- Journal of Endodontics

0099-2399/85/1105-0203/$02.00/0
JOURNAL OF ENDODONTICS
Copyright 9 1985 by The American Association of Endodontists
Printed in U,S,A.
VOL. 11, NO. 5, MAY 1985
The "Balanced Force" Concept for Instrumentation of
Curved Canals
El Concepto de "Fuerza Balanceada" para la
Instrumentacion de Conductos Curvos
James B. Roane, BS, DDS, MS, Clyde L. Sabala, BS, DDS, and Manville G. Duncanson, Jr., DDS, PhD
preparation of the associated pulp canal system becomes difficult. In fact, curvature introduces such complexity that total instrumenting concepts have been
developed to deal with the curved canal (1-5). Even
with these, it is commonplace for a canal located within
a curved root to be enlarged to a smaller final diameter
than it would be if it were located within a straight root.
Justification for such alteration is simply the fact that a
curvature introduces factors into preparation which, if
not controlled during enlargement, will cause transportation, ledge formation, and even perforation (1-4, 6,
7). The more severe the curvature of the root, the more
one tends to reduce the intended preparation diameter
in an effort to prevent irreversible damage of the canal
wall. Reducing the preparation size under such circumstances appears to be logical for two reasons: (a)
smaller diameter preparation means less cutting of the
canal walls and consequently a lesser likelihood for
expression of undesirable cutting effects and (b) small
diameter files are more flexible and therefore less likely
to cause transportation during enlargement.
The curvature problem appears to be solved by small
preparation diameters until one examines what that
solution sacrifices in canal debridement and reliability
of the final seal. Smaller preparation diameters reduce
the amount of mechanical and chemical cleansing of
the canal space. Several studies appear to indicate less
complete removal of debris when small nonflared preparations are used (8-10) while others relate to the
flushing effect of irrigants and indicate that irrigants are
not effective in washing debris from a small diameter
canal (11-13). Finally, Allison et al. (14)indicate that
preparation size/design has an influence upon the final
seal. They found that the best seal was achieved when
a stepback preparation was used.
The goals of an endodontic treatment are to remove
the canal's soft tissue contents as completely as possible, eliminate as completely as physically possible any
microbial elements, and create a situation within the
Canal curvature has always introduced complexity
into canal preparation. The "balanced force concept," developed by trial and error experimentation
over the past 12 yr, is proposed as a means of
overcoming the curvature influence. Its concepts
use force magnitudes in order to create control over
undesirable cutting associated with canal curvature.
Rotation is promoted as the means for maintaining
magnitude as a control and counterclockwise direction of rotation provides finite operator control. Diagrammatic evaluations, mathematical calculations,
bending moments, test canals, sectioned teeth, and
clinical radiographs are presented to document
each step of the concept. The concept comes to
fruition with the introduction of a new K-type file
design.
La curvatura del conducto siempre representb una
complejidad en la preparacion del mismo. El concepto de "fuerza balanceada" desarrollado a traves
de experimentacibn pot ensayo y error durante los
ultimos 12 a~os, se propone como un medio de
superar la influencia de la curvatura. Sus conceptos
usan magnitudes de fuerza a fin de Iograr el control
del corte indeseable asociado con la curvatura del
conducto. Se promueve la rotacibn como medio de
mantener la magnitud como control, y la direccion
de rotaci6n en sentido contrario a las agujas del
reloj permite un definido control al operador. Se
presentan evaluaciones diagram&ticas, calculos
matematicos, momentos de torsi6n, pruebas de
conductos, dientes seccionados y radiografias clinicas para documentar cada paso del concepto, que
se ve completado con la introducci6n de un nuevo
dise~o de limas tipo K.
Root curvature is a frequent occurrence in the human
dentition and when a curvature is present, endodontic
203
204
Journal of Endodontics
Roane et al.
canal that can prevent microbes or toxic substances
from passing through the canal system to the apical
supportive structures. To routinely and dependably accomplish these goals each time a canal is prepared, it
seems reasonable to demand the same completeness
of preparation for each canal regardless of whether it
is straight or curved. Variations in the size of preparations should occur in response to root or canal size
rather than the degree of root curvature. If such were
possible, specifically defined standardized canal preparations based on intracanal morphological needs and
not root curvature could be used. This is not presently
the case and to become capable of such preparation
will require advancement in the existing preparation
technology. Our goal of preparation based upon root
size rather than curvature was initially envisioned by
Roane in 1970 and, as a result of 12 yr of trial and error
experimentation, a preparation technique has been developed which appears to accomplish the specified
goals. This article is an attempt to describe that technique termed "the balanced force concept," to support
its validity, and to make its concepts available so that
others may also examine and test its validity.
RATIONALE
The balanced force concept was derived from the
physical law which states: for every action there is an
equal and opposite reaction. To develop the concept,
this law was used to identify and define actions and
reactions that occur during canal preparation in order
to study them and attempt to define a sequence of
events and motions that could be used to control
endodontic instruments during preparation. Successful
motions were retained and a preparation method defined which directed high magnitude forces against
small magnitude forces to develop a balance of action
to reaction, making it possible to ignore curvature during canal preparation.
To understand the balanced force concept it is necessary to study the design of preparation instruments,
develop a thorough knowledge of their characteristics,
and learn to recognize their complete capabilities as
well as their specific behavior during movement. With
this accomplished that knowledge may be used to
select an instrument which provides enough variation
in capabilities to allow the user a means of instrument
control when canal curvature is encountered.
Clinical usage and subsequent physical analysis indicate that it is best to select a triangular cross-section
K-type file. This type of preparation instrument offers
several advantages over other cross-sectional designs
and instrument types, when a balance of forces is being
sought. Most importantly, the K-type design provides
cutting edges with identical rake and clearance angles
(15) reg~dless of the direction of movement. Since
these angles remain unchanged by direction of ap-
proach with the cutting surface, the K-type instrument
may be used as a bidirectional cutting tool without a
loss of efficiency (16) (Fig. 1). Bidirectional cutting
means that the operator has two more cutting directions available with a K-type instrument than would be
available if a Hedstrom-type instrument were used.
Second, when a triangular configuration is selected the
cross-sectional mass of each file is reduced, the flute
depth is increased, and the magnitude of the bending
moment or "restoring force," as it will be referred to
here, is decreased.
A triangular file has a cross-sectional area or mass
that is 37.5% less than that of a square file of the same
standardized size. This point is verified by calculating
the area of two # 4 0 files, assuming one to be manufactured with a triangular and the other with a square
cross-section. The area of a triangular cross-section file
is determined using the equations b -- 3 R/1.732 and
A = b/4 x 1.732. In these equations b represents the
length of one side of an equilateral triangle, R the
circumradius or one-half of the cutting diameter, i.e. 1/2
of 0.40 mm, and A the area of the triangle. Substitute
the known values and find b = 3 (0.2)/1.732 -- 0.346
mm. With the value thus obtained for b, the second
equation may be solved for the cross-section area: A =
0.346/4 x 1.732 = 0.0499 mm 2. For a square file, the
FiG 1. W is a line drawn tangent to the circumference of cut c at a
point of contact with one cutting edge of a triangular shaped K-type
instrument. W is used to describe the clearance angles U and V into
the flute space associated with that cutting edge. They are equal and
are 60 degrees by definition. P is a tangent line drawn in a similar
fashion to intersect one edge of a square K-type instrument. It is
used to define the clearance angles Q and R which are equal and 45
degrees by definition. The sides of the square and triangle are lettered
a and b, respectively, and the areas are shaded for comparison. It is
easy to recognize that the square has a cross-sectional area which
is much larger than that of the triangle, i.e. 37.5% greater. The rake
angle for both the triangle and square may be measured as the angle
between the radial line d and the lines b and a, respectively. Both
shapes produce negative rake angles; however, the triangle enjoys a
15-degree more favorable angle. Neither the rake nor the clearance
angle is altered for either by the direction of movement.
Vol. 11, No. 5, May 1985
equations a = d/1.414 and A -- a 2 are required. In these
a represents the length of one side of the square, d the
cutting diameter of the file or 0.40 ram, and A the area
of the square. Substitute the known values and find: a
= 0.4/1.414 = 0.282 mm. By using that for a, the area
equation is solved thusly: A = (0.282)2 -- 0.0795 mm 2.
Results of the preceding calculations rounded to two
significant numbers indicate that when the cutting radius is 0.4 mm (i.e. a #40 file), a triangle has a crosssectional area of 0.05 mm 2 while a square has an area
of 0.08 mm 2. With the recognition that curvature could
be considered as a constant acting upon both shapes
and that the amount of metal, i.e. the cross-sectional
area, varies between the two shapes, then one shape
may be compared with the other using a percentage
ratio based upon their cross-sectional areas. By using
this method, it becomes apparent that a triangular file
has only 62.5% as much cross-sectional area as a
square file of equal cutting radius and if geometry had
no affect upon bending, the triangular file would be
expected to develop only 62.5% as large a resisting
load (restoring force) against the canal wall as a square
file assuming both were placed through the same curvature. Clinically speaking, a triangular file is predicted
to be more flexible and to apply a lighter restoring force
against the wall of a curved canal during preparation.
This means, that the triangular ground instrument will
be less likely to transport a canal wall during preparation. Its K-type bidirectional design allows the operator
to select any of four potential cutting approaches, i.e.
insertion, withdrawal, clockwise, or counterclockwise
rotation. Figure 1 provides a graphic comparison between the triangular and square, i.e. cutting edge configuration, rake, and clearance angles.
By recognizing that geometry has an influence upon
structural properties of materials, it is desirable to establish a relationship between the restoring force and
the cross-sectional geometry rather than only the area
as just previously considered. To do this it must be
recognized that the restoring force may be measured
through its opposing force, the angular deflection bending moment. Values have been experimentally deterI mined for bending moments between various geometric
shapes and were recently reported by Krupp et al. (17).
In their study, the amount of force required to produce
a 60-degree angular deflection of a triangular size 30
Unitek file was one-third less than that required to
produce an equal deflection of a square size 25 Unitek
file. Their graphs indicated that a bending momer)t
slightly greater than 40 g-cm deflected the size 30
triangular file 60 degrees while a bending moment
slightly greater than 60 g-cm was required to deflect
the size 25 square file equally. A decrease in the
restoring force reported as bending moment occurred
in spite of a greater cutting radius, i.e. larger instrument
size of the triangular file. The square file in this case
Balanced Force Concept
205
has 83% as large a cross-sectional area as that of the
comparable triangular file yet it generates about one
and one-half times as large of a restoring force, i.e.
bending moment of 60 versus 40 g-cm. By recognizing
that the restoring force is a statically applied load which
holds the files cutting surfaces against the curvature in
a single direction and that it is the load which is responsible for a straightening of the curve during
preparation, one can predict that a square file is more
likely to uncontrollably straighten the canal, to cut excessively at its tip, and therefore is more likely to ledge
a preparation. The difference in restoring force magnitude coupled with our unreported clinical and experimental observations lead us to conclude that a square
shape should be avoided whenever possible, but especially when using large instrument sizes.
The next step in understanding how to maintain
preparation size past a curve is learning to identify the
direction and location for expression of the restoring
force which is generated by elastic distortion within an
instrument when it is passed through a canal curvature.
Identification allows recognition of where the loads
concentrate and provides information useful in learning
how to prevent effective expression of those loads in
undesireable areas. It is important to learn to control
these forces in order to prevent them from becoming a
major influence during canal enlargement as their
expression can produce disastrous results. To accomplish control it is necessary to compare the magnitude
of loads, apply the rule that for every action there is an
equal and opposite reaction, and finally identify motions
which will direct the forces applied by the operator in
such a manner that they will mask those generated by
curvature. When such a balance is established, the
canal may be enlarged through its original axis, at least
in the apical third of the canal. In the process transportation is eliminated or at least displaced coronaUy where
it is less likely to have serious consequences. The
desired balance of forces can be generated simply by
rotating the instruments to produce canal enlargement.
This is true since rotation directs dentinal hardness
against the restoring force of the curved instrument
and simultaneously uses that hardness to create the
cutting loads. Rotation prevents expression of the curvature-generated restoring force via magnitude, at least
for a limited range of file sizes. Figure 2 illustrates the
balance of forces generated by rotation. The relative
magnitudes of the force generated from dentinal resistance and the restoring force generated in the instrument by a curvature are not defined; however, performance tests demonstrate the validity of the assumption
that the restoring force is of less magnitude than the
forces created from dentinal resistance (Figs. 3 and 4).
The magnitude of dentinal resistance is a function of
dentinal hardness and generates a force during preparation which remains relatively constant for each canal.
206
Roane et al.
FIG2. The enclosing circle representsdentinas it contactsthe cutting
edges of a file. R representsthe internal force applied by dentinal
hardnessas it is vectoredtoward the center of the instrument.S is a
restoringforce appliedagainstthe curvature by the file attemptingto
return to its original straight condition. S remains stationaryas the
instrument is rotated while R rotates with the blades. As long as R
remains greater than S, the instrumentwill not transport the canal;
however, shouldthe file be pulledout rather than rotated this formula
fails and S applies the primary cutting load. Transportation will
frequently occur undersuch conditions.
The magnitude of the restoring force developed
within a file is a function of the file's mass, geometry,
and composition as well as the radius and arc of the
instrumentation curve. Its expression is inversely related to the distance from the curve to the instrument
tip. Consequently, the restoring force is a variable force
and it will increase: (a) if the metal mass increases,
either as a result of shape or instrument diameter; (b)
if the radius of the canal curvature is decreased; (c) if
the arc of the canal curvature is increased; or (d) if the
distance from the curve to the file tip is decreased. By
using these relationships, one can identify specific canal
alterations that must be achieved in order to accomplish
large preparation sizes around curvatures without undesireable results, i.e. transportation, ledges, and perforations. Initially, the radius of the curve should be
increased by creating a canal access (1, 4, 5), i.e.
opening the coronal end and straightening the curvature
(Fig. 5). This effectively increases the radius and decreases the arc of the canal curvature by allowing the
instrument a straighter path to the apex. The resulting
straighter instrument generates a lesser restoring force
along its cutting edges and tip. The lighter loads produce less dentin removal and the canal is enlarged
using cutting pressures equal to that of smaller.instruments. As a consequence, larger diameter instruments
may be used before transportation, ledging, or perforation are likely. To avoid apical expression, primarily
ledging, the distance from. the curve to the instrument
tip may be increased by extending the file tip beyond
the apex before introducing the next larger instrument.
Journal of Endodontics
Overextension is generally considered to be undesirable and the consequences of leverage are best managed by modification of the instrument tip in order to
gain balance through the removal of the terminal cutting
points.
Modification of the instrument's tip is a recent innovation and has produced perhaps the most dramatic
change in instrument response within the concepts of
balanced forces. It entails removal of the cutting surfaces that primarily express the static restoring force
and therefore the surfaces that are primarily responsible
for canal transportation. Proper removal of these cutting points provides better instrument control than any
previously recognized method, including canal access
preparation (Fig. 5). The modified instrument is not
presently available on a commercial scale. A photograph of the modification is presented in Fig. 6 along
side that of a normal tip to emphasize the terminal
points. These points cut in response to a restoring
force produced when the curvature deflects the file
FIG 3. This electron micrographof a prepared canal in a mandibular
incisor reveals lingual movement of the preparation. The access in
this tooth appliedpressureto the file shaft whichshouldhavecaused
movement facially. The observationof a tapered preparationto the
lingual supports the balance of force magnitude concept. Missing
dentin to the lingualunbalanceddentinalresistanceto that side and
allowed the file to be pushed away from the F wall maskingentirely
the expressionof any restoringforce along the F wall.
Vol. 11, No. 5, May 1985
Balanced Force Concept
207
FiG 4. Presented are four cases that were prepared using the balanced force concept. The minimum apical preparation diameter was 0.45 mm.
The second molar in A was completed using standard files and tip overextension whereas the other three cases were completed using modified
instruments.
64 TE$
#LIDDEN
2hNhL
4CCE$$
A
FtG 5. This graphic representation A and actual molar canal access B illustrate how an access preparation alters the curvature's effect upon
enlarging instruments. B and C are the areas of dentin removal which are responsible for angular change. The change in instrument entry angle
before and after canal access is illustrated as the angle a and represents a reduction in the arc of the canal's curvature. Space created by the
access allows the canal curvature to be expressed in the instrument as though the radius of the curve had been increased. The curve becomes
more generalized and distributes throughout the canal length, thereby decreasing the curvature-related forces and their expression.
208
Roane et al.
Journal of Endodontics
is evident that use of the balanced force concepts,
especially with modified instruments, enables one to
prepare a curved acrylic canal much larger than classical techniques, without producing apical transportation.
File tip modification and use of the balanced force
~ s,," A
F
I
//
Z
FIG 6. Presented are two triangular K-type files. File A has a standard
tip with distinct points created when its cutting edges are terminated
with a standardized tip. File B is a prototype modified instrument with
a parabolic tip. This configuration eliminates the terminal points and
produces supporting triangular planes which distribute loads to keep
them below cutting magnitude. Penetrating capacity is maintained as
the flute depths are unchanged and the three original cutting edges
are replaced by six new edges.
from its passive or zero force straight line position (Fig.
7). The restoring force when present is transmitted to
the file tip through its metal shaft which acts as a lever
arm.
Altering the instrument tip removes its capability to
respond to elastic distortion in a concentrated area and
thereby mandates the expression of those forces over
the length of each cutting edge rather than at the file
tip. Thus, with the terminal points removed, internal
distortions established by the canal curvature generate
a restoring force which is proportioned over the cutting
edges and .dispersed enough to allow the relatively
larger magnitude of dentinal hardness to deny noticable
expression. Consequently, the file's straight profile becomes unapparent in the completed preparation. Introduction of tip modification introduces an ability .to enlarge a curved canal even along its inside wall completely to the apex. This ability does not appear to exist
within the methods tested to date using standard
ground K-type instruments. (Fig. 8). If the results seen
in Fig. 8 are'compared with those of Weine et al. (2), it
FIG 7. This hypothetical canal curvature is separated into its component parts. Point A is the axis of the curve, r is the radius, and a
defines its arc. When a theoretical file is passed around this curve, it
develops internal forces as a result of molecular compression and
expansion within its mass. Those forces, i.e. restoring forces, are
subsequently expressed at the file tip over the/ever FT. Changing
the curve by altering its arc or radius can alter the magnitude of the
restoring force, while changing the lever length by moving tip T away
from the curve helps to reduce expression by decreasing the leverage
advantage for the restoring force. Canal access may be used to
change the curvature while file overextension can be used to increase
the lever arm. File tip modification does not affect the lever; it simply
prevents expression of the generated forces. FZ indicates the file's
original straight condition and determines the zero force line, i.e. no
restoring force.
FIG 8. Presented are two acrylic canal models which have been
enlarged from an original diameter of size 20 through a file size 55
using the balanced force concept. Sample A was prepared with
standard instruments using 0.5-mm stepbacks every other file size.
It was the best of a series and shows slight outward transportation
apically. Sample B was prepared using modified files and no stepback.
It is typical of the series as no apical transportation was seen in the
entire group. Enlargement appears to have been accomplished along
both its inner and outer wall to the foramen.
Vol. 11, No. 5, May 1985
Balanced Force Concept
concept can enable one to easily enlarge a canal from
a size 20 to a size 55 file without recognizable transportation in the presence of rather significant canal
curvature.
209
f
TECHNIQUE CONSIDERATIONS
To utilize the balanced force concepts, instrumentation should be refined into placement, cutting, and
removal of each file using only rotary motions. Placement is accomplished using clockwise rotation (18) and
light, inward pressure. Cutting is accomplished using
counterclockwise rotation (19) and inward pressure
adjusted to match the file's strength, i.e. very light for
small instruments and heavy for very large instruments.
Cleaning or debris removal is accomplished using one
to two noncutting no pressure or slight outward pull
clockwise rotations. Cleaning is normally completed
only after the desired length has been reached and
maintained with counterclockwise rotation. Axial reciprocation or filing motion is used only to produce canal
transportation, to flare the coronal area, and during the
initial opening of calcified canals. Such motion voids the
balanced force formulation and its use should be limited
to extremely small diameter instruments or to that
portion of the canal which is coronal to the curvature,
except where transportation will serve to remove a
preexisting ledge or shelf. When unmodified instruments are used, the working depth should be shortened
by 0.25 mm with each change to the next larger size
instrument in order to prevent accumulation of tip cuts
at a single point. If accumulated, tip cuts will create a
mechanical ledge. The stepback precaution is not necessary when the tips are modified and filing action may
be used more freely without drastic loss of control.
The reason for clockwise placement and counterclockwise cutting may not be apparent at first. To
understand that concept, one has to again analyze file
design and postulate the reactions produced by variations of motion or direction. The cutting edges incline
down a K-type file shaft at approximately 45 degrees
from the vertical axis (Fig. 6). By standardized specification, the circumradius increases from the tip toward
the handle and the flutes spiral clockwise. The result is
simply that a load delivered by clockwise rotation pulls
away from the operator and moves the instrument
apically, while a load delivered by counterclockwise
rotation pushes toward the operator and moves the file
out of the canal (Fig. 9). As a consequence of this, the
operator may sense the total of all forces during counterclockwise rotation, torque plus outward movement,
while he or she senses only the torque portion of a
clockwise rotation. The unnoted inward movement in
that case places the instrument further into the canal
and excessively embeds its cutting edges into dentin.
The described movements are a consequence of vectored forces created when the applied torque interacts
pow , /
MOVEMENT
FIG 9. This illustration indicates the reactions which occur along the
blade inclines of a K-type file during clockwise and counterclockwise
rotation. Dentin strikes the incline opposite the rotating force, debris
is reflected to the dentin side, while a portion of the torque is vectored
causing the instrument to move into the canal when the torque is
clockwise and out of the canal when the torque is counterclockwise.
Inward movement is labeled power and is used to place the instrument into canals. Outward movement is labeled control. It is used to
incrementally disengage the cutting edges and may be opposed by
the operator pushing inwardly to produce a finite control over the
cutting force.
with the canal wall along the blade inclines. The resultant forces move the instrument into the canal when the
torque is applied clockwise and as the instrument
moves inwardly its standardized taper forces the cutting blades deeper into the canal walls. In other words,
at a given point within a canal, the cutting radius
increases as the file moves inwardly. Conversely that
radius decreases as the file moves outwardly. Inward
movement is a result of clockwise instrument rotation
while outward movement is a result of counterclockwise instrument rotation (Fig. 9). Hence, not only does
the operator sense the full load while rotating the
instrument counterclockwise but a load of dentin too
great to shear will cause the file to move outwardly and
that movement will simultaneously decrease the depth
of cutting edge penetration into the dentin. The decrease in penetration depth continues until the operator's applied inward pressure exceeds the shear
strength of the total engaged dentin and a cut results.
This relationship finitely adjusts applied force against
210
Journal of Endodontics
Roane et al.
the engaged dentin and gives the operator complete
control over each cutting action. With the use of this
system, the operator may recognize the accomplishment of a cut when a slight pop is felt. Continuing
counterclockwise rotation past 120 degrees, once dental cutting is recognized, enlarges the canal to the files
cutting diameter as each blade will have reached the
beginning position of another by that point. Further
counterclockwise rotation helps to ensure full diameter
enlargement and removal of dentin which may have
compressed away from the first blade pass. Upon
completing each cut, the file is again positioned for
cutting by using a clockwise placement stroke of onehalf or less revolution. Each placement is followed by a
counterclockwise cutting rotation. This sequence is
repeated until the working depth has been achieved
and the canal is fully enlarged by counterclockwise
rotation. When enlargement has been accomplished, a
final clockwise cleaning rotation is used to load canal
debris into the flutes and to elevate that debris away
from the apical foramen. Cleaning helps to prevent
debris accumulation within the canal where debris will
act to upset the balance of forces and also helps
prevent excessive loss of debris into the apical tissues.
Clockwise loading of debris is a nonenlarging motion
intended only to facilitate removal.
DISCUSSION
Utilization of the concept of balanced forces enables
one to produce enlargement of canals past severe
curvatures without compromise of enlargement concepts or preparation diameters. Examples of clinical
cases completed using the methodology described are
presented in Fig. 4. Without modified tips it is necessary
to extend the instrument slightly beyond the apex in
order to prevent transported walls and ledge formation.
When over extension is used, the tip must be retracted
to lie within the canal space at least one or two instrument sizes before completing the desired preparation
diameter in order to ensure establishment of a constriction or ledge. In those cases, the ledge created by the
last one or two instruments becomes the apical stop
and serves to prevent overextension of gutta-percha
during the filling procedure. Figure 4A presents a case
completed in this fashion. This process is neither completely accurate nor clinically desirable and it may be
eliminated in the future through the use of the modified
instruments. Extensive clinical trials of the modified files
have demonstrated this conclusion to be true. Several
cases are presented in Fig. 4 (B to D) which reveal the
clinical value of file tip modification.
CONCLUSIONS
The balanced force concept of instrumentation is
simply an expansion of the concept of reaming canals.
It differ~ primarily in that the cutting motion is intention-
ally counterclockwise and may be accomplished at any
level without blockage, especially when modified instruments are used. Instrument placement is accomplished
by clockwise rotation and is capable of producing significant loads on an instrument tip without requiring the
application of inward pressure by the operator. This
fact enables small instruments, i.e. # 8 and #10, to be
crushed past calcifications and allows one to open
calcified canals rapidly. In addition, this approach to
calcified canals appears to reduce the incidence of
secondary blockage from loosened particles.
The balanced force concept is similar to reaming in
the fact that clockwise rotation of each instrument must
be limited to no more than 180 degrees in order to
prevent overinsertion of the apical portion of the instrument into dentin. Such overinsertion causes the tip to
cease turning and allows the rotating force to unwind
the file coronal to that point and increases the likelihood
of instrument separation (20). Each placement load is
followed by a cutting motion, counterclockwise rotation
of 120 degrees or greater. This action completely enlarges the canal to the file diameter, frees the instrument, and prepares it for placement to a deeper depth
when the next clockwise rotation is supplied. Clockwise
placement and counterclockwise cutting rotations are
repeated until the desired depth or working length is
reached. On occasion the file becomes filled with debris
and will hesitate to accept the next placement motion.
When that occurs, the file must be removed, cleaned,
and then reinserted before instrumentation may progress. Upon reinsertion, preparation is continued until
the desired working depth is obtained and the canal
diameter has been enlarged by counterclockwise rotation of at least 120 degrees. A greater angle of rotation
is desired, i.e. one or two revolutions, but cannot be
safely accomplished in all canals. This is especially true
when an extremely sharp curvature exists as such a
curvature can easily cause fatigue failure and result in
instrument separation. Canals which are evenly curved
throughout their length produce little likelihood of fracture while those exhibiting sharp curvature concentrated in a small segment of the root require careful
rotation with minimum or limited clockwiseJcounterclockwise movement, i.e. 120 degrees either direction.
Enlargement is noticeably slower in such situations.
Sharp curvatures located primarily in the apical onethird of a canal present the greatest difficulty as they
do not allow much alteration of the curvature angle or
radius via canal access and their influence is expressed
through a very short lever arm.
To support the balanced force concept and establish
its safety, we have accumulated data relative to instrument failure during clinical use (20). In that study instrument damage was related to the direction of rotation
which produced the observed faults or failure in order
to determine the risk of instrument separation and its
relationship to the direction of rotation. The data ob-
B a l a n c e d Force Concept
Vol. 11, No. 5, May 1985
tained helps to explain our apparent disregard for reported counterclockwise instrument weakness (21, 22).
The findings of preparations produced in plastic canal
blocks using the balanced force concept will be presented in a subsequent article.
Dr. Roane is associate professor and chairman, Department of Endodontics,
University of Oklahoma College of Dentistry, Oklahoma City, OK. Dr. Sabala is
associate professor, Department of Endodontics0 University of Oklahoma College of Dentistry. Dr. Duncanson is associate professor and chairman, Department of Dental Materials, University of Oklahoma Health Sciences Center,
Oklahoma City, OK.
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