Fine lines on teeth: are enamel cracks harmful?

What those lines in the light are

Especially on the upper front teeth, hair-thin lines running up and down the tooth can show when light strikes at a certain angle. These are craze lines: surface cracks confined to the enamel, the outermost layer of the tooth. The American Association of Endodontists (AAE) describes them as extremely common in adult teeth, very shallow, painless and of no concern beyond appearance. A craze line does not remove any part of the tooth; its shape and function stay as they were.

Why they form

Enamel is very hard but does not flex. Over the years, chewing forces, clenching and grinding, a bite that does not meet evenly, nail biting or biting on hard objects leave small stresses in it. Most craze lines are a trace of this ordinary wear, which is why they are seen in adults and become more common with age. Acidic food and drink and tobacco are also listed among the things that affect the enamel surface.

Why an enamel crack rarely goes deeper

Enamel is made of millions of fine rods called prisms. In the outermost part of the enamel these rods run fairly straight; further in, they bend in different directions in bundles, and neighbouring bundles cross one another. When a section of tooth is examined under light, this arrangement appears as alternating light and dark bands, known as Hunter–Schreger bands. When a crack that started at the surface reaches this zone, it cannot run straight: it has to change direction and branch, and it slows down. Much of enamel's ability to keep most cracks within itself is attributed to this structure. Most of this knowledge comes from laboratory studies, and its significance inside the mouth is still being researched.

When to suspect a real crack

A craze line does not hurt; if there is pain along with the line, the situation is different. The AAE notes that pain from a cracked tooth is erratic, can come on when chewing or at the moment you release your bite, and can appear when the tooth meets very hot or very cold. These signs call for an examination:

• Brief, sharp pain on biting, especially when releasing the bite
• A sharp twinge that starts suddenly with cold or heat
• A darkening line that lengthens or extends towards the biting edge of the tooth
• Swelling of the gum around the tooth with the line

Darkening alone is not an alarm signal, because craze lines can also pick up stain over time. A painless line that has been on a front tooth for years and a line noticed on a back tooth together with pain on chewing may not be the same thing. At the examination the dentist shines light through the tooth from different directions: a crack reaching the dentine blocks the passage of light, whereas a surface craze line usually does not. A bite test can also show which point the pain comes from. The symptoms and treatment of cracked tooth syndrome are covered on a separate page.

Lines that pick up stain

A craze line is usually transparent and only visible when light strikes at the right angle. Over time, colourings from coffee, tea and tobacco settle into these fine gaps; the line starts to look yellowish or brownish and becomes more noticeable. A professional clean and polish can reduce surface staining, but stain that has settled inside the line does not always come out completely. Everyday ways to limit coffee, tea and wine stains are the subject of a separate article.

How whitening and bonding change the look

Because craze lines are harmless, doing nothing is also a valid choice; treatment is purely for appearance. Whitening lightens the overall shade of the teeth and can soften the stain in a line, but if the lines are deep or widespread it may not make a noticeable difference. Whitening does not close a line; it can only reduce the colour contrast. For a few lines that catch the eye, bonding can be considered: tooth-coloured composite filling material is bonded to the surface as a thin layer and the line sits beneath it. If there is a widespread problem with colour and shape across several teeth, veneers, thin shells bonded to the front of the teeth, may be discussed; they usually require removing some enamel, which cannot be undone.

To reduce new lines

• If you clench or grind your teeth, a night guard can be discussed at your examination.
• Do not bite on hard objects such as ice, nutshells or pens, and do not use your teeth to open packets.
• Nail biting puts repeated stress on the front teeth.
• At regular check-ups, have the lines checked for any change.

This content is for general information only and does not replace an examination.