37 The Nail and Finger Pulp

Nada N. Berry, Reuben A. Bueno, and Elvin Zook

37 The Nail and Finger Pulp

37.1 Introduction

The nail with its perionychium is a unique structure found only on the dorsal distal fingertips of primates. Other animals have adapted these appendages to serve different purposes (claws and hooves). Its flat appearance serves to assist in sensory perception through counter-pressure and accurate manipulation of small objects. It protects the sensitive fingertip and aids in thermoregulation. Loss of a nail leads to aesthetic deformity as well as loss of function.

37.2 Embryology

The Wnt signaling pathway is responsible for regulating dorsal/palmar digital orientation. Specifically, inactivation of the dorsally located Wnt7a pathway leads to ventralization of the dorsal surface, producing digits without nails and finger pads on both sides. Correspondingly in the ventral surface of the limb, gene engrailed-1 (En-1) blocks the Wnt pathway resulting in normal development of pulp structures. ¹

At 10 weeks of gestation, thickened epidermis called sole plate starts to expand and burrow proximally over the underlying dermis, creating the nail grove and laying down the nail-producing matrix. Within the superficial matrix, the cells differentiate into hard keratin, creating the nail plate. At 14 weeks, the plate starts to grow distally along the matrix. As the nail grows, more keratin is deposited, adding to the thickness of the nail while keeping the firm attachment to the underlying tissue. The advancement of the firm plate through the soft tissue creates the nail folds. The nail reaches the tip of the fingers by 32 weeks and in case of the toes by 36 weeks of gestation. ²

37.3 Anatomy

37.3.1 Surface Anatomy

Eponychium is the most proximal portion of the dorsal fingertip (▶Fig. 37.1). It allows attachment of tissues to the nail itself. Cuticle or nail veil provides a water tight seal to the underlying space. In cases where cuticle is disrupted and there is frequent water exposure, such as in dishwashers, chronic bacterial and fungal infections are common.

**Fig. 37.1** Surface anatomy of the fingertip.

Hyponychium is the junction of the nail to the most distal fleshy fingertip. It contains a keratinous plug that also seals off the underlying space. There are numerous leukocytes within the tissue of the hyponychium averting subungual infections.

Paronychium is the soft tissue that surrounds the lateral edges of the nail. The cells are arranged in an overlapping manor, i.e., as shingles on the roof. Even minor trauma in this area can introduce bacteria underneath the nail, leading to infection or paronychia. These are commonly treated with removal of a portion of or the entire nail under local anesthesia. ³ ^– ⁵

37.2.2 Longitudinal and Cross-sections of the Fingertip

The hard cornified structure overlying the dorsum of the fingertip is termed the nail plate (▶Fig. 37.2a). Underlying tissue that contributes to the thickness of the nail, called the nail, overlies the periosteum of the distal phalanx and originates distal to the terminal extensor tendon insertion. The two sections of the nail bed are germinal and sterile matrices. The nail fold contains dorsal roof and the ventral floor of the germinal matrix. The dorsal roof contributes to the smoothness and the shine of the nail (▶Fig. 37.2b). In instances of revision amputations, this portion of the matrix should be removed as well to avoid nail cysts.

**Fig. 37.2** (a) Longitudinal section of the fingertip. (b) Histological section of the sinus of the nail, nail root and nail halter of Morike, (c) As the cells with nuclei divide within the germinal matrix, they are pushed dorsally and distally. As this occurs, the cells flatten and elongate and the nuclei disintegrate. The addition of keratin layers forms a solid rigid sheath of the nail plate. (37.2a from: Schmidt H-M, Lanz U. Surgical Anatomy of the Hand. Stuttgart, Germany: Thieme; 2004).

When an injured nail bed results in persistent deformity, it may be reconstructed from a nail bed of another digit, usually a toe. It is important to note that sterile matrix can be reconstructed by splitting the sterile matrix of the donor digit, similar to the split thickness skin graft. By doing so, the donor deformity is minimal. On the contrary, reconstruction of the germinal matrix requires full-thickness harvest from the donor digit, producing donor morbidity. ⁶

Specialized and viable keratin-producing cells with intact nuclei are located within the germinal matrix. As the nail is pushed distally during growth, the cell nuclei degenerate (▶Fig. 37.2c). These cells without nuclei are located within the sterile matrix. They add keratin to the undersurface of the nail plate, making it thicker and provide firm adherence of the plate to the nail bed. Delineation between the two matrices is indicated with a lunula. Lunula is an arc of pale tissue just distal to the eponychium.

Areolar tissue of the volar pad provides softness of the finger pulp. There are multiple fibrous collagen bands within the pulp (▶Fig. 37.3). Their purpose is not to create the compartments but rather to adhere the skin to the underlying periosteum. These septa need to be released during drainage of a pulp abscess (felon). This area also contains copious sensory appendages that aid in touch perception.

**Fig. 37.3** (**a,b**) Cross-section of the fingertip showing the nail structures. (From: Schmidt H-M, Lanz U. Surgical Anatomy of the Hand. Stuttgart, Germany: Thieme; 2004).

There is a thickening of the epithelium on the dorsum of the nail resembling a halter that hold the nail in place. This area is connected to the phalangoglenoid ligament and to the proper phalangeal or interosseous ligament, also called the ligament of Flint (▶Fig 37.3, ▶Fig 37.4, and ▶Fig. 37.5).