Working length changes in curved canals after coronal flaring by using rotary files and hand file: An in vitro study.

AIM: This in vitro investigation examined the effect of early coronal flaring (CF) and late CF on the working length (WL) in curved root canals.
BACKGROUND: The objective of this study was to determine if canal length is altered as a result of CF in curved canals of molar roots. STUDY
DESIGN: The conditions compared were combinations of (a) stainless steel hand files using Gates Glidden (G. G.) drills (SS) versus nickel-titanium rotary files (Ni-Ti); and (b) early CF (flaring completed before WL determination) versus late CF (flaring completed after WL determination). Selected were 90 canals of extracted maxillary or mandibular first molars (mesial root of mandibular molars and the mesiobuccal root of the maxillary molars) from three groups. CF was accomplished for the SS group using G. G. drills and for the Ni-Ti group using rotary ProTaper and Hero Shaper files. WL was determined by a digital vernier caliper before CF, immediately after CF, and again after canal preparation. STATISTICAL ANALYSIS: A repeated measures analysis of variance (ANOVA) test and a Tukey's multiple prosthoc test were used for this study.
RESULTS: Results indicated that WL decreased for all canals as a result of canal preparation. The mean decrease in WL was significantly greater for the SS group (-0.77 ± 0.42 mm) than for the Ni-Ti groups (-0.33 mm ± 0.44). Less change in WL occurred in all groups when initial WL was determined after CF.
CONCLUSION: WL in curved canals consistently decreases during the course of instrumentation. Clinician should keep this in mind for better treatment outcome.

INTRODUCTION

Root canal working length (WL) is defined as distance from a coronal reference point to the point at which canal preparation and obturation should terminate. A number of methods for determining WL have been described; including the use of radiographs, electronic apex locators, and tactile discrimination.[1] Regardless of the method, once the appropriate WL has been established, maintaining consistency of that measurement throughout the course of endodontic treatment is crucial to the clinician's ability to instrument and obturate to the desired apical location. Should the WL change during the course of treatment, the depth of canal becomes unpredictable. This is particularly true in the instrumentation of curved canals.[2] Weine et al., found that changes in the WL were produced by straightening of the canal during the course of treatment.[3] Flaring the coronal portion of root canal before instrumenting the apical region may provide a number of benefits. The cervical preparation of curved root canals should ensure straight access to the canals and their apical curvature.[4] Straight line access (SLA) and coronal flaring (CF) are performed to minimize deflection of files to the first bend of the canal. It has been suggested that this straightening of the canal result in a decrease in canal length. Therefore, straight line access and CF should be performed before WL determination.[5] It has been observed that straightening of canal results in a decrease in canal length; therefore, CF should be performed before WL determination.[6] If WL is determined before CF, it results in significant increase in canal length.[7] The outcome would be inadvertent instrumentation beyond the prescribed end point. This over-instrumentation would damage apical tissues, overextension of obturating materials, and adversely affect the prognosis.[8910] Furthermore, an adequate coronal pre-enlargement accurately defines what initial apical instrument should be used[1112] and it results in a more precise anatomical diameter at the WL. However, it is not known if determining WL after initial CF of root canal has been completed, but before apical preparation, will minimize the change in WL that occurs during the instrumentation of curved canals.[8910]

The purpose of this in vitro investigation was to compare pre and post instrumentation WL in curved root canals prepared by (a) using early CF (flaring completed before WL determination) versus late CF (flaring completed after WL determination); and (b) using either stainless steel and nickel-titanium (Ni-Ti) instruments.

MATERIALS AND METHODS

This in vitro study was carried out in the Department Of Conservative Dentistry and Endodontics, Vasantdada Patil Dental College and Hospital, Sangli. For this study, 90 extracted human maxillary and mandibular, first and second molar teeth were selected. Teeth were included in the study if they had normal pulp chambers, patent root canals, no noticeable defects or abnormal root morphology, fully formed root apices without any sign of resorption, similar canal curvature (20-45°), and excluded if calcification or double curvatures of the canals were detected radiographically. Only the mesial root of mandibular molars and the mesiobuccal root of the maxillary molars were considered for the study.

Preparation of samples

Coronal access cavity preparation was made in each specimen by using a carbide bur #10 and #14 using a high speed air rotor hand piece. The distal roots of mandibular molars and palatal and distobuccal roots of maxillary molars were removed by using a diamond disc, so that the mesial root and the associated crown were separated from the remainder of the tooth.

Measurement of canal angulation

Canal curvature of the mesial root was determined by using the Schneider's method.[13] Roots with mesial-distal curvature between 20° and 45° were selected for this study.

Measurement of non-flared WL (WL before intracanal instrumentation)

The occlusal surfaces of each tooth were flattened on a model trimmer to provide a consistent reference point for WL determination. A #10 K file was introduced into the mesiobuccal canal of each specimen. The location on the root where the file first appeared at the apical foramen when viewed through a surgical operative microscope (Global Surgical Corporation, St. Louis, MO, USA) was noted. The root end was resected at, or slightly coronal to the level of the apical foramen using a flat diamond abrasive disk. The root end was placed against a flat glass barrier. A file was placed into the canal until it reached the apical glass barrier. Using the fattened coronal surface as a reference point, WL measurement was determined with the help of a digital caliper in micrometers. This length was designated as non-flared WL. This measurement was recorded three times in succession for each canal, and a mean value for WL was calculated.

Division of samples into respective groups

All the 90 specimens were ranked by degree of maximum canal curvature and divided into three groups. The roots demonstrating the smallest curve was assigned to group 1, the next smaller curve was assigned to group 2, the next smaller curve was assigned to group 3 till the teeth were divided among the three groups. The mean canal curvature per group was calculated and compared to verify similarities in curvature.

Canal instrumentation and measurements in each group

Group 1-Stainless steel group

Specimens in group 1 were treated as follows: The root canal was flooded with a solution of 5.25% sodium hypochlorite (Dentpro, Chandigarh, India). Apical patency was verified using a #10 K file. The canal was irrigated with 1 ml of 5.25% sodium hypochlorite, and G. G. Drill (Dentsply Maillefer, Ballaigues, Switzerland) no. 3, 2, and 1 were used for CF, following the sequence recommended by the manufacturer. The canal was irrigated and the distance to the apical glass barrier was determined as previously described. This length was designated the Group 1 flared WL.

The apical portion was prepared by step-back technique using hand instrument following the sequence recommended by Clem.[14] A #30 K file was used till full WL. Step-back instrumentation was initiated with a #35 K file placed 1 mm short of the flared WL. Each larger instrument was carried to a depth 1 mm short than previous file. Canal flaring was considered complete when the portion of the canal instrumented with hand blended with the portion instrumented with G. G. drills. After the step-back procedure, using #30 master apical file, group 1 Final WL was determined as described previously.

Group 2-Ni-Ti rotary ProTaper group

Each specimen in group 2 was instrumented using rotary ProTaper systems (Dentsply Maillefer, Ballaigues, Switzerland), following the full sequence recommended by the manufacturer. Once coronal portion of the canal was prepared with SX, S1 the canal was irrigated, and the distance to the apical glass barrier was determined as described previously. This length was designated as group 2 flared WL. S1, S2, F1, and F2 were then taken to full length. Final finishing was done by finishing file F3. After irrigation with 5.25% sodium hypochlorite and verifying apical patency, WL was measured as previously described and was designated as group 2 final WL.

Group 3-Ni-Ti rotary Hero Shaper group

Each specimen in group 3 was instrumented using rotary Hero Shaper systems (Micro-Mega, Besançon, France), following the full sequence recommended by the manufacturer. Restrictive dentin was removed using Endoflare in gentle brushing motion. Once coronal portion of the canal was prepared with n°20-0.06 taper to the two-third of WL, the canal was irrigated with 5.25% sodium hypochlorite and the distance to the apical glass barrier was determined as described previously. This length was designated as group 3 flared WL. Final finishing was done by using n°30-0.04 taper files. After irrigation and verifying apical patency, WL was measured as previously described and was designated as group 3 final WL.

RESULTS

Three WL readings were noted for each sample: (1) Non-flared WL (WL before intracanal preparation). (2) Flared WL (WL after CF preparation). (3) Final WL (WL after complete canal preparation).

Mean canal curvature and non-flared WL values were compared between groups by using one way analysis of variance (ANOVA) test. For each specimen, differences between the non-flared WL, flared WL, and final WL were calculated in Table 1. Within each group, a repeated measures analysis of variance test and a Tukey's multiple prosthoc test was used to determine the presence of statistically significant differences between (a) the non-flared WL versus the flared WL; (b) the flared WL versus the final WL; and (c) the non-flared WL versus the final WL. Change in WL due to early CF and late CF was calculated.

Table 1

Mean and standard deviation (SD) of working length (WL; in mm) according to different groups with non-flared WL, flared WL, and final WL

The mean canal curvature for group 1 was 28.47 ± 6.41°, group 2 was 29.17 ± 7.04° and, group 3 was 28.87 ± 6.60°. One-way ANOVA test was used to verify similarity between mean canal angulations of all the groups. No significant difference (P = 0.92) was found between mean canal angulation. One way analysis of variance (ANOVA) test was used to verify similarity between mean non-flared WL of all the groups shown in Table 2 and Figure 1. No statistically significant difference (P = 0.47) was found between mean non-flared WL. Tables 3 and 4 show values of difference of non-flared WL to flared WL, non-flared WL to final WL, and flared WL to final WL of all groups. Statistically significant difference (P < 0.05) was found between all the groups. But greater change in WL at all stages of canal preparation with stainless steel instruments [Figure 2]. In Table 5, Tukey's multiple prosthoc test showed that statistically group 1 was significantly different compared to group 2 and 3 (P < 0.05). There was no statistically significant difference between group 2 and 3 (P > 0.05).

Table 2

Comparison of three groups (stainless steel, ProTaper, and Hero shaper) with respect to non-flared WL, flared WL, and final WL by one way ANOVA

Figure 1

Comparison of three groups (stainless steel, ProTaper, and Hero Shaper) with respect to non-flared working lenght (WL), flared WL, and final

Table 3

Mean and SD of working length (WL; in mm) according to different groups with difference of non-flared WL to flared WL, non-flared WL to final WL, and flared WL to final WL

Table 4

Comparison of three groups (stainless steel, ProTaper, and Hero Shaper) with respect to difference of non-flared working length (WL) to flared WL, non-flared WL to final WL, and flared WL to final WL by one-way ANOVA test

Figure 2

Comparison of three groups (stainless steel, ProTaper and Hero Shaper) with respect to difference of non-flared WL to flared WL, non-flared WL to final WL, and flared WL to final WL

Table 5

Pairwise comparison of three groups (stainless steel, ProTaper, and Hero Shaper) with respect to difference of non-flared WL to flared WL, non-flared WL to final WL, and flared WL to final WL by Tukey's multiple posthoc procedures

DISCUSSION

The objectives of root canal preparation are removal of organic substrate from the canal system by chemomechanical methods and the three dimensional shaping of the root canal system into a continuously tapering preparation while maintaining the original outline form of the canal.[1]

Goerig et al.,[15] advocated shaping the coronal aspect of a root canal first before apical instrumentation is commenced. The authors list the following advantages like straighter access to the apical region, eliminates coronal interference, removes the bulk of tissue and microorganisms before apical shaping, allows deeper penetration of irrigants, minimizes or eliminates the amount of necrotic debris that could be extruded through the apical foramen and final apical instruments are unimpeded through most of their length. Sadeghi and Doago and Ibarrola et al., compared WL changes after SLA and different CF methods. They found that it is better to establish WL after SLA. Least changes in WL occurred with Ni-Ti rotary orifice shapers after CF.[716]

In the present study, CF in the stainless steel group was accomplished with Gates Glidden (G. G.) burs and in the Ni-Ti ProTaper group S1 and Sx and in the Ni-Ti Hero Shaper group with Endoflare were used. In this study; group 1, 2, and 3 differed in the relative amounts of WL change that occurred as a result of CF.

In group 1, initial CF had a major influence on length change. Of the overall 0.77 mm mean decrease in canal length, 0.44 mm occurred during initial canal flaring.

While in group 2, of the overall 0.14 mm mean decrease in canal length, only 0.05 mm decrease in canal length occurred during initial canal flaring.

In group 3, of the overall 0.13 mm mean decrease in canal length, only 0.06 mm decease in canal length occurred during initial canal flaring.

Change in WL due to preparation of coronal portion of the canal was significantly more by stainless steel instruments (P values < 0.05). The relatively large length change produced by the G. G. drills in group 1 may be due to the removal of the cervical bulge of dentin. CF with G. G. drills tends to create a straight line access to the midroot canal. Flaring in group 2 and 3 with ProTaper S1 and Sx and Hero Shaper Endoflare tended to follow the original canal contour and did not create the same type of straight-line access to the midroot canal.

In the study by Davis et al., similar results were obtained wherein mean decrease in WL was significantly greater for stainless steel group (−0.48 ± 0.32 mm) than for the Ni-Ti group (−0.22 ± 0.26 mm).[6] This supposition is also supported by the findings of both Dummer and Thompson[14] and Bryant et al.,[15] who reported that Ni-Ti rotary files caused minimal change in canal configuration.

Once CF was accomplished, change in WL occurred for all the groups during the shaping of apical portion of the canals. Thus, it can be observed that WL consistently changes as preparation of canal progresses towards the apex after CF. Change in WL due to preparation of apical portion of the canal was significantly more by stainless steel instruments.

The results of this study [Table 6] indicate that Ni-Ti rotary files can be expected to consistently produce a small decrease in WL, whereas the combination of G. G. drills and stainless steel files causes more than twice as much decrease in WL. The fact that 16.67% of the canals in group 1 lost at least 1 mm of length, seems to indicate significant potential for overinstrumentation and overfilling of curved canals when G. G. drills and stainless steel files are used, when WL is determined before flaring. None of the canals in group 2 and 3 lost 1 mm or more of length, indicating significantly less risk of overinstrumentation and overfilling when Ni-Ti rotary files are used, regardless of whether flaring is completed before or after WL determination.

Table 6

Number (percentage) of canals showing change in working length using final coronal flaring by all groups

Thus, the clinical impact of length changes that occur during instrumentation may be reduced if WL is verified as the canal preparation around curved portion of the canal progresses. Even if WL were shortened during cleaning and shaping procedures, length verification would permit adjustments to obturation material, and reduce the risk of overfill. Also, subsequent instrumentation should be done using the altered WL.

CONCLUSION

WL in curved canals is consistently decreased during the course of instrumentation. WL is decreased significantly more when a regimen of G. G. drills and stainless steel hand files are used compared to Ni-Ti rotary files. When rotary Ni-Ti files were used the reduction in WL was much smaller. CF before WL determination significantly reduces change in WL that occurs after instrumentation in curved canals, especially for stainless steel group. When rotary Ni-Ti files were used, the reduction in WL, although statistically significant, was much smaller. The clinician should know that the WL subsequently decreases during canal instrumentation. Keeping this factor in mind the cleaning and shaping should be done so as to provide better treatment outcome.