Pilonidal disease was first reported in 1833. This process was first described by Anderson in 1847 and later named pilonidal sinus by Hodges in 1880.  The word pilonidal derives from the Latin words pilus ("hair") and nidus (“nest”).
Sacrococcygeal pilonidal sinus is a common disorder among young adults. Observed most commonly in people aged 15-30 years, with a 3:1 male-to-female ratio, it occurs after puberty, when sex hormones are known to affect the pilosebaceous gland and change healthy body hair growth. The onset of pilonidal disease is rare in people older than 40 years. 
In the 1950s, pilonidal sinus disease was thought to be of congenital origin rather than an acquired disorder. The pilonidal sinus and abscess were thought to be secondary to a congenital remnant of an epithelial-lined tract from postcoccygeal epidermal cell rests or vestigial scent cells. Sinuses to the neural canal can occasionally extend to the dura, but these are rare and are located in the lumbar region rather than in the sacral region.
Pilonidal disease is now widely considered to be an acquired disorder, on the basis of observations that congenital tracts do not contain hair and are lined with cuboidal epithelium. The pilonidal cyst is also not a true cyst, in that it has an epithelialized walls and is more of a cavity, which makes a congenital origin less likely. The recurrence of the disorder after complete excision of the disease tissue down to the sacrococcygeal fascia and the high incidence of chronic pilonidal sinus disease in patients who are hirsute further support an acquired theory of pathogenesis.
In a census and survey of patients admitted to British hospitals in 1985 for treatment of pilonidal sinus disease, it was found that 7000 patients required hospitalization for an average of 5 days. The hospitalization of these patients for the treatment of pilonidal disease resulted in a loss of productivity and earnings, as well as a disruption of education, because patients recovered in the hospital. In the 1980s, Bascom found that at least 40,000 US soldiers were admitted to the hospital for an average of more than 5 days of inpatient care for pilonidal disease and associated complications.
Treatment options are now available that provide a rapid rate of cure and a lower recurrence rate and minimize the number of hospital admissions. Although numerous randomized clinical studies have evaluated different treatments, no clear consensus has been reached as to the optimal medical or surgical treatment of pilonidal disease.
A pilonidal sinus can occur in many different areas of the body, but most are found in the sacrococcygeal area, in the natal cleft, approximately 5 cm from the anus. The characteristic pilonidal sinus is a midline opening in the sacrococcygeal area in the natal cleft. Not uncommonly, the patient may have a series of openings in the midline or may have secondary lateral openings superior to the midline pit.
The sinus tract itself is smooth and lined with squamous epithelium. Eventually, the sinus tract leads to a subcutaneous cavity lined with granulation tissue and filled with nests of hair. The sinus tract openings are actually an extension of the deep cavity. This is why an abscess formation may present either in the midline or lateral to the midline.
After the onset of puberty, sex hormones affect the pilosebaceous glands, and subsequently, the hair follicle becomes distended with keratin. In pilonidal disease, a folliculitis develops, which produces edema and follicle occlusion. The infected follicle extends and ruptures into the subcutaneous tissue, forming a pilonidal abscess. This results in a sinus tract that leads to a deep, subcutaneous cavity.
In 90% of cases, the direction of the sinus tract is cephalad, which coincides with the directional growth of the hair follicle. This usually places the tracking follicle approximately 5-8 cm from the anus. In the rarer instance that the sinus is located caudally, it is usually found 4-5 cm from the anus. The caudal location of the sinus can be easily mistaken for a fistula-in-ano and should be considered before treatment. 
The laterally communicating sinus overlying the sacrum is created as the pilonidal abscess spontaneously drains to the skin surface. The original sinus tract from the natal (intergluteal) cleft becomes an epithelialized tube. The laterally draining tract becomes a granulating sinus tract opening.
Loose hairs are drilled, propelled, and sucked into the pilonidal sinus by friction and the movement of the buttocks whenever a patient stands or sits. This bending and sitting action tightens the skin of the gluteal cleft and lifts it from the fascia. The negative pressure drives hair deeper into the sinus and appears to be exaggerated with a deep gluteal cleft. 
Hair enters tip first, and the barbs on the hair prevent it from being expelled, causing the hair to become entrapped. Physical examination occasionally may reveal a tuft of hair emerging from the midline opening in the natal cleft. This trapped hair stimulates a foreign body reaction and infection. Rarely, foreign bodies other than human hair can cause this disease process. Rare case reports exist in which the hair did not come from the patient but instead came from a bird's feather, the type used to stuff feather bedding.
The etiology of pilonidal disease has been surrounded by controversy. In the 1950s, as noted (see above), the predominant thinking was that this was a congenital condition; however, current theories focus primarily on predisposing factors and acquired conditions. 
The incidence of pilonidal disease is also affected by hair characteristics, such as kinking, medullation, coarseness, and growth rate. White persons are affected more frequently than are African or Asian persons. Other factors affecting the disease's incidence are increased sweating activity associated with sitting  and buttock friction, poor personal hygiene, obesity, and local trauma, which help to explain why pilonidal sinus disease is common in army recruits.
Predisposing factors to pilonidal disease are believed to include the following  :
Sedentary lifestyle or occupation
Deep natal cleft
Family history of pilonidal disease
In an article examining the condition in Turkish soldiers, the incidence was found to be 8.8%, with the correlation factors known to be family history, obesity, being the driver of a vehicle, and the presence of folliculitis or a furuncle at another site on the body. 
The incidence rate of pilonidal disease is approximately 0.7%. Males are affected 2.2-4 times more frequently than females. During a population study involving college students, the incidence rate was found to be 1.1% (365 of 31,497 people) in males and 0.11% (24 of 21,367 people) in females. The onset of the disease is earlier in females, which may be due to the fact that puberty occurs earlier in females.
Irrespective of the method of treatment applied, very few patients are troubled with symptoms of persistent pilonidal disease beyond the age of 40 years. This is important to note because ultimately, cure is an almost inevitable result that comes with age, regardless of the choice of surgical management.
Few prospective, randomized, controlled trials have compared one treatment of pilonidal disease with another. However, one such study compared primary closure with excision in 110 patients and found that the former was associated with longer hospitalization, though it was also associated with earlier return to work.  In addition, the primary closure group had an increased risk of infection and a higher recurrence rate. Nevertheless, the authors suggested that both procedures have a place in treatment and that the choice depends on surgeon preference.
It appears that no definitive comparisons of outcomes are available as yet; however, some suggest that open drainage procedures have a lower risk of infection.
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