Barry Musikant
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 ERHAPS
the granddaddy of old concepts is the use of K-files rather than reamers.
Before the advent of stainless steel’s use for endodontic instruments,
endodontic instruments were fabricated from carbon steel, a type of steel
that has little resistance to torsional stresses. Given this severe limitation,
files made sense because they had to be used in a push-pull motion since
any type of rotation would immediately increase torsional stresses, which
would lead to separations. A push-pull motion is most efficient when the
flutes on the shank are horizontally oriented. In short, when push-pull
was the only safe motion, an instrument with the flute configuration of
a file made sense.
However, once the instruments were made from stainless
steel, rotation became feasible and the orientation of the flutes should
have been modified from the horizontal orientation of a file to the more
vertical orientation of a reamer simply because vertically oriented flutes
remove the dentin that the flutes engage more efficiently. Once the flutes
are more vertically oriented, there will be fewer flutes on a reamer than
on a file given equal cutting lengths. The fewer the flutes the less the
engagement, and the less the engagement the less the resistance as these
instruments negotiate to the apex.
An easy way to appreciate the advantages of more
vertically oriented flutes is to think of how a screw holds things together.
Screws have very horizontally oriented flutes which give the screw the
ability to engage maximally while minimizing the material removed as the
screw is embedded into the material. This is the exact opposite of what
is desired when using endodontic instruments, yet is far closer to the
orientation of a file than a reamer.
The lesson to be learned from the above discussion
is that reamers are far more efficient than files and are also safer because
less force need be applied to gain access to the apex. Less force puts
less stress on both the roots and the instruments. The consequences of
less force being applied are a reduction in distortion to the root and
less distortion and separation of the shaping instruments.
The use of ineffective, difficult-to-use files has
ramifications beyond their own weaknesses. When used in traditional techniques,
they performed so poorly that an alternative sytem called rotary NiTi looked
good by comparison. Rotary NiTi is used at the earliest moment because
of the severe limitations of K-files. It is here that a dilemma occurs.
To be used with relative safety, rotary NiTi requires the creation of a
glide path beforehand; therefore, ineffective traditional techniques could
not be totally eliminated, particularly in curved canals. The difficulty
in creating the initial glide path with the old K-files and then the relative
ease of using rotary NiTi after the glide path is created reinforces the
ineffectiveness of K-files.
The shame of this daily negative reinforcement is
the potential to equate the ineffective shaping ability of stainless steel
files to the far more efficient shaping ability of stainless steel reamers.
The reality is that reamers engage the tooth about 50 percent less than
files and their vertically oriented flutes remove far more dentin when
rotation or reciprocation is used. Relieving the reamers along their entire
length creates a continuous flat that reduces the engagement of this modified
reamer to about one-third that of a file. The flat also creates two vertical
columns of chisels that remove the dentin in both the clockwise and counterclockwise
motions employed in either manual wristwatch motion or a reciprocating
handpiece. A relieved reamer is also more flexible than a comparably sized
file because the greater number of flutes in a file work-hardens the shaft
more than a reamer, and the relieved reamer has a thinner cross-sectional
area, making the shaft of the reamer that much more flexible.
Because the use of K-files has been so central a
part of the teaching of traditional techniques, most dentists are not aware
of the advantages first of a regular reamer and the subsequent improvements
made by a relieved reamer which are then optimized with the use of the
reciprocating handpiece.
A more modern, but still flawed concept is the unlimited
potential of super-elastic NiTi. There is no question that given equal
cross-sectional areas, NiTi is about one-third the stiffness of stainless
steel. It is this flexibility that has changed endodontics dramatically
in the past 20 years. There are limitations to super elasticity, however.
If a rotary NiTi instrument must negotiate a coronal curve and an apical
curve on the way to the apex, most often the apical third of the canal
cannot be enlarged to more than a 25/06 taper before the outer wall of
the canal will start to be selectively shaped at the expense of the rest
of the canal. This is an example of transportation, one of many forms of
distortions that canals can undergo when being instrumented. The limitations
of rotary NiTi are a result of another property that NiTi possesses, namely
shape memory. As a result of its shape memory, the NiTi shank wants to
spring back to its original shape after being bent. If it cannot, it will
certainly apply more pressure against the outer wall of a curve, and this
pressure will increase the further apically the instrument is. Because
of the limitations imposed by shape memory, NiTi cannot be used with a
rotary engine in curved canals much beyond a 25/06.
Stainless steel, though stiffer, does not have the
property of shape memory. It can be pre-bent easily and retain that shape.
In fact, a thinner stainless steel reamer will often record the multiplanar
curves of a canal after it has been removed from the canal. This information
can be duplicated manually in instruments and then these instruments can
be negotiated to the apex through a combination of manual and engine-driven
reciprocation. The reciprocating handpiece, unlike a rotary handpiece,
limits the arc of motion to 30 degrees—an arc so limited that the envelope
of motion is not enough to distort the canals. Because of this limited
motion, pre-bent instruments with apical sizes far greater than 25 can
be negotiated to the apex without the concern for apical distortion that
would be part and parcel of rotary NiTi shaping in curved canals. An additional
advantage to a 30-degree reciprocating motion is that NiTi instruments
can also be pre-bent (although not as easily as stainless steel instruments
can be) and then used to shape the canal without further distortion. The
main advantage of NiTi is that within its limits it can shape curved canals
rapidly with minimal distortion. However, reciprocation can accomplish
exactly the same thing with the added advantage of the ability to pre-bend
and with the elimination of both torsional stress and cyclic fatigue, the
two factors that are most responsible for instrument separation when rotary
NiTi is used.
Rotary NiTi further hurts itself by substituting
a crown-down preparation as its first step after the creation of the glide
path. NiTi orifice openers will tend to stay centered, removing equal amounts
of tooth structure from both the inner and outer walls of the canal, widening
the canal while retaining the coronal curve. The presence of a coronal
curve is a factor in instrument separation as the NiTi instruments become
thicker and their tapers greater. In short, separation occurs coronally
because adequate access has not been achieved. This problem is compounded
because inadequate coronal access will place more stress on the apical
ends of rotary instruments as they attempt to negotiate the glide path.
The apical end of the glide path becomes less effective as the coronal
curve increases. Given inadequate coronal access, the instrument may mainfest
excess stress apically, leading to separation because NiTi instruments
do not follow the glide path apically as well when they become thicker.
Rotary NiTi has led to a cascading of events that
are unavoidable as long as these instruments are used the way they are.
Because they are vulnerable to separation from excess torsional stress
and cyclic fatigue, it is recommended that they be used only once, making
these systems very expensive. Minimizing torsional stress requires the
additional expense of autoreversing reduction gear electric motors. In
the more advanced models each instrument has its own specific torque. Yet
all the specificity built into these motors does little to help these instruments
shape curved canals with predictable safety. This is most obvious in the
recommendations of many advocates of these systems, which clearly state
that these rotary Niti instruments should not be used unless a glide path
is achieved through the entire length of the canal. There are also a number
of conditions under which the creation of a glide path is not sufficient
to allow the safe use of rotary NiTi. These include highly curved canals,
canals that join, and canals that end in s-shaped curves.
The new concepts that in my opinion should replace
some currently held beliefs include:
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Stainless steel reamers are superior to stainless steel files and relieved
stainless steel reamers are far superior to non-relieved reamers.
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The No. 2 Peeso is an excellent instrument to eliminate the coronal curve,
which in turn makes the rest of the instrumentation that much less challenging.
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Thirty-degree reciprocation allows relieved reamers (both stainless and
NiTi) to be pre-bent regardless of the diameter and taper of the instrument,
producing adequate apical preparations both mesio-distally and bucco-lingually
throughout the length of the canal.
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Reciprocation virtually eliminates torsional stress and cyclic fatigue,
effectively removing the two most important factors in instrument separation.
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The No. 2 Gates Glidden is an excellent instrument to shape the canals
to .65 mm within 3 mm of the apex.
We believe in these principles so strongly that we teach
free hands-on workshops so that you can determine for yourselves whether
these concepts have merit. Click here
for information about the next workshop.
January-March 2006
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FIGURE 1: Showing the horizontal
flute configuration of a file.
FIGURE 2: Showing the vertical
flute orientation of a reamer.
FIGURE 3: A wood screw’s
very horizontally oriented flutes make it similar to a file.
FIGURE 4: A relieved reamer,
showing the continuous flat that reduces engagement.
FIGURE 5: Photo of the flat
on the relieved reamer, creating two vertical columns of chisels.
FIGURE 6: Illustration demonstrating
NiTi’s shape memory.
FIGURE 7: Photo of stainless
steel instrument, which lacks shape memory.
FIGURE 8: Apical distortion
of rotary NiTi shaping in curved canals.
FIGURE 9: Showing the excess
torsional stress and cyclic fatigue of rotary NiTi instruments.
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