Endo file separation discussion

Fracture

Friday, January 27, 12

Torsional


Cyclic


JOE 2009


Review Article

Figure 2. The method described by Pruett et al (14) for describing canal
geometry using two parameters: radius of curvature and angle of curvature.
(Reprinted with permission [14]).
Figure 1. The degree of root canal curvature obtained using the method
described by Schneider for determining canal curvature using only one param-
eter to define the angle. A has an angle of 43 and B has an agle of 52, even angle of curvature and radius of curvature (Fig. 2). To determine these
though both angles measured according to the method of Pruett et al. equaled parameters, a straight line is drawn along the long axis of the coronal
60 degrees. Location of the curve along the canal will also change the straight portion of the canal. A second line is drawn along the long
measured angle. (Reprinted with permission [14]). axis of the apical straight portion of the canal. There is a point on
each of these lines at which the canal deviates to begin or end the canal
curvature. The curved portion of the canal is represented by a circle
An ideal model would involve instrumentation of curved canals in with tangents at these two points. The angle of curvature is the number
natural teeth. However, in such tests, a tooth can only be used once and of degrees on the arc of the circle between these two points. Angle of
the shape of the root canal will change during instrumentation, making curvature can also be defined by the angle formed by perpendicular
it impossible to standardize experimental conditions. As a result, several lines drawn from the points of deviation that intersect at the center of
devices and methods have been used to investigate in vitro cyclic fatigue the circle. The length of these lines is the radius of the circle and defines
fracture resistance of NiTi rotary endodontic instruments. the radius of the canal curvature defined in millimeters. This parameter
The aim of this review of the literature was to summarize and represents how abruptly a specific angle of curvature occurs as the

Review Article
a dynamic model without specifying at what distance from the tip of the
instrument the point of maximum curvature was located. Stainless steel
tubes with an inner diameter of 1.04 mm and a radius of curvature of 6
mm were used by Lopes et al (23). The authors used two different arcs
lengths because in the curved canals with the same radius, it is possible
that there are arcs (curved segments) with different lengths represented
by angles with different degrees. They used an arc of 9.4 mm corre-
sponding to an angle of 90 and an arc of 14.1 mm, corresponding
to an angle of 135 when using a 6-mm radius of curvature. They
used a straight coronal part of 10.6 mm and 5.9 mm, respectively, so
that the total length of the curved and straight parts was 20 mm. Bui
et al (24)Review Articlethat were constructed by bending
used artificial canals
a 16-mm gauge stainless steel Monojet blunt needle to a 5-mm radius
on curvature and angles of curvature of 30 , 45 , and 60 . The point of
maximum curvature was 7 mm from the tip of the instrument.
Similar to Pruett et al (14), Kramkowski et al (25) constructed
artificial canals by bending stainless steel tubing. Two canals were
bent to a 5-mm radius of curvature with angles of curvature of 45
and 60 . The center of the radius in the curved section of the canal
was 7 mm from the tip of the file. The artificial canals were inserted
into predrilled acrylic blocks for mounting in a fixed jig on the platform
of the cyclic fatigue instrument. The jig was placed at the opposite end of
the rotary handpiece fixed at a distance so that the files protruded
approximately 2 mm out of the end of the tube. Instruments were
rotated in the artificial canal with a consistent insertion and withdrawal
of 8 mm. Silicone spray (CRC Industries Inc, Warminster, PA) as a lubri-
cant and a debris-clearing agent was applied between each file tested.
Cylindrical tubes did not sufficiently restrict the instrument shaft,
which spring back into its original straight shape, aligning into a trajec-
tory of greater radius and reduced angle, as it has been speculated in
previous articles that have used this type of methodology (19–21,
26). Because of the inner diameter of the tubes (glass and metal) is
greater than that of the instruments, an instrument rotated in the tube
will follow a trajectory that is not predictable and without the parameters
of radius and angle of curvature and point of maximum curvature that
were established when constructing the artificial canals. Furthermore,
each instrument, depending on tip size, taper, design, pitch length,
and morphologic and geometric features, will follow its own trajectory
in tubes that do not sufficiently constrain the shafts of the instruments,
especially the smaller ones.
If instruments of the same dimensions follow different trajectories
in the test apparatus, a direct comparison between instruments of
different brands may be difficult to establish and the results obtained
may be unreliable and not consistent. Furthermore, it is unclear what Figure 5. A schematic drawing of the three stainless steel pins that con-
the predictability of these parameters of radius and angle of curvature strained the instrument into the curvature in the studies by Cheung et al
(32–36).
and point of maximum curvature obtained by bending a straight metal
or glass tube. Another problem with a loose-fitting canal is that the file
may ‘‘walk’’ or vibrate in that space, leading to a change in the magni-
tude of stress and 4. The inclined planevariations in the results.
Figure possibly leading to used in some studies to produce the curvature the beginning of the curvature will gradually move away from the long
Ounsiof al (27) have used a custom-designeda sloped steel model A groove
et a rotary instrument working against stainless metal block. axis of the shaft. For this reason, the choice of this point may vary greatly,
mimickingmachined into the face ofcanal space. Thethe file in place during testing. (Re-
a 2-mm-wide artificial the block keeps constant diameter of and the calculated angle may present great variability. Furthermore, it is
printed with permission [28]).
the cavity that reproduced the curved canal presents the same problems not possible to establish exactly the point of maximum curvature
of the tube-like devices in lacking the reproducibility of the actual because the physical and geometric features of the different instruments
trajectory followed by different files. curvature as done by Pruett et al (14). They
considering the radius of may determine different bending properties, so that the point of
In other studies,angles of curvature: 37 , instrument ,was produced study by
used four the curvature of a rotary 40.5 , 45 and 48 . In the maximum curvature may lie at different points and at a different distance
when worked against aal (30),metal block using a groove machined into the face of
Kitchens et sloped a 2-mm-wide groove was from the tip of each file.
the face ofathe block to keep the filesteelplace during polished chrome plating to
hardened 316 stainless in block with testing (28–31) As mentioned earlier, bending properties of different files may
(Fig. 4). The block file in place during testing.resist the operation of
keep the had sufficient hardness to Three angles of curvature were determine a different trajectory if the file is not constrained in a precise
an instrument. The different angles of Schneider’s method (14): 25 , 28 , and
used and measured using curvature used in these studies trajectory. If testing is completed for all different files at a given angle to
33.5 Ray et al to Schneider’s method (15).
were determined. according (31) used a highly polished area of a stainless steel ensure consistency, the bending properties of the different files deter-
Figure 3. A schematic drawing of curved glass or metal tubes used for fatigue Li et al (28, with used a sloped 15 to the horizontal plane, similar to Li et
block 29) an incline of carbon-steel block and calculated mining different angles of curvature, thus biasing the results and the
testing of NiTi rotary instruments. only the angle of29). At a maximum flexure, all files produced an angle of 28
al (28, curvature by Schneider’s method (15), without comparisons.
determined according to Schneider’s method (15). To limit these problems, Cheung et al (32–36) constrained the
Despite the radius of curvature having been recognized as the most instrument into a curvature using three stainless steel pins (Fig. 5).
JOE — Volume 35, Number 11, November 2009 important factor influencing cyclic Rotary Instruments 1471
Cyclic Fatigue of fatigue, these studies measured file They used three smooth cylindrical pins of 2-mm diameter from
Friday, January 27, 12 curvature according to Schneider’s method, which takes into consider- a high hardness stainless steel mounted in acrylic shims, which were
ation only the angle of curvature and not the more important radius of adjustable in the horizontal direction; the position of the pins deter-

Figure 12. An example of several instruments that follows the sa

JOURNAL OF ENDODONTICS Printed in U.S.A.
Copyright © 2004 by The American Association of Endodontists VOL. 30, NO. 3, MARCH 2004

Effect of a Separated Instrument on Bacterial
Penetration of Obturated Root Canals

Jeffrey L. Saunders, DDS, Paul D. Eleazer, DDS, MS, Ping Zhang, PhD, DDS, and
Susanne Michalek, PhD

The aim of this study was to determine the effect a of bacteria in more than 50% of the root canals in their study within
separated instrument has on the time required for 19 days and 42 days when using Streptococcus epidermidis and
bacterial penetration of obturated root canals. Proteus vulgaris, respectively. Khayat et al. (9) found complete
Twenty-six extracted human mandibular premo- bacterial penetration in an average of 28.8 days and 25.4 days for
root canals filled using lateral condensation and vertical conden-
lars with single canals were used in the study.
sation, respectively, when human saliva was introduced into the
Group 1 consisted of teeth that contained a sepa-
access cavities. No study has been performed investigating the
rated size 40 Profile rotary file and were obturated effect of bacterial penetration of root canals with broken files plus
with gutta-percha and zinc oxide eugenol sealer to gutta-percha and zinc oxide eugenol (ZOE) sealer.
the level of the separated file. Group 2 consisted of Because separation of instruments is a sequela of endodontic
teeth that were similarly obturated, but without a treatment, it would be useful to know how such occurrences affect
separated file. The negative control canals were the sealing ability of the obturation material. Knowing the answer
obturated and had the entire root surface sealed may affect a clinician’s decision whether to attempt separated
with nail polish. The positive controls were obtu- instrument retrieval. Through the endodontic literature, it is unclear
rated without sealer. Streptococcus sanguis was whether a fluted instrument separated in a root canal would allow
placed in the access chamber daily, and penetra- quicker penetration of bacteria than the same length of gutta percha
tion was determined when turbidity was noted in and ZOE sealer. The purpose of this investigation was to determine
the effect a separated instrument has on the time of bacterial
the culture broth. The results showed no signifi-
penetration of canals obturated with gutta-percha and ZOE sealer.
cant difference between the two experimental
groups.
MATERIALS AND METHODS

experimental group 2. The remaining six teeth were divided into
two control groups containing three teeth each. None of these teeth
contained a broken file.
For experimental group 1, 10 size 40 .04 Profile rotary instru-
ments were nicked with a #2 round bur 3 mm from the tip to
facilitate file separation at this point. The size 40 Profile rotating
at 300 rpm was then introduced with apical pressure into each
canal until separation occurred. After instrument separation, each
of the 10 teeth in this group was again radiographed to ensure that
the separation occurred in the apical third of the canal.
After instrumentation, the roots of all 26 teeth were coated with
two applications of fingernail polish. The apical 2 mm of the roots
were not covered with polish, except for the three teeth in the
negative control group, which had their entire root coated. All 26
teeth were then autoclaved.
All teeth in the negative control group and experimental group
2 were obturated to the working length with gutta-percha and Roth
811 sealer (Roth International, Chicago, IL) using the lateral con-
densation technique. A sterile technique was used for obturation of
all teeth. A heated plugger was used to remove coronal gutta-

fillings. Swanson and Madison (4) also found a significant vari-
ability when they examined the penetration of a dye through
obturated root canals.
The large range of time for penetration of the experimental
groups can possibly be attributed to variable root canal anatomy,
shape of canal preparation, and sealer type.
The presence of 3 mm of separated instrument did not speed up
or slow down penetration of bacteria when compared with the
normally obturated experimental group. This result implies that the
separated file did not compromise obturation of the root canal
space. This fact is surprising, because root canal anatomy is quite
variable and is not perfectly round like the separated instrument.
Also, the separated instrument has flutes, so it would be unlikely
to completely obturate the root canal space by itself. With sealer
extruded into the flutes, the separated file may become the equiv-
alent of any other obturation material.
The results of this study indicate that the separated instrument
itself does not play a large role in the sealing ability of the

2 Studies included:
Crump/Natkin 1970 - Spili et al 2005
199 cases

Incidence of separation:
Stainless Steel: 0.5-7.4 %
Niti: 0.4-3.7%

Does retention of a separated instrument,
compared w/ no retained instrument - result in a
poorer clinical outcome?


Assessment: 1 year is the earliest possible foully
time to determine whterh the lesion has healed

Inﬂuence of a preoperative RL
80.7% of lesions healed when a periapical
lesion was present
92.4% remanning healthy when no lesion was
present


Important ﬁnding - studies w/ no controls only
used the presence of a preoperative periapical
lesion to act as the main prognostic factor for
successful mgmt. of these cases


Highest proportion of instrument fragments
occurs in the apical third

Removal should be weighed against modest
beneﬁt


A dozen ways to prevent nickel-titanium
rotary instrument fracture
Peter M. Di Fiore, DDS, MS

D
uring the past 15 years,
nickel-titanium (NiTi)
rotary instruments have
become a part of the
ABSTRACT ✷
J
A D
A

✷

standard armamen- Background and Overview. With the

N
CON
tarium in endodontics. They are

IO
increased use of nickel-titanium (NiTi) rotary instru-

T
used extensively by generalists and

T

A
ments for root canal preparation in endodontics, N

I
U C

specialists to facilitate the cleaning A ING EDU 4
instrument fracture has become more prevalent. RT
and shaping of root canals,1 and it Extensive research has been conducted on the physical
ICLE
appears that with the increased properties and mechanical characteristics of NiTi rotary instruments, as
application of these instruments in well as the factors that can contribute to instrument failure. NiTi rotary
contemporary endodontic practice, instruments are subjected to torque and are susceptible to cyclic fatigue,
fractures have become more which are the main causes of instrument fracture. However, with an
prevalent.2,3 understanding of how these instruments function in preparing root
Fractured instruments are a defi- canals and by applying ways to reduce torque-generated metal fatigue,
nite hindrance to the goals of clinicians can use the instruments safely in clinical practice.
cleaning, shaping and filling root Results. The author presents 12 measures that clinicians can take to
canals,4,5 and they may adversely prevent NiTi rotary instrument fracture and discusses them in detail.
affect the outcome of endodontic Clinical Implications. NiTi rotary instrument fracture complicates
treatment.2,6-8 Techniques for the progress, and compromises the prognosis of endodontic treatment.
removing fractured instrument However, when clinicians take appropriate measures, rotary instrument
fragments from root canals have fractures can be prevented.
been described in the dental litera- Key Words. Nickel-titanium; rotary; instrument; fracture; prevention.
ture.9,10 However, removal of frag- JADA 2007;138(2):196-201.
ments may be impossible or imprac-
tical, especially when they are small
and located in the apical portion of
narrow curved root canals or when Dr. Di Fiore is an associate professor of endodontics and the director, Predoctoral Endodontics, New
repeated attempts at removal could York University, College of Dentistry, Department of Endodontics, 345 E. 24th St., New York, N.Y.
result in excessive enlargement of 10010. Address reprint requests to Dr. Di Fiore.

196 JADA, Vol. 138 http://jada.ada.org February 2007
Copyright ©2007 American Dental Association. All rights reserved.


File Characteristics


size, taper and cutting flute depths are important
factors that affect the torsional and bending prop-
erties of rotary instruments. 15-19

Size. A comparative study of the fatigue resis-
tance of NiTi rotary instruments of different sizes
and flute designs revealed that large instruments
were highly susceptible to fatigue failure.15

Research has demonstrated C A Las P R instru- I C E
C L I N I that an A C T
ment’s cross-sectional diameter increases, it

becomes less resistant to cyclic fatigue.18 After
d performing dynamic stress tests on various NiTi
Copyright ©2007as
rotary instruments, investigators found that American De
ware an instrument’s size and taper increase, the
re torque generated during rotation increases and
hap. the fracture time decreases.12,13
sures Taper. In cyclic fatigue-to-fracture tests of dif-
e risk
ferent NiTi rotary instruments, researchers found

torque generated during rotation increases and
the fracture time decreases.12,13
Taper. In cyclic fatigue-to-fracture tests of dif-
ferent NiTi rotary instruments, researchers found
that 0.06 taper instruments had less resistance to
fracture than did 0.04 taper instruments.19 Shen
and colleagues compared the types of failures
20

that occurred with NiTi rotary instruments of
various geometric designs and reported that a
very high percentage (21 percent) of the instru-
ments that fractured had progressively larger
tapers and a much lower percentage (7 percent)
had consistently even tapers. They
also noted that failures for progres-
taper and sively tapered instruments tended
flute depths to be fractures, whereas for evenly

after they have been used for one clinical case. drills.34,35
efficientl
CANAL CURVATURE ASSESSMENT funnel th
CANAL CURVATURE ASSESSMENT
The fracture potential of an instrument rotating entrance
in a curved canal becomes greater as the angle of root cana
curvature increases and the radius of curvature binding,
decreases.11,13,14 Zelada and colleagues30 and fore, pra
Martin and colleagues31 reported that during the introduc
preparation of root canals in extracted molar canal.
teeth, all instrument fractures occurred in
severely curved canals with angles of curvature MANUA
greater than 30 degrees. A careful preoperative Hand ins
radiographic examination with fine hand instru- sageway
ments in the canals will reveal the presence and progress
acuity of root canal curvatures. Therefore, when have dem
curvatures are present, the operator should be mentatio
wary of the possibility of a fracture and proceed ments, u
cautiously during root canal preparation. instrume
incidenc
ACCESS PREPARATION preparat
In the internal configuration of an adequate leagues36

tapered instruments failed rapidly with little
rotation.
Cutting flute depth. Instruments with deep cut-
ting flutes and progressively larger variable
tapers have rapidly changing cross-sectional
diameters along the entire length of their shafts.
These instruments develop high torque levels
that make them more prone to metal fatigue and
fracture. However, instruments that have shallow
cutting flutes, evenly tapered shafts and consis-
tently shaped cross-sectional areas are more
resistant to fracture. This is because the torsional
and bending stresses that develop during use are
distributed uniformly along these instruments’
entire length. 19-21
Therefore, practitioners should
be completely familiar with the mechanical fea-

their surfaces. It is important for clinicians to
16,22,23

realize that these pre-existing conditions asso-
ciated with the Instrument Use process may con-
manufacturing
tribute to the propagation of instrument fractures
during use.24,25 Cyclic fatigue and torsional testing
procedures that measured rotation time and
torque level at fracture have demonstrated that
used rotary instruments are significantly more
susceptible to fracture than are new ones.26,27
These findings are further supported by SEM
observations of used instruments that revealed
signs of deterioration, including surface cracks
that can progress to fractures with
further use.22-25,28 Sotokawa29 found
that by applying a systematic Practitioner
schedule for the disposal of enlarge roo

ACCESS PREPARATION


CANAL ORIFICE ENLARGEMENT


canal.
MANUAL INSTRUMENTATION
MANUAL INSTRUMENTATION
Hand instruments can create a smooth, open pas-
sageway for rotary instruments to follow as they
progress to the apical terminus. Three studies
have demonstrated that manual root canal instru-
mentation with fine stainless steel hand instru-
ments, used in a step-back manner before rotary
instruments were used, significantly reduced the
incidence of rotary instrument fracture during the
preparation of curved canals.36-38
Roland and col-
leagues and Patino and colleagues used fine
36 37

hand instruments to enlarge curved root canals in


canal instrumentation so that when the torque on
an instrument, rotating at a AND TORQUE IRRIGA
ROTATIONAL SPEED constant speed,
reaches a preset level, the motor automatically Irrigati
CONTROL and allows the
reverses its rotational direction plishin
file to be withdrawn before it locks and fractures SEM st
in the root canal.39 Gabel and colleagues40 investi- have de
gated the influence of rotational speed on the ated du
failure of NiTi rotary instruments for the prepa- root can
ration of root canals in extracted molar teeth and remova
found that instrument distortion and fracture ethylen
were four times more likely to occur at higher is a com
rotational speeds (333 rotations per minute) than prepara
at lower rotational speeds (167 rpm). Gambarini41 canal c
found that instruments used in low-torque motors ical ove
( 1 Newton per centimeter) were more resistant stresse
to fracture than those used in high-torque motors fore, du
( 3 N/cm). Therefore, practitioners should use should
electric motors set at low rotational speeds and gate ca
low torque levels during root canal preparation.
ROTAR
CROWN-DOWN TECHNIQUE The ma

CROWN-DOWN TECHNIQUe


IRRIGATION AND LUBRICATION


related failures during root canal preparation in ro
E
extracted teeth. The operator’s ability to sense
OPERATOR PROFICIENCY
and resist these binding and locking tendencies is A
a skill that can be obtained only with experience. N
Yared and colleagues, 39,52,53
in several extensive
m
investigations, showed that preclinical training in 19
the use of NiTi rotary instruments for the prepa-
ce
ration of root canals in extracted molar teeth was E
crucial for avoiding instrument fracture. There-
fil
fore, inexperienced operators should engage in
preclinical training exercises as learning experi- au
ences before using these instruments on patients, te
then proceed carefully in clinical practice as they in
20
gain experience.
ni
SUMMARY AND CONCLUSION
in
There are several measures that practitioners can
26

rs fore, during root canal preparation, practitioners
should lubricate instruments generously and irri-
gate canals copiously.
ROTARY INSTRUMENT MANIPULATION
ROTARY INSTRUMENT MANIPULATION
The manner in which NiTi rotary files are manip-
s ulated for preparing root canals is extremely
important. It has been shown that a cyclic axial
motion applied to rotary instruments during oper-
f ation was significant in preventing premature
fracture.51 Also, a pecking or pumping motion,
which lowers apical forces during root canal
In preparation, has been advocated by researchers
as an important way to prevent instrument
d binding and torque-generated cyclic fatigue.12-14
p- Li and colleagues14 tested the cyclic fatigue of
NiTi rotary instruments under static and
e- dynamic pecking motion conditions and found

techniqu
then proceed carefully in clinical practice as they instrume
2003;29(
gain experience. 11. Pru
nickel-tit
SUMMARY AND CONCLUSION 12. Sat
instrume
There are several measures that practitioners can 26(3):156
13. Ha
take to prevent NiTi rotary instrument fracture and cycli
during root canal preparation: instrume
14. Li U
davoid subjecting NiTi rotary instruments to endodon
excessive stress; tests. J E
15. Ch
duse instruments that are less prone to fracture; Buono V
dfollow an instrument use protocol; endodon
16. Ku
dassess root canal curvatures radiographically titanium
and instrument them carefully; 17. Xu
and bend
densure that the endodontic access preparation instrume
is adequate; 18. Ull
loads in
dopen orifices before negotiating canals; 2005;31(
denlarge root canals with fine hand 19. Yao
rotary ni
instruments; 55-7.


g dset rotational speed and torque at low levels;
duse the crown-down technique;
n dirrigate and lubricate root canals during
preparation;
dmanipulate rotary instruments with a pecking
or pumping motion;
dif inexperienced, engage in preclinical training
in the use of rotary instruments.
Instrument fracture is a serious iatrogenic
mishap that can complicate and compromise
endodontic treatment. It therefore is imperative
that clinicians using these instruments in prac-

Endo file separation discussion

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