The role of root canal instrumentation in successful endodontic treatment is to clean the canal of organic debris and to shape the canal for three-dimensional hermetic obturation and effective chemical irrigation. The ideal preparation for a root canal is a continuously tapered funnel shaped with the smallest diameter at the apex and the widest diameter at the orifice1). Apical enlargement is performed to ensure cleanliness and improve obturation2).

Rotary nickel-titanium (NiTi) instruments have been shown to prepare the root canal rapidly, and maintain the canal shape and working length with few aberrations during root canal preparation3). They are available in various designs that differ in tip and taper design, rake angles, helical angles, pitch, and presence of radial lands. Numerous studies have shown that NiTi rotary instruments can effectively produce a well-tapered root canal form sufficient for obturation, with minimal risk of transporting the original canal4-7).

Many rotary NiTi file systems have been introduced to the market. Most of these NiTi file systems - e.g. ProFile® (Maillefer, Dentsply, Ballaigues, Switzerland), K3™ (SybronEndo, California, USA), Hero Shaper® (Micromega, Besancon, France) - have a constant tapered shaft design, while they have various rake angles and radial lands8-10). The ProFile® can be categorized as a passive instrument, while the K3® and Hero Shaper® are active instruments with an active cutting motion11). Current evidences show that files with sharp cutting edges do not affect the instrument's ability to stay centered, even in curved canal3). Nevertheless, certain active file systems have the tendency to dangerously screw into canal, especially when they have a constant taper, pitch, and helical angle12). It has been claimed that continuous changing the pitch and helical angle over the length of the cutting blades of an instrument could reduce the screw-in effect13,14).

If screw-in effect has occurred during canal preparation, it is difficult to manage and preserve working distance especially in novice operator. Due to the over instrumentation by screw-in effect, operators could not make apical stop fits with the width of last apical shaping file tip at apical constriction area. It means that apical terminus width became wider than that of as we want and apical stop would be destroyed. Then ideal obturation would not be attained and it will block the way to successful endodontic treatment.

It is hard to find out the study that has compared screw-in effect of various NiTi file system in terms of the maintaining ability of apical terminus width of curved canals. Numerous authors have reported the advantages of rotary preparation with NiTi instruments over hand preparation, both for experienced and for inexperienced operators15,16). However, few studies have addressed the screw-in effect of NiTi file systems. This effect jeopardizes the safe usage of NiTi file, especially in novice operators.

The purposes of this study were to compare the apical terminus width of simulated root canal prepared with three NiTi file systems (ProFile®, Hero Shaper®, K3™) used by undergraduates for evaluation the effects of flute angle and pitch or radial land on reducing screw-in effect and to determine more adequate NiTi file system for inexperienced operators.

Fifty senior students at the Dental College of Pusan National University (Busan, Korea) with no practical experience using NiTi file systems in root canal preparation participated in this study. The regular curriculum instruction in root canal shaping procedures consists of six hours of lecture and eight hours of model (extracted tooth) exercises with stainless-steel files. It was all of experience before the procedures. In addition to the lectures on basic and practical endodontics of the regular curriculum, the students received two hours of lecture and watched video about the three NiTi file systems used in this study. Respective instrumentation procedures were demonstrated before the start of the experiment.

The students prepared simulated curved root canals in resin blocks (Endo Training Bloc; Dentsply Maillefer, Ballaigues, Switzerland) using the three NiTi file systems ProFile®, Hero Shaper®, K3™. The characters of each NiTi file system were summarized in Table 1.

Many rotary NiTi file systems have been introduced in the market. Rotary NiTi instruments have been shown to prepare the root canal rapidly, and maintain the canal shape and working length with fewer aberrations3). This is due to the combination of the crown-down instrumentation technique, and file design characteristics such as flexibility, flute design, and non-cutting tip17). With this various advantages, using of NiTi instruments has been increased clinically.

The ability to maintain the original canal curvature and produce a continuously tapering and conical form with the smallest diameter at the end-point of the preparation is one of the objectives of root canal preparation1). To control the working length is important in root canal therapy especially when there is loss of tactile sensation with NiTi rotary instrumentation, therefore the operators expert their best effort try not to overinstrument the canal18).

Certain active NiTi file systems have the tendency to dangerously screw into canal12). If screw-in effect has occurred during canal preparation, working distance could not be controlled. As a results, tip of the file pass through the apical constriction. This means that apical terminus width became wider than the tip's width of last apical shaping file at apical constriction area. And then apical stop would be destroyed. Manufacturers have claimed that continuous changing the pitch and helical angle over the length of the cutting blades of active file systems could reduce the screw-in effect than the file systems with constant pitch and helical angle13,14). But it is difficult to find the report of evaluation screw-in effect. This is necessary to investigate the screw-in effect of active file systems.

This study was designed to compare the apical terminus width of simulated root canal prepared with three NiTi file systems (ProFile®, Hero Shaper®, K3™) used by undergraduates for evaluation the effects of variable flute angle and pitch on reducing screw-in effect under the presumption of screwing could make large apical terminus width.

Johnson11) classified all rotary instruments as having active, semiactive, or passive cutting blades. The ProFile® system fits into the category of passive instruments. The slight negative rake angle and radial land make the file cut less aggressively than those having cutting blades - e.g. ProTaper®, Hero Shaper®, K3™ - and those having semiactive cutting blades - e.g. Quantec™19-21). Each file system has advantage and disadvantage according to its unique characteristic. These systems essentially differ from one another in the design of the cutting blades and taper of their files.

The ProFile® has three equally spaced U-shaped grooves, and central parallel core inside that may account for the enhanced flexibility. It has 20° negative rake angle at cutting edge and flat radial land to cut dentin in a planning motion. This U-file design of the ProFile® is claimed to have no side cutting ability and maintain the canal center well19,22).

The K3™ is a rotary instrument with radial land in combination with a radial land relief in combination with a positive rake angle, a flattened noncutting tip, and an asymmetrical constant tapered active file design with variable helical flute and variable core diameter. These are features that are claimed to enhance cutting efficiency, debris removal, and file guidance and strength13,23,24).

The Hero Shaper® has triple helix cross section with a positive cutting angle. It has no radial land23). It has variable pitch and helical angle of the cutting portion. According to manufacturer's report, increasing the blade pitch could remove debris more effectively, reduced threading-in phenomenon12,14).

The results of this study have shown that there was a significant difference in apical terminus width after instrumentation of each file system. ProFile® had significantly smaller terminus width than Hero Shaper® and K3™.

The reason for this is not obvious but may be explained by its unique file design and cutting ability. The U-file design and radial land of the ProFile® seem to contribute to reduce self-threading of the file into the canal as there is no side cutting ability. It means that the instrument design of the ProFile® system is able to preserve more conservative apical terminus width contrast to Hero Shaper® and K3™. The K3™ also has radial land, nevertheless, it made wider apical terminus width than ProFile®. This might be due to the restoring forces of the metal were the forces in a straight file in a curved canal attempt to straighten the file toward the outer curvature thereby preferentially removing material in this area17). Especially K3™ has variable core diameter, its file tip would be stiffer than other NiTi file system. Several studies have been reported that K3 file created slight canal transportations toward the outer aspect of the curvature in the apical region of the severely curved canal17,25).

And also this result is in disagreement with several observations of other studies in that only small mean changes in working distance occurred with rotary NiTi instruments3). This may be due to the operator's experiences. In other studies by experienced showed small amount of over preparation occurred in canal prepared using NiTi instruments17,18,20). But in the present study the operators had no experiences of using NiTi files. This may be able to affect the ability to control working distance and maintain finger rest.

Although manufacturers have insisted that variable pitch and helical angle could reduce the screw-in effect13,14), results of this study suggests that the radial lands do more important role in reducing screw-in effect. The ProFile® with U-file design which has no side cutting ability is more conservative in maintaining apical terminus width. Therefore it is recommended for first choice to undergraduate students who have no experience of using NiTi instruments. If inexperienced operator wants to use Hero Shaper® and K3™, they should pay attention to control working distance with definite finger rest during canal preparation for achieving accurate apical stop.

Under the condition of this presumptive study, active file system (Hero Shaper®, K3™) with variable pitch and helical angle seems to have more screw-in effect than passive file system (ProFile®) with constant pitch and helical angle. The radial lands of passive file system seem to play more important role in reducing screw-in effect. And ProFile® is more conservative in maintaining apical terminus width, it is recommended for first choice for undergraduate students.