O. A. Peters, C. I. Peters, K. Schonenberger & F. BarbakowEndodontic Division, Department of Preventive and Restorative Dental Sciences, University of California, San Francisco, USA.
Endodontic Department, University of The Pacific Dental School, San Francisco, USA.
Division of Endodontology, Department of Preventive Dentistry, Periodontology and Cariology, University of Zurich, Switzerland.Introduction.
It is generally believed that engine-driven or manually used nickel-titanium (NiTi) instruments produce better- prepared root canals than their stainless steel counterparts. Clinically, however, such instruments, and in particular the rotary types, do have a higher risk of separation (Barbakow & Lutz1997). Reasons for separation of rotary NiTi instruments include variations in canal anatomy, such as merging, curving, re-curving, dilacerating or dividing canals (Ruddle 2002). Specifically, retrospective analysis of routinely discarded NiTi instruments indicated two distinct fracture mechanisms, namely, torsional and flexural fractures (Sattapan et al. 2000a).
Both of these mechanisms may contribute, albeit not uniformly, to instrument separation. Smaller instruments may become wedged into constricted canal areas producing a so-called â€˜taper lockâ€™ effect. The torque required to rotate the shaft of â€˜taper lockedâ€™ instrument may exceed the alloyâ€™s torsional limit, leading to separation of a relatively small portion of the instrument tip (Sattapan et al. 2000a). On the other hand, continuous rotation of files incurved root canals requires the instrument to flex during every rotation, resulting in cyclic compression and elongation, which produces metal fatigue. Fatigue fractures typically occur at the crescent of any given curve, resulting in fragments of various lengths (Sattapan et al. 2000a). Unfortunately, both mechanisms have one major fact in common, that is, they are difficult, if not impossible, to predict clinically.
Manufacturers and clinicians have recommended discarding rotary instruments on a regular basis, e.g. after 10 canals (Yared et al. 2001, 2002), or even to consider them as single-use items to avoid cyclic fatigue. Other suggestions to avoid cyclic fatigue include limiting the use of rotary instruments whilst shaping root canals to between 10 or 20 s and not to remain in a canal once a certain working length has been reached (Ruddle 2002). In addition, torque-controlled electrical motors have been marketed recently to help clinicians to better identify when torsional limits are reached (Gambarini 2000). Another method to reduce torsional fracture is to modify the rotary instrumentâ€™s cross-sectional geometry, thereby increasing cutting efficiency and consequently reducing contact areas and torsional loads (Blum et al.1999a).This concept has resulted in marketing of new instrument types (e.g. ProTaper, Dentsply Maillefer, Ballaigues, Switzerland; FlexMaster, VDW, Munich, Germany), which are claimed to generate lower torque values. In particular, a nonradial landed and more effectively cutting cross-section has been designed for that purpose (Fig.1).
It is difficult to objectively analyse torsional loads and other physical parameters during preparation of curved canals due to underlying engineering principles (Peters & Barbakow 2002). Indeed, few reports have appeared detailing torque and force during rotary preparation of curved canals. Moreover, it has been suggested that canal geometry might influence rotary instrument performance in terms of shaping outcomes (Nagy et al. 1997, Peters et al. 2001).
Therefore, the aim of this study as a part of an ongoing project was to investigate torque and force generated by ProTaper instruments when curved canals in extracted maxillary molars were prepared. The effect of canal anatomy on physical parameters was also tested.
Figure 1. Scanning electron micrograph of ProTaper shaping file 1 detailing the instrument tip and the nonlanded cross-section (original magnification x150).
Figure 2. Construction of the torque testing device showing major components (A) force gauge, (B) torque sensor, (C) motor, and (D) linear drive.