G. M. Yared1 & G. K. Kulkarni
Discipline of Endodontics and Preventive Dentistry, Faculty of Dentistry, University of Toronto, Ontario, Canada.
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 Ni-Ti alloy instruments. In one study (Pruett et al. 1997), the authors investigated the effect of the angle and radius of curvature on cyclic fatigue of Lightspeed rotary Ni-Ti instruments. Ramirez-Salomon et al. (1997) evaluated the incidence of Lightspeed separation. Thompson & Dummer (1997, 1998) and Bryant et al. (1998) demonstrated that the incidence of Ni-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 showed proper tuition or experience was necessary to minimize the incidence of instrument separation (Barbakow & Lutz 1997, Mandel et al. 1999, Yared et al. 2001a).
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. (2001a) demonstrated that Ni-Ti rotary instrument failures are less likely to occurat a lower rotational speed.
Torque is another parameter that might influence the incidence of instrument deformation and separation. Different types of motors are used in conjunction with Ni-Ti instrumentation. When a high torque control motor is used, the instrument is very active and the incidence of instrument locking and consequently deformation and separation would tend to increase. With these motors, torque at failure of the instrument is often exceeded. Air motors do not allow torque control, and variation in air pressure could affect the rotational speed and consequently torque. For instance, a drop in air pressure would lead to a decrease of torque. The instrument would become less active and the operator would tend to force the instrument into the canal leading to deformation and separation.
Recently, a new generation of low and very low torque control motors has been introduced; torque values as lowas1and0 Ncm can be set on the low and very low torque control motors, respectively. These motors take into consideration the low torque at failure values of Ni-Ti rotary instruments. Wolcott & Himel (1997) showed that torque values at fracture for 0.04 taper ProFile nickel- titanium rotary instruments (PRI) sizes 15, 25 and 35 were: 0.21, 0.48 and 1.24 Ncm, respectively. According to Kobayashi et al. (1997), torque should be set between 0.40 and 0.80 Ncm for PRI to avoid instrument failure. Svec & Powers 1999 compared the torque at failure values of used and unused PRI and showed that torque at failure was 0.78, 1.06 and 1.47 Ncm for unused 0.04 taper PRI sizes 25, 30 and 35, respectively. Recently, Bortnick et al. (2001) demonstrated no significant difference in instrument deformation and breakage between air and electric motors. Yared et al. (2001b) evaluated in vitro the failure incidence of 0.06 taper PRI when used in conjunction with three different motors and a specific instrumentation technique. Their results indicated no difference amongst the air, high torque control and low torque control motors, when an experienced operator performed the canal preparation.
The purpose of this study was to evaluate the incidence of PRI failures when used by an inexperienced operator, under access limitations, in conjunction with air, high torque control, low torque control and very low torque control motors.