Definition and Overview

Septic or infectious arthritis is a medical condition that occurs when a joint (such as the knee or hip) becomes infected. It can be caused by bacteria, viruses, or fungi. These pathogens can invade the joints if they are able to enter the body through an open wound. The infection can also start in other parts of the body and travel to the joints via the bloodstream.

The joints are protected by the synovial fluid that keeps pathogens at bay. However, certain diseases, such as lupus and arthritis, can break down or weaken this defense. This allows pathogens to cause infection.

The condition can affect both children and adults. However, it is more common in certain groups of people. These include:

  • The elderly

  • People with a suppressed immune system. These include organ transplant patients and people with HIV/AIDS.

  • People with a history of joint damage and using prosthetic joints

There are 7.8 cases of septic arthritis for every 100,000 people in the United States and Europe every year. The most commonly involved joint is the knee (accounts for up to 50% of cases) followed by the hip (20%) and the shoulder (8%). The least affected are the ankle, wrist, and elbow.

Causes of Condition

The disorder occurs when pathogens find their way to the joints. The most common is staph bacterial infection. But the condition can also be caused by fungi and viruses. These organisms can enter the body through an open wound, injection, or through a surgical cut. Some types of infections (such as skin and urinary tract infections) can also spread to the joint when they travel through the bloodstream. Other causes of the condition are hepatitis, herpes and HIV viruses, and mumps.

Pathogens cause septic arthritis by damaging the joint’s articular cartilage. This is the white tissue that covers the ends of the bones where they come together to form the joints. This damage can cause the cartilage to erode and impair blood supply to the joint. This can happen as soon as three days from the onset of infection.

Key Symptoms

Septic arthritis can cause extreme pain. This can prevent a person from using the affected joint. Thus, many patients limp or have to use crutches or a wheelchair to move around.

The affected joint is usually swollen, warm to the touch, and appears red in colour. Other symptoms include fever, generalised weakness, and chills.

Who to See and Types of Treatments Available

The condition is diagnosed with the following:

  • Imaging tests - These tests are valuable tools in diagnosing conditions that affect the bones and joints. Doctors normally start with an x-ray. If abnormalities are found, more sensitive imaging tests are used to gather more information. These include a CT scan and an MRI.

  • Joint fluid analysis - Joint infection can be confirmed with this procedure. It uses a special type of needle to collect a small amount of fluid that surrounds the joint. The sample fluid is then sent to a lab for testing. If there’s an infection, the test will show abnormal levels of white blood cells.

  • Blood tests - The patient’s blood is also tested for infection. This test also uses a needle to collect a small amount of blood from a vein for testing.

Treatment involves draining the infected fluid surrounding the joint. This must be done immediately to stop further damage to the joint. This also helps prevent the infection from spreading to nearby tissues.

The procedure can be done using a needle or a flexible tube. However, some cases require open surgery. In general, doctors attempt to drain the fluid without making an incision. This is important because it reduces the risk of another infection. The infected fluid is drained two to three times per day. This can last between seven and ten days. If the infection does not clear up, open surgery is performed. It is also advised if nearby tissues are also infected and must be removed.

Antibiotics are also prescribed. These medications can be taken orally. However, in many cases, they are delivered via an IV. The therapy can last for up to two weeks.


  • Yokoe DS, Avery TR, Platt R, Huang SS. Reporting surgical site infections following total hip and knee arthroplasty: impact of limiting surveillance to the operative hospital. Clin Infect Dis. 2013 Nov. 57(9):1282-8.

  • Broy SB, Schmid FR. A comparison of medical drainage (needle aspiration) and surgical drainage (arthrotomy or arthroscopy) in the initial treatment of infected joints. Clin Rheum Dis. 1986 Aug. 12(2):501-22.

  • Corvec S, Illiaquer M, Touchais S, et al. Clinical features of group B Streptococcus prosthetic joint infections and molecular characterization of isolates. J Clin Microbiol. 2011 Jan. 49(1):380-2.

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