Ideal master apical file size

In my lectures, one question arises more than any other: ‘How large do you instrument your cases?’ In response, this column was written to help guide clinical decisions with regard to the creation of the ideal master apical file (MAF) size for general dentists in daily practice.

The MAF is the last file taken to the minor constriction (MC) of the apical foramen. Its diameter becomes the final prepared diameter of the canal at the MC. An ideal MAF preparation should provide debris removal throughout the canal, facilitate irrigation and actually cut dentine circumferentially along the canal walls in the apical third to the level of the MC.

It is essential that MAF creation does not transport the canal at any level or enlarge the MC. The final prepared canal shape should resemble a tapering funnel with narrowing cross sectional diameters. The larger the MAF, the greater the taper required throughout the canal to avoid parallelism of the prepared canal and optimise the hydraulic forces possible in warm obturation techniques.

Primarily and ideally, the MAF should be based on the pre-existing diameter of the MC. Large canals and large pre-existing MC diameters facilitate creation of larger MAFs. The converse is true. The diameter of the MC is determined through a process called ‘gauging’. Taking a hand K file to the MC and determining the hand file which resists displacement through the MC gauges the canal. For example, if a 25 or 30 hand K file resists displacement through the MC, this is the diameter of the MC and guides to a large extent the MAF.

Clinically, if the initial diameter of the foramen is equivalent to a 15 hand K file (which represents a very small MC and/or calcified canal), it is not necessary to prepare the canal to an MAF of 55-60 as might be appropriate in a larger canal. A MC diameter equivalent to a #8 hand file might be more appropriately enlarged to a #25-30. Alternatively, a MC that begins at a 25-30 diameter can and should be enlarged to a 50-60 MAF, if possible, given the curvature and thickness of the root walls.

The endodontic literature is absolutely clear that canals instrumented to a larger MAF are cleaner than those instrumented to smaller apical diameters. In other words, a canal taken to a 50 is cleaner than one taken to a 30 at the MC. A canal instrumented to a .04 taper and a 20 tip size, initial canal anatomy dependent, is going to be too small in the vast majority of clinical cases encountered, especially relative to the cleanliness which can be achieved to a .06 taper 45 or 50. A .04 taper 20-tip size will not generally allow adequate irrigation into the apical third or cut dentine circumferentially in the apical third to the MC.

The choice of MAF is also dependent upon the diameter and curvature of the entire canal; aside from the initial size of the MC. Enlarging a canal of significant curvature to a larger than necessary apical diameter risks strip perforation as well as instrument fracture and canal transportation in the form of ledging and ‘elbow’ creation on the outside of the canal curve amongst other problems. A long root will require proportionally more dentine removal coronally to maintain a continuous taper up the root, which can, in some clinical instances, predispose the root to fracture.

Other considerations

Achieving and maintaining apical patency is a fundamental principle in endodontics. Just as transportation of the canal path and over enlargement of the MC can be the precursor to an iatrogenic event, loss of patency can easily promote the same result. A canal blocked with dentine mud (pulp tissue, dental shavings, etc) can cause deflection of files from the canal path. Engaging the tip of a RNT file into a plug of debris at any canal level (coincident with other risk factors) can easily lead to rapid file breakage.

The above notwithstanding, only patency files (small K files, 6-10 K files) should be taken through the MC. Use of a rotary file beyond the MC, even in the most talented hands, can risk violating the foramen unnecessarily. Leaving the MC in its original size and position, as recommended above, provides an apical capture zone for obturation material and restricts its extrusion beyond the root end.

Based on the endodontic literature, the average diameter of the MC is approximately equivalent to a #28 ISO hand K file (if one existed). Transportation of the MC through its violation can enlarge the diameter significantly and lead to a host of untoward possible iatrogenic outcomes, including extrusion of filling materials, sealers, irrigants and canal contents, all undesirable events.

To create MAF sizes of the size discussed above, two methods can be recommended:

1. K3 alone (SybronEndo, Orange, CA). K3 comes in tip sizes that range across the various tapers available (.12, .10. .08, .06, .04 and .02) in 15-60 tip sizes. It is possible and efficient using this system alone to create the larger MAFs referenced here. K3 cuts well, is flexible, resists fracture and has excellent tactile control.

2. A hybrid technique blending LightSpeed RNT instruments (Discus Dental, Culver City, CA, USA) and K3. The hybrid technique advocates the larger apical diameters possible with LightSpeed instruments in the apical third and uses the shaping ability of the K3, primarily in the coronal two thirds. The hybrid technique has evolved because the LightSpeed is a smooth-sided non-tapered instrument, which engages dentine only at its tip. Such a design minimises engagement of dentine and as a result reduces torsional failure potential. In clinical practice, if the .06 K3 25 reaches TWL, with LS, it is relatively straightforward to instrument the apical third to a 50-60.

Much of the instrumentation above, especially in vital cases, might ideally be performed with File Eze (Ultradent, South Jordan, UT, USA) in the canal, a viscous EDTA gel that can hold pulp in suspension until it can be floated from canals during irrigation. Such a gel minimises any chance of the creation of blockages of pulp debris, especially in the apical third. File Eze can be delivered via a White Mac tip (Ultradent, South Jordan, UT, USA), which accurately places an ideal quantity of the gel into the pulp chamber. In clinical practice, the File Eze is placed, instrumentation proceeds, the canal is irrigated and the File Eze reapplied as needed. File Eze is generally employed throughout instrumentation of a vital tooth (or a necrotic tooth that has significant pulpal remnants) until the bulk of the pulp has been removed.

Traditionally, TWL was often based arbitrarily on statistical averages from the literature. More rationally, the choice of the ideal MAF is a decision taking into account the given canal anatomy, especially the pre-existing diameter of the MC. Creation of the most conservative MAF possible without exacerbating existing possible iatrogenic issues or creating new ones while providing the most ideal apical cleaning possible is desirable.

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