Barry Musikant
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TERMS OF what is generally considered the state of the art of endodontic
shaping, NiTi instruments used in a reduction-gear rotary engine are presently
at the head of the class. Their use has certainly improved the results
compared to what was routinely attained with the traditional use of stainless
steel files. Not only are the shapes more conducive to a better fill, but
the shapes are gained without hand fatigue, a major plus when one compares
the effort that went into shaping curved canals with hand files.
Rotary NiTi produced a quantum leap in quality while
reducing the effort needed to attain that quality. One could say that the
advocates of rotary NiTi actually popularized a standard for superior endodontics
that, heretofore, was only attainable by a few highly skilled endodontists
taking an excessively long time to produce.
Rotary NiTi works because NiTi instruments
have much greater flexibility than stainless steel files. It also works
because the configuration of the NiTi files is really not that of files
at all, but the configuration of reamers. The flutes on a NiTi file have
the more vertical orientation one sees on k-reamers. This makes sense because
the more horizontal flutes on a k-file are very inefficient when used with
a rotary motion. Horizontal flutes tend to groove the dentin rather than
remove it while also inefficiently planing the walls. It is still a mystery
that all the rotary NiTi files have the configuration of reamers while
the hand instruments used to establish the critical glidepath have the
configuration of files. If the former is such a plus, which it is, why
not use hand reamers initially?
A secondary problem arises because of the efficient
design of the NiTi files (which are really reamers). Attached to a rotary
engine, these files have the ability to aggressively engage the length
of the canal system. However, apical engagement potentially leads to torsional
stress, a factor that NiTi is highly vulnerable to. Those who develop rotary
NiTi techniques are well aware of this weakness and have established crown-down
techniques that minimize the development of torsional stress. NiTi is also
vulnerable to cyclic fatigue, defined as repetitive cycles of compression
and tension to the shank of the NiTi instrument as it rotates around a
curve. Excessive cyclic fatigue leads to separated instruments even if
torsional stress is completely eliminated, which it never is.
Minimizing cyclic fatigue for any given NiTi instrument
is only possible by their frequent replacement with new instruments. Over
the past two decades, the factors that lead to increased separation of
NiTi include a direct relationship to the degree of curvature encountered,
the abruptness of the curve, the thickness and taper of the NiTi instrument,
and the speed of rotation.
Eliminating the sources of rotary NiTi separation
has become the paramount goal of the advocates and manufacturers of these
systems. This goal is so dominant that it now supersedes the biologic needs
of the teeth being instrumented. Where 20 years ago it was recommended
to shape canals to a minimum of 35 and a taper of .08 or higher, as Dr.
Schilder originally recommended to meet the biologic needs for cleansing
the canal, today a mesio-buccal canal will most likely be shaped to a 20
or 25 with either an .04 or .06 taper, not because the shaping is adequate,
but because shaping to a smaller apical diameter with a lesser tapered
instrument produces a lower incidence of separated instruments. Tables
exist that clearly show the average width of canals 1 mm, 2 mm, and 5 mm
from the apex in the canals of all teeth. The mesio-distal width of a mesio-buccal
canal of a first molar is over .40 mm. at the 1 mm level making a 20 or
25 preparation inadequate. Preparations to this level may look good on
x-ray when they are obturated with a radiopaque material, but the walls
surrounding that fill have not been properly cleansed according to the
data established for the original dimensions of the canal before instrumentation.
In short, fills of this sort are reminiscent of silver point fills two
generations back. In their day, they looked good on x-ray, but they often
did not do the job.
Rotary NiTi is not only flexible to
a far greater degree than stainless steel, but also has shape memory. It
seeks to regain its straight-line configuration even after taking an appreciable
curve. It is highly resistant to plastic deformation and when finally deformed
NiTi instruments are far more prone to separation when exposed to torsional
stress and cyclic fatigue. While flexibility is an excellent quality in
an endodontic instrument, shape memory is not. Shape memory in instruments
of increasing thickness and diameter forces these instruments to work more
and more selectively on the outer walls of teeth, increasing the chances
of canal distortion in the apical third and elliptical shaping at the apex.
The distorting effects of shape memory along the walls of the canal along
with the fear of separation due to torsional stress and cyclic fatigue
have defined the more limited use of rotary NiTi in curved canals as the
effects of their properties have become better understood.
As we see, rotating NiTi exaggerates the weaknesses
of this metal. If a 45-degree horizontal reciprocating motion were substituted
for rotation, torsional stress and cyclic fatigue would be virtually eliminated.
However, shape memory would still be a problem.
On the other hand, the use of the reciprocating
engine offers us the chance to reexamine the increased use of stainless
steel. Unlike NiTi, stainless steel does not have significant shape memory.
In other words, it can be pre-bent to more readily adapt to the canal being
shaped. If a pre-bent stainless steel instrument were to be placed into
a rotary handpiece, the increased stiffness of the stainless steel would
tend to distort the apical preparation. However, when placed into a 45-degree
reciprocating handpiece, the pre-bent instruments scribe an arc of 1/8
of a full rotation, not enough to produce apical distortion because the
motion does not extend beyond the canal curvature that the instrument will
shape and widen. Therefore, the stiffness, a negative quality of the stainless
steel instruments, is compensated for by the use of the reciprocating handpiece
and their having been pre-bent.
Reciprocation is far closer to the balanced
force technique of canal instrumentation that has been recommended as a
way of keeping the instruments centered within the canal. In fact, it is
safer than the balanced force technique because the back stroke with the
reciprocating handpiece mills the dentin away rather than cleaving it,
mechanics that induce far less stress in the instruments. The reciprocating
engine substitutes a far larger number of reciprocating cycles with low
amplitude rather than having very few cycles with very high amplitude that
occur with balanced force. The end result is the same, only accomplished
much more safely with the reciprocating handpiece.
We hope we have established the fact that
stainless steel when used with the reciprocating handpiece has far more
versatility than rotary NiTi. The only challenge left is to design a reamer
that has the most advantageous architecture to be efficient and safe. We
already know that a reamer engages the dentin far less than a file and
the vertically oriented flute design of a reamer is more efficient than
the horizontally oriented flutes on a file when the motion is either rotation
or reciprocation which is nothing more than a series of short rotations.
The reamer can be designed to have even less engagement by placing a flat
along the entire length of the shank, which reduces the engagement by an
additional third. The incorporation of a flat creates two columns of chisels
with one cutting in the clockwise direction and the other in the counterclockwise
direction, making the instrument ideally designed for the reciprocating
handpiece. The overall lack of resistance creates superb tactile perception
either when used manually or in the reciprocating handpiece. And please
be aware that these instruments, called the SafeSiders®, are designed
to be used either manually or in the reciprocating handpiece.
The reduced engagement along the entire length of
the instrument’s shank affords the ability to have a cutting tip that will
pierce tissue rather than impacting it apically. Apical resistance will
let the dentist know if a sharp bend is being contacted, and the ability
to pre-bend them, negotiate to the apex with the pre-bent instrument, and
then attach to the reciprocating handpiece to produce effective apical
shaping simply, safely, and efficiently, gives the dentist a tool that
had not existed up until this time.
Because they are relieved, they are also more
flexible than non-relieved reamers. They are also more flexible because
the reamer design means that they have fewer flutes, which means they are
less work-hardened and even less prone than stainless steel files to distort
and fracture.
This discussion has attempted to bring some
deeper understanding to the interconnection between the design of instruments,
the metals they are made of, and the forms of delivery that are used to
make them function. It is our opinion that the SafeSiders approach used
with the EndoExpress® reciprocating handpiece is the most effectively
designed system, bringing both efficiency and safety to the highest levels
yet attainable.
September-October 2005
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This discussion
attempts to bring some deeper understanding to the interconnection between
the design of instruments, the metals they are made of, and the forms of
delivery that are used to make them function.
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