Discussion - References.
By removing the cervical interference, it was possible to insert a larger file size to the apical constricture. Hence, our present study has confirmed previous findings (Leeb 1983, Contreras et al.2001).The need for a larger file size to attain binding for both LS and K-file after coronal flaring is a further indication of the presence of cervical or mid root interference that initially prevents both types of instruments from reaching the apex. However, the ability of LS instruments to register a larger measurement than the K-file in the same canal may be owing to differences in instrument design and alloy properties. The short cutting blade, noncutting tip design and nontapered shaft of LS instruments (Fig. 3) may allow them to bypass the cervical constriction area or any premature binding site along the root canal. Furthermore, the flexible nickel-titanium alloy of LS instruments may allow them to bypass the canal curvatures more readily than a more rigid stainless steel file. Both the canal interference and the curvature may be the factors that govern a clinician’s ability to sense apical diameter with a file.
As in a previous study (Liu & Jou1999), the LS instruments registered larger apical file sizes than the K-files. However, the mean discrepancy between the two file types was unchanged after flaring (9.6 _10_2 mm vs. 9.3 _10_2 mm) rather than reduced (11 _10_2 mm vs. 7.3 _10_2 mm) (Liu & Jou 1999). In retrospect, one would expect the discrepancy between the two instruments to diminish after coronal flaring, as removing the cervical interference should allow a K-file to reach the apical 4 mm without obstruction. Possible explanations are a lack of canal taper (<0.02 mm mm_1 taper) and constrictions within the apical 4 mm, which allowed similar-sized LS but not K-files to pass through. Also large K-files may be too stiff to negotiate severe apical curvatures.
Two important issues need to be addressed in relation to the discrepancy of results obtained between the two instruments before and after flaring. Firstly, it is possible that repeated ‘sizing’ enlarged the canal even though the files were supposedly placed passively. Secondly, LS is not an instrument of any one determined shape that changes only in diameter. Rather, it is a series of instruments that show gradual shifts in both size and shape as the instrument size increases (Marsicovetere et al. 1996). Hence, part of the difference is possibly related to LS tip configuration because the maximum diameter of LS instruments may be further away from the pilot tip of the instrument than is the case for K-files. Conflicting reports (Marsicovetere et al. 1996, Schrader et al. 1998) suggest that LS instruments are undersized or oversized in relation to their nominal size. We measured the cutting heads of 26 LS instruments (from sizes 20- 50, two instruments of each size) using a micrometer. On average, instruments were slightly smaller than the stated size by approximately 1 ISO unit (0.01mm) and the difference varied from 0.5 to _3 ISO units. Thus, the results cannot be explained by size differences. Non etheless, it may not be appropriate to state that LS is more ‘accurate’ (i.e. closer to true canal diameter) than K-files in gauging the apical constricture.
Figure 3. A size 30 K-file with a corresponding size 30 LS instrument showing their different tip designs and taper (original magnification: 16x).
In the K-file group, the file size increased by approximately one file size after flaring rather than two file sizes as previously reported (Contreras et al. 2001). These differences may be due to variations in the types of teeth utilized in the two studies. Contreras et al. (2001) assessed only the mesial roots of mandibular molars, whereas the present study included a range of different canal types of varying size and curvature from both maxillary and mandibular premolars and molars. The values calculated are valid for ‘wider’canals, as the mean apical diameter of canals utilized was mostly file size 20 or 25 (i.e. K (without flaring), Table 2). Clinically, smaller canals may not be negotiable initially with a size 20 LS instrument.
In our study, 27 out of the 36 (i.e.75%) premolar canals sized by K-file recorded at least1 ISO file size larger after flaring. Furthermore, 31 out of 36 (i.e. 86%) premolar canals in the LS group registered larger file sizes after coronal flaring. This indicates that cervical interference is also present in premolar canals, and its removal will allow a larger file size to t to the apex. As a result, we would recommend early flaring not only in molar teeth but also in premolars.
Grossman et al. (1988) recommended that a root canal should be enlarged to at least three sizes beyond the first file size that binds at the apex to ensure adequate debridement. In our study, apical canal diameter determined with LS after flaring was approximately 3 ISO file sizes larger than with a K-file without coronal flaring. In other words, early flaring is essential and canals may need to be enlarged to a greater file size than previously accepted.
The LS manufacturer recommends early coronal flaring followed by individual sizing of each canal before determining the appropriate master apical rotary size. This usually results in a larger master apical file size. The typical master apical rotary sizes of 50-55 are recommended by the LS manufacturer (Lightspeed Technology Inc.) for the mesial canals of mandibular molars. These sizes are much larger than the ‘conventional’ master apical file sizes of 25-30 recommended by most authors for hand instrumentation techniques (Grossman et al. 1988, Weine 1989, Ingle et al. 1994, Torabinejad 1994,Walton & Rivera 1996,West & Roane 1998). Enlargement with stainless steel hand instruments larger than size 30 in mesial canals of mandibular molar will most likely lead to a high frequency of procedural errors, such as fledging, canal straightening, zipping, apical transportation and strip perforations. This is because most mesial roots of mandibular molars demonstrate curvature in both mesio-distal and bucco-lingual directions (Cunningham & Senia1992), and the dentine thickness is narrowest in the apical third region (Gani & Visvisian1999).
Finally, based on the results of our study and previously reported findings (Seidberg et al. 1975, Leeb 1983, Stabholz et al. 1995, Levin et al. 1999, Liu & Jou 1999, Contreras et al.2001), we conclude that if an operator wishes to determine an accurate canal WL and apical file size simultaneously, canal orifice enlargement should be performed first before placement of the measurement file. The better sense of apical diameter provides information that should result in better control of the biomechanical preparation. Furthermore, the use of LS instruments would register larger canal sizes than K-files in the apical constricture.
Early coronal flaring not only allows better sense of apical constricture and diameter, it may also facilitate cleaning by allowing the irrigant to work deeper, more quickly and more effectively into the apical third region (Ram 1977). The concept of apical enlargement is still poorly understood. To date, no study has shown the influence of apical enlargement on the success and failure of endodontic treatment. The feasibility of larger apical third preparation, especially in premolar and molar teeth, should be investigated. Questions such as whether the canals are cleaner with larger preparation (from both pulp remnants and dentine debris) and whether the roots are weaker if they are further enlarged, still remain to be answered.
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