Barry Musikant
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RECENT ARTICLE in the August 2006 issue of the JOE, “Removal of
Separated Files from Root Canals with a New File-removal System: Case Reports,”
authored by Yoshitsugu Terauchi et al, caught my attention, not because
of the introduction of a new retrieval method, but because of the discussion
of the causes of instrument separation as well as their effects on the
success rates that follow. The authors state that
“various factors have been associated with the fracture of
NiTi rotary instruments including operator experience, rotational speed,
canal curvature, instrument design and technique, torque, manufacturing
process, and absence of glide path. It has been noted that these
NiTi instruments frequently fracture in narrow, curved root canals.
The instruments usually separate by two different mechanisms: torsional
fatigue or bending fatigue. Torsional fatigue occurs when the instrument
binds against the canal walls and is usually associated with excessive
apical force applied during instrumentation. Bending fatigue is caused
by continuous stress applied to an instrument that is already weakened
by metal fatigue and breakage occurs when it reaches its point of maximum
flexure, where the stress is the greatest, and this is often seen in curved
canals.
“When an endodontic instrument fractures during
root canal treatment, this immediately hinders the clinician from thoroughly
cleaning and shaping the canal system, and thus compromises the outcome
of the treatment. In most cases it is difficult to determine the
true extent of how well the canal is disinfected when the instrument separates,
especially if short of working length and, therefore, it is important
to be able to bypass or retrieve the separated instruments without further
damage to the tooth structure. It has been shown that attempts at
removal of these files usually result in the removal of a large amount
of root dentin which ends up reducing the root strength by 30 to 40%.”
From this article, it would be difficult not to conclude
that separated instruments are a highly undesirable occurrence, one that
is best avoided. Yet, this article and many others emphasize ways
to overcome the damage that the weaknesses in these rotary systems produce
or place the blame for separation on the inexperience of the clinician.
Both of these approaches derive from the postulate that the rotary system
is unassailable and above reproach. I find this attitude entirely
counterproductive, placing fault on the wrong parties and misdirecting
energies from efforts where they could more productively be placed both
for the benefit of patients and the dentists doing the work.
Let’s start with a different premise.
We want an endodontic shaping system in which the instruments are not vulnerable
to separation no matter what the shape of the canal is. We know that
the main causes of separation with rotary NiTi are excessive torsional
stress and cyclic fatigue, which are produced by a series of full 360-degree
rotations (Figure 1). In a curved canal, after every 180 degrees
of rotation the side of the rotary NiTi instrument that was in compression
is now in tension and vice versa. The result of this repeated cycle
of compression and tension is the equivalent of bending a metal coat hanger
back and forth until it breaks. Why not eliminate the compressive
and tensile forces leading to bending fatigue failure by eliminating rotation?
Once rotation is replaced by 30-degree reciprocation, which is limited
to one twelfth of a full rotation or 5 minutes on the face of a clock (Figure
2), there is no longer any significant bending fatigue that can occur.
Torsional stress is also virtually eliminated because 30 degrees of clockwise
rotation is immediately followed by 30 degrees of counterclockwise rotation,
preventing the buildup of torsional stress. In short, the small envelope
of motion provided by a 30-degree reciprocating handpiece prevents the
binding of any endodontic instrument.
From a practical point of view, as soon as
the clinician switches from a rotary system to a reciprocating one, he
or she no longer needs to worry about the separations that occurred from
the potentially excessive stresses produced by torsional stresses and cyclic
fatigue because they no longer occur. The factors that contributed
to torsional stress and cyclic fatigue no longer are problems associated
with separated instruments. These factors include curved canals of
all degrees, narrow canals, bifurcating canals, or joining canals.
That is not to say that curved canals are not a challenge to shape adequately,
only that the process of shaping them no longer entails the need to focus
on the avoidance of separation as a side issue. Full attention can
now be given to focusing on the canals themselves rather than the potential
consequences of the canals.
In the courses we teach, we help dentists to
pay attention to the shape of the initial instruments as they come out
of the canal. A thin stainless steel reamer will record the location,
degree, and orientation of a curved canal. Stainless steel can easily
be prebent to conform to any canal shape that may exist. As long
as the prebent reamer is correctly oriented within the canal, the 30-degree
reciprocating engine will efficiently drive the reamer to the apex without
distortion due to the narrow envelope of motion.
This is a far more intelligent system than
one prone to separation and one in which full rotations require at a minimum
the use of weaker and far more expensive NiTi used in the way they are
most vulnerable. The superior intelligence of the SafeSiders®
system used in the reciprocating handpiece is evident to the user, who
no longer worries about a set of variables that affected him when he formerly
used rotary NiTi.
The intelligence of the SafeSiders system lies
both in the reciprocating motion and the relieved design of the reamers
that replace the K-files of old. In the same way that rotation produces
undesirable metallurgical stresses, a K-file engages dentin without efficiently
removing it. The SafeSiders replace inefficient K-files with far
more efficient relieved K-reamers. Less engagement means less resistance,
which means negotiating to the apex more easily. Fewer flutes in
the 16 mm of working length on the relieved K-reamer means more vertically
oriented flutes, which work best where the motion is reciprocating.
The relief along its entire working length makes the relieved K-reamer
far more flexible than a K-file. The fewer flutes along its length
also means the relieved K-reamer has been less work-hardened, further enhancing
its flexibility. In addition, we have developed a new tapered Peeso
called the Pleezer which straightens the coronal curves of all canals more
easily and with less resistance than the formerly used No. 2 Peeso.
Once the coronal curve is straightened, the majority of curved canals are
simplified to a j-shape as opposed to a c-shape, making subsequent enlargement
far less challenging. So effective is coronal straightening that
the resulting apical tactile perception becomes so keen that a cutting
tip that can pierce tissue rather than impact it is provided on the SafeSiders.
Please appreciate the fact that 30-degree reciprocation allows even an
instrument with a cutting tip to recognize when a blockage has occurred
and further prebending may be necessary.
The several innovations that constitute the
SafeSiders/EndoExpress technique for canal cleansing and shaping provide
the dentist with a safe, effective, and economical way of doing excellent
endodontics. One of the hardest concepts to digest is the fact that
not only do the SafeSiders provide a superior way to do endodontics, but
the cost of the system on an ongoing basis is about 90 percent less than
the cost of rotary NiTi. Yet this cost reduction is entirely in line
with an intelligent system. When a whole set of complexities is removed,
the cost of dealing with those complexities is also removed, and a system
devoid of those complexities not only becomes elegant and simple, but often
much less costly.
September - October 2006
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FIGURE 1: Full 360-degree
rotations produce excessive torsional stress and cyclic fatigue.
FIGURE 2: Thirty-degree reciprocation
is limited to one twelfth of a full rotation.
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