A comparative study of root canal preparation using ProFile .04 and Lightspeed rotary Ni–Ti instruments

Discussion

In the majority of studies dealing with automated root canal preparation only two or three parameters of automated preparation are investigated. This study presents data on every aspect that is important for a definite conclusion on the clinical usefulness of a rotary device: root canal cleanliness, straightening of root canal curvature, working safety, and working time. Since this is part of a series of investigations on different rotary Ni–Ti instruments, a comparison of all major Ni–Ti systems will be possible. A similar series of investigations on rotary Ni–Ti systems has been undertaken previously (Thompson & Dummer 1997a,b,c,d, 1998a,b, 2000a,b, Bryant et al . 1998a,b) using simulated root canals in resin blocks with different kinds of curvatures that allowed standardization of the experimental model. In the present investigation root canal preparation was performed in extracted teeth. Although the morphology of the root canals probably shows large variations, this seems to be the only way to evaluate the cleaning ability of a preparation technique, as in plastic blocks only canal blockage with resin debris may be studied.

Straightening of curved canals.
Several studies, meanwhile, have confirmed the ability of rotary Ni–Ti files to maintain original root canal curvature even in severely curved canals (Glosson et al . 1995, Knowles et al . 1996, Short et al . 1997, Thompson & Dummer 1997a,b,c,d, 1998a,b, 2000a,b, Bryant et al . 1998a,b, Schäfer & Fritzenschaft 1999, Hülsmann 2000, Hülsmann et al . 2001).
The ability of Profile .04 instruments to prepare curved root canals has been reported in several studies. Schäfer & Fritzenschaft (1999) prepared simulated root canals with root canal curvatures of 28 and 35 with the Profile .04/.06 system and HERO 642 instruments. HERO 642 showed a better centring ability with only few minor deviations from the original curvature. Conversely, Frick et al . (1997), in root canals in extracted teeth with curvatures of 30 and more, could show no difference between ProFile and Quantec with only minimal straightening and a clear superiority compared to hand instrumentation, whereas Kosa et al . (1998) reported on more apical straightening with Quantec files compared to ProFile instruments. In the investigation of Thompson & Dummer (1997b) only minimal straightening of severely curved canals could be detected, the mean apical transportation was 0.01–0.016 mm.
The excellent maintenance of root canal curvature by Lightspeed preparation also has been reported by several studies (Glosson et al . 1995, Knowles et al . 1996, Tharuni et al . 1996, Barbakow & Lutz 1997, Thompson & Dummer 1997c,d, Shadid et al . 1998). Thompson & Dummer (1997d) described a mean apical transportation of 0.06 mm using Lightspeed instruments. The increased flexibility of the Ni–Ti files and the safety tips with rounded transition angles is discussed as the main factor for the superior shaping ability of both systems investigated in this study. Interestingly, the preparation technique – step-back for Lightspeed and step-down for ProFile – seems to have no measurable influence on the degree of straightening, whereas in studies on stainless steel instruments the step-down-techniques proved to be clearly superior in terms of apical cleanliness, apically extruded debris and straightening (Ruiz-Hubard et al . 1987, Murgel et al . 1991, Luiten et al . 1995, Al Omari & Dummer 1995).
The comparison of the pre- and postoperative photographs of root canal cross-sections enables the evaluation of the most important requirements of root canal preparation, that is, the prepared canal completely includes the original canal and no unprepared areas are left. 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.
The results for postoperative cross-sections in the present study did not differ significantly for both systems investigated, which is in accordance with the study of Short et al . (1997). Nevertheless, both systems were not able to prepare all parts of the root canal system and left areas of uninstrumented canal wall. Superimposition of photographs of the pre- and postinstrumentation crosssectional form of the root canals showed that Lightspeed removed more dentine and left significantly less uninstrumented canal wall areas than Profile .04 in the coronal part of the root canal, whereas in the middle and apical parts of the root/canal no differences could be found. This probably is largely due to the use of size 70 Lightspeed instruments in the step-back phase of the preparation.

Cleaning ability.
Most studies on Ni–Ti instrumentation in the last few years have focused on centring ability, maintenance of root canal curvature, or working safety of these new rotary systems; little information is available on their cleaning ability.
Peters et al . (1997) described a homogeneous smear layer in all parts of the root canal system after Lightspeed preparation without a chelating agent and with tap water as the only irrigant. Park et al . (1998) and Kochis et al . (1998) reported on similar results for Profile .04 as for hand instruments, both leaving uninstrumented canal wall areas, debris, and smear layer. In a comparative SEM study, Profile .04 and Lightspeed were equally effective in the debridement of root canals (Peters et al . 1998). The slightly superior cleaning ability of the Lightspeed system might be due to the fact that the number of irrigations with sodium hypochlorite was 12 for Lightspeed, but only 10 for Profile .04 and the larger size of Lightspeed instruments in the step-back phase.
Compared to results of a previous study, the cleaning ability of Profile .04 and Lightspeed was worse than for Quantec 2000 or HERO 642, for which frequently clean root canal walls without debris, no or minimal smear layer and many open dentinal tubules could be detected under the SEM (Hülsmann et al . 2001). The main reason for the inferior cleaning ability of Lightspeed and Profile .04 may be the radial lands of the instruments which perform a planing action on the root canal walls rather than a cutting action that is claimed for Quantec 2000 and HERO 642. Medioni et al . (1999) confirmed the superior cleaning ability of Quantec 2000 when compared to HERO 642, ProFile, and hand instrumentation.