Nail Anatomy

The nail organ is an integral component of the digital tip. It is a highly versatile tool that protects the fingertip, contributes to tactile sensation by acting as a counterforce to the fingertip pad, and aids in peripheral thermoregulation via glomus bodies in the nail bed and matrix. ^{[1, 2]} Because of its form and functionality, abnormalities of the nail unit result in functional and cosmetic issues. The structures that define and produce the nail (nail plate) include the matrix (sterile and germinal), the proximal nail fold, the eponychium, the paronychium, and the hyponychium (see the images below). Collectively, the nail bed (sterile matrix), nail fold, eponychium, paronychium, and hyponychium are referred to as the perionychium.

See 15 Fingernail Abnormalities: Nail the Diagnosis, a Critical Images slideshow, to help identify conditions associated with various nail abnormalities.

The nail plate emerges from the proximal nail fold and is bordered on either side by the lateral nail folds (paronychium). The nail plate is composed of hard, keratinized, squamous cells that are loosely adherent to germinal matrix but strongly attached to the sterile matrix. ^[2] The nail fold, the most proximal aspect of the perionychium, is composed of a dorsal roof and a ventral floor. It is found approximately 15 mm distal to the distal interphalangeal joint (DIP). ^{[3, 4]}

The dorsal roof rests above the forming nail, and the ventral floor lies beneath the nail, immediately distal to the insertion of the extensor tendons. The ventral floor is the site of the germinal matrix and is responsible for 90% of nail production. The dorsal roof of the nail fold plays a role in housing cells that impart shine to the nail. The skin proximal to the nail that covers the nail fold is the eponychium. The tissue distal to the eponychium in contact with the nail represents the cuticle.

Extending from proximal to distal on the nail is a half-moon shaped white arc known as the lunula (see the image below). The lunula is the distal extent of the germinal matrix. ^[4] This characteristic color change is due to the persistence of nail cell nuclei in the germinal matrix; distal to this location, nuclei are absent, and the nail is transparent. ^[2] The area of the nail bed distal to the lunula is the sterile matrix. This is a secondary site of nail production and is tightly adherent to the nail plate and the periosteum of the distal phalanx.

A junction is formed between the sterile matrix and the fingertip skin beneath the nail margin. This area is referred to as the hyponychium. This region is susceptible to contamination from environmental interactions. A keratin plug acts as a mechanical barrier to protect against infectious inoculation. Also found within this keratin plug are polymorphonuclear leukocytes and lymphocytes contributing an immunologic barrier to the mechanical one established by the keratin plug. ^[2]

The arterial blood supply to the perionychium originates from the terminal branches of the radial and ulnar proper palmar (volar) digital arteries. These vessels originate proximal to the metacarpophalangeal joint from the common palmar digital arteries. The proper palmar digital arteries branch proximal to the DIP joint and give off a branch that travels dorsal to the DIP joint, supplying the superficial arcade that feeds the nail fold and proximal matrix. ^[3]

Development of the nail matrix begins in the ninth embryonic week from the nail anlagen. By week 16, the fetal nail is identifiable. ^[1] Nearly 90% of the nail plate is produced by the proximal half of the matrix, more specifically, the germinal matrix. As a result, more of the nail plate substance is produced proximally, leading to a natural convex curvature of the nail from proximal to distal. Ulnar and radial projections of the matrix extend proximally to form points or horns of the matrix (see the image below). ^[3] These lateral horns are attached to the dorsal expansion of the lateral ligament of the distal interphalangeal joint (DIP). ^[1]

Nail growth is separated into 3 areas: (1) germinal matrix, (2) sterile matrix, and (3) dorsal roof of the nail fold.

The germinal matrix has the following characteristics:

• It is found on the ventral floor of the nail fold; The nail is produced by gradient parakeratosis

• Cells near the periosteum of the phalanx duplicate and enlarge (macrocytosis)

• Newly formed cells migrate distally and dorsally in a column toward the nail

• Cells meet resistance at established nail, causing them to flatten and elongate as they are incorporated into the nail

• It initially retains nuclei (lunula); more distal cells become nonviable and lose nuclei

The sterile matrix has the following characteristics:

• The area is distal to the lunula

• It has a variable amount of nail growth.

• It contributes squamous cells, aiding in nail strength and thickness

• It has a role in nail plate adherence by linear ridges in the sterile matrix epithelium

The dorsal roof of the nail fold has the following characteristics:

• The nail is produced in a similar manner as the germinal matrix

• The cells lose nuclei more rapidly

• It imparts shine to the nail plate

Nail growth is estimated at 3-4 mm per month. Complete nail plate growth takes approximately 6 months. Certain factors increase the rate of the growth. These include longer digits, summer months, young persons (< 30 y), and nail biters. ^[2]

Abnormalities of the nail plate can be classified according to whether they are morphologic or related to nail color. These changes can be associated with systemic disease and provide an early clue to practitioners. ^{[5, 6]} Below are examples of nail plate changes and the associated disease processes that they represent.

Onycholysis

On examination, onycholytic nails are smooth and firm, and there is distal separation of the nail plate from the nail bed. Onycholysis is associated with the following conditions:

• Trauma, as seen in the following image

  Traumatic onycholysis.

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• Thyroid disease (especially hyperthyroid)

• Plummer's nail: Onycholysis of fourth or fifth nail associated with hyperthyroidism

• Anemia

• Erythropoietic porphyria

• Histiocytosis X

• Peripheral ischemia

• Leprosy

• Lupus erythematosus

• Pemphigus vulgaris

• Porphyria cutanea tarda

• Pellagra

• Pleural effusion

• Psoriatic arthritis

• Reiter syndrome

• Scleroderma

• Syphilis

• Aspergillus niger ^[7]

Clubbing

Nails with clubbing have the following features:

• Increased transverse and longitudinal nail curvature

• Fibrovascular hyperplasia of paronychium

• Lovibond angle (angle between dorsal surface of distal phalanx and the nail plate) is greater than 180º (see the image below)

  Lovibond angle.

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• Schamroth sign, which is a disappearance of the diamond shape created when the dorsal surfaces of thumb interphalangeal joints are opposed (see the following image)

  Left: Normal space between opposing thumbs. Right: The window is lost with nail clubbing.

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Clubbing is associated with the following conditions:

• Hemiplegia (when unilateral)

• Lung disease

• Inflammatory bowel disease

• Cardiovascular disease

• Liver disease

• Acquired immunodeficiency syndrome (AIDS)

Koilonychia

On examination, koilonychia findings include concavity of the nail plate and the appearance that a drop of water could be retained in the nail (see the following image).

Koilonychia is associated with the following conditions:

• Trauma

• Iron deficiency anemia

• Exposure to petroleum-based solvents

• Hemochromatosis

• Hypothyroidism

• Coronary artery disease

• Normal variant in infants

  Koilonychia.

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Onychomadesis

A proximal separation of the nail plate is the characteristic finding in onychomadesis (see the image below).

Onychomadesis is associated with the following conditions:

• Trauma

• Malnutrition

• Drug sensitivity

• Pemphigus vulgaris

• Kawasaki disease (see the Kawasaki Disease Diagnostic Criteria calculator)

• Hand, foot, and mouth disease

Beau lines

Beau line findings include a transverse depression in the nail plate secondary to temporary cessation of nail growth, as seen in the following image. Beau lines are associated with high fever, malnutrition, and poorly controlled diabetes.

Muehrcke nails

Muehrcke nails include the following features (see the image below):

• Transverse white bands parallel to the lunula

• Usually found in pairs and traverse the entire nail

• Form of leukonychia caused by abnormality in nail bed vasculature

  Muehrcke nails.

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Muehrcke nails are associated with the following conditions:

• Hypoalbuminemia

• Nephrotic syndrome

• Glomerulonephritis

• Liver disease

• Malnutrition

• Post chemotherapy

• Peutz-Jeghers syndrome

Lindsay nails

Findings on examination of Lindsay nails are as follows (see the following image):

• Proximal nail bed is white secondary to edema

• Distal nail bed pink or brown

• Abnormal melanin pigment

• Form of leukonychia caused by abnormality in nail bed vasculature

  Lindsay nail (half-and-half nail).

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Lindsay nails are associated with the following conditions:

• Renal disease (hemodialysis patients)

• Renal transplant

• Human immunodeficiency virus (HIV)

Terry nails

Terry nails demonstrate the following findings (see the image below):

• Most of the nail plate is white with a distal pink band

• All nails are affected equally

• The form of leukonychia is caused by abnormality in nail bed vasculature

  Terry nail.

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Terry nails are associated with the following conditions:

• Cirrhosis

• Chronic congestive heart failure (CHF)

• Adult-onset diabetes mellitus

• HIV

Red lunula

A red lunula is as its name describes (see the following image). In addition, the lunula may be absent, or an azure (blue) lunula may be seen.

Red lunula is associated with the following conditions:

• Alopecia areata

• Collagen vascular disease

• Prednisone use

• Cardiac failure

• COPD

• Cirrhosis

• Psoriasis

• Carbon dioxide poisoning

• Wilson disease (azure lunula)

• 5-fluouracil (5-FU) use (azure lunula)

Splinter hemorrhage

A splinter hemorrhage is a longitudinal extravasation of blood along the nail bed (see the image below).

Splinter hemorrhage is associated with the following conditions:

• Bacterial endocarditis

• Trauma

• Mitral stenosis

• Vasculitis

• Cirrhosis

• Scurvy

• Chronic glomerulonephritis

• Darier disease