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Written by Richard Mounce, DDS Saturday, 01 July 2006 00:00
The minor constriction of the apical foramen should ideally be maintained at its initial size and position after endodontic cleansing and shaping procedures. Simply stated, if the minor constriction of the apical foramen is maintained at its original size and position, then excellence in endodontic treatment can result. The converse is also true. This article identifies strategies to maintain the apical constriction at its original size and position in order to achieve the aforementioned benefits (to be described in detail).
In the broadest sense, maintaining the apical constriction allows (in a nonresorbed apex and/or fully mature apex) a natural boundary or point up to which the clinician can place irrigants, shaping files, and obturation materials, and beyond which, outside of patency files, it is vital that solutions and instruments not pass. In the most specific and simplistic sense, everything that is done to a tooth endodontically has a significant impact directly or indirectly on the apical foramen. If the apical third is not properly cleaned and shaped, significant debris can be left, which can lead to later failure.
If the minor constriction is violated and transported in some fashion, then cleanliness is compromised and obturation is significantly harder to carry out well. Even if the apical third is not transported, if any portion of the canal is mismanaged in some fashion, the entire result is put at risk, and the focal point of this failure is ultimately the apical foramen. For example, if the coronal and middle third shaping and irrigation are not appropriate, then it is likely that an inadequate amount of irrigation will reach the apical third, necessarily compromising cleaning and shaping. The vital importance of the apical third might be thought of as the summit of a mountain. Achieving all the steps in the entire process of instrumentation can allow achievement of a preparation that treats the apical foramen appropriately and allows ultimate achievement of the climb (ie, to reach the summit of excellent treatment).
PRINCIPLES OF INSTRUMENTATION
Maintenance of the apical constriction at its original size and position adheres to the 5 principles of instrumentation listed below:
(1) Maintain the apical foramen at its original size and position.
(2) Maintain the canal in its original position (ie, the canal is only enlarged circumferentially, and its position not moved).
(3) Keep the apical foramen as small as is practical.
(4) The final preparation should resemble a tapered funnel.
(5) Facilitate obturation, yet keep the canal as small as is practical.
It is a valuable but important aside to note what instrumentation to a size 50, 70, etc really means relative to the apical foramen. In the most concrete terms, instrumentation of a canal to a given size means that the largest diameter noted (50, 70, etc) is the diameter created in the canal at or to the minor constriction of the apical foramen, and not beyond it. In other words, the largest diameter used does not in any way enlarge or move the minor constriction. Knowing to what size to instrument the canal is an inexact science; for the purposes of this discussion, it is important to know the initial diameter of the canal (gauge the canal). From this initial measurement of the narrowest canal diameter, the clinician can then determine to what size the canal should be instrumented at the constriction in its final diameter. Such gauging can and should occur after the bulk of canal instrumentation is carried out.
There is strong evidence that the traditional diameters to which canals have been instrumented might be too small, and that larger apical diameters are desirable.1-9 While a comprehensive discussion of the creation of larger apical diameters is beyond the scope of this paper, it is possible to create such diameters safely in 2 ways, amongst other possible methods. The K3 rotary Ni-Ti file system (SybronEndo) and LightSpeed (LightSpeed USA) both individually, or in combination, can be used to safely instrument canals to a larger apical diameter and not transport this delicate canal space in any fashion.
A brief discussion of calcification as it relates to the apical foramen is also relevant. Calcification occurs in a coronal to apical direction, the same as the progression of pulpal disease. The clinical correlation to this is simple in that if the clinician can break through the canal to a given level where the canal is patent and negotiable, then the canal should be relatively easily instrumented up to appropriate sizes if the process is carried out properly. Suffice it to say that once such a calcification is broken through and the canal is negotiable, it has great merit to use small
K-files to enlarge the canal at its apical terminus to approximately a 15 K-file (create a glide path) before crown-down instrumentation procedures commence.
Observation of the principle that the apical foramen should remain at its original size and position is one part of a larger set of principles guiding instrumentation, allowing for ideal canal shaping irrespective of the brand of files or materials used. Regardless of the file brand or technique used to shape canals (hand or rotary Ni-Ti), the goals of instrumentation are the same, especially with regard to the apical foramen. In practical terms, maintenance of the apical constriction has one significant effect especially relative to obturation. If the constriction is maintained at its original size and position, it provides a natural barrier to the movement of obturation material. As such, when obturating material is compacted apically with heat and cold pressure, having the constriction in place forces material into all of the ramifications of the pulp space, especially relative to a cold lateral obturation technique.
CHARACTERISTICS OF THE APICAL FORAMEN
Figure 1. Stylized version of the apical foramen. (Courtesy of Dr. Arnaldo Castellucci.)
It must be realized that statistically the apical foramen has several characteristics that must be identified. Any given tooth may have multiple apical foramen, and these foramen may or may not have a typical anatomic configuration. Said differently, the apical foramen can be anything but regular or singular. A minor constriction may not actually be present in some cases either naturally or through resorption. The foramen pictured in Figure 1 is a stylized description for educational purposes. On average, the diameter of the minor constriction is approximately 0.20 to 0.30 mm, and the average distance from the minor diameter to the major diameter of the apical foramen is approximately 0.50 to 1.0 mm. The foramen is not necessarily coincident with the anatomic root end (ie, the anatomic apex). The clinical correlations of this will be discussed later in this paper. It is possible for the apical foramen to be located up to 4 mm from the anatomic apex, with an average distance of approximately 0.5 to 1.0 mm.10,11
This variety in anatomy provides many of the challenges present in cleansing and shaping the apical third of root canal systems. For example, if a canal has fins, cul-de-sacs, anastomoses, delta canals, and irregular anatomy in the apical third (which is, of course, the norm), the clinician is wholly dependent on irrigation in the apical third to float debris out of the canal and provide a bactericidal effect to remove microbes that are not mechanically removed from the apical third. Maintaining the apical constriction in instrumentation procedures is vitally important in this process because placement, control, and efficiency of irrigation in part depends on maintenance of the apical foramen at its original position and size. By contrast, an apical constriction that is transported and enlarged may bleed profusely as well as lose its ability as a natural barrier to the movement of irrigants and obturation materials. These are both scenarios that can and do provide morbidity of varying levels of seriousness.
It is also important to realize that each root of a multirooted tooth is, for the sake of instrumentation, a single canal and the net equivalent to a single-rooted tooth that has 1 canal. Viewed differently, an upper first molar that has 4 canals is the equivalent of 4 anterior teeth in the context that at least 4 individual apical foramina must be respected and addressed.
CLINICAL CORRELATIONS, METHODS TO KEEP THE CONSTRICTION AT ITS ORIGINAL SIZE AND POSITION
(1) Estimate the true working length prior to picking up a handpiece to begin access. Having a clear idea of what the tooth length is before beginning can make a significant difference in guiding the negotiation of hand files and glide path creation, especially in the apical third. In addition, having a preoperative estimate of the position of the apical foramen can keep the clinician from using too much force in the apical third that might otherwise push files beyond the constriction, possibly leading to transportation.
(2) Instrument in a crown-down fashion, shaping the apical third last. In other words, instrument the coronal third first, the middle third second, and the apical third last. Doing so provides the following: (1) optimal irrigation at all stages in the process, (2) ideal tactile sense, and (3) the greatest degree of control over the apical third from virtually every vantage point. The worst case scenario is to instrument the apical third without having negotiated the canal, irrigated the space fully, not have a working length, and no glide path created with hand files prior to the use of rotary
Ni-Ti files. Use of files in the apical third in such a scenario is the harbinger of the creation of œdentin mud” and blockage of dentin shavings and pulp debris that can easily occlude the canal irrevocably and dramatically increase the chance of an iatrogenic outcome.
(3) It is helpful to use an instrumentation sequence that stresses (in this order) negotiation of the canal with small hand files to learn the canalÃs diameter and curvature, achievement of patency, glide path creation (to approximately a 15 K-file to the true working length), rotary Ni-Ti file use, irrigation, recapitulation, and a repeat of this given sequence. While a comprehensive discussion of this sequence is beyond the scope of this paper, suffice it to say that this order of treatment (which is universal to many instrumentation techniques and rotary Ni-Ti file brands) can provide maximum efficiency and safety relative to other approaches of instrumentation. For example, if the canal is not recapitulated (patency is verified after irrigation), debris can easily build up in the apical third, which can cause iatrogenic outcomes. In addition, if a glide path is not created, the opportunity for rotary Ni-Ti instrument fracture rises dramatically. Both of the above outcomes will have a direct and significant effect on the minor constriction of the apical foramen, whose maintenance of its original size and position is vital for achievement of excellence in the creation of endodontic shaping and obturation.
Figures 2a and 2b. Clinical cases completed with the principles discussed.
(4) It is essential to maintain control of rotary nickel titanium files in the apical third. The author favors the K3 rotary Ni-Ti file (SybronEndo) for its tactile control, excellent cutting ability, and fracture resistance. Files that tend to screw into roots and/or that may not provide ideal tactile characteristics make the possibilities for transportation of the apical foramen greater. For example, in the distal root of many lower molars, the canals are relatively straight and the foramen is easily breached. Tactile control and slow, passive, deliberate movement of a rotary Ni-Ti file down the canal has value to avoid such apical perforation (Figures 2a and 2b).
Keeping the apical constriction at its original position and size has great value for the prevention of iatrogenic events, facilitating excellence in irrigation, instrumentation, and obturation, and providing the best possible clinical result.
1. Shuping GB, Orstavik D, Sigurdsson A, et al. Reduction of intracanal bacteria using nickel-titanium rotary instrumentation and various medications. J Endod. 2000;26:751-755.
2. Card SJ, Sigurdsson A, Orstavik D, et al. The effectiveness of increased apical enlargement in reducing intracanal bacteria. J Endod. 2002;28:779-783.
3. Rollison S, Barnett F, Stevens RH. Efficacy of bacterial removal from instrumented root canals in vitro related to instrumentation technique and size. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2002;94:366-371.
4. McGurkin-Smith R, Trope M, Caplan D, et al. Reduction of intracanal bacteria using GT rotary instrumentation, 5.25% NaOCl, EDTA, and Ca(OH)2. J Endod. 2005;31:359-363.
6. Peters OA, Barbakow F. Effects of irrigation on debris and smear layer on canal walls prepared by two rotary techniques: a scanning electron microscopic study. J Endod. 2000;26:6-10.
7. Falk KW, Sedgley CM. The influence of preparation size on the mechanical efficacy of root canal irrigation in vitro. J Endod. 2005;31:742-745.
8. Tan BT, Messer HH. The quality of apical canal preparation using hand and rotary instruments with specific criteria for enlargement based on initial apical file size. J Endod. 2002;28:658-664.
9. Albrecht LJ, Baumgartner JC, Marshall JG. Evaluation of apical debris removal using various sizes and tapers of ProFile GT files. J Endod. 2004;30:425-428.
10. Tamse A, Kaffe I, Littner MM, et al. Morphological and radiographic study of the apical foramen in distal roots of mandibular molars. Part II. The distance between the foramen and the root end. Int Endod J. 1988;21:211-217.
11. Dummer PM, McGinn JH, Rees DG. The position and topography of the apical canal constriction and apical foramen. Int Endod J. 1984;17:192-198.
Disclosure: The author receives an honorarium from Sybron Endo when he lectures for them. Otherwise, he has no financial interests in any company or product mentioned in this article.
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