A. SYNOVITIS.

Definition.—­By the term ‘synovitis’ is indicated an inflammation of the synovial membrane.  It may be either (a) Simple or Acute, or it may be (b) Purulent or Suppurative.

In the simple form there is little or no tendency for the affection to implicate the other structures of the joint, whereas in the suppurative form the joint capsule, the ligaments, and the bones soon come to participate in the diseased processes, giving us a condition which we shall afterwards describe as acute arthritis.

(a) SIMPLE SYNOVITIS.

1. Acute—­(Causes).—­Simple or acute synovitis is nearly always brought about by injury to the joint—­by blows or bruises, or by sprains of the ligaments.  At other times it occurs without ascertainable cause, and is then put down to the influence of cold, or to poisonous materials (as, for example, that of rheumatism) circulating in the blood-stream.

Pathology.—­Uncomplicated acute synovitis never causes death.  The pathological changes in connection with it have therefore been studied in cases purposely induced, and the animal afterwards slaughtered.  It is then found that, as in inflammation elsewhere, the synovial membrane is showing the usual inflammatory phenomena—­that it is thick and swollen as a result of the inflammatory hyperaemia and commencing exudation.  Later, the synovial fluid becomes increased in quantity, is thin and serous, and after a time is seen to be mixed with the inflammatory exudation poured into it.  We then find that it has lost its clear appearance, has become thick and muddy, and has floating in it flakes of fibrin.

If the case progresses favourably these materials are soon absorbed and resolution occurs.  In rarer cases the thickening and congestion of the membrane increases, and the articular capsule becomes so distended with the increased synovia and accumulated inflammatory discharges that a kind of chemosis occurs.  In other words, there oozes through, without actual rupture of the membrane, a thin, blood-stained, and purulent-looking discharge.

It is an important point to note that in cases of synovitis the fringes of the synovial membrane become swollen and blood-injected, forming noticeable red elevations at the margins of the cartilages.  It is then that the diseased condition soon spreads and runs into arthritis.

Further, it is important, especially with regard to the question of the degree of pain and lameness likely to be caused, to note that often granulations are thrown out upon the looser folds of the membrane.  As these increase in size they come to form fringed and villous membranous projections inserting themselves between the bones forming the articulation.  In such cases there is no doubt that the intense pain sometimes observed in these cases is due to pinching of these prolongations of the synovial membrane by the opposing bones of the joint.