Regardless of the technique used in cleaning and shaping, procedural
errors can occur. These
included loss of working length, apical transportation, apical
perforation, lateral stripping and instrument fracture.
Loss of working length has several causes. These include failure to have an
adequate reference point from which the corrected working length is determined,
packing tissue and debris in the apical portion of the canal, ledge formation,
and inaccurate measurements.
Apical
transportation and zipping occurs when the force of the file exceeds
the threshold for cutting dentin in cylindrical non-tapering curved canal . When
this apical transportation continues with larger and larger files, a “teardrop”
shape develops and perforation can occur apically on the lateral root surface
. Transportation in curved
canals begins with a size #25 file. Enlargement of curved
canals at the corrected working length beyond a size #25 file should be done
only when an adequate coronal flare is developed.
Instrument fracture occurs with torsional and cyclic
fatigue. Locking the
flutes of a file in the canal wall while continuing to rotate the coronal
portion of the instrument is an example torsional fatigue . Cyclic fatigue results when strain develops
in the metal.
Stripping perforations occur in the furcal region of
curved roots, frequently the mesial roots of maxillary and mandibular molars .The
canal in this area is not always centered in the root and prior to preparation
the average distance to the furcal wall (danger zone) is less than the distance
to the bulky outer wall (safety zone).
An additional factor is the concavity of the root.
Watch Winding
Watch winding is reciprocating back and
forth (clockwise/counterclockwise) rotation of the instrument in an
arch. It is used to negotiate
canals and to work files to place. Light
apical pressure is applied to move the file deeper into the canal.
Reaming
Reaming is defined as the clockwise, cutting
rotation of the file. Generally
the instruments are placed into the canal until binding is encountered. The instrument is then rotated
clockwise 180-360º to plane the walls and enlarge the canal space.
Filing
Filing is defined as placing the file into the canal and pressing
it laterally while withdrawing it along the path of insertion to scrape the
wall. There is very
little rotation on the outward cutting stroke.
The scraping or rasping action removes the tissue and cuts superficial
dentin from the canal wall. A
modification is the turn-pull technique. This involves placing the file to the
point of binding, rotating the instrument 90º and pulling the instrument along
the canal wall.
Circumferential filing
Circumferential filing is used for canals
that are larger and or not round. The file is placed into the canal and
withdrawn in a directional manner sequentially against the mesial, distal,
buccal, and lingual walls.
Standardized preparation
After 1961, instruments were manufactured with a standard
formula. Clinicians utilized
a preparation technique of sequentially enlarging the canal space with smaller
to larger instruments at the corrected working length. In
theory this created a standardized preparation of uniform taper. Unfortunately this does not occur. This technique
was adequate for preparing the apical portion of canals that were relatively
straight and tapered; however in cylindrical and small curved canals procedural
errors were identified with the technique.
Step-back Technique
The step-back technique reduces
procedural errors and improves debridement. After coronal flaring and
determining the master apical file (initial file that binds slightly at the
corrected working length), the
succeeding larger files are shortened by 0.5 or 1.0 m increments from the
previous file length . This step-back process creates a flared, tapering
preparation while reducing procedural errors.
The step-back preparation is superior to standardized serial filing and
reaming techniques in debridement and maintaining the canal shape.The step-back
filing technique results in more pulpal walls being planed when compared to
reaming or filing.
Step-Down Technique
The step down technique is advocated for cleaning and
shaping procedures as it removes coronal interferences and provides coronal
taper. Originally advocated for
hand file preparation it has been incorporated into techniques
employing nickel-titanium files. With
the pulp chamber filled with irrigant or lubricant the canal is explored with a
small instrument to assess patency and morphology (curvature). The working length can be established at this
time. The coronal
one third of the canal is then flared with Gates Glidden drills or rotary files
of greater taper (.06, .08, .10,). A
large file (such size #70) is then placed in the canal using a watch winding
motion until resistance is encountered. The process is repeated with
sequentially smaller files until the apical portion of the canal is
reached. The working length can be
determined if this was not accomplished initially. The apical portion of the canal can now be
prepared by enlarging the canal at the corrected working length. Apical taper is
accomplished using a step-back technique.
Passive Step-back
The passive step-back technique is a modification
of the incremental step-back technique. After the apical diameter of the canal has been
determined, the next higher instrument is inserted until it first makes
contact (binding point). It is then
rotated one half turn and removed . The
process is repeated with larger and larger instruments being placed to their
binding point. This
entire instrument sequence is then repeated. With each sequence the instruments drop deeper
into the canal creating a tapered preparation.
This technique permits the canal morphology to dictate the preparation
shape. The technique does not require
arbitrary rigid incremental reductions and forcing files into canals that
cannot accommodate the files. Advantages
to the technique include: knowledge of
canal morphology, removal of debris and minor canal obstructions, and a gradual
passive enlargement of the canal in an apical to coronal direction.
The diameter of rotary flaring instruments.
Size Gates-Glidden
Peeso-Reamers
#1 .5
mm .7
mm
#2 .7
mm .9
mm
#3 .9
mm 1.1
mm
#4 1.1
mm 1.3
mm
#5 1.3
mm 1.5
mm
#6 1.5
mm 1.7
mm
Anti-Curvature Filing
Anti-curvature filing is advocated during coronal flaring
procedures to preserve the furcal wall in treatment of molars . Canals are often not
centered in mesial roots of maxillary and mandibular molars, being located
closer to the furcation. Stripping
perforations can occur in these teeth during aggressive enlargement of the
canal space. Stripping
perforations occur primarily during use of the Gates Glidden drills . To prevent this procedural error, the Gates
Glidden drills should be confined to the canal space coronal to the root
curvature and used in a step-back manner . The Gates Glidden drills can
also be used directionally in an anti-curvature fashion to selectively remove
dentin from the bulky wall (safety zone) toward the line angle,
protecting the inner or furcal wall (danger zone) coronal to the curve . While this can be accomplished with the use
of hand files, it appears that directional forces with Gates Glidden drills is
not beneficial.
Balanced Force Technique
The balanced force technique recognizes the fact that
instruments are guided by the canal walls when rotated.Since the
files will cut in both a clockwise and counterclockwise rotation, the balanced
force concept of instrumentation consists of placing the
file to length and then a clockwise rotation (less than 180 degrees) engages
dentin. This is followed by a
counterclockwise rotation (at least 120 degrees) with apical pressure to cut
and enlarge the canal. The degree of
apical pressure varies from light pressure with small instruments to heavy
pressure with large instruments. The
clockwise rotation pulls the instrument into the canal in an apical
direction. The counterclockwise cutting
rotation forces the file in a coronal direction while cutting
circumferentially. Following the cutting
rotation the file is repositioned and the process is repeated until the
corrected working length is reached. At
this point a final clockwise rotation is employed to evacuate the
debris.
Nickel Titanium Rotary Preparation
Nickel titanium rotary preparation utilizes a crown-down
approach. The specific technique
is based on the instrument system selected.
One instrument sequence uses nickel titanium files with a constant
taper and variable ISO tip sizes . With this technique, a .06 taper is
selected. Initially a size .06/45 file
is used until resistance, followed by the .06/45, .06/40, .06/35,
.06/30, .06/25, and .06/20. In a
second technique, nickel titanium files with a constant tip diameter are
used. The initial file
is a .10/20 instrument, the second a .08/20, the third a
.06/20, and the fourth a .04/20 . For
larger canals a sequence of files using ISO standardized tip sizes of 30 or 40
might be selected. Using the crown
down approach creates coronal flare and reduces the contact area of the file so
torsional forces are reduced.
Final Apical Enlargement and Apical Clearing
Apical clearing enhances the preparation of the apical
canal, improves debridement, and produce a more definite apical
stop in preparation for obturation. Apical
clearing is generally performed when there is an apical stop and the master
apical file is less that a size #40 file.
If the apical configuration is open or a seat, apical clearing might
make the opening larger and potentiate the possibility of extrusion of the
obturation materials. Apical clearing
consists of two distinct steps: final apical enlargement and dry reaming.
Final apical enlargement is performed after the canal has
been cleaned and shaped. It involves
enlargement of the apical preparation three to five sizes
beyond the master apical file . The
degree of enlargement depends on the canal size and root curvature. In a
small curved canal enlargement may only be three sizes to decrease the
potential for transportation. In a
straight canal it can be larger without producing a procedural error. Since the prepared canal exhibits taper, the
small files at the corrected working length can be used to enlarge the canal
without transportation. Final
apical enlargement is performed with the irrigant and employs a reaming action
at the corrected working length. The last
file used becomes the final apical file.
Since the file is only contacting the apical 1-2 mm the walls of the
canal, the technique will result in a less irregular apical preparation. The canal is then irrigated. The smear layer is removed with a
decalcifying agent and the canal dried with paper points.
After drying the canals, the dry reaming is
performed. Dry reaming removes
dentin chips or debris packed apically during drying. The final apical file (or the
master apical file in cases where apical enlargement was not performed) is
placed to the corrected working length and rotated clockwise in a reaming
action.
Recapitulation
Recapitulation is important regardless of the technique
selected . This is accomplished by taking
a small file to the corrected working length to loosen accumulated debris and
then flushing it with 1-2 ml of irrigant.
Recapitulation is performed between each successive
enlarging instrument regardless of the cleaning and shaping technique.
Combination Technique
This technique combines coronal flaring,
nickel titanium rotary preparation, and the passive step-back technique Following
access, the canal is explored with a #10 or #15 file. If the canal is patent to
the estimated working length a working length radiograph can be obtained and
the corrected working length established .
In order to insure an accurate length determination a
size #20 file or larger should be used .
If a #20 file will not go to the estimated working length passive
step-back instrumentation can be performed by inserting successively larger
files to the point of binding and reaming. This removes coronal interferences and
creates greater coronal taper permitting larger files access to the apical
portion of the root.
After establishing the working length, Gates
Glidden drills are used for straight line access . A #2
Gates is used first followed by the #3 and #4.
In very narrow canals a #1 Gates may be needed. It is important to remember the size of the
Gates Glidden drills. If the
canal orifice cannot accommodate a size #70 file, passive step back should be
performed to provide adequate initial coronal space. To prevent stripping
perforations, the Gates should not be placed apical to canal curvatures. Generally the #2-#4 provides adequate coronal
enlargement and preserves root structure.
The use of nickel
titanium rotary instruments with greater tapers can also be used for this step
(.06, .08, and .10 tapers are common). The
Gates Glidden drills can be used in either a crown-down or step-back
sequence. Following use, the Gates
Glidden drill should be removed from the handpiece to prevent injury to the
clinician, assistant or patient .
Master Apical File
Emphasis has traditionally been placed on determining the
canal length with little consideration of the canal diameter in the apical
portion of the root. Since every canal
is unique in its morphology the apical canal diameter must be
assessed. The size of the apical
portion of the canal is determined by placing successively larger instruments
to the corrected working length until slight binding is encountered . Often the next larger instrument will not go
to the corrected working length. If it
does go to length a subjective estimation of the apical diameter must be
made depending on the degree of binding. This file will be the master apical file
(initial file to bind). It is defined as
the largest file to bind at the corrected working length following straight
line access. This provides an estimate
of the canal diameter before cleaning and shaping and it is the point where the
step-back preparation begins.
Nickel-Titanium Rotary
Once the master apical file is identified, the middle to
apical portion of the canal is prepared using nickel titanium rotary
instruments . Rotary
files are used with a crown-down approach to within 3 mm of the corrected
working length. Adequate coronal
taper is established when the .06/45 goes to within 3.0 mm of the
corrected working length. Using the crown down approach creates coronal
taper and reduces the contact area of the file so torsional forces are
reduced.
Recapitulation
Recapitulation is accomplished after each instrument used
in the canal by taking a small file to the corrected working length and then
flushing the canal with 1-2 ml of irrigant .
Step-Back Apical Preparation
When the body of the canal has been shaped, the apical
portion is prepared using standardized stainless steel or nickel titanium hand
files in a step-back process .
The first instrument selected for this portion of the shaping process is
one size larger that the master apical file (initial file to bind
slightly). Larger files are successively
shortened by standardized increments of 0.05 mm or 1.0 mm. Generally sequentially stepping
back to a file size of #60 or #70 will produce adequate flare and
blend the apical and middle thirds of the canal.
Apical Clearing
With a flared preparation from the orifice to the
corrected working length, the apical portion of the canal is enlarged. With a tapered preparation the canal
can be enlarged with a reaming action as the canal walls will keep the
instrument centered .
Box-4 The Combination Technique Steps
Canal negotiation
Working length
determination
Straight line access
Master apical file
determination
Rotary preparation of the
middle one third of the root
Apical step-back
preparation
Apical clearing
General Considerations – A Review
The following principles and
concepts should be applied regardless of the instruments or technique selected.
1.
Initial canal exploration is always performed with smaller files to gauge canal size, shape, and configuration.
2. Files are always manipulated in a canal
filled with an irrigant
or lubricant present.
3. Copious irrigation is used between each instrument in
the canal.
4. Coronal
preflaring (passive step-back technique) with hand instruments will
facilitate placing larger working length files (either hand or rotary) and will
reduce procedural errors
such as loss of working length and canal transportation.
5. Apical
canal enlargement is gradual, using sequentially larger files from
apical to coronal, regardless of flaring technique.
6. Debris
is loosened and dentin is removed from all walls on the outstroke (circumferential
filing) or with a rotating (reaming) action at or close to working length.
7. Instrument binding or dentin removal on insertion should
be avoided. Files are
teased to length using a watch winding or “twiddling” action. This is a
back-and-forth rotating motion of the files (approximately a quarter turn)
between the thumb and forefinger, continually working the file apically.
Careful file insertion (twiddling)
followed by planing on the outstroke will help to avoid apical packing of
debris and minimize extrusion of debris into the periradicular tissues.
8. Reaming is
defined as the clockwise rotation of the file.
Generally the instruments are placed into the canal until binding is
encountered. The instrument is then
rotated clockwise 180-360º to cut and plane the walls. When withdrawn the instrument tip is pushed
alternately against all walls. The pushing motion is analogous to the action of
a paintbrush. Overall, this is a turn and
pull.9. Filing is defined as placing the file into the canal and withdrawing it along the path of insertion to scrap the wall. There is very little rotation on the outward cutting stroke. The scraping or rasping action removes the tissue and cuts superficial dentin from the canal wall.
10. Turn pull filing involves placing the file into the canal until binding. The instrument is then rotated to engage the dentin and withdrawn with lateral pressure against the canal walls.
11. Circumferential filing is used for canals that exhibit cross sectional shapes that are not round. The file is placed into the canal and withdrawn in a directional manner against the mesial, distal, buccal, and lingual walls.
12. Regardless of the technique, after each insertion the file
is removed and the flutes are cleaned of debris; the file can then be
reinserted into the canal to plane the next wall. Debris is removed from the file by wiping it with an alcohol-soaked
gauze or cotton roll.
13. The canal is effectively cleaned only where the files actually contact and plane the walls.
Inaccessible regions are poorly cleaned or débrided.
14. Recapitulation
is done to loosen debris by rotating the master apical file or a smaller size at the corrected working length
followed by irrigation to mechanically remove the material. During recapitulation the canal walls are not planed and
the canal should not be enlarged.
15. Small, long, curved,
round canals are the most difficult and tedious to enlarge. They require extra
caution during preparation, being the most prone to loss of length and transportation.
16. Over enlargement of curved canals by files attempting to
straighten themselves will to lead to procedural errors .
17. Overpreparation
of canal walls toward the furcation may result in a stripping perforation
in the danger zone where root dentin is thinner.
18. It is neither desirable nor necessary to try to remove
created steps or other slight irregularities created during canal preparation.
19.Instruments, irrigants, debris, and obturating materials
should be contained within the canal. These are all known physical or chemical
irritants that will induce periradicular inflammation and may delay or
compromise healing.
20. Creation of an apical stop may be impossible if the apical
foramen is already very large. An apical taper (seat) is attempted, but with care. Overusing large
files aggravates the problem by creating an even larger apical opening.
21.Forcing or locking (binding) files into dentin produces
unwanted torsional force. This tends to untwist, wrap-up, either will weaken,
and break the instrument.
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