The terms hydrops, hydrarthrosis, and chronic serous synovitis are synonymous, and are employed when a serous effusion into the joint is the prominent clinical feature.  Hydrops may occur apart from disease—­for example, in the knee-joint from repeated sprains, or when there is a loose body in the joint—­but is met with chiefly in the chronic forms of synovitis which result from gonorrhoea, tuberculosis, syphilis, arthritis deformans, or arthropathies of nerve origin.

Arthritis is the term applied when not only the synovial membrane but the articular surfaces, and it may be also the ends of the bones, are involved, and it is necessary to prefix a qualifying adjective which indicates its nature.  When effusion is present, it may be serous, as in arthritis deformans, or sero-fibrinous or purulent, as in certain forms of pyogenic and tuberculous arthritis.  Wasting of the muscles, especially the extensors, in the vicinity of the joint is a constant accompaniment of arthritis.  On account of the involvement of the articular surfaces, arthritis is apt to be followed by ankylosis.

The term empyema is sometimes employed to indicate that the cavity of the joint contains pus.  This is observed chiefly in chronic disease of pyogenic or tuberculous origin, and is usually attended with the formation of abscesses outside the joint.

Ulceration of cartilage and caries of the articular surfaces are common accompaniments of the more serious and progressive forms of joint disease, especially those of bacterial origin.  The destruction of cartilage may be secondary to disease of the synovial membrane or of the subjacent bone.  When the disease begins as a synovitis, the synovial membrane spreads over the articular surface, fuses with the cartilage and eats into it, causing defects or holes which are spoken of as ulcers.  When the disease begins in the bone, the marrow is converted into granulation tissue, which eats into the cartilage and separates it from the bone.  Following on the destruction of the cartilage, the articular surface of the bone undergoes disintegration, a condition spoken of as caries of the articular surface.  The occurrence of ulceration of cartilage and of articular caries is attended with the clinical signs of fixation of the joint from involuntary muscular contraction, wasting of muscles, and starting pains.  These starting pains are the result of sudden involuntary movements of the joint.  They occur most frequently as the patient is dropping off to sleep; the muscles becoming relaxed, the sensitive ulcerated surfaces jar on one another, which causes sudden reflex contraction of the muscles, and the resulting movement being attended with severe pain, wakens the patient with a start.  Advanced articular caries is usually associated with some abnormal attitude and with shortening of the limb.  It may be possible to feel the bony surfaces grate upon one another.  When all its constituent elements are damaged or destroyed, a joint is said to be disorganised.  Should recovery take place, repair is usually attended with union of the opposing articular surfaces either by fibrous tissue or by bone.