Evaluating instrumentation techniques
With the continuous introductions of endodontic rotary files, recommended techniques for their use seem to proliferate even more rapidly. Although a desired canal shape can be prepared with virtually any series of instruments, voices of advocates confuse the choices with differing and sometimes conflicting approaches for accomplishing that shape. One is inclined to ask, “Can they all be right?”
Coupled with the fact that some endodontic files might become obsolete before the techniques for their use can be thoroughly evaluated, the important question becomes apparent: “Are there not common principles by which all present and future files can be used most effectively, efficiently and safely to conform to the operator’s treatment ideals?” The quest for the answer to that question culminated in one of the most ambitious NiTi rotary file research projects ever undertaken that spanned over five years and encompassed over 2,400 evaluations. Understanding the dynamics of the results of those evaluations has the potential to save thousands of hours of chair time, significantly increase production income, and provide satisfaction that one is mastering excellence.
Many who have incorporated rotary instrumentation into their practice understandably looked for a simple system of files and an easy technique that could be used in a routine manner. Many were attracted by claims that techniques, having the fewest instruments, facilitated canal preparation. That notion needs to be replaced with the realization that understanding the scientific principles of instrumentation needs to be the foundation of expertise rather than cookbook type instructions or recommendations that seem to suggest that one routine fits every canal anatomy. With understanding, there is no need to rely on the ability to decipher conflicting explanations of noted authorities. Neither is there a need to rely on time consuming and costly trial and error experience. It is easy to forget that it requires ten years to have ten years of experience.
Rudimentary to understanding the principles of rotary instrumentation are the interrelationships of file dimensions that are important for considering the threat of file failure due to the stresses of fatigue and/or torsion. (Fatigue results from the excessive stresses of repetitive compression and tension that occurs during rotation of a file around a curvature. Torsion is the axial force of being twisted when one part of a file rotates at a different rate than another part.) Some important interrelationships can be expressed as irrefutable statements of physics and are as follows:
1. Fatigue of a file increases with the square of the file’s diameter.
2. Fatigue of a file increases with the degree of curvature of the canal and the number of file rotations.
3. In a straight canal the ability of a file to withstand torsion increases with the square of the file’s diameter.
4. The torque required to rotate a file varies directly with the surface area of the file’s engagement in the canal.
5. A file with a more efficient cutting design requires less torque, pressure or time to accomplish canal enlargement.
In addition to using the above file dimension relationships in establishing our parameters for enhancing instrumentation techniques, we can now draw on trends that became apparent during our 2,400 evaluations. Although research evaluating endodontic instruments and techniques cannot result in absolutes, extensive research does provide the most significant evidence based predictability to be used for formulating instrumentation techniques. To test the validity of claims for file designs and techniques, a computerized clinical simulator was constructed to simultaneously measure torque, pressure and time, during various prescribed uses of types, sizes and tapers of instruments, to determine efficiency and the threat of file failure (Fig.1).
The simulator computer provides the means for precisely duplicating motions (US Robotics) designed to simulate clinical applications for comparing different instruments and techniques. In eliminating operator variability and subjectivity while conforming to operation procedures, computer programming can control the preparation parameters for the depth and the speed of file insertion and withdrawal, as well as the speed of file rotation. Not only can the stresses of the force of insertion and torsion of each individual file type, size and taper be measured under different circumstances, but also the stresses, using different file sequences, can be recorded in order to determine the least stressful and most expeditious technique design. All measurements are plotted over time to illustrate when and how stress occurs. The simulations can be applied to different canal dimensions and curvatures to determine if technique modifications are necessary. The logged data determine the methods for which each instrument may be used most effectively while minimizing the threat of failure. Technique solutions quickly become apparent. The results may be surprisingly different from what has been recommended. In fact, no published file technique conformed to all of the parameters for technique design suggested by the research.
As noted above, research cannot result in absolutes, but some observations became particularly evident in achieving efficiency while minimizing threats of failure. Some of the more important observations are as follows:
1. Advance a file into the canal with no more than 1 mm increments with insert-withdraw motions.
2. To advance a particular file the first 1 mm into a canal after it becomes engaged, a minimal specific pressure needs to be applied. If that pressure needs to be increased in order for additional advancement to occur or if a negative pressure (screwing-in force) is encountered, change to a different tapered file or circumferentially file coronal to this position.
3. File advancement into the canal should be able to occur at a rate of at least 1/2 mm per second without having to increase the pressure for insertions.
4. If a file has more than a 0.02 taper do not advance more than 2 mm beyond the preparation of the previous file if any part of the file is engaged in a curvature.
5. Apply no greater than 1 pound of pressure on any file while advancing into the canal.
6*. Except for 0.02 tapered files having a size diameter of 0.20 mm or smaller, do not engage more than 6 mm of the file’s working surface if any part of the file is engaged in a curvature.
7*. Beyond the point of canal curvature, the file diameter should be no greater than:
0.60 mm for a 0.02 taper,
0.55 mm for 0.04 taper,
0.50 mm for 0.06 taper,
0.35 mm for a 0.08 taper.
(This consideration is the result of testing for 45-degree curvatures having 8 mm radii and applies only to these dimensions for rotary NiTi files. File diameters should be smaller for more severe curvatures and can be adjusted larger for less severe ones.)
Although developing expertise in using rotary instrumentation depends upon a thorough understanding of the file dimension relationships and research observations presented, the first attempts at putting them all together can seem at first confusing and a cause for returning to a technique with fewer efficient results but one with a definite standard sequence. However, basic techniques procedures can easily be established that encompass all the considerations discussed above, but the resulting technique will probably be very different from those that are currently advocated. With closer examination of the suggested parameters for instrumentation, one’s concepts for what causes instrument stress and what results in efficiency may change. Space limitations for this article will not permit a discussion of all the parameters listed and their application to all types of files but let us consider the ramifications of 6* and 7* above. For example, many dentists assume you can conform to number 6* by extending the file preparation to a depth of not more than 6 mm into the canal. That assumption can certainly result in instrument breakage. Consider the resulting 7 mm engagement (exceeding the recommended 6 mm of engagement) that occurs when a size 25/0.04 taper file extends the preparation of a 25/0.06 file only 2 mm (Fig. 2).
Fig. 2: If the 25/0.04 file advances only 2 mm beyond the terminus of the 26/0.06 preparation, 7 mm of the file becomes engaged.
In a survey of the frequency of curvatures, the results of Schafter et al., (JOE, Roentgenographic Investigation of Frequency and Degree of Canal Curvatures in Human Permanent Teeth) indicated that of the mb2 canals in maxillary first molars examined, the median degree of curvature was 42 degrees with a 6.6 mm radius when viewed from the buccal and 14 degrees with a 9.2 mm radius when viewed from the mesial. This curvature is substantially greater than the standard selected in our research for testing. With these findings in mind, a thorough understanding of the concept stated in 7* probably contributes to the development of expertise of instrumentation more than any other.
The ability of a file to resist fatigue has an inverse relationship with the square of its diameter: Therefore, a severe apical curvature can be less threatening than a more moderate, more coronal curvature. Since the diameter of a file increases along its taper, determining the location of a canal curvature beyond which a file can advance is of paramount importance for the prevention of excessive stresses on the file. The first step is to determine if there is a curvature of any significance and how far the curvature is from the apex. Withdrawing the file used to establish the working length, and passively re-inserting it will indicate a curvature if it meets any resistance short of the working length since the canal is now larger than the file. The length of the canal to the curvature, the coronal zone, is measured and recorded with the same importance as determining the working length. The canal can now be divided into two zones, the canal portion short of the resistance defines the coronal zone and the portion beyond the resistance defines the apical zone (Fig. 3).
Fig. 3: The canal portion short of the resistance defines the coronal zone, and the portion beyond the resistance defines the apical zone.
The second step is to determine the distance each of the files having different sizes and tapers can safely be advanced around and beyond the curvatures and which size file will need to be used in the coronal zone to prevent any subsequent file from binding in that zone when used in the apical zone. In other words, as a means to minimize total file engagement, any file used in the apical zone should not bind in the coronal zone. The coronal zone offers no particular problem for enlargement because this is the straight part of the canal. By using the parameters suggested above for diameter limitations* (0.60 mm for a 0.02 taper, 0.55 mm for 0.04 taper, 0.50 mm for 0.06 taper, and 0.35 mm for a 0.08 taper), we can calculate if the diameter of a selected file would exceed our limitations. That determination can be calculated by using the following formula for the file we intend to use (Fig. 4).
Fig. 4: This formula can determine the file to use.
For instance, if we want to know how far we can advance a 25/0.06 file beyond the point of curvature, the parameters state the diameter should not exceed a 0.50 mm diameter that is reached when the file extends only 4 mm beyond the point of curvature (diameter limitation, 0.50, minus tip size, 0.25, divided by taper, 0.06, equals approximately 4). If that 4 mm distance coincides with the working length, we can safely use the 25/06 file to WL provided we first prepare the coronal zone large enough that the file would not engage in this portion of the canal when advanced to the WL. That would require enlarging the coronal zone slightly larger than the equivalent of a 50/0.06 file at the terminus of the coronal zone. This procedure also conforms to parameter 6* that states do not engage more than 6 mm of the file’s working surface if any part of the file is engaged in a curvature. If the WL is more than the 4 mm distance from the point of curvature, a smaller file size or taper must be used.
Although the first impression of this process may seem complicated, little practice is required to master this approach. The benefit is maximum efficiency, expediency and the virtual elimination of file failure.
If the clinician insists on following a routine technique sequence because consistency weighs heavily in his or her practice efficiency, then the following procedure encompasses all of our considerations for file sequencing for minimizing instrument stress while providing an efficient procedure for routine canals. Of course, the other parameters for the use of each file need to be followed. That technique sequence is as follows:
1. 25/0.06 to curvature.
2. 55/0.06 to curvature.
3. 25/0.02 to working length.
4. 25/0.04 to working length.
5. 25/0.06 1 mm short of working length.
6. 0.02 tapers or LightSpeed-type instruments should be used to enlarge any desired apical diameters greater than a size 25.
The exceptions to 'routine', of course, is any canal having an apical zone greater than 6 mm, a curvature more than 45 degrees and a radius less than 8 mm.
Although many of the accepted instrumentation techniques encompass few, if any, of the parameters listed above, working within the parameters can be done with impressive efficiency because minimum file stress is maintained. Each step of instrumentation can occur quickly without repetitive non-productive attempts. Following the parameters should provide the means to advance into the canal at least 0.5 mm per second with each file insertion. The total cumulative time can be extremely impressive when compared to other techniques. While following these procedures, one should keep in mind the purpose is to illustrate a means for providing the greatest expediency and efficiency for enlarging the canal space while virtually eliminating excessive instrument stress or failure—not to advocate a particular canal size or taper. The final canal dimensions should be adjusted to conform to the judgment of the operator and the requirements of the obturation technique used.
As one broadens the scope of understanding, skill is enhanced in a scientific manner and success becomes more predictable. The art of endodontics becomes the science of endodontics and expertise becomes the nature of the operator.
John T. McSpadden can be reached at email@example.com.
Editorial note: This article was originally published in Endo Tribune US, Vol. 3, Issue 3, 2008.