Jay Vuong
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THIS PRESUMABLY modern era of endodontics, many more dentists are experimenting
with the newer nickel titanium instruments only to encounter their limitations.
Because of the alloy’s flexibility, nickel titanium shaping instruments
can be sized with larger and varying degrees of taper. These
increased tapers and computer-aided flute designs have helped impart a
more uniform and predictable shape to the canal space, especially in the
apical half. Depending on the usage, the shape defined or refined
by existing NiTi instruments may be used to enhance the effectiveness of
cleaning and the ease of obturation through standardization.
With all of the positive attributes of nickel
titanium, why do many endodontists, including me, still rely heavily on
the traditional stainless steel instruments? Besides the substantial
increase in cost, one reason lies in the physical property of the nickel
titanium metal itself. Stated plainly, nickel titanium, although
flexible, has a tendency to fracture when strained, especially under the
torsional strain that occurs when instruments rotate in the confines of
tight, curved, and long canals. Predicting the likelihood that an
instrument will fracture is difficult. True, using newer instruments
can reduce separation rates. However, with such an increase in operational
cost, older instruments tend to “appear” usable unless we remind
ourselves of their age through the tedious process of marking the
instruments according to their number of uses and factoring in the additional
wear imparted with prior uses in difficult canals. Even with all
these precautions, NiTi instruments can still separate without any prior
visual evidence of distortion. Once separated in the canal, NiTi
instruments are often difficult to remove or bypass. The flex
of the metal makes them difficult to unwind out of the canal, especially
around a curve. Their flexibility also allows them to absorb the
energy of ultrasonics without dislodging their threads from the dentinal
walls. Rather, under ultrasonic vibration, the metal has a tendency
to chip away. The silver lining to this difficult situation
may be that the NiTi instrument often fractures “at the apex” when binding
is usually at its maximum, and that its radiographic opacity matches well,
if not inconspicuously, with adjacent gutta-percha. The film may
look good, but I still feel a little uneasy, especially in infected cases.
Another main reason why I still rely on stainless
steel instruments is that although NiTi instruments are good for shaping
once a pathway in the canal is established, they are not predictable penetrating
and gauging instruments. Using NiTi instruments in a rotary fashion
will only allow them to stay centered and penetrate the canal by screwing
their way into an existing pathway. Although this action should facilitate
the apical movement of these instruments in a crown-down fashion, there
are times when the existing path in the canal is irregular in anatomy.
This irregularity is a precursor to the instrument’s binding and separation.
The centering effect of these instrument predisposes them to remove dentin
indiscriminately on the furcation or depression side of the root as well
as dentin in the root’s thicker and safer zones. Unless a straightening,
anticurvature mechanism is used at the coronal level prior to deep NiTi
introduction, the situation becomes predisposed to strip perforations,
especially in curved, thin, and long canals. An operator’s tendency to
abandon the very important endodontic doctrine of straight-line access
becomes a compromising habit when one overestimates the abilities of nickel
titanium.
Through trial and error, especially
with rotary instrumentation, most endodontists still feel the need to explore,
measure, and establish the canal using traditional stainless steel instruments.
This exploration, measurement, and establishment of the main canal space
with stainless steel instruments is especially important with cases that
present with unusual pulpal anatomy or prior mishaps, such as blockage,
ledges, strip perforation, and apical distortions. These are the
very cases that require thoughtful manipulation, usually requiring
the tactile sensitivity of stainless steel instruments.
NiTi instruments can therefore be viewed
as “dumb” instruments mainly having the ability to ream the walls
of the canal. Their “smartness” is the shape that is imparted into their
design, which then can be imparted to the final canal shape.
Their flexibility doesn’t allow the operator to have optimal directional
sensitivity inside the canal space; rather, sensitivity becomes a measurement
of torque or resistance control—the operator becomes more preoccupied with
avoiding instrument fracture than exploring the pulpal anatomy. NiTi’s
amazing elastic memory, the ability to stay straight no matter how you
bend the instrument, is an asset as the instrument threads its way
into a cana; however, it becomes a hindrance if you need to prebend the
instrument or its tip in order to explore or bypass a ledge or blockage.
At larger instrument sizes, this elastic memory translates to a disposition
to strip perforations and apical distortion in straightening the canal
and is easily underestimated by the overconfident operator.
Taking some of these pluses and minuses of
the NiTi instruments as applied to endodontics, I have favored the older,
yet more reliable stainless stain instruments. Rather than using
NiTi instruments as a means to all ends, the recognition that
NiTis are used most effectively as sizing and shape refining instruments
has allowed me to use them more sparingly.
July-August 2001
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With
all of the positive attributes of nickel titanium, why do many endodontists,
including me, still rely heavily on the traditional stainless steel instruments?
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