What's Actually Inside a Tooth — and Why It Hurts When It's Exposed

A tooth is several different tissues stacked together, and each layer produces a different kind of pain when it is worn through, exposed or inflamed. Cold sensitivity, a sharp jolt on sweet food, a deep throb at night and soreness on biting are not the same problem with different volumes. They come from different tissues, and they mean different things.

This is what is inside a tooth, layer by layer, tied to sensations you have probably had.

If a tooth is giving you a symptom you cannot place, it is worth having it looked at rather than guessing at it.
Call (02) 9649 6468 · Book online · WhatsApp

What are the layers of a tooth?

From the outside in, and from the crown down to the root:

• Enamel — the hard outer shell of the crown.
• Dentine — the bulk of the tooth, sitting underneath enamel.
• Pulp — the nerve and blood supply, in a chamber at the centre.
• Cementum — a thin layer covering the root surface.
• Periodontal ligament — the fibres that attach the root to the bone.
• Bone — the socket that holds the tooth.

Only the outer two are what most people picture as "tooth". The rest is where most of the trouble happens.

Enamel: why early decay is completely silent

Enamel is the hardest tissue in the human body and is almost entirely mineral. It has no nerves and no blood supply. It is not living tissue in the way bone is.

Two consequences follow from that, and they explain a great deal.

Decay in enamel produces no symptom at all

A cavity that is still confined to enamel cannot hurt, because there is nothing in enamel to feel with. It is found on examination or on a radiograph, not by sensation. By the time a tooth aches, the process has usually moved past enamel. How far decay travels before you feel it follows that timeline.

Enamel does not grow back

Early softening of enamel can remineralise, helped by saliva and fluoride. But once enamel has actually broken down and cavitated, or been worn away by acid or abrasion, the body does not replace it. There are no cells in enamel to do the rebuilding. What is lost stays lost, and is either left, sealed or restored.

That is why prevention on enamel matters more than repair. Fissure sealants close off the deep grooves on biting surfaces before bacteria establish there, and fluoride strengthens the mineral that remains.

Dentine: why cold, sweet and air hurt

Dentine is softer than enamel, distinctly yellower, and makes up most of the tooth. It is also the layer that explains almost every sensitivity complaint.

Dentine is not solid. It is riddled with microscopic tubules that run from the outer surface inwards towards the pulp, and those tubules are filled with fluid. There are somewhere in the order of tens of thousands of them per square millimetre.

The hydrodynamic explanation, in plain terms

When exposed dentine meets cold, cold air, sweet food or pressure, the fluid inside those tubules moves. Cold makes it contract and draw inwards. Sugar draws it outwards osmotically. The nerve endings at the inner end of the tubules detect that movement, and the only signal they can send is pain.

So the nerve is not being touched by the cold drink. It is registering fluid moving in a tube. That is why the pain is sharp, immediate, and usually gone within seconds of the stimulus being removed — and why it can be provoked by something as slight as breathing in through the mouth on a cold morning.

Why the gum line is the usual place

Enamel covers the crown, but it stops at the neck of the tooth. Below that, the root is covered by cementum, which is thin and wears through readily.

So when gums recede, or when acid erosion strips the surface near the gum line, dentine is exposed with very little between it and the mouth. That is the commonest reason for cold sensitivity in adults, and it is why sensitivity so often appears along a whole row of teeth at once rather than in one spot. Sensitive teeth to cold: what it means works through the possibilities.

Dentine is also softer than enamel, so once decay reaches it, it spreads faster and undermines the enamel above it. A small hole on the surface can sit over a much larger cavity underneath.

Sensitivity that has been building for months is worth assessing, because the cause changes what helps.
Call (02) 9649 6468 · Book online · WhatsApp

Pulp: why this pain is severe and hard to locate

The pulp is the living core of the tooth: nerves, blood vessels and connective tissue, sitting in a chamber that runs down into the roots.

The critical detail is that the chamber is rigid and sealed. It is enclosed by dentine on every side, and it cannot expand.

Why inflammation there is so painful

Inflammation anywhere else in the body swells. A sprained ankle swells because the tissue has somewhere to go. Inflamed pulp has nowhere to go, so instead of swelling it builds pressure inside a sealed box.

That pressure presses directly on the nerve, which is why pulp pain is described the way it is: deep, throbbing, pulsing with the heartbeat, worse lying down, and severe out of all proportion to the size of the tissue involved.

Why a badly inflamed pulp does not simply recover

Blood enters and leaves the pulp through a very small opening at the tip of the root. When pressure inside the chamber rises high enough, it compresses those vessels and restricts the blood supply.

Restricted blood supply causes more tissue damage, which causes more inflammation, which raises the pressure further. Past a certain point that cycle does not reverse. The pulp is no longer able to heal itself, and the tooth needs either root canal treatment or removal.

Why the pain sometimes stops before things get worse

People often report that a severe toothache settled on its own and take it as a sign of recovery. Sometimes it is. Often it is not: the pain has stopped because the pulp has died, and dead tissue cannot signal.

The infection is still there, and it now has an unimpeded path out through the root tip into the surrounding bone. That is the stage at which an abscess forms, and the pain returns as a different sensation entirely.

Why you cannot tell which tooth it is

Pulp has no receptors for position or pressure, only for pain. The signal arrives at the brain without a return address. That is why patients can point to the general area but not the tooth, why upper and lower teeth on the same side get confused, and why the ache is sometimes felt in the ear, the temple or the jaw.

Cementum and the periodontal ligament: the layer that does know

A tooth is not fused to bone. It hangs in its socket, suspended by the periodontal ligament — a thin sheet of fibres running from the cementum on the root to the bone of the socket.

The ligament acts as a shock absorber, letting the tooth move a fraction of a millimetre under load instead of taking the force rigidly.

It is also densely supplied with pressure and position receptors. That is why you can tell whether you are biting a grain of rice or a soft chip, and why, when the ligament becomes inflamed, you can point straight at the guilty tooth without hesitation.

So the two pains are diagnostically useful in opposite ways. Pain you cannot localise tends to be coming from the pulp. Pain on biting that you can pinpoint exactly tends to involve the ligament, whether from infection reaching the root tip, from a crack, or from a filling sitting slightly high.

Bone: the layer that gives no warning

The bone around the roots is what actually holds teeth in. Gum disease destroys it slowly, and it does so with almost no symptom, because bone loss itself does not hurt.

A tooth or a set of gums can be well down this path before producing any symptom at all — which is the single most useful thing to understand about dental problems, and is set out in early gum disease signs people miss.

Bone does not regenerate on its own once it has been lost to periodontal disease. Treatment aims to stop further loss and stabilise what remains. General background is available from healthdirect.

What treatment matches which layer?

Each layer maps to a different kind of treatment, and the further in the problem has travelled, the more involved the treatment becomes.

• Enamel — prevention. Fissure sealants at $80 per tooth, and fluoride application, which is included within a check-up.
• Dentine — restoration. A composite filling is $250 to $450 depending on size and position. More on fillings.
• Pulp — root canal treatment at $1,600 to $2,200, or removal of the tooth. A simple extraction is $350 to $550 and a surgical extraction is $650 to $850, whether or not the tooth is a wisdom tooth.
• Ligament and bone — periodontal treatment. The initial phase of deep cleaning is $1,400 to $1,600, with maintenance at $400 every three months. More on deep cleaning.
• Assessment — a check-up with X-rays and a clean is $250.

All of those figures assume no health-fund contribution. A fund may contribute, and what it contributes depends on your policy, so check with your fund. A written quotation is always given before treatment begins, so nothing is a surprise. Full pricing is listed on the fees page.

Quick answers

Why does cold hurt but heat does not?

Short, sharp pain on cold that stops quickly generally points to exposed dentine. Pain triggered by heat, particularly if it lingers after the heat is removed, more often points to the pulp, and is assessed differently. Either way it should be examined rather than self-diagnosed.

Can enamel be rebuilt?

Early mineral loss can be reversed with fluoride and saliva. Once enamel has actually broken down, it does not grow back, because enamel contains no living cells to rebuild it.

My toothache stopped by itself. Is it fixed?

Not necessarily. Pain can stop because the problem has settled, or because the pulp has died. Those look identical from the outside and are distinguished by examination and testing, so a tooth that has had severe pain should still be checked.

Why can I feel exactly which tooth hurts sometimes and not others?

Because the periodontal ligament carries position and pressure information and the pulp does not. Pain you can pinpoint on biting usually involves the ligament. Pain you cannot place usually comes from inside the tooth.

What to do next

None of this replaces an examination, and it is not intended to let anyone diagnose their own tooth. What it should do is make the questions easier to ask. If you want to read further, how far decay travels before you feel it covers the enamel and dentine story, sensitive teeth to cold covers the dentine one, and deep cleaning covers the supporting structures.

If a tooth has been aching, waking you at night, or hurting to bite on, that is worth an appointment rather than a wait.
Call (02) 9649 6468 · Book online · WhatsApp

Written and reviewed by Humzah Aly, Oral Health Therapist. This article is general information only and is not a substitute for personalised dental advice. Please see a dental practitioner to discuss what is right for you.