Pilonidal sinus. Pilonidal sinus above a man's buttocks. A pilonidal sinus is a pit which forms when hairs in the area grow inwards into the skin. If the pit becomes infected, extensive painful abscesses may result. Surgical removal is the only way of treating the condition. All the tissue surrounding the affected area has to be removed and the wound is normally left open to heal slowly from the bottom upwards. During this time, it is important to keep the healing area clean, but the condition often recurs even after surgery.