Introduction, Materials and Methods.
Instrument separation and deformation are serious concerns in root canal treatment. During shaping, instruments might lock and/or thread (screw) into the canal. Locked instruments are subjected to high levels of stress, frequently leading to separation. Several studies have evaluated the influence of various factors on the fatigue life and resulting separation of endodontic Niâ€“Ti alloy instruments. In one study, the authors investigated the effect of rotational speed, angle of curvature, and radius of curvature on cyclic fatigue of Lightspeed rotary Niâ€“Ti instruments (Pruett et al . 1997). Ramirez-Solomon et al (1997) evaluated the incidence of the Lightspeed separation. Thompson & Dummer (1997a,b) and Bryant et al . (1998) demonstrated that the incidence of Niâ€“Ti rotary instrument deformation and separation was related to instrument design and instrumentation technique.
The influence of operator experience was assessed in three studies that demonstrated proper tuition or experience was necessary to minimize the incidence of instrument separation (Barbakow & Lutz 1997, Mandel et al . 1999, Yared et al . 2001).
Pruett et al . (1997) found that the rotational speed did not affect cyclic fatigue of Lightspeed instruments. Gabel et al . (1999) and Yared et al . (2001) demonstrated that failure of ProFile Niâ€“Ti rotary instrument was less probable at a lower rotational speed.
Torque is another parameter that might influence the incidence of instrument locking, deformation, and separation. Theoretically, an instrument used with a high torque is very active and the incidence of instrument locking and, consequently, deformation and separation would tend to increase. Whereas a low torque would reduce the cutting efficiency of the instrument, and instrument progression in the canal would be difficult, the operator would then tend to force the instrument and may encourage instrument locking, deformation, and separation. Recently, Yared et al . (2001) evaluated the influence of torque on the incidence of ProFile failures. According to Kobayashi et al . (1997) torque should be set between 0.39 and 0.78 Ncm for ProFiles to avoid instrument failures. Svec & Powers (1999) compared the torque at failure values of used and unused ProFiles. They demonstrated that torque values at failure were very low for both used and unused instruments.
The Greater Taper (GT) Niâ€“Ti rotary instruments (Dentsply/Maillefer, Maillefer, Switzerland) are manufactured in 0.06, 0.08, 0.10, and 0.12 taper. All the instruments have the same tip diameter of 0.20 mm and a maximum flute diameter of 1.0 mm. The influence of various parameters on the failure of GT rotary instruments has never been tested.
The purpose of this study was to evaluate the influence of rotational speed, torque, and operator experience on the incidence of locking, deformation, and separation of GT Niâ€“Ti rotary instruments during repeated simulated clinical use, exposure to 2.5% NaOCl and steam autoclave sterilization.
Materials and methods.
The present study included three parts that assessed, respectively, the influence of rotational speed, torque, and operator experience on the incidence of instrument locking, deformation, and separation. The same design was used in a recent study that evaluated ProFile instruments (Yared et al . 2001). Extracted human mandibular and maxillary first and second molars with mature apices and demonstrating curvatures ï€¾ï€ 25 ï‚°ï€ (Schneider 1972) were used; the teeth were kept in 10% formalin at 37 ï‚°ï€ C. Access cavities were prepared, the canal orifices located and the cavities irrigated with 2.5% NaOCl. Patency of the canals was determined with a size 06 K-type file (Dentsply/Maillefer, Ballaigues, Switzerland). Only canals having a snug fit with a .08 or 10 K-type file were included. The snugness indicated that the canal was narrow and suitable for inclusion in the study. The working length of each canal was determined by passing a size 06 file to the apical foramen and then subtracting 0.5 mm. Working length and reference points were recorded for each canal. Initial radiographs were taken from the buccal and proximal directions; exposure time and processing were standardized. The radiographs were used to detect canals that joined each other; in these cases only one canal was included in the study. The angle of curvature and the radius of curvature were determined on the initial buccal radiograph using the method of Pruett et al . (1997). Canals were ordered according to radius of curvature (least to greatest) then randomly and blindly assigned into three groups, such that all ranges of radii of curvature were equally represented in each group, and such that each group included 100 canals. In the three groups, GT instruments were used in a crown-down technique sequentially in a descending order of taper. The instruments were used in a handpiece in conjunction with a variable high torque motor (TC 3000 Maillefer) and used according to the following principles: the apical pressure exerted on the GT was light and each instrument was used for only a few s in the canal; the GT was used with small in and out movements. The canals were enlarged until a 0.08 taper GT reached the working length. Three to four recapitulations (waves) with GT taper 0.12â€“0.06 were required to complete cleaning and shaping of each canal. Preparation was judged to be complete when a 1â€“ 1 Machtou plugger (ISO size 50) penetrated to 5â€“7 mm short of the working length, and a fineâ€“medium guttapercha cone fitted 0.5 mm short of the working length. During shaping each canal was irrigated with 5 mL of 2.5% NaOCl using a 5/8-inch 27-gauge needle placed as far into the canal as possible without binding. The patency of the apical foramen was frequently checked by passing the tip of a size 08 file through the foramen. Before each use, the GT set (kit) was sterilized by steam autoclave for 5 min at 135 ï‚°ï€ C; the whole cycle of sterilization lasted 35 min; the same set was used in up to 10 canals. Ten sets of four GT taper 0.12â€“0.06 were included in each of the three groups. A ï‚´ï€ 2.5 magnification was used to check for instrument deformation after each passage. An operator blinded to the study performed the instrument inspection. Instrument deformation, separation and locking within each group were recorded. The number of canals shaped by each instrument was also recorded. In case of instrument deformation or separation, the instrument was replaced. The number of instruments required to complete the cleaning and shaping of the 100 canals in each subgroup was recorded. In case of instrument locking, the rotation direction was reversed to disengage the instrument and shaping was completed after examining the instrument for deformation. The locked instruments were reused in the following canals and not discarded. Statistical analysis was carried out with pairwise comparisons using Fisherâ€™s exact tests for each of the failure types. The SAS 6.12 program for Windows (SAS, Cary, NC, USA) was used; significance was set at the 95% level.
In the first part of the study, the rotational speed was fixed at 150, 250, and 350 r.p.m. for subgroups 1, 2 and 3, respectively. The torque generated by the motor was set at 20 Ncm. The same operator performed the cleaning and shaping procedures in all the canals of the three groups. The canals of subgroup 1 were first prepared, followed by subgroup 2, and then by subgroup 3. In the second part of the study, the rotational speed for the three groups was fixed at 150 r.p.m. and the same operator performed the preparation procedures. The torque generated by the motor was set at 20, 30, and 55 Ncm for subgroups 4, 5, and 6, respectively. The canals were prepared in that order. In the third part of the study, the rotational speed was set at 150 r.p.m. for the three subgroups (7, 8 and 9) and the torque generated by the motor was fixed at 20 Ncm. Different operators performed the endodontic procedures in the three subgroups. In subgroup 7, an endodontist experienced in the instrumentation technique performed the cleaning and shaping. In subgroup 8, the operator was trained on 30 clear resin endodontic blocks with curved canals and on 10 curved canals in extracted human molars. In subgroup 9, the operator was introduced to the technique without any training. The operators in subgroups 8 and 9 were general dentists whose degree of experience with respect to endodontics was limited to the training received in their undergraduate studies and to the experience acquired during 3 years of general practice.