Barry Musikant
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THOUGHT that it might be interesting to discuss what it takes to design
a reamer that functions well. The need for such a discussion was
brought home to me by a challenge that I received from an anonymous
poster on the DentalTown.com website who stated that reamers fabricated
from a triangular wire would be more efficient than the SafeSiders®
that are made from a square wire relieved along its length.
Although I thought that I had compared our design to
everything else on the market when we developed the SafeSiders, I did
not specifically remember comparing them to the triangular
configuration. So this past week for about two hours I compared
triangular-based instruments and the SafeSiders. The observations
that I made included these:
- The SafeSiders negotiated through a canal with less
resistance; that is, they negotiated the canal in a shorter time with
less apical pressure applied.
- The greater the curve, the more the SafeSiders configuration surpassed the triangular reamers.
I would have expected those results because a flat along the length of the instrument
- reduces engagement
- creates two vertical blades that cut in the clockwise
and counterclockwise directions, the exact motion produced by the
reciprocating handpiece.
- creates a place for debris to collect
- has the ability to line up with the long diameter of
an oval canal because it is the path of least resistance making the tip
an efficient pathfinder
- permits the incorporation of a cutting tip that can
differentiate between resistance from a tight canal and from hitting a
wall due to an abrupt curve—thanks to the reduced engagement along the
length of the instrument
Although I believe that these reasons are valid,
they do not explain the obviously increased resistance I experienced
with triangular based reamers. We then examined their action in
plastic blocks, and the reason for the reduction in efficiency became
somewhat more obvious. Flexibility is good, but only up to a
point. If flexibility becomes too great, it means that there is
little resistance along the length of the shank. When a canal is
tight, the first part of the reamer that engages within the canal is
going to be the part that is located most apically. The more flexible
an instrument is, the more easily it will bend, but it will also
undergo torsional distortion with less resistance. In practical
terms, this distortion means that the apical third of a very flexible
reamer is likely to bind apically, and while the bind may not be great,
it is more than the torsional resistance of the shank. In other
words, that portion of the shank coronal to the bound apical third will
continue to move. Fortunately, the reciprocating handpiece only
produces an arc of 30 to 45 degrees, which is not enough rotational
movement to cause separation of the apically locked instrument.
However, a locked instrument does no apical shaping unless the
instrument is freed up with frequent vertical movements, and the apical
progress is achieved much more incrementally. While the
instrument can be used, it has such minimal internal strength that it
has a narrow window of usefulness.
If one thinks about it, one realizes that even
stainless steel can be taken to the point where it is so weakened that
it starts to suffer from the same vulnerabilities as NiTi does.
Like rotary NiTi, these instruments must be used with smaller amounts
of apical penetration. Stainless steel instruments should not be
used in a lingering fashion, and—just as with rotary NiTi—crown-down
preparations will reduce the chances of exposing these instruments to
excess stress with the potential of separation.
The metal core of a reamer fabricated from square
wire—even with the relieved flat along its entire length—is thicker
than the core of a reamer fabricated from triangular wire. The
ratio of the surface area that an instrument is cutting to the inner
core will always be more favorable in a square-wire reamer than in a
triangular-wire reamer. The circumference of any instrument is pi
(3.14) times the diameter of the instrument, so the surface area in
contact with the walls of the canal is always going to be 3.14 times
the thickness of the instrument used. Given this multiple of
contact, it becomes obvious when making the comparison, that a thicker
inner core is a benefit from two points of view: strength and increased
resistance to torsional stress. The two benefits make the
square-wired reamer more effective. The net result is an
instrument that is stiffer than triangular-wire based reamers, but
still more flexible than standard square-wire based reamers and much
more flexible than non-relieved K-files. This reality—plus the
advantages produced by the flat in terms of reduced resistance and its
increased pathfinding potential—make this instrument an improved
vehicle for gaining access to the apex compared to all other designed
instruments.
The increased flexibility of the SafeSiders allows
the dentist to shape most canals without the need for prebending.
However, because the initial SafeSiders are stainless steel they record
the original curves, thereby telling the dentist the degree,
orientation, and location of the curves. With this information,
the dentist can readily prebend any SafeSiders reamer, manually place
it into any curve, even an abrupt one, negotiate to or close to the
apex depending upon the circumstances, and then reattach the reamer to
the reciprocating handpiece and let its 30–45 degree arc of motion
further shape the canal effortlessly without distortion while virtually
eliminating torsional stress and cyclic fatigue, the two factors
produced by rotary NiTi systems that are most responsible for
separations.
I had not addressed this challenge since the
introduction of the SafeSiders. In fact, to be honest, I
originally mistakenly thought that all K-reamers were triangular in
shape and was surprised when I finally realized that typical reamers
are in fact fabricated from square wire. As happens in life, at
times you get lucky. My oversight actually produced an instrument
that functions better because it is stiffer. Teaming the
SafeSiders with a 30–45 degree reciprocating handpiece achieved the
perfect coupling of two technologies that enhance each other.
What originally appeared to be a counterintuitive observation makes
sense when we think of the flexibility of endodontic instruments not
only in horizontal terms, but also in vertical terms.
As usual, I want any readers of this article to know
that I am still giving free one-on-one 2–3 hour workshops in our New
York City endodontic practice so that the systems we advocate can be
tried. I will make sure that I have some of the triangular-based
reamers on hand so that anyone who would like to take the course with
me could make the comparison firsthand. To set up a free
workshop, call me at 212-582-8161.
July - August 2007
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FIGURE 1: A cross-section of a square-wire-based instrument.
FIGURE 2: A cross-section of a triangular-wire-based instrument.
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