A comparative evaluation of the canal centering ability of three rotary nickel-titanium retreatment systems in the mesio-buccal canals of mandibular first molars using computed tomography.

BACKGROUND: During endodontic retreatment, relative difficulty exists in removing the filling material and maintaining the canal anatomy. Usage of nickel-titanium (NiTi) rotary retreatment instruments is widely accepted, but there is a lack of adequate literature evidence about their canal centering ability. AIM: To compare the canal centering ability of rotary NiTi retreatment systems.
MATERIALS AND METHODS: Mandibular first molars with mesiobuccal canals with canal access angle of 20-40° were used. Canals prepared until ISO 25, 0.06 taper. Obturated with three different techniques lateral compaction, Thermafil, and Resilon/Epiphany. Retreatment was carried using three different systems ProTaper retreatment, Mtwo R and REndo. Specimens were subjected to computed tomography analysis at coronal, middle, and apical third of the root canal preobturation and postretreatment procedure. STATISTICAL ANALYSIS USED: One-way ANOVA and post-hoc Tukey test.
RESULTS: No statistically significant difference with three retreatment systems. Variation existed among all the subgroups at the coronal, middle, and apical third of the root canal.
CONCLUSION: All retreatment systems with three obturation techniques showed eccentricity within acceptable limits. REndo, MtwoR showed better canal centering and ProTaper retreatment system showed tendency for eccentric canal preparation, especially in apical third.

INTRODUCTION

Nonsurgical retreatment includes disassembly and corrective procedures which are performed to properly clean, shape, and seal the root canal system.[1] The structural durability of the tooth following root canal therapy is based on the remaining tooth structure present. Aggressive instrumentation of root canal causes loss of dentin which may weaken the tooth.[2] The goal in both primary and retreatment procedure is to produce a preparation without any deviation from the original canal curvature and procedural errors. Nickel-titanium (NiTi) rotary retreatment systems enable faster removal of obturated materials with reduced patient and operator fatigue.[3] When compared to rotary NiTi systems, the canal centering ability of rotary NiTi retreatment systems is questionable due to the lack of literature evidence.

The most common root canal filling material requiring removal is Gutta-percha due to its wide use as an obturating material. Resilon derived from polymers of polyester has a handling property similar to Gutta-percha and has been proved to be best-removed using a combination of rotary instrumentation and chloroform.[45] The Resilon system is expected to form a Monoblock.[6] Carrier-based obturating systems like Thermafil can make canal centering more difficult for retreatment systems. Mandibular first molars are known to possess curvatures in bucco-lingual and mesio-distal views[7] and has been reported to be the most complicated of all canals for endodontic treatment.[8]

MATERIALS AND METHODS

Ninety extracted mandibular first molars with mesio-buccal canal access angle (CAA)[9] between 20 and 40 were selected [Figure 1].

Figure 1

Canal Access Angle determination: A - Root canal orifice, B - Root apex, C - Point of deviation of straight line drawn from root apex, CAA - Canal access angle

15 K-file was inserted into mesio-buccal canal until it was seen at the apex, 1mm was reduced from this length and working length was determined. Canals were prepared with RaCe to size 25, 0.06 taper as per the manufacturer's instructions at 500 rpm in a crown down fashion. Canals were irrigated with 2.5 ml of 5.25% NaOCl and recapitulated with No. 25 K-file.

All the specimens were subjected to spiral computed tomography (SCT). They were aligned so that the long axis was perpendicular to the beam. A three-dimensional imaging was performed using Dentascan (GE Electricals, Milwaukee, WL, USA). Three cross-sectional CT views at apical, middle, and cervial third were obtained at 3 mm intervals from the apex. The remaining dentin thickness from the lumen root canal to the most mesial and distal part of the root surface was measured[10] preoperatively as M1 & D1 respectively [Figure 2].

Figure 2

The preoperative remaining dentin thickness from the root canal wall to the root surface mesially and distally - M1 and D1, respectively

Specimens were divided into three groups of thirty (n = 30) to receive three different obturation techniques (lateral compaction, Thermafil, Resilon/Epiphany).

In lateral compaction — 25 0.02 taper Gutta-percha master cone was used with AHplus sealer and 20 finger spreader for compaction. Accessory cones were placed and compacted until spreader was unable to penetrate beyond the coronal third. Excess was sheared and compacted with a plugger, Thermafil-size 25 Thermafil verifier was verified. AHplus sealer was used. Size 25 Thermafil obturators were heated in Therma-Prep oven and inserted to working length slowly without twisting or forcing. Carrier was sheared at canal orifice with a red hot instrument, Resilon/Epiphany — size 25, 0.06 taper Resilon master cone was used. Epiphany sealer and thinning resin were mixed to adjust viscosity and coated into the canal. The cone was seated into the canal, and light cured for 40 seconds. The excess material was sheared at the canal orifice.

Further, the samples were divided into three subgroups based on the retreatment systems (ProTaper Retreatment, MtwoR, REndo)

Chloroform solvent was used in retreatment. All retreatment systems were used as per manufacturer's instructions. ProTaper retreatment - D1, D2, D3 were used at 500 rpm. The D1 ProTaper file was used to remove the filling material from the cervical third, D2 coronal third and D3 with light apical pulses of pressure until working length was reached.

MtwoR-were used at 300 rpm. The canals were instrumented to working length using MtwoR1 size 15 0.05 taper followed by R2 size 25 0.05 taper in brushing action with circumferential filing movement.

R-Endo-was used at 300 rpm. Size 25 0.04 taper Rm hand file was used with ¼ turn pressure directed toward the apex to create a pathway allowing the centering and alignment of subsequent instrument. Size 25 0.12 taper Re NiTi rotary file was 1-3 mm beyond the pulp chamber with circumferential filing. Size 25, 0.08 taper R1 was used at coronal third, size 25, 0.06 taper R2 at middle third and size 25, 0.04 taper R3 to working length through repeated apically directed pushing actions.

Irrigation with 2 ml of 5.25% NaOCl was performed throughout the procedure at change of each instrument. Samples were again subjected to SCT and evaluated at the same three levels as in preoperative topographs.[10] The remaining dentin thickness from the lumen root canal to the most mesial and distal part of the root surface was measured[10] postoperatively as M2 & D2 respectively [Figure 3].

Figure 3

The postoperative remaining dentin thickness from the root canal wall to the root surface mesially and distally - M1 and D1, respectively

The canal centering ratio[10] was calculated using the formula M1-M2/D1-D2 or D1-D2/M1-M2. The mean centering ratio indicates the ability of the instrument to stay centered in the canal. Result of 1 indicated perfect centralization capacity and closer the value to 0 the worse the ability. Statistical analysis was performed using one-way ANOVA using SPSS 2009, (Statistical package for social sciences) SPSS Inc, Chicago, USA. The level of significance was set at P < 0.05.

RESULTS

The specimens were named as follows for ease of explanation [Table 1].

Table 1

Grouping of specimens according to obturation and retreatment techniques

Statistical analysis for canal centering between all subgroups showed no statistically significant difference. The mean values showed that [Table 2] in coronal third subgroup, LR had the maximum canal centering ability followed by LM and LP, respectively, middle third LP followed by LR and LM, respectively, in apical third, LR followed by LM and LP, respectively, [Table 3] in coronal third subgroup, TM had the maximum canal centering ability followed by TR and TP, respectively, middle third TP followed by TR and TM, respectively, in apical third, TM followed by TR and TP, respectively, [Table 4] in coronal third subgroup, RR had the maximum canal centering ability followed by RM and RP respectively, middle third RM followed by RR and RP, respectively, in apical third, RM followed by RR and RP respectively.

Table 2

Distribution of mean and SD values of canal centering ratio (M1-M2/D1-D2 or D1-D2/M1-M2) at three levels with three different retreatment systems for lateral compaction group

Table 3

Distribution of mean and SD values of canal centering ratio (M1-M2/D1-D2 or D1-D2/M1-M2) at three levels with three different retreatment systems for Thermafil group

Table 4

Distribution of mean and SD values of canal centering ratio (M1-M2/D1-D2 or D1-D2/M1-M2) at three levels with three different retreatment systems for Resilon/Epiphany group

DISCUSSION

Retreatment of root-filled teeth is indicated when there is a persistence of disease resulting from microleakage, incomplete cleaning and shaping, technical shortcomings, or complex anatomy.[5] The ability of the instrument to keep centered inside the canal is essential to provide correct enlargement, without excessive weakening of root structure.[11] Canal centering ability becomes a critical aspect during endodontic retreatment to prevent deviation from the originally prepared canal.

Mesio-buccal canals of mandibular first molars are reported to be curved both in bucco-lingual and mesio-distal direction.[7] The selection of teeth with similar CAA was chosen to standardize the degree of severity of the canal curvature (20-40°) is important as the roots were considered to have severe curvature at the apical region. The smaller the CAA, greater is the apical curvature which would intensify the deformation and cause stress at the tip of the instrument leading to deviation and fracture, in addition, the obturating material may add to torsional fatigue.[9]

Lateral compaction of Gutta-percha has been used for years as a conventional root canal filling technique, and regarded as a reference point for comparison with other techniques.[12] Thermafil obturation system results in a denser, better-adapted obturation upon radiographic examination in curved canals.[13] During retreatment, the presence of carrier may possibly deflect the instrument leading to eccentricity. Resilon/Epiphany system a thermoplastic synthetic polymer performs like Gutta-percha and has similar handling properties. The core (Resilon) bonds to sealer (Epiphany) which bonds to the root dentin by resin primer.[6] Its sealant being a dual curable resin provides immediate coronal seal thereby forming a “Monoblock.”[1415]

The removal of Gutta-percha using hand files with or without solvents can be tedious, time-consuming process,[4] comparatively, the NiTi rotary retreatment systems may decrease patient and operator fatigue.[3] The radiographic evaluation,[16] though not destructive, only allows for two-dimensional evaluation of the root canal[17] whereas SCT has the advantages of precise three dimensional details of the root canal anatomy[18] with no sample destruction.[17]

In the lateral compaction Group, the better centering ability of REndo instrument in coronal third could be due to larger diameter pilot hole created by Rm hand file (25 size 0.04 taper) eliminating the coronal interferences of both gutta-percha and dentin. The positive rake angle and active cutting tip of the MtwoR might have allowed better canal centering. The ProTaper retreatment because of its negative rake angle, convex triangular cross-section and excessive taper makes it less flexible, and removal of dentin along with Gutta-percha[19] could have resulted in poor centering. In the middle third, the better centering of the ProTaper retreatment could be due to the specific flute design which has a capability to pull the GP into the flutes and direct it towards the orifice.[19] The poor centering of MtwoR could be due to the aggressive cutting nature, and active cutting tip compared to noncutting tips of the D2 ProTaper and R2 Rendo systems. The better centering of REndo in the apical third could be due to lesser taper of the last instrument R3 (size 25 0.04 taper) than the actual prepared apical size (size 25 0.06 taper) of the canal. The rounded core structure of R3 instrument[12] provided better flexibility for negotiation of the apical curvature. MtwoR and ProTaper retreatment system have lesser canal centering ability compared to REndo which could be due to greater taper of the instruments thereby inversely affecting the flexibility of the final instrument MtwoR2 (Size 25 0.05 taper) and ProTaper D3 (size 20 0.07 taper).

In Thermafil group, the coronal third MtwoR centered itself better because of positive rake angle and active cutting tip along with the grooved carrier to guide the instrument. The poor centering ability of REndo system was due to the features of the Rm hand file and Re rotary file which could not provide its advantages due to the presence of the plastic carrier. ProTaper retreatment deviated more due to the active tip of D1 instrument (size 30 0.09 taper) and the suggested 500-700 rpm which could cause the deflection of the instrument due to the carrier causing excessive dentin removal. The better centering with the ProTaper retreatment system in the middle third could be due to the flute design of the instrument which tends to pull the GP into the flutes and direct it toward the orifice.[19] This could also be due to frictional heat produced during instrumentation which plasticizes the GP and disengaging the carrier making it easier to retrieve[19] as observed. However, the other two retreatment systems caused deflection in the middle third due to the grooved plastic carrier which could not be disengaged and had to be removed manually. In the apical third, the retrieval of the carrier provides a pilot hole to retreat the apical third. The possible reason for the better centering of MtwoR and REndo is that with their instrument design providing lesser core structure, thus improving flexibility helping themselves center inside the canal better in the apical third but ProTaper retreatment centered poorly due to its convex triangular design and greater taper of 0.07.

In Resilon/Epiphany group, the better centering ability of REndo in coronal third could be due to a larger diameter pilot hole created by the proprietary Rm hand file (25 size 0.04 taper) by elimination of the coronal interferences of both Gutta-percha and dentin. The ProTaper retreatment system because of its negative rake angle, convex triangular cross section and excessive taper makes the instrument less flexible and its ability to remove dentin along with Gutta-percha[19] could have resulted in poor centering. The ProTaper retreatment instrument which tends to pull the filling material was unable to pull out Resilon due to the bonding between the materials forming a “Monoblock” leading to poor centering. In the middle third and apical third, the better centering ability of MtwoR than REndo and ProTaper retreatment is due to cutting tip present in the MtwoR instruments which helps to penetrate through the dense Resilon obturation whereas ProTaper D2 and D3 and REndo lacking an active cutting tip and thus unable to penetrate through the material causing defection and eccentric preparation of the canal.

CONCLUSION

All three NiTi rotary retreatment systems used with three different obturation techniques showed eccentricity at all three levels.

REndo and MtwoR instruments that have reduced core diameter and positive rake angle showed better canal centering.

ProTaper retreatment instruments with negative rake angle, increased core diameter and taper showed more tendency for eccentric canal preparation especially in the apical 3rd of the root canal.

REndo and MtwoR instruments with reduced core diameter could not disengage the plastic carrier of the Thermafil obturation system and needed to be manually disengaged.