Infection in a joint is called ‘Septic Arthritis’. Septic Arthritis is an emergency!
If a joint is affected then there will be severe pain and the joint will be hot, swollen and very tender. Even small movements cause severe pain. In addition, the patient will feel very unwell in themselves (clammy, sweaty, general malaise) and they will probably have a temperature.
If there is any infection in a joint then if it is left, the infection will destroy the articular cartilage in the joint, leaving the joint to become rapidly severely arthritic.
If septic arthritis is suspected in a knee then the joint should be aspirated (fluid drawn off with a needle) to get a sample, which will be sent to the labs to check for bugs (bacteria). Only after a sample is obtained (which should be done double-ASAP!) should the patient then be started on antibiotics (otherwise this can give a false-negative result for the aspiration). The antbiotics need to be high-dose intravenous antibiotics.
To leave a hot, painful, swollen, infected knee joint and to just give only antibiotics is a serious mistake! Any infected knee joint needs an urgent knee arthroscopy, ASAP! Under an anaesthetic, joint fluid samples + synovial biopsies should be taken, and then the knee should be washed out copiously, to get rid of all of the pus from inside the joint. This often ends up needing to be repeated more than once, and the patient will also at the same time need an extended course of strong antibiotics.
Even if septic arthritis is caught early and treated appropriately, it still often ends up leading to significant damage in the joint, with a far higher risk of the joint developing secondary osteoarthritis in the future. Also, with any infection in a joint or a bone, there is a risk that the infection may ‘hide’ dormant in the joint/bone, and sometimes the infection can recur spontaneously at a later date – sometimes even many years later.