QuestionSBA
QuestionChoice
A knee X-ray is often requested to assess for other differentials, such as chondrocalcinosis/fractures, or to assess if there has been any joint damage secondary to infection. However, this does not represent the most important initial investigation since a diagnosis of septic arthritis needs to be ruled out using a joint aspirate sent for microscopy, culture and sensitivity.
52984
c
Knee X-ray
QuestionChoice
Gout or pseudogout can present with an acutely hot and painful joint. However, it is not typical for the paediatric age range. Moreover, given the presentation, septic arthritis is the most important diagnosis to exclude; thus, an aspirate for microscopy, culture and sensitivity is the best first test.
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b
Joint aspiration and electron microscopy with polarised light
QuestionChoice
The most important diagnosis to exclude is septic arthritis. This happens to be the most likely diagnosis in this case due to the fever, inability to bear weight and recent chest infection, which could have been the origin of the organism that has seeded haematologically to the joint. A joint aspirate should be done with cultures to isolate the organism. Empirical antibiotics should be commenced while awaiting the results.
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a
Joint aspiration and microscopy, culture and sensitivities
QuestionChoice
This would be considered if a diagnosis of septic arthritis were confirmed. However, at this stage, the diagnosis is uncertain; thus, performing an aspirate in the first instance is the next best step.
52986
e
Send to theatre for surgical washout
QuestionChoice
A knee MRI would be a sensible investigation if there was a suspicion of an alternative diagnosis or if the joint aspiration were inconclusive. However, it would not represent an initial investigation.
52985
d
Knee MRI
QuestionComment
Septic arthritis has systemic upset
Transient synovitis is just ouch hurt after viral infection
24800
User
subscriber
Intravenous Prone
QuestionComment
overlying cellulitis is a contraindication to joint aspiration though...
43998
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subscriber
Botox Haemophilus
Concept
Chapter
# Summary

Septic arthritis is an infection of the joint characterized by acute inflammation and swelling. It is caused by a bacterial or viral pathogen that infects the synovial fluid. The most commonly implicated organism is Staphylococcus aureus. Noteworthy clinical signs include a tender, swollen joint with reduced mobility, often accompanied by systemic illness. Key investigations include joint aspiration for Microscopy Culture and Sensitivity, along with blood tests showing increased white blood cell count and elevated ESR/CRP. Management typically involves IV antibiotics, joint washout under general anaesthesia, and physiotherapy once the acute infection has resolved.

# Definition

Septic arthritis is an infection of the joint, specifically the synovial fluid. It is typically caused by a bacterial or viral pathogen and necessitates prompt medical intervention due to the high risk of joint damage and other severe complications.

# Epidemiology

Septic arthritis has an annual incidence of 4-10 cases per 100,000 patients in Western Europe. It can affect individuals of any age, though certain populations are at a higher risk due to underlying conditions.

# Aetiology

The most prevalent organism implicated in septic arthritis is Staphylococcus aureus. Other responsible organisms include:

-   Gonococcus: primarily in sexually active individuals
-   Streptococcus spp.
-   Gram-negative bacilli

Risk factors contributing to septic arthritis include:

-   Pre-existing joint diseases such as rheumatoid arthritis
-   Chronic kidney disease
-   Immunosuppressive states
-   Presence of prosthetic joints

# Signs and Symptoms

The clinical presentation of septic arthritis usually involves:

-   Acute onset of tender, swollen joint
-   Reduced range of joint movement
-   Systemic symptoms such as fever, malaise, or chills

# Differential Diagnosis

Differential diagnoses for septic arthritis include:

-   Gout and pseudogout: characterized by intense joint pain, redness, and swelling, often in the big toe or knee.
-   Osteoarthritis: presents with joint pain, stiffness, and sometimes swelling, generally improving with movement.
-   Rheumatoid arthritis: marked by joint pain, swelling, and stiffness, often symmetrical and worse after rest.
-   Lyme disease: may show erythema migrans rash, flu-like symptoms, and possibly migratory joint pains.

# Investigations

Diagnostic investigations for septic arthritis include:

-   Joint aspiration for Microscopy, Culture, and Sensitivity: The aspirate usually appears turbid and yellow, resembling pus.
-   Blood tests: elevated white cell count, high ESR/CRP
-   Blood cultures: to identify causative organisms
-   Imaging: X-ray of the joint may be performed to evaluate for osteomyelitis or other complications.

# Management

The treatment of septic arthritis involves:

-   IV antibiotics guided by local antibiograms and susceptibilities
-   Consideration of joint washout under general anaesthesia to remove infected material
-   Physiotherapy following the resolution of acute infection to restore joint function

Complications of septic arthritis can include:

-   Osteomyelitis: infection of the bone
-   Chronic arthritis: persistent joint inflammation
-   Ankylosis: joint fusion resulting in immobility

# References

[Click here for the BNF Treatment Summary for Musculoskeletal infections](https://bnf.nice.org.uk/treatment-summary/musculoskeletal-system-infections-antibacterial-therapy.html)

[Click here for the BMJ Best Practice Summary of Septic Arthritis](https://bestpractice.bmj.com/topics/en-us/486)
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Septic Arthritis
Topic
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Orthopaedics
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In cases of suspected septic arthritis, joint aspiration is crucial for diagnosis and guiding appropriate antibiotic therapy.
A 14-year-old adolescent presents with a red-hot swollen knee joint after a chest infection last week. He has been unable to walk due to the pain in his knee despite analgesia. He has a background history of type 1 diabetes managed with an insulin pump. On examination there is erythema overlying the joint and pain whenever the joint is moved in any direction. His temperature is 38.3 °C.

What is the most important next step?
a