Source: https://www.verywell.com/extensor-surface-1069321
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 forearm 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 of the arm is the outer arm, or the parts of the skin on the opposite side of the joint. Extensor surfaces do not touch. The legs are another example. The extensor surface of the leg is the front of the leg. The flexor surface is the back 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
Extensor Surfaces & Skin Conditions
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.
Psoriasis
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, 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 associated with psoriasis.
Symptoms of each condition develop separately, and in most cases psoriasis symptoms precede arthritis symptoms. Any joint on the body can be affected by psoriatic arthritis. Symptoms of psoriatic arthritis include stiffness of joints, fatigue, swelling of the fingers and toes, tendinitis, 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 herpes simplex and mycoplasma.
There are many symptoms associated with erythema multiforme, many of which are related to the skin. Specifically, round, bullseye-shaped lesions appear on extensor surfaces like the arms and legs.
Eczema
Eczema is a type of skin condition characterized by patches of rough, inflamed skin that 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 type of extremely itchy, persistent rash. 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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