Endodontic treatment can be thought of as a series of steps. Each step in the treatment sequence sets up for the next. If the goals of each of the series of steps are reached, then the following step proceeds without incident and the entire sequence of endodontic therapy goes well and achieves an acceptable clinical result.
After diagnosis, the first clinical step is access. The goals of access are as follows:
• expose and identify all the canals,
• obtain straight-line access for the subsequent instrumentation of the canals, and
• remove all pulp and calcified tissue in the chamber.
This article will discuss access using clinical examples that demonstrate not only proper access, but also improper access.
Figure 1. Small access preparation (a). Arrow points to ceiling still in place, blocking straight-line access for the canal (b).
Figure 2. Endodontic instruments in canals demonstrating straight-line access.
As in most other fields in dentistry, it is easier to learn by visualization rather than by explanation only. The first example is a maxillary single-rooted bicuspid. In an attempt to keep the access conservative the dentist has inadvertently left the ceiling of the canal toward the palatal aspect in place (Figure 1). This does several bad things. During instrumentation the endodontic instruments will have to bend around the ceiling. This is not straight-line access, and it places unnecessary strain on the instrument the farther the instrument is placed into the canal.
Figure 2 demonstrates what straight-line access on a mandibular molar should look like. Note that the reamers are not bending around the axial wall, and they come straight out of the canal. Lack of straight-line access sets up all the right ingredients for instruments, especially Ni-Ti files, to break. Also, necrotic tissue may be left under the ceiling toward the palatal. The dentist must not think about conservative access preparation, but rather must think about removing the entire ceiling of the chamber to expose the canals. Removing the ceiling in its entirety is the limiting factor in how large to make the access opening.
Figure 3. Access preparation demonstrates “mouse hole” effect on buccal, and ceiling overhang on palatal.
In Figure 3 we see the access cavity prepared for a maxillary first bicuspid. The 2 canals are exposed and evident. Some of the axial wall labial to the buccal canal is still present. The orifice is not totally situated on the floor; part of the orifice is in the labial axial wall. This is not straight-line access for this canal. The endodontic instruments will have to bend around the labial axial wall, causing additional strain on the instruments. On the palatal side note a small, thin piece of ceiling. This will also impinge on instrumentation. Both the mesial and distal axial walls are very rough. This creates extra shadows, which may make it difficult to identify the canals (in this case it does not). These walls should be smoothed with a non-end-cutting bur. We prefer a parallel-sided coarse diamond, like the Shaper from the PulpOut bur kit (Essential Dental Systems).
Figure 4. Underextended access opening.
Figure 5. Buccal overhang evident (a). Palatal overhang evident (b).
We see an ugly example of a bicuspid access opening in the example shown in Figure 4. The access is very underprepared. It should be extended in both the buccal and lingual directions. Certainly, this makes it difficult to detect the canals and determine how many exist. This is not straight-line access for instrumentation. In this view of the buccal we can see the overhang of the ceiling, and in the view of the lingual we can see the lingual overhang of the ceiling as well (Figure 5).
Figure 6. Underextended access preparation.
Figure 7. Ugly access opening: too small.
Figure 8. Ceiling still in place covering palatal canal, preventing straight-line access for instrumentation.
In the last example of bicuspid access we see once again that the access opening has not been extended far enough to the buccal or palatal (Figure 6). In this view we see that the palatal canal orifice is situated in both the floor and the palatal axial wall. This is not straight-line access, and will make instrumentation much more difficult. The axial wall to the left has been gouged and contains undercuts. The groove on the floor of the chamber needs to be opened up. Without opening it up, it is hard to tell if a third canal and tissue remnants remain. The easiest way to widen and open this groove is with an ultrasonic tip using some type of enhanced vision (loupes or a microscope).
The following are examples of molar access. The mistakes are the same; only the teeth have changed. Figure 7 shows a maxillary molar with a grossly underprepared access opening. This demonstrates the clinician’s lack of understanding of the goals of access preparation.
In Figure 8 the access preparation has not been extended enough to the palatal side. You can see that the palatal aspect of the palatal orifice is still slightly covered by the ceiling. This prevents straight-line access for instrumentation. The mesial aspect of the axial wall has not been extended farther mesially to expose a possible MB2 canal that is usually found in this area. The distal axial wall is rough and still has some ceiling in place. Underneath the ceiling is a possible area for tissue to hide.
Figure 9. Underextension of access with ceiling still covering buccal and palatal.
In the maxillary molar case shown in Figure 9 we can see that the outline is very rough and the ceiling has not been fully removed. There is still ceiling overlying the palatal and buccal canals and the disto-palatal axial wall. The outline has not been opened enough to remove the entire ceiling, and consequently it is hard to tell if there is a DB canal. When we remove the ceiling on the buccal it will reveal that this is a 2-rooted maxillary molar. The MB and DB roots have fused into one large root. When we look at the floor of the chamber we see that a black developmental line connects the 2 canals. The canals are directly opposite each other in a buccal-palatal line, and the buccal canal seems to be as large or larger than the palatal canal. All these clues add up to only a single canal on the buccal.
Figure 10. Groove off of MB canal where the MB2 can often be found.
In this last ugly maxillary molar case we can see that the access has not been opened enough to expose the MB2 canal, if it is present. In Figure 10 we can see the MB and DB canals and a groove extending out from the MB. Usually, the MB2 orifice will be found in this groove. The entire mesial-axial wall from MB to palatal canals should be pushed more mesially with a non-end-cutting diamond. Once this is done it is easy to open up the groove with an ultrasonic tip and expose the MB2 orifice.
Figure 11. Access preparation insufficiently opened toward distal canal (a). Endodontic explorer in DB canal partially covered by ceiling (b).
Figure 12. PulpOut bur deepening the preliminary outline of the access preparation (a). Fixed stop of the bur touching the cusp tips (b). The bur is now in the middle of the access chamber.
Figure 13. The shaper above the ceiling…no canals here (a). The shaper correctly placed on the floor of the chamber to begin pushing the axial walls back (b).
Figure 14. Bicuspid access after use of Pulpout bur (a). Completion of access preparation (b).
Figure 15. Maxillary molar preparation immediately after PulpOut bur use (a). Competed access preparation (b).
Figure 16. Preliminary access showing entrance to floor and ceiling all around (a). Final preparation with access extended to eliminate any ceiling overhangs (b).
The next case is very typical of a bad mandibular molar access preparation. Figure 11a shows an underextended access cavity outline form. From this view we do not have straight-line access into the MB and distal canals. On the distal canal we can see a darkening of the floor. We also see that the distal-axial wall has not been fully removed, leaving some ceiling above this mesial dark spot on the floor. Probing with an endodontic explorer into the mesial aspect of the distal canal reveals a second distal canal (Figure 11b). This view clearly shows the position of the distal-buccal canal with the distal overhang of ceiling still in place. Failure to find and debride this canal drastically reduces the odds of a successful endodontic case.
These next cases (the “good”) show ideal access preparations for a bicuspid and maxillary and mandibular molars. The depth is gained quickly with the PulpOut bur because there is no fear of perforation. This bur has a fixed stop at 7 mm. This measurement, based on anatomic research, will place the dentist in the pulp chamber quickly without fear of perforation when the bur is fully inserted, so that the fixed stop contacts the cusp tips and occlusal plane (Figure 12). Once most of the outline form is exposed using the PulpOut bur, the axial walls are pushed back using the non-end-cutting extra coarse diamond shaper bur to remove any remaining ceiling.
Figure 13a shows the “bad.” The diamond has been incorrectly situated on top of the ceiling. If we extend the axial walls, we will enlarge the access above the ceiling and never find the canals, since they are located on the floor. Figure 13b shows the “good.” Here we see the tip of the non-end-cutting diamond placed on the floor of the chamber. When we push the axial walls back and remove the ceiling we will automatically expose the canals. The axial walls are pushed back until all canals have straight-line access for instrumentation; the orifice for the canal is located only on the floor of the chamber, not partially in the wall of the chamber.
This case of “good” access on a bicuspid demonstrates the initial opening after use of the PulpOut bur (Figure 14). The 2 canals can be seen along with some ceiling still left in place. Figure 14b shows completed access preparation with extension of the axial walls to remove the ceiling and establish straight-line access.
Figure 15a shows the initial opening into a maxillary molar. Note that the ceiling is still intact all around the access, but is especially prominent on the mesial and palatal aspects. Figure 15b shows the final access preparation, which is extended to establish straight-line access for later instrumentation.
The mandibular molar in Figure 16a is shown immediately after initial access opening with the PulpOut bur. Once again note the ceiling still in place on all aspects of the chamber preparation. The non-end-cutting diamond will be placed on the floor of the chamber, and the axial walls will be cut to remove the ceiling. Figure 16b shows the completed access preparation with removal of the ceiling and straight-line access for instrumentation.
With the advent of specialized burs that help us turn a qualitative technique into a quantitative technique, the art of access preparation is becoming more scientific. With science comes the ad-vantages of speed, standardization, repeatability, and freedom from iatrogenic complications.
Deutsch AS, Musikant BL. Morphological measurements of anatomic landmarks in human maxillary and mandibular molar pulp chambers. J Endod. 2004;30:388-390.
Deutsch AS, Musikant BL, Gu S, et al. Morphological measurements of anatomic landmarks in pulp chambers of human maxillary furcated bicuspids. J Endod. 2005;31:570-573.
Lee MM, Rasimick BJ, Turner AM, et al. Morphological measurements of anatomic landmarks in pulp chambers of human anterior teeth. J Endod. Accepted for publication August 2006.
Disclosure: The authors hold 18 patents for co-inventing endodontic products for Essential Dental Systems, a company they co-founded.