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There 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, bicycling, long periods of sitting 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 parts of the body other than near the tailbone. 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.

Dec. 05, 2012