Pilonidal sinus
A pilonidal sinus is a small hole or "tunnel" in the skin. It usually develops at the top of the cleft of the buttocks, where the buttocks separate.
A pilonidal sinus is a small hole or channel in the skin that most commonly occurs at the top of the cleft between your buttocks.
The exact cause of pilonidal sinuses is unclear. It is generally thought they are caused by loose hairs that push into the skin.
Surgery is used if a pilonidal sinus becomes infected. If there is no infection, treatment may not be necessary.