hyponychium

The hyponychium is the distal part of the nail unit, comprising the thickened skin beneath the free edge of the nail plate. It lies at the tip of the finger, between the underside of the distal nail plate and the skin of the fingertip, and is bounded proximally by the hyponychial fold. The term comes from Greek roots meaning “below the nail.”

Anatomy and structure

The hyponychium is formed by the epidermis of the distal nail bed and fingertip skin, consisting largely of keratinized epidermis. It extends slightly beyond the free edge of the nail plate and helps create a seal between the nail plate and the fingertip.

Function

Its primary role is protective: the hyponychium forms a barrier that helps prevent entry of dirt and microorganisms under the distal edge of the nail plate and provides cushioning to the fingertip during grasping and manipulation.

Clinical relevance

Injury or inflammation of the hyponychium, termed hyponychial irritation or dermatitis, can occur with irritants or allergic contact reactions and may cause pain, swelling, or nail changes. Infections such as paronychia typically involve the surrounding nail folds rather than the hyponychium itself, but trauma to this region can contribute to subungual or periungual issues. The hyponychium should not be confused with the eponychium (the proximal nail fold or cuticle).

Overall, the hyponychium is a specialized distal nail unit component that protects the fingertip and helps maintain the integrity of the nail bed.