Septic Arthritis & Osteomyelitis

Septic Arthritis (infectious arthritis of a synovial joint), Osteomyelitis (infection of bone).

• These conditions are rare but can be life threatening (red flags). They are due to bacterial infection of the bone or joint and should be suspected with acute onset of: fever, unexplained limp, reluctance to use a limb or inability to weight bear, bone or joint pain with a hot swollen joint, bone or joint tenderness, or complete reluctance to move a joint or limb. 
• Early diagnosis and treatment are important to improve outcomes.
• The child with vertebral discitis can be difficult to diagnose and may be suggested by the child crying on spine flexion (e.g., nappy or diaper change).  
• These conditions may occur together, usually affect one joint but can be > 1 joint especially if the child is very young or immunosuppressed (by disease or treatment). With neonates and infants, they may not appear that unwell and may not always have a high fever. Careful examination and suspicion are important.
• Tuberculosis must always be suspected and especially with co-existent Human Immunodeficiency Virus [HIV].
• The risk of bone or joint infection is increased in sickle cell disease. Osteomyelitis is most frequently caused by Staph aureus, Strep pneumonia, Salmonella and Klebsiella. Long bones are most affected. Delayed diagnosis may lead to chronic osteomyelitis and avascular osteonecrosis. Septic Arthritis appears to be polyarticular, symmetrical, with a predilection for large joints and lower extremities. Pathogens are similar to osteomyelitis
• Travel history is important -  infections that can cause bone and joint infection or reactive arthritis vary across the world and with variable risk factors: e.g., Brucellosis from exposure to animals and unpasteurised milks (more common in the Middle East and Southern Europe), Lyme disease and exposure to ticks (more common in North Europe and North America), Arboviruses (e.g., Chikungunya, Zika and Dengue) are endemic in Asia, Latin America and Sub-Saharan Africa, Blastomycosis in Latin America.  The immunocompromised (from disease or treatment or co-existent infection with Human Immunodeficiency Virus [HIV] are at great risk.  More information about infections and their musculoskeletal features is available from the Centre for Disease Control and Prevention. 

Septic Arthritis - most common in the young (50% < 2 years), most commonly affects lower limb joints (knee, hip, ankle). Bacteria may reach the joint through blood spread (most common route), through the skin, or by local spread from an adjacent infected site. In neonates and infants bacterial infection can spread easily through blood vessels from bone into the joint as the metaphysis is intracapsular. In the adolescent with septic arthritis, and especially of the sternoclavicular joint or shoulder joint, IV drug abuse must be suspected. 

Osteomyelitis - is most common in infants and young children (< 2 years).  Infection tends to occur in the bone metaphysis. Most infections are blood spread from a primary site of infection (chest, ear, nose or throat, skin). Infection may also occur by direct inoculation (open wounds after trauma) or from local soft tissue infection.  Infection can be acute, subacute (over 2-3 weeks) or chronic. Abscesses may form in the bone. Chronic Recurrent Multifocal Osteomyelitis is a diagnosis of exclusion, is culture negative, and is now thought to be an autoinflammatory condition. 

The Table below lists the most common pathogens for both Septic Arthritis & Osteomyelitis (although pathogens are not always detected and blood or synovial cultures can be sterile).  Lyme disease is covered in the 'swollen joints' module.

Principles of management of septic arthritis and osteomyelitis 

• Prompt diagnosis, urgent washout and drainage of joint and rapid treatment with antibiotics.
• There is likely involvement of several clinical teams (paediatrics, orthopaedics, microbiology) to determine the management plan, whether to opt for bone aspiration / biopsy as well as joint aspiration, take the appropriate cultures and choice of antibiotics (given by intravenous route initially).
• There are usually high inflammatory markers (ESR, CRP, procalcitonin), high white cell count (neutrophils) and these guide clinical progress and response to treatment. CRP is a better predictor than ESR for acute infection.
• Radiographs may be normal or show fluid or soft tissue swelling or increased joint space. Bony changes may not be apparent for 2-3 weeks. Ultrasound may show joint effusion and is useful to aid aspiration. MRI is very sensitive to early changes and identify bone involvement. Isotope bone scan (increased uptake or hot spots) can be useful if MRI is not available.
• Antibiotics are usually given empirically whilst awaiting cultures. Antibiotics are usually continued for several weeks (IV) and then oral, pending clinical progress and blood tests (acute phase reactants - ESR, CRP - and white cell count).
• Analgesia and resting the joint / limb (splinting) are important.  

Pointers towards septic arthritis at the hip include Kocher's rules which can be useful in the child with hip effusion and differentiating between transient synovitis and septic arthritis. Kocher's rules include four factors as listed below. 

• Fever > 38.5 degrees centigrade.
• Non-weight bearing or pain with passive motion of the hip joint.
• ESR >40mm/hr.
• Serum White blood count > 12X10⁹/L.

The probability of Septic Arthritis increases with the number of these predictors being present; for example, zero factors present has a probability of septic arthritis being <0.2%. With 2 factors present the probability is 40% and with 3 factors present the probability is 93%. All 4 factors being present increases the probability of septic arthritis to >99%. 

It is noteworthy that Kocher's rules are not universally used (e.g., are not used in New Zealand) and are not useful to differentiate sepsis from acute rheumatic fever (ARF). In endemic areas for ARF, a monoarthritis is a common presentation and should be suspected with a raised ESR even if the white cell count is not elevated (see Further Reading).

Further Reading 

Royal Children's Hospital Melbourne Clinical Guideline (accessed April 2022): Bone and Joint Infection.