Fissure is a medical term having many meanings, but in dentistry it is used to refer to grooves in the teeth.

Fissure is a term used to designate the indentations in the chewing surfaces of the side teeth. This distinguishes them from the humps. Teeth usually have one main fissure, which run lengthwise along the tooth and is thus termed longitudinal fissure. Correspondingly, the many smaller crosswise fissures are called transverse fissures. Everyone's tooth structure is more or less the same in this respect.

There are minor differences, however, for example fissures can be very narrow. The animation shows a cross-section of a „normal“ fissure, which is easy to clean using a toothbrush. And now a cross-section of an abnormal fissure – a minor difference with grave consequences – and the reason why some people are more likely to develop fissure cavities than others, while some people do not get them at all- more on this in the same named video.

Bits of food and bacteria which cause cavities (kariogenic bacteria) can invade these fissures, and because they are so narrow this makes it difficult to remove the food and bacteria using a toothbrush. Moreover, the enamel around these fissures is usually thinner – with the result that cavities form, bacteria attack the thin enamel and invade the crown – leading to a fissure cavity. Fissure cavities are very difficult to detect, as they are not clearly visible (the surface of the enamel may even be completely intact) and a diagnosis is made even more difficult by the narrowness of the fissures. X-rays may not always help in detecting cavities either, since the thick layer of enamel in the crown of the tooth absorbs the rays and makes it difficult to detect smaller cavity lesions.

If a fissure is at risk of developing cavities, after the tooth comes through a dentist should use a thin resin or filling compound to seal it and prevent invasion by oral microflora (a procedure called fissure sealing). If the fissure has already been invaded, it must first be cleaned and then perhaps expanded by drilling (known as extended or invasive fissure sealing).