Comparative Evaluation of Efficacy of ProTaper® Universal Rotary Retreatment System for Gutta-percha Removal from Single Root Canals, Obturated with Two Different Techniques: In Vitro Cone-beam Computed Tomography Study

INTRODUCTION

Retaining natural dentition is the supreme goal of dentistry. In an attempt to fulfill this goal, retreatment of certain poorly obturated root canals is sometimes a necessity in endodontic practice. American Association of Endodontists (AAE) has defined retreatment as “the removal of root canal filling materials from the tooth, followed by cleaning, shaping, and obturating the canals. The terms “endodontic reintervention” and “endodontic revision” have also been employed for the same.¹

Sometimes, even after thorough instrumentation, substantial amount of root canal filling material remains in the canal. This leftover may lead to compromised seal of the new root filling; therefore, previous filling should be removed completely.²

Various methods which have been advocated in the literature for retrieval of root canal fillings are the following: by using Hedstrom files (H-files), stainless steel K-files, heat carrying instruments, and engine-driven rotary instruments like peso reamers, Gates-Glidden drills, retreatment files, ultrasonic tips, and lasers.^3–5 Studies have shown that the use of rotary instrument for removal of filling material is safer and consumes lesser time than hand instruments.^2,6,7

It has also been observed that rotary nickel–titanium (NiTi) instruments reduce operator’s and patient’s fatigue.⁸ In endodontics, one of the most commonly used rotary systems is ProTaper® NiTi rotary file system, which has been upgraded to ProTaper® Universal retreatment system (ProTaper® URS; Dentsply/Maillefer, Ballaigues, Switzerland). It consists of shaping, finishing, and retreatment instruments. Retreatment files are three in number and have various diameters at tip, different tapers, and lengths.^6,8–10

There are various root canal filling materials like silver points, gutta-percha (GP), resilon, mineral trioxide aggregate (MTA), and paraformaldehyde pastes which are available.¹¹ Gutta-percha, along with sealers is frequently used material for root canal obturation. There are different techniques used to obturate root canals with GP.³ Clinician can adopt any technique which is more promising to him. Among diverse techniques available, cold lateral condensation is most frequently used. Along with conventional lateral condensation technique, thermoplasticized GP technique has also gained popularity in the recent times.² Therefore for the present study, these two techniques had been considered.

Majority of previous studies which have been performed to assess root canal fillings, were invasive in nature. Methods like sectioning the teeth and then examining the root canal surfaces for remaining GP causes damage to the specimen as well as loss of GP material during sectioning.^2,7,12 Other radiographic methods lacked in providing correct volumetric analysis of root filling material.¹³ In such a scenario, cone-beam computed tomography (CBCT) is a technology which provides three-dimensional (3D) quantitative volume analysis of leftover root canal material and gives more accurate measurements. It is a nondestructive method which permits detailed visualization of the features without destruction of teeth.^2,6

MATERIALS AND METHODS

Fifty-two extracted intact permanent single-rooted maxillary central incisors were selected with fully formed apices. Radiographic evaluation was performed to confirm the patency of canals and to check for any possible internal resorption of teeth. The sample teeth were decoronated horizontally with a diamond disk (0.2 mm thickness) at the level of 2 mm coronal to cementoenamel junction (CEJ). A size-15, K file was introduced into the canal till apical foramen. Initial hand instrumentation of each sample was carried out till size-25, K-file. Later the canals were prepared with 5 ProTaper® Universal rotary instruments in the sequence recommended by the manufacturer (S1, S2, F1, F2, and F3).

RESULTS

Preoperative and postoperative values obtained from CBCT scanning were recorded and percentage of volume of remaining root canal filling material was calculated. The percentage of residual root canal filling material was calculated by using the following formula (Table 1):

DISCUSSION

In spite of performing good root canal treatment, failure rate of this treatment is around 8%.¹⁴ Under circumstances of failure of endodontic therapy, endodontist can opt and proceed with one of the following options: nonsurgical orthograde retreatment therapy, surgical endodontic therapy, and combined therapy consisting of both nonsurgical and surgical treatment.¹⁵ Most of the time, nonsurgical retreatment is preferred by patient as well as endodontist.^7,16

Reestablishment of correct working length and decontamination of root canals completely is a crucial step in root canal retreatment.¹⁷ But in spite of recent advances in endodontics, many studies have reported the presence of substantial amount of residual root canal filling material in the canals after instrumentation.^18–20 Therefore, the goal of retreatment shifts from complete retrieval to maximum retrieval of old filling.

Previous studies have concluded that rotary instruments are safer and take lesser time than conventional hand instruments, and it also reduces clinician’s fatigue.^12,21–23 Therefore, to contribute to the aforementioned data on rotary retreatment files, the present study on efficacy of one of the rotary retreatment files was formulated.

Previous investigations have employed variety of methods to calculate remaining root canal filling material in the canals after retreatment. These methods were radiographic examination, stereomicroscope evaluation, scanning electron microscopy, and CBCT, taking digital images after splitting teeth longitudinally, clearing techniques and micro-CT.^2,7,13 Radiographic images of sample provide only two-dimensional (2D) information of a 3D structure. Radiographs may be subjected to magnification and distortion.²⁴ Digital imaging technique results in loss of residual filling material during splitting of teeth longitudinally. To date, micro-CT represents the most precise method for this evaluation but it is extremely time-consuming. Therefore, 3D CBCT imaging, which is more promising and readily available to researchers, was adopted for present study to evaluate residual root canal filling material.²³ Though CBCT has its own limitations like streaking image artifacts in obturated root canals due to beam hardening effect,²⁵ it did not hinder the volume calculation; thus, the results of this study were not affected much.

Among the available variety of rotary retreatment files, ProTaper® Universal retreatment system (ProTaper® URS; Dentsply/Maillefer, Ballaigues, Switzerland) files have been proven to be one of the most popular and commonly used systems currently available in market.²⁶ The initial preparation of specimen teeth was carried out with ProTaper® rotary files; therefore for subsequent retreatment, ProTaper® URS was selected. Iriboz et al. carried out an in vitro research to compare efficacy of ProTaper® URS and Mtwo retreatment system and concluded that ProTaper® URS was superior to Mtwo retreatment system.²⁷ Studies performed by Siotia et al., Soares et al. have studied efficacy of ProTaper® URS in removing different root canal filling materials, resilon, and GP from the canals during retreatment and found out positive results.^28,29 Topcuoglu et al. had compared cyclic fatigue resistance of D-RaCe and ProTaper® URS files and found out that the canals with 45° of curvature showed no significant difference between both the retreatment systems.¹⁶

Present study demonstrated that it was difficult to retrieve obturated material thoroughly from the canals. Findings of this study are in accordance with previously performed studies.^2,4,6,9 In a previous publication, it was stated that ProTaper® URS leaves around 10–35% of root canal fillings in the canals after their use.²³ This study confirmed the above-mentioned data by presenting a mean value of residual volume 10.69% and 13.7% in groups I and II, respectively (Fig. 1). There was statistically significant variation among groups I and II with a p value being 0.004. To some extent, compromised retrieval property of ProTaper® URS can be attributed to dimensions of retreatment files.^2,23 ProTaper® URS is a set of three Ni–Ti rotary files which are labeled as D1, D2, and D3. Apical size, taper, and length of these files vary as follows: D1—30/0.09, 16 mm; D2—25/0.08, 18 mm; and D3—20/0.07, 22 mm. These files have a convex, triangular cross section, and D1 has an active working tip which assists its penetration into obturated canals. These files should be used in recommended sequential order which is D1, D2 and D3. Apical size and taper of the last file to be used (D3 file) is 20/0.07, and in the present study, the preparation of the apical part of the canal was carried out with finishing file F3 which has apical size and taper 30/0.09. Because of difference between dimensions of retreatment file and prepared canal, there is possibility that file could not engage the root filling material completely and failed to retrieve it. To obtain favorable results, reinstrumentation at working length with an instrument larger in size than the one used for primary treatment is necessary.²

A potential reason for gaining much more percentage of root filling material residues in warm thermoplasticized group than the other group can be drawn out from the following findings:

• Thermoplasticized GP when inserted in the canals is in alpha (α) phase. This phase of GP is more plastic in nature and thus can flow in small irregularities and dentinal tubules of the canals leading to a better seal of GP to the root canal walls.^23,30–32
• Thermoplasticized GP which gets wedged into irregularities of dentin cannot be easily removed.
• It has been reported that thermoplasticized GP technique resulted in an obturation with greater density.²
• During obturation, warm GP produces deeper penetration of sealer into tubules which again leads to superior seal formation when compared to cold GP compaction.³³
• In cold lateral condensation technique, the filling does not consist of homogeneous mass of GP. The final root canal filling is composed of many GP cones tightly compacted together, joined by tight frictional grip and sealer cements. The accessory and master cones are laminated because of which it remain separate.²
• In cold lateral condensation technique, GP is in beta (β) phase. GP in this phase cannot be compressed at room temperature. Therefore, accessory anatomy of canal like fins, isthmus, and apical deltas cannot be reached out and thus results in compromised seal when compared to warm GP compaction.³²

CONCLUSION

Following inferences can be concluded:

• ProTaper® URS did not remove root canal fillings thoroughly from either of the two sets of obturated canals. Thus, removal of root canal fillings should be clubbed with other techniques like photon-initiated photoacoustic streaming (PIPS) and passive ultrasonic irrigation (PUI) to get better results.^34,35
• ProTaper® URS was more efficient in removing GP from the canals obturated with cold lateral condensation technique when compared to warm thermoplasticized GP technique.

The findings of this in vitro study have to be correlated with the clinical cases, and for more precise results, more in vivo studies must be conducted.

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