For example, the front of the knee and the back of the elbow or forearm are extensor surfaces where inflamed skin and scaly patches of psoriasis may develop.
This article explores the location and function of extensor surfaces and which skin conditions often affect these areas.
Location and Function
In dermatology, the skin surfaces of a joint are described as either extensor or flexor surfaces, which correspond to muscles of the same name:
Extensor surfaces overlay extensor muscles, which cause a joint to open and extend. Flexor surfaces, also called flexural surfaces, overlay flexor muscles. The flexor muscles decrease the angle between bones on the side of the joint, such as bending the knee. Directed by a flexor muscle, the flexor surfaces are where folded skin can touch.
These descriptions are important in describing lesions and other skin changes, as location can help determine the cause of a skin disorder affecting a jointed body part.
Extensor and flexor muscles work in opposition to one another 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 (lower back) that allow you to bend forward and backward.
The surfaces overlying these muscles are traversed with capillaries that detect gross (large and obvious) sensations and nerves that deliver oxygen to tissues and detect subtle sensations. 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, or straightened beyond its healthy range of motion.
Adverse Conditions
The extensor surface is a common site of skin disorders, including:
Psoriasis Nummular eczema Dermatitis herpetiformis Erythema multiforme
Psoriasis and dermatitis herpetiformis are autoimmune disorders, while eczema and erythema multiforme are more closely related to an allergy or hypersensitivity reaction.
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% 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 somewhat 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 abrasions 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 an inappropriate immune system reaction, such as asthma and hay fever.
A combination of genetic, environmental, and immunological factors are believed to be at the root of this exaggerated response. This is unlike an autoimmune disorder in which cells are directly attacked.
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.
Dermatitis Herpetiformis
Dermatitis herpetiformis is a chronic skin disorder closely linked to 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.
Summary
Extensor surfaces are the skin surfaces outside a joint.
Inflamed skin or scaly lesions on extensor surfaces are often linked to autoimmune disorders, such as psoriasis and celiac disease, or hypersensitive or allergic reactions to medications or infections.
A Word From Verywell
There are many options to help clear up or minimize inflamed or itchy patches of skin, but to effectively do so you’ll need to know the cause. If you’re experiencing skin change, make an appointment with a healthcare provider, such as dermatologist who specializes in skin conditions.
