Root Canal Treatment: What Happens Inside the Tooth

What a Root Canal Treats

A root canal is the procedure that removes infected or inflamed pulp from inside a tooth and seals the space so the natural tooth can stay in the mouth. The pulp is the soft tissue at the centre of the tooth, made of nerves and blood vessels. It sits in the pulp chamber under the crown and continues down each root through narrow channels called root canals.

The pulp can be damaged by deep decay, a crack, a large or repeated filling, or a blow to the tooth. Once bacteria reach it, the tissue becomes inflamed and cannot recover on its own, because it is enclosed in hard walls with a narrow blood supply at the root tip. Left in place, the infection moves out through that root tip into the bone, where it can form an abscess.

The treatment belongs to endodontics, the branch of dentistry that deals with the pulp and the tissues around the root tip. The tooth itself is kept and only the damaged tissue inside it is removed. The canals are cleaned, shaped and filled with gutta-percha, a rubber-like material, and the crown of the tooth is rebuilt with a filling or a crown. The treated tooth no longer senses hot and cold, but it stays anchored in the jaw by its roots and the surrounding ligament and keeps chewing like its neighbours.

Signs That Point to Pulp Damage

Pulp damage usually shows itself through pain, although a tooth can also lose its pulp quietly and appear only on an X-ray. The typical sign is persistent toothache, especially when chewing or biting down. Sensitivity to hot or cold that lingers long after the drink or food is gone points to inflamed pulp, while a short twinge that fades within seconds is more often a surface problem.

Other signs involve the gum and the colour of the tooth. The gum around the tooth may become swollen or tender, and a small pimple-like bump can appear near the root, which is where pus from an abscess drains. A tooth that turns grey or darker than its neighbours, often months after an injury, may have a pulp that has died. A cracked or chipped tooth with exposed pulp needs assessment even before it hurts.

None of these signs confirms the diagnosis on its own, since several overlap with gum disease, a cracked tooth or sinus pressure on the upper back teeth. The dentist tests the tooth with cold, taps on it, checks the gum and takes a radiograph to see whether the bone at the root tip shows a dark area, the mark of an infection that has spread beyond the tooth.

How the Procedure Runs, Step by Step

A root canal follows the same sequence in almost every case. Examination and radiographs come first and show how far the infection has spread, how many canals the tooth has and how curved they are. The tooth is then numbed with local anaesthetic, and a thin rubber sheet is placed around it to keep saliva and bacteria out. The dentist opens a small access cavity through the top of the tooth to reach the pulp chamber.

Cleaning is the central step. The pulp is removed, and rotary or hand instruments clean and widen each canal along its full length while rinsing solutions wash out debris and bacteria. Once the canals are clean and dry, they are filled with gutta-percha and a sealer, and the access cavity is closed with a temporary or permanent filling.

Chair time depends mainly on the number of canals. A front tooth with a single canal takes roughly 30 to 45 minutes, a premolar with one or two canals about 45 to 60 minutes, and a molar with three or four canals 60 to 90 minutes or more. Severe infections and retreatment of an older root canal are usually spread over two or three visits, with a medicated dressing inside the tooth in between.

The last step is rebuilding the tooth. Front teeth with enough remaining structure can often be restored with a filling. Back teeth, and molars in particular, usually need a crown to protect them from fracture, because they carry the heaviest chewing load and have already lost structure to decay and to the access cavity.

Pain Before, During and After Treatment

Most of the pain linked with root canals comes from the infection, and the treatment is what ends it. Inflamed pulp in a closed space builds pressure, which is why such a tooth can throb or react strongly to heat. Removing that tissue removes the source of the pain.

During the procedure, local anaesthesia numbs the tooth and the tissue around it, so patients usually feel pressure and vibration from the instruments without sharp pain. A very inflamed tooth can take longer to numb, and more anaesthetic can be given if you still feel anything.

After the anaesthesia wears off, mild soreness around the tooth or some stiffness in the jaw for a few days is normal, because the tissue around the root tip has been disturbed. It responds to ibuprofen or paracetamol, cold compresses on the cheek and soft food for a few days.

Pain that gets worse over the following days instead of easing, new swelling in the face or gum, a raised temperature, or a bite that feels high are reasons to contact the dentist promptly. These can signal a flare-up of the infection or a restoration that needs adjusting, and both are easier to manage early.

Success and Tooth Survival in the Research

Root canal treatment works in most cases, and the exact figure depends on how success is defined. An updated review pooling 42 longitudinal clinical studies published between 2003 and 2020 put the success rate of primary root canal therapy at 92.6% (95% CI 90.5-94.8) under loose criteria, where the lesion around the root tip shrank, and 82.0% (95% CI 79.3-84.8) under strict criteria, where it resolved completely (PubMed 35334111). The same review found that biological factors had the greatest influence on results and that the technological method of instrumentation made no significant difference (PubMed 35334111).

Survival is a separate measure. It asks whether the tooth is still in the mouth, whatever the radiograph shows. A systematic review of 14 studies found pooled tooth survival of 86% at 2 to 3 years, 93% at 4 to 5 years and 87% at 8 to 10 years after non-surgical root canal treatment (PubMed 20158529).

That review also identified four conditions that significantly improved survival. In descending order of influence these were a crown restoration after treatment, contact with neighbouring teeth on both sides, the tooth not serving as an anchor for a bridge or removable denture, and the tooth being a non-molar (PubMed 20158529). This is why a treated molar is usually crowned, and why the restoration placed after the root canal matters as much as the canal work itself.

Retreatment, Extraction and Other Alternatives

When a root canal does not heal, the tooth can often be treated again. Reinfection usually comes from a canal that was missed or could not be fully cleaned, or from bacteria leaking in under a restoration that was placed late or has broken down. In retreatment the old filling material is removed and the canals are cleaned and refilled. If the problem sits at the root tip and cannot be reached from inside, an apicoectomy may be considered, a small surgical procedure that removes the root tip and seals the end of the canal.

The alternative to saving the tooth is extraction, followed by closing the gap, and each option has its own trade-off. An implant replaces the root in the bone but involves surgery and a healing period. A bridge avoids surgery but usually requires reducing healthy structure from the teeth on either side. A partial denture is removable and generally less stable when chewing.

Leaving an infected tooth untreated is not a stable option. The infection spreads through the root tip into the bone and soft tissue, which is how an abscess forms, and it eventually leads to loss of the tooth. When the remaining tooth structure, gum and bone are in reasonable condition, root canal treatment keeps the patient's own tooth; when they are not, extraction and replacement may be more predictable, and that decision is made at the examination.

Aftercare and the Final Restoration

Aftercare focuses on protecting the tooth until it is fully restored and keeping the area clean. Until the permanent filling or crown is in place, the tooth is weaker than before and a temporary filling can wear or crack, so hard food should not be chewed on that side.

Getting the final restoration on time is part of the treatment. A crown placed after a root canal ranked first, in order of influence, among the four factors linked with better tooth survival in the review of 14 studies (PubMed 20158529). Delaying it leaves the tooth exposed to fracture and gives bacteria time to leak back into the cleaned canals.

Daily hygiene is the same as for any other tooth. Brush twice a day, clean between the teeth with floss or interdental brushes, and keep the gum line around the crown clean, since new decay at the edge of a crown can let bacteria back into the tooth. Smoking slows healing around the root tip and should be avoided in the weeks after treatment.

Follow-up visits check that the tooth is comfortable and that the bone around the root tip is healing, which is confirmed on a radiograph over the following months. Persistent pain, new swelling or a loose restoration are reasons to return before the next scheduled visit.