A vital determinant in providing predictable endodontic results is the ability to accurately locate the minor constriction (MC) of the apical foramen (AF). The MC is a natural barrier present in root canal systems and represents the termination point for endodontic instrumentation, irrigation and obturation.
Maintaining the MC at its original position and size during instrumentation is key to reducing post-operative pain and swelling. It is correlated with long-term clinical success in combination with excellent cleansing and shaping and the provision of a post-endodontic restorative coronal seal.
Several anatomical observations help frame this discussion: roots can have more than one apical foramen and the location of the AF is variable on the root end. It is not uncommon for a root to have 3AF or more and these can be up to 4mm from the anatomic root end.
In addition, it is certainly possible for resorption, amongst other factors, to modify the usual anatomy of the AF. In other words, the AF can be present without a constriction or be highly variable in some instances due to deposition of cementum, for example.
On average, the MC is approximately .28mm across at its narrowest diameter and approximately .5 to 1.5mm from the anatomic root end. With these averages borne in mind, knowing the location of the MC is essential. It represents – and is a natural – ‘capture zone’ to the apical movement of gutta percha or bonded materials such as RealSeal (SybronEndo, Orange, CA, USA) during obturation, in addition marking the location at which instrumentation and irrigation should cease. This capture zone prevents and minimises extrusion of materials during obturation, especially if it is left untouched, as it should be.
The exact position of the MC can best be accomplished with a combination of two methods, one being the electronic apex locator (EAL) and the use of a bleeding point determination (BPD). BPD is a highly accurate method of determining the location of the MC.
A BPD is performed on a paper point of an appropriate taper relative to the canal being measured, while the concept of apical patency is directly relevant to BPD. Achieving and maintaining a canal that is open and always negotiable (not blocked with debris or transported in any way) from the orifice to the MC is a predictor of diminished post-operative pain and enhanced clinical success.
If the MC has been maintained at its original position and size and the canal path is clear of debris, once the bulk of the instrumentation has been accomplished the tip (1-2mm) of the paper point should spot with either clear fluid or haemorrhage. This marking should be easily reproducible.
In addition, the clinician can and should use an EAL to verify and confirm the BPD and vice versa. I use the Elements Diagnostic Unit (SybronEndo, Orange, CA) for its state-of-the-art electronics, ease of use and reliability. It has compared very favourably with other available systems in the endodontic literature.
Coincident to this EAL determination, the clinician should ideally make an estimate working length (EWL) determination prior to beginning the case. The EWL is usually very similar to the TWL later determined by an EAL. This statement is made with the caveat that the TWL will get shorter as the instrumentation proceeds.
In practical clinical terms, the clinician may need to take TWL measurements at numerous junctures (once a file will first reach the EWL, after the bulk of instrumentation has been achieved, after the final preparation before cone fit and any further times that the clinician may have any doubt as to the position of the MC) as the clinician needs to tangibly appreciate how much the TWL has shortened during instrumentation.
Inherent in this discussion is the vital importance of copious irrigation and frequent recapitulation. It is easy to determine TWL using an EAL and obtain a BPD if the canal is open and easily negotiable. The converse is true.
Blocked, transported and ledged canals are difficult in which to determine TWL, with either EAL or BPD. Clinically, one method to make EAL and BPD more predictable is to appreciate that rotary nickel titanium files generate a tremendous volume of dentine chips and, if irrigation and recapitulation is not frequent, ideally after every insertion of the file, blockage can occur easily and as such the reliability of EAL and BPD will be greatly reduced.
A clinical correlation to the discussion of the final determination of TWL is the common question as to what the most ideal apical size is, can, or should be. While the comprehensive answer to this question is beyond the scope of this column, several important relevant points can be made.
Once the canal is prepared to some minimal canal diameter as might happen clinically such as a .06 25, to the TWL determined by EAL and BPD, it has value to know what the actual diameter of the MC is at its narrowest width. If, for example, a hand K file is placed to the TWL and a 25-hand K file binds at the TWL, this is the diameter of the MC, i.e. it can be said that the canal is ‘gauged’.
Knowing this diameter has value. If the canal is a 25 at its narrowest diameter and the final master apical prepared diameter is only a 25, it is self evident that the canal space is not enlarged in the apical third to any clinical relevant diameter, i.e. the apical third is not adequately cleaned or shaped.
One of the values of a system like the K3 RNT system is that it is available in tip sizes that range from a 15 to 60 in .02, .04 and .06 tapers, taper dependent. This allows the creation of a larger master apical diameter. Clinically, if a canal gauges to a 25, it can easily be taken to a 40 or larger with K3. Endodontic literature is absolutely clear that canals are cleaner when instrumented to a larger master apical diameter due to both the mechanical action of the files and the improved volumes of irrigant that reach the apical third.
To what size a given root should be enlarged is an empirical balance between considering collectively: 1) how curved the canal; 2) how long the root; 3) if the canal system is negotiable and patent; 4) the initial size of the MC; 5) if the apex is open or immature or the tooth has been subjected to trauma; 6) risk of instrument fracture based on file tip size and taper amongst many such parameters. Sound clinical judgment and caution is advised.
In summary, appreciating the vital importance of maintaining the minor constriction of the apical foramen at its original position and size is a key determinant in the long-term success of treatment as well as minimising any post-operative discomfort.
Coincident to accurately determining the position of the minor constriction, knowing its initial diameter is important so that an ideal enlargement of this delicate space can be performed to a biologically relevant diameter above the minor constriction. I welcome your feedback and questions.