway for shaping files [15, 16]. Glide path preparation can be performed manually with stainless steel K-files or using rotary instruments [17]. However, K-files have disadvantages, including technique sensitivity, risks of canal transportation, alteration of the original canal anatomy, and increased debris extrusion [18]. Rotary glide path preparation, on the other hand, offers advantages such as reduced instrumentation time, better preservation of canal anatomy, decreased operator fatigue, and minimized apical debris extrusion [19].

Nickel-titanium (NiTi) rotary files have emerged as better tools for glide path creation [17]. The Proglider (PG) was launched in 2014, followed by the introduction of the Protaper Ultimate Slider (PS) in 2022. Both instruments utilize M-wire technology [20, 21]. Notably, PS features a shorter active area, fewer blades, and a parallelogram cross-section, whereas the Proglider is characterized by a square horizontal cross-section [15].

Rotary NiTi instruments have improved the efficiency of root canal shaping, outperforming stainless steel in flexibility and cutting efficiency, all while preserving canal geometry [22]. Gold treatments were introduced in 2014 when Dentsply Maillefer launched ProTaper Gold™ (PTG), an M-Wire instrument that undergoes a post-grinding heat treatment process. PTG system (Dentsply Maillefer, Ballaigues, Switzerland) is a heat-treated gold instrument, which is heat-treated after the file is made. PTG has convex triangular cross-sections, shaping files with progressive taper and finishing files with variable taper. This system includes 3 shaping files (#19.04, #18.02, dan #20.04) dan 3 finishing files (#20.07, #25.08, and #30.09]) [23, 24].

The Protaper Ultimate™ (PTUlt) system (Dentsply Maillefer, Ballaigues, Switzerland) represents a more recent innovation in gold heat-treated instruments. The shaping files have progressive tapers, starting at 5.5% at D4, increasing to 6% at D8, and then decreasing to 4.7% at D12 and 1.7% at D16. These files have cross-sections of 85° parallelogram from D0 to D8, and gradually increase to 80° parallelogram at D8 to D10 and remain at 80° from D10 to D16. Finishing files in this system exhibit specific taper profiles. F1 has a fixed taper of 7% from D0 to D3, which reduces to 6.5% at D4 and gradually decreases to 3.5% at D16. F2 tapers from 8% to 7%, then to 6.5% at D4, and further narrows to 3.3% at D16. F3 tapers from 9% to 8% [24, 25].

M-wire contains 508 Nitinol, which has been thermomechanically treated with specific temperature and tensile stress. M-wire is not completely austenite phase but has small amounts of martensite and R-phase depending on its manufacturing process [26]. M-wire exhibits higher flexibility, greater resistance to cyclic fatigue, and better mechanical properties compared to conventional NiTi [27, 28].

In 2018, United Dental Group, Shanghai, China, introduced the M3 Pro Gold (M3PG) system, which is a heat-treated gold Control Memory wire (CM wire) file. It has a non-cutting tip and a convex triangular cross-section. This system includes 5 files (#17.08, #20.04, #25.04, #25.06, dan #35.04). Control Memory Wire (CM-Wire) is NiTi with a lower nickel content (52%) compared to conventional NiTi. CM-Wire is preferred as a rotary instrument over conventional NiTi. It offers enhanced flexibility and resistance to cyclic fatigue while lacking the superelasticity of conventional NiTi [29].

Currently, there is a lack of research that directly compares the efficacy of the three leading systems, M3PG, PG and PTUlt, in terms of their ability to maintain canal centering and prevent root canal transportation. This study aimed to evaluate the capability of the aforementioned three systems after making a glide path using PG and PS in preserving canal centering and preventing root canal transportation.

2. MATERIALS AND METHODS

3. RESULTS

3.1. Canal Transportation

Transportation data analysis was conducted using a one-way ANOVA test. The results indicated significant differences between groups at points 0, 2, and 9 (p<0.05), so a post-hoc Tukey test was performed (Fig. 3). The post-hoc test at point 0 revealed significant differences between the PG+PTG group and the PG+PTUlt group. At point 2, significant differences were found between the PG+M3PG group and the PG+PTUlt group, as well as between the PG+Ult group and the PS+M3PG group. At point 9, significant differences were observed between the PG+M3PG group and the PG+PTUlt group. The results of the post-hoc test on transportation between other groups at other points did not show significant differences (Table 2).

Table 2.

Point of Measurement	Group		PG+PTUlt
Point 0	PG+PTG	Mean diff.	-0.081333
		Sig.	0.008*
Point 2	PG+M3PG	Mean diff.	0.111000
		Sig.	0.003*
	PS+M3PG	Mean diff.	0.093333
		Sig.	0.015*
Point 9	PG+M3PG	Mean diff.	0.111667
		Sig.	0.022*

Note: (*) indicates significance (p < 0.05).”

4. DISCUSSION

The primary objective of root canal preparation is to remove infected pulp tissue and necrotic tissue in the root canal. Another critical aspect is maintaining the original shape of the root canal while preserving healthy root dentin, ensuring a good long-term prognosis [2]. The process of shaping and cleaning are essential to the success of endodontic treatment, as they directly impact disinfection and filling procedures [4]. The complex anatomy of root canals makes shaping challenging, increasing the risk of inadequate disinfection and procedural errors such as canal transportation, ledges, zipping, and perforations [2]. Changes in the root canal morphology are often linked to unfavorable treatment outcomes [19].

The increased flexibility of NiTi instruments has contributed to shorter treatment times and reduced procedural errors, including zipping ledges and canal transportation. NiTi instruments have undergone various modifications, including heat treatments, to enhance their elasticity, flexibility, and resistance. NiTi alloys exhibit three distinct microstructural phases: austenite, martensite, and R-phase, depending on temperature. Austenitic NiTi is strong and rigid, while martensitic and R-phase NiTi are soft, elastic, and malleable. Thermomechanical treatment can stabilize the alloy in the martensitic phase, R-phase, or a mixed phase by changing transformation temperatures, thereby modifying the alloy’s characteristics [34, 35]. The gold color of Gold heat-treated instruments results from the heat treatment applied to the alloy and the titanium oxide layer that coats the instrument’s surface. This titanium oxide layer has a thickness of 100–140 nm [2, 27].

This study utilized three different types of gold-treated NiTi instruments. The M3PG system (United Dental Group, Shanghai, China) is a gold heat-treated file made of CM wire with a non-cutting tip and a convex triangular cross-section [29]. The PTG system (Dentsply Maillefer, Ballaigues, Switzerland) is also a gold heat-treated instrument, with heat treatment applied after the file is manufactured. PTG has a progressive taper and a convex triangular cross-section [24]. The PTUlt system (Dentsply Maillefer, Ballaigues, Switzerland) is a recently introduced gold heat-treated instrument. Its Shaper and Finisher files have an off-centered

The instrument has a parallelogram cross-section, while the other instruments in the system retain a convex triangular cross-section [25].

The final diameter of the files used in this study is size #25. Akhlaghi et al. reported that the size #25 files did not show a significant difference in bacterial reduction compared to files with larger apical diameters. However, Buchanan et al. stated that as the apical diameter increases, the risk of root canal transportation also rises [36].

Glide path files facilitate the enlargement of the root canal, improving shaping efficiency [31]. Recently introduced rotary NiTi files have proven to be more efficient and effective for creating glide paths. This study utilized two different glide-path files, PG and PS. PG is a rotary instrument made with M-wire technology and features a square horizontal cross-section. PS shares similarities with PG in tip size, surface finish, and helix angle. However, the PS has a shorter active area, fewer flutes, and a parallelogram cross-section, distinguishing it from the PG [15].

The superimposition method was used in this study to evaluate the shaping ability of each file group. This method was chosen due to its validity based on the body of evidence [31]. This study used double-curved endodontic blocks (#10.02, Dentsply Maillefer, Ballaigues, Switzerland). Acrylic resin blocks are validated experimental models for analyzing root canal preparation, offering standardized curvatures, and facilitating imaging before and after treatment [32, 37].

Extracted teeth were not selected in this study due to their non-standardized dentin hardness and irregular anatomy, which would hinder repeatable testing [30]. However, acrylic resin blocks have a limitation: the heat generated during preparation can soften the resin, increasing the risk of instrument fracture. Irrigation was performed after each file change, and recapitulation with a #10 file was conducted to reduce heat and clear resin debris [15]. Root canal transportation exceeding 0.3 mm is considered detrimental to the prognosis of root canal treatment. In this study, all groups exhibited canal transportation; however, none exceeded the 0.3 mm threshold [38].

This study found that the PTG file combined with PG was significantly better at preventing root canal transportation than the PTUlt file with PG at point 0. This aligns with findings from the study conducted by Sharawy et al., who reported that PTG caused less root canal transportation in the apical third compared to PTUlt. The M3PG file used with PG was significantly better at preventing transportation than PTUlt with PG at point 2; however, its performance was inferior at point 9. This is the first study to compare these files in terms of root canal transportation, and the differences between these two files can be attributed to variations in their alloy type, taper, tapering design, and cross-sectional shapes. PTUlt uses M-wire alloy, while M3PG uses CM-wire alloy. These findings are consistent with research by Biradar et al. and Kishore et al., who found that CM-wire files produced less transportation in the apical and middle thirds of the root canal compared to M-wire files [39, 40]. McSpadden et al. stated that instruments with greater flexibility are less likely to cause transportation [40]. This is supported by the statement of Pongione et al, that CM-wire has superior flexibility and resistance to cyclic fatigue compared to other superelastic NiTi technologies [41]. Additionally, Oh et al. observed that CM-wire retains almost 45º of curvature after being bent and released, whereas M-wire exhibits spring-back behavior after the external force is removed [42-44].

A smaller taper of .06 on the M3PG file may explain its superior ability to prevent root canal transportation in the apical third. Schafer et al. reported that increasing taper reduces instrument flexibility, thereby increasing the likelihood of root canal transportation [45]. On the other hand, the reduced taper from D4 to D16 in the PTUlt file may be the reason for its advantage in preventing transportation in the coronal third compared to the fixed taper of M3PG. In the three groups utilizing PS for glide-path preparation, no significant differences were observed across all measurement points compared to other groups.

Regarding the centering ratio, the M3PG file with PS demonstrated better centering ability than PTUlt with PG at points 2 and 6. While the centering performance of these two files has not been compared before, differences in alloy type, taper size, tapering design, and cross-section likely contribute to the variation. M3PG uses CM-wire alloy, while PTUlt uses M-wire alloy. These findings align with studies by Biradar et al. and Kishore et al., which found that CM-wire demonstrates superior centering ability in the apical and middle thirds of the root canal compared to M-wire [39, 40]. Oh et al. stated that CM-wire is better suited for curved root canals, while gold-treated M-wire performs well in straight or calcified canals [44]. This is attributed to the ability of CM-wire to adapt to root canal anatomy effectively, although it has a lower maximum torque load compared to M-wire [42]. The differences in glide path used in the two groups may also influence the results, as the two glide path instruments used have distinct cross-sectional designs. To date, no studies have compared the maximum torque load and cyclic fatigue of M3PG, PTG, and PTUlt files.

In this study, the use of acrylic resin has a limitation in the form of differences in hardness compared to natural teeth. Therefore, future studies can build on this research by using samples from natural teeth.

CONCLUSION

This study found significant differences in canal centering and root canal transportation. The M3PG file with PS demonstrated significantly better centering ability compared to PTUlt with PG in two-thirds of the root canal. Moreover, the M3PG file with PG showed significantly better performance in preventing root canal transportation than PTUlt with PG in the apical third of the root canal. However, in the coronal third, the performance of the M3PG file with PG was less favorable. In conclusion, M3PG can be a good choice in curved root canals, while PG and PtUlt can be a good choice in straight root canals or calcified root canals.

REFERENCES