Tactile perception in endodontics

Is it encountering an impediment such as a solid wall or is it lodged in a tight canal? Is the tip of the instrument entering a round or oval canal?

Superior tactile perception is a direct result of the instruments design and how it is used. A reasonable analytic task is to determine what endodontic instrument designs and techniques enhance tactile perception. For sure, the information conveyed from the tip of the instrument will become increasingly clear as the engagement along length is reduced. If there is a great deal of engagement along length, exactly what the tip of the instrument is encountering becomes murky.

In that light the typical K-file design consisting of 30 horizontally oriented flutes along length (Fig. 1) will engage the walls of the canal significantly more than a reamer with 16 more vertically oriented flutes (Fig. 2).

If both the reamer and the file are made from a square wire, the reamer with 16 flutes will have a total of 64 contact points while the file with 30 flutes will have 120 contact points. The greater the number of contact points the greater the engagement and the increase in resistance to apical negotiation. Increasing resistance along length reduces the tactile perception at the tip.

The more horizontal orientation of the flutes engage the dentin rather than cut it when used with the recommended watch-winding motion. The file design is similar to that of a screw and, like a screw, tactile perception at the tip is secondary to engagement along length. While the goal of a screw is engagement, that is not the goal of an endodontic-shaping instrument and the more horizontally oriented flutes along the length of a file are counterproductive to the goals the dentist wants to achieve.

Ideal tactile perception tells the dentist when a solid wall is hit. The dentist differentiates this type of engagement from being in a tight canal by the degree of tugback. No immediate tugback means the dentist is hitting a solid wall. Immediate tugback means the dentist is most likely in a tight canal that will allow him to progress to greater depths using either a tight watch-winding motion or via the instrument’s use in the 30-degree reciprocating handpiecce.

I emphasize the word ‘immediate’ because a solid wall continuously being pecked at with an instrument will start to produce tugback simply because the repeated pecks into a solid wall will start to establish its own man-made pathway.

Knowing an impediment has been encountered tells the dentist he must remove the instrument, place a small bend at the tip and attempt to manually negotiate around the impediment. Once around, the dentist leaves the instrument at the newly negotiated depth and reattaches it to the reciprocating handpiece for what is generally smooth and rapid negotiation to the apex.

Fewer more vertically oriented flutes increase tactile perception. Fewer flutes also make the instrument less work-hardened, which in turn makes the instrument more flexible, another feature that enhances tactile perception. Placing a flat along the entire working length further improves tactile perception by further reducing engagement along length while making the instrument even more flexible. Those 64 contact points are now reduced to 48 (Fig. 3).

A cutting tip is an additional feature that improves tactile perception. Unlike a non-cutting tip that has the potential to impact pulp tissue, a cutting tip tends to pierce it. There is no concern about a cutting tip creating its own pathway because the degree of motion is limited to either a tight watch winding stroke or the 30-degree arc generated by the reciprocating handpiece.

Compare this approach to the use of K-files and the subsequent use of rotary NiTi. The K-files are poorly designed to enhance tactile perception because they engage excessive amounts of tooth structure along length. Their horizontally oriented flutes are designed to engage, not cut, and the great number of flutes resulting from twisting the wire more times produces a stiffer instrument incompatible with superior tactile perception.

Rotary NiTi is now used in a crown down fashion where the goal is to determine when excessive resistance is encountered along length, not at the tip. In fact, the tips of these rotary NiTi instruments don’t engage apically until the shaping procedure is almost completed and then rarely exceeds a diameter of apical preparation beyond what was established by the K-files.

Relieved reamers not only supply more accurate information differentiating a solid impediment from a tight canal, but can also differentiate between a round and oval canal. Some advocates of rotary NiTi have gone to great lengths to explain how much the apical end of a canal should be preparted by using such terms as “tuning” and “gaging” where the apical prepartion is determined by the presence of clean dentin filings on the flutes of the rotary instruments.

Tuning is to first see filings. Gauging is to take the diameter up to the point where the filings are clean. If clean filings are present, rotary NiTi advocates take this as clear evidence that the canals have been shaped adequately to assure clean walls circumferentially.

However, two factors make us hesitate in accepting “tuning” and “gauging” as effective and predictable procedures. First the literature clearly demonstrates a high incidence of canals that are oval in their apical anatomy.^1-3

Secondly, a symmetrical instrument, like all rotary NiTi instruments, cannot differentiate between a round and oval canal. Only an asymmetrical instrument, one with a flat along its length, can make that determination (Fig. 4). When a symmetrical instrument produces dentinal filings at the tip of the instrument it may only mean that the filings have been removed from the smaller diameter of an oval canal producing no information about the wider diameter. The literature has reported that the wider portion of an oval canal is at times three to five times that of the smaller diameter.³ Those using rotary NiTi instruments will not know this.

Given the increased vulnerability of rotary NiTi to breakage as the tip size and taper of the instruments increase, the perception of small preparations being adequate for cleansing and irrigational purposes is a comfortable one even though there is much evidence to counter these perceptions. From a practical point of view, the smallest apical preparation that allows for effective irrigation is a 30 with a 35 apical preparation strongly recommended. There are a number of articles that closely correlate the degree of apical preparation with reduced bacterial count and reduced bacterial count is closely associated with higher success rates.^4-5

From the above discussion we can see where superior tactile perception gives us the tools to first differentiate between hitting a solid impediment and negotiating a tight canal. The former situation produces no immediate tugback on the instrument. The latter does. No tugback is our clue to immediately remove the instrument from the canal, prebend it at the tip and seek to manually negotiate around the present impediment.

This differentiation is all important in not deviating from the correct canal path by making our own. The cutting tip of relieved reamers confined to a tight manual watch-winding motion or the 30-degree reciprocating handpiece easily negotiates to the constriction and then 0.5 mm beyond to assure patency throughout the shaping procedure, which in turn keeps the instruments centered, minimizing the chances of canal transportation.⁶ By instrumenting 0.5 mm beyond the constriction through a 25, the canals can be opened to the constriction to a minimum of 35, 40 1 mm back and then overlayed with a 25/06 taper without distortion while assuring a space large enough to be well-irrigated with the NaOCl. Non-distortion is a result of the modified balanced force that is generated when a tight watch-winding motion is employed.

In the same way, the 30-degree reciprocating handpiece mimics this tried and proven manual motion in keeping the tips of the instruments well-centered while negotiating curved canals.

Rotary NiTi is a motion that can never provide new information about what is occurring at the tip of the instrument. When using rotary NiTi any information on the apical anatomy of a canal has first been attained by employing K-files, instruments that are designed and used in ways that as this discussion has attempted to address are incompatible with shaping the canals distortion-free and accurately assessing their apical anatomy, at times erroneously giving the dentist the impression that they are narrower than they may, in fact, be.

When one considers that relieved reamers are used in a way that assures long life, virtually eliminating separation, give the dentist more accurate information to determine how wide the canals should be shaped to, have the flexibility to be used both manually and in the reciprocating handpiece and is supported by a growing body of research that clearly demonstrates superior results are attainable while reducing costs per use by 90 percent compared to rotary NiTi, it is no surprise this alternative approach is garnering more and more enthusiastic attention. Clinical examples are shown in figures 5-7.

For more information regarding this highly effective and safe approach please contact me at my free online forum, www.endomailmessageboard.com.

References are available upon request at s.rendon@dental-tribune.com.

 

About the author

Dr Barry Lee Musikant is a member of the American Dental Association, American Association of Endodontists, Academy of General Dentistry, The Dental Society of NY, First District Dental Society, Academy of Oral Medicine, Alpha Omega Dental Fraternity, and the American Society of Dental Aesthetics. He is also a fellow of the American College of Dentistry (FACD). As a partner in the largest endodontic practice in Manhattan, Musikant’s 35-plus years of practice experience have crafted him into one of the top authorities in endodontics.