Ingrown toenail (onychocryptosis) is a fairly frequent problem resulting from either growth of nail fold inwards into the nail bed or abnormal embedding of the nail plate into the nail groove, causing significant discomfort. Although often thought to be synonymous, the terms ingrown nail and paronychia refer to different conditions. Both can cause significant discomfort. Ingrown toenails may cause pain with ambulation. See the image below.
Retronychia is a rare variant of ingrown nail in which the nail plate is embedded into the proximal nail fold with subsequent inflammation of the proximal nail fold. 
Ingrown nails result from an alteration in the proper fit of the nail plate in the usual nail groove. Sharp spicules of the lateral nail margin develop and are gradually driven into the dermis of the nail groove. The nail acts as a foreign body. An inflammatory response occurs in the area of penetration, leading to erythema, edema, purulence, and development of granulation tissue.
The normal distance between the nail groove and the border of the nail is 1 mm. A thin epithelial layer covers the nail groove and protects it from irritation. With an increase in pressure on the nail bed and nail groove, an epidermal breakage occurs, with subsequent inflammation, pain, and infection. 
Ingrown nails generally occur as the result of poorly fitted footgear. However, this may be caused by prior trauma to or abnormal shape of the nail margin. 
Types of ingrown nail include the following  :
Neonatal: Occurs as a result of delayed overgrowth of the free nail margin on the tip of the toe and can be managed conservatively (see the first image below)
Infantile: Congenital form results from malalignment of the big toenail or as a result of hypertrophy of lateral nail fold
Adolescent : Most common cause is a narrow nail bed, resulting in ingrowing distal lateral nail
Adult: Most common cause is pressure that causes sharply bent lateral margin of the nail plate
Distal embedding from a big toe nail that is too short
Pincer nail (see second image below)Neonatal ingrown nail. Courtesy of Dermatology Research and Practice (Haneke, E. "Controversies in the Treatment of Ingrown Nails." Dermatology Research and Practice. 2012; 2012:783924).Pincer nail. Courtesy of Dermatology Research and Practice (Haneke, E. "Controversies in the Treatment of Ingrown Nails." Dermatology Research and Practice. 2012; 2012:783924).
Of all nail problems, this is the most common. Toenails are affected much more commonly than fingernails. The lateral margins of the great toe are most frequently affected.
In the United Kingdom, 10,000 cases per year have been reported.
No racial predilection is noted.
Ingrown nail has a reported male-to-female ratio of 3:1. In reported cases of retronychia, a female predominance has been noted. 
The condition is observed in people of all ages but is most common in the second decade of life. Ingrown nails become much more common as children begin bearing weight on their feet and wearing shoes, although congenital onychocryptosis has been described, [5, 6] as have cases in infants. 
Prognosis is excellent. Complete healing is expected. In general, mortality is not associated with ingrown nails. Morbidity is chiefly the result of infection of the tissues. If neglected, abscess formation (paronychia) may occur or spread and lead to osteomyelitis, systemic infection, sepsis, or amputation.
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