Canal preparation is one of the major steps in root canal treatment and is directly related to subsequent disinfection and fillings.1 The goal of root canal shaping is to form a continuously tapered shape with the smallest diameter at the apical foramen and the largest at the orifice so as to allow effective irrigation and filling and not change the original canal curvature.2

Because of the stiffness, stainless steel instrument is likely to cause canal aberrations such as zips, elbow, ledges and perforations.3,4 During the late 1980s and 1990s, various types of endodontic instruments were developed with nickel-titanium alloys.5 The introduction of nickel-titanium alloy for the mechanically driven endodontic instruments has greatly simplified shaping the root canal systems.5 These new instruments exhibit more elastic flexibility in bending and torsion,5 and are also markedly superior to stainless steel instruments in terms of angular deflection and maximum torque to failure.6 These instruments have been found to be better than stainless steel instruments in maintaining the original anatomy and the shape and position of the apical foramen.7-9

However, despite the evident advantages of the new technique, Ni-Ti rotary instruments have several unexpected disadvantages. One of these is tendency to screw into the canal.10 This tendency is particularly accentuated especially when continuously rotating nickel-titanium instruments are used.11 Because rotary instruments rotate with electric power, control of the working length is more difficult during rotary instrumentation than during hand instrumentation.10 For this reason, the screw-in effect during rotary instrumentation may cause overinstrumentation beyond the apical foramen. Undoubtedly instrumentation beyond the apical foramen reduces the success rate.5,12-15

Some kinds of factors have been investigated to be involved in the screw-in effect of Ni-Ti rotary files. Cross-sectional geometry, helical angles, pitch length and taper of files are included in these factors.16 Diemer et al. evaluated the effect of pitch length on the torsional stress and tendency to screw-in using two instruments with same cross-section (triple helix; 6% taper).16 Their results showed that increasing the pitch length decreased torsional load sharing and the tendency to screw-in. Schräder et al. compared torques and forces developed by two sequences of nickel-titanium rotary instruments, using only .04 taper instruments or a combination of .04 and .06 tapers, respectively.17 Their conclusions were an instrumentation sequence encompassing various tapers seems to be safer in torsional and fatigue failure compared to sequence that used only one single taper.

The concept of maximizing cutting efficiency by minimizing surface contact area of the instrument with root canal wall has led to development of variations in taper. It has been suggested that increasing instrument taper while maintaining the same size tip maximize cutting efficiency.8 However, as the taper increases, the stresses of the instrument were also greater.18

In the previous research, the screw-in effect among several Ni-Ti rotary file systems was compared.10 In addition, the impact of varying instrument taper on screw-in effect needs to be investigated.

Therefore, the purpose of this study was to evaluate the influence of taper on the screw-in effect of nickel-titanium rotary files.

Table 2 shows the mean and the standard deviations of screw-in force according to the taper of each file systems.

The more tapered instruments generated more screw-in forces in Profile group (p < 0.05). In K3 group, 0.08, 0.10. and 0.12 tapered instruments showed more screw-in force than 0.04 tapered one, and 0.08 and 0.12 tapered instruments showed more screw-in effect than 0.06 tapered one (p < 0.05). 0.12 tapered file of Profile group produced significantly greater screw-in force than any other instruments in the study (Figure 2).

Root canal instrumentation with highly flexible rotary Ni-Ti instruments improved preparation quality, determined by better centering ability.3 This may lead to superior obturation quality and probably to better clinical outcomes.16 However, Ni-Ti instruments are clinically prone to screw-in depending several factors and this may in turn compromise clinical outcomes. To overcome or control this effect in the canal, factors associated this effect need to be evaluated. In the previous study,10 the screw-in effect among several Ni-Ti rotary file systems was evaluated. The present study evaluated the effect of the instrument taper, which is one of the factors that may influence on the tendency of screw-in. The result showed a tendency that more tapered instrument generated more screw-in force.

In the present study, simulated canals in resin blocks were used to eliminate variations in canal anatomy without using lubricants which may limit the number of variables and avoid stress measurement because of axial or transversal irrigation force.19

One special characteristic of a Ni-Ti rotary instrument is increasing the taper from standardized instrument of 0.02 mm/mm to variable greater tapers up to 0.12 mm/mm. Torque generated and apical force applied were measured in a study18 using an engine-driven Ni-Ti file, Quantec Series 2000 (Tycom Corp., Irvine, CA, USA) in extracted human teeth. The torque was dependent on the tip size and taper of each instrument, and the canal size. They found that both the torque and apical force were increased with increasing taper (0.02, 0.03, 0.04, 0.05 and 0.06) of the file. However, there was no close relation between torque and apical force. Another study17 using Profiles in simulated canals showed that both the contact area and torque were increased with increasing taper (0.04 and 0.06) and size (15, 20 25, 30, 35 and 40) of the instruments. The generated apical force were also slightly increased with increasing taper of the instruments, but not with the size of them in the study. In the present study using Profile and K3, the result was similar with the above mentioned studies in that the screw-in effect was increased with increasing taper (0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) of the instruments. Since torque is a force that is generated between the canal wall and instrument, if the torque is increased, the files might have increased resistance in entering the canal. The contact area also might give resistance to the instrument in entering the canal. However, screw-in force might not increase with increasing resistance. Therefore, it seems that torque might be increased due to contact area is increased with increasing taper or size of the instruments. However, screw-in effect might not be related to the torque directly. On the contrary, screw-in effect might increase with less resistance with less contact area which may be related to the instrument taper.

In the previous study,18 each canal was instrumented sequentially using all files of #1 to #10. In the present study, the smaller tapered instrument was used earlier in each simulated root canals followed by next tapered instrument. Thus, the instrumentation sequence is different from the study, but the results are similar. Thus, instrumentation sequence seems not much significant to influence on the results.

Feature of file design may affect screw-in effect. ProFile and GT rotary files have radial lands with U-shape and noncutting tips. K3 instrument is reported to have a slightly positive rake angle in combination with a peripheral blade relief for reduced friction and a third radial land to help prevent screwing in.20 These file systems are similar in respect to have flattened or modified cutting edges to limit screw into the canal. In the present study, the tendency of increasing screw-in force with increasing taper was shown in both Profile and K3 groups. However, it was less clear in K3 group. It may need further investigation whether its design is related to it because it was said that a third radial land was designed to help prevent screwing in.20 In the present study, 0.12 GT rotary file produced significantly greater screw-in force than any other instruments in this study. With repeated experiment, the result was similar in the study. Because the authors cannot explain it clearly, it needs to be elucidated on the reason. One possibility may be the less contact area to the canal wall with bigger taper in the triple U cross-section geometry. Turpin et al. used theoretical models to show that triple U cross-sections generate localized stresses not only at the cutting edge but also at the center of the instrument.21 These stresses are much more that those exerted on a triple helix of the same size and diameter but without any flat radial land area.

In conclusion, within the condition of the present study, engine-driven instruments with more taper seem to produce more screw-in effect in both Profile and K3 systems. More attention should be paid when using more tapered rotary instrument to prevent or reduce the screw-in forces so as not to hamper the endodontic success.22,23 Further studies are needed to investigated on other factors that may be involved in the screw-in effect and to find techniques to control this effect during rotary instrumentation.