Causes

Infection occurs when foreign organisms (e.g. bacteria, viruses or fungi) enter the body and invade the host tissue. These organisms can enter the knee directly through an open wound, spread from infected areas nearby or travel to the joint via the blood stream.

Risk factors

Various factors may increase the risk of developing an infection in the knee.

Traumatic:

  • Cuts / Open Wounds
  • Animal / Insect Bites
  • Invasive medical procedures including surgery or injection

Non-traumatic:

  • Urinary Tract Infections
  • Fragile Skin (e.g. Eczema or Psoriasis)
  • Immunosuppressed (e.g. Rheumatoid Arthritis, Lupus, HIV)
  • Weakened Immune System (e.g. Chronic Kidney or Liver Disease)
  • Existing Joint Problems (e.g. knee osteoarthritis or gout)

Types of infection

Knee infections can be classified by the location of infection or the specific organisms involved:

Septic Arthritis is inflammation of a joint caused by a bacterial infection. The bacteria reach the inner lining of the joint (synovial membrane) and trigger an inflammatory response. Septic arthritis is considered a medical emergency as delayed treatment can lead to significant joint destruction, amputation or death. Septic arthritis commonly affects the knee and usually occurs in one joint only (monoarthritis).

Osteomyelitis is inflammation of a bone caused by an infectious organism. Osteomyelitis commonly affects the long bones that form the knee joint and is considered a medical emergency as the infection can spread to other areas of bone or the knee joint itself (septic arthritis).

Infective bursitis is inflammation of a fluid filled sac (bursa) outside the knee joint. The sac in front of the kneecap (prepatellar bursa) is particularly susceptible to infection in those that regularly kneel, as bacteria can enter the area through a lesion in the skin.

Lyme disease is a bacterial infection (Borrelia borgdorferi) transmitted by ticks. If the infection spreads to the knee it can trigger an inflammatory response in the joint, which is termed Lyme arthritis. The knee is the most commonly affected joint but multiple joints can be involved at the same time (oligoarthritis). In contrast to septic arthritis, Lyme arthritis may not cause significant knee pain.

Reactive arthritis (Reiter’s syndrome) is an inflammatory reaction in a joint, secondary to an infection in a different part of the body; it is important to note that reactive arthritis is not caused by an infection in the affected joint itself. The infection is typically based in the urinary or gastrointestinal system and when the immune system responds to defend the body, it may ‘accidentally’ attack normal joints. Individuals with the HLA-B27 gene are more likely to develop the condition.

Presentation

Infection triggers an inflammatory response and the associated signs/symptoms (pain, swelling, heat, redness and difficulty moving the joint) can develop rapidly within 24 hours. Individuals may have a fever (raised temperature) or feel generally unwell, but not all infections produce signs/symptoms (subclinical infection) and can remain in the body undetected. Gout and CPPD disease can present with similar symptoms and it is important to differentially diagnose these conditions.

Reactive arthritis classically presents with a combination of inflammatory signs/symptoms in the eyes, urinary system and knee joint, giving rise to the mnemonic ‘can’t see, can’t pee, can’t bend your knee’.

Click here to download the reactive arthritis infographic (printable version)

Diagnosis

Specific investigations are considered the gold standard for diagnosing different types of infection,

For definitive diagnosis of septic arthritis and infective bursitis, fluid samples are taken from the knee with a needle. In septic arthritis, the fluid is taken from inside the knee joint, while in infective bursitis the fluid is withdrawn from the swollen bursa. This joint fluid is then analysed using a specific laboratory test (gram stain and culture) to confirm the presence or absence of organisms.

Unlike septic arthritis, Lyme arthritis cannot be reliably diagnosed by analysing fluid taken from the joint. Instead, blood samples are taken from a vein and tested for specific antibodies produced by the body when the bacteria causing Lyme disease is present (IgB antibodies to B. burgdorferi). This test is called enzyme linked immunosorbent assay (ELISA) and those with positive or uncertain test results are subsequently tested by Western blotting to confirm the diagnosis.

Osteomyelitis is usually suspected following a combination of blood tests and imaging (XRay, ultrasound scan, MRI or CT) but definitive diagnosis requires biopsy of the bone and analysis (bone gram stain and culture).

There is no specific diagnostic test for reactive arthritis, but a combination of clinical signs/symptoms can be used to determine the likelihood of the condition. Individuals presenting with inflammation of the knee, eyes and urinary system are likely to have reactive arthritis. Since reactive arthritis is caused by an infection elsewhere in the body, swabs from the throat and urinary system, urine and stool samples or fluid drained from the affected knee joint can be analysed to detect an infectious organism.

Treatment

Once organisms are identified from the relevant testing procedures, appropriate antibiotics can be started to fight the infection. These antibiotics may be administered orally or through a vein (intravenously), with antibiotic treatment typically lasting 2-6 weeks. The knee joint may also require a surgical wash out (lavage) and removal of infected tissue (debridement), to ensure the organisms have been eliminated from the joint. In cases where there is risk of the infection spreading, amputation of the affected limb may be necessary.

In reactive arthritis, oral non-steroidal anti-inflammatories or intra-articular corticosteroid injections may be required to control inflammation. Medication to suppress the immune system may also be necessary to prevent further damage to the knee joint.

Recovery

Recovery time can vary depending on the severity of the infection.  Early treatment is vital to help minimise joint damage and reduce the spread of infection. Infections that are left untreated can progress to sepsis, a potentially life-threatening condition that causes multi organ failure.

Signposting

Patients should be referred to Accident and Emergency urgently if a knee joint infection is suspected.

Acknowledgements

Written by: Richard Norris & Daniel Massey, The Knee Resource

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