The nail unit is a complex structure located on the dorsal surface of the fingers and toes. It has two main functions - protection and sensation. The nail unit protects the digits from trauma and assists in providing tactile sensation. In order to understand the anatomy of the nail unit, its component parts and clinical correlations must first be examined.
The nail unit consists of the nail plate and the surrounding soft tissues. The nail plate is the outer portion of the nail unit which is formed by multiple layers of keratin. It is considered to be both hard and flexible, as well as being transluscent. The nail folds protect the proximal and lateral margins of the nail plate, and the nail bed (or sterile matrix) lies directly underneath the nail plate. This nail bed attaches to the distal phalanx, providing a smooth surface for the nail plate to slide over (it does not contribute to the growth of the plate itself). The germinal matrix is an area of soft tissue that is located proximal to the sterile matrix. The cells found within the germinal matrix divide and become keratinized, forming the nail plate. This continuous cell division pushes the nail plate over the bed as it matures.
The hyponychium is located distal to the nail bed and is situated underneath the free edge of the nail plate. An eponychium (or cuticle) is a layer of stratum corneum which extends between the skin of the finger and the proximal nail plate. The lunula is a white, 'half- moon' shaped appearance of the germinal matrix that is visible through the proximal nail plate.
A nail bed injury is a term that is used to describe damage to the soft tissue that lies underneath the nail plate, which is made up of both the nail bed and the germinal matrix. There are two main mechanisms of injury which can cause a nail bed injury. Crush injuries occur when a finger is caught in a door or when it is struck directly by a hammer. Lacerations occur when a circular saw is involved in an injury.
Whenever a nail bed injury is suspected, it is essential that an x-ray is performed on the affected finger in order to assess for any bony injury. In cases where the nail bed has been lacerated, surgical repair is often recommended to improve the cosmetic appearance of the new nail that is growing through. During this procedure, the nail is removed and the laceration is repaired using absorbable sutures. If the injury is severe enough, it can take up to six months for the new nail to fully grow through and the finger may be sensitive to cold temperatures during this time.
