The perionychium, or nail complex, is a highly specialized structure that is critical to normal digit function . The nail complex allows for improved manipulation of small or fine objects, helps regulate perfusion, contributes to tactile sensation, protects the fingertip , and is possibly the most important structure of the fingertip in regards to aesthetics.

The anatomy of the perionychium includes the nail bed (sterile and germinal matrices), the paronychium, the eponychium, the nail fold, and the hyponychium . The nail plate itself is a hard, kertinaceous three-layered structure formed by contributions from multiple components [2, 3]. The perionychial components may be considered generative or formative of the nail plate, or supportive of the nail plate as structures that frame, sheath, and otherwise support the nail (Fig. 1.2).

The generative components are the nail bed epithelial structures frequently referred to as sterile and germinal matrices, though this may be an oversimplification. In reality, a dorsal, intermediate, and ventral nail matrix can be described, though this classification is debated [3–8]. What is frequently called the “germinal” or “germinative” matrix is designated the combined intermediate and dorsal matrix. The intermediate matrix consists of the matrix at the proximal aspect of the nail plate which is adherent to the deep surface of the nail plate and extends distally to the lunula. This is the primary zone of keratinization and nail plate formation. The lunula, or the distal extent of this zone, is the demarcation between the more pale-blue-gray germinal matrix and the more pink ventral matrix, frequently referred to as the sterile matrix . The germinal matrix color is thought to be distinctly different due to the light scattered by the larger nuclei of the highly synthetic cells of the intermediate/germinal matrix as it produces the majority of the nail plate and its keratin. In reality, the entire matrix is of this color, though only the distal portion is visible from beneath the eponychial fold. The intermediate matrix is thought to produce the majority of the nail plate with up to 20 % of the remaining nail plate coming from the ventral matrix, though this is controversial [3–5, 7]. The proximal aspect of the intermediate matrix transitions to the dorsal matrix, which is the deepest and most proximal portion of the proximal nail fold. This produces the most superficial layer of the nail plate and is also considered a component of the germinal matrix . The most proximal extent of the combined germinal matrix is typically less than 1 mm away from the terminal fibers of the terminal tendon of the distal phalanx . At the lunula, the distal or ventral matrix begins, which has also been referred to as the “sterile” matrix, though arguments against this terminology can be made, and instead, it will be referred to as the nail bed epithelium (NBE). It is argued that this layer may contribute to approximately 20 % of the nail plate as the nail plate becomes thicker as it grows out distally over the ventral matrix/NBE. The NBE does appear to transition from a more matrix-like zone proximally to a more nonproliferative epithelial-like zone distally. The germinal matrix is thicker and has a rete pattern of attachment to the nail plate, whereas the NBE has a ridge pattern of attachment distally and is thinner. The NBE produces a thin keratinizing layer that moves distally along with the nail plate as it continuously grows to provide adherence and continued distal growth. This tight adherence contributes to the more full-thickness avulsion of NBE with nail plate avulsion than the germinal matrix, which is more typically spared a full-thickness avulsion. The ventral matrix or NBE terminates at the onychodermal band .

Histologically, the germinal matrix is distinct from NBE. Both lack a granular layer to the epithelium, but the germinal matrix is thicker and interdigitates with the nail bed fibrocollagen. Mamelons (basaloid buds) are more prominent proximally and centrally than laterally and are oriented toward the tip of the digit. The NBE is not a self-replicating structure. It is created by the matrix and moves distally with the nail plate as previously stated [7], and 81 % of the cells within the nail plate appear to be provided by the proximal 50 % of the nail matrix [2] .

What is frequently referred to as the eponychial fold is more precisely called the “dorsal nail fold” or the “posterior nail fold” (PNF) which blends with the radial and ulnar (both called “lateral”) nail folds. The dorsal nail fold forms a pocket called the proximal nail fold for the germinal matrix and the nail plate root. The external skin or dorsal skin of the PNF is formed by a continuation of the dorsal digit skin that is thin and lacks hair and sebaceous adnexae. The ventral component of the fold has a much thinner and flattened epithelium. This ventral component forms the roof and the matrix forms the floor of this pocket beneath the fold. These two separate epithelial layers are separated by the forming nail plate itself and are frequently stented apart from one another after PNF and/or germinal matrix injuries to prevent formation of synechiae. The proximal 75 % of the ventral epithelium of the PNF is the eponychium, which performs onycholemmal keratinization to produce a thin layer of keratin on the superficial or dorsal surface of the forming nail plate. This forms the “true cuticle” of the nail which is deep to and extends distally on the growing nail plate past the “false cuticle.” The false cuticle is a softer keratin-composed structure formed mostly by the dorsal and less by the ventral surfaces of the PNF . It is not adherent to the nail plate. However, the true cuticle is adherent to the nail plate and protects the PNF pocket by sealing it off from the outside environment. If the dorsal matrix or the transition zone between the dorsal matrix and the eponychium is damaged, the nail will lose its polished appearance and appear dull and rough.

As the eponychium may be considered a transition zone from true skin to the germinal matrix component of the dorsal matrix of the nail fold, the hyponychium is a transition zone from the distal NBE to the skin of the fingertip . They both form nail/skin confluences that completely surround the nail proximally (eponychium) and laterally/distally (hyponychium) to protect the generative components of the nail. The proximal hyponychium begins where the NBE terminates and blends into epithelium of the fingertip skin. The proximal hyponychium produces the solehorn, which functions to allow separation of the ventral or deep surface of the nail plate while still sealing the NBE. The proximal hyponychial attachment to the nail plate is visible through the nail plate as the onychodermal band, which corresponds to the most proximal point of attachment of the fingertip skin stratum corneum to the nail plate. The distal hyponychium is a narrow rim of transitional post-separation skin. The skin of the distal groove is included in this zone and the skin of this zone lacks the ridges that comprise fingerprints. This region has a deep band of connective tissue called the anterior ligament which anchors the hyponychial dermis to the periosteum of the ungual process of the rim of the distal phalanx tuft .

The nail plate is supported by the NBE and intermediate matrix, but these structures are also supported by fibrocollagenous structures termed the nail unit support system (NUSS) which lay between the matrix structures and the dorsal cortex of the distal phalanx . Though the nail bed structures are epithelial structures, there is no true dermis below them and instead the NUSS forms their supportive platform. It directly attaches the epithelium to the deeper phalanx with ligamentous attachments made up of fascial and periosteal tissues. The tissue deep to the epithelium that attaches the matrix to the phalanx that is in the position of what would normally be dermis is termed “corium” and is made up of dense fibers created largely by modified periosteum of the dorsal distal phalanx ungual process. Proximally, the NUSS has longitudinally oriented fibers that form a fibrous sheath around the nail matrix and forms nail root ligaments. The nail root also maintains attachment to the phalanx by more oblique fibers that fuse with the fibers from the dorsal expansions of the radial and ulnar collateral ligaments and the radial and ulnar lateral interosseus ligaments (LILs) over the lateral tuberosities of the base of the distal phalanx . The proximal NUSS is somewhat loose and more delicate. In the intermediate zone beneath the NBE, large bands of collagen are arranged vertically that connect the periosteum to the NBE longitudinally. The “nail bed corium” (NBC) describes this region. The NBC has an abundance of blood vessels and glomus bodies in its longitudinally oriented framework, with its capillary bed ending at the onychodermal band. Distally, the NUSS has looser and more obliquely oriented fibers that angle distally toward the hyponychium . The anterior ligament is a crescentic band of fibers that also extends from the rim of the ungual surface to help stabilize the distal NBE and hyponychium .

The extraordinary fibrous structure provides the distal digit its significant anatomical support. The vast fibrous structure and network enables for balanced motion and stability of the distal interphalangeal (DIP) joint connection of the nail system, protection from injury, and the ability to grasp. The DIP joint is composed of both ligaments and tendons, while robust ligamentous and fascial reinforcement fastens the nail bed and skin to the concealed bone. Together, all of these different complex units of anatomy and the network they compose interrelate and contribute to the support of the joint and the unified distal digit.

LILs (“paraterminal ligaments”) spread from the lateral tuberosities at the origin of the distal phalanx base to the spines of the ungual system. This ligamentous network provides reinforcement for the posterolateral nail bed fibroconnective tissue, lateral fortitude to the digit, and a guarded neurovascular network. The ligamentous network is composed of fibers which are connected with the collateral ligament and attaches to the flexor and extensor tendons .

Dense collateral ligaments spread from divots on the lateral aspect of the condyles of the middle phalangeal head to the posterior aspect of the lateral tuberosities of the distal phalanx. Accessory collateral ligaments run bilaterally from the middle phalangeal head to the sides of the volar plate. These collateral ligaments send dorsal expansions that combine into the proximal nail bed connective tissue. They provide as a safeguard to the proximal matrix. Serving as an anchor, a “bone-nail” ligament develops and attaches the nail root to the lateral tuberosities.