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May 2009 | Dental Products Report
Clinical 360°: endodontics

The new standard
for ideal apical size

Achieving enhanced apical sizes efficiently and predictably.

by Dr. Richard E. Mounce

The set-up

FIGS. 1-2 Canals prepared to enhance apical diameters. The MB1, MB2 and DB canals were prepared to a #35 (Fig. 1) and the palatal was prepared to a #40 (Fig. 2). The DB and palatal were obturated with SybronEndo’s RealSeal One Bonded Obturators and the MB1 and MB2 were obturated with SybronEndo’s RealSeal and SystemB technique.

“Much has been written about cleaning, obturating and coronal seals, but apical size receives less emphasis. This article discusses leaving the foraminal size as small as is practical and making the ‘ideal apical preparation.’ Dr. Mounce also discusses the cautions of creating ideal preparations."
—Dr. John West, team lead

There is controversy as to the ideal master apical diameter (MAD) of root canal preparations.

Traditional rationales for MAD choice were a reflection of the materials and methods that have been available over the decades. For example, prior to rotary nickel titanium (RNT), Gates Glidden drills and hand files were used to enlarge canals. As a result, smaller apical preparations tended to be the norm because of the tendency for large hand files to transport canals apically. With the advent of RNT, larger apical preparations became technically easier to prepare.

This column addresses my rationale for creating larger apical sizes and discusses one way to accomplish this clinical goal with RNT.

Studies of size
From a literature-based viewpoint, there is very little research to argue that smaller apical diameters are superior to larger ones. Quite the contrary, the endodontic literature is absolutely clear that larger MADs create cleaner canals than smaller ones. In essence, a #45 MAD creates a cleaner canal than a #30, a #35 MAD creates a cleaner canal than a #20, etc.

It is noteworthy that across the endodontic literature, the average initial diameter of the minor constriction (MC) of the apical foramen is 0.28 mm. If preparations are arbitrarily kept below this size, not all of the walls of the preparation are likely being touched during instrumentation to the MC.

Enhanced MADs provide mechanical removal of debris and bacteria in the apical third and improve the quantity of irrigation that reaches the apical third. In addition, MAD enhancement promotes easier cone fit and improved sealer hydraulics.

Ideal preparation
The ideal terminal extent of instrumentation is the MC and no further. To stop at the MC and not carry shaping instruments beyond the MC fulfills one of the principles of endodontic canal preparation that is, to keep the MC at its original position and size. The MC is a natural boundary for instrumentation, obturation and irrigation during root canal procedures.

Treatment is predictable when this natural boundary is left alone during endodontic therapy. Conversely, all manner of extrusion and transportation occurs when the MC is altered. All irrigation, instrumentation and obturation should be taken to the MC and not beyond in both vital and necrotic cases. The only instruments that pass the MC should be patency files, (small hand K files #6, #8 and #10). Patency files assure the canal is open and negotiable throughout the entire process.

Master cones are fit to the MC as well and down packed at this level. Care is taken to assure the master cone has tugback at the MC. The cone should remain at its original position and not slide through the constriction when subjected to heat and pressure during down packing. Tugback is the resistance of the master cone to coronal displacement when a gentle upward pressure is placed on the master cone.

Preparing enhanced apical diameters
From a clinical viewpoint, larger MADs can be prepared safely and efficiently, in part, by first avoiding strip perforation and other iatrogenic problems. In essence, while it is desirable to prepare larger MADs, this enhancement should not come at the risk of untoward outcomes that diminish long-term success or promote vertical fracture. Care should be taken to make sure the final prepared taper is correct for the complexity of the root form. To avoid transportation of the MC, it must be correctly located and, other than patency files, all instrumentation be taken to the MC, and not beyond.

To simplify canal preparation conceptually, it has value to break the process into two components: the “basic preparation” and the “enhanced preparation.”

A “basic preparation” represents the master apical taper (the largest taper that will be created through the length of the root) and some smaller master apical diameter. For ground RNT this is usually a .06 working taper. Systems such as the Twisted File (TF) from SybronEndo offer larger tapers, usually a .08. For many clinicians, the initial diameter in the basic preparation is approximately a 20-25-tip diameter, depending on the case complexity.

At this stage, if clinicians are to fit a cone and obturate the case, they are imposing onto the root an arbitrary MAD. This MAD has no relevance to the anatomy of the root being enlarged. In essence, it is a one-size-fits-all preparation that is unlikely to circumferentially remove dentin at the MC.

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