Discussion.
The purpose of this study was to compare the relative efficiency and shaping ability of rotary K3 nickel-titanium instruments with stainless steel hand K-Flexofiles. K-Flexofiles were chosen as controls since it is well known that out of all hand instruments used in a rotary motion, these instruments displayed the greatest cutting efficiency (Tepel et al.1995) and most ideal shape (Al-Omari et al.1992, Schafer et al.1995).Moreover, the shaping ability of some rotary nickel-titanium instruments has already been compared with that of the K-Flexofiles (Kumet al.2000, Bertrand et al.2001, Park 2001, Schafer 2001, Schafer & Lohmann 2002a,b). Because there is no evidence in the literature that K-Flexofiles are better used in a linear or rotational working motion (Bishop & Dummer1997), these instruments were used in a reaming motion as the present K-file has a triangular cross section. The K-Flexofiles were not used in a crown-down or balanced-force manner since a pilot study showed that these techniques did not result in better instrumentation results compared with the sequence used in the present study.
This study described the shaping abilities of the instruments under strictly controlled laboratory conditions, using clear resin blocks. Use of simulated canals in resin blocks does not reflect the action of the instruments in root canals of real teeth. However, resin blocks allow a direct comparison of the shaping ability of different instruments (Schafer et al. 1995). A major drawback of using rotary instruments in resin blocks is the heat generated, which may soften the resin material (Kum et al. 2000), and lead to binding of cutting blades and separation of the instrument (Thompson & Dummer 1997c, Baumann & Roth 1999). Thus, owing to the nature of the resin, care should be exercised in the extrapolation of the present results on failure of the K3 instruments to the use of these instruments in real root canals, where dentine is involved (Thompson & Dummer1997c).
In comparison with the stainless steel K-Flexofiles, rotary K3 instruments achieved better canal geometry, showed less canal transportation and straightening (Figs 3 and 4) and created fewer canal aberrations (Table 5), both in canals with 288 and 358 curves. The ability of rotary nickel-titanium instruments to maintain the original shape of curved canals has been con- firmed by several studies (Thompson & Dummer 1997a,b,c,d, Baumann & Roth 1999, Kum et al. 2000, Park 2001, Schafer 2001, Schafer & Lohmann 2002a). However, similar to other rotary nickel-titanium instruments, the K3 files created as light canal transportation toward the outer aspect of the curvature in the apical region of the canals, especially in those having 358 curves (Thompson & Dummer 1997d, 1998, 2000b, Baumann & Roth1999, Park 2001, Schafer & Lohmann 2002a). The canal transportation may be due to the root canal preparation with instruments of greater taper, since these are stiff as compared with those of ISO taper (Kum et al. 2000, Thompson & Dummer 2000b). In contrast, canals enlarged manually with K-Flexofiles showed a more marked transportation toward the outer aspect of the curves and, although not statistically significant (Table 4), had a higher incidence of canal aberrations (Al-Omari et al. 1992, Bishop & Dummer 1997, Kum et al. 2000, Park 2001, Schafer 2001, Schafer & Lohmann 2002a).
In several studies, the shaping ability of different rotary nickel-titanium instruments and flexible stainless steel hand K-files have been compared. These studies corroborate the findings of the present investigation in that in severely curved simulated canals the use of different rotary nickel-titanium instruments such as ProFiles (Kum et al. 2000, Park 2001), GT files (Park 2001), Hero 642 files (Schafer 2001) and FlexMaster (Schafer & Lohmann 2002a) resulted in less canal transportation, fewer canal aberrations and less instrumentation time compared with stainless steel K-Flexofiles.
In the present study, none of the canals became blocked with resin shavings, and none of the canals showed overextension of preparation. Thus, the only changes of working length was a loss of working distance. In general, it was possible with both types of instruments to control the working distance well (Table 4). This finding is in agreement with several observations of other studies in that only small mean changes in working distance occurred with rotary nickel-titanium instruments (Kum et al. 2000, Thompson & Dummer 2000a, Schafer & Lohmann 2002a). On the whole, it is questionable whether the small changes of working length observed in the present study may have any clinical significance. These changes may be due to minor canal straightening during canal enlarge mentor lack of length control by the operator (Thompson & Dummer 2000a).
The mean time for canal preparation was recorded, and included instrument changes within the described instrumentation sequences. Both, in the 288- and 358- curved canals, the K3 instruments were significantly faster than hand preparation with K-Flexofiles (Table 3). This is in agreement with the findings of several authors that instrumentation times with rotary nickel-titanium instruments are substantially faster than with stainless steel hand instruments (Thompson & Dummer 1997 a,b,c, Kum et al. 2000, Thompson & Dummer 2000a, Schafer 2001, Schafer & Lohmann 2002a).
During the present study, no fractures occurred with K-Flexofiles, whereas 11 K3 instruments separated. It is worth emphasizing that these all were 0.04 taper instruments (Table 2). To date, no data is available on the torsional properties of the K3 instruments, therefore noexplanation can be given why these particular instruments were susceptible to separation. Related to the total number of K3 instruments used, a fracture rate of approximately 3% (11 out of 373 K3 files used when all instruments were used to enlarge one canal only) resulted. However, related to the total number of 48 simulated canals enlarged with these instruments, a separation rate of approximately 23% occurred. In comparison with previously published studies conducted under the same experimental conditions as used in the present investigation, the separation rate of K3 files was considerably higher compared to the fracture frequency of Hero 642 and FlexMaster instruments (Schafer 2001; Schafer & Lohmann 2002a). Thus, when using K3 files, according to the instrumentation sequence described in the present study, the separation rate of K3 instruments was higher than previously reported fracture rates of newer rotary nickel-titanium instruments (Thompson & Dummer 1997a, Baumann & Roth 1999, Kum et al. 2000, Thompson & Dummer 2000a). Since up to now, the manufacturer has not recommended a particular preparation sequence for severely curved canals, further research is necessary to investigate the influence of different instrumentation sequences on the separation rate of K3 instruments.

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