<unk>  is most commonly caused by a bacterial infection. Bacteria can enter the joint by: * The bloodstream from an infection elsewhere (most common) * Direct penetration into the joint (arthrocentesis, arthroscopy, trauma) * A surrounding infection in the bone or tissue (uncommon, from osteomyelitis,<unk>  bursitis, abscess). Microorganisms in the blood may come from infections elsewhere in the body such as wound infections, urinary tract infections, meningitis or endocarditis. Sometimes, the infection comes from an unknown location. Joints with preexisting arthritis, such as rheumatoid arthritis, are especially prone to bacterial arthritis spread through the blood. In addition, some treatments for rheumatoid arthritis can also increase a person's risk by causing an immunocompromised state. Intravenous drug use can cause endocarditis that spreads bacteria in the bloodstream and subsequently causes<unk> . Bacteria can enter the joint directly from prior surgery, intraarticular injection, trauma or joint prosthesis. Risk factors In children, although<unk>  occurs in healthy children and adolescents with no co-occurring health issues, there are certain risk factors that may increase the likelihood of acquiring<unk> . For example, children with renal osteodystrophy or renal bone disease, certain hematological disorders and diseases causing immune suppression are risk factors for childhood<unk> . The rate of<unk>  varies from 4 to 29 cases per 100,000 person-years, depending on the underlying medical condition and the joint characteristics. For those with a<unk>  joint, 85% of the cases have an underlying medical condition while 59% of them had a previous joint disorder. Having more than one risk factor greatly increases risk of<unk> . * Age over 80 years * Diabetes mellitus * Osteoarthritis * Rheumatoid arthritis. Risk of<unk>  increases with anti-tumor necrosis factor alpha treatment. * Immunosuppressive medication * Intravenous drug abuse * Recent joint surgery * Hip or knee prosthesis and skin infection * HIV infection * Other causes of sepsis Organisms Most cases of<unk>  involve only one organism; however, polymicrobial infections can occur, especially after large open injuries to the joint.<unk>  is usually caused by bacteria, but may be caused by viral, mycobacterial, and fungal pathogens as well. It can be broadly classified into three groups: non-gonococcal arthritis, gonococcal arthritis, and others. *Non-gonococcal arthritis – These bacteria account for over 80% of<unk>  cases and are usually staphylococci or streptococci. Such infections most commonly come from drug abuse, cellulitis, abscesses, endocarditis, and chronic osteomyelitis. Methicillin-resistant Staphylococcus aureus (MRSA) may affect 5 to 25% of the cases while gram negative bacilli affects 14 to 19% of the<unk>  cases. Gram negative infections are usually acquired through urinary tract infections, drug abuse, and skin infections. Older people who are immunocompromised are also prone to get gram negative infections. Common gram negative organisms are: Pseudomonas aeruginosa and Escherichia coli. Both gram positive and gram negative infections are commonly spread through the blood from an infective source; but can be introduced directly into the joint or from surrounding tissue. It often affects older people, and often happens suddenly, involving only one joint. Joint aspiration cultures are positive in 90% of cases, while only 50% of blood cultures yield any organisms. *Gonococcal arthritis – Neisseria gonorrhoeae is a common cause of<unk>  in people who are sexually active and under 40 years old. The bacteria is spread through the blood to the joint following sexual transmission. Other symptoms of disseminated gonococcal infection can include migration of joint pain, tenosynovitis and dermatitis. Synovial fluid cultures are positive in 25 to 70% of the cases while blood cultures are seldom positive. Apart from blood and joint cultures, swabs from urethra, rectum, pharynx, and cervix should also be taken. Polymerase chain reaction (PCR) is another useful way of identifying gonococcal infections if diagnosis is difficult and clinical presentation is similar to reactive arthritis. *Others – Fungal and mycobacterial infections are rare causes of<unk>  and usually have a slow onset of joint symptoms. Mycobacterial joint infection most commonly affects hip and knee joints, caused by reactivation of past mycobacterial infections, with or without signs and symptoms of tuberculosis in lungs. Synovial fluid cultures will be positive in 80% of the cases. However, acid fast smears are not useful. Histology is not specific to myocobacterial infection as there are other granulomatous diseases that can show similar histology. Borrelia burgdorferi, a bacterium that causes lyme disease, can affect multiple large joints such as the knee. Confirmation of Lyme disease is done through enzyme-linked immunosorbent assay (ELISA) followed by confirmation using Western Blot test. It cannot be cultured from synovial fluid. However, PCR testing yields 85% positive result from synovial fluid. Viruses such as rubella, parvovirus B19, chikungunya, and HIV infection can also cause<unk> . *Prosthetic joint infection – Artificial joint infection are usually caused by coagulase negative Staphylococci, Staphylococcus aureus, and gram negative bacilli. Concurrent infections by multiple organisms is also reported in 20% of the cases. The risk factors of prosthetic joint infections are: previous fracture, seropositive rheumatoid arthritis, obesity, revision arthroplasty, and surgical site infections. List of organisms * Staphylococci (40%) *Staphylococcus aureus – the most common cause in most age groups. Can be caused by skin infection, previously damaged joint, prosthetic joint or intravenous drug use. * coagulase-negative staphylococci – usually due to prosthetic joint * Streptococci – the second-most common cause (28%) *Streptococcus pyogenes – a common cause in children under 5 * Streptococcus pneumoniae * Group B streptococci – a common cause in infants * Haemophilus influenzae * Neisseria gonorrhoeae – the most common cause of<unk>  in young, sexually active adults. Multiple macules or vesicles seen over the trunk are a pathognomonic feature. * Neisseria meningitidis * Escherichia coli – in the elderly, IV drug users and the seriously ill * Pseudomonas aeruginosa – IV drug users or penetrating trauma through the shoe * M. tuberculosis,<unk> lmonella spp. and Brucella spp. – cause<unk>  spinal arthritis * Eikenella corrodens – human bites * Pasteurella multocida, bartonella henselae, capnocytophaga – animal bites or scratches * Fungal species – immunocompromised state * Borrelia burgdorferi – ticks, causes lyme disease * Spirillum minus, Streptobacillus moniliformis– rat bites
Septic arthritis
Osteoarthritis
Knee arthritis
Gout
Compression arthralgia
Rheumatoid arthritis
Inflammatory arthritis
Reactive arthritis