Retronychia refers to the ingrowth of the proximal nail plate into the proximal nail fold accompanied by persistent paronychia. Retronychia can happen at any age and to a person of either sex, but it is most common in adult women. The halluces are most commonly affected due to their propensity for repeated trauma. Retronychia may occur bilaterally. Retronychia is classified into 2 stages: In stage 1, there is xanthonychia (yellowing of the nail plate) with mild paronychia and interrupted nail growth. In stage 2, there is severe paronychia and proximal nail plate elevation.

A history of preceding trauma, such as local trauma, ill-fitting footwear, and physical and sporting activities, is common. Retronychia may be associated with systemic conditions, including arthritis, pregnancy and postpartum period, compartment syndrome, and thrombophlebitis. Anatomical risk factors including lateral deviation and reflex hypertension of halluces have been reported as well. When the fingernail is affected, the most common preceding events include cast immobilization and brachial plexus injury, and multiple nails are often involved. It is postulated that these risk factors cause local tissue hypoxia leading to the proximal nail plate separating from the matrix. Then, when the new nail plate grows out, the old one becomes raised above it and embeds into the proximal nail fold. This could lead to multiple nail plates building up on top of one another underneath the proximal nail fold.