Root Canal Instrumentation with a Patency Technique
Peter Velvart, DMD
An important aspect of endodontic therapy is the mechanical
cleansing and shaping of the root canal system.1,2 In order to
maximally clean the canal, its shape must be maintained in all dimensions. To
achieve the ideal preparation of the root canal, the dentin should be evenly
removed in all directions and at all levels in the canal (Figure 1).
Unfortunately, when curved canals are treated, the instrumentation methods can transport
the apical, or root-ward, portion of the canal toward the outside of the curve
and the coronal portion toward the inside (Figure 2).3
Straightening of curved canals has been attributed to large-sized
stainless steel files being too rigid so they tend to straighten the canals 4,5
as well as dentin debris deflecting the instrument away from the original path
of the canal. This results in a
straightened canal and occasional perforation.
In order to minimize these complications, modifications in
instrumentation6-9 and techniques have been suggested10-12
such as the anti-curvature technique,11 the step-down technique,13
and the balanced-force technique.14 Until recently, stainless steel
was the metal from which all files were produced. Recently, nickel-titanium
files have been introduced. The nickel-titanium alloy exhibits
"superelasticity,"15 which enables it to return to its
original shape even after substantial deformation. That means nickel-titanium
instruments can be rotated beyond a curve in the canal, resulting in less
straightening of the original canal shape.
To reduce canal transportation and the loss of length of a
calcified and/or curved canal, the "patency instrumentation"
technique has been developed for use with nickel-titanium files. In this
process, a file with a 0.02 taper is placed to the correct working length
(Figure 3). Once this has been accomplished, the canal is instrumented using a
file with the same apical size but different taper (0.023) until it is loose
(Figure 3). Since the tip size is identical to the previous file but the taper
is different, the instrument cuts only in the coronal area and not at the tip.
This subsequently maintains the patency of the apical canal and significantly
reduces the potential of creating an apical dentin plug, losing canal length,
and deflecting the file during instrumentation. The instrumentation is
continued with a file of the same apical size but 0.026 taper (Figure 3). Files
with 0.02 tapers of the next larger diameter are then used to the full length
of the canal so that complete cleaning of the canal is achieved.
Since minimal dentin is produced and the prepared coronal canal
space allows the debris to escape in a coronal direction, this should
theoretically reduce the potential of straightening the canal or loss of
Standardized plastic blocks with 45° curvature and initial patency
of a size #10 were used to simulate the root canal. The blocks were divided
into four groups of 10 blocks each to compare four instrumentation methods. The
canals in Group 1 were instrumented with stainless steel files with a non-cutting
tip using a balanced-force technique without pre-flaring. The second group used
the patency instrumentation technique. The first, second, and third files had a
taper of 0.02, 0.023, and 0.026, respectively. Stainless steel files sized #10
through #25 and nickel titanium filed sized #30 through #40 were used. Group 3
canals were instrumented using the balanced-force technique and the stainless
steel files with a cutting tip. The canals in Group 4 were instrumented with
stainless steel files using a sharp, cutting tip and a push/pull
circumferential filing technique with a quarter-turn motion. This group served
as the standard control.
The statistical analysis was completed with an analysis of
variance. The mean canal length loss and t-test
comparisons of the results were subsequently analyzed (Figure 4). The patency
instrumentation technique used by Group 2 produced the least loss of canal
length with a mean of 0.3 mm ± 0.54. Statistically, this was superior to all other
techniques tested (P \< 0.01).
Group 1’s balanced-force technique with safe end tips was superior
to both Group 3, the balanced-force technique with a cutting tip (1.15 ± 0.58
versus 2.4 ± 0.516, P \< 0.01),
and Group 4, the push/pull technique with a cutting tip (1.15 ± 0.58 versus 2.8
± 0.35, P \< 0.01). Groups 3 and 4
did not differ significantly from each other.
The loss of length in the cutting tip groups was rather extensive
and presumably an exaggerated representation of the clinical environment. Since
all the techniques were performed under identical conditions, it may be assumed
that their relative efficiency will be similar when used in vivo.
It appears that the instrumentation of the coronal aspect of the
canal prior to the apical part is beneficial and accounts for the improved
performance of the technique in comparison to the traditional balanced-force
and push/pull procedures. In addition, the presence of the file in the apical
portion of the canal might act as a physical barrier to prevent the movement of
cut dentin apically, thus maintaining the patency of this critical portion of
the canal. When coronal instrumentation is complete, the apical part of the
canal is instrumented. Since the apical third is patent, the nickel-titanium
files appear to easily negotiate the curve to the apex. As the files are only
instrumenting a small portion of the canal, significantly less dentin is
produced than with the other techniques.
The cutting tip's ability to engage the dentin wall and "make
its own canal" made the balanced-force technique more effective with the non-cutting
tip than the cutting tip. Use of the latter often resulted in canal
straightening, ledging, and loss of length. A potential difficulty with the non-cutting
tip as described in the balanced-forced technique may be that the canal is not
cleaned to its full length, although the instrument appears to approach the
foramen. Obviously, this is not biologically desirable. With the patency instrumentation
technique, the apical canal is not initially cleaned, either, but will be once
the coronal canal has been filed and space has been created.
This study indicates that the patency instrumentation technique
with files of variable taper and non-cutting tip is a superior technique. Both
the push/pull and balanced-force techniques that used cutting tips caused a
clinically unacceptable degree of canal length loss. While the design of
endodontic instruments continues to be refined, additional studies must be
completed to determine the most effective manner of utilizing these tools
during root canal therapy.
- Grossman LI. Endodontic Practice. 10th
ed. Philadelphia, PA: Lea & Febiger; 1981:200-236.
- Schilder H. Cleaning and shaping the
root canal. Dent Clin North Am 1974;18(2):269-296.
- al Omari MA, Dummer PM, Newcombe RG,
Doller R. Comparison of six files to prepare simulated root canals. 2. Int
Endodont J 1992;25(2):67-81.
- Lentine FN. A study of torsional and
angular deflection of endodontic files and reamers. J Endodont 1975;5:181-191.
- Pettiette M, Metzger Z, Phillips C,
Trope M. Endodontic complications of root canal therapy performed by dental
students with stainless-steel K-files and nickel titanium hand files. J
- Miserendino LJ, Moser JB, Heuer MA,
Osetek EM. Cutting efficiency of endodontic instruments. Part I: A quantitative
comparison of the tip and fluted regions. J Endodont 1985;11(10):435-441.
- Miserendino LJ, Moser JB, Heuer MA,
Osetek EM. Cutting efficiency of endodontic instruments. Part II: Analysis of
tip design. J Endodont 1986;12(1):8-12.
- Powell SE, Simon JH, Maze BB. A
comparison of the effect of modified and nonmodified instrument tips on apical
canal configuration. J Endodont 1986;12(7):293-300.
- Roane JB, Powell SE. The optimal
instrument design for canal preparation. J Am Dent Assoc 1986;113(4):596-597.
- Weine FS, Kelly RF, Lio PJ. The effect of
preparation procedures on original canal shape and on apical foramen shape. J
- Abou Rass M, Frank AL, Glick DH. The
anticurvature filing method to prepare the curved root canal. J Am Dent Assoc
- Walton RE. Current concepts of canal
preparation. Dent Clin North Am
AC, Michelich RJ, Schultz HH. Instrumentation of root canals in
molars using the step-down technique. J Endodont 1982;8(12):550-554.
- Roane JB, Sabala CL, Duncanson MG Jr. The
"balanced force" concept for instrumentation of curved canals. J
- Stoeckel D, Yu W. Superelastic NiTi wire.
Wire J Int 1991;3:45-50.