Barry Musikant
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 ECENT
endodontic literature is emphasizing the need for wider apical canal preparations
to insure adequate irrigation throughout the length of the canal.
The minimum apical preparations should not be less than a 30 and often
to widths as wide as a 50 or 60. The rationale is that wider apical
preparations will mechanically remove more tissue from the canals both
in the bucco-lingual and the mesio-distal dimensions and allow a greater
quantity of irrigant to come into intimate contact with the pulp tissue
remnants, dissolving them in the process. Any remaining tissue is
a source of food for bacteria. The more of this substrate that is
removed, the less potential remains for future bacterial growth, assuming
that all sources of leakage have been eliminated.
Recent morphometric measurements of canals by Wu
and Wesserlink have described many anatomic situations where the mesio-distal
dimensions of a canal are less than half those of the bucco-lingual dimensions.
For example, on average according to their data tables, the mesio-distal
dimension of the MB canal of a maxillary first molar 1 mm from the apex
is approximately .22 mm, while the bucco-lingual dimension at the same
level is .43 mm—nearly twice as wide. A canal opened to a 30 1 mm
from the apex would on average not touch the buccal and lingual walls of
the canal. We would not know this because our x-rays limit our vision
of canals clincally to a mesio-distal view for the most part. Nor
would the instruments we are most familiar with give us any clue to the
oval nature of these canals.
Most instruments used in endodontics are symmetrical
in design. A symmetrical instrument in an oval canal will produce
the same resistance as it rotates (Figure 1). An asymmetrical instrument,
on the other hand, one with a flat along its entire working length, will
produce less resistance when the flat is aligned with the long diameter
(Figure 2) and more resistance when aligned with the short diameter (Figure
3), confirming to the dentist that the instrument is, indeed, in an oval
canal. Being able to differentiate between a round and oval canal
gives the dentist the knowledge to further widen oval canals in an attempt
to remove tissue tucked into the buccal and lingual recesses of the oval
canal as well as creating canal shapes that are more easily irrigated.
The need for greater apical preparations for improved
irrigation runs into a conflict when rotary NiTi instruments are used.
It is well documented that rotary NiTi instruments have a greater tendency
to separate as the apical dimension and taper of the instrument and curve
of the canal being instrumented increase. Today, it is rare to have
a rotary NiTi preparation exceeding a 25/06 in a significantly curved canal.
In short, the risk of mechanical failure trumps the biologic needs of the
tooth. Furthermore, the shape memory property of NiTi instruments
precludes the use of greater tapered instruments in highly curved canals
because of their tendency to selectively remove tooth structure from the
outer wall of the canal. This potential for transportation increases
as the apical dimension and taper of the instrument and curve of the canal
being instrumented increase. Separation and transportation are two
powerful reasons curved canals are not shaped more aggressively with rotary
NiTi.
Interestingly, stainless steel instruments, although
much stiffer than comparably sized NiTi instruments, have advantages over
NiTi that make them more suitable for achieving greater tapered shapes
without apical distortion. Stainless steel is readily bent to any
shape. It does not have the property of shape memory that springs
NiTi back to its original shape. Any stainless steel instrument may
be bent to a shape that conforms to the shape of the canal. In fact,
initial stainless steel instruments record the shape of the canal, giving
the dentist the ability to see the degree, location, and direction of any
curves that the initial instruments have negotiated. This is valuable
information that the dentist can use when adapting thicker stainless steel
instruments to these curved canals.
These precurved instruments are then used manually
to negotiate the initial curves of the canal. Once the initial curve
is negotiated, these manual instruments are then attached to a 30-degree
reciprocating handpiece that drives these instruments to the apex.
Because the envelope of motion is constricted to 30 degrees of motion,
1/12 of a circle, these prebent instruments have little or no potential
for distortion. The net result is that stainless steel instruments,
prebent and used with the confined motion supplied by the reciprocating
handpiece, can widen canals to any dimension required without separation
and transportation being a concern. While this takes a bit of practice,
the entire learning experience is accomplished without separation being
part of the learning curve.
In fact, the stainless steel instruments are optimized
by having a reamer design with fewer and more vertically oriented flutes
than a file. They are also fabricated from triangular wire as opposed
to the square wire files are made from. The result is a reamer that
engages the walls of the canals far less than a file and at the same time
removes dentin from the walls of the canal far more efficiently when used
in a reciprocating (wristwatch-winding) motion. The reamers are also
more flexible because fewer flutes make the instruments less work hardened.
The combination of fewer and more vertically oriented flutes as well as
fewer contact points is further enhanced by a flat placed along the entire
working length of the reamers, making these reamers even more flexible
and less engaging. A cutting tip allows the reamers to pierce any
apical debris rather than impact it. If a wall is encountered, resistance
along the length of the instrument is so negligible that the dentist immediately
knows an obstacle has been encountered, directing him to remove the instrument,
prebend it, and negotiate manually around the obstacle, reattaching the
reamer to the reciprocating handpiece and proceeding to the apex.
The demand for greater apical preparations is increasing
because a major improvement in irrigation is the creation of an apical
vacuum produced by the placement of a suction tip at the most apical preparation
of the canal. This condition can be achieved only if the preparation
is prepared to a minimum of a 35. With the vacuum placed at the tip
of the root, the irrigation fluids—most notably NaOCl—can be delivered
coronally and then pulled apically by the most apically placed suction
tip. Traversing the length of the tooth, the NaOCl will remove the
debris in the central as well as the lateral canals as it travels apically.
This will be further enhanced by the use of warm NaOCl, which is far more
efficient at dissolving tissues than room-temperature NaOCl.
Since the chemical dissolution of bacteria-supporting
tissue is being recognized as a critically important adjunct to successful
endodontics, it will become mandatory to employ greater tapered shapes
and wider apical preparations supporting chemical dissolution.
July - August 2006
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The
chemical dissolution of bacteria-supporting tissue is being recognized
as a critically important adjunct to successful endodontics.
FIGURE 1: A symmetrical instrument
in an oval canal will produce the same resistance as it rotates.
FIGURE 2: An asymmetrical
instrument with a flat along its entire working length will produce less
resistance when the flat is aligned with the long diameter.
FIGURE 3: An asymmetrical
instrument with a flat along its entire working length will produce more
resistance when aligned with the short diameter.
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