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By Mayo Clinic staffThere is some disagreement about what causes pilonidal cysts. Most pilonidal cysts appear to be caused by loose hairs that penetrate the skin. Friction and pressure — skin rubbing against skin, tight clothing, long periods of sitting, bicycling or similar factors — force the hair down into skin. Responding to the hair as a foreign substance, the body creates a cyst around the hair.
This explanation accounts for rare cases of pilonidal cysts that occur in other parts of the body. For example, barbers, dog groomers and sheep shearers have developed pilonidal cysts in the skin between fingers.
Another possible explanation is that normal stretching or motion of deep layers of skin causes the enlargement and rupture of a hair follicle, the structure from which a hair grows. A cyst then forms around the ruptured follicle.
Some children are born with a small indentation in the skin (sacral dimple) near the top of the cleft of the buttocks. Rarely, a sacral dimple may become infected, essentially forming a pilonidal abscess.