Root canal retreatment: when is it needed, and how successful is it?

In root canal treatment, the inflamed or infected nerve and blood vessel tissue inside the tooth (the pulp) is removed, and the canals are cleaned, shaped and filled so that they are sealed. Most treatments work: in a study of insurance data in the USA following 1.46 million teeth for 8 years, 97% of the teeth were still in the mouth; events such as retreatment, root-end surgery and extraction added up to 3%, and most were seen in the first 3 years after treatment. In teeth where it does not work, however, the inflammation at the root tip (apical periodontitis) continues or starts again.

The American Association of Endodontists (AAE) lists the reasons why initial treatment fails as follows: narrow or curved canals not being treated during the first procedure, complicated canal anatomy going undetected, a delay in placing the crown or permanent filling after treatment, and the restoration on top not preventing bacteria in the mouth from getting inside the tooth. New problems can also arise years after successful treatment: new decay that exposes the root filling, a crown or filling that breaks or opens at its edge, and a crack in the tooth.

These reasons fall into two groups. In the first group the problem is inside the tooth: a canal that has not been cleaned, a filling that stops short, bacteria leaking back in. In this group retreatment makes sense, because it reaches the source of the problem directly. In the second group the problem is in the structure of the tooth: a vertical crack in the root, or a perforation in the root wall that cannot be repaired. In this group, however well you clean the canals, the pathway for bacteria does not close. Before the decision to retreat, the main task is to understand which group the tooth is in.

The two most often quoted studies on the success of retreatment come from the Eastman Dental Institute in London. In the 2008 systematic review, using the pooled data of 17 studies published between 1961 and 2005, the success rate of retreatment was found to be 77%. The review showed that the presence of an area of inflammation (a lesion) at the root tip before treatment, the size of the lesion, how close to the root tip the root filling ended and the quality of the restoration on top were the factors determining success.

In the prospective study the same team published in 2011, 750 teeth were retreated and followed for 2 to 4 years. The rate of complete healing at the root tips was 80% with retreatment and 83% with initial root canal treatment; the authors found the two rates similar. The proportion of teeth still in the mouth after 4 years was also almost the same in the two groups (95.3% and 95.4%). The treatments in this study were carried out by postgraduate endodontic students; it shows not that the results can be carried over unchanged to every practice, but that in the right case retreatment is not markedly worse than initial treatment.

The same study also listed one by one the findings that make healing easier and harder. Those that increase the chance of healing: no lesion, or only a small lesion, at the root tip before treatment; no discharge pathway (fistula) in the gum; the canal being able to be opened and cleaned right to its end; no flare-up of pain or swelling between sessions; the filling material not extruding beyond the root tip; and a satisfactory, sealed filling or crown on top of the tooth. The absence of a perforation in the root or tooth wall was also favourable.

The findings that affect whether the tooth stays in the mouth are a little different. In the second part of the 2011 study, a history of diabetes and systemic steroid treatment, pain before treatment, a narrow but deep pocket in the gum, a discharge pathway and, in retreated teeth, a pre-existing perforation increased the risk of losing the tooth. Having a cast crown on the tooth rather than a temporary one, having a neighbouring tooth on each side, and the tooth not being the last in the row, on the other hand, reduced the risk. In teeth with pain before treatment, the risk of tooth loss in the first 22 months was around 3 times higher.

The evidence comparing retreatment with its alternative, root-end surgery, is limited. A Cochrane review writes that surgery may show slightly higher healing in the first year, but that at 4 years of follow-up the results of the two methods were similar, and that there is little evidence for a sound decision. In a 2015 meta-analysis, the pooled success rate was 92% for microsurgery and 80% for retreatment; the difference was significant in follow-ups shorter than 4 years, and not significant in follow-ups longer than 4 years.

The breakdown below becomes clear after an examination and an X-ray and, if needed, a three-dimensional image. Seeing yourself in the second list does not mean the tooth will definitely be lost; it means the option to be discussed changes.

The sequence below is based on the steps described by the AAE. The number of sessions and the time taken vary with the structure of the tooth, the type of old filling and the state of the inflammation.

1. 1

   Diagnosis: looking for the source of the problem

   The dentist asks about the history of pain in the tooth, when the root canal treatment was done and the restoration on top; carries out tapping and pressure tests, measures the gum pocket and takes an X-ray. If a missed canal or a crack is suspected, they may ask for a cone beam CT scan. The key question is whether the problem is inside the canal or in the structure of the root.

2. 2

   Anaesthetic and isolation

   The area is numbed with a local anaesthetic. The tooth is surrounded with a rubber sheet (rubber dam) to protect it from saliva and bacteria in the mouth.

3. 3

   Re-entering the tooth

   The top of the tooth is opened to reach the canals. If there is a crown on top, sometimes access is made through the crown, and sometimes the crown and the post inside the canal have to be removed. This step can be the hardest and longest part of the procedure.

4. 4

   Removing the old filling and searching for canals

   The old root filling is removed. According to the AAE, at this stage the dentist carefully examines the tooth for additional canals or new infection. Finding a missed canal can explain why the initial treatment did not work.

5. 5

   Cleaning, shaping and disinfection

   The canals are cleaned and shaped again; most of the cleaning is done by the irrigating solutions put into the canal. If the infection is heavy, medication may be placed in the canals for a while and the tooth closed temporarily, with treatment completed at the next session.

6. 6

   Filling the canals again

   The cleaned canals are filled again and the top of the tooth is closed with a temporary filling.

7. 7

   Permanent restoration

   The tooth is soon closed with a permanent filling or crown. In the study by Ng and colleagues, having a cast crown on the tooth rather than a temporary one was one of the factors reducing the risk of tooth loss. Waiting for months with a temporary cover lowers the chance of retreatment working.

Alongside retreatment, the AAE also lists root-end surgery and extraction as options. The right order usually starts with the least invasive, but the condition of the tooth can change the order.

Retreatment is a safe and sensible step for most teeth, but it has its limits. We write the points below so that you can discuss them whichever dentist you see.

The first few days after retreatment are similar to the initial root canal treatment. Healing of the bone at the root tip, however, takes months and can only be monitored with check-up X-rays.

1. The day of the procedure and the next few days

   After the numbness wears off, the tooth may be tender and uncomfortable when you bite. The NHS writes that after root canal treatment the area may feel swollen and tender, that this will settle within a few weeks, and that paracetamol or ibuprofen can be taken.

2. Between sessions

   If treatment is spread over more than one session, the tooth stays closed with a temporary filling. Not biting hard food on that side during this period helps protect the temporary filling and the tooth.

3. Permanent restoration

   Once the canal has been filled again, closing the tooth with a permanent filling or crown should not be delayed.

4. Check-up X-rays

   In the study by Ng and colleagues, retreated teeth were checked once a year and followed for 2 to 4 years. Your dentist may ask for images at similar intervals to see whether the lesion is getting smaller; complete healing can take more than a year.

We do not give figures here, because the scope of retreatment varies greatly from tooth to tooth. The items that determine the price are:

Type of tooth and number of canals

Retreating a single-rooted front tooth and a molar with three or four canals is not the same job; the time and number of sessions differ.

Need for removal

If the post inside the canal or the crown on top needs to be removed, the procedure takes longer and the crown also has to be remade.

Imaging and specialist

A three-dimensional CBCT scan, and the procedure being done by an endodontic specialist under a microscope, affect the scope.

Permanent restoration

The permanent filling or crown made after retreatment is a significant item in the total cost.

The next step if it fails

If retreatment does not work, root-end surgery, or extraction and an implant in its place, may come up. Take this possibility into account when making the decision.