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:
- All caries, overhanging restorations, and unsupported tooth structure should be removed.
- All canals should be visible in one mirror view.
- 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.
- 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.