Introduction

Endodontic procedures require adequate cleansing and shaping and an acceptable three dimensional obturation of root canal systems. Vertical root fracture and cracks may appear after canal enlargement, filling especially lateral compaction , post space preparation, retreatment and ultrasonic instruments (1).

These consequences have seen during and after root canal therapies and may finally lead to treatment failures or even tooth loss (1). Therefore, early diagnosis of predisposing factors and efforts to reduce their incidence is necessary.  Numerous studies have reported that root canal preparation with nickel-titanium (NiTi) rotary instruments leads to crack formation in root dentin(2-4). Rundquist showed that cracks originate from the apical third (5).

Cracks or craze lines may initiate from the external surface of root without extending into the root canal lumen, or they may extend from inside the canal toward the outer surface, without reaching it. Alternatively, they may extend from the most inner to the most outer surface and make a connection between pulp and periodontal ligament (6). Occurrence of such lesions should be prevented as far as possible.

ProTaper files are well known rotary instruments with Convex Triangular Cross Section that performed well in different studies with regard to shaping ability (9, 10).

RaCe instruments with a triangular cross-sectional design and alternating cutting edges (straight sector varies with twisted sector) are claimed to prevent from screwing the instrument into the root canal thus reducing intraoperative torque values (11).

 Because of their increased tapering, their active cutting movements are facilitated and thus more dentin is removed in coronal areas in comparison to other rotary systems and maintained the original canal curvature more than ProTaper (12).

NiTi Tee instruments consist of six Ni-Ti files (Coronal Shaper, K-type and five files with a modified S-profile) with varying tapers for use with the crown down method. Like most rotary instruments is a rounded noncutting tip with S-type flute design that is characterized by two 90° cutting edges with no radial lands. Flutes at base are twice the size of flutes at the tip.

This three NiTi root canal instruments represent three different cross-sectional designs with large variations in the depth of the flute; apply in crown down technique (13).

Hyeon-Cheol Kim showed that stiffer file designs generated higher stress concentrations in the apical root dentin during preparation of the curved canal, which increases the risk of dentinal defects. Thus, stress levels during shaping and fracture susceptibility after shaping vary with instrument design (14).

Also the final apical preparation size is an important factor in root canal cleanliness. However, enlargement of the apical region with larger instruments may cause excessive crack formation and root canal transportation. There is currently no consensus on the optimum final apical preparation size (15) Several studies have examined and compared the crack formation in various Rotary, Recircal and Hand preparation systems (3, 8, 9, 16, 17).  They claimed that the tip design, cross-section geometry, constant or progressive taper type, constant or variable pitch, and flute form might also contributed to defect formation. Yoldas et al reported the fracture rates up to 60%.  The relatively low flexibility of the rotary files and repeated instrumentation of the roots reported the two reasons for the high degree of crack formation in this study (8).

To our knowledge, no studies have been performed to evaluate dentinal cracks that may cause by three multi rotary files with the same number of files and the same application method but various cross sections. Thus the aim of this study was to compare the Niti Tee, RaCe and Protaper instruments on dentinal crack formation after root canal preparation of mesial roots of mandibular molars.

Material and methods

In this in vitro study, 50 extracted first mandibular molars were selected.

The inclusion criteria were mandibular molars with roots of 0-20 degrees of curvature. The teeth were examined radiographically and molars with previous dental treatment, open apex or root resorption and severe anatomic variation were excluded from the study.

Root surfaces were cleaned from calculus and tissue remnants and disinfected in NaOCl 2.5% ( Bojneh Co. Iran) by 1 minute immersion. After disinfection, the teeth were kept in normal saline.

The distal roots of the specimen were cut at a 11mm distance from the apex by a low speed saw (Isomet, Buehler, Ltd, Evanston: IL, USA (accompanied by water spray as coolant. The roots were observed by   stereomicroscopy and light transmission by 12-fold magnification to detect any possible cracks or defects. Defective teeth were excluded from the study and replaced by sound molar roots.

The working length was determined by a K-file #15 (Dentsply, Maillefer, Switzerland (. The file was introduced into the canal until the tip could be seen through the apical foramen. Working length was established by 1mm reduction from the acquired length.

 To simulate PDL, the silicon impresion material (Colten Whaledent AG Altsatten Switzerland) was used so that 10 mm of the roots were placed in 5 × 5 puty blocks (Altsatten Switzerland).

The roots were randomly divided into three experimental groups of 15 and one control group of 5 teeth.

 In group 1, the roots were prepared by ProTaper universal rotary system (Densply Maillefer. ballaigous. Switzerland (coupled with reduction gear rotary handpiece driven by electric torque device (VDW co, Munich, Germany) according to manufacturer’s instruction up to F3 (30/0.09) file. Sx file and then S1 and S2 were initially used with 300 rpm and 3 N/cm torque for flaring the coronal portion. Then, F1, F2 and F3 were used to the working length with a gentle apical pressure. Each file was used for preparation of 3 canals.

In group 2, roots were prepared with RaCe (FKG Dentaire, La Chaux-de-Fonds, Switzerland) rotary system using an electric motor with torque control (VDWco, Munich, Germany) according to the manufacturer's instructions with 600 rpm and 2N/cm as follow: ≠10/0.04, ≠35/0.08, ≠25/0.06, ≠25/0.04, ≠25/0.06 and ≠30/0.06. Each file was used for preparation of three canals.

In Group 3, mesial roots were prepared by NiTi ee (Sjöding Sendoline, Kista, Sweden) file system according to manufacure’s instruction with 300 rpm and 2N/cm torque as follow; first #30/0.08  used for preflaring of the coronal and middle of the canal and creating a straight line access, Then #30/0.06 to 2/3 of working length, #30/0.04 to apical one-third, #25/0.04 to the working length, and then #20/0.04, #25/0.04 and #30/0.04 and finally #30/0.06 were used for apical enlargement. Each file was used for preparation of three canals.

  In group 4 (control group) the tooth left unprepared.

  During preparation, each canal were intermittently cleansed by sodium hypochlorite 2.5% (Bojneh Co. Iran), with a 27 gauge needle.

As final irrigation in all experimental groups (1, 2 and 3) 1 ml of EDTA 17% (Ariadent, Tehran, Iran) for 30 seconds and 5.25%  sodium hypochlorite were used for removal of smear layer.

 After instrumentation, the teeth were removed from the simulated block and washed with tap water. The Samples were cut horizontally At 3, 5 and 9 mm distances from the apex using a low spead saw (Isomet:BuehlerLtd.Lake Bluff.IL) of 0.13 mm thickness and water spray as coolant. After cleansing the surface, the samples were examined with a digtal stereomicroscope (BX43, Olympus) at a 40X magnification and the images were taken. Samples were evaluated by 2 observers to assess presence of any microcracks.

The samples were placed in "no crack" group, in the lack of any craze lines or microcracks either at the external surface of the root or at the internal surface of the root canal wall toward dentin. ‘‘crack group’’ was defined if any craze lines, microcracks, or fractures were present in root dentin.

All 50 specimens were assessed and the incidence of cracks on dentin was reported in number and percentage. The operators were blind to the grouping of samples. In cases of disagreement between observers, the samples were re-evaluated so that consensus was acquired.

Chi-Square test was used to find significant differences in crack incidence between the study groups.

Results

  In this study, 50 teeth were prepared by Protaper, RaCe and NiTiTee rotary systems in equal groups of 15. Cross sections were evaluated for presence of microcracks. Also, in each system, 45 cuts were evaluated in 15 teeth at 3 mm, 5 mm and 9 mm distance from the apical region. No crack was observed in the control group.   There was no defect in any of the groups at 3 mm sections in this study.

Dentinal defects were observed in 3 samples (20%) of Protaper group that two cases of them were as complete crack (figure 1), 2 samples (13.3%) of RaCe group and 4 samples (26.7%) of NiTi Tee group.

Chi-square and Kruskal Wallis test showed no significant differences in the frequency of cracks at 3, 5
and 9 mm sections from apex between three experimental groups (P=0.76). Of total of 135 sections in three rotary systems, the defects were observed in 9 samples (20 %).

Comparisons between Protaper, RaCe and NiTiTee rotary systems in producing dentinal defects are shown in table 1.

Figure 1. Microscopic view of complete crack in the dentinal wall

Table 1. Distribution of dentinal defects in different root canal preparation systems

* The numbers in parenthesis denote the number of teeth with cracks in 3, 5 and 9-mm sections, respectively