I can’t reach the apex!

1. I can’t reach the apex! What do I have to do?
» By: Dr. Roger Rebeiz
Docteur en Sciences Odontologiques, Prof. d'endodontie
Introduction
This article describes, in details, the technique that helps the dentists to safely reach the apex
with the use of endodontic instruments in difficult root canal treatment cases. The study of
retreatment is not included in this article.
After reading carefully this article, readers should have a clear understanding of the sequence
and the use of endodontic instruments for secure and effective shaping and cleaning whenever
they face difficulties in exploring and flaring the entire canal’s length.
Once the coronal access is achieved, the stainless steel hand files can directly reach the apex,
determine the working length and prepare the canal entirely in the case of straight canals,
while in a narrow, curved, or long canal, the exploration file encounters obstacles in its route to
the apex where it becomes unable to progress apically. The operator may interpret that as an
“apical calcification”!, however these obstacles are not a sign of calcification, but a sudden
curvature, a division, or a narrowing of the canal, as actually, a canal cannot be patent
coronally and calcified apically, because the calcifications, as the pulp diseases, progress in the
coronal apical direction.
At the initiation of a root canal treatment, the files get stuck along the walls of an unshaped
canal which prevents the file’s apical progression. The shaping of the canal’s coronal part
removes the restrictive dentin and eliminates the contacts of the file from the highest coronal
walls. After this stage, the file which couldn’t reach the apex initially can advance, as the only
contact with the canal is found on the tip of the file.
Accommodation of the Cavity Access
Primarily the operator should check the cavity access which may require further enlargement or
more divergence occlusally to eliminate any structure that may retard the direct access of the
instruments to the orifices.
Flaring the Canal's Coronal Part
The instruments chosen for the flaring of the canal’s coronal part include a combination of hand
stainless steel files and rotary which can be the Gates Glidden drills or nickel-titanium
instruments.
A) Flaring the canal’s coronal part with stainless steel hand files and Gates Glidden
drills
A series of hand K-files #10 to #35 is used in a “step-back” modality, to enlarge the canal’s
coronal half or two-thirds, until the Gates Glidden drills are able to proceed with the shaping of

2. the canal’s coronal space.
K-files: As a general rule, all the K-files must be precurved before placing them in a canal. The
degree of the precurving depends on the radiographic appearance of the root curves. A
precurved file makes its way more easily through the obstacles that it my encounter. The File
must be precurved in a gentle and gradual way at its 2 to 3mm end; it must not be bent.
The K-files can be used in 2 different ways, “push-pull” motion or “watch-winding and pull”
motion:
a- “Push-pull” motion: The K-files are conducted into the canal until they meet resistance with
dentin walls, then a push and pull motion of 2 to 3mm distance is performed. This movement,
also called filing motion, optimizes the cutting capability of the K-files and produces dentinal
debris. If this movement is done brutally, the push action can plug this debris, causing an
apical blocking of the canal which compromises its shaping and cleaning. The filing motion of
the K-files must be quite gentle to prevent the instrument from plugging the dentin particles.
b- “Watch winding and pull“ motion: to prevent the grave risk of obstructing the canal, the file
should principally work on the withdrawal. The file is introduced into the canal until it meets
friction with the canal walls, then it is rotated in the canal in a quarter turn (90°) clock-wise
direction (engaging the file), followed by a 90° counter clock-wise rotation (cutting dentin)
keeping the instrument at the same depth, at last the file is withdrawn (disengaging and
cutting) few millimeters back. After that, the file is again passively introduced until it meets
with dentin walls so the “watch-winding and pull” motion is repeated. This operation of file’s
passive insertion and cutting withdraw is repeated 3 to 5 times until the file becomes free
inside the canal; only then the bigger file is introduced to accomplish the same operation.
The author of this article advocates using the K-files in “watch winding and pull” motion during
the stage of the flaring of the canal’s coronal part. The push and pull movement is applied in
other situations that will be described further in this article.
The canal lubrication favors the work of the files within the canal. As long as the canal’s entry
and the coronal third are not enlarged enough to permit the passage of the irrigation needle
inside the canal, the action of the files stays optimal in the presence of a viscous chelator as
RC- Prep (Premier Dental Products), Glyde (Dentsply-Millefer)….
The K-files #10 to #35 are used by step-back; each file works on a shorter level than the
previous one, which creates a space in the canal to permit the Gates Glidden drills fit properly.
Gates Glidden drills: Despite the fact that the Gates Glidden are called “drills”, they have a
non-cutting tip to prevent perforation; on the other hand they have a side –cutting action along
their equator.
The Gates Glidden may be used for either one, or the three following levels and roles:
1- Enlarging the canal orifice: The enlargement of the canal entry establishes a loose pathway
starting from the pulp chamber to the canal, and simplifies subsequent cleaning and shaping
procedures.
2- Eliminating cervical interferences: The mesial root of lower and upper molars presents an
abrupt cervical curvature due to the change of direction between the pulp chamber and the

3. root canal. This variation of direction shapes a triangle of dentin. The files flex around this bend
as long as the coronal curve exists. The use of the Gates Glidden is a very efficient way to
eliminate this cervical interference and straighten the path of the files toward the canal’s
coronal third.
3- Flaring the straight coronal part of the canal: Used correctly, the Gates Glidden are very
effective for enlarging the straightaway coronal part of the canal.
The recommended rotational speed for a safe use of the Gates Glidden goes from 750 to 1000
rpm. The attributes of Gates Glidden instruments are best appreciated using a gear reduction
hand piece with a high-torque. The use of the Gates Glidden consists in the succession of the
following moves:
- First, the Gates Glidden is inserted without force inside the coronal straight part of the canal
until it faces resistance. Generally, this occurs in the coronal third of narrow and curved canals
or the coronal half or two thirds in straight and large canals. This operation may be done
without rotation for a safer use or when it comes to a non experimented operator, and with
rotation for the experimented one.
- Then the Gates Glidden is withdrawn about 1millimiter until it is free to move within the
canal.
- Finally it is applied carefully against a selective canal wall and removed in a cutting withdraw
motion, without ever simultaneously having contact with the entire circumference of the canal.
In round–shaped single rooted teeth, the canal is in a central position and the wall thicknesses
are approximately the same, buccolingually and mesiodistally. Consequently, the
circumferential enlargement of all the canal walls can be used confidently. On the other hand,
some root canals, especially the mesial root of the lower and upper molars present a
buccolingual concavity on their distal surface, and the canals are not in a central position, but
closer to the distal wall which is a thin area called “danger zone”. In these roots,
circumferential enlargement should be avoided, since it would lead to excessive thinning of the
root or even to perforation from stripping in the area of the bifurcation. The Gates Glidden
must be constantly worked against the mesial wall (“safety zone”) of the curve, away from the
bifurcation area. This procedure is called anticurvature filing method.
This phase of the canal’s coronal part flaring would typically include Gates #1, 2, 3 which
should be used serially and in a step-back modality, so that each successively larger drill works
shorter than the preceding smaller one.
The # 1 (diameter 0.50mm) has the only function of preparing the passage to the followed #2
Gates (diameter 0.70mm) which eliminates the cervical triangle of dentin, provides the
continuity between the cavity access and the root canal, and effectively enlarges the coronal
third of the canal. The “bud” of a #3 Gates (diameter 0.90mm) is used to enlarge the opening
of the canal orifice. A Gates Glidden drill accomplishes its work after two to three passages into
the canal root.
The Gates # 4, 5, and 6 are not used within the canals; they are used as burs in the pulp
chamber to eliminate pulpal horns in certain cases.
The use of a Gates-Gildden drill produces a large amount of dentin mud, so the canal must
always be irrigated after its use (Fig. 1) (Fig. 2).

4. » (Fig. 1): A lower first molar with narrow and
curved canals in the mesial root.

5. A: Preoperative radiograph / B: Radiographic
appearance of the access cavity / C: The # 15
K-files are unable to advance to the apical one
third of the canals because of large
interferences in the curve of the coronal third,
and the contact along the walls of the canals.
Note the distal inclination of the instrument's
shaft in the mesial root / D: The coronal two-
thirds have been enlarged by a series of K-files,
and the curve of the coronal third has been
eliminated by the Gates Glidden. The # 15 K-
files now have a straight line access to the
radiograph terminus of the canal. Note the
mesial displacement of the instruments shaft
/E: Postoperative radiograph.

6. » (Fig. 2): The misuse of the Gates-Glidden
leads to errors in the root canal shaping.
A: Postoperative radiograph of a lower first
molar. The brutal and circumferential use of
Gates-Glidden within the canal causes a
perforation in the area of the bifurcation
/ B:Postoperative radiograph of a lower first
molar. Lack of application of the anticurvature
filing method with a Gates-Glidden # 2 leads to
stripping in the area of the bifurcation in the
mesial root / C: Postoperative radiograph of a
lower first molar. The Gates-Gildden # 3 is
inserted with force inside the distal canal. The
obturation of this canal demonstrates the
incorrect use of the Gates, which has left its
impression on the middle third of the canal root
/ D: Postoperative radiograph of a lower
second premolar. A repeated placement with
pressure of Gates # 2 and 3 at the same canal
root level lead to the formation of the ledge.

7. To illustrate the article, the author selects the lower premolars, as single rooted teeth, and the
mesial root of lower molars which are thin and present curvatures on many plans.
At this stage the exploration file which couldn’t reach the apex before the flaring of the canal’s
coronal part don’t have any more contact with canal walls except on the tip, and can then
advance.
B) Flaring the canal’s coronal part with stainless steel hand files and nickel-titanium
rotary instruments
The practitioner may accomplish a more conservative shaping of the coronal two- thirds by
using nickel-titanium rotary instruments than by utilizing the Gates Glidden drills.
Prior to manipulating nickel-titanium rotary instruments safely, stainless steel hand K-files # 10
and 15 must be sufficiently applied to confirm or create a space larger than the first rotary
instrument’s tip that will be employed in the canal.
K-Files # 10 and 15 are used with a “watch winding and pull” movement within any part of the
canal until their motion is free. The enlarged depth of the 15 K-file is measured and its length is
transferred to the nickel–titanium rotary instruments.
According to the nickel-titanium system used, the shaping of the canal’s coronal two-thirds
might be accomplished with the use of one instrument, if this instrument has a variable taper
(ProTaper), or with more rotary instruments if they have a constant taper.
The use of the nickel-titanium rotary instrument into the canal is as follows: The instrument
rotates while penetrating the canal, then is introduced about 2 mm with a light apical pressure,
then withdrawn 1mm, reintroduced about 2 mm deeper or until the cutting flutes have contact
with the canal walls, withdrawn 1mm, introduced again 2 or 3 mm, withdrawn 1mm and so on
until the instrument reaches the depth enlarged by the 15 K-file, or until the instrument meets
resistance.
This action is performed to prevent any engagement of the rotary instrument with the canal
walls, which may stop the instrument and modify its continuous rotation to a twisting motion
which can lead to the breakage of the instrument.
The rotary instrument should be removed from the canal after 4 to 6 seconds of intracanalar
use, regardless of whether it is cutting or not.
If the rotary instruments have a constant taper, the shaping of the coronal two- thirds needs
more than one instrument, about three generally. They will be used in a crown –down
sequence, from largest to smallest size. Example: 25 taper .12 or .10 or .08 to 25 taper .06 to
20 taper .06 or 25 taper .04. The larger tapers eliminate the coronal obstructions, the
succeeding smaller tapers will go deeply towards patent depth reached previously by the 15 K-
file. Each instrument cuts the dentin along a short space which diminishes the risk of the
instrument’s engagement with the canal walls, and consequently its breakage. The instruments
with sharp cutting edges (for example “HERO”, “Flexmaster”, “Race”…) are more effective in
cutting dentin than the instruments with radial lands (“ProFile”, ”GT”…).
When a rotary instrument stops advancing easily in apical direction, the operator should
immediately remove it from the canal and examine the accumulated debris on the instrument
to understand the workload done and the cause of the block-out.
1- If a large portion of the working part of the instrument is filled with dentinal debris, it means
that the work carried out by this instrument has been significant; the operator should clean the

8. file (with alcohol gauze) to eliminate the debris which fill its blades and make them less
efficient at cutting, then introduce a 15 K-file to confirm the existence of the previously
established glide path, and irrigate to flush debris out of the canal before reintroducing the
rotary instrument.
2- If only the apical part of the instrument is filled with dentinal debris, this means that the
instrument was cutting in a very narrow canal. In this case the operator should use K-files # 15
and 20 until they are free and able to create more space inside the canal in order to make it
easily accept the tip of the nickel–titanium instrument, irrigate the canal and at last reintroduce
the rotary instrument to continue its work, or dropdown to an instrument smaller in taper and
tip size, and so continue shaping. Knowing that this nickel-titanium instrument endured
accumulated efforts on its tip; its reuse can be risky.
3- If the instrument stops progressing and is not filled with debris, this can be due to one of
following conditions:
- The instrument is too stiff to flex around a curve,
- The canal is narrower than the tip of the instrument,
- The canal is blocked with dentinal debris,
- The canal has an irregularity (a ledge for example),
- The root canal anatomy has a sudden change in its direction.
The operator should turn back to the manual instrumentation. This operation will be described
later in this article. (Fig. 3) (Fig. 4)

9. » (Fig. 3): A lower first molar with narrow and curved canals
in the mesial root.
A: Preoperative radiograph
B: A 15 K-file is unable to reach the apex of the canal.

10. C: The enlarged depth of the 15 K-file is measured and its
length is transferred to a nickel-titanium rotary instrument.
The instrument is directed away from furcal wall concavities
D: Intraoperative radiograph after the flaring of the canal's
coronal part. The 15 K-file which couldn't reach the apex
initially makes easily its way to the apex. Note that the
reference point of the stop is the cusp associated with the canal,
and the closeness of the instrument's s to the mesial wall of the
cavity access
E: Postoperative radiograph. The shaping of the canals was
performed with the help of rotary nickel-titanium
accomplishing a conservative shaping.
» (Fig. 4): Postoperative radiograph of a lower first molar.
The misuse of rotary nickel-titanium instruments conducts to
excessive thinning of the mesial root walls in the "danger
zone”.

11. While performing the in and out movement, the nickel-titanium rotary instrument is
manipulated in a withdrawn action, like a “brush”, to laterally and selectively cut dentin on the
outstroke. The rotary instruments are directed consecutively on all the canal walls in round –
shaped single rooted teeth, away from thin walls in ribbon-shaped canals, and deliberately
away from root furcal concavities walls in multi rooted teeth.
Note that:
- The pulp chamber and the canal are flushed copiously with sodium hypochlorite during and
after the use of each nickel-titanium rotary instrument.
- The nickel-titanium rotary instruments should be used at a constant speed varying from 250
to 350 rpm. The use of an electric torque control motor with auto reserve is desirable.
When the canal’s coronal two-thirds are shaped by one of the techniques previously described,
the access can be focused on the apical one-third. Apart from certain root canals’ anatomical
configurations, it is reasonable to assume that a 10 and 15 hand K-file are able to reach the
apical foramen.
If the file is free in the canal but is making no apical progress, the canal has probably:
- An anatomical complexity, as an abrupt apical curvature or a divided canal in an unseen third
dimension,
- Iatrogenic errors as a ledge, a block, or a fractured instrument.
Abrupt apical curvature:
If the canal has an abrupt curvature in the apical one-third, the operator may face an obstacle
that prevents him from reaching the foramen with the K-file #10 or 15. He should then not
force the file trying to overcome the obstruction; on the contrary, he must withdraw the file
from the canal, give it a greater and exaggerated degree of precurvature at its tip, and repeat
the exploration. If the file reaches the foramen with difficulty, the clinician must not remove it
from the canal before it is “free” within the canal. If removed, the operator might not be able to
replace it easily at the same point. Once at the foramen, the K-file requires small push-pull
motions with no rotary component until it becomes free in the canal. The short push and pull
movements help to restrict the precurved portion of the file to operate in the curved portion of
the canal and thus to preserve as much as possible the precurvature that has been previously
given. If the operator applies long filing motions the file would pass alternately from the curved
to the straight portion of the canal and lose its precuvature. (Fig. 5) (Fig. 6)
The operator should renew the lubricating of the canal before moving to a larger file.

12. » (Fig. 5): Lower second premolar with an
abrupt apical curvature oriented in the
plane of the radiograph.
A: Preoperative radiograph demonstrating
a straight root. The position of the apical
foramen is not able to be located on a
preoperative radiograph / B: Postoperative
radiograph. An apical curve of about 90°
directed distally can be appreciated
radiogarphically. The apical third of the
root canal was cleaned and shaped after
the enlargement of the coronal two-thirds
had been performed.

13. » (Fig. 6): Lower right second premolar
with an apical curve non oriented in the
plane of the radiograph.
A: Preoperative radiograph demonstrating
a slight curve in the apical third of the
canal root / B: After the shaping and
cleaning of the coronal two-thirds, straight
# 10 K-file was not able to reach the apical
foramen / C: A precurved file at its tip
reached the apical foramen. The
unidirectional stop placed on the
instrument and oriented in the direction of
the precurvature showed that the apical
foramen is oriented distobuccally. The
intraoperative rdaiograph shows, only, the
distal plane of the apical curve
/ D:Postoperative radiograph, two years
recall.
Divided canals:

14. A single canal may bifurcate at any level of its root and so end up with two separate foramens.
This division creates a sharp curvature in the trajectory of the canal system. The two canals of
the bifurcation should be shaped to allow the Gutta-Percha cones to fit in the separate
foramen. (Fig. 7)
» (Fig. 7): A single canal may bifurcate at
any level of its root and so, end up with
two separate foramens
A: Postoperative radiograph of a lower first
premolar with a single root showing a
canal bifurcated at the coronal third level
of its root / B: Root canal bifurcation at the
middle third level of its root / C: Root canal
system showing a trifurcation at the apical
third level of its root.
The shaping of this root canal system is performed according to the following steps:
1- A series of pre-bent K-file sizes #10 – 30 is used from smaller to larger in a step-back
operation beneath and above the bifurcation, to enlarge the canal’s common part and explore
the curves of the bifurcation. These files are used in a watch-wind and pull motion.
2- Then Gates Glidden #1 and 2 are used to flare the common coronal part of the canal
system.
3- At this stage the # 10 K-file still cannot reach the apex in both canals. To allow it to travel
freely to the apex, the curves of the divided canals should be smoothened with the series of K-
file sizes #10– 30 previously used. This operation is called recapitulation. A recapitulation is
the reintroduction and reapplication of instruments previously used. At the first recapitulation,
the K-file #10– 30 descend to positions more apical than they had descended before, because
each file meets a canal’s part that has been enlarged by the instruments used at the previous
passage. After three or more recapitulations with the series of K-files, the # 10 which couldn’t
arrive at the apex initially can reach it in both canals. (Fig. 8) (Fig. 9)

15. » (Fig. 8): The shaping of a divided root
canal system with the help of stainless
steel hand files.
A: Preoperative radiograph of a lower first
premolar. During a previous operation a
small file was broken at the middle third
level of the root / B: Intraoperative
radiograph with a file installed at the apical
third with another file encountering
obstacles in its route to the apex because
of the sharp curvature created by the
division of the canal / C: After the use of a
series of pre-bent K-files beneath and
above the canal root bifurcation, the
previous exploring file descended more
apically than before / D: After three
recapitulations with the series of K-files,

16. the exploration file which couldn’t reach
the apex initially can reach it now in both
canals / E: Postoperative radiograph. (The
proximity of the two canals prevents the
clear demonstration of their obturation).
» (Fig. 9): The shaping of a divided root
canal system with the help of stainless
steel and rotary nickel-titanium
instruments
A: Preoperative radiograph shows a
division of the canal at the middle third
/ B: The curvature is blunted and

17. smoothened during the shaping of the
canal’s coronal two-third with the help of
stainless steel hand files / C: The nickel-
titanium rotary instruments are able to
follow a canal that divides, after the
elimination of the abrupt curves, as shown
in this intraoperative radiograph.
/ D:Postoperative radiograph, two years
recall.
The deeper the bifurcation is in an apical level, the more difficult it becomes to shape the two
separate canals. It is important to notify that nickel-titanium rotary instruments may not be
able to follow a canal that divides in abrupt curves.
Apart from the sudden variation of the canal’s trajectory, the operator may not be able to reach
the apex due to an iatrogenic obstruction caused by a procedural error made during cleaning
and shaping. This obstruction can be a ledge, a block, or a fractured instrument. Assuming that
a canal exists, the exploration of the entire canal should be done if possible despite the time
consuming and the efforts often hopeless.
Ledge:
A ledge is an artificial dentin irregularity created by error on the surface of the root canal wall
preventing the progression of the instruments to the apex of a previous patent canal. Ledging
is caused by an improper use of shaping instruments, as:
- An insertion of rigid and uncurved hand stainless steel instruments shorter than of the
working length, with excessive apical pressure. The canal wall is either gouged or a beginning
of a false route is created, which results in ledge formation,
- A repeated placement with pressure of rigid nickel- titanium rotary instruments those are
unable to enter and shape the abrupt curved apically,
How to manage a ledge?
Early recognition or ledge formation is advantageous. Ledges created by # 25 or 30 file are
more difficult to bypass than one created by a smaller file, because the shelf created by the
larger instrument is more likely to prevent penetration beyond the ledge.
Ledges might either be reduced or eliminated by using K-files #10, 15, and 20 if possible. The
technique for managing ledges is:
1- A 10 K-file with a very short accentuated apical precurvature and a unidirectional rubber
stop is placed on the instrument and oriented to the direction of the precurvature. The file is
placed into the canal and turned circumferentially until it negotiates the canal. Once the tip of
the file is apical to the ledge, it is operated with very short push and pull movements, assuring
on staying apical to the ledge. When the #10 file moves freely, slight longer push and pull
strokes will be able to reduce the ledge,

18. 2- A 15 K-file overcurved upon its apical millimeters (to compensate for some losses of
curvature once in the canal), with a unidirectional stop is introduced with the precurvature
facing the direction of the canal’s curvature, in the hope of negotiating the canal, then a push
and pull motion of 1 to 2 mm distance is performed until the file moves freely.
3- When the ledge can be predictably bypassed with #15, draw the way for the # 20 K-file to
improve the diameter of the canal’s glide path.
The canal is always flooded with a viscous chelator or irrigation solution.
Not all ledges can be removed. Rotary nickel–titanium instruments should never be used to
bypass the ledge and negotiate the canal. These instruments are not used into the canal until
the ledge has been bypassed by a # 20 hand file which creates a space so that the tip of the
nickel–titanium instrument can passively follow the glide path.
Canal Blockage
Intracanal debris, pulp residues, and dentin particles, may accumulate and be plugged,
especially when an abrupt push and pull motion is performed with rigid and uncurved hand
instruments within the apical foramen.
The essential prevention of the canal blockage is to regularly break-up debris and keep the
canal patent. To ensure the canal patency and avoid the dentin mud accumulation, the dentist
should introduce regularly a #10 K-file beyond the working level of the files: to the apical zone
after each file which works in the coronal or middle third of the root canal, and through the
foramen when the files work in the apical third. A voluminous irrigation of the root canal space
should be done after the use of each instrument to flush –out the loosened debris.
Once a block occurs, the technique for trying to manage it is as follows:
- A 45 degree curve is placed at the apical, 2 to 3mm, of # 10 K-file with a unidirectional
rubber stop, and is used to bore through the dentinal block. The files of 21 mm length are
preferable than a 25 mm length because they bend less. The file is inserted into the canal until
its contact with the blockage, then a turn and apical push motion is performed in a
circumferential rotation in the hope of detecting a catch into the debris, or a space between the
particles and the canal. Continuous tries are necessary. The file is replaced with another new
file as soon as it bends.
- If a catch is felt or the tip of the file bypasses the obstruction, the file should not be removed;
it must be used with short push and pull amplitude (1 to 2 mm). This instrument will be able to
slightly move more freely, after performing work for about two to three minutes. Only then,
slight longer pushes and pulls, 2 to 3 mm amplitude, are done. Finally, 3 to 4 mm push and
pull is performed until the file is able to slide easily to the apex. Here the file should not be
removed, and a radiograph must be taken to verify its position.
Chelating agents are used to facilitate the file penetration.

19. » (Fig. 10): The misuse of the endodontic
instruments may create a canal blockage in
curved canals
A: Postoperative radiograph demonstrates
mesial root canals, shaped in a false route,
due to the persistent efforts performed to
negotiate the apical third of the canal after
the creation of a canal blockage.
Depending on the severity of the blockage and the patience of the operator, these efforts might
or not lead to bypass the blockage. When failure to penetrate the blockage occurs after
persistent efforts, the operator cleans and shapes the canal at a new working length
determined by to the blockage. (Fig. 10)
Conclusion
Difficult advance of the files is often due to “hang-up” in the coronal two-thirds of narrow and
curved canals. The flaring of the canal’s coronal part before commencing the cleaning and
shaping of the canal’s apical third eliminates the contacts of the file with the highest coronal
walls and allows a deeper entry, as the only contact with the canal is found on the tip of the
files. The curves of the coronal third must be straightened with the help of rotary instruments,

20. Gates Glidden or nickel titanium files. The curves of the middle third must be smoothed by
following the rules which vary according to the instruments used as well as the type of the
curve. While the instruments are making their way to the apex, the instrumentation requires
care and attention to avoid ledge and block.