Biologic master apical file sizes

There is considerable evidence in endodontic literature that the traditional master apical file sizes employed in root canal preparation are too small, and do not result in a “clean” canal with common irrigation techniques.

There is considerable evidence in endodontic literature that the traditional master apical file sizes employed in root canal preparation are too small, and do not result in a “clean” canal with common irrigation techniques. Said differently, instrumenting an “average” molar canal to a .04 taper, 20-tip size rotary preparation (especially without significant taper in the more coronal aspect of the canal) does not predictably allow adequate irrigation to reach the apical region of the canal. Nor does it allow removal of apical dentin circumferentially to provide optimal disinfection.

Endodontic literature clearly states that canals instrumented to the minor constriction of the apical foramen to larger master apical file sizes, relative to smaller ones, produce cleaner canals. References on this are available on request. The average minor constriction of the apical foramen is approximately a size 25 to 30 at the apical constricture before instrumentation is ever started. If the average apical preparation is this size, it can be argued that canals are not instrumented adequately with such sizes in the apical third, and that more instrumentation is required.

Larger master apical file sizes allow greater volumes of irrigant to reach the apical third. This facilitates placement of irrigants so that they can function. In addition, larger master apical file sizes have the potential to mechanically remove much, if not all, of the tissue present at the given canal level in the apical third. There is no conclusive evidence that I am aware of that shows smaller instrumented apical diameters are in any way superior to larger ones. To state that larger diameters are not important or are immaterial to the potential for endodontic success, in my opinion, is to turn a blind eye to an overwhelming body of evidence to the contrary. And yet, this apparently is what is being recommended by many of the commonly available rotary systems because they lack larger tip sizes.

Several cautionary points should be made here. Creation of larger apical diameters, if not done appropriately, could lead to greater iatrogenic potential since more instrumentation would need to be done. This carries with it an increased risk of file separation. In addition, length control is even more important. As larger apical diameters are created, if an apical perforation or transportation is made, it will be bigger proportionately than one created to a smaller diameter. For example, an apical transportation with a 30 master apical file obviously is not as large as one created to a 60. But with excellent technique and mental focus, potential misadventures - like any clinical errors - can be avoided.

So how are larger master apical diameters created? Canal instrumentation can be conceptualized in a simple manner. There is the basic preparation, which is the standard taper and tip size that traditional instrumentation has gone to - usually a 30-tip size and a .04 or .06 taper for an average molar canal. If one chooses to instrument canals to a “biologic” or larger apical diameter, further instrumentation is required to achieve the benefits of such diameters. One way to proceed is to gauge the apex when this basic preparation is reached and achieved. Gauging the minor constriction of the apical foramen involves using a hand K-file to determine the diameter of the foramen before final instrumentation in the apical third. Such a .02-tapered hand file should resist displacement through the apical foramen at the true working length. For example, if the canal is gauged to be a 25 at the apex, the canal can be instrumented up to approximately a 50 or larger (as desired) to provide the greatest cleansing possible from such diameters. This occurs because of enhanced irrigation and the removal of dentin where it really matters. There are two common methods available to create these larger apical diameters. One is the Lightspeed system (Lightspeed Technologies, San Antonio, Texas). The other is with the larger tip sizes in the K3 system (SybronEndo, Orange, Calif.).

I expect much will be written on this subject in endodontic literature in coming years. I will address this subject, and its manifestations, in future Endo File columns.

As always, I welcome your feedback and questions on this topic.

Dr. Richard Mounce is in private endodontic practice in Portland, Ore. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material also is available as audio CDs and a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via e-mail at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.

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