Septic arthritis

Question 1

What causes it?

Answer 1

It results from the infection of the joint with pyogenic organisms, most commonly Staphylococcus aureus. Gram-negative organisms are more common in the elderly or immunosuppressed. Joints become infected by direct injury or by blood borne infection from an infected skin lesion or other site.

Question 2

What risk factors are there?

Answer 2

Prosthetic joints
pre-existing joint disease
recent intra-articular steroid injection
diabetes mellitus.

Question 3

How does it present?

Answer 3

Hot, painful swollen, red joint, which has developed acutely.

There may be a fever and evidence of infection elsewhere.

In the elderly and immunosuppressed and in RA the articular signs may be muted and a high index of suspicion is needed to make the diagnosis.

In 20% the septic arthritis involves more than one joint. Prosthetic joint infections may be early (within 3 months), or delayed/late.

Early infection presents with wound inflammation or discharge, joint effusion, loss of function and pain. Late disease presents with pain or mechanical dysfunction.

Question 4

Investigations

Answer 4

Joint aspiration and synovial fluid analysis:

• WCC
• Gram stain (but may be negative in bacterial infection) and culture
• Polarized light microscopy for crystals (in gout and pseudo gout)

Bloods: FBC, erythrocyte sedimentation rate, CRP, blood cultures

X-rays of the affected joint are of no value in the diagnosis because normal initially. Loosening or bone loss around a previously well, fixed prosthetic implant suggests infection.

Swab of urethra, cervix and anorectum if gonococcal infection a possibility

Question 5

Treatment

Answer 5

Pending sensitivities: Flucloxacillin 1-2g 6 hourly i.v. (erythromycin or clindamycin if penicillin allergic) and oral fusidic acid 500mg 8 hourly.

Add gentamicin in immunosuppressed patients to cover Gram-negative organisms.

Modify treatment depending on culture and sensitivity and continue with two antibiotics for 6 weeks (initial 2 weeks IV).

Adequate joint drainage: by needle aspiration, arthroscopy or open drainage.

Always refer infection of a prosthetic joint to orthopaedic surgeons.

Immobilise joint in acute stages, mobilize early to avoid contractures. NSAIDs for pain relief.

Question 6

Conditions that would present similarly

Answer 6

Fibromyalgia
Inflammatory arthritis
Osteoarthritis
Fracture of bone adjacent to joint, major ligamentous injury (recent + old injuries)
Bursitis
Cancer