Designing endodontic instruments for success

The ideal is rarely met, but the correct design and utilization of the instruments will go a long way in approaching that ideal. The present state of design and utilization of the instruments that are now being marketed by the major manufacturers and advocated by their well-paid lecturers do not come close to the ideal, as will be clearly pointed out in this article.

Let’s start with the primary goal, gaining initial patency of the canals from the orifice to the apex. The vast majority of dentists will use K-files to start shaping the canal. If they do not use NiTi either in rotation or asymmetric reciprocation, then they will use K-files for the entire procedure. If they do use NiTi in one form or another, they will most likely shape the canal to a minimum of 20 prior to incorporating NiTi. In either case, the K-files are poorly designed to initially traverse the small patent pathways that may be present and to then enlarge them in a sequential manner.

Why do we say poor design? K-files have 30 predominantly horizontal flutes along their 16 mm of working length. If used with a watch-winding motion, these horizontally oriented flutes will engage and disengage the dentinal walls without actually removing any dentin until the pull stroke is applied. It takes two strokes to shave dentin away when using K-files — the twist that supplies the engagement, and the pull that shaves away what has been engaged.

With the flutes engaging dentin along its length, the K-files produce a poor tactile perception of exactly what the tip of the instrument is encountering. If unaware that the tip of the instrument is encountering a solid wall, it is all to easy to ledge at this point, creating a manmade defect that may be a great obstacle to further accurate apical negotiation.

When the pull stroke is applied to shave dentin away in a curved canal, there is a marked tendency to selectively shape to the outer wall. When the instrument is repeatedly inserted into the canal, these same horizontally oriented flutes tend to impact debris apically, creating blockages and loss of length.

Recognizing the loss of length encourages the dentist to twist the K-file into the canal to gain greater depth and then pull up to shave what has been engaged. Done repeatedly in a canal that has been unknowingly blocked apically leads to canal transportation and, if carried further, to frank perforation. At a minimum, the original canal anatomy has been lost, something NiTi cannot compensate for even if used after the glide path has been established.

Many dentists have encountered the problems described above. Yet, they have been taught that such problems are a result of their inadequate use of the instruments, that expertise in their proper usage would avoid such problems. While this statement has some merit, it avoids the more important possibility that superior design and utilization might produce more predictable desirable results in a simpler manner.

Such would be the case if the K-file with its highly compacted horizontal flutes were replaced with reamers, instruments with half the number of flutes along the same 16 mm of working length and a flute orientation that is twice as vertical. If such an instrument is used with the same watch-winding stroke, the blades will immediately shave dentin away from the canal walls rather than first engaging them.

The pull stroke is not required to shave dentin away from the wall, and in fact the vertically oriented flutes would do a poor job of it if it were required. Unlike the K-files, pulling up on the K-reamers will not selectively remove dentin from the canal outer walls.

The reamers produce a superior tactile perception of what the tip of the instrument is encountering, because there is significantly less engagement along length and what engagement is present is being removed with the first clockwise motion of the instrument. Knowing when the instrument is encountering a solid wall directs the dentist to remove the instrument, bend it at the tip and manually negotiate around the impediment.

The reamers supply the vital information for the dentist to do what is necessary for non-distorted shaping in curved calcified canals. The K-files do not. While the advantages of reamers are based on a combination of the more effective shaving of dentin and less engagement along length, these advantages are further enhanced by placing a flat along its entire working length, further reducing engagement and creating two vertical columns of chisels that shave dentin away in both the clockwise and counterclockwise motion that enhance the removal of dentin via the vertically oriented flutes. Canals that prove difficult to negotiate with K-files are far less resistant when negotiated with relieved reamers.

These reamers may also be employed in a 30-degree reciprocating handpiece that mimics a manual instrument in motion, but at a much higher frequency. The result is rapid and accurate instrumentation of the canals.

One might think that the above description of the relieved reamers compared to K-files would lead to their substitution as the choice of instrumentation for creation of the glide path, and indeed for some dentists this is exactly the case. Yet, so effective are the relieved reamers — whether used manually, in the 30-degree handpiece or some combination of both — that there is no reason to stop their usage at a 20 or 25. They may be used to create the entire canal space, eliminating any use of NiTi in either rotation or asymmetric reciprocation.

By eliminating the need for NiTi, several advantages are gained. An instrument limited to a tight watch-winding stroke or a 30-degree arc of motion generated by the handpiece completely eliminates the two factors most responsible for instrument separation, torsional stress and cyclic fatigue. As a result, separated instruments are no longer a concern, even when shaping the most tortuous of canals.

With separation anxiety removed, the dentist is far more confident in aggressively applying the instrument against all the walls of canals, be they round or oval. Buccal and lingual tissue extensions in oval canals have been shown to be often untouched when using rotary or asymmetric reciprocating NiTi, leaving tissue behind that can adversely affect the success of the procedure. This is less likely to be the case when using relieved reamers, because they will be used aggressively against all the walls of the canal and can safely be used to instrument the apical preparations of canals to a minimum of 35 in most situations.

A preparation of 35 is not arbitrary. Research has clearly shown that 35 is the minimum preparation required for effective irrigation, a step that removes any tissue remnants chemically that have not been removed mechanically while more effectively removing the smear layer and opening up the dentinal tubules, a step vital for the deeper penetration of bacteriocidal cements into the dentinal tubules where bacteria may still reside.

With the data clearly demonstrating the shortcomings of NiTi in the shaping of oval canals, that in itself might be enough to seek out the better ways our alternative techniques suggest. However, recent research over the past three years has now implicated NiTi used in great arcs of motion in the production of micro-fractures along the length of the canal. Separate studies have shown that micro-fractures can coalesce and propagate to the point of inducing vertical root fractures.

These micro-fractures are not infrequent occurrences. Depending upon the rotary system used, they will induce micro-fractures anywhere from 25 to 60 percent of the time they are employed. This same research has shown that short arcs of motion and hand instrumentation do not induce micro-fractures. Along with the fact that short arcs of motion — be they manual or engine-generated — do not produce instrument separation, we can make the overall observation that the safety of both the instruments and the canals that they are shaping will increase wherever a manual watch-winding motion and/or 30-degree reciprocation is substituted for full rotation or asymmetric reciprocation.

What we have attempted to broach is the misplaced sanctity of K-files as the given initial instrument in canal shaping. It is illogical and not supported by common sense and simple mechanical principles. Rotary NiTi, initially thought of as the answer to the deficiencies of K-files, has been proven to produce more problems than they solve. Under any circumstances there are alternatives to rotary NiTi that allow for safer but more aggressive instrumentation while eliminating instrument separation, which in turn removes the need for rapid replacement.

Multiple use of relieved reamers is the norm and saves dramatic amounts of money compared to the single usage of NiTi instruments that are an absolute requirement so strongly advocated by the major manufacturers of these products.

When instruments are designed and utilized correctly, you will find that the more you learn how to use them, the more they will be used. This is just the opposite of NITi, where the more you learn how to use them, the more selectively they are used. Just to make things absolutely crystal clear, more selective use means less. And reduced usage as a result of greater experience in their use is proof positive they were not designed correctly from the onset.

Some things to think about.

Note: This article appeared in Endo Tribune U.E. Edition, Vol. 7, No. 9, October 2012 issue.