Discussion - References.
Torque is one amongst many parameters that might influence the incidence of instrument locking, deformation, and separation. Theoretically, an instrument used with a high torque (motor) is very active and the incidence of instrument locking and consequently deformation and separation would tend to increase. Where as 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. Torque control motors allow the setting of torque generated by the motor. In low torque control motors, torque values set on the motor are supposed to be less than the value of torque at deformation and at separation of the rotary instruments. In high torque control motors, the torque values are relatively high compared to the torque at deformation and at separation of the rotary instruments. During rootcanal preparation all the instruments are subjected to different levels of torque. If the level of torque is equal or greater than the torque at deformation or at separation the instrument will either deform or separate. Theoretically, with the low torque control motors, the motor will stop from rotating and can even reverse the direction of rotation when the instrument is subjected to torque levels equal to the torque value set on the motor. Thus, instrument failure would be avoided, where as, with high torque control motors, the instrument torque at deformation and at separation would be reached before the relatively high torque value set on the motor. Consequently, the instrument would deform and separate. However, the results of a recent study (Yared et al. 2001b) did not indicate a difference between high and low torque motors with respect to instrument failure. In that study, the operator was experienced with the use of PRI.
This is the first comprehensive study that evaluated the incidence of Ni-Ti rotary instrument failures when used in conjunction with different types of motors and under access limitations.
The authors had the impression that it would be more interesting to compare the motors using used ProFile. Differences in the incidence of instrument locking, deformation and separation amongst the different motors could have been masked if new instruments were used in each canal. For this reason, each set was used in resin blocks according to the described technique and by the same operator. The number of recapitulations for each instrument was in a similar range when preparing the five root canals in resin blocks.
In the present study, apical enlargement was kept as small as practical according to the principles of Schilder (1974) and because in the opinion of the authors any greater apical enlargement should have been completed with hand instruments.
As in recent studies (Yared et al.1999, 2000, 2001a), it was impossible to standardize the number of recapitulations (waves). Canal width and anatomy influenced the frequency of recapitulations needed before a size 25 PRI reached the working length. However, 3-5 recapitulations were sufficient for all the canals.
The instruments were inspected for deformation with 2.5_ magnification after each passage in the canal. In a previous study, we noted that deformations went un detected if magnification was not used (Yared et al. 2000). The use of a dental operating microscope, where available, might decrease the incidence of false negatives (undetected deformations).
The manufacturer recommends using the ProFile instruments at speeds between 150 rpm and 300 rpm. The rotational speed was setat170 rpm. Previous studies have shown significantly less Ni-Ti rotary instrument failures at slower speeds (Gabel et al. 1999, Yared et al. 2001a).Yet, it should be noted that the use of a slow speed could have masked any differences amongst the motors.
The torque value (10 Ncm) set on the motor of the second group was the lowest allowed by this motor. In the third group, the torque value was set at 2 Ncm. This motor allowed different low torque settings (1, 1.5, 2 and 3 Ncm). In a pilot study, we noticed that at settings equal to or lower than 1.5 Ncm the rotation of the les was frequently reversed. With the air motor torque control was not possible.
Wolcott & Himel (1997) showed that the torque at fracture for 0.04 taper ProFiles sizes 15, 25 and 35 were: 0.21, 0.48 and 1.24 Ncm, respectively. In laboratory experiments, Kobayashi et al. (1997) indicated that the torque threshold for the auto-torque reverse mechanism in the TriAuto ZX(Morita, Japan) for PRI taper4%should be set between 0.39 and 0.78 Ncm. Svec & Powers (1999) showed that torque at failure was 0.78, 1.06 and 1.47 Ncm for unused 0.04 taper ProFile rotary Ni-Ti instruments sizes 25, 30 and 35, respectively. Consequently, it was expected that the present study would show a significantly higher incidence of instrument failures especially with the air, high torque control and low torque control (Nouvag) motors. With the air motor, torque control was not available. Instruments could have been subjected during instrumentation to torque levels exceeding their torque at failure values. Also, with the air motor, the rotational speed might not be constant. A sudden increase in the speed (due to air pressure variation) would result in a rapid increase of the torque to which the instrument was subjected (Kobayashi et al. 1997) leading to instrument failure. The results of the present study confirmed those of Bortnick et al. (2001) who demonstrated that the incidence of instrument deformation and separation is similar for air and high torque control motors.
Ingroups2and3, the torque setting on the motors was relatively high (10 and 2 Ncm, respectively) (Wolcott & Himel 1997, Kobayashi et al. 1997, Svec & Powers 1999). Expectedly, the incidence of instrument deformation and separation was significantly higher with these motors than with the very low torque control motor. In a recent and similar study, instrument separation did not occur with the low torque control motor (Nouvag Micromotor TCM Endo 2, Nouvag, Goldach, Switzerland). The difference can be explained by variation in the methodology: inexperience of the operator and instrument stress due to access limitations in the present study.
Sattapan et al. (2000) showed that torque at failure was significantly higher than torque during instrumentation. In the present study, the instruments were used with a slight apical pumping as in the study of Sattapan et al. (2000); thus, it seems that torque during instrumentation by an inexperienced operator could have been higher in the four groups than the torque at failure of the specific instruments. This fact would account for the high incidence of instrument failure in groups 1-3. In group 4, the very low torque control motor contributed to limit the incidence PRI failure.
A high incidence of instrument deformation and separation occurred in groups 1-3. The operator probably exerted excess apical pressure on the PRI (Kobayashi et al. 1997) and/or used them for too long in the canal. Consequently, these PRI locked into the canal and were subjected to a high level of torque; failure occurred instantly. In group 3, the instruments were subjected to torque greater than their torque at fracture which is probably less than the torque set on the motor (Wolcott & Himel 1997, Svec & Powers 1999). So, the instrument fractured before the motor reversed the rotation direction. In group 4, the torque set on the motor was probably less than the torque at fracture of the PRI. Consequently, the motor reversed the direction of rotation. High stress on the instrument was avoided reducing the incidence of instrument failure. This aspect is very important especially for students and inexperienced operators and in clinical situations where the instruments might be subjected to increasing stress due to limitations of access.
The results from Table 2 indicated that the size 15 instruments were more prone to deformation than the other instruments when used in conjunction with air, high torque and low torque control motors. Also, there was a trend toward deformation with the smaller instruments except when a very low torque motor was used. Also, the size15 instruments separated more frequently Than the other instruments when an air motor was used (Table 3). Consequently, it may be prudent to consider the size15 ProFile with a 0.06 taper a disposable instrument. Similar findings were reported by Bortnick et al. (2001) with respect to the size 20 ProFile with a 0.04 taper.
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