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 »  Home  »  Endodontic Articles 7  »  Failure of ProFile Ni-Ti instruments used by an inexperienced operator under access limitations
Failure of ProFile Ni-Ti instruments used by an inexperienced operator under access limitations
Materials and methods.

Extracted human mandibular and maxillary first and second molars with mature apices and mesial or buccal roots demonstrating curvatures greater than 258 (Schneider 1972) were used; the teeth were kept in 10% formalin at 37 8C. Access openings were made, the canal orifices located and the chamber irrigated with 2.5% NaOCl. Potency of the canals was determined with a size 6 K-type le (Dentsply Maillefer, Ballaigues, Switzerland). Only canals having a snug t with an 8 or 10 K-type le 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 6 K-type le to the apical foramen and then subtracting 0.5 mm. The working length and reference points were recorded for each canal. Initial radiographs were taken from the buccal and proximal directions and 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 four groups, such that all ranges of radii of curvature were equally represented in each group, and such that each group included 30 canals. Wax was placed on the tip of the roots of each tooth. Each tooth was mounted in a maxillary resin arch containing all maxillary (resin) teeth except for tooth 17. The mandibular molars included in the study were also mounted in the maxillary resin arch with the mesial canals directed bucally. The maxillary and the mandibular resin arches were mounted in a phantom head. The degree of mouth opening of the phantom head was standardized for the four groups. A rubberdam was placed and retained on the experimental tooth with a clamp.
In the four groups, 25 mm long PRI with a 0.06 taper, sizes 40-15 were used in a crown-down technique sequentially in a descending order of size. In group 1, the PRI were placed in an air motor (Model 324, Micro- Mega, Besanc¸ France) with a contra-angle (6 :1) reduction handpiece, set at 166.67 rpm. Placing the handpiece of the air motor on the‘Turtle’setting and placing the indicator on 1000 the speed is determined. Air pressure at full rheostat depression was 40 psi. In group 2, the PRI were used in a handpiece in conjunction with a high torque motor (NouvagTC 3000, Nouvag, Goldach, Switzerland). The torque on this motor was set at 10 Ncm. This motor allows torque to be set at very high levels varying from 10 to 55 Ncm. The rotational speed was set at 170 rpm. In group 3, the PRI were used with a low torque motor (Nouvag Micromotor TCM Endo 2, Nouvag, Goldach, Switzerland) that allowed low torque values (settings 1,1.5, 2 and 3). According to the manufacturer, these settings correspond to torque values of 1, 1.5, 2 and 3 Ncm, with a range between 1 and _12 Ncm (corresponding to setting 8).The speed on this motor was set at170 rpm. This motor had an auto-torque reverse function. In group 4, the PRI were used with a new generation very low torque control motor (Tecnika, ATR, Pistoia, Italy). This motor allows very low torque values (<1 Ncm). It also has an auto-torque reverse function. This motor was used with the torque values set by the manufacturer. The speed on this motor was set at 170 rpm.
The series of PRI were used according to the following guidelines: the apical pressure exerted on the PRI was light and each instrument was used for only a few seconds in the canal; the PRI were used with small in and out movements. Only one buccal canal was prepared in each molar. After the first sequence of PRI, the apical width of the canal was checked with a size 15 hand le. At this stage, canals were excluded from the study if the size 15 le reached the working length and was loose in the canal. The canals were then enlarged until a size 25 PRI reached the working length. Three to five recapitulations (waves) with PRI sizes 40-15 were required to complete cleaning and shaping of each canal. Preparation was complete when a 1-1 Machtou plugger (ISO size 50) penetrated to 5-7 mm short of the working length, and a fine medium gutta-percha 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 0.625-in. 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 8 K-type le through the foramen.
Before each use, the PRI set (kit) was sterilized by steam autoclave for 5 min at 135 8C; the whole cycle of sterilization lasted 35 min. One set of PRI sizes 40-15 was used for each canal. Thirty sets of six PRI taper 0.06 instruments were included in each of the four groups. A 2.5x magnification was used to check for instrument deformation after each passage. An operator blinded to the study performed the instrument inspection. Instrument deformation and separation within each group were recorded. In case of instrument deformation before completing the cleaning and shaping, the instrument was replaced. In case of instrument separation, the cleaning and shaping was stopped. Prior to instrumentation of canals, each set of PRI was used according to the sequence, by an endodontist experienced with the instrumentation technique, to prepare five curved canals in endodontic resin blocks. Each set was then blindly assigned to one of the three groups if all the instruments did not show any deformation or separation.
The same operator, a 4thyear undergraduate student, performed the cleaning and shaping procedures in all the canals of the four groups. This student, not familiar with the Ni-Ti rotary instrumentation, followed a theoretical course on the technique and practiced on seven endodontic resin blocks. The canals in group 4 were first prepared followed by the canals in groups 3-1.
Only the overall number of deformed and separated instruments in the different groups was analyzed statistically for significance with chi-square tests.