Decay is the most common reason a tooth ends up needing root canal treatment. Most decay is dealt with by a filling. A small proportion reaches far enough to involve the nerve.
Decay is the gradual loss of tooth structure caused by acid. Bacteria in plaque produce that acid every time they are fed sugar.
Enamel dissolves slowly at first. Once decay passes through enamel into the softer dentine underneath, the process speeds up. The hole in the tooth then grows more quickly.
The nerve, or pulp, sits at the centre of the tooth. Dentine separates the pulp from the outside, and dentine carries tiny tubules running inwards.
Shallow decay stays well away from the pulp. Deeper decay lets bacteria and their by-products travel down those tubules, and the pulp responds by becoming inflamed.
Inflammation alone does not mean a tooth needs root canal treatment. A pulp that is irritated but still healthy often settles once the decay is removed and a well-sealed filling is placed.
The question is whether the inflammation is reversible. Sensitivity that disappears within seconds suggests an irritated pulp; sensitivity that lingers for minutes suggests inflammation that will not settle.
A tooth that aches unprompted, or wakes someone at night, has usually passed the point where a filling alone will settle the tooth.
Once bacteria reach the pulp itself, the tissue cannot recover. Root canal treatment removes the affected tissue, cleans and disinfects the canal system, then seals the canals.
Working under a dental operating microscope matters here. Decay near the pulp sits in a small space, and the difference between sound and affected dentine is easier to judge under magnification and strong light.
Not every tooth with deep decay is uncomfortable. A pulp can lose vitality quietly, with the first sign appearing on a routine X-ray rather than as a symptom.
Silent decay is one reason bitewing X-rays matter. Decay between teeth stays invisible to the naked eye until the process is well advanced.
A tooth that has had deep decay removed has lost structure. Back teeth take heavy loads, and a well-sealed permanent restoration is what keeps the tooth working.
Your own dentist provides that restoration. Where a large amount of tooth has gone, a cusp-covering restoration is often advised.
A lower first molar where decay had reached the nerve, with infection at the end of both roots by the time treatment began.