Discussion - References.
There is accumulating evidence suggesting that rotary NiTi instruments facilitate root canal preparation with minimal or no canal transportation (Peters et al. 2001, Ruddle 2002). However, instrument separation might occur more frequently with rotary systems than with conventional hand instruments and there remains a clinical concern even after in-depth introductory courses (Barbakow & Lutz1997).
Consequently, physical parameters governing the fracture mechanisms of rotary endodontic instruments are of considerable interest. Sattapan et al. (2000a) outlined two distinct fracture mechanisms, i.e. torsional fracture and flexural fracture. Torsional fractures occur when the apical portion of a rotating instrument is forced into narrow root canals. Friction increases at this point, high torque is required to rotate the instrument and the fragile instrument tip is subjected to excessive torque (Blum et al. 1999b). This effect has been described as â€˜taper lockâ€™ since it might occur with similarly tapered instruments of varying tip diameters rather than with variably tapered instruments (Yared et al. 2002). Consequently, one of the design characteristics of ProTaper instruments is a variable taper along the cutting part of the instruments (Fig.1).
Repeated bending in curved canals causes metal fatigue and subsequent instrument separation (Sattapan et al. 2000a). In vitro studies have indicated that rotary NiTi instruments have a predefined fatigue life and are able to withstand between 250 and 500 rotations in simulated metal canals with 908 curvatures and 5 mm radii (Haikel et al. 1999, Gambarini 2001, Peters & Barbakow 2002).
Operator-related factors and clinical ability are also important factors related to instrument separation (Yared et al. 2002) and consequently, electric motors and handpieces have been developed to simplify the use of NiTi rotary instruments. Although some of these handpieces are equipped with torque-limiting systems, operators will use varying apically directed force and speed of insertion. Furthermore, times required to prepare canals must differ greatly and, consequently, the risk of fatigue fracture will vary.
Whilst the problem of cyclic fatigue of NiTi instruments has been described in several studies under varying in vitro situations, little is known about the torque and force generated by these instruments during root canal preparation. Most of the latter studies are limited to the preparation of straight canals in anterior teeth and this is due to the construction of the respective torque transducers (Blum et al. 1999 a, b, Sattapan et al. 2000b). However, a different approach to assessing torque and force generated by rotating ProFile .04 instruments in curved single-rooted teeth has been described recently (Peters & Barbakow 2002). This was achieved by incorporating a dynamic torque sensor into the rotational axis, with the result that torque is then measured between the motor and the shank of the endodontic instrument.
In the current study, ProTaper instruments were used to shape canals in extracted maxillary molars whose internal morphology had previously been assessed using micro computed tomography (Peters et al. 2000). Overall, mean maximum peak torque scores were 2.2 N cm and this result was lower than the 2.82 and 2.5 N cm previously recorded for Quantec (TycomCorp., Irvine, CA, USA) and for ProFile .04 instruments (Dentsply Maillefer), respectively (Sattapan et al. 2000b, Peters & Barbakow 2002).
The present study appears to be the first to prove that torque is correlated not only to apically exerted force, but also to preoperative canal volume. Hence, preparation of narrow and constricted canals could subject rotary instruments to higher torsional loads. At the same time, apically directed force increased similarly when narrow canals were prepared. In fact, apically directed forces in excess of 10 N were exerted on finishing files in some severely curved and narrow canals. However, this amount of force did not produce excessive torque and did not cause instruments to fracture. The reason why these instruments did not fracture was probably related to a patent glide path being present at all times. For that purpose, we recommend, as a guideline, to enlarge canals to at least a size 015 K-File prior to the use of ProTaper instruments at WL.
Moreover, four instruments separated in pilot experiments and it might be speculated that specific instruments have their individual optimum force and that the forces used in the present study were acceptable for some rotary instruments, but may have been excessive for the delicate, differently designed ProTaper shaping instruments.
In the current study, the numbers of rotations during simulated shaping of root canals were also counted in order to address cyclic fatigue. This parameter was correlated significantly to canal volume and behaved in this respect similarly to torque and applied force. However, wide ranges were recorded for all five instruments and in particular, for shaping file 1. In one case, more than 50 rotations were required to prepare a â€˜constrictedâ€™ canal. Earlier experiments using the same testing device had indicated that tapered rotary instruments (GT 20/ .08, Dentsply Maillefer), which have a cross-sectional dimension at D3 similar to a shapingfile1, fractured after 250 rotations when tested in a simulated canal with 908 curve and 0.5 mm radius. Consequently, it might be advisable to discard ProTaper shaping files after four to five constricted canals.
Sattapan et al. (2000a) speculated that fatigue fracture might play a major role in Quantec instruments, but others have reported that cyclic fatigue was a minor factor for ProFile instruments when used correctly and discarded regularly (Yared et al. 2001). Apparently, the instrumentsâ€™ cross-sectional geometry does play an important role in cyclic fatigue.
However, torsional and flexural fractures are not mutually exclusive categories and cyclic fatigue simulation using metal canals cannot closely resemble clinical conditions because only minimal friction occurs between the rotating instrument and the holding device. Cyclic fatigue might occur differently if instruments are subjected to apically directed force and rotate in curved canals. Clinically, supra-threshold torque does occur for a limited time period only (Peters & Barbakow 2002) and it might be more appropriate to calculate the product of torque and time (area under torque curve in Fig. 3) in order to assess the amount of stress the instruments have been subjected to.
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