Barry Musikant
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 UCCESSFUL
ENDODONTICS results in teeth that are comfortable under function and regrow
bone where bone loss has previously occurred. Inflammation within
the periodontal ligament is usually the reason endodontically treated teeth
are uncomfortable. Inflammation is incompatible with bone regeneration.
Inadequate debriding, overinstrumentation, and overfilling either initiate
inflammation or do not resolve preexisting inflammation.
Fractured instruments are one cause of inadequate debriding. If
the apical portion of the canal is blocked early on by a fractured file,
irrigating and cleansing apical to the broken instrument will be impossible
unless the dentist can remove or negotiate around the separated segment
(Figure 1). The incidence of fracture is directly related to the
instrument’s resistance to torsional and flexural stress, which in turn
is related to the sequence of instruments used. The greater the
amount of dentin each instrument must cut, the greater stress that instrument
may encounter.
Crown-down rotary NiTi poten-tially subjects the
initial instruments to excessive torsional and flexural stresses.
The torsional stresses increase as the instrument engages and cuts a greater
length of canal. The flexural stresses increase as the instrument deviates
from a straight line (Figures 2 and 3). Torsional and flexural stresses
together create a stress load that is greater than the sum of their individual
loads. NiTi offers far less resistance to these stresses than stainless
steel. NiTi is flexible, but soft. Where stainless steel can
selectively cut into dentin in a directed fashion, NiTi can only mill dentin
away while staying centered. A centered canal preparation is far
more likely to encounter a strip perforation on the distal aspect of a
mesiobuccal root than is a canal prepared by a No. 2 Peeso purposefully
directed away from the furcation and the distal aspect of the mesiobuccal
root.
The sharper and harder the metal, the easier the
task of cleaving dentin from the canal wall during instrumentation.
Stainless steel is approximately four to five times harder than NiTi.
NiTi can cut dentin only in very small increments. If it engages
more than the minimum amount of dentin at any one time, it is unable to
cleave the dentin from the canal walls. Rather, the dentin engages
the flutes of the file or reamer and “sucks” the instrument into the tooth
without cleaving off the dentin, subjecting the instrument to excessive
torsional and flexural stresses.
This potential stress problem is aggravated by the
crown-down sequence of instrumentation. Crown-down implies going
from thicker to thinner instruments as the canal is negotiated apically.
How far apically to go with the initial instruments is not well-defined.
If a .12 tapered rotary NiTi instrument .20 mm at the tip initially negotiates
6 mm into the canal, the diameter of this instrument at the orifice is
0.92 mm (6 x .12 + .20). This width can only be gained gradually
and carefully by using a light touch and a rapid up-and-down motion of
the instrument, making certain never to engage too much dentin at any one
time.
A system that uses a sequence that does not define
a clear-cut method of instrumentation, further limited by its lack of strength
and cutting efficiency, is one that cannot predictably shape the great
variety of canal curvatures, lengths, and widths that will be encountered.
Inadequate instrumentation that results from separated instruments would
logically increase the incidence of inflammation in the periodontal ligament,
leading to success rate lower than that for teeth shaped to the same taper
without instrument separation.
Although the traditional step-back sequence of instrumentation
required modifications to produce a wider tapered canal preparation, step-back
defines the amount of dentin to be removed at any one time much more specifically
than crown-down does. For example, if the canal is already opened
to the apex to a No. 20 reamer or file, the width of the apical 12 mm of
the canal are as shown in Figure 4.
If the canal is sequentially widened from No. 25
through No. 40 using a 1 mm step-back technique, the maximum amount of
dentin removed from instrument to instrument is .03 mm/mm, as shown in
Figures 5 through 8, below. This is less than 1/10 th the incremental
dentin removed with the initial crown-down rotary NiTi instruments (see
Figure 9, below). Furthermore, the instruments doing the cutting
are tough, inexpensive stainless steel, highly resistant to fracture.
The step-back also minimizes hand fatigue and canal distortion because
the instruments go a shorter distance as they become thicker and stiffer.
Optimization of the modified step-back technique
is encapsulated in the EZ-Fill® SafeSider™ Instrumentation System,
a series of predominantly stainless steel instruments that are designed
with a patented non-interrupted continuous flat along the entire cutting
length. The flat further facilitates rapid negotiation to the apex
in the following ways:
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The instrument does not engage the dentin along the flat. Less engagement
means less resistance, which allows for a lighter touch while negotiating
more rapid access to the apex.
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Two vertical blades are created that sweep the dentin from the flutes into
the open space.
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The instrument is more flexible, allowing it to negotiate curves more efficiently.
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Instruments last longer because they are used with less force and in conjunction
with the No. 2 Peeso reamer.
The last two instruments in the sequence are NiTi and
are used manually. They too have the flats and are used in such a way that
they are subject to minimal torsional and flexural stress. The EZ-Fill
SafeSider System provides a hand test to determine whether or not the NiTi
instruments are strong enough to be used in the tooth without fracturing.
If they do not fracture in the hand when bent approximately 90 degrees,
they will not break in the tooth if used in the prescribed manner.
The two NiTi instruments quickly provide an .08 taper that is consistent
with complete debridement and thorough irrigation, standard requirements
for endodontic success.
The SafeSiders provide a design and a sequence that
together give the dentist the ability to perform complete shaping to at
least an .08 taper, fit a medium gutta-percha point, and, with the use
of the bi-directional spiral and EZ-Fill Epoxy-resin cement, create a total
three-dimensional obturation that is at least the equivalent of far more
expensive and complicated techniques.
Every aspect of the EZ-Fill SafeSider Instrumentation
and Obturation System falls well within the safe capacity of each instrument
to perform its task. Consequently, excellent results are easily,
speedily, economically, and predictably achieved.
September-October 2001
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FIGURE 1: Fractured instrument
blocking apical portion of the canal.
FIGURE 2: Flexural stress
increases as the instrument deviates from a straight line; the arrow indicates
the point of stress and potential fracture.
FIGURE 3: Flexural stress
will be increased if the instrument must negotiate past an overhang; arrow
A indicates overhang preventing continuous straight line access; arrow
B indicates point of greatest curvature on outside wall of canal.
FIGURE 4: Showing a canal
opened to the apex to a No. 20 reamer or file; arrow indicates the thickest,
most engaged part of NiTi, most prone to fracture.
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