Table of ContentsView AllTable of ContentsAdverse ConditionsPsoriasisNummular EczemaErythema MultiformeDermatitis HerpetiformisFrequently Asked Questions
Table of ContentsView All
View All
Table of Contents
Adverse Conditions
Psoriasis
Nummular Eczema
Erythema Multiforme
Dermatitis Herpetiformis
Frequently Asked Questions
Extensor surfaces are skin surfaces on the outside of a joint that are common regions for lesions or other skin changes in certain skin disorders.
For example, the front of the knee and the back of the elbow or forearm are extensor surfaces where inflamed skin and scaly patches ofpsoriasismay develop.
This article explores the location and function of extensor surfaces and which skin conditions often affect these areas.
Location and Function
Indermatology, the skin surfaces of a joint are described as either extensor or flexor surfaces, which correspond to muscles of the same name:
Location ExamplesExtensor SurfacesBack of the elbowFront of the kneeKnucklesFlexor SurfacesInside the elbowBack of the kneeFolds underneath knuckles
Location Examples
Extensor SurfacesBack of the elbowFront of the kneeKnuckles
Back of the elbow
Front of the knee
Knuckles
Flexor SurfacesInside the elbowBack of the kneeFolds underneath knuckles
Inside the elbow
Back of the knee
Folds underneath knuckles
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 thelumbar 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 andinflammationdue 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 ishyperextended, or straightened beyond its healthy range of motion.
The extensor surface is a common site of skin disorders, including:
Psoriasis and dermatitis herpetiformis areautoimmune disorders, while eczema and erythema multiforme are more closely related to anallergyorhypersensitivity reaction.
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 theelbows, 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.
DermNet /CC BY-NC-ND
Moreover, the skin of the knees and elbows are frequent sites of trauma, including abrasions andcontusions. Over time, this causes the skin to thicken, a condition known asepidermal 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.
6 Common Triggers for Psoriasis
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.
This photo contains content that some people may find graphic or disturbing.See PhotoDermNet /CC BY-NC-ND
This photo contains content that some people may find graphic or disturbing.See Photo
This photo contains content that some people may find graphic or disturbing.
Skin Infections Associated With Eczema
Common drug triggers include barbiturates, penicillin, phenytoin, and sulfonamide. Viral and bacterial infections likeherpes simplex virusandMycoplasma pneumoniaecan also incite a reaction.
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 theKoebner response.
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 evidence from the University of California, San Francisco that gluten can trigger flares in up to 20% of people with psoriasis, suggesting a common genetic link.
How Psoriasis and Celiac Disease Are Linked
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 asdermatologistwho specializes in skin conditions.
It is usually on flexor surfaces but there is a coin-shaped type called nummular eczema that is found on extensor surfaces.Learn MoreEczema Symptoms
It is usually on flexor surfaces but there is a coin-shaped type called nummular eczema that is found on extensor surfaces.
Learn MoreEczema Symptoms
Psoriasis can occur anywhere. Plaque psoriasis, which is the most common type, typically occurs on extensor surfaces. A rare type called inverse psoriasis occurs on flexor surfaces.Learn MoreInverse Psoriasis
Psoriasis can occur anywhere. Plaque psoriasis, which is the most common type, typically occurs on extensor surfaces. A rare type called inverse psoriasis occurs on flexor surfaces.
Learn MoreInverse Psoriasis
Dermatitis herpetiformis looks like a cluster of bumps that can appear similar to mild acne or eczema. It is usually itchy.Learn MoreDermatitis Herpetiformis Photos
Dermatitis herpetiformis looks like a cluster of bumps that can appear similar to mild acne or eczema. It is usually itchy.
Learn MoreDermatitis Herpetiformis Photos
Learn MoreIs Gluten Hiding In Your Medications?
A combination of blood tests and a skin biopsy, or sample, to look for immune proteins called immunoglobulin A (IgA) antibodies.
8 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Di Meglio P, Villanova F, Nestle FO.Psoriasis.Cold Spring Harb Perspect Med. 2014;4(8):a015354. doi:10.1101/cshperspect.a015354Matard B, Cavelier-Balloy B, Reygagne P.Epidermal psoriasiform hyperplasia, an unrecognized sign of folliculitis decalvans: a histological study of 26 patients.J Cutan Pathol.2017;44(4):352-357. doi:10.1111/cup.12892American Academy of Dermatology Association.Eczema types: nummular eczema overview.Trayes KP, Love G, Studdiford JS.Erythema multiforme: recognition and management.Am Fam Physician. 2019;100(2):82-88.National Institute of Diabetes and Digestive and Kidney Diseases.Dermatitis herpetiformis.Bhatia BK, Millsop JW, Debbaneh M, Koo J, Linos E, Liao W.Diet and psoriasis, part II: celiac disease and role of a gluten-free diet.Journal of the American Academy of Dermatology. 2014;71(2):350-358. doi:10.1016/j.jaad.2014.03.017Dopytalska K, Sobolewski P, Błaszczak A, Szymańska E, Walecka I.Psoriasis in special localizations.R.2018;56(6):392-398. doi:10.5114/reum.2018.80718Antiga E, Caproni M.The diagnosis and treatment of dermatitis herpetiformis.Clin Cosmet Investig Dermatol.2015;8:257-65. doi:10.2147/CCID.S69127
8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Di Meglio P, Villanova F, Nestle FO.Psoriasis.Cold Spring Harb Perspect Med. 2014;4(8):a015354. doi:10.1101/cshperspect.a015354Matard B, Cavelier-Balloy B, Reygagne P.Epidermal psoriasiform hyperplasia, an unrecognized sign of folliculitis decalvans: a histological study of 26 patients.J Cutan Pathol.2017;44(4):352-357. doi:10.1111/cup.12892American Academy of Dermatology Association.Eczema types: nummular eczema overview.Trayes KP, Love G, Studdiford JS.Erythema multiforme: recognition and management.Am Fam Physician. 2019;100(2):82-88.National Institute of Diabetes and Digestive and Kidney Diseases.Dermatitis herpetiformis.Bhatia BK, Millsop JW, Debbaneh M, Koo J, Linos E, Liao W.Diet and psoriasis, part II: celiac disease and role of a gluten-free diet.Journal of the American Academy of Dermatology. 2014;71(2):350-358. doi:10.1016/j.jaad.2014.03.017Dopytalska K, Sobolewski P, Błaszczak A, Szymańska E, Walecka I.Psoriasis in special localizations.R.2018;56(6):392-398. doi:10.5114/reum.2018.80718Antiga E, Caproni M.The diagnosis and treatment of dermatitis herpetiformis.Clin Cosmet Investig Dermatol.2015;8:257-65. doi:10.2147/CCID.S69127
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Di Meglio P, Villanova F, Nestle FO.Psoriasis.Cold Spring Harb Perspect Med. 2014;4(8):a015354. doi:10.1101/cshperspect.a015354Matard B, Cavelier-Balloy B, Reygagne P.Epidermal psoriasiform hyperplasia, an unrecognized sign of folliculitis decalvans: a histological study of 26 patients.J Cutan Pathol.2017;44(4):352-357. doi:10.1111/cup.12892American Academy of Dermatology Association.Eczema types: nummular eczema overview.Trayes KP, Love G, Studdiford JS.Erythema multiforme: recognition and management.Am Fam Physician. 2019;100(2):82-88.National Institute of Diabetes and Digestive and Kidney Diseases.Dermatitis herpetiformis.Bhatia BK, Millsop JW, Debbaneh M, Koo J, Linos E, Liao W.Diet and psoriasis, part II: celiac disease and role of a gluten-free diet.Journal of the American Academy of Dermatology. 2014;71(2):350-358. doi:10.1016/j.jaad.2014.03.017Dopytalska K, Sobolewski P, Błaszczak A, Szymańska E, Walecka I.Psoriasis in special localizations.R.2018;56(6):392-398. doi:10.5114/reum.2018.80718Antiga E, Caproni M.The diagnosis and treatment of dermatitis herpetiformis.Clin Cosmet Investig Dermatol.2015;8:257-65. doi:10.2147/CCID.S69127
Di Meglio P, Villanova F, Nestle FO.Psoriasis.Cold Spring Harb Perspect Med. 2014;4(8):a015354. doi:10.1101/cshperspect.a015354
Matard B, Cavelier-Balloy B, Reygagne P.Epidermal psoriasiform hyperplasia, an unrecognized sign of folliculitis decalvans: a histological study of 26 patients.J Cutan Pathol.2017;44(4):352-357. doi:10.1111/cup.12892
American Academy of Dermatology Association.Eczema types: nummular eczema overview.
Trayes KP, Love G, Studdiford JS.Erythema multiforme: recognition and management.Am Fam Physician. 2019;100(2):82-88.
National Institute of Diabetes and Digestive and Kidney Diseases.Dermatitis herpetiformis.
Bhatia BK, Millsop JW, Debbaneh M, Koo J, Linos E, Liao W.Diet and psoriasis, part II: celiac disease and role of a gluten-free diet.Journal of the American Academy of Dermatology. 2014;71(2):350-358. doi:10.1016/j.jaad.2014.03.017
Dopytalska K, Sobolewski P, Błaszczak A, Szymańska E, Walecka I.Psoriasis in special localizations.R.2018;56(6):392-398. doi:10.5114/reum.2018.80718
Antiga E, Caproni M.The diagnosis and treatment of dermatitis herpetiformis.Clin Cosmet Investig Dermatol.2015;8:257-65. doi:10.2147/CCID.S69127
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