Creating the Access Cavity for a tooth's root canal procedure.

- The steps of the procedure. | Goals and requirements. | What it's like for the patient. | Complications, issues, concerns, perforations.

What is an "access cavity" preparation?

As a starting point for performing your tooth's endodontic therapy, your dentist must first gain entry into it's nerve space inside. Making this opening in your tooth is termed creating the "access cavity."

Diagram of where a tooth's access cavity is made.

The space accessed is composed of the tooth's pulp chamber and individual root canals. And it's this interior tooth area, in its entirety, that will be treated during your procedure.

How the access cavity is crafted is more important than you probably realize.

While the individual steps of performing this task are simple for your dentist, and probably little noticed by you, the results of the process dictate factors involving both your tooth's endodontic treatment and the task of rebuilding it afterward.

  • The access cavity must provide entry to all needed portions of the tooth, both from a standpoint of visualization and also physical access for the instruments and materials that the dentist will use ...
  • ... but not result in the removal of any more tooth structure than is absolutely necessary. (Doing so would tend to weaken the tooth or unduly complicate rebuilding it after its procedure has been completed.)

As such, this portion of your root canal procedure plays a substantial role in helping to create a successful treatment outcome.


The steps of creating an endodontic access cavity preparation.

Note: Since this is the formal beginning of your tooth's root canal procedure (in the sense of working directly with the interior of your tooth), this process is only begun after it has been properly numbed up and isolated by way of placing a rubber dam.

Step #1: Choosing the opening's location.

Where on the tooth will the hole be drilled?

A primary objective of creating an access cavity is that it provides straight-line access to the tooth's individual root canals.

Why straight-line?

Anyone who has seen a root canal file knows that they are long, straight objects and might just assume it takes a relatively straight path to be able to insert them into a tooth. That's basically true. But it's really only part of the reason.

Just as important is this. As a file is worked inside a canal, any aspect of the tooth that deflects or directs forces to it (which is what happens if it rubs against the walls of a too-small access cavity) will affect the contours, motion and cutting action of the instrument's lower (working) end, typically in deleterious ways. Unencumbered straight-line access helps the dentist avoid these complications. (Ingle)

The access cavity allows straight-line access to the root canals.

Diagram of a molar's endodontic access cavity.

(Note: A rubber dam is in place so contaminates can't enter the open tooth.)

Due to this requirement, and based on the anatomy of the root canal systems of teeth:

  • With back teeth (molars and premolars) - The access cavity is created on the occlusal (chewing) surface of the tooth.
  • With front teeth (incisors and canines)- The opening is made on the lingual (backside) surface of the tooth.

And while there can be exceptions to these general rules, for the vast majority of cases this is where the opening is placed.

Step #2: Drilling the access cavity.

It's a simple matter for your dentist to create the hole in your tooth. They'll simply use their dental drill (the same one they would use if placing a filling for a tooth).

What will they find inside?

The area accessed is generally a hollow space composed of the tooth's pulp chamber and individual root canals. With normal healthy teeth, this area is filled with pulp tissue (tooth nerve).

What remains of the pulp at the time of your procedure will vary depending on the current status of your tooth. It might be "vital" (live nerve tissue is present), "necrotic" (the nerve has died, often no obvious remnants of it are present) or some status in between.

 

A tooth's pulp chamber is hollow space. The dentist may or may not find it filled with nerve tissue.

Picture of access endodontic access cavity in a molar.

The openings into the tooth's root canals can be seen on its floor.

Your dentist's goals.

As they develop the opening's shape and contours, your dentist will have several issues on their mind.

  • Does it reveal the locations where the tooth's root canals can be expected to be found?
  • Does it provide the needed straight-line, unencumbered access to them.
  • Does it remove aspects of the tooth's pulp chamber that might serve as a harbor for debris and contaminates?

    As an example, note in our graphic above that the top portion of a tooth's pulp chamber has pointed projections (pulp horns). As we've illustrated, the access cavity needs to extend to and include these spaces, so no pockets of debris are left behind.

Additional tasks.

Included in the process of creating the access cavity is the need to make decisions about, and deal with, any dental restoration or tooth decay that is present.

  • All tooth decay must be removed from the tooth. - Reasons why include: Decay harbors bacteria and assorted debris. Any portions of the tooth that have been structurally compromised by a cavity might fail (break off, crack) during the treatment process, thus allowing the entry of contaminates into the tooth. Fluids might seep through or past the decayed areas.
Picture of a tooth's endodontic access cavity preparation.

This tooth's filling, and the cavity that undermined it, were removed when making its access cavity.

  • All fragile and/or loose portions of the tooth must be removed. - This includes both actual tooth structure and/or whatever dental restoration is present.

    A main goal here is simply that of predictability. The dentist needs to know that what exists will remain intact, and won't leak fluids, during the tooth's root canal process or between appointments.

    Sometimes leaving some portion of the tooth's existing dental restoration makes a reasonable choice. If instead it's entirely removed (see picture), and especially if a large portion of the tooth is now missing, the dentist may feel that placing some type of temporary filling will aid in the predictability of providing the tooth's work.

  • What about making the access cavity directly through an existing dental crown? - This option is a possibility. There are however enough issues and considerations associated with this choice that we have dedicated an entire page to this topic: Performing root canal treatment on teeth that already have a crown.

What is making the access preparation like for you (the dental patient)?

As we mention above, a dentist doesn't start the process of creating the tooth's access cavity until after it has already been numbed up. So you shouldn't feel anything, at least of any significance.

What will you feel?

The local anesthetics ("shots") used in dentistry are characteristically only effective in preventing the sensation of pain, not pressure. So you can expect to feel the vibrations that your dentist's instruments create as they are used.

  • You'll notice the light vibrations of their "high speed" drill as it's used to initially penetrate through your tooth's hard enamel layer. (Exactly like you would feel when having a filling placed.)
  • Once the general size and shape of the access cavity has been developed, your dentist will likely switch out the type of handpiece (drill) they use to a "slow speed" one (like when they widen and flare the individual openings of each root canal). These drills tend to create a slower, sometimes heavier, level of vibration.

For most people, that's about all that will be noticed.

What might you feel?

While this isn't the usual course of events, we think it's important to mention that with some types of cases, and some scenarios, the potential for the patient to feel pain does exist. We discuss this issue on this page: Does having root canal treatment hurt?

Because of this potential, it only makes sense that at the onset of a patient's appointment they establish with their dentist some type of sign that indicates that a problem has been noticed. That way the dentist will know to stop immediately and tend to it. The use of a predetermined hand signal is common.

 
An access cavity in the chewing surface of a molar.

The access cavity of a molar.

Step #3: Refining the access cavity's shape.

From a purely technical standpoint, defining when the process of creating a tooth's access cavity has been completed and when its next stage of treatment has been begun (cleaning and shaping its canals) could almost always be debated.

Suffice it to say that the cavity's shape will be continually refined as each new need is identified.

  • If during the tooth's treatment the potential for additional canals is suspected, the access cavity may have to be enlarged when searching for them.
  • While instrumenting a canal, if the dentist finds that the confines of the opening restrict the file's freedom (as discussed above), they'll simply trim that portion of the tooth back.

Adjustments like these should take just a few moments to accomplish and be an non-event for the patient.


Complications associated with access cavities.

Root perforations.

Making an initial opening in a tooth and then expanding it so it reveals the opening of each of the tooth's root canals isn't always an easy, straightforward task.

As examples, sometimes instead of being hollow, the tooth's pulp chamber is substantially calcified. Or some of the tooth's root canals may not join with the chamber in the expected locations.

Difficulties for the dentist.

When an atypical situation exists, the dentist will need to use their drill inside the tooth more extensively. And as they continue to trim away more and more of the internal aspect of the tooth, knowing how close they are getting to the root's outer surface is impossible to know precisely.

(With challenging cases, the landmarks inside a tooth that are routinely relied upon as reference points may not, or may no longer, exist. Also, with a rubber dam in place, understanding the orientation of a severely broken down tooth can be quite difficult to interpret. With each of these scenarios, the dentist's drill may not be trimming precisely where they expect.)

Perforations.

When dealing with these types of difficult cases, the dentist's drilling as they create the access cavity may result in the creation of a perforation (a hole that extends out the side of the tooth's root).

Treatment.

The extent to which this is a problem will have to be evaluated by your dentist. In some cases a successful repair is possible.

A case's prognosis will typically depend on the size and location of the opening, when it was diagnosed and treated, and the materials used to make the repair. MTA (mineral trioxide aggregate), a biocompatible compound high in calcium and phosphorus, is currently considered the material of choice.

Smaller perforations, located further down on the tooth's root, tend to offer a more favorable treatment prognosis. Also, perforations that are identified and treated promptly (a strategy that helps to prevent secondary inflammation and eventual infection of the tissues surrounding the site) also tend to have a more favorable outcome.

 Reference: 

Ingle JI, et al. Ingle's Endodontics. Chapter: Retreatment of Non-Healing Endodontic Therapy and Management of Mishaps - Linked above.

Excessive tooth structure removal.

With every case, the dentist's goal will be one of keeping the overall size of a tooth's access cavity as conservative as possible.

The challenge.

Once a tooth's endodontic work has been completed, it will need to be restored. And making an access opening that's oversized might make doing so more difficult. Also, removing an excessive amount of tooth structure could conceivably weaken the tooth.

That's not to say that all access cavities should be "small." Instead, and simply, no preparation should be larger than the dentist determines is necessary to perform the tooth's work successfully. (We discuss issues with access cavities that are too small above.)

Picture of a comparatively large endodontic access cavity.

This access cavity is very large because tooth decay and the tooth's filling needed to be removed.

Not all access preparations can be kept conservative.

Few teeth that end up requiring root canal treatment are pristine. Most have been affected by some type of trauma (e.g. extensive tooth decay, fracture).

Since the creation of an access cavity always includes removing all tooth decay, and may include removing the tooth's current restoration too, some access openings will necessarily wind up encompassing a large portion of the tooth.

Rebuilding the tooth.

What remains of the tooth will dictate what type of final restoration should be placed, such as a dental crown vs. a filling.

It's important to note that the choice that's made can have a substantial impact on the long-term success of the tooth's procedure. We discuss this issue in detail on our page: Rebuilding your tooth after its root canal treatment.

 

Last revision/review: 1/21/2019 - Page created.

 
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