Skin Conditions Affecting Extensor Surfaces

Why Psoriasis and Other Disorders Affect Knees and Elbows

In dermatology, the extensor surface is the area of skin on the outside of a joint. The muscle that causes a joint to open and extend is aptly called the extensor muscle. Examples of an extensor surface include the front of the knee or the back of the elbow or forearm.

By contrast, the flexor surface is the skin on the side of a joint that folds. Directed by a flexor muscle, the flexor surface can be described as areas where folded skin can touch, such as the inside of the elbow or the back of the knee.

These descriptions are important in describing the location—and likely cause—of any skin disorder affecting a jointed body part.

Function

Extensor and flexor muscles work in opposition to the other and are situated on opposing sides of the shoulder, upper arm, elbow, forearm, wrist, hand, fingers, hip, thigh, knee, foot, and toes. There are even extensors and flexor muscles in the neck and along the lumbar spine that allow us to bend forward or backward.

The surfaces overlying these muscles are traversed with capillaries and nerves that deliver oxygen to tissues and detect gross and subtle sensations, respectively. Those on extensor surfaces are more prone to injury and inflammation due to the action of bending a joint.

Every time you bend an elbow, for example, the capillaries and nerves are stretched around the bone and joint. The same does not occur on the flexor surface unless the joint is hyperextended.

Adverse Conditions

For reasons not entirely understood, the extensor surface is a common site of skin disorders, including psoriasis, nummular eczema, dermatitis herpetiformis, and erythema multiforme. Of these, psoriasis and dermatitis herpetiformis are autoimmune disorders, while eczema and erythema multiforme are more closely related to an allergy or hypersensitivity reaction.

Extensory surface skin conditions
Illustration by Brianna Gilmartin, Verywell

Psoriasis

Psoriasis is a common autoimmune disorder characterized by the overproduction of skin cells called keratinocytes in the outer layer of skin (epidermis). Plaque psoriasis, also known as psoriasis vulgaris, is the most common type, accounting for up to 90 percent of cases. It can affect any part of the body but most often develops on the elbows, knees, and lower back.

Why these specific surfaces are affected remain something of a mystery. It has been proposed that the repetitive stretching of these tissues with everyday use makes them a common site of chronic inflammation.

Moreover, the skin of the knees and elbows are frequent sites of trauma, including abrasion and contusions. Over time, this causes the skin to thicken, a condition known as epidermal hyperplasia.

Not surprisingly, skin trauma and localized inflammation are two of the key triggers of psoriasis. Moreover, the development of hyperplasia increases the density of cells in the epidermis, providing more "targets" for an autoimmune assault.

Nummular Eczema

Eczema, also known as atopic dermatitis, is an inflammatory condition characterized by the appearance of scaly patches of itchy skin. The term atopic is used to describe diseases caused by inappropriate immune system response, such as asthma and hay fever. This is unlike an autoimmune disorder in which cells are directly attacked.

Eczema is believed to be caused by a combination of genetic, environmental, and immunological factors, leading to a normal, albeit exaggerated immune response.

While eczema overwhelming favors flexor surfaces, one type—known as nummular eczema—causes coin-shaped, scaly plaques on the extensor surfaces of the arms, legs, and hips.

As with psoriasis, nummular eczema is not well understood by scientists. However, it is believed that a hypersensitive reaction causes lipids (fat) to leach from the epidermis, causing dryness and well-defined areas of redness and inflammation.

Erythema Multiforme

Erythema multiforme is an acute, self-limiting condition caused by a hypersensitive reaction to infections, medications, and other triggers. Common drug triggers include barbiturates, penicillin, phenytoin, and sulfonamide. Viral and bacterial infections like herpes simplex virus and Mycoplasma pneumoniae can also incite a reaction.

Erythema multiforme is characterized by bullseye-shaped lesions on the extensor surfaces of the arms and legs, including the fingers and toes. The pattern of rash is described as zosteriform, meaning constrained within a specific nerve territory on the skin (dermatome). This suggests that the aggravation of nerves, common on extensor surfaces, may promote the development of erythematous lesions.

Erythema multiforme can also appear along the lines of a previous skin injury, a condition referred to as the Koebner response.

Dermatitis Herpetiformis

Dermatitis herpetiformis is a chronic skin disorder closely linked celiac disease and gluten sensitivity. It is characterized by an intensely itchy cluster of blisters on extensor surfaces (as well as the scalp, groin, and buttocks).

Dermatitis herpetiform is caused by the accumulation of immunoglobulin A (IgA) in the epidermis. For reasons unknown, gluten can cause IgA to activate, triggering localized inflammation and the development of eruptive lesions.

As autoimmune disorders, celiac disease and gluten sensitivity both share similarities with psoriasis (and can often co-occur). This includes the preponderance of lesions on extensor surfaces, including the knees and elbows.

There is even evidence from the University of California, San Francisco that gluten can trigger flares in up to 20 percent of people with psoriasis, suggesting a common genetic link.

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