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Azerbaycan Saytlari

 »  Home  »  Endodontic Articles 1  »  Influence of rotational speed, torque and operator proficiency on failure of Greater Taper files
Influence of rotational speed, torque and operator proficiency on failure of Greater Taper files
Discussion and References



Discussion.
This is the first comprehensive study that has evaluated the influence of rotational speed, torque, and operator experience on the incidence of locking, deformation and separation of GT Ni–Ti rotary instruments during repeated simulated clinical use, after exposure to 2.5% NaOCl and with autoclave sterilization.
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 a recent study (Yared et al . 2001) it was impossible to standardize the number of recapitulations (waves). Canal width and anatomy influenced the frequency of recapitulations needed before a 0.08 taper instrument reached the working length. However, 3–4 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 undetected if magnification was not used (Yared et al . 1999). The use of a dental operating microscope, where available, might decrease the incidence of false negatives (undetected deformations).

Rotational speed.
The torque value set on the motor in this part of the study was empirically chosen on the basis, although not confirmed by Yared et al . (2001), that high torque values would not be safe. The motor used in this study allowed torque to vary between 10 and 55 Ncm; these are extremely high levels. In laboratory experiments, Kobayashi et al . (1997) indicated that the torque threshold for the auto torque-reverse mechanism in the Tri Auto ZX (Morita, Japan) for ProFile rotary instruments taper 4% should be set between 0.39 and 0.79 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.
Instrument locking, deformation, and separation did not occur in any of the canals shaped with GT instruments rotated at 150 and 250 r.p.m. Some instruments used at 350 r.p.m. locked. Rotational speed in the present study did not influence the incidence of GT failures. The results of the present study did not agree with those of Gabel et al . (1999) and Yared et al . (2001); however, different instruments were tested in those studies. The difference between the results of the present study and those of Yared et al . (2001) could be attributed to the taper of the instruments. GT instruments used in crowndown would be subjected to lower stress levels at their tip than the 0.06 taper Profile instruments (Blum et al . 1999). Consequently, the incidence of deformation and separation would decrease. Moreover, Thompson & Dummer (1997a, b) demonstrated that the design of the instrument and the specific instrumentation sequence adapted have influence on instrument failure. Kobayashi et al . (1997) did not find any difference between 240 and 280 r.p.m.; but these rotational speeds are too close and the instruments tested were ProFiles. Pruett et al . (1997) found that cyclic fatigue of Lightspeed Ni–Ti rotary instruments was not affected by the rotational speed. However, Lightspeed instruments have a completely different design and behave in a different way.

Torque.
Instrument locking, deformation, and separation did not occur with any of the three subgroups.
Based on the results of previous studies (Yared et al . 1999, 2000, 2001), the first part of the present study and the work by Gabel et al . (1999) lower speeds were deemed safer than a higher speed with respect to instrument failures. As a result, a speed of 150 r.p.m. was chosen in this part of the study.
As mentioned earlier, the motor used in this study allowed a torque range between 10 and 55 Ncm. Clearly, the significance of this part of the study would have been enhanced if more torque values were evaluated.
When the torque value set on the motor is greater than the maximum torque at failure of the instrument, separation may occur if the instrument is locked. Svec & Powers (1999) demonstrated that torque at failure values for ProFiles are relatively low. According to Kobayashi et al . (1997) the torque values for the ProFiles should be set between 0.39 and 0.79 Ncm. Although the torque values in this study were great, neither deformation nor separation occurred with the GT rotary instruments. This was probably due to the technique, the strict adherence of the operator to the clinical guidelines, and to the minimal load exerted on the instruments. Yared et al . (2001) had also reported similar findings with Profile instruments. Kobayashi et al . (1997) demonstrated that increased load on the instrument resulted in torque increase. Sattapan et al . (2000) demonstrated that when Ni–Ti rotary instruments were used with a slight pumping motion, apical load exerted during instrumentation was relatively low and 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). This fact would account for the excellent results obtained in the present study. Also, the results of Sattapan et al . (2000) would confirm that the technique of instrumentation influences the incidence of instrument failures (Thompson & Dummer 1997a,b).
Recently, motors that set torque values at minimal levels (less than 1 Ncm) have been introduced. These motors allow the torque value to be set at a level lower than the maximum torque at failure. In view of the results obtained in this part of the study (no deformation and separation with very high torque), these motors may not be useful for experienced operators. On the other hand, their use would be beneficial with less experienced operators and with students, especially if the torque is set at a level lower than the yield point. These motors would also be useful in canals having small radii of curvature.

Operator proficiency.
Instrument separation did not occur with the experienced endodontist, nor with the trained operator, confirming the results of previous studies (Yared et al . 1999, 2000, 2001) and of parts 1 and 2 of the present study performed under the same conditions. This fact demonstrates the reliability of the instrumentation technique when the technical guidelines are respected as well as the importance of preclinical training and experience. The incidence of locked, deformed and separated instruments with the untrained operator confirmed the significance of training. Although no statistically significant difference was detected amongst the three operators with regard to instrument separation, the results showed a trend toward separation with the untrained operator. This operator probably exerted excess apical pressure on the GT instruments (Kobayashi et al . 1997) and/or used them for too long in the canal. Consequently, the instruments locked into the canal and were subjected to a high level of torque, with the result that instrument separation occurred on two occasions. Other clinical factors, such as a severe canal curvature, narrow canal diameter (Sattapan et al . 2000), and tilting of the handpiece so that the file becomes diverted from the long axis of the canal would also lead to increased load on the instrument, resulting in separation (Kobayashi et al . 1997). The results of part 3 confirm those of Barbakow & Lutz (1997), Yared et al . (1999, 2000, 2001) and Mandel et al . (1999) that training is necessary to avoid complications.
Interestingly, in a similar study using ProFiles (Yared et al . 2001), the incidence of failures with an untrained operator was significantly higher than with the untrained operator in the present study. This difference was probably related to the degree of experience in endodontics of the untrained operators in the two studies.
In the three parts of the present study, an experienced operator cleaned and shaped 700 curved canals with the GT instruments (subgroups 1–7). Instrument deformation and separation did not occur but instrument locking occurred at 350 r.p.m. This finding confirms the reliability of the technique and the results of a recent similar study using ProFiles at 150 r.p.m. (Yared et al . 2001). Thompson & Dummer (1997a) showed that approximately one ProFile deformed per canal, a phenomenon that was thought to be due to the tendency of the instrument within the canal to bind, with the result that the continuous rotation of the handpiece wound up the cutting blades. In their study, simulated canals in resin blocks were used; they also used Profile rotary instruments with a 4% taper. Instrumenting canals in resin blocks is also more difficult than in human teeth. Blum et al . (1999) demonstrated that ProFiles taper 4% are mostly active at their tip and so are subjected to a high level of stress. In another study, Thompson & Dummer (1997a,b) demonstrated clearly that the design of the instrument and the specific instrumentation sequence adopted would have an influence on instrument failure.

References.

Barbakow F, Lutz F (1997) The Lightspeed preparation technique evaluated by Swiss clinicians after attending continuing education courses. International Endodontic Journal 30, 46-50.
Blum JY, Machtou P, Micallef JP (1999) Location of contact areas of Profile Ni-Ti rotary instruments in relation to the forces developed during mechanical preparation of extracted teeth. International Endodontic Journal 32, 108-14.
Bryant ST, Thompson SA, Al-Omari MAO, Dummer PMH (1998) Shaping ability of ProFile rotary nickel-titanium instruments with ISO sized tips in simulated root canals. Part 1. International Endodontic Journal 31, 275-81.
Gabel WP, Hoen M, Steiman HR, Pink FE, Dietz R (1999) Effect of rotational speed on Nickel-Titanium file distortion. Journal of Endodontics 25, 752-4.
Kobayashi C, Yoshioka T, Suda H (1997) A new engine-driven canal preparation system with electronic canal measuring capability. Journal of Endodontics 23, 751-4.
Mandel E, Adib-Yazdi M, Benhamou L-M, Lachkar T, Mesgouez T, Sobel M (1999) Rotary Ni-Ti Profile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. International Endodontic Journal 32, 436-43.
Pruett JP, Clement DJ, Carnes DL (1997) Cyclic fatigue of nickel-titanium endodontic instruments. Journal of Endodontics 23, 77-85.
Ramirez-Solomon D, Soler-Bientz R, de la Garza-Gonzalez R, Palacios-Garza CM (1997) Incidence of Lightspeed separation and the potential for bypassing. Journal of Endodontics 23, 586-7.
Sattapan B, Palamara JEA, Messer HM (2000) Torque during canal instrumentation using rotary nickel-titanium files. Journal of Endodontics 26, 156-60.
Schilder H (1974) Cleaning and shaping the root canal. Dental Clinics of North America 18, 269-96.
Schneider SW (1972) A comparison of canal preparations in straight and curved root canals. Oral Surgery 32, 271-5.
Svec TA, Powers JM (1999) Effects of simulated clinical conditions on Nickel-Titanium rotary files. Journal of Endodontics 25, 759-60.
Thompson SA, Dummer PMH (1997a) Shaping ability of ProFile 0.04 taper Series 29 rotary nickel-titanium instruments in simulated root canals. Part 2. International Endodontic Journal 30, 5-18.
Thompson SA, Dummer PMH (1997b) Shaping ability of Quantec Series 2000 rotary nickel-titanium instruments in simulated root canals. Part 1. International Endodontic Journal 31, 259-67.
Yared G, Bou Dagher F, Machtou P (1999) Cyclic fatigue of Profile rotary instruments after simulated clinical use. International Endodontic Journal 32, 115-9.
Yared G, Bou Dagher F, Machtou P (2001) Influence of rotational speed, torque, and operator’s proficiency on ProFile failures. International Endodontic Journal 34, 47-53.