This condition is produced most directly by the opening of joints and the occurrence of septic decomposition in their fluids. But it also occurs sometimes by exposure to cold, from injuries, and from the extension of phlegmonous inflammations from surrounding parts.

In its anatomical details, the inflammation resembles closely that of the pleura and pericardium. The synovial membrane and cartilages are covered with fibrinous exudation, and the cavity contains serous fluid in which flakes of fibrine are visible. This fluid is sometimes very considerable and distends the joint. In this stage the disease may resolve and the joint return to its normal condition. On the other hand, especially if the joint has been laid open and exposed to decomposition, the inflammation may go on to suppuration. The synovial membrane becomes swollen and dull, being infiltrated with inflammatory cells, and gradually converted into granulation tissue. But if the inflammation is very acute, we may have suppuration by the mere exudation of leucocytes without much change in the synovial membrane. When the disease has gone on to the formation of granulation tissue it is now in a subacute condition which is apt to be prolonged. The inflammation also extends beyond the synovial membrane. In the cartilage the cells undergo active proliferation, and the matrix breaks down; thus softening occurs, and ulcers form. There is inflammation of the bone, a rarefying ostitis. The ligaments also take part in the inflammation, they are softened and, with the synovial membrane, take part in the formation of granulation tissue. The periosteum is inflamed and new bone is formed, so that irregular projections occur near the ends of the bones. With all this there is, of course, usually an abundant purulent discharge from the joint, which may weaken the patient and prove fatal, perhaps with amyloid disease.

If the inflammation subsides, the various masses of granulation tissue develop into connective tissue, and, by the contraction of this, great rigidity of the joint may be produced. The granulation tissue lining the joint also to a great extent coalesces, and the result is a partial or complete obliteration of the joint. The bones thus become finally united by fibrous or osseous adhesions, and a permanent anchylosis is the result.