Pterygium is a nail abnormality that is either dorsal or ventral depending on the site of involvement.

Dorsal pterygium occurs when the proximal nail fold fuses with the underlying matrix and, subsequently, with the nail bed. As a result, the nail plate is divided into 2 lateral segments that progressively decrease in size as the pterygium widens. Complete involvement of the matrix and the nail bed produces a total loss of the plate and a permanent atrophy of the nail apparatus. Dorsal pterygium is most commonly acquired. It arises from conditions involving the proximal nail fold, such as lichen planus, burns, trauma, radiodermatitis, cicatricial pemphigoid, graft-versus-host disease (GVHD), toxic epidermal necrolysis (TEN), pemphigus foliaceus, and Raynaud phenomenon.

Ventral pterygium (pterygium inversum unguis) consists of a distal expansion of the hyponychium, which anchors to the undersurface of the nail plate, ultimately resulting in the obliteration of the distal nail groove. Ventral pterygium is more common in females. It usually arises from conditions involving the hyponychium, such as systemic connective tissue diseases (eg, systemic sclerosis, systemic lupus erythematosus, Raynaud phenomenon), scarring in the vicinity of the distal nail groove, use of formaldehyde-containing hardener, subungual exostosis, stroke, and trauma. Ventral pterygium may also be idiopathic, congenital, or sometimes familial.

Both ventral and dorsal pterygium predominantly involve the fingernails. Aside from the nail dystrophy, patients may complain of pain, particularly in the case of trimming the nails of a ventral pterygium. There may also be functional consequences, such as difficulty in buttoning a shirt or picking up a coin.