Summary: Anatomy

•The nail unit can be divided into several components: nail matrix, nail plate, supporting structures (nail bed and phalangeal bone), nail folds, cuticle, and hyponychium.

•Main arteries and nerves run longitudinally along both sides of the digit and have dorsal and ventral branches.

28.2Anatomy

Knowledge of nail unit anatomy is critical for management of tumors beside and underneath the nail. The nail unit and adjacent structures important for Mohs surgery will be summarized.

28.2.1 Nail Matrix

The nail matrix generates the nail plate and is the most proximal portion of the nail unit. The nail matrix connects to the ventral surface of the proximal nail fold, and a portion of the nail matrix can be seen by the clinician as the lunula. The nail matrix can be divided into proximal, intermediate, and distal components. The proximal matrix produces the superficial nail plate. The intermediate matrix produces the majority of the nail body. The distal matrix produces a portion of the nail bed epidermis. Laterally, the matrix continues proximally to form a “horn” on either side.

28.2.2 Nail Plate

The nail plate is formed by the nail matrix and is composed of a hard multilayered keratin sheet.

28.2.3Supporting Portion: Nail Bed and Phalangeal Bone

The nail bed lies beneath the nail plate and begins from the distal edge of the nail matrix and continues to the hyponychium. The nail bed’s pink color is due to its vascularity, and the space between the nail bed and the

underlying phalanx is only a few millimeters thick, without any underlying subcutaneous tissue or fat [1, 2].

28.2.4 Nail Folds

The nail folds surround the nail plate on three sides, and consist of a proximal nail fold and two lateral nail folds. The proximal nail fold has a dorsal and ventral surface. The dorsal surface is seen as the skin along the nail unit’s proximal border. The ventral surface of the proximal nail fold lies beneath the dorsal surface and becomes contiguous with the proximal nail matrix. The lateral nail folds frame the lateral edges of the nail plate.

28.2.5 Cuticle

The cuticle, also known as the eponychium, sheaths the most proximal nail plate. The cuticle seals the pocket between the dorsal proximal nail fold and the nail plate. By doing so, pathogens are prevented from entering this region.

28.2.6 Hyponychium

The hyponychium is the most distal component of the nail unit and is located under the free edge of the nail plate and seals the nail bed along the distal groove.

28.2.7 Arteries and Nerves of the Digit

The main digital nerves run longitudinally on both the radial and ulnar sides of the digit and have both dorsal and ventral branches. The main digital arteries (the volar and dorsal digital arteries) run adjacent to the nerves and provide contralateral circulation via their many anastomoses [3]. The volar digital arteries reach the pulp of the fingers and originate from the superficial palmer arch to provide the primary arterial supply of the digit. The dorsal digital arteries primarily supply the proximal region of the fingers and branch off the dorsal metacarpal artery of the dorsal carpal arch. The thumb has aberrant arterial connections resulting from a lack of a direct bilateral blood supply. Therefore, when operating in this region, caution is recommended [3].