What causes root canal failure (7 proven reasons)?

What causes root canal failure

Root canal failure is uncommon among patients. The success rate of endodontic treatment is up to 86-98%. However, anatomic (structural) differences and the extent of infection in patients at the time of therapy are the most common causes of root canal failure.

The diagnosis of root canal failure is based on the following criteria:

  • Return of pre-treatment symptoms, such as pain or swelling.
  • The presence of a round black area around the root of the treated tooth on the x-ray. 

Your dentist or root canal dentist (endodontist) will evaluate the success of treatment based on how well the root canal is done or any signs of infection on an x-ray.

What do root Canal Failure symptoms feel like?

If you experience one or more of the following symptoms in a tooth canal-treated tooth, consult your dentist:

  •  Severe pain in the tooth while chewing or biting.
  • Spontaneous pain in the tooth 
  • A boil on the gums.
  • A broken tooth 

What are the steps Of Root Canal Treatment

Before going into detail about the causes of root canal failure, it is essential to understand the steps of root canal therapy. 

The following are the steps involved in root canal treatment:

Step 1

It involves measuring the length of root canals with files to completely clean and fill the canals.

Step 2

The root canals are cleaned and shaped with endodontic files (long needle like instruments) to remove the infected pulp tissue, bacteria, and canal irregularities. The canals are simultaneously flushed with a disinfectant solution with a long needle syringe.

Step 3

The canals are filled with coagulated latex sticks (GP points) and a sealant (paste) to seal gaps between the GP points.

What causes root Canal Failure?

There are several reasons for root canal infection or reinfection:

Incomplete removal of infection

One of the most common cause of root canal infection is the persistence of bacterial infection. Improper canal disinfection allows bacteria to replicate, leading to root canal failure.

Also, complex canal anatomy, such as isthmus (a communication between two canals — filled with pulp) evades mechanical and chemical cleaning. Also, bacteria lodged in porous dentin are often difficult to remove and can lead t root canal reinfection.

Improper Sealing Of The Tooth

A well-sealing filling and crown on a root-canal treated tooth are prerequisites for a successful root canal treatment. Any breach in the seal provides a portal of entry to the bacteria present in the ambient environment.

Trouble with instruments

A small or improperly opened tooth can cause the instrument breakage in the canal. 

Files are needle-like instruments of varying sizes, designed to remove the infected pulp and smooth the canal walls to achieve an adequate seal with a filling.

The tooth opening needs to be large enough to allow placement of files into the canals without bending.

A broken instrument in the canal poses difficulty in cleaning the canal below the fractured instrument. However, endodontists (root canal specialist) handles these cases well as they are specialized in retrieving a broken instrument or bypass the instrument and clean the canal.

Untreated or missed canals

It is not an uncommon practice to miss a canal during root canal treatment.

It primarily occurs in molars, where the one root, one canal formula is frequently overruled. Most, molar roots have two or more canals which are narrow and difficult to access. An x-ray reveals the canal anatomy of the tooth. Sometimes, a canal lies behind another and, therefore, can be missed.

Moreover, a less-than-satisfactory tooth opening makes it difficult for the dentist to locate the extra canals.

The inability to treat all the canals is one of the causes leading to treatment failure. The uncleaned canals harbor bacteria, leading to root canal failure.

Short or over-extended root fillings

The quality of root canal filling cannot be overlooked. The success rate of treatment plunges in patients with under or over-fillings of canals. However, over-extended fillings are four times more likely to fail than under-fillings.

The study revealed that 65% of patients with poor-quality root filling and 42% with missed canals showed root canal failure. 

Perforations in the root canals 

Perforation of pulp chamber or canal walls during canal disinfection exposes the tooth to external enviroment leading to infection. Perforations account for 10% of root canal-treated teeth and frequently occur in lower molars. 

Re-Treatment Of Root Canal failure

During the retreatment, the old root fillings are removed with files. The root canals are again cleaned and disinfected, and filled by a new filling.


Root canal failure is diagnosed if the severe pain or swelling returns and the treated tooth root apex shows a black round (radiolucent) area on the x-ray. The causes leading to root canal failure involve: 

  • Incomplete cleaning of canals
  • An improper seal of the tooth from a filling or crown
  • An instrument breaks in the canal.
  • Untreated or missed canals
  • Short or over-extended root fillings

Regular dental checkups aid in assessing the outcome of treatment and should be scheduled at least on an annual basis to monitor any changes.

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