A. Kfir, E. Rosenberg, O. Zuckerman, A. Tamse & Z. Fuss
Department of Endodontology, The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel.

Introduction.
Thorough debridement of the root canal system followed by a complete seal is an important step in root canal treatment (Schilder 1974). During instrumentation, iatrogenic procedural errors, such as zipping, canal transportation, ledging and root perforations, may occur, especially when the instruments are rigid (Cattoni 1963, Walton & Torabinejad 1996, Bakland 1991). These aberrations appear more frequently in student clinics when using traditional instruments and techniques (Hayes et al. 2001).
Nickel-titanium (NiTi) instruments were introduced to alleviate procedural errors (Walia et al. 1988). They allow better negotiation of the curved root canal, with fewer procedural errors (Pettiette et al.1999). Along with improved alloys and design, such as blunt tips, flexible shanks and less aggressive flutes (Hankins & El Deeb 1991, Saunders & Saunders1992), a number of canal preparation techniques have also been developed, e.g. the ‘serial step-back technique’ (Clem 1969) and ‘balanced force technique’ (Roane et al. 1985). These techniques have been shown to produce preparations that maintain the original curve of the canal (Bolanos & Jensen1980). Recently, automated rotary NiTi instruments were introduced that significantly reduce the incidence of procedural error and reduce treatment time (Roig-Cayon et al. 1997, Thompson & Dummer 1997, Spangberg 1998).
The numerous innovations in the field of root canal treatment present dental schools with a new challenge, that is, to expose students to the numerous types of instruments and techniques for preparing root canals and, at the same time, to minimize procedural errors. A new‘ 8-step method’ for canal preparation was developed at our dental school that integrates traditional instruments and techniques with modern ones. The method includes conventional (K- and H-files), rotary (Gates-Glidden), and tapered and nontapered NiTi instruments (Profile and Lightspeed). Steps 1 and 2 include traditional access cavity and coronal flaring using hand instruments with a filing motion or rotary Gates-Glidden reamers sizes1, 2, 3 and tapered NiTi Pro- file rotary instruments.
Determination of working length is established in step 3 using radiographs or an electronic apex locator. The apical preparation in narrow canals is prepared using reaming or filing motions up to size 25 with stainless steel K-files (step 4).With larger sizes, only a reaming motion is recommended with K-files, orwith rotary NiTi Lightspeed files (step5).Hand instruments with reaming or filing motions and rotary NiTi Profile files are used to prepare the middle third of the canal to blend the coronal flare with the apical preparation (step 6). Obturation of the root canals is completed with gutta-percha points and AH26 as a sealer using the lateral condensation technique following adaptation of the master guttapercha point (steps 7 and 8). Junior students are exposed to the traditional hand instruments and Gates-Glidden reamers associated with the new techniques, whilst senior students are also exposed to NiTi rotary instruments.
The aim of the present study was to compare the incidence of procedural errors during root canal preparation by junior dental students in patients using the new ‘8-step method’ versus the traditional ‘serial step-back technique’.

Materials and methods.
Junior dental students treated 291root canals of all tooth types in an undergraduate clinic within the School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel. The traditional ‘serial step-back technique’ was performed in 142 root canals by all students in one class and the new‘ 8-step method’ in 149 root canals by all students in another class. The students of each class had the same experience following training on extracted teeth using the method performed in their class. The treated canals were the first clinical cases for each student.

The following steps were carried out with the ‘serial step-back technique’:

1. Access opening using cylindrical diamond (Strauss, Netanya, Israel) or tungsten carbide (SS White, Lakewood, NJ, USA) drills.
2. Working length determination using radiographs and electronic apex locator if needed.
3. Canal enlargement at full working length to size 25- 30 followed by serial step-back preparation using only a filing motion with K-files (VDW, Munich, Germany) in an apical to coronal direction. Every instrument beyond size 25-30 was inserted1mm short of the length of the previous instrument up to average size 45-50. In large canals, e.g. palatal roots of maxillary molars, further coronal flaring was carried out using Gates-Glidden reamers size 2 or 3.
4. Master cone verification to length and size.
5. Canal obturation using the lateral condensation technique with gutta- percha cones (VDW, Munich, Germany) and AH-26 (Dentsply DeTrey, Konstanz, Germany) as a sealer.

The following steps were carried out with the new‘8- step method’:

1. Access opening using cylindrical diamond or tungsten carbide drills.
2. Enlargement of the coronal third of the canal before working length determination using a filing motion with K- or H-files (VDW, Munich, Germany) and a reaming motion with K-files or with rotary Gates-Glidden reamers (VDW, Munich, Germany).
3. Working length determination using radiographs and electronic apex locator if needed.
4. Enlargement of the apical third of narrow canals using both reaming or filing motions with K-files up to size 25.
5. For canals larger than size 25 and instrumented canals to size 25, further enlargement of the apical third of the canal using only the ‘balanced force technique’ with K-files size 30 and larger, up to the appropriate size for each canal as presented in Table 1.
6. Enlargement of the middle third of the canal to blend the coronal flare with the apical preparation using a reaming motion with K-files larger than the master apical file.
7. Master cone verification to length and size.
8. Canal obturation using the lateral condensation technique with gutta-percha cones and AH-26 as a sealer.

All hand instruments were stainless steel, and during root canal preparation, repeated irrigation with 0.5% sodium hypochlorite and saline were carried out. Pre and postoperative and working length radiographs for each tooth were recorded. Two independent endodontists were asked to detect the following procedural errors radiographically: canal transportation, root perforations and canal obstructions without knowing which method was performed. Patients’ files were also evaluated separately for procedural errors recorded during treatment and included, even if not evident, in the radiographs. The results were statistically analysed using the ‘binomic test for comparison of proportion’.

Table 1. Average master apical file for the '8-step method'.


Average master apical file (MAF) of different root canals as suggested when using the '8-step method' for preparing root canals. Values determined according to initial diameter of each canal, tactile feeling and root canal anatomy. It is recommended only when a reaming motion is performed in the apical third of the root canal.

Results.
The results of the different procedural errors are presented in Fig.1.With the ‘8-step method’, significantly (P < 0.0001) more root canals (136, 91%) maintained their original shape with no deviation compared to the conventional ‘serial step-back technique’ (87, 61%). The following procedural errors were detected in the‘8-step method’: 10 canals with transportation (5%) and five root perforations (2%); there were no canal obstructions. With the ‘serial step-back technique’, significantly (P < 0.0001) more errors occurred: 28 canals were transported (17%), 10 had root perforations (7%) and16canals had obstructions (6%).
The percentage of canals maintaining their original shape with both techniques in anterior and posterior teeth is presented in Fig. 2.All anterior teeth maintained the original canal shape when treated with the ‘8-step method’. Only 78% of the anterior teeth had no deviation when prepared with the ‘serial step-back technique’. In molars, 88% maintained the original curve when treated with the ‘8-step method’ and only 49% when prepared with the ‘serial step-back technique’ (P < 0.0001).

Figure 1. Procedural errors examined during instrumentation of root canals by junior dental students using the '8-stepmethod' (n =149) compared with the'serial step-back technique' (n =142). A significant difference was observed between the two groups (P < 0.0001).



Figure 2. Maintenance of the original root canal shape during instrumentation of anterior and posterior teeth by junior dental students using the'8-step method' (n =149) compared with the'serial step-back technique' (n =142). A significant difference was observed between the two groups (P < 0.0001).

Discussion.
There are a variety of innovative instruments and techniques available to prepare root canals. However, each has its advantages and disadvantages. The acquisition of knowledge to allow the selection of the appropriate instruments and techniques for improved quality and efficient treatment requires time and experience. Dental schools worldwide are faced with the challenge of providing undergraduate students with the skills to prepare root canals safely and to expose them to a diversity of new instruments and techniques within a limited time period, normally 2-3 years.
The new ‘8-step method’ was developed to integrate the various instruments and techniques in one method. During the 3-year programme, undergraduate students are exposed to traditional stainless steel instruments as well as the modern rotary NiTi ones. Furthermore, the students become familiar with different instrumentation motions in the root canal (e.g. filing and reaming) and different techniques (e.g. ‘serial step-back technique’ or ‘balanced force technique’). After graduation, they are expected to recognize the advantages and disadvantages of each instrument, motion or technique and select which is best for their practice needs.
Flaring of the coronal third of the root canal before determining the working length has several advantages (Morgan & Montgomery 1984, Stabholtz et al. 1995). It removes coronal curvatures, improves tactile feel and allows irrigation of the coronal two-thirds of the canal to minimize extension of infected debris beyond the apical constriction. With this method, the unskilled student could block the canal. Therefore, students in this school were instructed in traditional techniques to determine the working length first and then instrument the canal to the apical constriction. With the‘8-stepmethod’, each student practices flaring of the coronal third with hand or rotary instruments on extracted teeth before treating patients. In the present study, the careful approach by the unskilled student prevented obstructions with the ‘8-step method ’compared to the traditional ‘serial stepback technique’. It should be emphasized that the blunt tip design of modern hand and rotary instruments make coronal flaring easier and safer (Miserendino et al. 1985, 1986).
Preparation of the apical third using the ‘8-step method’ is divided into two steps. The narrow canals are first instrumented to size 25 and then evaluated by both the student and instructor for procedural errors. The canal is further enlarged according to average initial diameter, tactile feeling, anatomy of the root canal and type of instrument used. The average size of the master apical file (MAF) is limited for some techniques and instruments to size 25 for simple canals. However, some instruments are designed to enlarge safely the apical third of the root canal to average sizes within the range of 40-60 depending on the initial diameter of the root canal. The present study demonstrated that preparing the apical third to larger sizes did not necessarily increase the incidence of the procedural errors provided the correct method was selected. For example, the average size of MAF for the MB root of maxillary molars was 35 using the ‘balanced force technique’ with the ‘8-step method’ and only 25 for ‘serial step-back technique’, and the procedural errors detected in the new method were significantly less.

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