Nail unit (anatomy)

The nail unit or nail apparatus refers to a group of distal digital structures involved in the function and support of the nail plate.

Gross anatomy

Structures of the distal phalanx composing the nail unit include ¹:

• nail plate
• nail matrix
• nail bed
• periungual soft tissues
  • eponychium (cuticle)
  • nail folds
    • proximal
    • lateral 
  • hyponychium

Location

The nail unit occupies much of the superficial, dorsal phalanges.

Relations

The trilaminar, keratinized nail plate is anchored firmly to the nail bed on its volar surface; it extends from the proximal eponychium, passing over the lunula of the nail matrix and meeting the onychodermal band as it approaches the hyponychium distally at its lone free margin. The nail plate is anchored laterally and proximally by overlapping specialized ridges of skin ⁴.

The nail matrix is covered by skin proximal to the proximal nail fold and eponychium, extending roughly half the distance between the nail fold and surface landmarks of the distal interphalangeal (DIP) joint ⁹. Its convex distal margin is in continuity with the nail bed, deep to the most proximal extent of the nail plate, and may be visible as the lunula. 

Function

The nail matrix is the progenitor for both the nail bed and nail plate, with the proximal matrix being far more robust in terms of proliferation ⁸. 

Arterial supply

The arterial supply to the digits of the upper extremities originates from the radial and ulnar arteries which branch and variably contribute to the carpal arches, forming the:

• superficial palmar arch
  • ​formed by the ulnar artery completing the vascular arcade by anastomosis with the superficial branch of the radial artery
  • ​forms the three common digital arteries which divide to form proper digital arteries
• deep palmar arch
  • contributes to the common digital arteries
  • origin of the princeps pollicis artery
• dorsal metacarpal arch
  • communication with the palmar vessels around the proximal interphalangeal joint

The proper digital arteries communicate through numerous transverse arches and an arcade of paired dorsal and palmar arches formed just distal to the insertion of the flexor digitorum profundus muscle; this dorsal arcade (dorsal nail fold arch) supplies the proximal nail fold, extensor tendon insertion, and has some contribution to the matrix. Subsequent anastomosis with the palmar arch results in the formation of the middle and distal matrix arches and an underlying capillary network perfusing the nail bed, nail matrix, nail folds and hyponychium. 

Venous drainage

• venous anatomy is highly variable, but typically involves paired superficial and deep venous systems which communicate via several anastomotic vessels
• the nail matrix has a propensity to drain into a dorsal, midline conduit
• the lateral nail folds into adjacent vessels running longitudinally

Innervation

• the volar (index, middle, and long) or dorsal paired digital nerves provide sensation to:
  • proximal nail fold
  • nail bed
  • nail matrix

Radiographic features

Ultrasound

Orthogonal planes (e.g. transverse, sagittal) should be obtained with consideration for the use of an acoustic standoff medium. A linear probe should be utilized. High frequency ultrasound should be specifically considered. 

The nail plate appears as a trilaminar structure, with the hyperechoic outer layers representing the dorsal and ventral plates and the intermediate layer the anechoic interplate space. Note should be made of the hyperechoic, smooth bony cortex of the distal phalanx in the far field. The nail bed will be found deep to the nail plate and superficial to bone; it will be continuous proximally with the nail matrix which can be identified due to higher echogenicity and location beneath the eponychium ¹³. 

Note should be made of specific structures in the periphery such as:

• distal interphalangeal joint
• point of insertion of the extensor tendon to the cortex of the distal phalanx
  • continuity between nail matrix and tendon insertion may allow spread of disease or inflammation from one structure to the other ¹¹
  • signs of related enthesopathy may include:
    • loss of normal fibrillar echotexture
    • presence of calcification
    • erosions, enthesophytes

Structures that may warrant specific measurements or observations include:

• nail plate
  • diameter, as measured from ventral to dorsal plate, should not exceed 0.65 mm
  • loss of the interplate space
  • ventral and dorsal plates should be smooth, homogenous, and continuous 
• nail bed depth
  • measured from phalanx cortex to ventral plate
  • values >2 cm likely abnormal

MRI

• test of choice for suspected neoplasm involving a component of the nail unit

Clinical importance

Allows non-invasive, detailed imaging of structure such as the nail bed, underlying distal phalanx, nail plate thickness, and matrix with particular attention to:

• assessment for a subungual mass, neoplastic disease
  • glomangioma ¹
  • subungual exostosis
• presence of phalangeal cortical disruption in the context of a likely overlying nailbed disruption indicative of an open fracture ¹¹
  • the presence of a Seymour fracture should specifically be considered in a pediatric patient ⁸
• tendinous injury or tendinopathy
  • absent attachment of the extensor tendon to the distal phalangeal base could indicate tendon rupture with subsequent retraction
  • this should be dynamically sought as a rounded, mixed echogenicity lesion more proximally
    • substrate for a jersey finger injury
  • associated cortical irregularity, retracted distal end of tendon, hyperechoic intratendinous structure; concern for avulsion fracture
• onychomycosis
• spondyloarthropathies
  • appears to help differentiate psoriatic arthritis similar presentations (e.g. rheumatoid arthritis)
  • appears to be helpful from a disease worsening prediction standpoint