October 1, 2010
by Giuseppe Cantatore, Elio Berutti, Arnaldo Castellucci MD DDS
Rotary NiTi instruments have revolutionised endodontics, allowing even the less experienced dentist to create perfectly truncated-conical shaping in harmony with the original anatomy, and improving the prognosis even of the most complex cases. Many “in vitro”1-13 and “in vivo”14-17 studies show quite clearly that the nickel-titanium alloy is greatly superior to stainless steel, since with NiTi instruments even canals with very accentuated curvatures can safely be shaped without the risk of creating ledges or of straightening the original curves. Numerous studies16,17 have shown that, with the use of NiTi, even the inexperienced dentist can obtain better results than by using stainless steel. However, the use of NiTi has one serious drawback, in that it carries a higher risk of the instrument’s breakage compared to stainless steel.18 The influence of various factors on breakage of rotary NiTi instruments has been extensively studied and it has been found that breakage usually depends on torsional19-25 and on bending stresses.21-23,26,28
Bending stress essentially depends on the original canal anatomy, on the radius of curvature of the canal, the speed of rotation and the flexibility of the instrument, the presence of intra-canal interferences, sharp changes of trajectory, as for example occurs in the case of merging root canals. The endodontist can do very little to reduce this type of stress.
Torsional stress depends on numerous factors: the area of contact between the instrument blade and the canal walls, the pressure the operator exercises on the handpiece, the diameter of the instrument section and that of the lumen of the canal in which it is working, the taper, the diameter of the tip of the instrument, the portion of the instrument that is subjected to torsion, the intrinsic strength of the instrument (thus on the design of its cross section), the design of its blades, and lastly on the torsion applied to the instrument.27,28 In short, breakage occurs if the canal section is smaller than the tip of the instrument that cannot cut the dentine, and what is known as “taper lock” occurs. This is followed by deformation and instrument breakage.
Analysis of NiTi instruments broken due to torsion shows that most breakages occur in the last few millimetres, where the taper is less and the diameter smaller.24,25,27,28 Consequently, the tip of the smallest NiTi files presents the highest risk of breakage due to torsion, from which it must be protected using a low torque value, reduced axial pressure and above all by avoiding the tip’s engaging against the dentine walls.
Numerous studies have evaluated the causes of breakage of NiTi instruments and have concluded that a marked reduction in the breakage rate of rotary instruments can be achieved when their use is preceded by preliminary manual enlargement and the creation of a “glide path”, that is a pathway with smooth canal walls along which the NiTi instruments can easily slip and slide to reach the working length.
It has frequently been reported that manual pre-flaring is very important to reduce the incidence of breakage. For example, the study by Berutti et al.32 evaluated the influence of manual pre-flaring and torque on the incidence of breakage of ProTaper instruments. The study used 400 plastic blocks, divided in two groups. All the simulators were shaped with the ProTaper files, but in one group the use of rotary instruments was preceded by manual pre-flaring up to a K File # 20. The result showed that after preflaring the ProTaper instruments were able to shape a markedly larger number of simulators before breaking (Figs. 1a, 1b).32
This and other studies33 underline the fact that the favourable repercussion of manual pre-flaring and creation of the glide path depend to a great extent on the reduced risk of “taper lock” of the thin and fragile tip of the instrument.28-33 Thus the canal must be enlarged at the foramen to a diameter greater than or at least equal to that of the tip of the first rotary NiTi instrument that will be used at that depth. It is also important to remember that all rotary NiTi instruments available on today’s market have non-active tips that are therefore not capable of cutting the dentine effectively.
Pre-flaring and creation of the glide path are usually done by hand with stainless steel instruments. This is the last manual phase of the entire shaping procedure, and, especially for the general practitioner, the most difficult phase and that in which the most dangerous errors can be made, that can cause the entire treatment to fail (ledges, foramen transportations, dentine plugs). Hand stainless steel instruments involve numerous disadvantages, due to their relative rigidity and their tip that in many cases is aggressive, so that in curved and/or calcified canals they can easily produce ledges or transportation.34
To avoid these dangerous errors, a new set of only three rotary nickel-titanium instruments has been developed. They are known as PathFile (Dentsply, Maillefer), and their purpose is to facilitate pre-flaring and creating the mechanical glide path (Fig. 2). The new rotary PathFile (Dentsply, Maillefer) were designed to create the glide path quickly and safely, thus eliminating the last manual phase in which the general practitioner can commit errors, and providing the expert endodontist with a tool that can transform difficult cases into extremely simple ones.
The PathFiles consist of only three rotary instruments with the following characteristics:
1) Tip diameter: the diameters are, respectively, 0.13, 0.16 and 0.19 mm. The gradual increase in tip diameter (similar to that of ProFiles Series 29, Dentsply Tulsa Dental Specialties) facilitates their progression, without the need to apply strong axial pressure, as on the contrary would occur if they possessed the ISO standard measurements, i.e. 10, 15 and 20. Size 15 is in fact 50% wider than size 10 and, moreover, there will be no advantage in following a size 10 hand instrument with a rotary instrument of the same size.
2) Length: PathFile are available, in lengths 21, 25 and 31 mm.
3) Tip design: the tip is rounded rather than cutting, to avoid ledges and zips (Fig. 3a).
4) Section design and cutting capability: PathFile have square section. This is easy to manufacture, with an essential design that has been used very widely and tested over long periods in hand files. This strong cross section of PathFile increases the resistance to torsional stress despite the small diameter and small taper. The four cutting edges increase their efficacy, including long and calcified canals.
5) Distance between blades: the distance between the blades has been optimised to increase the instruments’ strength and at the same time their ability to remove debris.
6) Flexibility: the flexibility of PathFile is guaranteed by the nickel titanium alloy and by their small taper, which is only 0.02. Their good resistance to bending stress is also due to these characteristics (Fig. 3b). Moreover, the high flexibility enables the original anatomy to be followed and maintained during the delicate phase of creating the glide path (Figs. 4a, b, c). Thanks to this, the general practitioner will no longer need to use the rigid stainless steel K-files, which are frequently the source of errors that may be irreparable: ledges, dentine plugs and canal and apical foramen transportation.
7) Safety: the working length is undoubtedly one of the most important aspects of the entire sequence of endodontic treatment. In the initial phases, the working length may vary due to canal enlargement, which increases the radius of the curves. PathFile are instruments that forgive these initial errors since they have the advantage of not cre
ating ledges if the working length is too short, and not creating foramen transportation if the working length is by mistake too long (Figs. 5a, b, c).
8) Efficiency: the efficiency is given by the 4 blades of the instrument, that ensure excellent cutting capability. This means that PathFile can be used at a speed of 300 rpm and at a very high torque, approximately 5-6 N/cm.
9) Simplicity of use: the PathFile’s big advantage is that the dentist has only to negotiate the canal to the foramen with a # 10 K-file before using them. It is obvious that, with such a thin and flexible instrument, it is almost always possible to reach the end of the canal without difficulty. Even the less experienced dentist can eliminate the last manual phase in which practice and skill in using endodontic instruments is indispensable if errors are to be avoided, that are sometimes irreparable. For endodontists, PathFiles are trusty friends that can transform a complex endodontic anatomy into a simple case, that can be almost entirely treated with rotary NiTi instruments (Figs 6a, b, c, d).
PathFile were immediately subjected to studies to evaluate their efficacy. The research by Berutti, Cantatore, Castellucci et al., recently published in the Journal of Endodontics,34 is one such significant study. The study compared changes to canal radius of curvature and incidence of canal aberrations, using hand stainless steel K-files or nickel-titanium rotary PathFile in “S” shaped and dual curve plastic blocks. The influence of the operator’s expertise was also investigated. The canals of 100 training blocks were coloured with Indian ink and photographed pre-instrumentation (Figs. 7 – 11). Preflaring was performed by an endodontist with PathFile (group 1) or with hand stainless steel K-files #10-15-20 (group 2); an unskilled clinician performed preflaring in another set of blocks with PathFile (group 3) and with hand stainless steel K-files (group 4). The blocks were photographed after the pre-flaring and the pre- and post- instrumentation images were superimposed to evaluate the results.
The radius of curvature pre- and post-instrumentation was measured in each block. The variation of the radius of curvature is a significant parameter to verify the instrumentation’s ability to maintain the original anatomy. To avoid measurement mistakes, the percentage increase between pre- and post-instrumentation radius was calculated. A high percentage means a significant alteration of the original anatomy, whereas a low percentage means a shape in harmony with the original anatomy.
Differences in canal curvature modification and incidence of canal aberrations were analysed respectively with the Kruskall-Wallis plus post hoc tests, and by the Monte Carlo method (P<0.05). The PathFile groups demonstrated significantly less modification of the curvature (P < .001) and fewer canal aberrations (P < .001). No expertise-related difference was found within the groups prepared with PathFile (P > 0.05), whereas the unskilled clinician produced more conservative shaping with PathFile than did the expert with manual pre-flaring (P < .01). The conclusion drawn by this study was that the unskilled dentist using PathFile obtains better results in terms of respecting the anatomy and maintaining the apical curvature, compared to the expert dentist using stainless steel hand K-files (Figs. 12a, b). With regard to the formation of ledges, these were completely absent in groups 1, 2 and 3, whereas they were found in group 4, that is in the blocks prepared by the unskilled dentist with stainless steel hand files (Fig. 13).
In a second part of their study, Berutti et al.34 also analysed the time taken to perform pre-flaring in relation to the type of instrument and the dentist’s experience, finding that the time was significantly shorter in the groups using PathFile (P < 0.001), the difference between expert and unskilled operator not being statistically significant (Fig. 14).
At the National Congress of the S.I.E. (Italian Endodontic Society) which was held in Turin in 2008, Greco and Cantatore presented an interesting study that evaluated “in vitro” the difference in penetration ability of radiopaque irrigant solutions in the case of pre-flaring with conventional hand stainless steel instruments (stainless steel K-files # 10, 15 and 20) and with rotary instruments in NiTi (PathFile #1, 2 and 3). The results showed a statistically-significant difference in the penetration of the irrigant in the middle and apical thirds of the narrow canal (mesial canals of mandibular molars and buccal canals of maxillary molars) using PathFile # 1 and 2 compared to hand instrumentation with stainless steel K-files # 10 and 15 (Figs 15a, b). (Figs 16a, b). The significance disappeared with the last and largest instruments: PathFile # 3 and stainless steel K-file # 20 (Figs. 17a, b). The authors of the study concluded that mechanical pre-flaring would appear to facilitate the flow of the irrigating solution compared to the use of hand stainless steel K-files. This research throws light on a new characteristic of PathFile: their ability to remove the content of the root canal together with the debris produced while working (Figs. 18a – h). This highly important characteristic is common to all rotary NiTi instruments and is also responsible for the almost complete lack of extrusion of debris beyond the apex while using PathFile.
With regard to the sequence and procedure for using PathFile clinically, it is necessary to make a distinction, depending on the presence or not of coronal interferences (Figs 19 – 21).
In case there is no coronal interference after an initial negotiation with a stainless steel hand K-File, # 08 or # 10, in the presence of a chelating agent (e.g. Glide or RC Prep), check the working length with this instrument in the canal, using an apex locator and a radiograph; the three PathFile can now be used at the same length, completing pre-flaring in just a few seconds. Having obtained a foramen of diameter size .19, the rotary system generally used (ProTaper, GT X, Twisted Files or any other) can now be taken to the same length with no risk. These instruments will, in complete safety, correctly shape the canal, finding the glide path on which to slide and the apical foramen of their same size, if not larger.
In case the working length is taken with a hand K-File # 15, it is obvious that the sequence can start with the PathFile # 2.
On the other hand, if the K-File used to take the working length was a # 20, it is still advisable to use the PathFile # 3, because this instrument, even though it is smaller in size, could facilitate the removal of the intra-canal content, for instance, removing the entire pulp tissue, avoiding collagen blockages.
All PathFiles must be used at the speed of 300 rpm with torque of approximately 5 N/cm and with gentle movements back and forth, until the working length is reached. The use of a relatively high torque is not dangerous, considering the strong square section of the instruments and in the light of the results of the study by Berutti et al.32 in which the use of a high torque was found to enable NiTi instruments to shape a considerably greater number of canals before breaking.
The time required to take the three PathFile to the working length is normally very short, and never more than 3-5 second per instrument. Longer times are not helpful but at the same time they are not dangerous, because thanks to their high flexibility, PathFile do not transport the foramen if a mistake is made in determining the working length. After the use of each instrument it is advisable to apply abundant irrigation, even if PathFile do not tend to accumulate smear layer or cause apical obstructions.
If the canal cannot initially be negotiated to the foramen, due to the presence of coronal interference or very abrupt curves in the apical one third, then PathFile should be used in step back mode, taking them to the point where the canal can receive them without engaging the tip. This coronal pre-enlargement is used to make room to other instruments, like ProTaper S1 or GTX 20 .06, that will be then used to remove the coronal interferences and to pre-enlarge the canal so that the pre-curved stainless steel instrument later can maintain its precurvature. Negotiation is then performed with a pre-curved # 10 K-File, the working length is taken and the procedure continues as described above (Figs 22 – 23).
In conclusion, in virgin vital or necrotic teeth, provided it is possible to take a # 08 or # 10 hand file to the foramen to determine the working length, the rotary NiTi instruments can be used immediately and all the old stainless steel hand instrumentation can be eliminated, thus also eliminating all the previous risks of creating ledges or obstructions.
Since, as has been said, PathFile also facilitate the penetration of irrigants towards the apical one third right from their first use, and at the same time transport pulp and debris in a coronal direction, it is clear that they enable post-operative pain to be reduced. In this fashion two significant goals are reached: greater comfort for the patient and the possibility of completing treatment in a single session, which, as has been reported,36 increases the success rate.
On this line, Berutti, Castellucci, Cantatore et al., have begun a study to verify the incidence of post-operative pain in patients after pre-flaring and the glide path have been created with PathFile vs. stainless steel hand K-files. Statistical significance has not yet been achieved, probably because the series is still very small at this early stage of the study, but the trend is towards a lower incidence of post-operative pain in patients in whom PathFile are used.37
Before they were placed on the international market, PathFile were tested clinically in full by dentists for over one year, confirming that they are a valid help above all in shaping difficult canals, with very accentuated curvature, since they make it possible to create a glide path without the slightest risk of foramen transportation or of creating ledges.
It may be concluded that PathFile, the new rotary NiTi instruments, open up a new era in the instrumentation of root canals, enabling the glide path to be created easily and safely including by the less experienced general dentist. They are also a valid help to the endodontist who, by using PathFile, can transform a complex canal anatomy into an easily treated canal (Figs. 24 -26). OH
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