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Septic Knee Arthritis

If a knee joint suddenly gets hot, red and swollen, the physician must rule out an acute septic knee arthritis. In about 70% of the cases this is due to Staphylococcus aureus.

The common pus bug that is contained in boils, pimples, infected wounds etc. can on occasion travel internally to other areas in the body such as a knee joint. When this occurs, it usually happens on one knee only.

Diagnostic tests

The physician will do a joint aspiration under sterile conditions and send the sample to the lab. The culture will take a day or two, but the immediate Gram stain will often suggest the cause already as a Gram positive staining will identify Staphylococcus aureus. On the other hand, if the Gram stain shows Gram negative cocci, the diagnosis in all likelihood is gonorrhea. This is one of the major venereal disease bugs.

Identification of bacteria involved

This bacterium likes to invade the body and often migrates to one of the knee joints, where an acute septic joint is caused. Finally, with a throat infection with Streptococcus the Gram staining of joint fluid likely will show oval shaped Gram positive bacteria lined up in chains of 3 to 5 bacteria, which is typical for this group of bacteria. Although the final diagnosis will eventually be available from the culture report, the physician will treat on clinical grounds as waiting for the results could permanently damage the joint surfaces by these very toxic bacteria.

 Septic Knee Arthritis

Septic Knee Arthritis

Antibiotic treatment is done with appropriate antibiotics that are initially usually given intravenously and then orally for a period of time. The associated disease from which the bacteria originated has to also be treated simultaneously (Ref.1,p.1147 and 1156).



1. The Merck Manual, 7th edition, by M. H. Beers et al., Whitehouse Station, N.J., 1999. Chapter 157.

2. ABC of rheumatology, second edition, edited by Michael L. Snaith , M.D., BMJ Books, 1999. Chapter 4.

3. The Merck Manual, 7th edition, by M. H. Beers et al., Whitehouse Station, N.J., 1999. Chapter 270.

4. Wheeless’ Textbook of Orthopaedics:

5. Goldman: Cecil Textbook of Medicine, 21st ed.(©2000)W.B.Saunders

6. Ferri: Ferri’s Clinical Advisor: Instant Diagnosis and Treatment, 2004 ed., Copyright © 2004 Mosby, Inc.

7. Rakel: Conn’s Current Therapy 2004, 56th ed., Copyright © 2004 Elsevier

Last modified: June 22, 2019

This outline is only a teaching aid to patients and should stimulate you to ask the right questions when seeing your doctor. However, the responsibility of treatment stays in the hands of your doctor and you.