Dr. Richard E. Mounce

	The minor constriction (MC) of the apical foramen is a critical landmark in endodontic treatment. If the clinician keeps the MC at its original position and size, properly shapes and disinfects the canal above this landmark and places a coronal seal, clinical success and healing are predictable.

	Maintaining the MC at its original position and size has numerous benefits that include:

		Reducing extrusion of irrigants, sealer and obturation materials that would otherwise be forced out a transported apex.

		Optimizing irrigation hydraulics and flow.

		Optimizing obturation hydraulics and movement.

		Providing simpler and more efficient cone fit.

	In both vital and non-vital cases, I instrument, irrigate and obturate cases to the MC. This defined end point is the 0.0 reading on an electronic apex locator (EAL). I do not back away from the MC for “safety” and try to minimize extrusion of either sealer or the core obturation material by such an action.

	This concept of preparing, irrigating and obturating to the MC should be contrasted with working short of the 0.0 EAL reading. Working short of the MC intentionally leaves uncleaned and unfilled space.

	In the best-case scenario, this concept assumes the clinician will never work short of this already corrected and subtracted 0.0 EAL/Total Working Length (TWL) location. It assumes it is better to leave uncleaned and unfilled space than to potentially have a sealer puff and a small amount of extruded core obturation material even when the MC remains at its original position and size.

	Subtracting 0.5 mm to 1 mm from the TWL for “safety” is unnecessary and arbitrary, and it introduces more clinical error than it prevents. It is the clinical harbinger of blockage and all manner of iatrogenic events including file separation. If the clinician is diligent in determining the true position of the MC, respects this position, keeps the MC at its original position and size, fits a cone to this location, and down packs correctly, there is no reason to fear the movement of either the cone or surplus sealer through the MC.

	The MC accurately can be located with the following protocol:
	

Pre-operatively, the length of the tooth should be determined. The estimated working length (EWL) gives the clinician some idea of the anticipated TWL. When a hand K file (HKF) reaches the EWL, the clinician should feel a tactile “pop.” This pop is correlated with the other measurements of TWL to be taken later, as the TWL and the location of the tactile “pop” at the apex using HKFs should be essentially identical. After achieving straight-line access, removal of the cervical dentinal triangle, when the first HKF reaches the EWL, an EAL can be placed onto the file,and the first electronic apex location of TWL taken.

When the first rotary nickel titanium file (RNT) reaches the TWL another electronic measurement of TWL should be taken.

When the last RNT file reaches the TWL, another electronic measurement of TWL should be taken.

If the canal is patent and negotiable, insertion of a paper point to the MC/TWL should reveal a small spot of moisture or hemorrhage at this level. The spotting should be reproducible with several paper points.

TWL determined with these methods should confirm each other.

	In essence, the EWL should be very close (if not identical) to the first electronic determination of TWL which should be very close (if not identical) to the final TWL (after canal preparation) with the caveat that TWL usually shortens as the canal is prepared. It is a common finding that the final determination of TWL taken with a bleeding point on a paper point (mentioned above) is likely to be shorter by 0.5 mm to 1 mm depending on the degree of canal curvature.

	Traditionally, TWL has been determined radiographically, but radiographic determination has inherent limitations. Specifically, radiographs are limited by the fact that using this method only allows the clinician to make guesses as to the position of the MC by interpreting the distance of a metal object (a file) to the anatomic apex. This distance is then correlated with anatomic averages of the distance between the anatomic (radiographic apex) and the tip of the file.

	This is inherently inaccurate and has nothing to do with the true position of the MC. The MC can be up to 3 mm from the radiographic apex. Using this interpretive radiographic method is inherently inaccurate and a distant second in clinical usefulness to the other methods described here. Determining the position of the MC accurately and instrumenting, irrigating and obturating to the MC and not beyond will lead to more cleaned and filled space within the tooth and fewer iatrogenic events.

	This column has addressed where, how and why to terminate endodontic canal preparation. Emphasis has been placed on an exact determination of the position of the MC, achievement and maintenance of apical patency and instrumentation, irrigation and obturation to the MC.

Articles

 The minor constriction (MC) of the apical foramen is a critical landmark in endodontic treatment. If the clinician keeps the MC at its original position and size, properly shapes and disinfects the canal above this landmark and places a coronal seal, clinical success and healing are predictable.  Maintaining the MC at its original position and size has numerous benefits that include: