Diagnosing the need for root canal treatment using x-rays.

- What are the signs a dentist looks for? | Pictures. Example radiographs. | What causes the signs to show? Why do they indicate that endodontic therapy is needed? | The need for 3D (Cone Beam) vs. 2D x-ray imaging.


Link to periapical radiolucencies section.

Other signs
on x-rays.

Link to other radiographic signs of root canal section.

One of the most valuable aids that a dentist has in diagnosing a tooth's need for root canal therapy is x-ray examination. To the point that it seems unlikely that any evaluation of a suspect tooth wouldn't need to include at least some, probably two or more, radiographs (x-ray pictures).

We show you what your dentist looks for.

With today's use of digital radiography, it's not uncommon that the dental patient gets to see the x-rays that have been taken of their tooth, right there on their dentist's chairside monitor.

So if you get a chance to see your films, or at least peak over your dentist's shoulder as they view them, this page explains about and gives examples of the kind of signs they're searching for when it comes to making a determination that your tooth needs endodontic therapy.

Signs on x-rays that indicate that root canal treatment is needed.

As a dentist views their patient's radiographs, they'll have a checklist running through the back of their mind of the details that may be visible on the pictures, that if they're there are important clues to discover.

For the most part, these signs will be ways the x-ray's image varies from what would be expected for a tooth in perfect health.

  • Sometimes these changes will be so minute that they're quite open to interpretation.
  • At other times they'll be so obvious that they constitute a slam dunk when it comes to making a diagnosis that the tooth requires root canal treatment.

Here's what a dentist looks for:

a) Periapical radiolucencies.

One of the most informative signs that shows up on an x-ray that indicates that a tooth possibly requires endodontic therapy is a dark spot that's centered around the tip of the tooth's root.

A periapical radiolucency.

An x-ray of a tooth needing treatment. A radiolucency is shown.

To a dentist, this is proof positive that endodontic treatment is needed.

  • Dentists refer to this type of dark spot as a "radiolucency."
  • One centered on the tip of a tooth's root (like in our picture) is referred to as a "periapical" radiolucency. (The term periapical refers to the area surrounding the tip of a tooth's root.)
What does the presence of a periapical radiolucency indicate?

The dark spot itself indicates that something has created a change in the density (hardness) of the bone in that area.

In the case of endodontic problems:

  • It's usually a sign of an infection housed in the nerve space inside the tooth.
  • As byproducts from the infection leak out (via the opening at the tip of the root where the tooth's nerve used to enter) it triggers a response in the surrounding tissues.
  • An aspect of this change is a decrease in the mineral content of the bone in the area (hence its dark appearance on the x-ray).

    (The formal classification of the actual lesion that forms can include: apical periodontitis, apical granuloma, acute apical abscess or radicular cyst.)

Other signs that are obvious in this x-ray.

In this particular radiograph, the presence of advanced tooth decay is also obvious. And between the two (the deep cavity and the radiolucency at the tip of the tooth's root, which equates with cause and effect), the dentist can feel essentially 100% confident that a diagnosis for root canal treatment is accurate.

Periapical radiolucency associated with a tooth that has failed root canal treatment.

A periapical radiolucency associated with failed root endodontic therapy.

Other situations where an x-ray may reveal an endodontic radiolucency.

Since failed root canal treatment is typically associated with the presence of infection, when that situation has occurred a radiolucency is frequently present too. (See picture.)

Diagnosis confusion.

Unfortunately, when it comes to viewing x-rays of previously root canalled teeth, observing a radiolucency isn't as diagnostic as it is with cases like the one above.

That's because there can be other explanations for why the dark spot (bone tissue change) exists, not the least of which is that the original lesion simply has not yet healed fully.

This confusion simply means that in order to make a confident diagnosis, other types of testing and evaluation must be performed.

Additional details about radiolucencies of endodontic origin.

Why isn't a radiolucency always evident on an x-ray of a tooth that needs root canal therapy?

It's possible for a tooth to have advanced endodontic pathology but when an x-ray is taken of it everything about it and it's surrounding tissues looks normal and healthy.

This conundrum is easy enough to explain:

  • The bone tissue changes that show up on a radiograph take time to develop (see below for further details).

    So it may be that other signs and symptoms have developed that have warranted an investigation of the tooth. But radiographically it's simply too early to observe any indication of its developing condition.

  • It may be that due to the nature of the tooth's condition (including low virility), that whatever associated changes have occurred are so minute that they're not readily detectable.

Possibly only somewhere on the order of 55 to 65% of root canal cases initiated because of pulpitis (a state where the tooth's nerve tissue is still alive but inflamed) show evidence of a periapical radiolucency at the time a diagnosis for endodontic therapy is made. (Mortazavi 2016) [page references]

Why does an x-ray show a dark spot?

As alluded to above, a radiolucency shows up on an x-ray because the bone in that region is less dense (it contains less mineral content, or else there is an actual void in the bone tissue in that area).

When dental x-rays are taken:

  • Areas of high density show as white regions (referred to as radiopacities). That's because the density of the object blocks the x-rays, and as a result that part of the x-ray film/sensor is shielded and remains unexposed, thus the object appears white or light in color.
  • In areas of low density, the x-ray beam passes through the structure easily, thus exposing/triggering the x-ray film/sensor. As a result, that portion of the picture appears darkened (referred to as radiolucent areas).

When will a radiolucency (bone changes) show on an x-ray?

Before a developing lesion will show up as a dark spot on a radiograph, the bone in the affected area must have finally reached a point where at minimum around 7% of its mineral content (like calcium) has been lost, and possibly as much as 30 to 50% is needed. (Bender 1982)

And because it takes time for change to occur, x-rays taken early on during a nerve's demise may not reveal any noticeable signs, and thus not assist with the tooth's diagnosis.

Other reasons for apical radiolucencies.

Further complicating the interpretation of x-rays is the fact that not all "dark spots" are caused by conditions that mandate root canal therapy.

Some of these conditions are totally benign and therefore require no attention, which means that a dentist must be careful in how much weight they assign x-ray evaluation toward forming their diagnosis.

Other changes on x-rays that may indicate that a tooth has endodontic problems.


Every tooth is held in place by a ligament that surrounds its root. This is referred to as the tooth's periodontal ligament or "PDL".

Any radiographic changes that take place with the space the PDL occupies or the adjacent bone to which it attaches can be a sign of nerve problems within the tooth and a need for root canal treatment.

PDL and Lamina Dura changes on an x-ray that indicate root canal treatment is needed.

Changes with the PDL space and Lamina Dura suggest that root canal is needed.

a) Widening of the periodontal ligament space.

If a portion of the PDL space shows signs of widening, it may be an indication of developing nerve tissue pathology.

When endodontic problems are involved, this space becomes wider as it fills with exudates (fluid byproducts) created by the state of inflammation that exists with the pulp tissue (nerve) inside the tooth.

b) Changes in the lamina dura.

The surface layer of the bony socket that encases a tooth's root, and to which it's periodontal ligament is attached, shows as a white outline on dental x-rays.

This layer of dense bone is referred to as the lamina dura. And when portions of it on an x-ray appear less evident or else thickened, it can be a sign of the bone's response to the degeneration of the tooth's nerve that's underway.

A tooth whose surrounding bone shows condensing osteitis.

c) Condensing osteitis.

Chronic inflammation associated with pulpal pathology can trigger a response where additional bone is deposited in the area surrounding a tooth. This is referred to as condensing osteitis.

Since this region now has increased density (more bone tissue has been deposited), it appears as a diffuse white (radiopaque) region on x-rays. It's generally centered in the region of the tip of the tooth's root(s).

Our picture shows such an instance. The source of the tooth's irritation is a deep filling that comes close to its nerve.


These are the first signals.

The changes just described are the earliest signs of developing pulpal pathology. And they're more commonly seen on x-rays than a fully mature periapical radiolucency as described initially on this page.

A paper by Mortazavi (2016) states that in regard to radiographic evidence:

  • PDL widening is the most common finding associated with endodontic pathology, being present in 46% of cases.
  • Changes in the lamina dura is the second most common finding. 20% of cases show a loss and 18% thickening.
  • Evidence of condensing osteitis is present in 12% of cases.

X-rays often show an obvious cause for a tooth's endodontic problems.

While not a specific indication that a tooth requires root canal therapy, finding some type of obvious pathology on an x-ray (such as an area of advanced tooth decay) makes it that much easier for a dentist to be confident in their diagnosis for it.

X-rays frequently reveal the presence of large cavities, periodontal problems (gum disease), the presence of large deep fillings, and obvious problems with the way previous root canal treatment was performed.

It's possible and common that any one, or some combination, of these types of events has played a role in the demise of the tooth's nerve tissue. Identifying them helps to complete the picture for the dentist because both cause and effect are evident on the x-ray.

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Additional details about x-rays used to diagnose endodontic conditions.

What kind of x-ray is needed?

The type of radiograph that a dentist takes when evaluating a tooth for endodontic problems is referred to as a periapical x-ray.

The term periapical refers to the fact that the picture shows the tooth, especially it's entire root portion (the term apical specifically refers to the "tip of the root").

Actually it's not just seeing the tip of the root that's important. An area of the surrounding bone tissue (along the lines of 1/4 inch beyond the tooth) must also be visible in the picture for it to be of diagnostic quality.

How many x-rays of your tooth will be needed?

When diagnosing the possible need for endodontic therapy, your dentist will probably end up taking at least two pictures of your tooth, possibly more. Each one will be taken from a slightly different angle.

Beyond the second picture just allowing the dentist a chance for a "second opinion," the change in angle helps them determine if what they see on the picture is truly associated with the tooth in question.

Digital vs. film x-rays.

Just as advances have been made in photography, similar changes have taken place in dental radiography too. Whereas historically x-ray pictures were only taken using film, nowadays your dentist may use a digital sensor instead.

One technique isn't necessarily better or more diagnostic than the other. But digital x-rays certainly offer many advantages [quicker to take, the digital image can be manipulated for easier interpretation (i.e. contrast, enlargement, etc...), x-rays can be precisely duplicated and sent electronically].

Cone Beam Computed Tomography (CBCT)

Rather than just the usual technique of taking two-dimensional (2D) x-rays that all dentists use, a more advanced form of radiographic evaluation involves the use of cone beam computed tomography (CBCT).

This technique creates a 3D representation of your tooth and surrounding structures so they may be evaluated in greater detail. However due to the expense of the needed equipment, it's more common to find this type of imaging being used in the offices of endodontists (root canal specialists) as opposed to your general dentist.

Uses for CBCT radiography.

a) Case diagnosis.

While a tooth's condition should initially be investigated using traditional (2D) radiography, in some instances, like when evaluating difficult to diagnose cases, the additional information gained from 3D imaging may provide significant benefit.

As an example, a study by Uraba (2016) found that when evaluating teeth that due to anatomical considerations are characteristically more difficult to examine radiographically (upper incisors, canines and molars), that the use of CBCT was able to identify signs of an endodontic problem (periapical lesions, see pictures above) in 20% more cases.

b) Preoperative case planning.

CBCT imaging may be useful in evaluating a tooth's root canal system on a pre-op basis. Especially if other types of x-ray evaluation have suggested that the configuration of the tooth's root canal system displays anomalies that might complicate its treatment.

Generally speaking however, CBCT evaluation should only be considered for cases where there is good reason to expect that the increased detail that it can provide is actually needed. It is not a technique that should be used routinely for cases.

c) During treatment.

During the course of performing a tooth's endodontic therapy, difficulties may crop up that CBCT evaluation may be able to help clarify and resolve.

Obviously this can aid in treatment success. But the use of 3D imaging might also assist the dentist in completing the tooth's procedure in a more conservative fashion. (For example, the openings of additional canals might be located radiographically, as opposed to the dentist needing to search for them by progressively trimming away more and more of the tooth.

d) Postoperatively.

Post-procedure use of CBCT imaging might be the only way to accurately diagnose post-treatment complications such as overlooked canals or root cracks.

CBCT imaging should be reserved for special situations.

As useful as 3D technology can be, performing a scan exposes the patient to a higher level of radiation than when conventional 2D x-ray technique is used. There is also the issue of its added expense.

In light of this, the current recommendation of several prominent organizations (American Academy of Oral and Maxillofacial Radiology, American Association of Endodontists, and European Society of Endodontology) is that its use should be considered to be an adjunct to conventional low-dose dental radiography (traditional two-dimensional dental x-rays).

The latter two organizations have position statements that emphasize that the use of CBCT radiography should not be routine but instead only used for cases where important factors justify its use and the cost-effectiveness for the patient has been taken into consideration.

(Parirokh 2017)


Written by: Animated-Teeth Dental Staff

Last revision/review: 9/7/2018 - Minor revision. Content added.

Content reference sources.


Topic Menu ▶  Root Canal Treatment



Are 3D x-rays really needed?

For my tooth's root canal my dentist says that 3d x-rays are needed. It's a pretty big added expense. It cost a lot too when they removed my son's wisdom teeth. Can I skip this?


Of course it's not really possible for us to answer your question. Only the dentist treating you can really make that determination.

The position of several dental organizations listed above is that Cone Beam (3D) evaluation should never be performed routinely for root canal cases. The issue is not just the cost but also the increased exposure to radiation that the patient receives.

We have to assume that evaluation of your tooth began initially with some type of (routine/traditional) 2D x-ray exam. And when sharing those pictures with you, we would think that the dentist might have pointed out what they noticed on those films that suggests that there are anomalies in your tooth's root canal system that indicated the need for 3D imaging to clarify. If they didn't, you might ask.

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