Pilonidal sinus commonly occurs as a swelling in the lower back between buttock folds. Inside the swelling you will find tuft of hair embedded. Current understanding is that fallen body hair from the back act as minute arrows and driven into the skin of the involved area. This swelling can frequently get infected and need surgical drainage of collection of pus. It is commonly seen in hairy males but infrequently you will find this condition is females.
Pilonidal sinus can come back even after the surgical excision and there are number of operations proposed to treat this condition. Karydakisprocedure is one of the operation which has better success in treating pilonidal sinus where groove in the natal cleft is flattened so that hair will not be driven into the overlying skin.