In retronychia, the proximal nail plate becomes embedded into the proximal nail fold with loss of the normal alignment of the nail matrix and nail plate. Growth of new nail pushes the old nail upwards, leading to stacking of multiple generations of nail plates.
Retronychia most often affects the great toenails. It is caused by repetitive microtrauma to the nail, most often by wearing tight shoes or doing activities that lead to toenail trauma.
In the acute stage, retronychia is painful and manifests with inflammation of the proximal nail fold, yellow or white discoloration of the nail plate (xanthonychia), and granulation tissue formation. Clinical features that help differentiate retronychia from paronychia include the presence of xanthonychia and swelling of the proximal nail folds rather than the lateral nail folds because the lateral nail folds are affected much less often in retronychia than in paronychia.
In the chronic stage, the inflammation resolves, but the nail plate becomes thickened and can separate from the nail bed (onycholysis). Layers of nail plates can become stacked, and the cuticle is missing.
The diagnosis of retronychia is clinical. Periodic acid-Schiff (PAS) stain or culture of the nail plate is often negative. Although the nail plate itself may have a positive PAS result if the nail plate is analyzed after removal, the onychomycosis is often not the primary problem but rather secondary to the trauma to the nail that caused compromise and inflammation. Diagnosis includes noticing the layered nail plate, absent cuticle (in chronic), and absent subungual debris.
(See also Acute Paronychia Acute Paronychia Paronychia is infection of the periungual tissues. Acute paronychia causes redness, warmth, and pain along the nail margin. Diagnosis is by inspection. Treatment is with antistaphylococcal antibiotics... read more 
and Chronic Paronychia Chronic Paronychia Chronic paronychia is recurrent or persistent nail fold inflammation, typically of the fingers. (See also Overview of Nail Disorders.) Chronic paronychia is an inflammatory disorder of the nail... read more 
.)
Treatment
Corticosteroids and nail removal
In the acute stage, treatment options include topical and intralesional corticosteroids. In the chronic stage, treatment is nail removal (1 Treatment reference Retronychia is a less common form of paronychia, which is infection of the periungual tissues. Acute symptoms include pain, inflammation of the proximal nail fold, xanthonychia, onycholysis... read more 
).
Avoidance of microtrauma, for example, by wearing shoes with more space around the toes to relieve pressure on the affected toenail, is important.
Treatment reference
1. Costa TPE, João AL, Lencastre A: Retronychia: A paradigm shift? Skin Appendage Disord 6(5):268–271, 2020. doi: 10.1159/000509370
