Septic Arthritis

Septic arthritis is infection of a native joint space.

Bacterial replication and the host inflammatory response can destroy articular cartilage rapidly, making septic arthritis both an infectious and orthopaedic emergency.

The knee is the most commonly affected large joint in adults, followed by the hip, shoulder, ankle and wrist.

Common organisms include:

• Staphylococcus aureus
• Streptococci
• Gram negative bacilli in selected hosts
• Neisseria gonorrhoeae in sexually active younger adults

Risk increases with:

• Older age
• Diabetes
• Immunosuppression
• Recent joint injection
• Skin infection
• Intravenous drug use
• Pre existing joint disease
• Bacteraemia

Gächter arthroscopic classification

Stage I

• Turbid joint fluid
• Synovial hyperaemia
• Possible petechiae
• No radiographic destruction

Stage II

• Severe synovitis
• Fibrin deposits
• Purulent fluid
• No established radiographic destruction

Stage III

• Thickened synovium
• Adhesions
• Compartment or pouch formation

Stage IV

• Aggressive pannus
• Cartilage invasion and destruction
• Subchondral osteolysis, erosions or cystic changes

Stages I and II, and many stage III infections, can usually be managed arthroscopically. Advanced stage III and stage IV disease may require open debridement.

Typical presentation is:

• Acute severe joint pain
• Restricted active and passive movement
• Effusion
• Warmth
• Swelling
• Fever, which may be absent

Pain with passive motion in all directions strongly suggests an intra articular process.

The absence of fever does not exclude infection.

Any acutely painful swollen joint should be treated as possible septic arthritis until infection is adequately excluded.

Differential diagnoses include:

• Gout
• Pseudogout
• Haemarthrosis
• Reactive arthritis
• Inflammatory arthritis
• Osteoarthritis flare

Crystals in synovial fluid do not completely exclude simultaneous bacterial infection.

Joint aspiration

Aspiration is the central diagnostic investigation.

Send synovial fluid for:

• White blood cell count
• Differential
• Gram stain
• Aerobic and anaerobic culture
• Crystal analysis

A synovial WBC above approximately 50,000 cells/µL with a high neutrophil percentage strongly supports infection, but lower values do not exclude septic arthritis.

Culture is more important than any single cell count threshold.

Obtain blood cultures when systemic infection or haematogenous spread is possible.

Except in sepsis or septic shock, obtain synovial fluid and blood cultures before starting antibiotics.

Imaging

Plain radiographs: baseline joint status and exclude fracture or advanced destructive disease.

Ultrasound: detects effusion and assists aspiration, particularly hip.

MRI: useful for:

• Adjacent osteomyelitis
• Deep joint infection
• Abscess
• Sacroiliac infection
• Difficult diagnostic cases

Do not delay drainage of an obviously septic accessible joint solely to obtain MRI.

Management requires:

• Joint drainage
• Antimicrobial therapy
• Treatment of bacteraemia or primary source
• Early rehabilitation once infection is controlled

Empirical antibiotics

After cultures in a stable patient:

A practical adult regimen where MSSA and streptococci predominate is:

Cefazolin 2 g IV every 8 hours

plus, where significant Gram negative risk exists:

Ceftriaxone 2 g IV every 24 hours.

If MRSA risk is significant:

Vancomycin IV, using weight based loading when severe and subsequent AUC guided dosing.

In septic shock, begin broad therapy immediately after blood cultures without waiting for joint aspiration if this would delay resuscitation.

Drainage

Large joint septic arthritis usually requires operative washout.

Arthroscopic lavage and debridement is appropriate for most knee, shoulder and selected hip infections.

Perform:

• Complete joint inspection
• Removal of pus
• Fibrin debridement
• Synovectomy according to severity
• Thorough saline irrigation

Advanced infection with:

• Dense adhesions
• Extensive cartilage destruction
• Bone involvement

may require open arthrotomy.

Hip septic arthritis

The adult native hip usually requires urgent operative drainage because:

• Joint is deep
• Needle aspiration alone gives unreliable source control
• High intra articular pressure threatens cartilage and femoral head

Repeat washout

Failure to improve after drainage should prompt urgent reassessment.

Concerning features are:

• Persistent fever
• Persistent or rising CRP
• Ongoing purulent drainage
• Persistent bacteraemia
• Reaccumulating effusion
• Continued severe joint pain
• Deteriorating systemic status

These indicate possible inadequate source control and may require repeat arthroscopy or arthrotomy.

Antibiotic duration

After adequate drainage and clinical improvement, IV treatment can often be continued for approximately 1 to 2 weeks, followed by an active oral agent.

A further 2 to 4 weeks of oral therapy is a reasonable reference for uncomplicated adult native joint infection.

Longer therapy is required with:

• Adjacent osteomyelitis
• Endocarditis
• Difficult organisms
• Immunosuppression
• Inadequate source control

Rehabilitation

Begin joint motion as soon as:

• Infection is controlled
• Drains are removed
• Wound permits movement

Prolonged immobilisation causes severe stiffness and muscle wasting.

Major complications include:

• Rapid cartilage destruction
• Secondary osteoarthritis
• Osteomyelitis
• Joint stiffness
• Instability
• Sepsis
• Death
• Need for later arthroplasty