Gianluca Gambarini
University of Rome "La Sapienza", Dental School

In Vitro Analysis of Canal Centering Using New Nickel-Titanium Rotary Endodontic Instruments.

Abstract
The use of nickel-titanium rotary instruments has demonstrated the ability to reduce canal transportation. The aim of the present study was to evaluate and compare new innovative NiTi intruments on the achievement of a continously tapered preparation with maximum respect of the original anatomy.Twenty moderately curved resin blocks were divided into two groups. Group A specimens were prepared using ProFile .04 and .06 Tapers. Group B specimens were prepared using GT Rotary Files (Maillefer,Baillagues,Switzerland). Pre- and post-instrumentation canal shapes were recorded using a digital video camera, magnified 100 times and measured. Canal transportation (the difference between the canal configuration before and after instrumentation) was determined both for the inner and outer side of the curvature in 1 mm steps.

Data showed that both instrumentation techniques provided excellent shaping results There was no statistically significant difference between the two groups (ANOVA). A minimal deviation from the original canal shape was observed in all cases. A tendency to enlarge the inner part of the curvature in the middle third and to enlarge the outer part in the other two thirds was recorded in most cases. Apical transportation was ,however, minimal.. File design of ProFile and GT Rotary Files is quite similar (non cutting tip, flat outer edges), which can reasonably explain the similar shaping results.

Introduction
The preparation of curved canals represent a considerable problem for the endodontic practitioner. Inadvertant errors, such as ledging and zip formation, occasionally occur, adversely altering root canal morphology and thus detrimentally affecting the prognosis for the endodontic treatment. Various instruments and preparation techniques have been proposed to minimize this problem.

Thanks to the favourable mechanical properties of the alloy, NiTi instruments are rapidly becoming very popular. New, innovative engine-driven rotary systems using NiTi instruments of various design have been developed during recent years to facilitate cleaning and shaping procedures. When new root canal instruments with a unique design which differs markedly from conventional files are produced, several characteristics need to be investigated and tested to allow an efficient and safe clinical usage.

The ProFile .04 and .06 Tapers nickel-titanium instruments have a non standard .04 mm/mm or .06 mm/mm taper, a U-shaped cross-section, a non-cutting safety tip and are designed with radial lands. These are flat surfaces that are thought to reduce the tendency of the instrument to screw into canal.

A recent development in file design has produced new NiTi rotary instruments (ProFile GT Rotary Files, Maillefer, Ballaigues, Switzerland), with a special design. The instruments have a #20 conically shaped (non-cutting) tip and four different predifined tapers (.06, .08, .10 and .12). It means that the taper of ProFile GT Rotary Files can vary from three (.06) to six times (.12) greater than that of ISO (.02) standardized files.

The use of nickel-titanium rotary instruments has fully demonstrated the ability to reduce canal transportation. In addition, file design can be very different (non-cutting or cutting tips, positive or negative cutting angles, different design in cross-section) and can significantly influence the degree of canal transportation.

The aim of present study was to evaluate and compare new innovative NiTi intruments on the achievement of a continously tapered preparation with maximum respect of the original anatomy.

Materials and Methods
Twenty moderately curved resin blocks were divided into two groups. The diameter and the taper of the simulated canals were equivalent to a n.15 root canal instrument.

Group A specimens were prepared using ProFile .04 and .06 Tapers with the following crown-down operative sequence: .06 n.25, .06 n.20, .04 n.15, .04 n.20, .04 n.25. Group B specimens were prepared using GT Rotary Files with the following crown-down operative sequence: .12 n.20, .10 n.20, .08 n.20, .06 n.20. Instruments were used in accordance with technique recommended by the manufactured. Instruments were rotated at 350 rpm in the TCM Endo (Nouvag) electric motor. Each canal was irrigated with 1 ml of distilled water after each instrument. All procedures were performed by the same operator. The artificial canals were photographed before the start and after completion of canal preparation.

Pre- and post-instrumentation canal shapes were recorded using a digital video camera (Sony SC-100 E) and stored on the computer (Pentium III). The assessment of preparation shape was done with the computer programme Corel Draw 7. The pre- and post-instrumentation canal shapes were magnified 100 times and measured canal trasportation. Canal transportation (the difference between the canal configuration before and after instrumentation) was determined both for the inner and outer side of the curvature in 1 mm steps using the Corel Draw 7 programme. The pre-instrumentation tracing were alligned with that of the post-instrumentation image and the post-instrumentation canal outline was traced by the same indipendent observer. This allowed direct mesurement by pre- and post-instrumentation canal.

The collected data were analyzed by ANOVA.

Results
The results for both groups are shown in Tables 1 and 2 which illustrate mean material remove (mm) and SD at the different measuring points after instrumentation of simulated curved canals for each group. Data were collected every mm, starting 1 mm from the apex in the inner and outer canal walls.

No instrumentation technique exhibited no canal transportation, but a slight tendency to straighten the canals. A minimal deviation from the original canal shape was observed in all cases. There is no statistically significant differences between the two groups. Apical transportation was minimal for both ProFile and GT Rotary Files groups.

Discussion The results of the present study confirm the findings of previous investigations,which demonstrated not significant differences in canal transportation between mechanical instrument system (Glosson et al. 1995). Overall, both instrumentation techniques provided excellent shaping results: there is no statistically significant difference between the two groups. A tendency to enlarge the inner part of the curvature in the middle third and to enlarge the outer part in the other two thirds was recorded in most cases. The direction of canal transportation was quite predictable confirming previous study on S. S. instrument made by Weine (1975), who demonstrated transportation toward the inner curvature occurred in the middle segment and the outer curvature in the apical segment. Apical transportation was found minimal. Apical enlargement was also minimal. This may be due to the fact that master apical files were sizes n.20. Moreover, nickel-titanium instruments have a low cutting efficiency (it is very difficult to cut the resin materials). Therefore, the minimal amount of transportation may be due to the decreased cutting efficiency. On the contrary rotary motion has been slown to be very effective in maintining the original canal shape; these findings may also results in a minimal amount of canal transportation. File design of ProFile and GT Rotary Files is quite similar (non cutting tip, flat outer edges), which can reassonable explain the similar shaping results.

Conclusions
Both instrumentation techniques provided excellent shaping results, developpng a continuously tapering canal preparation. A minimal canal transportation was observed in the moderately curved artificial canals, but apical foramen was maintained centered. It may concluded that the tested Ni-Ti rotary instrumentation techniques were shown to minimize the tendency for all preparation techniques to transport the prepared canal away from the original axis. There was no statistically significant difference between the two groups . File design of ProFile and GT Rotary Files is quite similar (non cutting tip, flat outer edges), which can reasonably explain the similar shaping results.

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