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 »  Home  »  Endodontic Articles 10  »  Smooth flexible versus active tapered shaft design using NiTi rotary instruments
Smooth flexible versus active tapered shaft design using NiTi rotary instruments
Discussion - References.



Discussion.
In recent years, the performance of NiTi rotary systems has been compared with those of traditional stainless steel hand instruments. However, owing to the dissimilarity in use, design and alloy of both groups, this type of study rarely reveals the influence of one specific feature. Additionally, coronal flaring with Gates-Glidden burs may reduce the ability to detect canal transportation because only the apical one-half of the canal remains to show the effects of instrumentation.
The present study evaluated two NiTi rotary systems (i.e. Lightspeed and GT-rotary) that were used in continuous mechanical rotation with no preparing with Gates-Glidden burs. For that reason, variation in canal preparation was attributed to a difference in instrument design (i.e. smooth flexible vs. active tapered). Furthermore, mesial canals of the same roots were used for both techniques to eliminate the variables encountered in root canals in different teeth (e.g. curvature, dentine hardness, canal diameter and length), thereby reducing the crucial amount of samples. The determination of canal curvatures and initial (PRE) root-canal volumes by software confirmed the similarity of the two experimental groups. All canals showed moderate global curvatures (ranging from 143 to 1778) as seen in regular clinical circumstances. The acceptance of more severe curvatures would increase the number of samples and the working load .
Despite efforts to ensure two distinct mesial canals, 83% of the roots hadanisthmus or excessively prolonged fin in at least one level. Parts of these prolongations and fins are impossible to reach with endodontic instruments. For that reason, main areas of the root canals were initially determined through visually fitting an oval figure (long/short diameter ratio _ 2) within the total canal contour on the level of each cross-section, whilst blocking out the remaining surrounding part (Fig.6). Visualization of canal shape before (PRE) and after (POST) instrumentation provided impressions of global preparation characteristics. In both groups, no canal aberrations occurred. For all canals, the irregular initial shape had become more gradually tapered during preparation, although objective criteria to determine canal flow and taper were missing.
Far more interesting is the amount of quantitative information on instrumentation effects that was obtained. Under the conditions of the present study, statistical analysis (multiway factorial anova) could not reveal any difference between the ‘Smooth’ (LS) and ‘Active’ (GT) group as for the amount of transportation, net transportation and centring ability. The lack of significance between the two groups may be a consequence of an insufficient number of samples. After all, canal preparation characteristics may be dictated more by anatomy than by the difference in instrumentation method. Although a high degree of similarity between the two groups was confirmed, the variety of root-canal anatomy within the groups produced relatively high dispersion of the data. On the other hand, any unobserved differences between the two instrumentation methods may, therefore, be so small that they are irrelevant for clinical practice.

Figure 6. Representation of the blocking-out process: The main area of the upper root canal was initially determined through visually fitting an oval figure (long/short diameter ratio _ 2) within the total canal contour, whilst blocking out the remaining surrounding part (yellow).

he main area of the upper root canal was initially determined through visually fitting an oval figure

By contrast, significant differences were noted when grouping was made with respect to direction. At the coronal section, more dentine was removed towards the furcation area. This could be attributed to the fact that no coronal preparing or anti-curvature relocation of the canal was performed with Gates-Glidden burs prior to NiTi rotary instrumentation. The remaining bulk of cervical dentine at the mesial side may force the flexible NiTi file to the distal just below the root-canal orifice resulting in more dentine removal towards the furcation. However, the resulting degree of net transportation was fairly small and therefore, no strip perforations or danger zones were created. This is important because mesial roots of mandibular molars are subject to perforation (Lim & Stock1987).
At mid-curvature, net transporation and centre movement was towards the outer side of the curvature. This finding was consistent with previous work of Thompson & Dummer (1997b,c,1998a,b, 2000) regarding preparation characteristics of ProFile .04 Taper Series 29, NT Engine/McXim, Mity Roto 3608 and Naviflex, Quantec Series 2000 and Hero 642. However, after Lightspeed instrumentation, the same investigators found centre movement towards the inner aspect of the canal at all positions except for the end-point (Thompson & Dummer 1997a). At the apical section in this study, the direction of net transportation was to the mesio-buccal or mesio-lingual direction, thus towards the outer side of the curve. Once more, the values for net transportation were fairly small and therefore no zipping or ledging could be found. Centring ratios for preparation with stainless steel instrumentation techniques were reported ranging from 0.21 to 0.47 (Leseberg & Montgomery 1991). In the present study, the highest mean centring ratio was only 0.15. Glosson et al. (1995) reported similar centring ratios of 0.12 for the apical, and 0.16 for the middle level for the Lightspeed group.
Finally, it is important to notice that, in general, dentine was removed in all directions at all levels, indicating that most areas of the root canal were touched. On the other hand, it should be emphasized that parts of the prolongations and fins were initially blocked out. Ideally, when experiments are carried out to compare different preparation techniques, the diameter of the apical preparation is of importance. In the current experiment, canals assigned to the ‘Smooth’ (LS) group had size 45 master apical rotaries and the maximum apical preparation diameter in the ‘Active’ (GT) group was size 35-40. The reason for this discrepancy is three-fold. First, morphometric video analysis of the Lightspeed system demonstrated that the diameter of the instrument’s head was frequently under-sized (Table 5) (Marsicovetere et al. 1996). Second, the dissimilarity in instrument design (Table 5) combined with the difference in use (step-back vs. crown-down) explains the fact that expected apical root-canal dimensions after preparation are comparable (Table 6). Third, the perpendicular reslices were made at equally spaced intervals and the most apical reslice was within 1-2 mm from the apical terminus because equal distribution of measuring points over the total length of the root presented amore individual approach than measuring at fixed distances from the apex.

Table 5. File dimensions of both systems.

File dimensions of both systems

Table 6. Expected apical root-canal dimensions after preparation.

Expected apical root-canal dimensions after preparation

Total dentine removal ranged from 0.97 _0.39 mm for the ‘Smooth’ (LS) group to 1.43 _0.85 mm for the ‘Active’ (GT) group. Peters et al. (2001) found similar values using a comparable method in maxillary root canals. When considering dentine removal along the root-canal axis, the GT-rotary system removed significantly more dentine in the apical half. This finding might indicate the creation of a better deep shape when using a more tapered file design.
The present study evaluated root-canal geometry using microfocus computer tomography (XMCT). This technique is nondestructive and, if combined with appropriate software, provides data in three dimensions based on high-resolution images of extracted human teeth under natural conditions (Bergmans et al. 2001). The use of XMCT in endodontic research is new. Recently, more suitable measurement software has become available on a prototype basis, allowing evaluation of matched specimens before and after preparation (Peters et al. 2000, Bergmans et al. 2001). The software used in the present study provides objective quantitative information because the examiner has no role in the calculating process. The processing takes place based on previously determined standardized criteria.

References.

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