The effects on the joint vary in severity.  In the milder forms, there is engorgement and infiltration of the synovial membrane, and an effusion into the cavity of the joint of serous fluid mixed with flakes of fibrin—­serous synovitis.  In more severe infections the exudate consists of pus mixed with fibrin, and, it may be, red blood corpuscles—­purulent or suppurative synovitis; the synovial membrane and the ligaments are softened, and the surface of the membrane presents granulations resembling those on an ulcer; foci of suppuration may develop in the peri-articular cellular tissue and result in abscesses.  In acute arthritis, all the structures of the joint are involved; the articular cartilage is invaded by granulation tissue derived from the synovial membrane, and from the marrow of the subjacent bone; it presents a worm-eaten or ulcerated appearance, or it may undergo necrosis and separate, exposing the subjacent bone and leading to disintegration of the osseous trabeculae—­caries.  With the destruction of the ligaments, the stability of the joint is lost, and it becomes disorganised.

The clinical features vary with the extent of the infection.  When this is confined to the synovial and peri-synovial tissues—­acute serous and purulent synovitis—­there is the usual general reaction, associated with pyrexia and great pain in the joint.  The part is hot and swollen, the swelling assuming the shape of the distended synovial sac, fluctuation can usually be elicited, and the joint is held in the flexed position.

When the joint is infected by extension from the surrounding cellular tissue, the joint lesion may not be recognised at an early stage because of the swollen condition of the limb, and because there are already symptoms of toxaemia.  We have observed a case in which both the hip and knee joints were infected from the cellular tissue.

If the infection involves all the joint structures—­acute arthritis—­the general and local phenomena are intensified, the temperature rises quickly, often with a rigor, and remains high; the patient looks ill, and is either unable to sleep or the sleep is disturbed by starting pains.  The joint is held rigid in the flexed position, and the least attempt at movement causes severe pain; the slightest jar—­even the shaking of the bed—­may cause agony.  The joint is hot, tensely distended, and there may be oedema of the peri-articular tissues or of the limb as a whole.  If the pus perforates the joint capsule, there are signs of abscess or of diffuse suppuration in the cellular tissue.  The final disorganisation of the joint is indicated by abnormal mobility and grating of the articular surfaces, or by spontaneous displacement of the bones, and this may amount to dislocation.  In the acute arthritis of infants, the epiphysis concerned may be separated and displaced.

When the joint is infected through an external wound, the anatomical features are similar to those observed when the infection has reached the joint by the blood-stream, but the destructive changes tend to be more severe and are more likely to result in disorganisation.