Craze Lines Are Not Cracks

Somebody photographs their tooth, notices fine vertical lines, and assumes the worst. A dentist looking at the same photograph:

Craze lines for sure (enamel only cracks), but all the discoloration is likely to be decay forming around an old silver filling. r/askdentists

Enamel only cracks. Two different things in one image, and the alarming-looking one was the harmless one.

Craze lines are normal

Fine, shallow, vertical lines confined to the enamel. They don’t reach the dentin, they don’t involve the nerve, and they don’t do anything.

Nearly every adult has them. Enamel is a brittle ceramic that spends decades absorbing chewing force and temperature swings between hot coffee and cold water, and microscopic surface cracking is what a brittle material does under that. They become more visible with age and more visible with staining, because the line picks up pigment and darkens.

Treatment: none. They are noticed rather than developed.

A real crack behaves differently

Three signs separate them, and the first is the useful one.

Pain on release, not on pressure. This is the classic sign of a cracked tooth. Biting down pushes the two halves apart slightly; letting go lets them snap back, and the movement irritates the nerve. So the sharp jolt comes as the pressure comes off.

New sensitivity to cold in a tooth that never had it.

A line that catches a fingernail, or one you can see change.

And crucially: a crack you can feel is not the same as one you can see. Cracks in back teeth run down through the chewing surface where nothing is visible, which is why a tooth can be badly cracked and look perfectly normal.

It doesn’t automatically mean a crown

The best news in this whole subject comes from somebody living it:

I had the same type of crack by on the other side of tooth so basically the middle of my tooth! Only it was cracked half off they just bonded it and that was 25 years ago! Never had an issue so you could start there r/askdentists

Twenty-five years on a bond. That’s not typical of every case, and it demonstrates that the answer isn’t always the most expensive one.

The rough logic of what gets recommended:

Bonding works for a shallow crack, especially in a front tooth, where chewing forces run along the tooth rather than trying to split it.

A crown is the answer when the crack runs into a molar or premolar under heavy vertical load. A crown works by wrapping the tooth — it holds the pieces together rather than filling the gap, which is why a filling on a cracked back tooth often fails.

A root canal and a crown if the crack has reached the pulp.

Extraction if it has gone below the gum line into the root, because there is nothing left to wrap.

Which is why the timing matters:

Immediately. This tooth probably already can’t be saved from what I’m seeing… but on the chance that a root canal and crown is still possible, the more you wait the more an extr r/askdentists

Cracks travel. A crack in the crown of a tooth is fixable. The same crack six months later, having propagated down into the root, is an extraction. Nothing else in dentistry has quite that character — a cavity gets bigger, but a crack changes category.

What actually causes them

Overwhelmingly one thing, and it happens overnight:

Cracked a tooth as in physically damaging them enough that they get a crack in them. Already done it once and had to have a crown put in. r/bruxism

Have had 2 crowns in 2 years on the same cracked tooth r/bruxism

Both from a forum about grinding, which is where these threads live.

Chewing applies force in bursts of a second or two, a few hundred times a day. Clenching applies it for hours, continuously, with no food in between to cushion it. Enamel is very hard and not very tough — it handles compression well and splitting badly — so sustained lateral load is close to the worst thing you can do to it.

Which is why a night guard is a cheaper object than the crowns it prevents. It doesn’t stop the clenching; it absorbs the force so the tooth doesn’t.

And this is what it costs when nothing changes:

I wake myself up every morning by extreme tooth grinding. A crown came loose and I cracked two other teeth, and im worried any expensive dental work will be ruined. r/bruxism

Two other causes worth knowing

Large old fillings. A tooth with a wide amalgam filling has been hollowed out — the remaining walls are thin, and thin walls flex. A great many cracks start at the edge of a big old restoration.

Temperature. Enamel expands and contracts, and hot then immediately cold is a stress cycle. Chewing ice is the classic offender, because it combines cold with a hard object.

And you probably won’t notice it first

I’m booked in for a crown next month, I didn’t actually notice it, my dentist caught it during a check-up, I wouldn’t have know haha r/bruxism

That’s the common way it goes, and it is an argument for check-ups that has nothing to do with decay. A crack found at a routine appointment is a crown. The same crack found because it started hurting has usually reached the pulp.

The short version

Fine vertical lines in the enamel are craze lines. Everybody has them, they do nothing.

A real crack hurts when you release a bite rather than when you press, and it can be invisible.

And cracks travel, which is the one thing that makes waiting expensive. A crack in the crown is fixable. The same crack in the root is not.