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
Several factors increase the risk of developing an infection after trauma. These include:
- Cuts and open wounds/fractures
- Penetrating injuries
- Contaminated penetrating objects
- Animal or insect bites
- Invasive medical procedures including surgery or injection
- Underlying medical conditions (e.g., diabetes, compromised immune system)
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 sacs in front of the kneecap (prepatellar or infrapatellar bursa) are particularly susceptible to infection after direct contact or in those that regularly kneel, as bacteria can enter the area through a lesion in the skin.
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 pus discharging from the wound, a fever (raised temperature) or feel generally unwell, but not all infections produce signs/symptoms (subclinical infection) and can remain in the body undetected.
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.
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).
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.
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 could cause 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