The link between injury or joint trauma and joint biomechanical stressing in Psoriatic Arthritis (PsA) is recognised. The observations that PsA may start at the enthesis has lead many investigators to conclude that, on the "balance of probabilities" excessive physical stressing or injury may be an inciting event. This page summarises the link to injury and physical stress and PsA.
|This figure shows the traditional immune system model for psoriasis and Psoriatic Arthritis.
According to this model the disease starts when the immune cells called T lymphocytes mount an
attack against a shared structure in the skin and the joint lining (synovium).
Black circles = lymphocytes
Red on left = skin psoriasis
Red in joint = joint lining inflammation
Psoriasis and some other Dermatological Diseases are linked to mechanical injury of the skin. This is called the Koebner or Isomorphic response .
Psoriasis also has a tendency to occur over sites of maximal physical stressing in the skin. This is likely why psoriasis may be prominent over the elbows and knees.
Nail disease in psoriasis is commonest in the thumbs and index fingers reflecting the fact these are used the most. Dermatologists often tell patients to keep their nails well-trimmed to minimise damage .
Going back well over 50 years there have been repeated reports of injury and subsequent PsA development . This was termed the "Deep Koebner phenomenon". We now know that the enthesis is a site of both bone and soft tissue microdamage.
Recent UK based and Canadian studies have shown a link between injury and subsequent PsA development [4,5].
Subjects with psoriasis who are overweight are more likely to develop PsA. Obesity leads to an increased level of inflammation and may also increase joint stressing and microdamage .
|This figure shows the biomechanical model for Psoriatic Arthritis. Microdamage around the entheses and adjacent bone is responsible for triggering joint inflammation. Microdamage over stressed areas of skin likewise triggers skin inflammation. The immune system can respond in a different way to events taking place in these two sites and this accounts for the different severity of skin and joint disease.|
Mechanical stressing of normal mouse entheses leads to an arthritis with many features of PsA. This is clear proof of concept of a primary enthesitis being linked to physical stressing of the joints .
The "eggshell skull rule" may be applied in this setting whereby minor injury may precipitate PsA.
Eggshell skull rule definition which is relevant to the magnitude of injury that may be linked to PsA onset.