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

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


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.

In fact, it seems unlikely that any evaluation of a suspect tooth wouldn't need to include at least some (probably two or more) conventional radiographs (traditional 2-dimensional x-ray pictures). And for especially problematic cases, the availability of 3-dimensional imaging may prove to be indispensable.

This page will give you an idea of what your dentist looks for when they read x-rays.

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

So if you do get a chance to see your films, this page explains about and gives examples of the kinds of signs they're searching for as they read them and formulate an opinion about your tooth's need for endodontic therapy as a solution for its problems.

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 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'll constitute a slam dunk when it comes to making the 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 on 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 is used to refer to that general area that surrounds the tip of a tooth's root.)

    In terms of shape, the lesion frequently has a "hanging drop" appearance.

What does the presence of a periapical radiolucency indicate?

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

In the case of endodontic pathology:
  • It's a sign of infection located in the nerve space inside the tooth.
  • As byproducts from the infection leak out of the tooth (via the opening at the tip of the root where the tooth's nerve used to enter), they trigger an inflammatory response in the surrounding tissues.
  • An aspect of this response is one where bone tissue is sacrificed from immediately around the point of exit from the tooth (hence the formation of the dark spot that's seen on the x-ray). And as this space forms, it fills with immune cells whose purpose is to defend against the irritants leaking from the tooth.
  • In essence, the formation of an endodontic periapical radiolucency is evidence of the person's body creating a line of defense against the spread of bacteria and infection byproducts from within its associated tooth.

Section references - Hargreaves

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

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 that their diagnosis for endodontic therapy is made.

Section references - Mortazavi

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 having 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 having 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 has been lost, and possibly as much as 30 to 50%, depending on the type of bone (cancellous vs. more dense cortical bone) that exists in the affected area.

And because it takes time for this level of 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.

Section references - Bender

Differential diagnosis - Other reasons for apical radiolucencies.

Unfortunately, not all radiolucencies are simple to interpret.

Easy cases.

On the x-ray above, a large cavity in the tooth is obvious. So with that case, 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.

But other "dark spots" seen on films are much more difficult to interpret.

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

A periapical radiolucency associated with failed root endodontic therapy.

Confusion with previously treated teeth.
Since failed root canal treatment is typically associated with the presence of persistent inflammation due to infection harbored within the tooth, when failure has occurred, a radiolucency can be expected to be present too. (See picture.)
But discovering a dark spot on an x-ray doesn't necessarily mean that a problem exists. There can be other explanations too.

For example, it may be that the tooth's original pre-treatment lesion simply hasn't fully healed yet. Or possibly what is seen is scar tissue (healthy remnants of the original lesion that are still present).

Confusion with healthy teeth.

Further complicating the interpretation of x-rays is the fact that some radiolucencies associated with teeth are due to totally benign conditions and therefore require no attention at all.

Of course, all of this potential for misdiagnosis simply means that in order to make an accurate determination, other types of testing and evaluation must be performed by the dentist too, not just taking x-rays.

b) Other changes visible 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 within the space that 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.

1) 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.
Differential diagnosis.
As a point of concern for the dentist, other causes can explain PDL widening.

One of these is as simple as a tooth that has increased mobility, like that that can result from tooth clenching. (This habit can also cause tooth sensitivity, which might also be misdiagnosed as a symptom indicating the need to perform endodontic therapy.)

2) 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. (See picture above.)

A tooth whose surrounding bone shows condensing osteitis.

3) 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 early signals.

The changes just described are the first 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 states that in regard to radiographic evidence:
  • PDL widening is the most common finding associated with endodontic pathology, being present in 46% of cases.
  • Identifying 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.

Section references - Mortazavi

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

While not necessarily indicative on its own, identifying some type of obvious tooth-related pathology on an x-ray makes it that much easier for a dentist to be confident in their diagnosis.

Radiographs of teeth that require root canal treatment frequently reveal the presence of large cavities, periodontal problems (gum disease), the presence of large deep fillings, or obvious problems with the way previous endodontic treatment was performed.

Identifying these types of conditions helps the dentist to complete their diagnostic picture because both cause and effect are evident on the x-ray.

That's an important aid.

We've brought this point up because it's been shown that a dentist's ability to accurately diagnose a tooth's need for root canal treatment, based solely on the minute changes they've visualized on their patient's x-rays, is questionable.

That's because reading radiographs can be highly subjective. A point that was documented by a study by Goldman that evaluated dentists' interpretation of the same set of films. It determined that:

  • The dentists' diagnosis was only in agreement 50% of the time.
  • When the same set of films was shown to each dentist several months later, they only concurred with their original diagnosis in 72 to 88% of cases.


So clearly, it's important for a dentist to corroborate what they feel they have seen on their patient's x-rays with other clinical observations and testing.

Section references - Goldman

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 with dental radiography too. Whereas historically x-ray pictures were only taken using ("old fashion") film, nowadays your dentist may use a digital sensor instead.

Neither is better. One is more convenient.

There is no overwhelming body of evidence that supports the notion that one method is superior to the other in terms of being more diagnostic. Having said that, digital x-rays do offer some advantages and options that conventional films cannot.

  • Since no developing process is involved, digital x-rays can be viewed immediately after they're taken.
  • Digital x-rays offer the option of image manipulation (changing contrast levels, color enhancement, reverse imaging, image subtraction, etc...), although this ability doesn't necessarily result in more accurate image interpretation.
  • Digital x-rays are more precisely duplicated and sent electronically.
So if your dentist still only takes traditional radiographs, it doesn't mean that you aren't in good hands.

Section references - Ingle

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 associated structures so they can be evaluated in greater detail (via cross-sections that show the tooth and its surrounding bone sectioned at varying angles). 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 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.

Section references - Uraba

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 a 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 blindly 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 the 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.

Section references - Parirokh


 Page references sources: 

Bender IB. Factors influencing the radiographic appearance of bony lesions.

Goldman M, et al. Reliability of radiographic interpretations.

Hargreaves KM, et al. Cohen's Pathway of the pulp. Chapter: Pathology of the periapex.

Ingle JI, et al. Ingle's Endodontics. Chapter: Diagnostic Imaging

Mortazavi H, et al. Review of common conditions associated with periodontal ligament widening.

Parirokh M, et. al. Treatment of a Maxillary Second Molar with One Buccal and Two Palatal Roots Confirmed with Cone-Beam Computed Tomography.

Uraba S, et al. Ability of Cone-beam Computed Tomography to Detect Periapical Lesions That Were Not Detected by Periapical Radiography: A Retrospective Assessment According to Tooth Group.

All reference sources for topic Root Canals.


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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