The nail unit

 The nail unit is a complex structure located on the dorsal surface of the fingers and toes. It has two main functions:

Protection – protects the digits from trauma

Sensation – assists with tactile sensation

In this article, we shall look at the anatomy of the nail unit – its component parts and clinical correlations.

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Fig 1 – Anterior view of the nail unit.

Components of the Nail Unit

The nail unit consists of the nail plate and the surrounding soft tissues:

Nail plate – outer portion of the nail unit, formed by layers of keratin. It forms a hard,

yet flexible, translucent plate.

Nail folds – skin that surrounds and protects the proximal and lateral margins of the nail plate

Nail bed (sterile matrix) – lies underneath the nail plate, attaching it to the distal phalanx. The nail bed provides a smooth surface for the growing nail plate to slide over (it does not contribute to plate growth itself).

Germinal matrix – area of soft tissue proximal to the sterile matrix. Cells within the germinal matrix divide and become keratinised to form the nail plate. Continuous cell division within the matrix ‘pushes’ the nail plate over the bed as it matures.

Hyponychium – the area distal to the nail bed, situated underneath the free edge of the nail plate.

Eponychium (cuticle) – layer of stratum corneum which extends between the skin of the finger and proximal nail plate.

Lunula – white ‘half-moon’ appearance of the germinal matrix through the proximal nail plate.

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Fig 2 – Lateral view of the nail unit.

Clinical Relevance: Nail Bed Injury

A nail bed injury refers to damage to the soft tissue underneath the nail plate – the nail bed and germinal matrix.

There are two main mechanisms of injury:

Crush – e.g. finger caught in door or direct blow from a hammer

Laceration – e.g. circular saw injury

An x-ray of the affected finger is required to assess for any bony injury (these injuries are often associated with a fracture of the distal phalanx).

In cases where the nail bed is lacerated, surgical repair can be carried out to improve the cosmetic appearance of the new nail growing through. The nail is removed, and the laceration repaired with absorbable sutures.

Following a nail bed repair, it takes approximately 6 months for the new nail to fully grow through and the finger can be sensitive to cold during this time.