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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 length.

 

Materials and Methods

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.

 

Results

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.

 

Discussion

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.

 

Conclusion

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.

 

References

  1. Grossman LI. Endodontic Practice. 10th ed. Philadelphia, PA: Lea & Febiger; 1981:200-236.
  2. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am 1974;18(2):269-296.
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  10. Weine FS, Kelly RF, Lio PJ. The effect of preparation procedures on original canal shape and on apical foramen shape. J Endodont 1975;1:255.
  11. Abou Rass M, Frank AL, Glick DH. The anticurvature filing method to prepare the curved root canal. J Am Dent Assoc 1980;101(5):792-794.
  12. Walton RE. Current concepts of canal preparation. Dent Clin North Am 1992;36(2):309-326.
  13. Goerig AC, Michelich RJ, Schultz HH. Instrumentation of root canals in molars using the step-down technique. J Endodont 1982;8(12):550-554.
  14. Roane JB, Sabala CL, Duncanson MG Jr. The "balanced force" concept for instrumentation of curved canals. J Endodont 1985;11(5):203-211.
  15. Stoeckel D, Yu W. Superelastic NiTi wire. Wire J Int 1991;3:45-50.
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