Discussion - References.
Comparison of different root canal preparation systems requires standardized conditions and the collection of data on all important aspects of performance for a definite conclusion on the clinical usefulness of a rotary device to be determined. In this study, several parameters such as postoperative root canal shape, cleanliness, working safety and working time were investigated. This study is one of a series of investigations on different rotary NiTi instruments that should allow a comparison of all the major NiTi systems. In this series of studies (Hulsmann et al. 20 01, Versumer et al. 20 02),mesial root canals of extracted mandibular molars were used. A similar series of investigations on rotary NiTi systems has been undertaken by the group of Thompson & Dummer (Thompson & Dummer 1997 a,b,c,d, 1998 a,b) using artificial root canals in resin blocks with specific curvatures. The advantages of such simulated root canals are a standardized canal form and reproducible conditions. The disadvantages of plastic blocks such as the difference between the hardness of dentine and the plastic as well as the abrasion behaviour have been described (Lim & Webber 1985, Miserendino et al.1988). In the present study, root canal preparation was performed in extracted teeth to evaluate parameters such as the appearance of the dentinal surface and the accumulation of smear layer and debris. On the other hand, parameters such as degree, angle and location of root canal curvature, initial diameter of the root canal and shape of the cross-section were standardized. However, the shape of the cross-section at the root canal orifice was controlled optically. Only teeth with a bucco-lingual distance twice as long as the mesio-distal distance were included into the study which is similar to the criteria used by Wu et al. (2001) and Wu & Wesselink (2001), who investigated only teeth with an internal long : short diameter ratio of >1.6 and >2, respectively. Cross-sections.
The aim of the present study was to examine whether the buccal and lingual extensions of oval root canals could be prepared completely with highly flexible NiTi instruments. The comparison of the pre- and postoperative photographs of root canal cross-sections enables the extent of dentine removal to be evaluated quantitatively and qualitatively. Bramante et al. (1987) were the first to develop a method for the evaluation of changes in root canal diameter. Using a modification of their method, pre- and postinstrumentation photographs of the root canal diameter may be superimposed and deviations between the two root canal outlines can be measured. As the diameter of a root canal is not constant from the orifice to the apex, the roots were sectioned horizontally at 3, 6 and 9 mm from the apex, respectively, allowing inspection of all thirds of a root canal.
Superimposition of photographs of the pre- and postinstrumentation cross-sectional forms revealed that all three systems showed the best results in the apical third with only a few sections having unprepared lingual or buccal extensions. This, of course, is due to the fact that most oval distal root canals become more round towards the apical third of the root; this has been confirmed in a recent study on extracted teeth (Wu et al. 2000). Preparation with ProFile .04 was superior in the apical region compared to Lightspeed and Quantec SC (Table 2), but in all three parts of the root canals, no significant differences between the three NiTi systems could be found. The middle and coronal cross-sections were increasingly irregular and frequently showed circular bulges (Fig. 2 a, b). All three systems performed relatively poorly in these two sections of the root canals, probably because of their flexibility, frequently not allowing the operator to force them in to the lateral extensions. The design of the instruments with safe tips and radial land may have resulted in a self-centering movement of the files along the initial lumen of the canal. The buccal and lingual extensions of the oval root canals therefore often remained unprepared, and a circular bulge somewhere in the centre of the root resulted (Fig. 3 a,b). The best results were obtained with Quantec SC instruments.
Figure 2.(a) Cross-section before preparation.
(b) Postoperative cross-section showing an unprepared lateral extension and an irregular canal shape.
Figure 3. (a) Preoperative cross-section.
(b) Postoperative cross-section showing a massive circular bulge within prepared lateral extensions.
The total amount of non instrumented canal areas was rather high (19.2%). It should be explained that in this study, Lightspeed instruments were used in a circumferential filing motion rather than a pecking motion as recommended by the manufacturers. Indeed, they recommend that oval canals should be treated as two canals with the buccal and lingual extension representing a separate canal. Preparation of these two canals is advised to a size when the two canals overlap in the middle of the root. In the recent literature, no data on postoperative cross-sections of distal root canals of lower molars could be found, but the findings are in accordance with the results of previous investigations by Wu & Wesselink (2001), who, following preparation of oval canals in mandibular incisors with the balanced force technique, reported uninstrumented extensions in 65%of the canals. Ina recent study on the quality of root fillings in oval canals using extracted premolars, Wu et al. (2001) also found a high percentage of unprepared and unfilled buccal and lingual recesses. Barbizam et al. (2002) confirmed these findings in a study on preparation of flattened root canals in mandibular incisors. Root canal cleanliness.
In the present study, postoperative root canal cleanliness was investigated only for the buccal and lingual extensions of the oval canals. None of the three NiTi preparation techniques resulted in complete cleaning of these areas of the canal walls. The majority of the canals achieved a score of 2 for debris and a score of 3 for smear layer. No significant differences between the three techniques could be found. As all techniques had been used with an identical irrigation regimen, this tends to reflect the fact that the irrigation regimen is more responsible for canal cleanliness than preparation techniques or instruments. The SEM evaluation revealed that neither irrigants nor instruments could remove sufficient debris and smear layer from the recesses. In a comparative SEM study, ProFile .04 and Lightspeed were shown to be equally effective in the debridement of root canals (Peters et al. 1998). In previous studies, it was shown that the cleaning ability of ProFile .04 and Lightspeed was worse than for Quantec SC or HERO 642. Following preparation with HERO and Quantec, clean root canal walls without debris, no or minimal smear layer and many open dentinal tubules could be frequently detected under the SEM (Hulsmann et al.2001, Versumer et al. 2002). The main reason for the inferior cleaning ability of Lightspeed and ProFile .04 probably will be the radial lands of the instruments which have a planning action on the root canal wall rather than a cutting action as with Quantec SC and HERO 642. Medioni et al. (1999) confirmed the superior cleaning ability of Quantec SC when compared to HERO 642, ProFile .04 and hand instrumentation. On the other hand, Barbizam et al. (2002) reported superior results for the manual crown-down technique using stainless steel K-files compared to ProFile .04 rotary preparation of flattened root canals in mandibular incisors. In the present investigation, no significant differences between the three NiTi systems could be found. Cleanliness of recesses in oval canals may be enhanced by use of sonic or ultrasonic irrigation techniques which remove debris but do not affect the smear layer when only used with water as irrigant. Therefore, sodium hypochlorite or chelating agents such as EDTA should be selected. When an ultrasonic unit is used for irrigation, the file is best directed towards the extensions (Lumley et al.1993). Working safety.
Generally, many authors reported on the fact that NiTi instruments fracture more frequently when forced with variable speed of rotation, overuse of instruments and excessively high rotational speed (Barbakow & Lutz 1997).Additionally, the operator’s experience with specific systems may be related to the frequency of instrument fractures. As recommended by Gambarini (2000), the use of a low-torque endodontic motor with constant speed for each file of any NiTi system instead of a high torque motor might help to reduce the risk of instrument fracture.
The Lightspeed system has been described to be safe for the preparation of curved root canals (Thompson & Dummer 1997 a, b, Versumer et al. 2002). In the present study, two instruments fractured during the forced attempt of a circumferential filing motion which is not in accordance with the manufacturers’ recommendation. In an evaluation of Thompson & Dummer (1997c, d), no fractures occurred with the ProFile .04 system.
Concerning the fracture frequency of Quantec SC instruments, little information is found in the literature. Thompson & Dummer (1998a) had one fractured and three deformed instruments. No apical blockages and loss of working length occurred in their study. In the present investigation, no fractures, but two apical blockages, could be detected. In recent studies on preparation of curved root canals using different NiTi instruments in 50 root canals, three instrument fractures, three apical blockages and eight cases of loss of working length occurred with the Quantec system (Hulsmann et al. 2001). Following preparation with ProFile .04, three instruments separated whereas no procedural errors occurred with the Lightspeed system (Versumer et al. 2002). Working time.
The finding that Lightspeed instrumentation took significantly more time than ProFile .04 and Quantec SC preparation to a large extent will be due to the fact that the number of files for the three systems greatly differs (Lightspeed, 20; ProFile .04 and Quantec SC, 10). Following the protocol of this study, the number of irrigations for Lightspeed was 12, ProFile .04 and Quantec SC 10. Clinically, the difference will be even more evident as time for instrument changes and adjustment of the stopper systems has to be added. On the other hand, preparation time for each single instrument was shorter for Lightspeed, due to the reduced contact zone between instrument and root canal wall.
Overall, the ability of NiTi systems to shorten working time compared to hand preparation or to automated root canal preparation using different endodontic hand pieces with conventional stainless steel files has already been confirmed in a number of previous studies (Esposito & Cunningham1995, Hulsmann et al. 20 01).
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