Causes of root canal treatment failure. - Technical and procedural reasons why endodontic therapy may fail. -

Procedural difficulties and errors. - What can go wrong? | Challenges and complications posed by the tooth's anatomy or condition.


Link to endodontic failure due to missed canals section.


Link to endodontic failure due to tooth cracks section.

Reasons why root canal treatment fails.

This page explains various issues and shortcomings that are frequently cited as being factors in the failure of a tooth's endodontic therapy.

As you'll find out, some of these are diagnostic and procedural issues/errors that your dentist might have handled more successfully. (In some cases, you, the patient, may have even played a role in creating the problem.)

Other issues involve factors related to the tooth, its root canal system or condition that any dentist, or even specialist, would have difficulty overcoming, if possible at all.

Fundamentally, why cases fail.

While the specific cause of any one tooth's endodontic failure will vary, unsuccessful cases typically involve some type of issue that results in a deficiency in one of the following fundamental areas:

  • 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 clinical standards needed. Or issues associated with the tooth's preoperative condition have created treatment obstacles.

Examples of specific reasons why root canal treatment may fail.

1) Missed canals.

Different types of teeth (molars, premolars, canines, incisors) characteristically have differing numbers of roots and root canals. But unfortunately for the treating dentist, there are no hard rules about the configuration that actually exists.

  • Specific roots of some types of teeth are well know for having, or frequently having, multiple canal configurations, and because of this should always be suspected of having more than one.
  • Even beyond what might generally be expected, it's always possible, no matter how rare, that the anatomy of a tooth's root canal system is simply atypical.

The back root (root "a") of a lower molar may have one or two canals.

Picture of an x-ray showing a lower molar's root canal anatomy.

If it has two but both aren't found, its root canal treatment will fail.

A dentist's due diligence.

The problem/question that arises is how much effort a dentist should reasonably expend in searching for these, possibly rare, variations.

As a point of fact, additional canals are frequently tiny in size and as such difficult to identify. Additionally, they may have a location inside the tooth that's strange or unexpected.

  • At minimum, looking for possible variations takes additional time. Although, this should just be a minor consideration for the dentist.
  • Worse, searching a tooth exhaustively can involve trimming away aspects of its interior that can result in structural weakening. Of course, this is especially disappointing when no additional canals are discovered.

For these reasons, it's easy enough to understand why a dentist might not be astoundingly inquisitive if the configuration they have already discovered lies within the parameters of what can be considered normal for the tooth involved.

The underlying problem.

The crux of this issue is simply that any untreated (overlooked, undiscovered) canals, no matter how minute in size, will remain a locus of persistent infection.

And as such, that leaves the expectation that they will lead to the failure of the tooth's root canal treatment.

Picture of a dentist using a surgical microscope during root canal treatment.

An endodontist using a surgical microscope.

Incidence rates.
  • A study by Iqbal 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 use a surgical microscope as a visual aid in their search for additional canals. Also, a specialist is more likely to be familiar with what variations might exist, and more likely to be able to adequately treat very tiny canals. See discussion and link below.)

  • A study by Hoen evaluated 337 failed root canal cases and determined that overlooked canals played a role in 42% of them.

Section references - Iqbal, Hoen

Animation showing lateral and accessory root canals.

Variant root canal anatomies.

2) Unfilled accessory and lateral canals.

The anatomy of the root canals inside a tooth can display variations.

  • An accessory canal is any branch of one of a root's main canals that leads off to an exit point on the root's surface.

    Somewhat arbitrarily, accessory canals are typically defined as canal branching found in the apical end (tip portion) of a tooth's root (last 1/3 of the root or so).

  • A lateral canal is also technically an accessory canal. But, and again arbitrarily, usually defined as branching that occurs in the upper 2/3rds of a tooth's root.

    As a point of difference, lateral canals often run horizontally from the central canal directly to the root's side. In comparison, accessory canals might be considered to have a more split-off/branched configuration (see animation).

The underlying problem.

With both of the above variations, it's common that the division of the canal isn't identified (they typically aren't observed on x-rays). And even when detected, it may be difficult, or even impossible, for the dentist to adequately treat (clean, shape, fill) the branch.

If that's the case, and just like with missed canals discussed above, the result will be one where some of the tooth's root canal system is left ineffectually treated. As a result, the deficient portion can provide a location where infection can persist and therefore act as a continued irritant to the tissues that surround the tooth's root, ultimately leading to case failure.


The root canal systems of any and all teeth have the potential to include accessory canals in their configuration. (They form when tissues that play a role in root formation become entrapped during the calcification process.) And their existence is quite commonplace.

An evaluation of 493 extracted teeth by Ricucci determined that 75% of them had accessory/lateral canals.

The question then arises, if they are so common, and they are difficult to clean and fill, why aren't they a dominant factor in root canal failure.

As one might expect, studies suggest that the size of the accessory canal and the contents they harbor (such as microorganisms and their byproducts) are key determinants in the outcome for the tooth.

Section references - Ricucci

A cracked molar.

Some cracked teeth may not be treatable.

3) Cracked roots.

Cracks that extend into 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 invaded by bacteria, the infection that results due to their colonization can't be cleared up. It will be a persistent source of irritation to the tissues that surround the tooth's root.

As such, the tooth's outlook is one of case failure. Short of amputating the affected root (if this option exists at all), case retreatment/further treatment is generally not possible.


There are several issues that can make dealing with cracks especially difficult:

  • When performing a tooth's endodontic therapy, a dentist may be unaware that a crack exists (they can be very difficult to identify), or underestimate the extent of the ones they see. Either way, the tooth's problems won't be resolved by its treatment.
  • It may be that the crack that has caused the tooth's failure formed after it received its endodontic treatment.

    This might be because an inadequate "final" restoration was chosen for the tooth (such as one that didn't provide a strengthening effect), or it's permanent restoration was not placed soon enough (the damage occurred while the tooth's provisional restoration was still in place).

  • It's also possible for what seemed to be a minor, manageable crack at the time of the tooth's treatment to progress.

    Beyond cases like those just mentioned, this scenario is even possible with seemingly appropriately restored teeth (e.g. crown placement), if the chewing or clenching forces they're subjected to are intense enough.

Section references - Torabinejad

4) Inadequate root canal seal.

The integrity of the seal created inside a tooth during its root canal procedure is an important factor in treatment success.

  • It serves as a barrier to the seepage of bacteria and other contaminates into or out of the tooth.

    (This is important in preventing recolonization of the tooth's root canal system by microorganisms. Also, any leakage from the tooth would serve as a source of constant irritation to the tissues that surround its root.)

  • The filling materials physically occupy the space of the root canal system. (When the full extent of a root canal is not filled in and sealed, tooth space remains that can harbor bacteria.)

It may be that the tooth's intact seal has deteriorated over time. Or that the placement of its filling materials resulted in shortcomings initially (due to underfilling, the presence of voids, etc...). And while there can be technical reasons why underfilling a canal has occurred, deficient fills can generally be expected to correlate with the level of clinician expertise.

A study of 90 failed cases by Iqbal determined that 1/3rd involved underfilling the tooth's root canal space.

Section references - Iqbal

5) Overextension of the tooth's filling material.

Studies have shown that in cases where the materials used to fill in and seal a tooth's root canal system extend out beyond its root's tip (overfilling, overextension), the likelihood of endodontic failure is increased. (Tabassum)

  • In an evaluation of 337 failed root canal cases, Hoen determined that overfills played a role in 3% of them.
  • Iqbal's evaluation of 90 failures found that overfilled canals played a role in 10% of cases.
The cause.

The basic problem involved with this issue is one of bioincompatibility. Due to the presence of the materials, a foreign body/inflammation response may be triggered in the tissues surrounding the root's tip.

This response doesn't occur in all cases. And in fact, it's possible for normal and complete postoperative healing to occur in the presence of an overfill.

As a final point, while it's true that in clinical practice unforeseen events sometimes occur, generally speaking the ability to properly confine root canal filling materials within a tooth must be considered a factor that correlates with the clinician's level of experience and skill. (See specialist vs. general dentist discussion below.)

Section references - Tabassum, Iqbal, Hoen

6) Inadequate coronal seal.

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

This phenomenon is referred to as "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. The study by Hoen cited above reported that 13% of failed cases involved complications with coronal leakage.

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

Beyond the clinician-associated issues mentioned above, other technical/procedural errors and complications can arise and have a detrimental effect on the treatment outcome of a tooth. This might include:

  • Failure to treat the entire length of the tooth's canals. - An important aspect of the root canal process is establishing the length of each of a tooth's individual root canals, and then treating this entire distance.

    Falling short can result in leaving debris and contaminates behind that can be a constant source of irritation to the tissues that surround the tooth's root, thus resulting in treatment failure.

  • Problems caused when shaping the canals - A dentist's use of root canal files may inadvertently create an internal configuration that deviates from normal canal anatomy. (Applicable terms include: canal ledging, apical transportation or zipping.)

    This kind of alteration can make the process of cleaning and/or sealing the affected canal(s) difficult or impossible.

  • Perforations - When using drills or files, a dentist may inadvertently create a hole (perforation) that penetrates the side of the tooth's root.

    Depending on the size and location of the perforation, some can be repaired successfully. However, in some instances the presence of the opening may make it difficult or impossible for the dentist to slide their tools and sealing materials beyond that point, thus inhibiting complete (proper) canal cleaning and sealing.

  • Broken instruments - The files that a dentist uses to clean a tooth's root canal system sometimes break. It's generally attributed to manufacturing defects, fatigue from usage, or with rotary instruments, creating a situation that places too much torque on the file.

    As a worst case scenario, the broken piece may be lodged inside the tooth and cannot be retrieved. If so, while leaving the fragment inside the tooth is never the dentist's first choice, the point during the treatment process when the incident occurred may be a mitigating factor.

    If the file separation has occurred after the canal's cleaning process has already been completed, then possibly the canal can still be adequately sealed even with the fragment present. If the incident occurred during the cleaning process and inhibits its completion, the prognosis for the tooth's treatment is much less favorable.

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The study of 90 endodontic failures by Iqbal cited above determined that about 6% of the cases could be attributed to problems associated with perforations, and 7% broken instruments.

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, the study by Iqbal cited above that evaluated failed cases determined that roughly 80% of them had been attempted 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 found on our failed root canal statistics page, this might be an especially prudent choice for certain types of teeth (like molars).

Additionally, 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 pathology.

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 (external to the tooth) locus of infection will be a persistent irritant to the surrounding tissues.

The Iqbal study cited above suggests that the success rate of teeth having this initial condition (vs. those that don't) may be lower, on the order of 20%.

A possibly needed solution.

In cases where the dentist's evaluation of the quality of the tooth's previously performed root canal treatment seems acceptable, the solution for this situation may be a minor surgical procedure referred to as an "apicoectomy with retrograde filling."

During this procedure, the tip of the tooth's root is trimmed away (an apicoectomy is performed). The exposed root canal opening on this trimmed surface is then sealed by placing a filling (a "retrograde" filling).

10) 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 typically considered less structurally sound than they were originally, possibly substantially so. 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.