Common Extensor Surface Skin Conditions

An extensor surface is the skin on the opposite side of a joint. In other words, it's the surface of an extensor muscle: a muscle that flexes when it bends and extends when it straightens.

Take the elbow, for example. When you bend your elbow, the skin on the inner forearm and inner upper arms touch. That's known as a flexor surface, or the parts of the skin that touch when a joint bends. The flexor surface of the leg is the back of the leg.

The extensor surface, however, is the opposite of a flexor surface. Extensor surfaces do not touch. The extensor surface of the arm is the outer arm, and the extensor surface of the leg is the front of the leg. 

Types of Extensor Surfaces

Extensor muscles exist in the arms, hands, legs, and feet. They, specifically, include:

  • Arm and shoulder
  • Forearm and elbow
  • Hand and wrist
  • Fingers
  • Thigh and hip
  • Leg and knee
  • Toes

Associated Conditions

Extensory surface skin conditions
Illustration by Brianna Gilmartin, Verywell

There are several different skin conditions that seem to favor taking hold of extensor surfaces. These conditions include psoriasis, erythema multiforme, eczema, and dermatitis herpetiformis. Take a closer look at each particular condition, below. 


Psoriasis, one of the most common skin diseases, involves the overproduction of keratinocytes in the epidermis, which, in turn, increases the cell turnover rate. There are several different types of psoriasis, all of which can appear nearly anywhere on the body.

Chronic stationary psoriasis, which is also referred to as psoriasis vulgaris, is the most common type of psoriasis. It commonly appears on extensor surfaces such as the knees and elbows. Plaque psoriasis almost exclusively affects extensor surfaces, like the elbows.

Psoriatic arthritis is a type of arthritis that's associated with psoriasis. Symptoms of each condition develop separately, and in most cases, psoriasis symptoms precede arthritis symptoms. Any joint in the body can be affected by psoriatic arthritis. Symptoms of psoriatic arthritis include stiffness of joints, fatigue, swelling of the fingers and toes, tendonitis, lower back pain, and even conjunctivitis.

Erythema Multiforme

The exact cause of the skin condition erythema multiforme is unknown, but it's believed to be caused by an allergic reaction or infection. Certain medications including barbiturates, penicillin, phenytoin, and sulfonamides can trigger a reaction, as can illnesses like the herpes simplex virus and mycoplasma.

There are many symptoms that are associated with erythema multiforme, and several are related to the skin. Specifically, round, bullseye-shaped lesions appear on extensor surfaces like the arms and legs.


Eczema is a type of skin condition that's characterized by patches of rough, inflamed skin and seems to overwhelmingly favor flexor surfaces, but it can appear on extensor surfaces. In cases of nummular eczema, circular, well-defined, scaly plaques appear on extensor surfaces including the arms, legs, and hips.

Dermatitis Herpetiformis

Dermatitis herpetiformis is a somewhat uncommon rash that's extremely itchy and persistent. It's thought to appear due to an autoimmune disorder, and recent research suggests a link between the skin condition and celiac disease. It's characterized by itchy bumps and blisters that appear on extensor surfaces such as the elbows and knees.

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Article Sources
  • American Academy of Family Physicians. "The Generalized Rash: Part I. Differential Diagnosis."
  • U.S. National Library of Medicine. National Institutes of Health. "Erythema Multiforme 410."

  • U.S. National Library of Medicine. National Institutes of Health. "Dermatitis Herpetiformis 410"