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Strategic synergies: the relationship between straight line access and apical third shaping

Oct. 19, 2009
Adequate access is a vital first step in attaining tactile control over all aspects of cleaning and shaping, especially in the apical third. Dr. Richard Mounce addresses straight-line access and its importance in all phases of endodontic cleanings and shaping.

By Richard E. Mounce, DDS

Adequate access is a vital first step in attaining tactile control over all aspects of cleaning and shaping, especially in the apical third. Alternatively, deficient access is the harbinger to iatrogenic events, undetected caries, fractures, leaking restorations, etc. — all of which can predispose the procedure to clinical failure.

Ideal access requires that:

  1. All caries, overhanging restorations, and unsupported tooth structure should be removed.
  2. All canals should be visible in one mirror view.
  3. Crowns should be removed before endodontic treatment. This said, there are certainly cases where access through a crown is the most practical course of action.
  4. The cervical dentinal triangle should be removed.
After access is complete, if the tooth does not have four walls with which to hold irrigant, composite can be used to hold the solution in the pulp chamber. Doing so optimizes both irrigation and tactile control. Having four walls also assures solid rubber dam clamp contact. The clinician should anticipate the number of roots and canals before access. Access may need modification, especially in teeth with variable anatomy. For example, lower canines have two canals approximately 6% of the time. As a result, finding a single canal in a lower canine that is not symmetrical to the external root form is a clear sign that the existing access has not located all canals. Upper first bicuspids have three roots approximately 5% of the time, and access will need modification from the two-rooted variety. Access rarely looks like the stylized pictures found in textbooks. For example, if the clinician is accessing a lower second molar that is inclined to the mesial and doing so through a bridge, access will necessarily be inclined to the distal in order to avoid cervical perforation as a result of misdirected access. In essence, tipping should cause the access to be redirected down the long axis of the roots. Many crowns do not reflect the underlying position of the roots, and instead only function to match the occlusion. Crowns are poor indicators of the position of the canals that lie below, because they are made to match the occlusion and may bear little resemblance to the anatomy that was originally present. Risk factors and anatomical variations should be considered before access to avoid iatrogenic events. Common risk factors for access include (among others): tooth rotation, root curvature, root calcification, cusp location, bone convexities, crowns, calcification, tooth inclination, fused roots, dens in dente, posts and pins, etc. As a result of these various challenges, while being made, the access size and shape may need to change considerably from that anticipated at the start of treatment.While access can be too large, it rarely is. Compromise in making the access too small is the far greater error relative to making access too large. Access that is too small virtually assures a less-than-optimal cleaning and shaping of the root canal system, lack of tactile control, missed canals, and substandard irrigation. Common denominators can be found in less than desirable access preparations, primarily a lack of line angle extension for straight-line file entry into the orifice. Overhanging dentin as a result of inadequate access can cause instruments to deflect off of canal walls during insertion. Lack of extension causes files to undergo more cyclic fatigue and torsional stress than they would otherwise be required to bear. As a result, file fracture, canal blockage, canal transportation, etc., all become much more likely in this scenario. Coincident to achieving the correct access outline form and canal location is preparation of the ideal orifice size. Overpreparation of the orifice through the indiscriminate use of Gates Glidden drills or other orifice openers predisposes the tooth to perforation as well as subsequent vertical fracture. In essence, the clinician is called upon to create the correct aperture through which to subsequently enlarge canals, but not excessively so. Clinically, with a system like the Twisted File*, in many canals the clinician can prepare both the orifice as well as the entire final canal taper with a single instrument. For the average lower molar, mesial root, the .08/25 TF can prepare the entire master apical taper and also the final orifice shape. For larger and less complex roots such as the palatal canal of an upper molar, the .10/25 TF can act as both the orifice shaper and canal-shaping file. In a similar manner, for the most complex and curved canals, the .06 TF can act as the both the orifice opener as well as the canal-shaping file.
Cases instrumented with the Twisted File* and RealSeal* bonded obturation.ArmamentariumExcellent straight-line access is far simpler if accomplished through the surgical operating microscope (SOM) (Global Surgical, St. Louis, Mo.). Every aspect of the process is more predictable with the magnification and visualization of the SOM. Its value in all aspects of endodontic treatment as well as access and orifice management cannot be overstated. Access bur kits can be helpful. Some kits, such as the LA Axxess kit, are universal in that they have all the needed burs for even the most challenging clinical cases. The LA Axxess kit includes a round diamond porcelain access bur, a transmetal bur for metal, three round burs (2, 4, and 6 sizes), a course diamond football-shaped bur for gross tooth structure removal, and two diamonds for fine axial preparation. The kit also contains a safe ended diamond for refinement of access walls and stainless steel line angle burs that can be used as orifice shapers as well as to create straight-line access.
The LA Axxess kit*
The LA Axxess kit 2.0*Used in a slow-speed handpiece, these line angle burs (of three varying tip sizes) are safe ended at the tip and do not cut on their ends. Line angle burs are used with the same precautions as Gates Glidden drills. As the line angle bur is inserted, it should be allowed to enter the coronal third passively. It is never forced. The line angle burs can be used in a brushing motion up and away from the furcation so as to remove the cervical dentinal triangle and remove restrictive dentin over the orifice. Using the line angle burs passively with a brushing motion will minimize the chances of perforation and/or excessive removal of coronal third tooth structure. Risk of perforation is greatest at the distal aspect of the mesial root of a lower molar and the distal aspect of the mesial buccal root of the upper first molar. The LA Axxess kit is available in a modified 2.0 version without the aforementioned prepping diamonds and line angle burs, and yet contains a safe ended diamond for high-speed alignment of access walls. Access is made in the context of the given tooth being entered. The clinician should be aware of the diagnosis of the tooth being started. If the tooth is vital, clinicians should anticipate that they will see hemorrhage. If the tooth is necrotic, they should expect either a dry chamber or non-vital pulp tissue. While this might seem obvious, neglecting to appreciate that the pulp chamber has been reached can lead to excessive removal of dentin along its floor or leave the pulp chamber unroofed. In addition to being cognizant of the pulpal diagnosis, especially in non-vital teeth that are covered by crowns, correctly managing a calcified orifice, once uncovered, is essential. Clinically, this means that in the most challenging cases, #6, #8, and #10 pre-curved hand K files are used to assure that the canal is open, patent, and negotiable to the apex before bringing orifice openers of any type into the canal. Using orifice openers too fast and forcefully into a calcified canal can easily lead to blockage and other iatrogenic issues. Reciprocating a #6 hand K file with the M4 Safety Handpiece* can easily and efficiently prepare the canal diameter of a #8 hand K file, a #8 hand K file to the canal diameter of a #10, etc.
The M4 Safety Handpiece*The M4 mimics the watch-winding motion of hand K file enlargement by reciprocating a hand K file 30 degrees clockwise and 30 degrees counterclockwise. The M4 is used in an E-type coupling in an electric motor at the 18:1 setting at 900 rpm. The (#6, #8, #10) hand K file that binds at the TWL or the estimated working length is first fed into the canal by hand to this level. The M4 is then placed onto the hand K file and with a 1 mm to 3 mm vertical amplitude of motion, the hand K file is reciprocated for 15 to 30 seconds. After the file begins to reciprocate freely (after the 15 to 30 seconds), the canal is irrigated and recapitulated, and the next larger hand K file is placed to length and reciprocated until the canal is enlarged to a #15 hand K file size equivalent. At this stage, a glide path has been created and rotary nickel titanium file enlargement commences. Straight-line access and its importance in all phases of endodontic cleanings and shaping have been addressed. Emphasis has been placed on making endodontic access large enough that the files inserted into the canal can be placed into the straightaway portions of the root without displacement off of the canal walls. The LA Axxess kit has been discussed as well as the capability of the Twisted File to shape the canal orifice after canal location. I welcome your feedback. *SybronEndo, Orange, Calif.
Richard E. Mounce, DDS, lectures globally and is widely published. He is in private practice in endodontics in Vancouver, Wash. Contact him at [email protected].