Oral Health Group
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Negotiating Curvatures: Rounding the Bend

November 1, 2005
by Richard Mounce, DDS


Negotiation of curved canals in endodontic treatment might be thought of as a series of very small steps that collectively produce the desired endodontic result. Said differently, to obturate a curved canal with an ideal shape requires patience and the achievement of many minor objectives to arrive at the desired end point. This article will examine and detail many of the small but essential steps needed to help clinicians negotiate canal curvatures safely and efficiently irrespective of the file system used.

As a starting point, it must be remembered that all canals are curved to some extent, even ones that appear radiographically straight. Canal curvature is only visible radiographically in a mesial to distal direction and as such provides a misleading two-dimensional image relative to the true curvature that curves both buccal to lingual as well as blends with the radiographically evident mesial to distal curvature. All canals are at risk of iatrogenic events if not managed properly irrespective of radiographic appearance and conversely, if treated well, even the most technically challenging roots can be instrumented to a very high standard.

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Luck is not a factor in creating excellence in cleaning and shaping curved root canal systems. Patience, a pre-operative assessment of many factors related to case difficulty and carrying out the many aforementioned minor steps (described below) can all go a long way toward managing curvature.

ESSENTIAL STEPS

Pre-operatively, the case must be assessed for the many difficulty factors that may be encountered. Aside from just the given curvature anticipated radiographically (taking into account the unseen buccal to lingual curvature) access difficulties, root length, calcification, apex architecture and maturity, etc., amongst other challenges must also be considered. Such assessment can guide the clinician to perhaps decide that a given tooth is beyond their skill level and require referral or to evaluate strategies for avoidance of a potential iatrogenic event (Fig. 1).

For example, if a canal is severely curved at mid root instead of apically, the incidence and risk of rotary nickel titanium file breakage in this area is high. This should alert the clinician to be especially careful to negotiate the canal by hand first and create a glide path (described below) to help determine the taper and tip size to be used as the master apical file size. Such a small radius of curvature at mid root can certainly lead to separation from cyclic fatigue (and to a lesser degree torque failure) and larger tip sizes in .06 tapered instruments (or larger tapers than .06) are used at great peril.

If rotary files are to be used in such a case, it might be better, case dependent, to minimize the maximum taper used below the curvature to .04. A blanket recipe of taking all teeth to a minimum .06 (minimum tip size 25) taper as recommended by some, will likely lead to unnecessary separations. The important strategy is for the clinician to see the potential problem before it occurs and take evasive action (Figs. 2A & B).

Once access has been made, instrumentation of the coronal third first, middle third second and apical third last is ideal (crown down instrumentation). Ideal irrigation and orifice shaping allows greater volumes of irrigation and unfettered tactile access into the middle and apical thirds (or level of significant curvature) of the canal system for both hand files and rotary files. Said succinctly, it is not possible to predictably instrument the apical third and manage difficult curvatures if the pathway leading up to the curvature or apical third is not handled well first. Especially for curvatures in the apical third, creating an ideal shape to the canal and removing all debris above the level of the curvature first will allow the clinician the most ideal tactile sense to negotiate and instrument the given challenge.

It may be necessary to spend significant time with hand files to establish patency and a glide path (Fig. 4). It is essential for the clinician to take as much time as needed to adequately negotiate the curvature initially by hand. Rushing to use rotaries to save time is a sure fire method to fracture instruments. Except in unusual circumstances, rotary files should not be used as pathfinders, hand files should.

Having significant quantities of K files available is a must. For an average molar case, irrespective of curvature, I use approximately one pack of 6s, one pack of 8s and one pack of 10 K files. These files are always pre-curved by hand before entry into the canal. In a tight and constricted canal, a small K file might be used just one or two strokes. The flutes of K files dull easily and from a cost versus efficiency basis there is little value in sterilizing K files for multiple uses or using dull files.

If calcification or resistance to advancement is encountered, while possible, it is rare that a #6 or even an #8 K file will cause blockages or accumulation of dentin mud apically. That said, after each insertion of the #6 or #8 file in the aforementioned negotiation of patency and glide path creation, irrigation in the canal should be undertaken so as to clear whatever chips have accumulated.

The desired goal of negotiation and creation of patency is to clear a pathway for the subsequent rotary files. For vital teeth it is essential to have a viscous EDTA gel in the canal such as FileEze (Ultradent, South Jordan, UT, USA) to emulsify the dislodged pulp in the canal and hold it in suspension until it can be irrigated out. This gel is very helpful to avoid apical blockages of pulp that might otherwise be pushed down in the canal if not emulsified nor removed by irrigation as instrumentation progresses.

The motion used to finesse these small files apically bears mention. The motion of file insertion is gentle and deliberate. Even the amount of pressure that the file is held with has clinical relevance. First off, the file should be gripped passively and gently, not so that the fingers blanch when using the file. Gently and delicately holding the file allows the file to transmit as much tactile information as possible to the clinician’s fingers. Secondly, the file should have a very subtle bend placed by hand over the distal half of the cutting flutes (the tip end). Placing a curvature on the file allows it to more easily negotiate the preexisting curvatures. The file should be inserted slowly down the canal feeling for restrictions, calcifications and curvatures. If the file drops to the apical foramen easily, a larger file can be selected for the negotiation. If it does not flow easily down the canal, it may be necessary to redirect the file slowly and deliberately until the true canal path can be found or chose a smaller file.

The goal of early exploration with hand files described above is to a) familiarize the clinician with the curvature, calcification and length of the canal and b) to simply open up the canal for its instrumentation with rotary files. Said differently, in the early exploration of the canal, the clinician is not trying to instrument the canal as much as to create a platform (a glide path) for the subsequent RNT files that will ‘do the heavy lifting’ of instrumentation. In other words, all K file use in the early exploration of the canal will be done so simply to allow the clinician to determine canal anatomy and facilitate glide path creation. This is an absolutely essential step in the creation of endodontic excellence. Skipping the step or not giving it the time and attention it deserves is problematic.

Once the canal has been negotiated to the true working length, even to a #10 K file, it has great value to then use a .02-tapered rotary file such as the Quantec SC .02/15 and/or the K3 .02/15 (SybronEndo, Orange, CA, USA). Such a file should be rotated at 350-600 RPM with a very smooth and gentle motion. If the file resists apical progression, it should not be forced. Once the file is removed from the canal, its flutes shoul
d be wiped and the canal irrigated. Use of the .02 rotary files in this manner can serve to create a pathway or refined glide path upon which subsequent rotary file use can proceed. Creating a glide path first diminishes the chances for rotary nickel titanium file separation.1-2

As an aside, it is noteworthy that the advent of rotary nickel titanium files has in some ways created obsolescence for some hand files and a new and essential vitality for others. For example, the small K files 6-15 are more relevant than ever. As mentioned, using rotaries without first creating a glide path is contraindicated and contributes significantly to fracture. Alternatively, in my empirical opinion, especially above a #20 K file, there is little value for K files as cleansing and shaping instruments.* Put differently, at or above a #20 K file, the stiffness of these instruments makes them especially susceptible toward creation of ledges and packing debris apically, especially if they are not used with a balanced forces technique.3

While it is beyond the scope of this paper to discuss the balanced forces technique, it represents one method (and exception) where larger K files might be used apically to create larger master apical file sizes. Rotaries can be used predictably and almost effortlessly if used appropriately in almost any canal anatomy with minimal risk of fracture and preclude the more time consuming use of hand instruments (Figs. 3-5).

While this is an article about negotiating curvatures, it bears mention as to which rotary system is most universally applicable for the greatest variety of canal anatomy to be treated. In the author’s hands, empirically, this is the K3 rotary system (SybronEndo, Orange, CA, USA). The file, due to its asymmetrical design and fixed taper and various tip sizes has virtually unlimited application across a wide range of clinical cases. Said differently, clinically, in years of use, having tried every file on the market in North America, I have not felt that another file had better qualities in my hands to enhance shaping, fracture resistance or cutting ability, centering, etc. K3, at this point in time satisfies the requirements of rotary preparation very well. References 4-14 provide a sampling of the available literature on K3. The K3, blended with hand instruments, used appropriately, as discussed in this paper create a powerful resource for management of curved canals.

* Another exception is the use of hand instruments to gauge the apex to determine the diameter of the foramen after initial preparation and give a guide to the final created diameter that is most clinically appropriate for the given clinical case.

Dr. Mounce has no commercial interest in any of the products mentioned in this paper.

Oral Health welcomes this original article.

REFERENCES

1.J Endod. 2004 Apr; 30 (4): 228-30. Influence of manual preflaring and torque on the failure rate of ProTaper rotary instruments. Berutti E, et al.

2.J Endod. 2003 Sep; 29(9): 567-75. Analysis of mechanical preparations in extracted teeth using ProTaper rotary instruments: value of the safety quotient. Blum JY, et al.

3.J Endod. 1985 May; 11(5): 203-11. The “balanced force” concept for instrumentation of curved canals. Roane JB, Sabala CL, Duncanson MG Jr.

4.J Endod. 2005 May; 31(5): 373-5. The shaping effects of three nickel-titanium rotary instruments in simulated S-shaped canals. Yoshimine Y, Ono M, Akamine A.

5.J Endod. 2005 Feb;31(2):114-6. The influence of a manual glide path on the separation rate of NiTi rotary instruments. Patino PV, Biedma BM, Liebana CR, Cantatore G, Bahillo JG.

6.Int Endod J. 2005 Jan;38(1):2-7 Effectiveness of different techniques for removing gutta-percha during retreatment. Masiero AV, Barletta FB.

7.Int Endod J. 2004 Sep;37 (9):593-601 Shaping ability of ProFile and K3 rotary Ni-Ti instruments when used in a variable tip sequence in simulated curved root canals. Ayar LR, Love RM.

8.J Endod. 2004 Apr;30(4):234-7. K3 Endo, ProTaper, and ProFile systems: breakage and distortion in severely curved roots of molars. Ankrum MT, Hartwell GR, Truitt JE.

9.Clin Oral Investig. 2004 Jun;8(2):102-10. Epub 2004 Feb 4. Appearance of the root canal walls after preparation with NiTi rotary instruments: a comparative SEM investigation. Prati C, Foschi F, Nucci C, Montebugnoli L, Marchionni S.

10.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003 Dec;96(6):757-63. Bending properties of rotary nickel-titanium instruments. Schafer E, Dzepina A, Danesh G.

11.Aust Endod J. 2003 Aug;29(2):75-8.Torsional properties of new and used rotary K3 NiTi files. Yared G, Kulkarni GK, Ghossayn F.

12.Int Endod J. 2003 Apr;36(4):288-95. Progressive versus constant tapered shaft design using NiTi rotary instruments. Bergmans L, Van Cleynenbreugel J, Beullens M, Wevers M, Van Meerbeek B, Lambrechts P.

13.Int Endod J. 2003 Apr;36(4):262-6. Factors influencing the fracture of nickel-titanium rotary instruments. Martin B, Zelada G, Varela P, Bahillo JG, Magan F, Ahn S, Rodriguez C.

14.Int Endod J. 2003 Mar;36(3):208-17. Efficiency of rotary nickel-titanium K3 instruments compared with stainless steel hand K-Flexofile. Part 2. Cleaning effectiveness and shaping ability in severely curved root canals of extracted teeth. Schafer E, Schlingemann R.


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