A root canal is not a straight tube
In the centre of each tooth lies a soft tissue carrying nerves and blood vessels, the pulp, which runs down through narrow canals inside the root. These canals are often curved or flattened, and there may be fine side branches, narrow passages joining two canals and small offshoots near the root tip. The dentine around the canal, the layer beneath the enamel, is full of microscopic tubules. When the pulp becomes inflamed and dies, bacteria settle into these crevices and usually live as a sticky, self-protecting layer on the surface, known as a biofilm.
Files shape the canal but cannot touch every wall
Root canal treatment (the field of dentistry it belongs to is called endodontics) uses fine, flexible metal instruments called files. They widen the canal into a smooth shape and scrape away most of the inflamed or dead tissue, along with some of the infected dentine on the walls. But a file works by rotating and is round in cross-section: it cannot touch every wall of a flat or oval canal, and it cannot enter side branches or narrow passages at all. Widening the canal further is no answer either, because the root becomes thinner and weaker. Perhaps the most important thing filing does is open a path for the rinsing solution to reach deep into the canal.
The rinsing solution does the real disinfecting
At every stage of filing, the canal is rinsed with a liquid. The most widely used is sodium hypochlorite: a dilute solution of the active ingredient in household bleach, prepared for medical use. It kills bacteria, dissolves tissue remnants and, being a liquid, can flow into crevices the instrument never touches. The clinical guideline of the European Society of Endodontology (ESE) lists hypochlorite alone, or hypochlorite followed by EDTA, among the options that may be considered.
- Sodium hypochlorite: kills bacteria and dissolves remnants of living or dead tissue.
- EDTA: dissolves the thin layer of debris the files leave on the walls and opens the mouths of the dentine tubules.
Why rinsing takes time
For a solution to work, it has to reach the bacteria and tissue, stay there for a while and be replaced. Hypochlorite is used up as it dissolves tissue, so the canal is refilled again and again during treatment. The fine needle that delivers it is held a little short of the root tip and never wedged into the canal, so that the liquid does not escape into the tissues outside the root. As a result, the liquid in the narrow zone near the root tip is exchanged only slowly. Much of the long stretch in the chair when nothing seems to be happening is, in fact, this rinsing.
What activation does
Activation means setting the rinsing solution in the canal in motion by vibration. An ultrasonic or sonic tip creates rapid currents and microscopic bubbles in the liquid, which is expected to carry the solution further into side branches and onto the biofilm on the walls. In some clinical studies, patients whose rinse was activated had slightly less pain over the first day or two, but the certainty of that evidence is low to moderate. The ESE guideline notes that no comparative evidence shows these add-on methods lead to better healing than conventional needle rinsing, and for teeth with inflammation at the root tip it makes a weak recommendation against adding them routinely. Activation can be useful, but it does not decide success on its own.
Why treatment sometimes takes two visits
A Cochrane review pooling the studies that compare one visit with two or more found no difference in healing on X-rays a year later, although the share of people reporting pain in the first week was higher after single-visit treatment. The ESE guideline recommends a single visit where all the necessary steps could be completed properly in that session. A second visit usually comes up when that condition is not met: the canal keeps weeping and cannot be dried, the anatomy turns out more complex than expected, time runs out, or keeping the mouth open becomes too hard. How much pain is normal afterwards is the subject of a separate article.
The rubber dam and a tight seal
Keeping the cleaned canal from being contaminated again matters as much as the cleaning itself. That is why a rubber dam is placed around the tooth for the whole procedure: a thin sheet with a hole in the middle that leaves only the tooth being treated exposed. The American Association of Endodontists (AAE) regards it as an essential standard of root canal treatment. The sheet keeps bacteria in saliva away from the canal and stops small instruments and rinsing solution from being swallowed or inhaled. Finally, the canals are filled and the tooth is sealed on top. If the filling on top stays temporary for a long time or leaks at its edges, bacteria can find their way back into the cleaned canal.
Frequently asked questions
What if I feel burning or taste something bitter during treatment?
With the rubber dam properly seated, the rinsing solution should not reach your mouth, although a faint smell of chlorine is normal. If you notice a bitter taste or a burning feeling on your tongue or lips, raise your hand and say so straight away. The edge of the dam is checked again and your mouth is rinsed if needed.
What does sudden, severe pain during rinsing mean?
Rarely, the rinsing solution can pass out through the root tip into the surrounding tissues. This usually shows itself during the procedure as sudden, severe pain and swelling that grows quickly; the dentist stops and starts managing it at once. If, once you are home, the swelling in your face grows, or you have a fever, difficulty swallowing or difficulty breathing, seek care without delay, at an emergency department if necessary.
I react to bleach or chlorine. Should I mention it?
Yes, mention it before treatment. A true allergy to hypochlorite is rare, but if you have had a reaction before, the dentist can consider a different rinsing solution. These solutions do not dissolve tissue equally well, so the cleaning plan changes accordingly.
I am allergic to latex. Can a rubber dam still be used?
Yes. Latex-free dams are available and do the same job. It is enough to mention your allergy at the start of the appointment; gloves and other materials are then chosen accordingly.
Can I breathe under the rubber dam if my nose is blocked?
The dam covers the mouth but leaves the nose free, and most people breathe comfortably through it. If you have a cold or a nose that often gets blocked, say so at the very start. One edge of the dam can be loosened, short breaks can be taken, or the appointment can be postponed until your nose clears.
Can an X-ray show that the canal is clean?
Not directly. An X-ray shows how close the filling comes to the root tip and how densely it fills the canal, not whether the bacteria are gone. If there was a dark area at the root tip caused by inflammation, healing shows as that area shrinking over time, which is why follow-up X-rays are usually taken months after treatment.
Sources
This article draws on the European Society of Endodontology's clinical guideline, a Cochrane review comparing single and multiple visits, the AAE position statement on dental dams and an umbrella review of pain after treatment. Evidence on activation methods is still limited and mixed.
- British Dental Journal (European Society of Endodontology) — Endodontic S3-level clinical practice guidelines: the European Society of Endodontology process and recommendations
- Cochrane Database of Systematic Reviews — Single versus multiple visits for endodontic treatment of permanent teeth
- American Association of Endodontists — AAE Position Statement – Dental Dams
- Journal of Clinical Medicine — Post-Operative Pain After Endodontic Instrumentation, Irrigation and Obturation: An Umbrella Review of Systematic Reviews Published from 2016 to 2025
This content is for general information only and does not replace an examination.
Edited by
Papatya Dental
This article draws on the clinical experience of the Papatya Dental team and is reviewed for accuracy before publication.
- Last updated:
- 2026-09-21
- Editorial contact:
- icerik@papatyadental.com