Discussion - References.
ProTaper instruments were recently introduced and embody two new concepts. Firstly, in cross-section, the instruments do not have a U-file design and secondly, the instrument’s shaft has variable tapers along its cutting surface (Ruddle 2001). This concept minimizes the number of instruments per set and is claimed to decrease torsional loads by reducing the frictional surface, thereby increasing cutting efficiency. No previous reports have evaluated the shaping ability of ProTaper Ni-Ti instruments using mCT. The mCT is emerging in several endodontic research facilities as a nondestructive and accurate method to analyse canal geometry and the relative effects of shaping techniques (Rhodes et al. 2000, Bergmans et al. 2001, Gluskin et al. 2001, Peters et al. 2001b). Accuracy and reproducibility of the system used in this study has been verified previously (Peters et al. 2000). This project is part of a larger study testing torque and force produced when canals are shaped using extracted human maxillary molars in a special testing device (Peters & Barbakow 2002).
Originally, ProTaper sets included five instruments, namely, Shaping files 1 and 2 and Finishing files 1-3, and such sets were used in the present study. However, an additional instrument (Shaper X) was subsequently introduced, whose task is to relocate canal orifices and shape the coronal part of a canal. In the present study, this coronal preflaring was completed with Gates Glidden drills. Whilst the modified cutting flute of ProTaper instruments might reduce friction and consequently torque (Blum et al. 1999), this design may also increase the incidence of procedural errors and overall canal transportation. The present study addressed this question and furthermore evaluated the effect of canal anatomy on preparation outcome.
No obvious procedural errors were detected in this study, confirming findings reported in two earlier studies, in which four other Ni-Ti preparation systems were evaluated using mCT (Peters et al. 2001 a, b). However, when transportation was expressed as CM shifts, varying degrees of canal straightening were recorded with ProTaper, as was the case with other previously described preparation techniques (Peters et al. 2001 a, b).
Overall canal anatomy, as described by volumes, surface areas and SMI, was statistically similar in the present study, compared to canals evaluated earlier using the same analytical methods (Peters et al.2001 a, b). Pro- Taper preparation removed dentine volumes varying from1.40 to1.76 mm3 compared to a preoperative canal volume of 2.42-5.27 mm3.Whilst some of these values for individual canals are lower than those previously described (Nielsen et al. 1995), the differences are probably due to varying regions of interest (ROI). However, calculations for a perfect 10-mm long cone with a 0.25- mm tip diameter and a.10 taper result in a theoretical file volume of 4.42 mm3. This result should correspond to the volume of a perfectly prepared canal of similar dimensions. In principle, it seems intriguing to refer to these volumes when deciding on irrigation parameters (Yamada et al.1983).
In this study, canal diameters were described as ‘thickness’, which was calculated by fitting spheres into reconstructed canals as described previously (Peters et al. 2000, 2001a). Specifically, maximum local sphere diameter relates to a specific file tip size, which a clinician would select to gauge the apical region (Ruddle 2002). ProTaper instruments adequately opened canals 5 mm from their apices, with sizes varying from 0.65 to 0.79 mm. Spreaders and pluggers with size 0.5-mm tips could readily be used during obturation of root canals with such apical preparations. Deep instrument penetration is considered critical for both lateral (Alison et al.1979) and vertical (Ruddle 2002) compaction. Canal ‘thickness’ is also an important parameter when considering how far into a canal irrigation needles can be safely inserted to allow for back flow.
Although procedural errors were not obvious, some canal transportation was evident in the present study. In fact, CM shifts were slightly larger than those reported previously (Peters et al. 2001a) for rotary instruments with a U-file cross-sectional geometry. This is possibly significant when the smaller apical sizes of ProTaper instruments (0.25-0.30 mm) are considered. Importantly, there was no significant difference in apical transportation when the findings for ‘wide’ canals were compared to those of their ‘constricted’ counterparts. However, results suggested that coronal preflaring with relocation of canal orifices was sufficient to avoid apical preparation errors. Whilst some canal straightening occurred, there was again no difference between the various canal types (p, db, mb) when graded as ‘wide’ or ‘constricted’.
The impact of preoperative canal anatomy was most prominent when assessing the amount of uninstrumented canal areas after preparation. Canals graded as ‘wide’ had significantly larger untouched areas (Fig. 4; Table 4), amounting to 43-49% of their total compared to their ‘constricted’ counter parts. Similar findings have been reported earlier for other techniques using mCT reconstructions (Peters et al. 2001a) as well as from canal cross-sections (Tucker & Wenckus 1997). It has been proposed that canals be prepared to sufficiently large apical sizes, firstly to optimize irrigation and disinfection (Ruddle 2002), and to facilitate elimination of microbes mechanically (Dalton et al. 1998). However, the clinical significance of the parameter ‘prepared surface’ is not yet clarified considering that viable microbes penetrate deeper into dentinal tubules and may persist during root canal treatments (Peters et al. 2002).
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