Initial Negotiation of Difficult Canals

Endodontics - Tips and tricks
4 Mar 2016

Cleaning and shaping the root canal is a critical phase in root canal treatment. For years, root canal instrumentation has been performed with hand instruments. Currently, NiTi rotary instrumentation has become the standard of care for shaping root canals in endodontics (Bird DC et al. 2009) and general dentistry practices (Locke M et al. 2013, Parashos P & Messer HH 2004). Owing to the flexibility of NiTi instruments driven by a rotary handpiece and a crown-down approach, the canal preparation is more efficient and faster with less procedural errors (Glosson CR et al. 1995). Recently, mechanized unequal reciprocation (Yared 2008) with a single file has been introduced as an alternative to complete rotation. To date, numerous studies have demonstrated the validity of this clinical concept (Grande et al. 2015, Plotino et al. 2015).

Initial Negotiation of Difficult Canals: Towards A New Paradigm Shift
An original article by
Pierre Machtou DDS, MS, PhD & Clifford J Ruddle, DDS, MS,
Ghassan Yared, DDS, MS

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Instrument fracture is a potential risk during shaping with engine-driven NiTi instruments (Tzanetakis et al. 2008). To minimize this mishap and get predictable results, the non-cutting tip of the first shaping file should not be locked in the canal, implying that the cross-section of the canal should be bigger than the instrument ( Berutti E et al. 2009). As a result, rotary files should not be used unless a sufficient, smooth, secure and reproducible tunnel from the canal orifice to the apical foramen (Glide path) has been previously created (West JD 2010). It is usually agreed that a size .15 or .20 stainless steel (SS) K-file should be used and glide passively to the terminus of the canal prior to using any rotary or reciprocating shaping files. Knowing that there is 50% increase between a ?.10 K-file and a ?.15 K-file, and 30% between the ?.15 K-file and the ?.20-K, the stiffness of these files may render their use challenging when attempting to treat constricted and curved canals (Ruddle CJ, 2014). Iatrogenic errors such as blockage or canal transportation may occur irrespective of the motion used and regardless of the clinician’s proficiency. Accordingly, dedicated NiTi glide path rotary instruments have been launched to replace hand SS K-files. The first glide path system available on the market was the Pathfile system (Dentsply) which includes three instruments (13 .02, 16 .02 and 19 .02) (Berutti et al. 2009). Since then, several brands (Capar ID et al. 2015) offer rotary glide path files, some with a reduced number of instruments:

Dentsply Pathfiles 3 instruments
Micro Mega G-files 2 instruments
J. Morita EndoWave 3 instruments
FKG Scout-Race 3 instruments
SS White V Glide Pathfile2 instruments
Clinician’s ChoicetX-Plorer Series 3 instruments
Dentsply ProGlider 1 instrument

These glidepath NiTi systems have been shown to be safe and more efficient than SS K-files (Berutti et al. 2009). Nevertheless, negotiating the canal to its terminus with a ?.10 SS K-file is strongly advocated to create a smooth canal pathway prior to using any NiTi glide path rotary system (Ruddle CJ, 2014). In fact, the first goal is to achieve an initial glide path with a super loose ?.10 K-file. Thus, the actual problem is not about selecting and using one of the different glide path systems available but to be able to negotiate difficult canals to length in a predictable way with a small K-file.

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Initial negotiation with SS hand instruments

It must be understood that canal preparation consists of three distinct stages: initial canal negotiation, creation of a glide path and canal shaping.

Initial canal negotiation is a key step of root canal preparation and, certainly the most difficult task to achieve in some clinical situations. It is usually performed with a small precurved file used with a watch-winding / pull motion. In long, narrow, constricted or severely curved canals this procedure may be demanding and not always successful, even in the hands of skilled clinicians. Initial canal blockage, ensuing ledge and apical transportation are needless complications but do occur frequently and may jeopardize the treatment outcome. To overcome these iatrogenic events, it is important to understand why the first scouting file cannot reach the full working length. Four individual or combined causes may be raised ( West JD, 2010):

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1-The first cause of blockage is the accumulation of dentin mud resulting from the dentin and organic components detached during the apical progression of the scouting file (Ibarolla JL 1997). Generally, when managing blocked and/or ledged canals, the shortest file that can reach working length is selected. Shorter instruments provide more stiffness and result in greater tactile control. The armamentarium needed to successfully negotiate through a canal blockage is generally a series of 21 mm-length stainless steel files in sizes 0.06, 0.08, and 0.10 mm. Importantly, the obstructed and blocked canal is filled with full-strength sodium hypoclorite or a lubricant, such as Glyde, ProLube, or RC Prep.

The first step is to precisely locate the level of the block, then preshape that portion of the canal that is coronal to this blockage. During this procedure, care should be exercised to avoid contacting the block. An abrupt curve is placed on the apical extent of a size 0.06 or 0.08 K-type file. This precurved file can now be passed through the pre-enlarged canal and will reach the obstruction. C-Pilot files (VDW, United Dental Manufacturers, Inc; Tulsa, OK) are available in sterile blister packs of 6 instruments and are recommended because they have a high resistance to bending, due to a special thermal hardening process (Lopes HP et al. 2011). The precurved file is placed in contact with the block. A single back-and-forth reciprocating motion is used in conjunction with light apical pressure. The file is immediately removed from the root canal with an envelope movement. The goal is to disintegrate the block with the tip of the file. Irrigate with 1.0 ml of NaOCl, repeat the same procedure three times, and then discard the file. A new file is selected and precurved, as the reciprocating action described, dulls the tip of the file. Indeed, a new file ensures a sharp tip, which is mandatory for a successful outcome. The same handle motion and file movement is continued with as many files as required until a light catch is felt. At this stage, a size 0.08 C-Pilot file is usually selected and utilized, as described, to expand the pathway following the use of the size 0.06 K-files. In some clinical situations, up to a dozen files may be needed to negotiate through and bypass a blockage. Once the canal has been negotiated and patency established, the file is repeatedly used in a slip, slide, and glide manner to refine, expand, and smooth the glide path. Patience and perseverance are oftentimes rewarded.

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2- A second cause is related to the use of a too large instrument compared to the canal cross-section. In some instances, when the first light resistance is felt during apical progression of the initial file, moving to a smaller size allows a deeper penetration

3- In curved canals, the pre-curvature of the file does not always match the curvature of the canal. In this case, a sharper curvature must then be placed on the file

4- At last, too much dentin constriction in the coronal area of the canal prevents the file to move forward. It is then necessary to preshape the patent portion of the canal.

At this stage, the clinician must be proficient with the different motions to be used during initial negotiation.
– To follow the canal pathway, a light touch is required to enhance tactile feedback. The precurved file should slip, slide and glide passively to light resistance
– The envelope of motion ( Schilder, 1974) is the first motion to be implemented with an .08 K-file. As soon as a slight resistance is encountered, the file is rotated clockwise while it is withdrawn. Such action cuts the canal dentine and organic interferences that hinder its progression. One ml of NaOCl irrigation is immediately delivered to remove residual debris. The sequence envelope-irrigation is repeated as needed to reach the apical foramen. As the file is always working on the outstroke, without any pressure, the procedure is safe and successful in most instances.
– If the file stalls at the same level after two envelope attempts, a watch-winding/pull motion followed by 1ml NaOCl irrigation is then recommended. To create more space, the next larger K-files .10 and .15 are used where they can go passively with the same watch-winding/pull motion. This allows the first initial .08 K-file to move deeper.
– When the apical terminus of the canal has been reached, an up and down motion with a short amplitude (2mm) is the only motion permitted in this area. The amplitude of the file is progressively increased until reaching 4mm. Once the file is able to slide and glide freely, the file is removed, straightened and dropped inside the canal. It should go to length unrestricted, confirming thus that the initial glide path has been secured,.
– In case of a severe apical curvature, once the .10 K-file is free at the working length, using the balanced force motion ( Roane JB et al. 1985) with a ?.15 Flexofile or Nitiflex is the safest way to progress apically and manage the situation.

Initial negotiation with short angle reciprocating engine-driven SS files
When Reciproc files were launched in 2010, G. Yared was the first to advocate canal preparation with a single Reciproc R25, without initial negotiation. He stated that a large majority of cases could be managed this way. At that time, this concept was regarded controversial and triggered endless debates among clinicians. Three years later, a study by De Deus et al (2013) validated this approach and demonstrated the ability of the Reciproc R25 to reach the full working length of straight and moderately curved canals without a glide path in a large proportion of cases.
Anyway, reciprocation must be seen as the natural movement for initial negotiation of root canals either by hand or mechanized instrumentation.
The watch-winding motion is a reciprocal motion but it is quite difficult for the practitioner to monitor both the pressure and the clockwise (CW) and counter-clockwise (CCW) angulations given to a file. Resulting mishaps have been already mentioned in the first section.
The M4 Safety™ handpiece (Kerr Endodontics) uses mechanized equal reciprocation with 30° angles and is efficient to scout canals and create a glide path with SS small K-files. An E-type connection allows the operator to lock a regular hand K file into a chuck. But the initial scouting file has to be manually inserted to length before securing it to the handpiece (Mounce RE 2008). In addition, the angles of reciprocation are still too large to avoid transportation, fatigue and torsional stresses when larger files are used.
The Race ISO 10 (FKG, La Chaud de Fond, CH) is another mechanized attempt to achieve initial canal negotiation. The Race ISO 10 files are 10/.02 , 10/.04 and 10/.06 NiTi rotary instruments dedicated to initial negotiation and glide path preparation. The advocated torque is 1N/cm and the setting speed is 800 rpm. Owing to their alternating cutting flutes, there is no screwing effect. A small coronal pre-enlargement is occasionally needed to optimize the outcome. Although efficient, the results are not 100% predictable and the limitations are related to the high flexibility of the NiTi alloy in case of sharp apical curvature and/or in the presence of an intracanal obstruction.
In a quest of a predictable and safe technique to catheterize difficult canals, it is important, at this stage, to define the requirements needed to achieve this goal: 

– It should be possible to track the canal pathway from the orifice to the canal terminus, irrespective of the angle of incidence of the coronal curvature, even in constricted, severely curved or S-curved canals.
– The technique should use a small scouting file, flexible but strong enough to resist torsional stress and cyclic fatigue. Besides, the file should have a good cutting efficiency, should not be twisted when meeting an intracanal obstruction and could be re-used. SS K-files are best suited to meet these objectives and cost less than NiTi files.
– The technique should eliminate the risk of fracture and avoid blockage and canal transportation.
– The technique should be able to bypass a canal blockage or a ledge.
– The technique should be forgiving and resist apical pressure in case of resistance to apical progression.
The endurance limit ( EL) is a new concept in endodontics which allows the use of engine –driven root canal instruments with no risk of fracture. It implies that, for very low stress, the service life of a metallic instrument, expressed in number of N cycles (cycle = loading stress or strain and releasing) is very high. It is considered that an instrument has an “infinite” lifetime when it can be subjected to 10 6 or 107 .cycles. Rather it will experience a failure one day, but it will not be by fatigue, but by another phenomenon: wear, accidental overload, corrosion…Therefore, the endurance limit can be defined as the stress value below which it can be considered that the instrument does not break by fatigue.

In 2004, Best et al. assessed the endurance limit of a 30/.06 taper rotary Profile instrument  and could establish a torsional fatigue profile, predicting an endurance limit reflective of a particular angular deflection ( 2,5° ). At 2,5°, it took approximately 21 days until 106 cycles were reached without instrument fracture.
Recently, Yared (2014, unpublished data) was able to determine the EL at 6° CW and 1°CCW for small reciprocating SS files. These very small angles provide an extremely controlled and precise work of the instruments far beyond what is possible to expect when they are used by hand. A slow speed was set by trial and error between 25-50rpm to allow, in combination with the small angles, an effective cutting action without a screwing-in effect. Therefore, initial negotiation with unequal mecanized reciprocation becomes possible, efficient and safe regardless of the presence of calcified canals, ledges and apical hooks (Yared G, 2015). The technique is straightforward and forgiving even for inexperienced practitioners. In a lubricated canal with NaOCl or Glyde, short pecking motions are required during the scouting phase and the file can be precurved when too much resistance is felt during apical progression. Pressure can be safely applied to overcome obstacles in constricted canals. Due to the small angles of reciprocation, taper lock and file separation are eliminated. Preliminary laboratory and clinical tests, have shown amazing and predictable results in the most difficult anatomical situations. The working time is drastically reduced to obtain apical patency and, oftentimes, several unexpected portals of exit in the apical area can be negotiated.
At the present time, the technique encompasses a motor prototype just as specific SS engine-driven files with a hardening process

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