Advanced Endodontics

SHAPING ROOT CANALS (continued)

How does decreasing the percentage taper over a portion of a file's cutting blades serve to improve flexibility? By reducing the amount of alloy or anything else?

The ProTaper Finishing files F1, F2, and F3 have fixed tapers of 7, 8 and 9%, respectively, in their apical extents. However, from D4-D14 they have decreasing percentage tapers even though each cross-sectional diameter gets larger over length. If you build a file with a fixed taper over length, it will be much larger, more stiff and less flexible --- plus it will indiscriminately continue to enlarge the coronal two-thirds of the canal that has already been optimally prepared. In other words, the smaller the percentage taper, the more flexible the file -- As an example a 20/.04 file is more flexible than the 20/.06 or 20/.08 files.

Why do the finishing instruments have different lengths in their active parts? As I could see, F1 has 17mm, the F2, 16mm and F3, 15mm. Are these lengths correct? What is the purpose?

The Finishing instruments have 14mm of active blades. As the machining wheel begins to disengage from the file, you will note another 1 or 2 mm of "apparent" blades because of the machining process. The F1, F2 and F3 have fixed tapers of 7, 8 and 9%, respectively, between D0-D3 then decreasing percentage tapers between D4-D14. From a clinical standpoint, when used as directed, the Finishers only work towards their terminal extents and will not engage or further enlarge the coronal one-half of a canal. So, the more shank-side blades do not serve to cut dentin; rather auger debris, and the total number of flutes is irrelevant. Additionally, ProTaper files are the only files in the world that do not have a fixed or uniform taper over the length of their blades. ProTaper Shapers and Finishers have multiple different tapers over their blades to increase efficiency, safety and flexibility, and this unique machining process will, at times, create a different number of flutes. Don't worry!

Which apex locator do you recommend?

The main thing to look for when choosing an electronic apex locator is whether or not it can reliably work inside canals with different solutions or exudates. The Root ZX (J. Morita), Endex (Osada) and AFA (SybronEndo) will function properly in all fluids and are extremely accurate and reliable. I personally prefer and use the Root ZX.

What suggestions do you have for keeping the apical opening smaller?

Regarding keeping the foramen as small as practical: Indeed, it is so easy to needlessly over prepare the foramen.

My suggestions are:

  1. Pre-enlarge the coronal two-thirds first to make a more direct path to the foramen.
  2. Attain working length (WL) after pre-enlargement so when you record WL, it will be less likely to change.
  3. Frequently monitor WL by using an electronic apex locator before and during apical one-third finishing procedures.
  4. Off-angled radiographs give a greater appreciation for root anatomy.
  5. Utilize paper points to measure the consistent drying point.
  6. Use rotary files a little short. The "RT" is a position already minutely beyond the foramen, then as we pre-enlarge, the length of the canal progressively shortens depending on original curvature.
When finishing the apical one-third with hand files, can the same results be obtained with either the #20-#60 hand files stepped back 1/2 mm or the ProTapers finishing files used as hand files? If the same result can be obtained, which do you prefer?

Yes, a similar tapered preparation may be achieved with either of these techniques. The F1, F2 and F3 ProTaper finishing files have tapers in their terminal extents of 7%, 8% and 9%, respectively. However, research has shown that any given ProTaper finishing file creates a little bigger space than its own percentage taper. For example, after using the F1, F2 or F3, the actual shape is typically about 8, 9 or 10%, respectively. The finished tapered preparation of a canal utilizing the 20-60 files stepped back 1/2 mm represents a 10% tapered shape mathematically. Studies have shown that tapered shapes of at least 6% in conjunction with a sufficient volume of irrigant over an adequate interval of time can efficaciously circulate and clean into the canal anatomy. Therefore, either of the above techniques serves to provide the "deep shape" necessary.

With regard to my personal preference, I consider both techniques equally useful. I am quite comfortable using hand files (20-60) as that used to be all that existed. However, with the advent of ProTaper and the ability to convert rotary files into hand files, we now have an instrument that has a taper which can create a predefined shape. This eliminates the need to systematically step back each file, as is the case with 0.02 tapered 20-60 files. In summary, I use and advocate both techniques -- The sequence you should select first is purely dependent on which one you feel more comfortable with.

When confronted with an "irregular glide path" you mention stepping back the #20-#60 hand files 1/2 mm, or utilizing a ProTaper manual file, to finalize deep shape rather than using rotary. If the canal appears to have, for instance, an apical diameter of #30, would the step-back sequence then be #30-#70?

Yes and no. In general, it is not necessary to go above a 0.02 tapered size 60 file when stepping back. In your example, the sequence of files I typically use to gauge, tune and finish the apical one-third would be 30-60; however, it's not wrong to utilize a size 70 file if it is used carefully. On a side note, in more open systems with an irregular glide path, after using a few larger sized hand files, the glide path smoothes, refines and oftentimes allows for the safe use of larger sized rotary files.

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