May 1, 2014
by Peter Z. Tawil, DMD, MS, FRCD(C); Derek J. Duggan, BA, BDS, MS
Each day, clinicians all over the world treat a wide variety of root canal systems, with often-complex canal configurations. The primary objectives of endodontic treatment are the optimal removal of bacteria and their substrate while preserving the original shape of the root canal system and the original location of the apical foramen (Schilder, 1974).
Achieving these objectives in curved roots can be compromised due to one of several risks: instrument separation, canal transportation, ledging, apical zipping and strip perforation of any given canal. In 1988, Walia introduced Nickel Titanium (NiTi) rotary files in endodontics, which helped to address some of these challenges.
TRADITIONAL NICKEL TITANIUMNickel Titanium is an alloy composed of approximately 55 percent nickel and 45 percent titanium, and can be elastically deformed up to three times more than stainless steel before it deforms permanently. In addition, NiTi alloys have a much lower modulus of elasticity than stainless steel, meaning that the forces exerted on root canals by a deformed NiTi file are much less compared to a similarly sized stainless steel file. This results in less iatrogenic errors such as strip perforations, zipping and ledging of canals.
Traditional NiTi exists in two different structural forms at room temperature. The unstressed file is in austenite form. When a bending/rotation force is applied, the alloy transforms into it’s martensitic form which allows the file to be bent using only minimal force. When the force is removed, the file reverts to its original shape in its austenite form. NiTi instruments have experienced a continued evolution in design in the flute, helical angle and pitch, cross sectional shape and tip design.
Despite the metallurgical advantages of nickel titanium over stainless steel, the inherent memory in traditional nickel titanium used to instrument a curved canal has always been a concern and there has been a “need to develop instruments with greater flexibility, no metal memory, increased resistance to fracture, and excellent cutting efficiency” (Dan Zhao et al 2013). One has to keep in mind that when a file rotates inside a canal, it goes through torsional and bending stresses, causing compression and flexion within the metal. This results in instrument fatigue, which can lead to a sudden instrument separation (Yum et al 2011, Zuolo et al 1997).
THE GLIDE PATHTo minimize iatrogenic complications, clinicians should first explore the root canal system with small pre-curved stainless steel files (usually size 6, 8 and 10 files of .02 taper). Once canal patency has been obtained, the canal can be enlarged with larger hand files (size 15 and 20 files of .02 taper) to establish a “glide path” before the first crown down rotary instrument is introduced into the canal (Berutti et al 2004, Patiño et al 2005). The glide path is defined as a smooth passage that extends from the canal orifice on the pulp chamber floor to the opening at the apex of the root (West 2006).
The main purpose of this glide path is to create a canal diameter that is the same size (or bigger) then the first tapered crown down rotary instrument that is used (Berutti et al 2009). To optimize safety in instrumentation, the more severe the curvature of a given canal, the wider the glide path should be from orifice to apex. The tip of your first tapered crown down rotary instrument should move freely down the canal walls serving only as a pilot without engaging the walls.
Establishing a glide path has been done using various methods. Articles have described the use of hand stainless steel K-files (Berutti et al 2004, Mounce 2005). Other authors have advocated the use of reciprocating handpieces along with stainless steel K-files (Kinsey et al 2005, Van der Vyver 2011). A more recent method of establishing glide path in cases with severe curvatures has been described using rotary NiTi files (Berutti et al 2009), which we will be discussing later in this article. Many instruments have been introduced to the market to serve this purpose: PathFiles™ (DENTSPLY/Maillefer), G-files™ (Micro-Mega), Scout-RaCe files (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and X-plorer canal navigation nickel-titanium files (Clinical Research Dental, London, ON.).
NEW METALLURGYTo optimize the instrumentation of curved canals in a safe predictable manner, a revolutionary metal has recently been introduced to the market. Controlled Memory files (CM NiTi™) are manufactured with a new Nickel-Titanium metal alloy, which has been subjected to a patented thermo-mechanical process. These files demonstrate martensitic properties at room temperature (Shen Y et al 2013) — something not observed with conventional NiTi metal. The result is a file with tremendous flexibility and virtually no memory. This remarkable memory-free property allows the instrument to adapt to root canal curvatures, following the root canal anatomy without creating undesirable lateral forces on the outer canal walls (Fig. 1).
Furthermore, these files can be pre-curved in cases with limited access and can be pre-bent to allow the clinician to bypass ledges (Sides 2012) prior to attaching a rotary handpiece to the file handle (Fig. 2).
FIGURE 2A & B. Prebending a Controlled Memory (CM NiTi™) rotary file.
Traditional NiTi is in austenite form at room temperature, which exhibits shape memory. This means that traditional NiTi files will always try to straighten in a curved canal resulting in over-instrumentation, canal straightening and unfavourable stresses, which can lead to cyclic fatigue failure.
Moreover, CM NiTi™ has been shown in the literature to be up to 600 percent more resistant to fatigue failure than conventional NiTi (Ya Shen et al 2011, Ya Shen et al 2012, Dan Zhao et al 2013). Typhoon Infinite flex files (Clinical Research Dental, London, ON.) is a CM NiTi™ file with an efficient cutting design. To further reduce the stress on the instrument and in order to preserve dentin in the cervical portion of the root, Typhoon Infinite flex files have a 12mm cutting zone instead of the traditional 16mm.
This file has increased torsional strength with increased resistance to cyclic fatigue and is more likely to unwind than separate. The “unwinding” of the flutes is a valuable warning to clinicians so that they may discard the instrument before separation occurs (similar to what clinicians have been used to with stainless steel hand files). The Typhoon Infinite flex files have a triangular cross section and a variable pitch, further increasing their flexibility and minimizing the stress on the files.
The following presents a simple technique that addresses curved canal shapes simply and efficiently using the X-plorer nickel-titanium files that are used to create the glide path followed by Typhoon™ Infinite Flex nickel-titanium files to complete canal enlargement (Clinical Research Dental, London, ON.)
CLINICAL INSTRUMENTATION TECHN
IQUE(1) Achieving Straight Line Access
After access is created and all canals have been located, initial orifice shaping is achieved using either traditional Gates Glidden drills or the Typhoon Instigator orifice opener (size 25/08), made with CM NiTi™.
(2) Creating a Reproducible Glide Path
Canal exploration is ideally first achieved with a No. 8 Stainless Steel hand file that is taken to length, followed by a No.10 Stainless Steel hand file. To complete the glide path, size 15 and 20 Stainless Steel hand K-files can be taken to length if the canal is straight. Whenever a clinician is dealing with a curvature, after a No.10 Stainless Steel hand file is taken to length, the glide path should be enlarged with flexible rotary NiTi files such as the X-plorer Rotary Canal Navigation files (Clinical Research Dental, London, ON.) as shown in Figure 3. These files are used at 400 rpm with 200g-cm torque with a delicate in-out movement until working length is reached following this recommended sequence (Fig. 3):
• 15/01 to length
• 20/01 to length
• 20/02 to length
• 25/02 to length (optional for severe curvatures)
FIGURE 3. X-plorer Rotary Canal Navigation files.
One has to keep in mind that prior to proceeding with the X-plorer files, patency should be established with at least a No. 8 Stainless Steel Hand file. To verify this, a No.8 Stainless Steel hand file must be placed at working length, withdrawn by hand and pushed back to working length without any difficulty. A 10mm fluted cutting zone characterizes the X-Plorer glide path files. This short cutting zone ensures that less of the file is engaged during glide path enlargement, minimizing both file stresses and the risk of file separation (Sides 2012).
LUBRICATION AND IRRIGATIONThroughout all phases of canal instrumentation, Sodium Hypochlorite should be used generously and can be supplemented with a lubricant like Tunnel Vision (Clinical Research Dental, London, ON.) chelating gel (Fig. 4). Tunnel Vision is a water-based EDTA gel, which effectively holds canal debris in suspension while lubricating the instrument being used. In combination with sodium hypochlorite, its effervescent action facilitates efficient canal cleansing and debridement. It should also be stressed that between each instrument used in the canal, a generous volume of fresh NaOCI should be introduced in a light pumping motion, and recapitulation of the canal should be performed with a No.10 Stainless Steel hand file. When this is done, effective exchange of irrigant in the root canal system is ensured, optimizing canal disinfection and helping to maintain patency (Fig. 4).
FIGURE 4. Tunnel Vision chelating gel.
CROWN DOWN AND APICAL INSTRUMENTATIONThe following is a four-file instrumentation sequence for Typhoon that is safe and predictable (Used at 400rpm with 200-275 g-cm torque using a delicate in and out pecking movement):
• 35/06 to resistance
• 30/04 to resistance or length
• 25/06 to resistance or length
• 20/04 to length
Repeat the sequence (Fig. 5) to achieve the desired apical size
FIGURE 5. Typhoon Crown Down sequence.
Note that this sequence uses alternating tapers to prevent “taper lock”. For larger canals, Typhoon has 04 taper files up to size 50. While the sequence discussed in this section will address most endodontic cases, clinicians can also mix Typhoon CM NiTi™ files in a hybrid fashion with some traditional NiTi files. Hybrid techniques uses a stiffer traditional NiTi file to instrument the straight portion of the canal and new generation memory free files like the Typhoon CM NiTi™ files for the curvier apical portion of the root.
CLINICAL CASE Figures 6 through 18 are a sequence of a clinical case where the instrument system described in this article was used:
FIGURE 6. Pre-operative radiograph showing a severe curvature of the MB canals.
FIGURE 7. Nitial access showing the inflamed hyperaemic pulp.
FIGURE 8. Given the limited opening that this patient had, this picture show the gentle pre-bending that was applied to the Typhoon file to help position it in the mesiobuccal orifice.
FIGURE 9. 10 & 11. Access pictures showing DB1, DB2, MB1, MB2 and P canals after instrumentation with Typhoon files was completed.FIGURE 9.
FIGURE 12. Cone fitting radiographs showing the severe curvatures of the MB canals.FIGURES 13 & 14. Searing of the Gutta-Percha after warm vertical compaction.
FIGURE 15. Searing of the Gutta-Percha after warm vertical compaction.
FIGURE 16. The orifices were sealed with purple composite and the access was then closed with IRM.
FIGURES 17 & 18. Post-operative radiographs showing the instrumentation and the preservation of the severe curvatures of the mesial canals that was facilitated by an established glide path and the enhanced flexibility of Typhoon Controlled Memory files (CM NiTi™).
CONCLUSIONWhile each case presents different challenges and obstacles, and there is no one-size-fits-all sequence that will address all canal shapes and sizes in endodontics, this article describes a straightforward instrument sequence for Typhoon that is safe and predictable. The development of Controlled Memory files (CM NiTi™) has helped to address concerns of instrument fatigue and failure when using NiTi files.
Clinicians should respect the limitations of each instrument and to take sufficient time to learn how to use them following the manufacturer’s instructions. By adopting a technique that encompasses a proper glide path sequence as discussed in this article and using new metal technologies such as Controlled Memory files (CM NiTi™), instrumentation of curved root canal systems should be more consistent, more predictable and safer than it has been in the past. OH
Dr. Peter Zahi Tawil started out his college career in Mechanical Engineering at McGill University in Montreal. He then pursued his studies in Dentistry at the Université de Montréal where he received his DMD in 2004. He currently works in private practice and teaches as a Clinical Professor at the University of North Carolina at Chapel Hill. He also serves as a volunteer with the 1000 Smiles Dental Project in Jamaica. Dr. Tawil is a mentor for the College of Diplomates of the American Board of Endodontics and an examiner for the Royal College of Dentists of Canada.
Dr. Derek Duggan received his dental degree from Trinity College Dublin, Ireland in 2001. Following four years of both private and hospital-based practice, Dr. Duggan attended UNC, Chapel Hill, NC, where he received his Masters in Endodontics in 2008. Following a period of private practice in Ireland, Dr. Duggan returned to the United States and has served as a full-time faculty in the Endodontic Department at UNC School of Dentistry since 2009.
Oral Health welcomes this original article.
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