Root Canal Infection – 5 Proven Reasons

root canal infection
Tooth cut in half shows root canals

Root canal infection is uncommon among patients. The success rate of endodontic treatment success rate is up to 86-98%.  However, there are some anatomic (structural) differences and the degree of infection among people at the time of therapy that can interfere with the success of root canal therapy.

Root canal failure is based on the criteria of return of symptoms present before the treatment and the presence of a black round area around the root of the treated tooth on an x-ray. Your dentist or root canal dentist will evaluate the success of treatment based on how well the root canal is done or any signs of infection on an x-ray.

Symptoms of root canal infection

If you have one or more of the following symptoms in a tooth, you may need a root canal:

  •  Severe pain while chewing or biting.
  • A deep cavity or a hole in a tooth that is painful.
  • Spontaneous pain.
  • A boil on the gums.
  • A broken or cracked tooth.
  • Persistent pain or sensitivity to hot or cold, even after the sensation has been removed.
  • Swollen or tender gums.

steps of Root canal treatment

Before going into the detail about the causes of root canal infection, it is important to understand the steps of root canal therapy:

Step 1: To measure the length of root canals to clean and fill them till the end of the root.

Step 2: To clean the canal and shape it and remove any irregularities to ensure a proper seal. The canals are mechanically cleaned with instruments called files and disinfected with a solution.

Step 3: Fill the canals with coagulated latex sticks and a sealant (in the form of a paste) to seal gaps between GP points.

Reasons for root canal infection

There are number of reasons for the root canal infection:

Persistence of infection

One of the foremost causes of root canal infection is the persistence of bacterial infection. Improper cleaning of the root canal is a leading factor leaving infectious remnants behind.

Bacteria hidden in the root canal areas such as isthmus (communication between two canals and contains infected pulp) or in porous dentin may evade mechanical and chemical cleaning

Improper seal of the tooth with a filling

A well-sealing filling and crown placed on the top of root canal filling are important to prevent the ingress of bacteria that are present in the ambient environment. Any breach in the seal can potentially infect the root canal.

Trouble with the instruments

The motorized files use to clean canals break if the tooth is not properly opened. Files are needles of varying sizes, designed to remove the infected pulp and smooth the canal wall to achieve an adequate seal with a filling.

The canal opening needs to be visible to allow uninterrupted placement of files into the canals without bending.

A broken instrument in the canal causes difficulty to clean the canal below the broken instrument. However, an endodontist (a root canal specialist) knows how to retrieve the broken instrument or bypass the instrument and clean the canal.

Untreated or missed canals

It is not an uncommon practice to miss a canal while carrying out the root canal treatment.

It occurs especially in the back or molar teeth where one root, one canal formula is frequently overruled by the fact that the number of canals is more than the number of roots. 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. These uncleaned canals harbor bacteria and cause root canal infection.

Short or over-extended root fillings

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

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

Re-treatment of root canal infection

During the retreatment, the old root filling is removed and canals are cleaned and disinfected. it is then followed by a new filling.


 The root canal infection is the return of symptoms you had prior to the treatment. The causes are listed below

  • Incomplete cleaning of canals
  • Improper seal of the tooth from a filling or crown
  •  Instrument break while cleaning the root canal and is stuck there
  • 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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