The exact cause of pilonidal sinuses is unclear. It is generally thought they are caused by loose hairs pushing into the skin.
They could also be caused by deep layers of skin being stretched and moved, leading to a hair follicle rupturing.
Hair follicles are the small holes that an individual hair grows out of. Pressure and friction on a hair follicle could somehow damage it, causing a pilonidal sinus.
If a hair follicle becomes blocked, it can become enlarged and then burst. A broken hair may push into the skin, leading to an infection.
This may explain why pilonidal sinuses are common around the buttocks, as sitting and driving will cause pressure and friction in this area.
There have been reports of hairdressers developing pilonidal sinuses on their hands. This may be caused by hair becoming trapped in moist, damaged skin between a hairdresser's fingers.
Infection of the pilonidal sinus
If a broken hair pushes into the skin, the skin becomes irritated, red and swollen. Bacteria can quickly infect this skin. The cleft between the buttocks is an ideal place for bacteria to spread, as it is often moist and warm.
When the skin is infected, the immune system (the body's natural defence against infection and illness) tries to fight the bacteria, which results in a collection of pus known as an abscess.
Some things are known to increase your risk of pilonidal sinuses, including:
- obesity – having a body mass index (BMI) of 30 or above
- age – pilonidal sinuses can occur at any age, but are more common in young adults between the ages of 15 and 40
- having an above-average amount of body hair, which may be why more men are affected than women
- having coarse and curly body hair
- a previous injury to the affected area of skin – for example, from a fall
- having a deep cleft between your buttocks
- having a family history of the condition – more than one-third of people have a family member with the condition
- having a job involving a lot of driving or sitting down for long periods
During the Second World War, thousands of army jeep drivers developed pilonidal sinuses. The condition became so widespread it was nicknamed "jeep seat", or "jeep disease".
As so many men doing the same sort of job developed the condition, this suggests environmental factors must play a role in its development. These factors could include:
- wearing restrictive clothing – such as army uniforms
- repetitive motion – such as bouncing around in the seat of a jeep
- poor personal hygiene