Evaluation of Root Dentin Thickness and Smear Layer Removal Efficacy of Two Novel Nickel Titanium Rotary Instruments - An In Vitro Cone-Beam Computed Tomography and Scanning Electron Microscopy Study.

OBJECTIVE: This study aimed to compare the two nickel-titanium (NiTi) instruments on root dentin thickness using cone-beam computed tomography (CBCT) and ability to remove the smear layer using scanning electron microscope (SEM).
MATERIALS AND METHODS: The effect of two NiTi rotary instruments such as XP Endo shaper and ProTaper Gold (PTG) in retaining adequate dentin thickness and debridement (i.e., removal of smear layer) was evaluated in vitro using CBCT and SEM. After cleaning and shaping of the root canal with adequate irrigation, the effect of two NiTi instrument systems at 3 mm (apical), 6 mm (middle), and 9 mm (coronal) levels on buccal, lingual, mesial, and distal aspects in terms of amount of dentin removal and smear layer removal was evaluated. The data were analyzed using student's unpaired t-test with a significance of P ≤ 0.05.
RESULTS: Both NiTi instrument systems had no statistically significant difference in amount of dentin removal at all three levels and on all four aspects. In smear layer removal, PTG had significantly better efficacy compared to XP Endo shaper at the coronal third of root canal.
CONCLUSION: Instrumentation with XP Endo Shaper and PTG showed a similar amount of dentin removal. PTG has a significant effect on smear layer removal compared to XP Endo Shaper only at the coronal third of root canal. Copyright:

INTRODUCTION

A layer of organic and inorganic substance covering instrumented canal walls which has an amorphous irregular and granular appearance is called as smear layer.[1] As a sequelae of instrumentation (hand and rotary), formation of dentinal debris and smear layer is unavoidable in all root canal cleaning and shaping.[2] It is controversial that whether the smear layer must be removed or retained. Smear layer has a protective role against the penetration of intracanal bacteria but limits the penetration of antibacterial irrigating solutions into the dentinal tubules and reduces the seal between restoration and dentinal walls.[3] Hence, elimination or reduction of smear layer is recommended with proper instrumentation and adequate irrigation.[2] Not only smear layer but also excessive removal of dentin during root canal preparation affects the prognosis of root canal treatment.[4] There is a direct relationship between the residual dentin thickness and strength of the root.[5] Thus, preservation of sound dentin (at least 1 mm of root dentin should remain along the root length) is of utmost importance.[6] The possible outcomes of excessive removal of root dentin are strip perforations and vertical root fractures, especially in danger zones (thin-walled root).[7]

Since the introduction of nickel–titanium (NiTi) rotary instruments with various capabilities, assessment of its efficacy in smear layer removal seems to be necessary. NiTi instruments have a greater ability for cleaning the coronal and middle portion of the root canal system rather than the apical segment.[8] Many approaches have been developed by the manufacturers to improve the flexibility of NiTi rotary endodontic instruments. These approaches include modifying the instruments cross-sectional design, and enhancement in the manufacturing process or the use of new alloys that provide better mechanical properties.[9]

Recently, ProTaper Gold instruments (PTG; Dentsply, Tulsa Dental Specialties, Tulsa, OK, USA) have been introduced. They are developed with proprietary advanced metallurgy and have a progressively tapered design that is claimed by the manufacturer to enhance cutting efficiency with improved flexibility and safety.[10] XP Endo Shaper (XPS; FKG Dentaire, La Chaux-de-Fonds, Switzerland) is another such new instrument which is manufactured using MaxWire (Martensite-Austenite Electropolishing-Flex, FKG) alloy. MaxWire alloy allows the instrument to change its phase according to changes in temperature (FKG Dentaire 2015) and claims increased flexibility.

Cone-beam computed tomography (CBCT) image shows the exact location and anatomy of the root canal system. In addition, it has been validated as a tool to explore root canal anatomy which assesses the remaining dentin thickness as the amount of dentin removed.[11] Scanning electron microscopy is a viable method to determine cleanliness of the root canals by assessment of smear layer.[12]

Hence, the purpose of this study was to compare two different rotary instrumentation systems, namely PTG and XP Endo Shaper on the amount of root dentin thickness and ability to remove the smear layer using scanning electron microscope (SEM) and CBCT.

MATERIALS AND METHODS

Ethical clearance

The study protocol was approved by the Institutional Ethics Committee, before commencing the study (CSP/18/APR/68/108).

Sample selection and standardization

Thirty human mandibular single-rooted premolars extracted for periodontal reasons were used in this study. Unrestored teeth with mature apices, without any evidence of resorption, or crack and canal curvature within 10°–30° were included using Schneider's method.[13] Teeth with more than one root canal, calcified root canal, internal resorption, and root curvature beyond 30° were excluded following radiographic examination. Then, the teeth were cleaned of soft tissue debris, calculus followed by sectioning at the level of cementoenamel junction using a low-speed diamond disc in conjunction with water was done. The patency of the canals was verified with a #10 K file (Mani Inc., Utsunomiya, Japan). Samples were then randomly divided into two experimental groups (n = 15) according to the instrumentation systems as follows:

Group 1: Preparation with XP Endo Shaper and

Group 2: Preparation with PTG.

Preinstrumentation cone-beam computed tomography

Sectioned samples were mounted in molds made of addition silicone dental impression material (Zhermack, Dentsply Sirona). Markings were made at 3 mm (apical), 6 mm (middle), and 9 mm (coronal) from the apex on the samples, and guttapercha points #25 were embedded into the impression material mold to coincide with these three markings [Figure 1] so as to enable these reference points to be visualized in the subsequent CBCT scans. The scans were obtained using CBCT [Planmeca Promax 3Ds, Helsinki, Finland] unit with a limited field of view (with a scan time of 12 s, 90 kVp, 10mA). Root dentin thickness was measured at buccal, lingual, mesial, and distal surfaces of samples at 3 mm, 6 mm, and 9 mm of the root canal.

Figure 1

Marking at 3 mm (apical), 6 mm (middle), and 9 mm (coronal) of sample with guttapercha

Root canal instrumentation

A size #10 K file was used to determine the working length and confirmed with a radiograph. The glide path was created using a hand stainless steel #15 K file. NiTi instruments were operated using an endodontic motor (X Smart, Dentsply Maillefer) with a 16:1 reduction handpiece with ethylenediaminetetraacetic acid (Glyde File Prep, Dentsply Maillefer) lubricant. In Group 1, the root canals were instrumented using XP Endo Shaper till #30.04 and in Group 2, instrumentation was carried with PTG system till F3 (#30.09). Speed and torque settings were adjusted according to manufacturer instructions in both the groups. Between each instrumentation, 2 ml of 3% sodium hypochlorite was used followed by a final rinse with 10 ml of 17% ethylenediaminetetraacetic acid.

Postinstrumentation cone-beam computed tomography

Samples were then subjected to CBCT investigations with the same exposure parameters as at the time of preinstrumentation which allowed pre- and postoperative image comparison on buccal, lingual, mesial, and distal aspects of samples at 3 mm, 6 mm, and 9 mm of the root canal to evaluate the root dentin thickness [Figure 2].

Figure 2

Root dentin thickness evaluation using cone-beam computed tomography

Scanning electron microscope

Longitudinal grooves were made on the external surface of the roots with a diamond disc and then splitted into two halves with a chisel. Cross-sectional markings were made at 3 mm, 6 mm, and 9 mm with a straight fissure bur to identify the apical, middle, and coronal sections. Samples were then mounted on a brass stub and gold sputtered to examine under the SEM at a magnification of 1000×. Images were then obtained at the level of apical, middle, and coronal third for each sample, respectively [Figures 3 and 4]. The images were evaluated by two independent observers who were unaware of the experimental groups to which the samples belonged. Smear layer grading was carried out as described by Hulsmann et al.[14]

Figure 3

Scanning electron microscope image of Group 1 (XP Endo Shaper) (a) 3 mm (apical), (b) 6 mm (middle), and (c) 9 mm (coronal)

Figure 4

Scanning electron microscope image of Group 2 (ProTaper Gold) (a) 3 mm (apical), (b) 6 mm (middle), and (c) 9 mm (coronal)

Statistical analysis

The obtained data were entered into the Microsoft spreadsheet and they were analyzed using Statistical Package for the Social Science [SPSS] software version 20.0 [IBM, Armonk, NY, USA] Student's unpaired t-test was used to compare root dentin thickness and amount of smear layer among the two experimental groups. P value was set at <0.05 for statistical significance.

RESULTS

There was no statistically significant difference in the root dentin thickness between the two groups at all the four aspects (buccal, lingual, mesial, and distal) and at all the three levels (3 mm, 6 mm, and 9 mm) of the root canal [Table 1]. Smear layer scores for both the groups at the 3–6 mm levels of the root canal were not statistically significant. However, the scores were significant at the level of 9 mm (P = 0.006), which indicates the better smear layer removal by PTG compared to XP Endo Shaper [Table 2].

Table 1

Comparison of mean amount of dentin removed in the specimens among the two groups

Aspect	Level	Groups	n	Mean±SD	P
Buccal	9 mm (coronal)	1	15	0.112±0.120	0.806
		2	15	0.080±0.118
	6 mm (middle)	1	15	0.203±0.189
		2	15	0.124±0.155
	3 mm (apical)	1	15	0.135±0.161
		2	15	0.117±0.181
Lingual	9 mm (coronal)	1	15	0.275±0.171	0.323
		2	15	0.262±0.215
	6 mm (middle)	1	15	0.237±0.183
		2	15	0.194±0.161
	3 mm (apical)	1	15	0.216±0.170
		2	15	0.161±0.217
Mesial	9 mm (coronal)	1	15	0.255±0.228	0.366
		2	15	0.219±0.184
	6 mm (middle)	1	15	0.174±0.161
		2	15	0.146±0.139
	3 mm (apical)	1	15	0.128±0.160
		2	15	0.164±0.174
Distal	9 mm (coronal)	1	15	0.150±0.171	0.967
		2	15	0.172±0.260
	6 mm (middle)	1	15	0.088±0.143
		2	15	0.132±0.178
	3 mm (apical)	1	15	0.110±0.160
		2	15	0.096±0.118

SD: Standard deviation

Table 2

Comparison of mean smear layer in the specimens among the two groups

Site	Group	n	Mean±SD	P
Coronal (9 mm)	1	15	2.40±0.51	0.006*
	2	15	1.50±0.97
Middle (6 mm)	1	15	2.70±0.82	0.108
	2	15	2.00±1.24
Apical (3 mm)	1	15	3.30±0.48	0.111
	2	15	2.80±0.78

*Significant at ≤0.05, Student’s unpaired t-test was performed. SD: Standard deviation

DISCUSSION

For a successful endodontic treatment, adequate instrumentation and cleaning along with removal of the smear layer is very essential. The effective but not aggressive instrumentation should result in judicious removal of infected canal content both inorganic and organic via copious irrigation, this eventually allowing the sealer and the filling materials to seal the root canal space three dimensionally.[15]

One of the property of NiTi instruments such as flexibility can be improved by methods like electro-polishing, electro-discharge machining and thermal treatment. And this property can address both mechanical and biological objectives of shaping of root canal system.[16]

PTG has been introduced to offset the limitations of its predecessors the ProTaper universal system. It has a convex triangular cross-section and a variable progressive taper. The manufacturer claims that PTG has been developed with proprietary advanced metallurgy and has a progressively tapered design to enhance cutting efficiency and safety. The gold metallurgy allows for “safer shapes” with these files which “follow” existing canal walls and carve only the necessary dentin to replicate the shape of the finishing file.

A recently introduced endodontic rotary instrument, XP Endo Shaper, is manufactured from a MaxWire alloy. Due to this metallurgy, the file tends to change its shape according to the various temperatures. During the Martensite phase (cooling), the file stands straight with the size #30 and initial taper of 0.01. However, when submitted to body temperature, it changes to the Austenite phase that can achieve a final preparation size of #30/0.04 with minimal stress to dentinal wall and adapting easily to canal irregularities.

On evaluating the root dentin removal, Group 1 (XP Endo Shaper) has produced more amount of dentin removal on comparing with Group 2 (PTG), across the root canal (3 mm, 6 mm, and 9 mm) in all the four aspects (buccal, lingual, mesial and distal) examined. However, the results were not statistically significant.

The flexible nature of the alloy of the PTG enhanced through a proprietary heat treatment technology imparts a reduced restoring force. This allows the instrument to remain centered in the canal.[17] This, in turn, allows a lesser amount of dentin removal, thereby retaining a greater amount of remaining dentin thickness. In addition, the heat treatment of the alloys of the PTG instruments may predispose these instruments to plastic deformation and disruption of cutting edges during use, thereby reducing its cutting ability resulting in a more conservative cutting efficiency.[10]

XP Endo Shaper being a conservative single file with enhanced flexibility provides an adaptive spring-like contact to the root canal walls and the booster tip of the instrument with six cutting edges enables the file to enlarge the canal from an apical size of #15–#30 in a single step without the need of the traditional incremental increase in apical size.[18] However, if we have to attribute certain reasons for the XP Endo Shaper removing more dentin than Group 2 (PTG), it could be related to the presence of six cutting edges as against the noncutting tip of PTG which functions only as a guide to allow easy penetration with minimal apical pressure.

The results of the SEM analysis have shown lower smear layer scores of Group 2 (PTG) at 9 mm level, which could be because of the culmination of the shaping with the F3 (9% taper) instrument, which might have resulted in reducing the amount of untouched areas.[19] The reasons for the higher smear layer scores with XP Endo Shaper can be attributed to the use of a single-file sequence with a comparatively decreased taper ranging from 1% to 4% which might not have contacted all the walls of the root canal.[20]

The strength of the present study is that it throws light on the fact that instrumentation systems could benefit from being flexible and at the same time having greater progressive tapers that touch the maximum amount of the canal walls rendering them open to effective irrigation to strike a balance between optimum shaping and cleaning strategies.

CONCLUSION

Instrumentation with XP Endo Shaper and PTG showed a similar amount of dentin removal. Thus, both can be considered as safe instruments to be used during shaping procedures with no danger of excessive dentin removal. PTG has a significant effect on smear layer removal compared to XP Endo Shaper only at the coronal third of root canal.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.