Shaping ability of ProTaper Gold, One Curve, and Self-Adjusting File systems in severely curved canals: A cone-beam computed tomography study.

AIM: The aim of the study was to evaluate and compare canal transportation (CT), centering ability (CA), and volumetric changes in severely curved canals prepared using ProTaper Gold (PTG), One Curve (OC), and Self-Adjusting File (SAF) systems via cone-beam computed tomography (CBCT).
MATERIALS AND METHODS: Sixty mesiobuccal canals of maxillary molars were selected. Pre- and postinstrumentation CBCT scans were taken in the same position. CT and CA were calculated at 1 mm, 4 mm, and 7 mm from the apex; change in volume for whole canal was measured and analyzed statistically.
RESULTS: SAF showed the least mean CT at all the levels with no statistically significant differences at 1 mm and 4 mm when compared to other groups while statistically significant differences were observed at 7 mm with PTG and OC. Regarding CA, SAF better maintained canal centricity than PTG and OC at all the levels assessed, though the differences were not statistically significant except at 7 mm where statistically significant difference was observed between SAF and OC. SAF removed less volume of dentin followed by PTG and the highest removal was shown by OC.
CONCLUSION: SAF showed promising behavior with the least CT, most centered preparation, and minimal dentin removal. Copyright:

INTRODUCTION

Root canal shaping is a crucial step of endodontic treatment as it determines the effectiveness of subsequent procedures of canal disinfection and obturation. Unfortunately, an aberrant canal curvature results in inadequate debridement or asymmetric shaping leading to untoward events such as canal transportation (CT), ledge formation, perforation, generation of abnormal root surface strain, and microcrack induction on the root surface with resulting root fractures.[1]

CT is “the 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.” The lower the radius and the greater the degree of curvature, the higher is the likelihood of CT.[2]

Centering ability (CA) can be defined as the ability to keep the instruments centered to provide a correct enlargement, without weakening of root structure.[3] The more curved the canal is, the more the instrument receives constraint and less centered it is.

Root canal anatomy and instrument design influence change in volume of the canal postinstrumentation along with CT and CA. Dentin removed at the time of instrumentation is an important factor to avoid procedural errors.

In recent years, there have been noticeable advances in the design and alloy of NiTi rotary instruments such as noncutting tips, radial lands, different cross-sections, and varying tapers to enhance the instrumentation ability. Thermomechanical processing is frequently employed to optimize the microstructure and transformation behavior of NiTi alloys, which eventually has greater influence on the mechanical properties.[4] Recently, two such thermally treated file systems have been introduced, namely ProTaper Gold (PTG) and One Curve (OC).

PTG (2015) (Dentsply, Tulsa Dental Specialties, Tulsa, OK, USA) has been metallurgically enhanced through proprietary heat treatment technology.[5] Files have a variable taper design and exhibit the exact geometries as ProTaper Universal (PTU) but show increased cyclic fatigue resistance and flexibility than PTU.[6]

OC (2018) (Micro Mega, Besancon, France) is a single-file system utilizing proprietary C-wire heat treatment with controlled memory and prebendable property, which helps in conserving canal curvature. The manufacturer claims that its variable cross-section along with continuous rotation ensures excellent cutting efficiency and a perfectly centered trajectory.[7]

Despite the popularity of NiTi rotary files, many systems appear to leave big amount of uninstrumented dentin areas, especially in nonround canal cross-sections.[8] To overcome this limitation, a system called Self-Adjusting File (SAF; ReDentNova, Ra'anana, Israel, 2010) was introduced which has a mesh-like structure without an internal core.[9] SAF is a hollow file designed as a compressible, thin-walled, pointed cylinder comprising 120-μm thick NiTi lattice which can three-dimensionally adapt itself to the shape of the canal.[10] This hollow file allows for uninterrupted irrigation of the root canal, with additional activation of the irrigant through its vibrating motion creating turbulence in the canal.[11]

Till date, research has been carried out to evaluate the shaping ability of SAF in oval or mild to moderately curved canals, but very few studies have analyzed the CT and CA of SAF in severely curved canals.

The use of cone-beam computed tomography (CBCT) has emerged as a powerful noninvasive tool to study root canal preparation. It provides three-dimensional (3D) reproduction of the tooth and allows better pre- and postoperative evaluation as the images are provided in orthogonal as well as in oblique planes for better assessment of root curvatures and CT.[12]

As it is necessary to consider the correlation of root canal anatomical aspects and mechanical action of the instrument to ensure endodontic treatment success, the present study was planned to evaluate and compare the shaping ability of PTG, OC, and SAF systems in terms of CT, CA, and volumetric changes in severely curved canals using CBCT.

The null hypothesis tested was that there would be no differences between the three file systems regarding the aforementioned parameters.

MATERIALS AND METHODS

After obtaining approval from the institutional ethical committee, sixty extracted human maxillary permanent molars were selected. All the collected samples were thoroughly cleaned of all tissue fragments by immersion in 2.5% sodium hypochlorite solution followed by storage in 0.1% thymol solution until use.

Teeth included had fully formed apices, severely curved mesiobuccal canals with more than 25° curvature (Schneider's technique). If two mesiobuccal canals were present, they had independent foramina and fully formed apices. Radiovisiographs were taken in both buccolingual and mesiodistal directions for identifying two separate mesiobuccal canals and calculating angle of curvature [α, Figure 1], radius of curvature [r, Figure 2], and length of curved part of canal [k, Figure 2].

Figure 1

Calculation of angle of curvature (α)

Figure 2

Calculation of radius of curved canal (r), root canal shown as bold line

r was calculated according to the formula: r = S/(2sinα).

k was calculated using the formula: k = (4πα/360°) × r2.

Data collected were subjected to statistical analysis, to ensure sample homogeneity at baseline [Table 1].

Table 1

Data represented as mean±standard deviation

Groups (n=20)	Angle of curvature	Radius of curvature	Length of curved part
PTG (I)	35.616±5.934	5.462±1.240	6.379±1.201
OC (II)	31.536±5.799	6.297±1.577	6.622±0.813
SAF (III)	33.316±3.610	5.780±0.908	6.650±0.701
P a	0.234	0.347	0.776

aOne-way ANOVA test, level of significance at, P<0.05. PTG: ProTaper Gold, OC: One Curve, SAF: Self-Adjusting File

Coronal access was made using an Endo-Access bur (Dentsply Maillefer) in a high-speed airotor handpiece and canal exploration was done. Patency was checked using #10 K-file. Samples were decoronated with a diamond disk to obtain standardized mesiobuccal root length of 14 mm. Working length was determined using #15 K-file, established 1-mm short of foramen. Samples were embedded in resin molds of size 4 cm × 2 cm. Long axis of specimens was parallel to long axis of mold, to ensure standardization. Preinstrumentation images of each sample were recorded using CS 3D imaging software (CBCT – Carestream 9300 Premium) at three levels – 1 mm, 4 mm, and 7 mm from apex.

A number from 1 to 60 was assigned to each tooth, and the samples were randomly allocated to three experimental groups according to the file systems used (n = 20):

In Group I, samples were prepared using PTG till F2 (25/0.08). In Group II, samples were prepared using OC (25/0.06), while in Group III, samples were instrumented using the pre-SAF Orifice Shaper (40/0.10, ReDentNova) with gentle strokes followed by pre-SAF1 – 15/0.02 and pre-SAF2 – 20/0.04 and further followed by 1.5-mm SAF (ReDentNova, Ra'anana, Israel) with an in-and-out transline motion using EndoStation. The instrumentation was carried out in each sample for 4 min.

Samples were irrigated with 2 ml of 5.25% sodium hypochlorite, followed by 1 ml of 17% ethylenediaminetetraacetic acid for removal of smear layer and with a final flush of normal saline. Recapitulation was done using #10 K-file to maintain the apical patency.

Postinstrumentation, samples were scanned under similar conditions as initial scans.

CT was determined by comparative evaluation of linear measurements in axial sections at the abovementioned levels in pre- and postinstrumentation scans using the following equation:[13]

CT = (a1− a2)−(b1− b2)

where a1 and a2– shortest distance from mesial edge of root to mesial edge of uninstrumented and instrumented canal, respectively, and b1 and b2– distance from distal edge of root to distal edge of uninstrumented and instrumented canal, respectively. Result of “0” indicates no CT and other than 0 means transportation has occurred. Negative value indicates transportation toward distal and positive value indicates transportation toward mesial.

CA was determined using formula:[13]

CA = (a1− a2)/(b1− b2) or (b1− b2)/(a1− a2).

If differences are unequal, smaller value is considered as numerator. Therefore, values equal to 1 indicate perfect CA of instrument and values closer to 0 indicate instrument's lower ability to maintain central axis of canal.

Volume increase (mm3) was determined for each canal by subtracting uninstrumented canal volume from instrumented canal volume, using volume calculation tool of software – OnDemand3D licensed dental software (CyberMed, Seoul, Korea).

Statistical analysis

Data were collected and subjected to analysis using the Statistical Package for Social Sciences (SPSS) version 21 (IBM Corp., Armonk, NY). Overall intergroup comparison was done using Kruskal–Wallis test and post hoc pairwise comparison was done using Mann–Whitney U-test. The level of statistical significance was set at 0.05.

RESULTS

Canal transportation

At 1 mm, Group III showed the least value followed by Group I and Group II with approximately equivalent values, with no statistically significant differences among the groups.

At 4 mm, Group III showed the least CT followed by Group I which was further followed by Group II. However, differences were nonsignificant among the groups.

At 7 mm, Group III showed the least CT followed by Group II and Group I. Statistically significant differences were observed between Groups I and III and Groups II and III. However, no statistically significant difference was observed between Group I and Group II [Table 2].

Table 2

Intergroup comparison of canal transportation and centering ability

	Mean±SD
	1 mm	4 mm	7 mm
CT
Group I	0.0800±0.04216	0.0500±0.07071	0.1600±0.06992a
Group II	0.0800±0.07888	0.0900±0.05676	0.1500±0.05270a
Group III	0.0600±0.05164	0.0400±0.05164	0.0500±0.05270b
P	0.698	0.140	0.002*
CA
Group I (PTG)	0.6167±0.20861	0.8000±0.26989	0.5233±0.11337
Group II (OC)	0.6667±0.29397	0.5833±0.22567	0.4333±0.11653a
Group III (SAF)	0.7000±0.25820	0.8000±0.25820	0.7500±0.26352b
P	0.811	0.106	0.009*

*P<0.05. Different alphabets (lower case) highlight statistical significance pairwise. CT: Canal transportation, CA: Centering ability, PTG: ProTaper Gold, OC: One Curve, SAF: Self-Adjusting File, SD: Standard deviation

Centering ability

At 1 mm, Group III showed the most centered preparation, followed by Group II and further followed by Group I, with no statistically significant differences among the groups.

At 4 mm, Group I and Group III showed approximately equivalent values with most centered preparations, followed by Group II, although the values failed to reach the level of significance.

At 7 mm, Group III showed the most centered preparation followed by Group I and further followed by Group II. Significant differences between Groups II and III were noted. No statistically significant differences were observed between Groups I and II and between Groups I and III [Table 2].

Volume increase

In Graph 1, the highest volume increase was observed in Group II followed by Group I and the least value in Group III, with statistically significant differences among the groups.

Graph 1

Intergroup comparison of volume increase

DISCUSSION

Selecting a NiTi instrument with high flexibility during the preparation of severely curved canals provides better adaptation of the file to the canal morphology and its ability to pass through the curvature, thus reducing the probability of iatrogenic errors.[14]

Besides metallurgical processes, tremendous modifications in the alloy type, cross-sectional design, taper variation, number of files, and sequence of instruments have taken place. Hence, the current study was designed to evaluate and compare the shaping ability in severely curved mesiobuccal canals of maxillary molars at three transverse sections from the apex using PTG, OC, and SAF systems using CBCT.

In the present study, natural teeth were used over acrylic resin blocks, since they closely mimic clinical situations. Acrylic resin does not emulate the anatomic variations.[15] Moreover, hardness of the resin block is half of that observed in the dentin in natural teeth. Furthermore, root canal instrumentation studies in resin blocks showed increased deviation or CT than canal preparations in natural teeth.[16]

Schneider's technique was used to measure and calculate the angles & the radii of the curvatures and the length of the curved root as it is the most commonly used radiographic method.[17]

During the biomechanical preparation of curved canals, increasing the apical diameter can cause undesired canal irregularity. Moreover, mesiobuccal canals have foramina with diameters ranging from 0.18 to 0.25 mm.[18] Therefore, the master apical file size was kept #25 for PTG and OC while 1.5-mm SAF was selected according to manufacturer's instructions.

In the present study, three cross-sectional levels were chosen: 1 mm, 4 mm, and 7 mm from the apex to represent the apical, middle, and coronal thirds of root canals where curvatures with high susceptibility to iatrogenic mishaps usually exist.

CBCT has emerged as a powerful nondestructive tool which provides images in orthogonal planes as well as oblique planes for better assessment.[8] Therefore, in the present study, CBCT tool was used to evaluate the shaping performance of the tested systems and assessment of alterations in root canal volume before and after instrumentation.

According to the results of the present study, at 1 mm, SAF showed minimum CT which could be attributed to the absence of central metal shaft making the file extremely flexible and pliable, exerting uniform lateral pressure on the dentinal walls. This is true both circumferentially and longitudinally.[11]

Various studies have been conducted by Peters et al., evaluating CT using different instruments with the same experimental setup in maxillary molar canals. It was observed that SAF had shown overall lesser CT scores than other file systems – K file, GT files, lightspeed, ProFile, ProTaper, and FlexMaster.[19202122] A study by Burroughs et al. also showed the least mean CT by SAF as compared to Profile Vortex and Typhoon files in simulated S-shaped root canals.[23] In a study by Pawar, SAF instrumentation resulted in more centered preparation than PTN and WO, when used for instrumenting nonround root canals.[24]

In the present study, PTG showed slightly more CT than SAF with no significant differences which could be due to the proprietary thermal treatment and file design of PTG that increases its flexibility. The file design includes nonlanded convex triangular cross-section and modified guiding tip with an angle of approximately 39°. The instrument also has a continuously changing helical angle and pitch over its entire cutting surface that prevents the instrument from screwing into the canal. These features could help to improve flexibility.[15]

In the present study, OC showed comparatively higher apical transportation than SAF but with no significant differences. The satisfactory results of OC may be attributed to the higher flexibility because of the heat-treated C-wire technology and its design features having variable cross-section which enable the effective shaping while maintaining initial canal anatomy.[7]

Furthermore, in our study, PTG and OC showed approximately equivalent CT values at apical third with no statistically significant differences among the groups. This could be explained by their manufacturing processes (metallic alloy and thermal treatment) and design pattern, which clearly affects their stress–strain distribution patterns and bending behaviors, decreasing their tendency to straighten in curved canals.[4]

According to Wu et al., apical transportation is only clinically relevant when it is >0.300 mm, compromising sealing ability of root canal filling material. In the present study, none of the apical transportation values surpassed this limit.[25]

In the present study, at 4 mm, the least CT was observed by SAF followed by PTG and maximum value of OC with no statistically significant differences among the groups which could be attributed to the instrument design along with the metallurgy, which has already been discussed above.

At 7 mm, the least CT values observed with SAF could be due to its minimally invasive approach. The reason for higher CT of PTG and OC at coronal portion can be attributed to the sharp cutting action of the instruments in the root canal. Moreover, as files were used to the full extent of the canals (including the curved portion), they are prompted to rectify the curvature by creating wear on the opposite wall, explaining higher transportation in the straight portion.[26]

Al-Gharrawi and Abbas also revealed that SAF in canals with 15° curvature showed the lowest mean CT at coronal third in comparison to BioRace and ProTaper groups, though the differences were statistically nonsignificant.[27]

OC, being a single-file system with S-shaped cross-section, has a lower core area compared to convex, triangular cross-sectional design of PTG, thus showing comparatively greater flexibility and lower transportation values than PTG.[14] Furthermore, one flare used as the orifice shaper in Group II has been manufactured through heat treatment of T-wire which makes it even more flexible.[28]

In our study, PTG showed the highest CT which is in accordance with the literature available. Pinheiro et al. observed the highest transportation values at coronal third by PTG when compared with ProDesign S, Hyflex CM, Hyflex EDM, and ProDesign Logic.[29] However, Singh et al. also concluded that PTG presented significantly higher CT values as compared to 2Shape in severely curved canals at all the levels.[30]

In a study, Razcha et al. showed significantly higher CT values of OC than HCM at the coronal level.[31]

Regarding the CA, the mentioned results can be attributed to the design of the instrument along with the metallurgy, which has already been discussed above.

Razcha et al. recorded no statistically significant differences among HCM, HEDM, Wave One Gold, and OC, when the mean centering ratios were compared.[31]

Volume increase is determined by the taper and diameter of the file and flexibility and cutting efficiency of the instrument. Therefore, overall volume increase is likely a cumulative effect of coronal flaring, glide path preparation, and the action of the final file. The higher standard deviation values observed in Groups I and II may be attributed to the fixed size and taper of the solid core file systems which shape the root canal irrespective of the original canal size.

The highest volume increase showed by OC file system could be attributed to a variety of different cross-sections along the active length of the file which offers an improved cutting action in the root canal. This is in accordance with the study conducted by Ghoneim in which OC showed more dentin removal than OS file with no statistically significant difference between them.[32]

In the present study, PTG showed comparatively lesser values than OC which could be due to variable taper design along the length in PTG file as compared to constant taper in OC.

While, in a study by Tomer et al. in severely curved root canals, PTG showed a greater amount of dentin removal compared to NeoEndo and Revo-S which was explained by the sharp cutting edges of the convex triangular cross-sectional design and its flute design that combines multiple tapers within the shaft up to 19%.[33] Furthermore, Singh et al. showed that the PTG removed dentin more aggressively at all the levels of root canal when compared to 2Shape.[30]

In our study, the least values of volume increase were shown by the SAF which makes SAF safer in terms of maintaining dentin integrity, making the samples prepared with it more resistant to vertical fracture.[9] This is in concordance with a study conducted by Serefoglu and Piskin, showing lesser mean volume changes in SAF than PTU in maxillary molars.[34]

CONCLUSION

Within the limitations of this in vitro study, the following conclusions can be drawn from the study:

All the three experimental file systems respected the anatomy of the severely curved canals within acceptable limit

SAF system better maintained canal centricity than PTG and OC at all the levels assessed, though the differences were not statistically significant except at coronal level where statistically significant difference was observed between SAF system and OC

SAF system removed less volume of dentin during instrumentation, followed by PTG file system, and the highest dentin removal was shown by OC

Out of the three tested file systems in severely curved canals, SAF system showed promising behavior with the least CT, most centered preparation, and minimal dentin removal. This combined with the lower tendency to straighten curved root canals, makes using SAF safer in terms of maintaining dentin integrity.

However, further in vivo studies are required to test the impact of shaping ability of these file systems in severely curved canals.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.