Comparative evaluation of the effectiveness and fracture rate of three pathfinding nickel-titanium rotary instruments, Mtwo, OneG, and ProGlider, in mechanically negotiating moderately curved molar canals to the full working length.

AIM: To evaluate the scouting ability of three pathfinding nickel-titanium rotary instruments in moderately curved molar canals.
MATERIALS AND METHODS: Ninety maxillary and mandibular molars were collected and were divided randomly into three groups each having 15 maxillary and 15 mandibular molars. The teeth were mounted on a specific apparatus for simulation of dento-alveolar socket. After access cavity preparation, the canals were negotiated with 08 K-file. Glide path preparation was performed using Mtwo or ProGlider or OneG instruments. The number of teeth in which the file reached the full working length (RFWL), number of passes required, and number of fractured instruments were recorded and subjected to statistical analysis using Chi-square, Kruskal-Wallis, and Mann-Whitney tests.
RESULTS: There was a significant difference between the groups (P < 0.05). The RFWL of ProGlider was significantly higher than that of Mtwo (P = 0.005) and OneG (P = 0.037). The percentage of fracture with ProGlider was 6.67% and both Mtwo and OneG had 26.67% frequency. The difference was not statistically significant (P > 0.05). There was significant difference in the number of insertion passes in maxillary distobuccal (P < 0.05), mandibular mesiobuccal (P < 0.01), and distal (P < 0.01) canals. ProGlider was the most effective pathfinding instrument due to its file's design and a progressive taper of 2%-8%. This achieves a greater preflaring of the coronal and middle portions of the root canal, rendering the advance of the instrument toward the apex easier.
CONCLUSION: ProGlider performed more efficiently and with less instrument breakage in scouting moderately curved canals of molar teeth.

INTRODUCTION

The goal of root canal instrumentation is to obtain a continuous tapering funnel flowing with the shape of the original canal from the coronal access to the apex.[1] It is an important step of the treatment as it influences the efficacy of subsequent procedures in endodontic therapy.[12] Most of the procedural problems associated with achieving ideal shaping of curved canals were due to the stiffness of stainless steel instruments.[3] Even though nickel-titanium (NiTi) instruments are stronger and more flexible than their stainless steel counterparts, fractures may still occur within their elastic limit.

Fracture is the most common complication that occurs during the use of rotary NiTi instruments,[4] and the fear of fracture is the biggest restraint to the adoption of the technology by clinicians.[5] NiTi instruments fracture mainly because of bending normal stresses (failure by fatigue) and torsional shear stresses (failure by torque). The risk of taper lock might be reduced by performing coronal enlargement[6] and creating a glide path before using NiTi rotary instrumentation, both manual and mechanical.[7] Absence of glide path establishment may result in ledge formation, canal blockage, transportation, zip formation, or perforation.

Preflaring and canal scouting are the crucial steps where the clinician might more commonly encounter procedural difficulties.[8] Glide path creation is an important consequential clinical step that influences the successful fulfillment of the mechanical and biological goals for shaping canals. Glide path in endodontics is defined as a smooth, though possibly narrow, tunnel or passage from the coronal orifice of the canal to the radiographic terminus or electronically determined portal of exit.[9]

It is a refinement of the original canal anatomy, which allows a safer passage of further used mechanical shaping instruments. Glide path creation has been widely recommended as a compulsory clinical step to improve the safety and efficiency of rotary NiTi instruments by preventing the taper lock phenomenon, diminishing fracture rates, and preventing shaping errors, thus increasing the instrument's life span.[10]

Berutti et al. advocate that the diameter of the canal after glide path preparation should be at least one size larger than the tip of the first rotary file used to prepare the canal.[7] West recommends a minimum of a “super loose” size 10 K-file.[9] The international protocol for glide path management is to further expand the pathway to an apical size of at least 0.15 mm. When a size 15 hand file is at length, sufficient space exists to safely accommodate the tip of the first mechanical (reciprocating or rotary) shaping file.[11]

A dedicated mechanical glide path file may be used to expand or preshape any given canal prior to utilizing greater tip diameter and tapered shaping files. Preshaping secured canals with devoted files improves the safety and efficiency of all shaping files that have significantly larger D0 diameters and tapers along their active portions. A variety of dedicated mechanical glide path files are marketed over the years. Most of them are either a 2- or 3-file sequence, in which each file has a traditional, fixed tapered design over the length of its active portion. Recently, single mechanical glide path files were introduced such as ProGlider (DENTSPLY Tulsa Dental Specialities, Dentsply International, Inc. Dr Johnson city TN, USA) and OneG (Micro-Mega SA rue du Tunnel, BESANCON FRANCE).

Despite the considerable amount of research conducted on root canal preparation, the literature lacks studies comparing the efficacy of different pathfinding systems in reached the full working length (RFWL). Therefore, the purpose of this study was to compare the effectiveness and fracture rate of three pathfinding NiTi rotary instruments (Mtwo 10/.04, OneG 14/.03, and ProGlider 16/.02) in mechanically negotiating moderately curved molar canals to the full working length. This study was conducted to compare the scouting ability of Mtwo, OneG, and ProGlider pathfinding instruments in moderately curved root canals of molar teeth.

MATERIALS AND METHODS

Freshly extracted maxillary and mandibular first and second molars were collected, cleaned, and stored in distilled water. Preoperative digital radiographs were taken from different angles for each tooth to assess the morphology and internal anatomy of tooth. Only mandibular molars with a single distal canal were included, whereas any existing second canal in the mesiobuccal (MB) root of the maxillary molars was excluded from the experimental procedures.

Inclusion criteria

Maxillary and mandibular first and second molars

Moderately curved root canals.

Exclusion criteria

Carious teeth

Double distal canal in mandibular teeth

Severe root curvature.

The curvature angle of the canals was determined according to the method described by Schneider.[12] Canals were considered straight if canal angle was <5°, moderately curved when angle was 5°–20°, and severely curved when angle was >20°. Forty-five mandibular and forty-five maxillary molars with moderately curved canals were selected. These teeth were randomly divided into three groups. Each group had 15 maxillary and 15 mandibular teeth.

After gaining root canal access, each tooth was mounted on a particular apparatus that simulated the alveolar socket and allowed connection of the metal lip clip of an apex locator. This apparatus was fabricated by pouring alginate in an appropriate sized plastic container. The tooth was mounted along with a metal pin, and alginate was allowed to set, and then the metal pin was removed. This created space for attachment of the metal lip clip.

Glide path preparation

Canals were negotiated with an ISO size 08 K-file. Working length radiograph was taken using no. 10 K-file. The apex locator (Dentsply Maillefer Ballaigues Suisse Switzerland) file holder was clipped to the upper part of the selected pathfinding instrument mounted on a handpiece, powered by an electric endo motor.

Glide path preparation was then performed in respective groups using Mtwo (10/.04, 280 rpm and 1.2-Ncm torque), or ProGlider (16/.02,300 rpm and 5-Ncm torque), or OneG (14/.03, 250–400 rpm) instruments, in full clockwise rotation, using a gentle in-and-out motion of about 1- to 2-mm amplitude. After every three insertion passes, the instruments were cleaned, and the root canals were irrigated with 2 mL of 3% sodium hypochlorite between each step. This was attempted in all the canals. RFWL was confirmed when the apex locator emitted a continuous sound alert.

When resistance was encountered, the auto-reverse mode of the motor was activated automatically, preventing the instrument from being advanced any further, indicating NOT RFWL. Intraoral periapical radiographs were taken. This preparation procedure was performed in all the groups [Figures 1 and 2]. Observations such as the number of teeth in which RFWL was achieved and RFWL was not achieved, number of passes required to RFWL, and number of fractured instruments were recorded, tabulated, and subjected to statistical analysis using Chi-square, Mann–Whitney, and Kruskal–Wallis tests.

Figure 1

Radiographic images of molars with root canals classified as reached full working length

Figure 2

Each tooth was mounted on a particular apparatus

RESULTS

In Mtwo, OneG, and ProGlider, 16 (53.33%), 19 (63.33%), and 26 (86.67%) instruments RFWL, respectively. There was a significant difference between the groups for the number (%) of pathfinding instruments that RFWL (P = 0.018). In pair-wise comparison, there was no significant difference (P = 0.432) between Mtwo and OneG for the number of files that RFWL. The number of RFWL files in ProGlider was significantly higher than that of Mtwo (P = 0.005) and OneG (P = 0.037). There was no significant difference in maxillary (P > 0.126) and mandibular (P > 0.177) teeth that RFWL. There was no significant difference in maxillary MB (P = 0.735), distobuccal (DB) (P = 0.452), and palatal (P = 0.555) canals that RFWL, as well as in mandibular MB (P = 0.126), mesiolingual (ML) (P = 0.086), and distal (P = 0.669) canals that RFWL [Tables 1, 2 and Figure 3].

Table 1

Group-wise, arch-wise, and canal-wise comparisons of number (%) of pathfinding instruments namely MTwo, OneG, and ProGlider that reached the full working length and of fractured instruments

Types and canals	RFWL				Fracture
	MTwo, n (%)	OG, n (%)	PG, n (%)	Chi-square test	Mtwo, n (%)	OG, n (%)	PG, n (%)	Chi-square test (P)
Teeth with all canals classified as RFWL	16 (53.33)	19 (63.33)	26 (86.67)	P=0.018*PairwiseM Two and OG (P=0.432)Mtwo and PG (P=0.005**)OG and PG (P=0.037*)	4 (26.67)	4 (26.67)	1 (6.67)	0.564
Tooth type
Maxillary	10 (66.67)	10 (66.67)	15 (100)	P=0.126	3 (20.00)	2 (13.33)	0 (0.00)	0.495
Mandibular	6 (40)	9 (60)	11 (73.33)	P=0.177	1 (6.67)	2 (13.33)	1 (6.67)	0.966
Maxillary canals
MB	13 (86.67)	13 (86.67)	15 (100)	P=0.735	1 (6.67)	1 (6.67)	0 (0.00)	0.937
DB	12 (80)	12 (80)	15 (100)	P=0.452	2 (13.33)	1 (6.67)	0 (0.00)	0.669
Palatal	15 (100)	13 (86.67)	15 (100)	P=0.555	0 (0.00)	0 (0.00)	0 (0.00)	-
Mandibular canals
MB	8 (53.33)	10 (66.67)	14 (93.33)	P=0.126	0 (0.00)	1 (6.67)	0 (0.00)	0.880
ML	9 (60.00)	14 (93.33)	14 (93.33)	P=0.086	1 (6.67)	0 (0.00)	0 (0.00)	0.880
Distal	15 (100)	14 (93.33)	13 (86.67)	P=0.669	0 (0.00)	1 (6.67)	1 (6.67)	0.937

*Significant P<0.05. OG: OneG, PG: ProGlider, RFWL: Reached the full working length, MB: Mesiobuccal, DB: Distobuccal, ML: Mesiolingual, **Highly significant

Table 2

Comparison of number of insertion passes for reached full working length in maxillary and mandibular canals between groups

Groups	Maxillary						Mandibular
	MB		DB		Palatal		MB		ML		Distal
	Mean±SD	Minimum-Maximum	Mean±SD	Minimum-Maximum	Mean±SD	Minimum-Maximum	Mean±SD	Minimum-Maximum	Mean±SD	Minimum-Maximum	Mean±SD	Minimum-Maximum
MTwo	3.38±1.19	2-6	3.17±0.58	2-4	1.80±0.56	1-3	4.38±1.19	3-6	4.00±1.23	2-6	2.47±0.64	2-4
OG	2.62±0.87	1-4	2.58±1.00	1-4	2.00±0.58	1-3	2.60±1.17	1-5	3.14±1.10	2-5	2.07±0.27	2-3
PG	2.80±1.32	1-6	2.33±0.72	1-4	1.73±0.88	1-4	3.29±0.91	2-5	3.36±0.93	2-5	1.77±0.60	1-3
Kruskal-Wallis test (P)	0.190		0.013*		0.349		0.009**		0.210		0.008**
Mann-Whitney U-test (P)
MTwo and OG	-		0.095		-		0.008**		-		0.040*
MTwo and PG	-		0.004**		-		0.030*		-		0.008**
OG and PG	-		0.185		-		0.074		-		0.088

*Significant P<0.05. OG: OneG, PG: ProGlider, SD: Standard deviation, MB: Mesiobuccal, DB: Distobuccal, ML: Mesiolingual

Figure 3

Graphs depicting. (a) Comparison of number (%) of pathfinding instruments that reached full working length in different groups. (b) Comparison of number (%) of pathfinding instruments fractured in different groups

There was no significant difference for the number (%) of pathfinding instruments fractured, between the groups (P = 0.564); among the arch, maxillary (P = 0.495) mandibular (P = 0.966); in canals maxillary MB (P = 0.937), DB (P = 0.669) and mandibular MB (P = 0.880), ML (P = 0.880), distal (P = 0.937).

There was a significant difference for the number of insertion pass for RFWL in maxillary DB (P = 0.013), mandibular MB (P = 0.009), and distal (P = 0.008) canals. In maxillary DB canal, there was a statistically significant difference (P = 0.004) between Mtwo and ProGlider. In mandibular MB canal, there was a statistically significant difference in Mtwo and OneG (P = 0.008) and Mtwo and ProGlider (P = 0.030). In distal canal, there was a statistically significant difference in Mtwo and OneG (P = 0.040) and Mtwo and ProGlider (P = 0.008).

DISCUSSION

Many studies have shown that a glide path preparation procedure prior to the use of mechanical instrumentation technique can reduce the frequency of separation of NiTi instrumentation systems.[71314] Such previous enlargement should be performed with fine hand instruments, which creates a smooth glide path for subsequent use of the larger rotary instruments.[1516] Small manual scouting instruments lack rigidity and have high flexibility. Disadvantages of the use of these instruments are buckling, deformation, separation, and more number of instruments.

Many dedicated mechanical NiTi instruments have been designed exclusively for glide path preparation purposes. Mtwo (ISO size 10 with 4% taper) has cross section of an “italic S” with two cutting blades. It has a constant helical angle and a greater mechanical resistance together with a tendency to advance into the canal. The flutes are deeper moving from the tip to the handle, thus increasing the capacity to remove debris coronally. It is used at a speed from 250 to 350 rpm and at a torque of 2 Ncm.

ProGlider utilizes M-wire technology and has increasing tapers from 2% to more than 8% along its active portion. A progressively tapered design over the active portion of a single file reduces the potential for the screw effect. The ProGlider file has a diameter of 0.16 mm at D0 and 0.82 mm at D16. ProGlider is used at a speed of 300 rpm and at a torque of 4.0–5.2 Ncm.

OneG has an innovative asymmetrical cross section. The three cutting edges are situated on three different radiuses relative to the canal axis. It has a diameter of 0.14 at D0 and a taper of 3%. A variable pitch between each cutting edge limits the screwing effect. OneG is used at a speed of 400 rpm and at a torque of 1.2 Ncm.

During mechanically advancing a pathfinding instrument into the canal up to working length, the file is largely subjected to stress as a consequence of two factors namely cyclic fatigue and torsional stress (taper lock phenomenon).[917] Cyclic fatigue appears to be less important than torsional stress on the incidence of instrument separation as the pathfinding instruments are generally extremely flexible because of the low taper and small tip size.[9]

Curved canals present a well-known clinical challenge to achieving proper mechanical instrumentation. In the present study, molar teeth with moderately curved canals (5°–20°) were chosen as they represent the type of root canal configuration most commonly observed in clinics.

In the present study, the tested pathfinding files exerted a significant impact on the frequency of teeth with all root canals classified as RFWL, whereas either tooth (maxillary/mandibular) or canal (MB, DB, ML, palatal, or distal) types had no influence on the performance of the instruments. Overall 53.33% of Mtwo, 63.33% of OneG, and 86.67% of ProGlider RFWL in moderately curved root canals of molar teeth.

ProGlider appeared as the most effective pathfinding instrument, producing the highest frequency of canals classified as RFWL (86.67%). This outcome may be the result of its file's design, which has a tip size (ISO #16) and a progressive taper (0.02–0.08) compared with the other tested systems. This design has been found to achieve a greater preflaring of the coronal and middle portions of the root canal; this, in turn, can render the advance of the instrument toward the apex easier.

There was no difference in the number of insertion passes required to RFWL in maxillary MB, palatal, and mandibular ML canals. However, there was difference in the number of insertion passes required to RFWL in maxillary DB (P < 0.05), mandibular MB (P < 0.01), and distal (P < 0.01) canals. The number of insertion passes required for Mtwo was higher than that of ProGlider; however, there was no difference between OneG and ProGlider.

In this study, the frequency of file separation was not similar among the ProGlider and other two files. The lowest percentage frequency of fracture (6.67%) was observed with ProGlider, and both Mtwo and OneG had 26.67% of frequency. However, the difference was not statistically significant (P > 0.05). There was also no significant difference of fracture frequency in tooth type (maxillary/mandibular) and canals.

A study[18] compared the cyclic fatigue resistances of NiTi rotary glide path files, ProGlider and OneG, and concluded that cyclic fatigue resistance of ProGlider is higher than that of the OneG file. ProGlider has a square cross section, whereas OneG has triangular and Mtwo has italic “S-” shaped cross sections. ProGlider's gradually increasing taper structure provides larger metal core mass. It works in the mid and coronal regions of the canal, removing coronal obstructions. M-wire alloy has improved mechanical properties, including higher flexibility and higher resistance to cyclic fatigue and torsional stress, which possibly explains a smaller number of instrument fracture in ProGlider group in the present study. This correlates with the results of a study conducted by Elnaghy and Elsaka[14] to compare the resistance to cyclic fatigue, torsional stress, buckling, and bending of ProGlider instruments with PathFile pathfinding NiTi rotary instruments.

De-Deus et al.[10] evaluated the effectiveness and fracture rate of four pathfinding NiTi rotary instruments in mechanically negotiating moderately curved molar canals. The highest and lowest frequencies of RFWL canals were observed in the ScoutRace and Pro-Design groups (P < 0.05), respectively, whereas the Mtwo and ProGlider groups showed intermediate results (P > 0.05). These findings are contrary to the findings of the present study. This difference may be attributed to the utilization of 2% tapered electropolished ScoutRace as compared to 3% OneG which are used in the present study. ProGlider showed more fracture rate than the Mtwo, ScoutRace, and ProDesign. This may be due to the pecking motion used as compared to gentle in-and-out insertion passes used in the present study.

In clinical practice, cyclic fatigue will increase in curved canals and torsional fatigue will increase in small and obliterated canals (tip lock). The type of NiTi files (design, taper, and size) and the instrumentation technique (crown down or step back and brushing or pecking motions) may overcome these anatomic challenges and prevent file separation.

CONCLUSION

Within the limitations of this study, ProGlider performed more efficiently and with less instrument breakage than OneG and Mtwo in mechanically scouting moderately curved canals of molar teeth.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.