A New Method of Canal Enlargement

Traditionally, rotary Ni-Ti (RNT) choices for canal enlargement were either “safe” or “efficient” and rarely, if ever, both simultaneously in all anatomies. Safe files were fracture-resistant but may not have cut as effectively as their efficient counterparts. Efficient files cut well and yet broke more often and had a greater risk of canal transportation. This inherent lack of both safety and efficiency is now a thing of the past. The introduction of the Twisted File (TF [Sybron-Endo]) represents a huge step forward for endodontic canal preparation. It is truly safe and efficient. TF is very difficult to fracture if used as directed and has excellent cutting efficiency without undue risk of transportation.
Inherent to the challenges manufacturers face is that grinding nickel titanium has always had to be performed against the grain structure of the metal. Grinding has the potential to create microcracks that can later be the focal point for failure due to ex-cess torsion and cyclic fatigue. Some brands of files have been electro-polished to reduce the effects of these microcracks (in theory); yet such electro-polishing may reduce the sharpness of the cutting edge and removes some amount of the surface layer of the metal. A comprehensive summary of the design features of all RNT files available in North America today is beyond the scope of this article. Suffice it to say that the designs currently available have been dictated, to one extent or another, by the limits imposed by grinding nickel titanium, among other secondary factors.

THE HISTORY BEHIND TF TECHNOLOGY

Figure 1. The grain structure of the Twisted File (TF) that is never cut across in its manufacture.

Nickel titanium in endodontics was introduced because the metal is superelastic. In its natural resting state, the material is in a crystalline structure known as austenite. When subjected to rotation in the canal, it is superelastic in that it undergoes a transformation to a crystalline structure known as martensite. When the stress is relieved, the material reverts to the crystalline structure of austenite. The transformation is superelastic in that the material regains its original shape when it reverts to its original crystalline structure.
Through a proprietary process of machining, heating, cooling, twisting, reheating, and cooling, TF is never ground against the grain structure (Figure 1). Through the first heating process, nickel titanium is placed in a state known as R phase (a state that might be thought of as a crystalline structure that is intermediate between austenite and martensite). While in R phase, the material can be twisted very precisely. The second cooling phase makes the precise twisting permanent while bringing the material back into the austenite crystalline structure. Twisting nickel titanium does not disrupt the grain structure of the metal and allows it to keep its inherent strength. The final TF file is subjected to a surface conditioning that maintains its surface hardness and properties.

ADVANTAGES OF TF

Figure 2a. The Twisted File.	Figure 2b. The Twisted File Large Pack Assortment (.10, .08, and .06 tapered TF).
Figure 2c. The Twisted File Small Pack Assortment (.08, .06, and .04 tapered TF).	Figure 3. The noncutting pilot tip of the TF.

TF is extremely resistant to fracture and very flexible, and has excellent cutting efficiency. Many TF cases can be done with a single file, 2 files, or at most 3 files. It is simple to use, has a minimal learning curve, and can be easily integrated into both specialty and general practice (Figure 2a). TF also tracks canals very well and moves apically without undue pressure or force. Finally, TF shows very little, if any, tendency to “screw in.”
TF is available in 5 tapers (.12, .10, .08, .06, and .04) and a fixed 25-tip size. Aside from these individual packs, TF is packaged in a Large Pack Assortment (.10, .08, and .06) and a Small Pack Assortment (.08, .06, and .04), and is available in 23- and 27-mm lengths (Figures 2b and 2c). TF has a noncutting pilot tip (Figure 3) and can be used with a wide variety of sequences for most clinicians and anatomies.

CROWN-DOWN TF TECHNIQUE

Figure 4. Clinical case treated with a single .10 TF file.	Figure 5. Clinical case treated with 2 files, the .10 TF and .08 TF.

Figure 6. Clinical cases treated with 3 files, crown-down, .12, .10, and .08.

Crown-down technique will predominate as the most effective sequence if a single-file technique is not preferred for the given anatomy. In the crown-down TF technique, the files are used from larger tapers to smaller. For an average molar case, this means that the .10 TF is inserted first, and if needed, the .08 TF and .06 TF follow. For many cases, a single file will be needed, sometimes 2 files, and less frequently, all 3 of these tapers (.10, .08, and .06). It is uncommon that 3 tapers of TF would be needed for an average molar. For larger canals, the initial file in the coronal one third would be a .12 or .10 taper. For medium canals, the initial taper in the coronal third would be .10 or .08. For small canals, the initial taper in the coronal third would be .08 or .06. For a multiple-file crown-down technique, the next smaller TF taper would follow the first taper used at the orifice. The .12 TF is used more as an orifice opener and is generally only taken down the roots below the orifice of very large canals (eg, the palatal roots of upper molars or the distal roots of lower molars—only when it is easily accepted and the root has no significant fluting).
In a single-file technique, a single TF file can be used to enlarge the entire canal. With passive insertion, canal preparation can be accomplished in this manner. Many wide-open and unrestricted straight canals (upper anterior teeth) can be easily enlarged with a single .12-tapered TF file. Many medium-sized canals (bicuspids) generally can be treated with a .10-tapered TF. Many small canals (lower anteriors) can be treated in a single-file technique with a single .08 TF.
How does the clinician know if the TF should be used as a single file to the apex or if the root should be enlarged crown-down? The first TF entered into the root should be wiped free of debris, and the canal should be irrigated and recapitulated. If it can be reinserted passively and one can make apical progress, assuming that it is an appropriate taper for the entire root length, then the clinician can advance the single TF file apically to reach the apex in usually an average of 3 to 4 insertions. If, for whatever reason, the single TF does not want to advance apically (easily and passively), the clinician should select the next smaller TF taper (Figures 4 to 6).
TF requires (as does all RNT file use) straight-line access and initially that the canal be negotiable to a Nos. 10 to 15 hand K-file. The canal should be lubricated at all times, ideally with sodium hypochlorite or chlorhexidine, depending on whether the tooth is vital, necrotic, etc.
The canal should be irrigated and recapitulated after TF insertion whether the clinician is moving crown-down or as a single file technique. Any electric motor currently being used for RNT files (corded or cordless) can be used with TF. While individual preferences will vary, SybronEndo recommends that TF should be run at 500 rpm. While a surgical operating microscope (Global Surgical) is not required to use TF, using a SOM during all phases of endodontics raises the level of care provided due to the commanding visual and tactile control that the clinician has over the enlargement of the canal.

TF FAQS:

How many times can I use a TF?
SybronEndo recommends single use of TF. I have used them in 3 to 5 canals.

How do I fit a cone with a TF? Are there matching master cones?
SybronEndo has introduced matching TF gutta-percha and TF paper points. A clinician who wants to custom-fit his or her own gutta-percha cones to TF preparations can do so. In addition, the .06 No. 20 RealSeal cones can be used as a universal, bonded obturation cone for TF preparations. This taper and tip size cone can be trimmed to achieve tug back in the vast majority of roots encountered.

If I see stretching of the TF file, do I have to discard it?
TF is designed to be a single-use file, which means it can be used in one canal or one tooth, whether that tooth has one canal or 5 canals. The surest way to maximize the functionality, efficiency, and fracture resistance of TF is to utilize it as a single-use file. If the file shows any signs of deformation, it should be discarded.

Can TF be used in any kind of anatomy? Are there any teeth or curvatures in which you would not use TF?
When used appropriately, TF can be used in any kind of anatomy, and the learning curve for TF is short. The tactile control needed for TF is much the same as with existing RNT systems. The cutting efficiency and the fracture resistance of TF will be a revelation to clinicians. TF wants to move apically and does not require the apical pressure to advance it that existing RNT files do. All of the above notwithstanding, techniques might need minor modification depending on the severity of the curvature. For example, if the curvature is extreme, after achieving straight-line access, irrigating copiously, and recapitulating frequently, the entire canal might be prepared to the TWL to a .04-tapered TF. Then, the whole preparation could be subsequently enlarged to a .06-tapered TF. The final prepared taper must be balanced against the risk of perforation, removal of only the needed dentin to facilitate irrigation, and obturation with the achievement of a continuous taper.

How do I know if I should use multiple TF files or if a single-file technique is indicated?
If a canal will allow apical advancement without undue pressure, TF can be inserted as far as the file will go passively. The TF file should be withdrawn, the flutes wiped, the canal irrigated and recapitulated, and the TF reinserted. In such a series of insertions, irrigations, and recapitulations, with a single file the entire canal can be shaped for many roots. If the given TF encounters undue resistance, the next smaller taper TF can be used. For the majority of molars that will be encountered in general practice, the combination of .10 and .08 TF will be used in tandem to reach the apex of the given root if the clinician opts to use multiple TF files of diminishing taper. Part of the answer to the question involves the ideal master apical taper that the clinician seeks to achieve, giving respect to the degree of curvature that the canal possesses. As long as the initial TF file inserted is not forced and advances passively, it can be taken to the apex if the taper being prepared is appropriate for the given root.

Figure 7a,b. Clinical case with warm carrier-based materials that were removed with TF.

Can I use TF for retreatment of pastes, gutta-percha, warm carrier-based devices, etc?
Yes! TF can be used to remove gutta-percha, pastes, warm carrier-based devices, etc. Rotational speeds for such uses are increased slightly, from 900 to 1,500 rpm. However, endodontic retreatment is primarily a specialist procedure and should virtually always done under the SOM. Clinical judgment is called for and caution is always advised when performing these advanced clinical functions (Figures 7a and 7b).

What is the optimal rotational speed for TF?
SybronEndo recommends a rotational speed of 500 rpm. After the clinician becomes familiar with TF, he or she may feel comfortable utilizing enhanced rotational speeds.

What if I want to create a larger master apical diameter than a 25? How can I do that?
Larger apical diameters can be created in any way in which the clinician chooses. TF is fully compatible with any other RNT file system desired. After choosing the desired taper that is taken to the true working length, most clinicians will fit a cone. If the clinician wants to create a larger master apical diameter, then the clinician can gauge the apex and use the size determined to create a larger master apical diameter as desired. For example, if the canal gauges to a No. 20, the final apical preparation can be taken to a No. 35, as desired.

What if I have a tooth longer than 27 mm? How do I cope with that clinically?
To begin with, few teeth (especially treated in general practice) are longer than 27 mm. Of course, such longer roots are found occasionally and are often an indication for referral. The simplest way to deal with this event is to reduce the occlusal reference point as needed. This can help extend the reach of the TF as indicated.

If I were to buy a few TF files to try them out, which ones would they be?
The TF files that will be used most often are the .10 .08 and .06. These tapers will usually be used in a single-file technique and in a 2-file tandem.

The .12 TF will be used mostly in straight and open canals such as the palatal canals of upper molars. As an orifice opener, the .12 TF will be used primarily at the orifice level only to allow access for the subsequent TF files, and not taken into the mid root region unless used to the apical third in wide-open canals. 

The .04 is used more for initial canal enlargement after the creation of a glide path. It can easily negotiate even the most severe curvatures if used correctly and can create the minimal enlargement needed to allow a subsequently larger taper (.06 or .08) to be created to length.

I am happy with my existing rotary files. Why should I change my instrumentation?
TF is a completely new paradigm in RNT instrumentation. Without the limitations of ground file technology (microcracks, lack of cutting efficiency, etc), R-phase technology allows files to be used with a safety and efficiency that was never before possible. For example, in some canals, one can often perform an enlargement of the entire canal in 1 to 3 insertions of the single TF instrument. In other anatomies, it is possible to shape the entire canal with 1 to 3 insertions of a .10 TF and 1 to 3 insertions of a .08 taper. This represents a real advancement because it is virtually impossible to create, for example, .08 taper in the apical third of molar roots, especially around a curvature with ground RNT files.

 

TACTILE CONTROL FOR TF

TF insertion is gentle and passive. The motion is continuous and controlled. The motion of insertion takes approximately 2 to 3 seconds. TF is not left spinning around a curvature; it is either being inserted or withdrawn at all times. Insertion takes place to resistance, and then the file is withdrawn. In my hands, by the time the tactile sensation of resistance is felt and the file is withdrawn, the flutes of 3 to 4 mm of the file are filled with chips. The chips and debris on the flutes are removed, and the canal irrigated and recapitulated. The same TF will be inserted again in a single-file technique, and a smaller TF will be in-serted in a multiple-file crown-down technique.

USEFUL TF UTILIZATION TIPS

Figure 8. Multiple handpiece attachments for a cordless motor, each with a different TF inserted. Changing out the attachments is much more efficient than changing an individual file.	Figure 9. The M4 Safety Handpiece.

TF files can be placed in multiple handpiece attachments ready for insertion into the corded or cordless handpiece male connector (Figures 2a to 2c). The insertion of attachments is faster and more efficient than putting individual files into the single attachment. Attachments with the preset files should be kept in the order of larger to smaller tapers. In this way, if they are used and put back on the dental tray in that same order, they can be more easily delivered back into the tooth, if and when they are needed again (Figure 8).
Blending the M4 Safety Hand-piece attachment (SybronEndo) with TF creates efficiency. However, the clinician does not need the M4 to use TF. The M4 Safety Handpiece is an attachment that fits onto any E-type electric motor attachment (Figure 9). The M4 can reciprocate a hand K-File/reamer in a reciprocating 30-degree clockwise and 30-degree counterclockwise motion. Clinically, if a canal is calcified and its negotiation by hand is difficult and time consuming, the M4 can be a significant aid. To use the M4, a No. 6 hand K-File is placed just at, or slightly beyond, the minor constriction. The file is left in the tooth under the rubber dam. The M4 is attached onto the hand K-File while in the tooth and activated. With a 1- to 3-mm vertical amplitude motion, the file, while attached to the M4, is moved slowly up and down until the file is free of resistance. This enlargement usually takes approximately 15 to 30 seconds. A No. 6 hand file used in this manner can usually enlarge a canal to approximately a No. 8 hand K-File diameter. Similarly, a No. 8 can often enlarge a canal to the diameter of a No. 10, etc. Once the minimal diameter of a No. 10 hand K-File is created in the tooth, the TF is inserted into the canal in the coronal third and enlargement begins.

CONCLUSION

A new method of canal enlargement has been introduced. The Twisted File (TF), by virtue of a proprietary process of heating, cooling, and twisting, special surface conditioning, and the fact that the file is not created by grinding across its grain structure, has created a file with excellent fracture resistance and cutting efficiency. This new technology in file fabrication allows us to facilitate single-file canal enlargement for a wide variety of canal systems.

Dr. Mounce lectures globally and is widely published. He is in private practice in endodontics in Vancouver, Wash, and offers intensive, customized, endodontic single-day training programs in his office for groups of 1 to 2 doctors. For more information, contact Dennis at (360) 891-9111 or e-mail RichardMounce@MounceEndo.com.

Disclosure: Dr. Mounce is on the advisory board of SybronEndo. He receives honorarium for some aspects of this work, lectures for example.