Pilonidal sinus is currently considered an acquired condition with a familial predisposition, characterized by acute or chronic inflammation in the subcutaneous tissue of the coccygeal region. Its incidence is increasing worldwide. Males are more than twice as likely to be affected. The peak incidence from puberty to the fourth decade of life explains the high socioeconomic impact due to limitations in education and professional life.
The current understanding is that cut hair fragments from haircuts at the back of the head fall along the posterior sweat groove into the intergluteal cleft, stand upright, and then penetrate the skin. A deep anal cleft and the movement of the buttocks facilitate the penetration of the fallen sharp-edged hair fragments into the skin. Most hairs found in the pilonidal sinus nest are 5-10 mm long and cut on one or two sides. Patients with pilonidal sinus have hairs with significantly stiffer hair shafts. The penetration of the hair depends on the orientation of the scales. A foreign body granuloma develops, which no longer heals spontaneously. It can be asymptomatic or become infected and thus clinically noticeable. Although trauma does not cause a pilonidal sinus, a blunt trauma can edematously close a singular asymptomatic tract, leading to an initial event with retention and pain or secretion.
Heavy hair growth seems to promote pilonidal sinus disease, with the theory being that heavy intergluteal hair holds the fallen cut hairs in place longer, allowing for increased impalement due to prolonged exposure. Therefore, good personal hygiene with regular bathing and showering seems to be associated with a lower incidence of the disease, as it ensures the regular removal of sharp cut hair fragments from the anal cleft.