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Azerbaycan Saytlari

 »  Home  »  Endodontic Articles 5  »  Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals
Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals
Discussion - References.



Discussion.
Amongst several factors, success or failure of endodontic therapy depends on the quality of canal preparation (Pettiette et  al. 2001). Canals can readily be prepared in vitro with nickel–titanium rotary instruments such as ProFile .04 to have smooth and continuously tapering walls (Bryant et  al. 1998a, 1998b). These preparations also minimize procedural errors such as apical zipping or ledging and improve obturation and thereby reduce apical leakage (Wu et  al. 2000). Moreover, rotary instruments allow larger apical preparations (Schrader et  al. 1999) with little or no canal transportation (Portenier et  al. 1998). However, the increased risk of separation of nickel–titanium instruments remains a significant problem for many clinicians. Although some authors reported a high number of instruments showing plastic deformation after use with few separations (Thompson & Dummer 1997a, 1997b), others documented a higher incidence of fractures (Zuolo & Walton 1995, Mandel et  al. 1999).
Previous studies described findings related to bending moments and torsional properties (Serene et  al. 1995) as well as the cutting efficacy (Schäfer & Lau 1999) of nickel–titanium hand instruments. However, it is more difficult to study these properties correctly whilst preparing canals with rotary instruments. Technically, torque is measured in relation to a rotational axis, which should coincide with the instrument’s long axis (Fig. 1). Consequently, a resulting torsional moment can be measured whilst the instrument rotates in a straight canal. This principle was recently utilized in experiments by Blum et  al. (1999a) and Sattapan et  al. (2000a). Unfortunately, the results of these tests do not hold for curved canals, whilst nickel–titanium rotary instruments were primarily developed to shape curved canals. Furthermore, the incidence of instrument separation, clinically, is probably higher in severely curved than in straight canals.
It was decided to design and construct a platform to allow torque and force measurements in curved canals (Figs 1, 2). Using sensors with high resolution and regular calibration procedures, the amounts of torque that the motor transferred to the instrument’s shank could be recorded, in real time, during canal preparation. The amount of torque generated clearly depends on the size of the contact areas between the instruments and the canal walls, as was recently demonstrated (Blum et  al. 1999b). Clinically, however, the size of the contact area is unknown and it can vary following various instrumentation sequences.
Torsional load to separation as described by the ISO 3630–1 test was also recorded using the torque-testing device. The selected ProFile .04 instruments separated at torque scores varying from 3.7 Nmm to 32 Nmm. These results are in accordance with those stipulated by the ISO norm and with other documented findings (Table 4, Silvaggio & Hicks 1997). Furthermore, when torsional strain was plotted against deflection angles, curves typical for nickel–titanium (Rowan et  al. 1996, Thompson 2000) were constructed (Fig. 5).

Figure 9. Bar diagrams detailing the number of revolutions counted whilst preparing various canal types with ProFile .04 rotary instruments. Significant differences within one instrument size are indicated by horizontal bars (P < 0.05 or 0.01).

Bar diagrams detailing the number of revolutions counted whilst preparing various canal types with ProFile
Table 4. Literature review of torque scores assessed with different measuring set-ups.

Literature review of torque scores assessed with different measuring set-ups

Results listed above the shaded line detail load to fracture according to the ISO 3630-1 test, whilst results below the shaded line detail torques generated during canal preparation.
a. Recalculated into SI units: 1 gcm ~0.1 Nmm (1 Nmm = 1 mNm).

In contrast to these standard measurements, there were significantly higher torsional moments for most ProFile .04 rotary instruments tested during the simulated canal preparations. Interestingly, the highest torsional moments were generated in straight canals in plastic blocks, whilst the lowest scores were recorded in canals in extracted teeth (25 Nmm vs. 14 Nmm). This again emphasizes the importance of size of contact areas and the differences in surface texture between plastic blocks and extracted teeth. In addition, rather high torques were recorded during apical preparation in curved canals in plastic blocks using size 35 and 40 ProFile instruments. This finding might explain the high incidence of instrument deformation and fracture in laboratory studies (Thompson & Dummer 1997b) and during continuing education courses. Table 4 lists previously reported torque scores found either according to the ISO 3630–1 test or during canal preparation. Differences in the scores between the present paper and those reported by Blum et  al. (1999a, 1999b) may be due to varying instrumentation sequences and larger apical preparations in the current study. Moreover, the ‘endographe’ used by Blum et  al. (1999a, 1999b) was developed to assess forces developed during hand instrumentation (Blum et  al. 1997). The ‘endographe’ is similar to another device (Sattapan et  al. 2000a), both of which use conventional dental handpieces. This is not the case in the currently described torque-testing device.
Some confusion occurs when using the unit ‘gcm’ to describe torque. Because both ‘gcm’ and ‘Nm’ have been used, scores were recalculated to compare findings reported in the literature and summarized in Table 4. Sattapan et  al. (2000a) described torsional moments for Quantec instruments that were similar to the current results. It is evident from Table 4 that very low scores (<0.3 Nmm) have also been found (Blum et  al. 1999a, 1999b).
In the current study, apically directed loads of up to 7.5 N were recorded for all larger instruments. Lower scores were found in straight canals in plastic blocks compared to the scores recorded in canals in extracted teeth. Irrespective of the type of rotary instrument, all manufacturers recommend using only light pressure. Unfortunately, the word ‘light’ is not numerically defined. This is reflected when forces as low as 0.1 N were demonstrated for size 2 Quantec 2000 instruments (Sattapan et  al. 2000a). In contrast, the current study confirms earlier findings for apically directed forces (Blum et  al. 1999a). However, it is important to note that despite relatively high apical forces in the present study, no ProFile instruments separated during canal preparation. A recent retrospective analysis of separated instruments indicated two main modes of failure, namely, torsional and flexural types (Sattapan et  al. 2000b). Flexural fractures may arise from minute surface defects within the instruments (Eggert et  al. 1999, Luebke et  al. 2001) and occur after cyclic fatigue. This effect has been extensively tested utilizing various experimental conditions (Pruett et  al. 1997, Haikel et  al. 1999, Yared et  al. 1999, 2000). In order to compare cyclic fatigue, reported as number of revolutions to failure, a tempered steel phantom similar to that recently described (Haikel et  al. 1999) was incorporated in the torque-testing device. The results reported in the current study for this criteria for some selected ProFile.04 instruments agree with recent findings reported for these instruments (Haikel et  al. 1999, Yared et  al. 2000). In addition to the static standard cyclic fatigue test, an oscillating ‘pecking’ motion was also carried out using a second group of ProFile .04 instruments. Unexpectedly, the pecking motion did not significantly enhance the lifespan of size 30 and 45 ProFile instruments. When compared to the number of revolutions actually occurring during canal preparation, a safety factor indicates that at least 5–10 curved canals can be prepared with a set of ProFile .04 instruments. Original data recorded during the current study showed that any instrument generated torque for roughly 50% of the time it rotated. In other words, it is unlikely that cyclic fatigue can occur if a rotating instrument does not significantly contact canal walls. Consequently, cyclic fatigue is not necessarily the main reason for instrument failure.
Interestingly, significant differences were found between the three types of canals used in the current study. Although canals in plastic blocks presented highly standardized in vitro conditions, their canal cross-sections and surface properties differ from that of canals in extracted teeth. Moreover, the effect of the complicated three-dimensional anatomy on canal preparation must also be considered (Peters et  al. 2001a, 2001b). Likewise, operator experience resulting in a wide range of apical forces may also be a factor (Mandel et  al. 1999).
To summarize, the current study is the first to report torsional moments during rotary preparation of curved root canals. Further experiments with automated feed are required to detail the effects of instrument design or sequence on torsional moments and apically directed forces. In the future, the torque-testing device described in this paper should be combined with three-dimensional analysis to study the relationship between root canal anatomy and physical parameters during rotary preparation.

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