Like pyaemic arthritis this is due to an irritant which is present primarily in the blood and affects the structures of the joints like other connective tissue structures. The result is an acute inflammation accompanied by serous and sometimes by fibrinous exudation into the joint. The synovial membrane is injected and swollen, but in most cases the inflammation passes off without leaving any permanent change.

In exceptional cases the inflammation lingers in one or more joints, just as it does in the heart when the valvular structures are affected by the same rheumatic poison. In that case the chronic inflammation produces thickening and rigidity of the ligaments, sometimes with fibrous union of the bones.