Root canal treatment - Complications, Problems, Failures. -

What can go wrong? The signs and symptoms of failed endodontic therapy. | Reasons for failure. Factors and statistics. | Solutions. What options do you have?

Signs of

Link to signs of root canal failure section.

Causes of

Link to why endodontic treatment can fail section.

Common problems with root canals.

While the root canal therapy that's been performed for your tooth will hopefully last you a lifetime, complications, and even outright treatment failures, can and do occur.

We've divided our discussion that covers this possibility into the following subjects:

A) How can you tell if your root canalled tooth has a problem?

Many of the signs and symptoms of failed or failing root canal therapy are the same as those that signaled the tooth's original need for treatment. They frequently include:

  • Tenderness, sensitivity to percussion (tapping on the tooth) and/or pain.
  • It's possible that some amount of swelling may be present. Or the tooth, or a persistent gum boil near it, has a bad taste or odor coming from it.
  • These symptoms may be constant or intermittent, possibly even disappearing completely for much of the time.

    Their intensity may wax and wane (usually corresponding with the level of activity of the infection that has caused them).

We discuss these signs in more detail, along with the various forms they frequently take, here: How can you tell if your tooth needs root canal treatment?

a) If your root canalled tooth isn't symptom free, it should be evaluated.

For the most part, if you have a tooth that's had root canal treatment, and it continues to have or has started to show pretty much any type of symptom, it should be examined by your dentist.

What's normal.

The usual expectation after the completion of endodontic therapy is that after an initial period of healing the treatment has been able to resolve those issues responsible for creating an irritating effect on the tissues that surround the tooth's root.

And in the vast majority of cases, if this result has been successfully achieved the tooth will be totally asymptomatic (without symptoms).

b) If something seems wrong, how soon should you have your tooth checked out?

As a general rule, if you notice that something seems wrong with your root canalled tooth you should have it examined by your dentist sooner rather than later.

That's because teeth that have endodontic problems are unpredictable and have the potential to flare up at any time.


Infection typically plays a role in root canal failure, and how active it may become at any one point in time can't be predicted.

As a worse case scenario, a long-standing low-grade infection that has only caused minor symptoms may shift into an acute phase, bringing with it intense pain and significant swelling.

Just because this potential exists doesn't mean that it will happen. But it does mean that there's absolutely no good reason to delay in contacting your dentist's office and seeking attention. Once you've done so, they can make a determination about the urgency of your needs.

In the case where you can't be appointed immediately, the simple act of phoning in a prescription for antibiotics for you (either to be started immediately or to have on hand if needed) can reduce your risk for trouble significantly.

Some signs of endodontic failure can only be detected by your dentist.

An x-ray showing evidence of failed root canal treatment.

The dark spot (radiolucency) at the tip of this tooth's root suggests that a problem exists.

c) Not all problem teeth display symptoms you can detect.

Some teeth that have failed endodontically won't display any symptoms that the patient really notices.

A common scenario is one where from the patient's point of view their tooth seems perfectly fine. But during x-ray examination (possibly taken as part of a routine dental checkup, or as planned monitoring of the tooth's work) one of the films suggests that a problem exists.

Usually what the dentist has discovered is referred to as a "radiolucency," like the one shown in our illustration.

The grey area of successful treatment.

The point that a tooth's work might be classified as a failure even though it remains quiet brings up the issue of tooth survival vs. case success.

In endodontic terms, survival refers to a lack of symptoms while a classification of success is associated with the more rigorous standard of both an absence of symptoms and evidence of periradicular (around-the-root) tissue healing (as in no radiolucency is present).

d) Having symptoms doesn't always indicate endodontic failure.

It's possible that the symptoms you have noticed are not associated with your tooth's root canal work at all. Here are some possibilities:

  • Referred pain - The nerve that services a tooth services other teeth and anatomical structures too. It's possible that a dental or medical problem elsewhere along the run of the nerve just happens to create a sensation that feels like it's coming from your root canalled tooth.
  • Phantom pain (deafferentation pain) - This type of tooth sensation is similar to phantom limb pain amputees sometimes experience.

    It stems from some type of traumatic episode that has occurred in relation to the nerve that services the tooth, like when pulp (nerve) tissue is removed from a tooth during its root canal therapy. Possibly phantom pain occurs in as many as 3% of such cases. (Marbach 1993)

(Nixdorf 2010) [page references]

B) Reasons why root canal treatment fails.

The specific cause of any one tooth's endodontic failure usually boils down to some combination of the following factors:

  • The cleaning aspect of the tooth's procedure has been incomplete or ineffectual. If so, infection inside the tooth will persist and ultimately result in case failure.
  • The seal created for the tooth (either during its procedure or by its final restoration) has not been successful in keeping contaminates from seeping into or out of the tooth.

    The seal might have been deficient initially, or has since deteriorated. Either way, the lack of a seal will allow persistent infection to exist within the tooth.

  • Technical shortcomings or complications have prevented the completion of the tooth's work according to the standards needed. Or issues associated with the tooth's preoperative condition have created obstacles.

Specific reasons why root canal treatment may fail.

Listed below are examples of how the above problems may have become a factor in an endodontically treated tooth's failure.

The root canal system of a tooth may branch.

The shape of some canal systems makes them harder to treat.

1) Branched canals (unfilled canals).

Root canals sometimes fork, resulting in one or more offshoots (see picture). In many cases the resulting branches may be very tiny and as a result both hard for the dentist to detect and treat.

If any division of a canal is not identified, the result will be one where one branch is treated while the other is totally overlooked. The untreated portion of the canal will serve as a source of infection that will act as a persistent irritant to the tissues that surround the tooth's root, ultimately leading to case failure.

2) Missed canals.

It's not terribly uncommon for a tooth to have more individual root canals than is normally expected. (Some tooth roots are well known for having, or potentially having, more than one canal. With others, this may be a rare event.)

These additional canals may be very tiny and their presence easily missed when not anticipated. And if a canal is overlooked, it will remain untreated and a locus of persistent infection.

  • A study by Iqbal (2016) determined that missed canals are a major cause of root canal failure (around 18% of failed cases), and most commonly associated with treatment provided by general dentists as opposed to specialists. (It's common for an endodontist to have a surgical microscope that they'll use to search for extra canals.)
  • Hoen (2002) evaluated 337 failed root canal cases and determined that overlooked canals played a role in 42% of them.

A cracked molar.

Some cracked teeth may not be treatable.

3) Root cracks.

Cracks that have formed in a tooth's root can be colonized by bacteria. But unlike root canals that can be cleaned and sealed off, there's no way to treat the minute spaces created by cracks.

That means that once they've been colonized by bacteria the infection that results can't be cleared up. It will be a persistent source of irritation to the tissues that surround the tooth's root.

Compounding this type of situation, when performing endodontic therapy a dentist may be unaware that a crack exists (they can be very difficult to identify), or underestimate the significance of the ones they see. Either way, all of the tooth's problems won't be resolved by its treatment.

In other cases, the crack that has caused the tooth's failure formed after it received its endodontic treatment. (Possibly because an inadequate "final" restoration was placed on the tooth, see below.)

4) Inadequate interior seal.

The integrity of the seal created inside a tooth during its root canal treatment is an important determinant in treatment success. It serves as a barrier to the seepage of bacteria or other contaminates into or from the tooth that would be a constant irritation to the tissues that surround it.

The seal may have deteriorated over time, or been faulty initially (due to underfilling, the presence of voids, etc...). A study of failed cases (Ibqal 2016) determined that 1/3rd involved underfilling the tooth's root canal space.

5) Overextension of the tooth's filling material.

The findings of some studies suggest that if the material that's been used to fill in and seal a tooth's interior extends out beyond its root's tip, the likelihood of endodontic failure is increased. (Tabassum 2016)

Hoen's (2002) evaluation of 337 failed root canal cases reported that overfills were involved in 3% of them.

The ability to properly confine these materials could be expected to be a function of the operator's level of experience and skill (see specialist vs. general dentist discussion below).

6) Inadequate coronal seal.

A defective or inadequate final restoration (the "permanent" one placed after the completion of treatment) can allow bacteria and other contaminates to reenter a tooth. (The x-ray graphic above shows an example of this situation.)

This phenomenon is termed "coronal leakage" and it is a major cause of root canal failure. For more information, use this link: What is Coronal Leakage?

Even the highest quality root canal work can't survive (resist reinfection of the root canal space) if its tooth's permanent restoration doesn't provide an adequate seal. A study by Hoen (2002) reported that 13% of failed cases involved complications with coronal leakage.

7) Other types of technical shortcomings with the tooth's procedure.

Beyond the operator issues mentioned above, other technical complications can effect the outcome of a tooth's procedure. This might include:

  • Problems caused when shaping the canals - A dentist's use of root canal files may inadvertently create contours within the tooth that subsequently makes this space difficult to adequately clean or seal.
  • Perforations - Using drills or files, a dentist may inadvertently create a hole out the side of the tooth's root.
  • Broken instruments - It's not terribly uncommon for a root canal file to break while being used. In some cases the broken portion may be lodged inside the tooth and cannot be retrieved.

A study by Iqbal (2016) determined that about 6% of failed cases could be attributed to problems associated with perforations, and 7% broken instruments. (Note: The sample size of this study was only 90 patients).

8) Lack of clinician expertise.

Research suggests that treatment performed by endodontists (root canal specialists) tends to have a higher success rate than that provided by general dentists. For example, a study by Iqbal (2016) evaluated 90 failed root canal cases and determined that roughly 80% of them had been completed by general dentists. (See additional statistics below.)

Why referral to a specialist may make sense.

Any dentist can tell you, providing endodontic therapy for some teeth will prove to be amazingly straightforward, and then for others surprisingly involved. Unfortunately, a tooth's level of difficulty can't always be predicted.

For this reason, some dentists may feel they can boost their patient's chances of success by referring suspect cases on to an endodontist before complications arise. Per the data in our table below, this might be an especially prudent choice for certain types of teeth (like molars).

Our page "Endodontist vs. General Dentists- Which makes the best choice, and when?" discusses this issue in detail.

A periapical radiolucent lesion.

A tooth with a periapical radiolucency.

Teeth having pre-op radiolucencies may be more likely to fail.

9) Preoperative tooth conditions.

A tooth's initial status may play a role in the ultimate success or failure of its endodontic work. One such concern involves teeth that have a "periapical radiolucent lesion" (see picture).

These types of lesions may continue to harbor bacteria despite the successful completion of the tooth's root canal treatment. If so, this locus of infection will be a persistent irritant to the surrounding tissues.

The success rate of teeth having this initial condition (vs. those without) may be lower on the order of 20% (Iqbal 2016).

C) Contributing / Complicating factors.

It's possible that your tooth's root canal treatment has been successful but the tooth itself has problems due to other factors.

a) The tooth has broken or fractured.

Teeth that have undergone root canal treatment are seldom as structurally sound as they were originally. And for this reason, they often require the placement of a dental crown for strengthening and protection.

If an endodontically treated tooth does break, it's not always a big problem.

  • Assuming that the damage is confined to just the crown portion of the tooth (not its root), it's quite likely that the tooth can be rebuilt. (In some instances, the repair may require the placement of a dental post and core.)
  • If the crack extends into the tooth's root, an evaluation will need to be made to determine if the likelihood of making a successful repair seems possible (see above).

b) The tooth has extensive decay or gum disease.

Just like any other tooth, teeth that have had root canal treatment are at risk for the formation of tooth decay and gum disease. And if allowed to advance, either of these conditions can ultimately lead to the tooth's loss.

How likely is your tooth's root canal therapy to fail?

Overall, research studies suggest that endodontic therapy has a generally high success rate.

a) Treatment success rates.

Chen (2007)

This study monitored over 1.5 million root canalled teeth over a 5 year period. It determined that:

  • Roughly 8% of cases failed and were resolved by extracting the tooth.
  • Of the teeth that survived, 4% had experienced root canal failure but were salvaged by retreating them.

(These findings suggest to us that the overall success rate of a tooth's original treatment lies on the order of 88%.)

Raedel (2015)

This study reviewed a dental insurance database to determine the outcome of over 500,000 root canal cases where the treatment had been provided by general dentists. It calculated a 3-year survival rate of about 84%.

b) Treatment by general dentists vs. endodontists.

It seems logical to speculate that the extra training a root canal specialist (endodontist) receives positively influences the outcome of their work. Research seems to confirm this:

  • Background information included in a paper by Ibqal (2016) states that success rates for root canal work performed by general dentists runs on the order of 65% to 75%. Whereas for specialists, this number lies around 90%.
  • A small study involving just 350 teeth (Alley 2004) found a success rate of 98% for therapy performed by endodontists vs. 90% for cases completed by general practitioners.

Another study (Lazarski 2001) evaluated the outcome of over 100,000 root canal cases (each tooth was followed over a minimum time frame of 2 years).

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It reported a similar success rate for work completed by both specialists and general dentists. But noted that the specialist group treated a substantially greater percentage of molars (multi-rooted teeth often having a very complex canal system) whereas the generalist group more single-rooted, typically easier to treat, teeth. (See our "failure rate by tooth type" table below for a comparison.)

Possibly you could conclude from this study that:

  • The similar success rate achieved by specialists while treating more difficult cases suggests that the extra training and experience they have plays a valuable role in treatment outcome.
  • But this extra experience may not be needed for simple cases.

c) Failure- Retreatment vs. extraction.

The Lazarski (2001) study mentioned above also provided some insight in regard to the ultimate outcome for endodontically treated teeth that had experience failure.

Out of a group of over 4000 failed root canal teeth, 59% of cases were resolved by extraction while the remaining 41% seemed suitable candidates for salvaging via some form of endodontic retreatment.

d) Incidence of root canal failure by tooth type.

We ran across three studies that included data about endodontic failure by tooth type.

Iqbal (2016) - As a part of its evaluation of 90 failed root canal cases, this study reported the following failure rates:

Distribution of failed root canals by tooth type.
4.4% of cases ... Upper incisors
3.3% of cases ... Upper canines (eyeteeth)
15.5% of cases ... Upper premolars (bicuspids)
44.4% of cases ... Upper molars
5.5% of cases ... Lower incisors
1.1% of cases ... Lower canines
5.5% of cases ... Lower premolars
20.0% of cases ... Lower molars

Burry (2016) - As a part of it's investigation of an insurance database, this study evaluated groups of teeth that had been treated by general dentists that had since experienced root canal failure (those that had developed problems at 1, 5 and 10 years after completion, over 338,000 teeth total). The failure rate per tooth type was similar for all three groups.

Distribution of failed root canals by tooth type.
19% to 20% of cases ... Incisors and Canines
32% to 34% of cases ... Premolars
46% to 47% of cases ... Molars

Hoen 2002 - This study evaluated 337 teeth whose initial root canal treatment had failed.

Distribution of failed root canals by tooth type.
20% of cases ... Incisors and Canines
22% of cases ... Premolars
58% of cases ... Molars

Together, these studies seem to suggest that:

  • Anterior teeth (incisors and canines) tend to experience failure less often than premolars and molars.
  • Teeth that most frequently have a single root canal (incisors, canines, lower premolars) tend to have the lowest failure rates.
  • Teeth frequently/typically having multiple canals (upper premolars, upper molars, lower molars) tend to have the highest percentage of failures.
  • Molars in general and possibly upper molars in particular (the type of tooth typically having the greatest number of canals, 3 or more) have the highest failure rate by far.

    (Not only does a larger number of canals present greater challenges but many additional canals are small and curved, thus making them difficult to both identify and treat.)

This data makes it easy to understand why a general dentist might be more inclined to refer the treatment of teeth that have a relatively more complex anatomy (molars, especially upper molars) to a root canal specialist.

What's the fix for failed root canal treatment?

If you have a tooth whose root canal therapy has failed, you really only have two options: 1) Retreatment or 2) Extraction. (Retaining the tooth doesn't make an appropriate choice.)

a) Endodontic retreatment.

Failed root canal cases can often be retreated. A decision to proceed with this option would simply depend upon your dentist's judgment about its chances of success.

In most cases retreatment simply involves repeating the same procedure that was performed originally, with the exception that additional time will be required to remove the previously placed canal sealing material. (Your dentist may offer to perform the retreatment process, or may feel that the expertise of an endodontist is required.)

We now discuss this topic here: Root canal retreatment.

b) Tooth extraction and replacement.

Besides retreatment, the only other option for a tooth having failed root canal work is to extract it. This option might be chosen because retreating the tooth is not possible, or only offers a low probability of success.


While just removing the tooth may seem the simplest solution, doing so usually makes the poorest choice. Your dental health is best preserved by replacing missing teeth.

  • We discuss the different tooth replacement methods that exist (along with their cost considerations) on this page.
  • Because it has become such a common alternative to root canal therapy in general, we discuss considerations associated with a tooth extraction and replacement with a dental implant approach in detail on this page.

Timing your next step.

Whichever decision is made, your follow-up treatment should be performed within the time guidelines recommended by your dentist.

Following their examination, they can gauge how much urgency appears to be involved. As a precaution, your dentist might write you a prescription for antibiotics so you already have it on hand if conditions with your tooth worsen before your definitive treatment can be performed.

Why you mustn't delay.

Teeth that have failed endodontic treatment are unpredictable due to the fact that they typically harbor infection, which has the potential to flare up (create pain and/or swelling), possibly significantly so, without warning.



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An old root canal filled tooth that hurts!

Almost 40 years ago I had an abscess on an upper incisor and it was treated with root canal. Over the years it went grey so I had a veneer. Recently (after eating chocolate) it started to hurt, a sharp pain and sensitivity to heat. My dentist has X rayed it and the root canal seems to be intact. My query: I thought this tooth was dead so how can I be experiencing pain?
Thanks (and for your website)


We're not really going to be able to shed any light on your specific situation. In your case, your solution is to continue to monitor your tooth, and continue to document what types of stimuli cause your discomfort. Keep reporting this information to your dentist. They'll ultimately figure things out. Not all diagnoses are quick, easy or one-visit affairs. The diagnostic services of an endodontist is sometimes needed for difficult cases.

The discoloration of your treated tooth that you noticed is commonplace and not necessarily associated with what you are going through now.

Our page about reasons why root canal treatment can fail explains different problems that may crop up. With previously treated, historically successful teeth, some examples are:

1) The seal created by the tooth's original work may have deteriorated and now bacteria are able to reenter the tooth.

2) A previously treated tooth may develop a crack, which can allow an entry point for bacteria.

While an x-ray will often provide valuable information, it may take time for a tooth's condition to advance to a point where the signs associated with its pathology finally show up on one. Hence the need to maintain contact with your dentist and let them to continue to monitor and reevaluate your situation as they feel is indicated.

Beyond what you mention, it's common that a problematic tooth will develop tenderness, swelling and/or a persistent pimple in the region of its root. So watch for that, because if you notice that it will will help in making the diagnosis. Good luck.

Thank you

I'm so grateful to you to have given such a detailed explanation for my strange tooth.
I will indeed keep in close consultation with my dentist and I have taken note of the various factors you have described.
Thanks again

JS Scotland.

Root Canal Retreatment

Thank you for providing the detailed information for this article. It was both informative and helpful. I appreciate the effort.

Thanks SW.

We fear that you wouldn't have read this page unless you're having problems with a tooth, so good luck with getting the difficulty associated with it resolved.

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