1. Causes unknown.

2. Metastatic and bacterial arthritis.

3. Secondary arthritis (lymphangitis, erysipelas, phlegmon).

All those acute affections which are not caused by trauma we classify further, according to the nature of the exudation, into serous, serofibrinous, fibrinous, seropurulent, and purulent joint affections. The purulent may be more or less freely purulent, and the prognosis varies accordingly. The cases of so-called catarrhal purulence, in which staphylococci are often found, are of least significance. Streptococci give rise to more serious inflammation, sometimes with putrefying pus mixed with gas, and in such cases there is much greater danger of infection. Gonococci give very varying inflammatory products (further details below).

* It would be perhaps more correct to say "directly traumatic." A trauma may lead to hydrarthrosis, to circumscribed capsulitis, to arthritis deformans, or may even cause the development of a tubercular focus.