The hyponychium is the skin under the free edge of the nail that seals the gap between the nail plate and the fingertip, protected by a keratin plug with immune cells that keeps out dirt and microbes.
Also known as: quick, skin under the nail, distal nail seal, hyponychial skin
The hyponychium is the strip of skin under the free edge of the nail, where the plate lifts away from the fingertip. It closes the space between nail and finger. A plug of keratin containing immune cells protects the seal, so dirt and microbes that gather under the nail cannot get in (Medscape).
The hyponychium is sensitive. Pushing a tool under the nail to clean it, or cutting nails too short, can break the seal and hurt. In some people the hyponychium grows forward along the underside of a long nail. This is common in people who wear long nails or extensions for a long time and makes shortening them uncomfortable. Filing back gradually over several weeks is gentler than cutting in one go.
Two problems start here. Onycholysis is the nail plate lifting from the bed at the free edge, often after trauma, infection, thyroid disease or psoriasis, and is a reason to see a GP or dermatologist (Mayo Clinic). Fungal infection also enters at the free edge, where the seal has failed.
In a manicure the hyponychium is left alone. Polish is capped over the free edge but not brushed underneath. Cuticle oil applied under the tip as well as at the base keeps the area supple.