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To test the validity of claims for file designs, a computerized clinical simulator was constructed to simultaneously measure torque, pressure, and time, during the prescribed use of instruments, to determine efficiency and the threat of file failure. The simulator A computer provides the means for precisely duplicating motions designed to simulate clinical applications for comparing different instruments rather than using a very specific circumstance for testing.
While eliminating operator variability and conforming to operation recommendations, 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 stress of the force of insertion and torsion of each individual file 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 approaches.
All measurements are plotted over time to illustrate when and how stress occurs. Rather than measuring the over-all flexibility of the file, the simulator device can be used to measure dynamic flexibility, recording the resistance to bending as a rotating file progresses onto an inclined plane or simulated curved canal. The measurement occurs over time as different diameters and cross section configurations of the file transverse a curvature.
The logged data help determine the methods for which each instrument may be used most effectively while minimizing the threat of failure. The simulations can be applied to different anatomies and technique solutions quickly become apparent, rather than having to rely on subjective and time consuming trial and error experience that lack the benefit of controls. An examination of the results puts the manufacturer’s technique recommendations in perspective, validating or invalidating their claims. Identifying technique enhancements and file design improvements become more feasible. The results can be used to substantially enhance efficiency and may be surprisingly different from what has been recommended.
Irrigation and lubrication can reduce torque requirements by as much as 400% compared to rotating in a dry canal. However, shorter strokes of insertion can be more effective than carrying the rotating file to greater depths into the canal with fewer insertions, even with irrigation. The percentage of the file engaging dentin plays an important role in influencing the torque reduction resulting from lubrication or clearing of debris by irrigation.
When the rotating file becomes engaged for more than a few millimeters, the interface of irrigation is reduced between the surface of the canal and the file. This has little effect in reducing torsion, since any irrigation has little opportunity to penetrate the additional distance with the file due to the rotating file moving the irrigating solution in a coronal direction. This is especially the case when the irrigation is intermittent rather than constant and the insertion of the file is continuous rather than being done in shorter strokes. The use of a handpiece, having a tubular connection with an irrigation pump, can be beneficial even with a short stroke technique, not so much for lubricity, but for the elimination of debris before its accumulation contributes to resistance of rotation.
The cutting ability of a file is primarily the result of its cross-section design when the taper and technique are the same. The angle of incidence of the blade and the width of the land, shown in cross-section perpendicular to the blade, are the best indications for comparing different files for cutting ability. The helix angle certainly should be considered in conjunction with the cross-section design in order to maximize the efficiency of a particular file. It is important to note that comparisons of cutting abilities of some files often change at different diameters along the working surface due to changes in the ratio of the depth of flute and file diameter, the width of lands, and the helix angle. Cutting ability is synonymous with cutting efficiency. However, the term cutting efficiency of a file would more appropriately be reserved for describing the entire working surface that becomes engaged rather than a measurement at one diameter.
What tactile sensations indicate efficient or effective rotary instrumentation? How an instrument feels offers little information about the collective stresses on a file. Contrary to the practitioner’s usual reliance on the tactile sensations of torsion for conventional hand files, stress on rotary files, as the result of the force of cutting, can most accurately be determined by testing. The results can be very different from the indications of tactile sensations. Since variations in torsion (those rotational forces that would urge the handpiece to move in a counterclockwise direction if the file remained in a stationary position) are difficult or impossible to feel, the tactile sensations of a rotary file are primarily due to variations in pressure, which can offer indications for a needed response. For instance, when applied pressure results in the negative pressure of screwing-in, then an immediate response of removing the file from the canal is needed in order to prevent the stress of excessive engagement. Or when a greater pressure is required for continued advancement into the canal then the file should be removed in order to avoid ledging or subjecting the file tip to excessive stress. With no comparative basis for applied pressure, the uninformed user may likely select a file with inefficient scraping edges for having a smoother feel than one with more efficient less stressful cutting edges.
Cutting effectiveness, it should be pointed out, depends on more than just the sharpness of the cutting blades. It is also the result of the angle of incidence, helix angle, taper and flute design and the relationship each of these instrument features has with the file sequence and technique used.
Often underestimated in importance is the fact that to rotate a file can be due to the force of caused by the blades becoming engaged by cutting into the canal wall to form chips without dislodging them. When the blades are parallel or the helix angles are the same along the working surface, the file becomes an effective screw. Torsion results when any screwing-in force is resisted and can cause the file to become locked in the canal. Parallel blades and more spirals are a carry-over from the hand file design when instruments were twisted during manufacturing and the ‘watch-winding’ motion was used as a reaming technique. Screwing-in forces can actually be augmented with sharper cutting angles further complicating the formula for efficiency. Several file manufacturers utilize accelerating helix angles to reduce the screwing-in forces. For instance, a 22-degree helix angle at the tip of the file may be graduated to a 45-degree angle at the handle end. As both ends of the working surface are engaged, the different angles have different screwing-in rates, feed rates, and much of the torsion of screwing-in is reduced. However, most of the working surface of the file with graduating helix angles needs to be engaged before the different blade angulations have much benefit in canceling out some of the screwing-in forces and when most of the file is engaged that in itself increases torsion. If only a small portion of the file becomes engaged, the screwing-in forces may require most of the force for rotation. The operator may notice this pulling-in force if only a short portion of the file becomes engaged in a constriction.
The speed of rotation can have a significant influence on the screwing-in forces and the resulting torsion. The optimum rotation speed for efficiency remains approximately the same independent of the canal anatomy. Slowing the speed of rotation as a precautionary measure can result in additional torsion because the screwing-in forces may increase by allowing the blade to become too deeply engaged to dislodge the chips that are formed. The most efficient design incorporates only the minimum number of spirals that are necessary to effectively remove debris. In doing so,more positive cutting angles can be utilized and the decrease in the number of spirals of the blade reduces the amount of its engagement. The result is a reduced required torque for rotation and increased efficiency.
Employing different tapers can be one of the most important methods of limiting file engagement.The nomenclature, however, for describing the techniques for using tapered files can be confusing to the novice in conceptualizing the action that is occurring if one is only considering an individual file rather than a sequence of files. If a smaller tapered file is inserted into the preparation of a larger tapered canal, only the apical portion of the file initially becomes engaged, yet the technique is termed. On the other hand, crown-down is more meaningful when referring to that portion of the canal being first enlarged during a sequence from large tapers progressing to smaller tapers. Conversely, if a larger tapered file is inserted into a smaller tapered canal, the file initially engages and prepares only the coronal aspect of the canal and yet the sequence technique is called a approach. The distinction is important in order to keep in mind which part of the file is being engaged and is being stressed. In either technique, one advantage in changing from one taper to another is that the initial engagement is minimal and any increase in engagement is gradual, thus enabling an opportunity to more accurately interpret variations in resistance as the file progresses into the canal. That opportunity might not be available when using files having the same taper since this approach can quickly result in full engagement with minimum apical advancement.
The operator is better served if progress into the canal is terminated before maximum engagement occurs. One of the most important considerations for rotary instrumentation is utilizing the advantage of changing file tapers. The reduction of stress on the instruments can significantly be achieved by minimizing engagement with this technique.When using the crown-down technique, only the file tip, the portion least resistant to torsion failure, initially becomes engaged. When using the step-back technique, a larger diameter, the more torsion resistant portion of the file initially becomes engaged. In contrast, if the canal preparation of one file is followed with a file having the same taper, file engagement is maximized, increasing the torque requirement that increases the stress on the file.
The primary consideration is to set parameters for preventing file failure and eliminating unnecessary or counterproductive actions. By observing the situations that usually have a high incidence of file separation, we can then test procedures to effectively avoid those situations. Once effective procedures are identified, then the most efficient approach can be determined. The incidence of file failure during testing indicates instrumentation should encompass considerations for all of the following parameters for rotary instrumentation:
- Advance into the canal using no more pressure than was required to advance the first 1mm.
- Engage no more than 6mm of a file if it is engaged in a curvature (the exception would be a size 20-.02 or smaller).
- Advance a file into the canal with no more than 1mm increments with insertion/withdrawal motions.
- Advancement into the canal should be able to occur at a rate of approximately 0.5mm/s without increasing the pressure of insertion.
- Follow the use of one file with a file having a different taper.
When any of the parameters cannot be met, changing to a file having a different taper in the technique sequence will usually enable re-adherence to the parameters. These parameters, by necessity, require subjective and arbitrary judgments, since there is no one point at which file breakage definitely occurs or definitely does not occur. However, careful examination during extensive testing indicates that any exception to these parameters should be undertaken with the cautious awareness of the operator.