N.B.: MEDICAL EMERGENCY = treatment necessary within 48 hours to prevent irreversible permanent joint damage!

Risk factors: advanced age, immunocompromised state, rheumatoid arthritis, intraarticular injection, prosthetic joint

Transmission: inoculation ← trauma / recent instrumentation; bacteremia → hematogenous seeding to large joints of shoulder, hip, knee

Organism:

• Staphylococcus aureus (with 31% most common)
• Gonorrhea (multifocal septic arthritis in young adults; indistinguishable from tuberculous arthritis, but more rapid)
• Brucellar arthritis (indistinguishable from tuberculosis, slow infection)
• Salmonella (commonly associated with sickle cell disease / Gaucher disease)

1. neonates, infants: group D streptococcus
2. <4 years of age: Haemophilus influenzae, Streptococcus pyogenes, S. aureus
3. >4 years of age: S. aureus
4. >10 years of age: S. aureus, Neisseria gonorrhoeae
5. adults: S. aureus

Pathophysiology:

1. lytic enzymes in purulent articular fluid → destruction of articular + epiphyseal cartilages
2. pus in joint → increased intraarticular pressure → compromised blood flow → osteonecrosis

Age: most commonly encountered in neonates

Location: shoulder, hip, knee, elbow, ankle; multifocal involvement common in the very young

• painful joint, fever

Character of aspirated synovial fluid:

• frankly purulent / turbid fluid
• WBC >20,000 / mm³ with >90% PMNs
• positive result on Gram stain; positive culture for bacteria
• joint effusion

CT:

• bone erosions around joint
• fat-fluid level in the absence of trauma (SPECIFIC>)

MR:

• intense enhancement of hypertrophied synovium
• perisynovial edema of adjacent soft tissue
• subtle bone marrow signal alterations adjacent to articular surface: decreased on T1WI + increased on T2WI + enhancement ← reactive marrow edema (DDx: osteomyelitis with more obvious SI alterations)

Cx:

1. Bone growth disturbance (lengthening, shortening, angulation)
2. Osteonecrosis = avascular necrosis
3. Chronic degenerative arthritis
4. Ankylosis

DDx:

1. Transient synovitis (no MR-signal alterations in subarticular bone marrow)