A pilonidal sinus is an infected tract under the skin between the buttocks. Treatment commonly involves an operation. After the operation, you should keep the area free of growing hair by regular shaving or other means of hair removal.
Pilonidal means a 'nest of hairs'.
A sinus tract is a narrow tunnel (a small abnormal channel) in your body. A sinus tract typically goes between a focus of infection in deeper tissues to your skin surface. This means that the tract may discharge pus from time to time on to your skin.
The exact cause is not clear. There are various theories. For example, one theory is that the
develop from a minor abnormality you were born with, in the skin between the buttocks. This may
the condition tends to run in some families. Part of the abnormality in this part of your skin
may be that
the hairs grow into your skin rather than outwards.
Another theory is that you develop skin dimples (skin pits) in the skin between your buttocks. These may develop as a result of local pressure or friction causing damage to the small structures below your skin which are responsible for making hairs (the hair follicles). Because of local pressure, growing hair in your natal cleft may become pushed into your skin pits.
Whatever the cause, once hair fragments become 'stuck' in your skin they irritate it and cause inflammation. Inflamed skin quickly becomes infected and so a repeated (recurrent) or persistent infection tends to develop in the affected area. The infection causes the sinus to develop which often contains broken pieces of hair. (A similar condition can occur between the fingers of hairdressers, caused by customers' hairs entering moist, damaged skin.)