The objective of root canal preparation is to clean and shape the root canal system, while maintaining the original configuration. After root canal preparation, the canal shape needs to be a continuously tapering, conical, funnel-shaped canal with the smallest diameter at the end-point and the largest at the orifice.1)

During the late 1980s and 1990s, various types of endodontic instruments were developed with nickel-titanium alloys to effectively achieve the above mentioned objective of root canal preparation. Walia et al.2) initially investigated the endodontic files fabricated from nickel-titanium blanks. They found that, due to the low modulus of elasticity, nickel-titanium files have two to three times more elasticity than stainless steel files. Reports on the efficacy and shaping ability of nickel-titanium rotary files have demonstrated their potential for root canal treatment.3-6) Nickel-titanium instruments have a decreased tendency for canal transportation and therefore remain better centered.7) Most studies have expected that these new instruments may enhance endodontic treatment with respect to both its quality and speed.

On the other hand, there are some behavioral aspects that operators need to understand in nickel-titanium rotary instrumentation. Tendency to screw into the canal is one of them. This phenomenon is accentuated during root canal preparation especially when continuously rotating nickel-titanium instruments are used. Because rotary instrumentation lacks tactile sense, control of the working length is more difficult during rotary instrumentation than hand instrumentation. 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.8-11)

Gutierrez et al.12) using scanning electron microscopy, found that overinstrumentation of root canals of human teeth provoked different types of apical cementum perforations and/or the production of zipping. By chance, many root canals can be inadvertently prepared beyond the apical foramen, which can be deleterious to the periapical tissues to heal in teeth with infected canal. Bacteria must have been pushed out of the foramen whenever a file penetrated the apex of an infected root canal. If bacteria and dentinal chips on the flutes of the files are pushed out of the main foramen, they may remain in the periapical tissues when the instrument is withdrawn from the root canal. Bacteria were seen attached firmly on the dimples of the resorptive lacunae, which correspond to inflammatory resorption of the root.13) Histopathological studies demonstrated overinstrumentation may cause persisting inflammatory response and postoperative pain or trigger a foreign body reaction.14,15) The high percentage of bacteria in the flutes of files used in overinstrumented human teeth with infected root canals carry a potential risk for postoperative pain, clinical discomfort and flare-ups.

Because the screw-in effect of nickel-titanium rotary instruments may cause these overinstrumentation-induced side effect, the screw-in effect of each rotary instruments needs to be evaluated. Therefore, the aim of this study was to compare the screw-in effect among several nickel-titanium rotary file systems.

Sixty simulated resin root canals in clear resin blocks (REF A 0177, Dentsply-Maillefer, Ballaigues, Switzerland) were used, which have 34 to 35 degree curves and a mean canal length of 18 mm. They were divided into 6 groups according to the instrument used. Nickel-titanium rotary file systems were K3™ (SybronEndo, Glendora, CA, USA), M_two (VDW GmbH, München, Germany), NRT with safe-tip and one with active tip (Mani Inc., Shioya-gun, Japan), ProFile® (Dentsply-Maillefer, Ballaigues, Switzerland) and ProTaper® (Dentsply-Maillefer, Ballaigues, Switzerland) (Table 1). To have the same taper and tip size of the instrument, instruments with taper of 0.06 and size 20 were selected for each system. For ProTaper®, S2 was selected because it has size 20. All canals were prepared using a reduction speed contra-angle with an electric motor (SurgiMotor II, Aseptico Co., Woodinville, WA, USA). Ten canals were prepared in each group using a constant speed of 300 rpm. The contra-angle handpiece was mounted on the newly designed device and single pecking movement was generated. The depth of penetration each time was 1.0 mm from the first point that the file met the canal wall.

The screw-in force is shown in Table 2. ProTaper® produced significantly more screw-in effects than any other instruments in the study (p < 0.001). K3™ produced significantly more screw-in effects than M_two and ProFile® (p < 0.001).

There was no significant difference in screw-in effect among M_two, NRT, and ProFile® (p > 0.05). There was no significant difference between NRT with active tip and NRT with safe tip either (p > 0.05).

Some factors may influence on the tendency of screw-in. When continuously rotating mode is used, nickel-titanium instrument can screw into the root canal. Various parameters were suggested to limit the tendency to screw-in.16) Constant pressure, slight pecking movement, controlled rotational speed and torque, and instrument cross-sectional design were included in these suggestions. Li et al.17) recommended the use of a slow, continuous pecking movement to reduce stresses. They stated that a pecking motion may be a crucial factor in preventing the breakage of nickel-titanium rotary instrument. The pecking movement is intended to regularly disengage the instrument and allow the instrument to return to its normal state before continuing the preparation.17) In this study, a pecking distance of 1 mm was used. This is a suggested distance by most of the manufacturers commonly in clinical situation. If constant pressure is applied, the instrument may be gradually screwed into the canal. However, when light force is applied, this phenomenon can be reduced.18)

Various aspect of file design may affect screw-in effect. Every permanent rotating system has the tendency to screw into the canal. To overcome this problem, cutting edges of nickel-titanium rotary systems were flattened, modified, or shortened. ProFile® nickel-titanium rotary instruments have radial land. According to manufacturers, each instrument was designed to reduce the screw-in effect: the flattened edges of ProFile®, the modified edges of K3™, and the varying the taper of ProTaper®.

Screw-in effect is one of the draw-backs in nickel-titanium rotary instrumentation. To overcome or control this in the canal, the amount of the force needs to be evaluated in each instrument system. In the present study, some instruments showed significantly greater screw-in effect than others. Some instruments showed similar screw-in effect with others. This result may be needed for the clinician to know before choosing any instrument system for the rotary instrumentation. Also these information may alert the clinician to be careful not to overinstrumentate due to the screw-in effect during instrumentation. In the present study, six types of instruments were used. Their designs are different each other. Their designs may be taken into consideration for the explanation of the difference in their screw-in effect.

First, file systems in the study have different cross-sectional geometry. ProFile® and NRT have flat radial lands. M_two and ProTaper® have sharp blade. K3™ has unique asymmetrical radial land. Existence of flat radial lands may be related to reducing the screw-in effect. This finding is consistent with Oh et al.19) They states that active file system with variable pitch and helical angle had more screw-in effect than passive file system. It seems that radial lands play more important role in reducing screw-in effect.

ProFile® and NRT have flat radial land and negative rake angle. M_two has sharp blade and negative rake angle, ProTaper® has sharp blade and positive rake angle while K3™ has unique asymmetrical radial land and slightly positive rake angle. Therefore, positive rake angle may be related to the screw-in effect.

Second, file systems in the study have different helical angles. The angle of the helix corresponds to the angle between the blades and the central axis of the instrument. ProFile® (26°), NRT (41°), K3™ (30°) files have constant helical angle, while ProTaper® (15°<) and M_two (25°<) files have variable helical angle. Therefore, there seems no significance with the helical angle in the study.

Third, files in the study have various pitch length. M_two file has the longest pitch length at apical 2 mm, followed by ProFile®, ProTaper®, NRT, and K3™. In this study, there seems no significance with pitch length.

Diemer and Calas20) stated that a varying helical angle and a longer pitch may help in preventing screw-in. Their explanation was that a longer pitch reduces the helical angle, which in turn considerably reduces screw-in. Their explanation is inconsistent with the result in present study. But, they used one kind of Ni-Ti instrument (HERO, Micro-mega, Besancon, France) that have the same cross-sectional geometry and different pitch length.

From the result of the present study, it was concluded, therefore, that there was significant differences of screw-in effect among the tested nickel-titanium rotary instruments. The radial lands and rake angle of nickel-titanium rotary instrument might be the cause of the difference.