Geometric and dimensional characteristics of simulated curved canals prepared with proTaper instruments

Objective This study identified which regions of ProTaper instruments work during curved root canal instrumentation. Material and methods Twelve ProTaper instruments of each type, S1, S2, F1, and F2, were assessed morphometrically by measuring tip angle, tip length, tip diameter, length of each pitch along the cutting blades, and instrument diameter at each millimeter from the tip. Curved canals in resin blocks were explored with manual stainless steel files and prepared with ProTaper instruments until the apical end following four distinct sequences of instrumentation: S1; S1 and S2; S1, S2, and F1; S1, S2, F1, and F2. Image analysis was employed for measuring canal diameters. The diameters of the canals and diameters of the instruments were compared. Data were analyzed by one-way ANOVA and Tukey’s test. Results No statistically significant difference was found between the canals and instrument diameters (p>0.05). The largest diameters in the end-point of the instrumented canals were obtained with F1 and F2 instruments and in the initial and middle thirds with S1 and S2 instruments. Conclusions All instruments worked at the tip and along their cutting blades, being susceptible to fail by torsion, fatigue, or the combination of these two mechanisms.


INTRODUCTION
According to their manufacturers, ProTaper nickel-titanium rotary instruments (Dentsply/ Maillefer, Ballaigues, Switzerland) were designed to improve cutting efficiency, flexibility, and safety, being developed for instrumentation of difficult, constricted, and severely curved canals with a few "shaping" and "finishing" instruments.
The shaping instruments S1 and S2 have

MATERIAL AND METHODS
Twelve ProTaper instruments of each type, S1, S2, F1, and F2, were used, totalizing 48 instruments, which were examined with a microscope (Mitutoyo TM 500, Tokyo, Japan) at   instruments. S1, F2, and F3 instruments increased 45% on the first pitch length, and S2

RESULTS
and F1 instruments showed a smaller increase, around 35% and 23%, respectively. The increase in length from the second to the 8th pitches in S1 and S2 instruments was 8.5% and 12%, respectively. From the eighth to the last pitch, these instruments showed an increase of 21.5%.    Figure 4 for an F1 instrument. In S1 and S2 instruments, finer microcracks, as well as signs of fretting, smoothed, or scratched surfaces were found until D 7 ( Figure 5).

DISCUSSION
International standards are provided to establish manufacturer's guidelines followed to   instruments. It can also be observed that the largest diameters next to the end-point were obtained with F1 and F2 instruments working until the full length, while most of the shaping of the orifice and middle portions was carried out by S1 and S2 instruments. However, S1 and S2 instruments also enlarge progressively the end third of the canals. In fact, Peng, et al. 11 (2005) evaluated S1 instruments discarded after clinical use and found that these instruments fractured at a mean distance of 3.67 mm from their tips.
In the present study, SEM images revealed that the simulated clinical use of the ProTaper instruments produced alterations on their surfaces. Instruments S1 and S2 showed cracks next to the region of maximum canal curvature and in regions more distant from the tip. These results suggest that the instruments S1 and S2 are subjected to torsional stresses in the coronal and middle thirds because of their largest diameter in these regions, besides flexural stresses at the tip and at the curvature of the canals in the apical region, being thus prone to fail by two distinct mechanisms: fatigue in the apical portion and overloading in torsion in the coronal and middle thirds. It must be remembered that S1 and S2 instruments are the first to work in the whole extension of the canal, and thus their tip should act in curved canals whose apical portion is not adequately widened.
The F1 and F2 instruments were developed to shape the apical third, but they also expand the shape into the middle and coronal thirds of the canal 6,13 . These instruments present a decrease in their lifetime in relation to S1 and S2 instruments because they work actively in the apical third of the canals, being thus subjected to higher deformation amplitudes due to the

CONCLUSIONS
The largest diameters in the end-point of the instrumented canals were obtained with F1 and F2 instruments, while most of the shaping in the initial and middle thirds was performed by S1 and S2 instruments. However, all instruments worked at the tip and along their cutting blades, being prone to fail by torsion, fatigue, or by the combination of these two mechanisms.