Septic Arthritis

Definition

  • Joint inflammation due to infection by micro-organisms

Incidence

  • All age groups
  • Usually children
  • 50% < age 3
  • M=F
  • Any joint
  • Knee is most common joint affected overall
    • Infants
      • Hip
    • Children
      • Knee
    • Adults
      • Large Joints
    • IVDU
      • SCJ & SIJ

Pathogenesis

  • Two Routes
    • Haematogenous
      • Distant focus
      • Seeds synovial membrane
    • Direct
      • Extension
        • Osteomyelitis
          • Neonates (70% have OM & septic joint) & children
          • From adjacent focus of OM
          • Intra-articular metaphysis
            • Proximal femur, humerus, elbow
    • Transphyseal vessels in neonates
    • Overlying Soft tissue Infection
      • Inoculation
      • Penetrating injury
      • Iatrogenic

Predisposition

  • Three factors
    • Host
      • Immunodeficiency from diabetes, alcohol, malnutrition, etc
    • Site
      • Previous joint trauma, RA, previous HCLA, etc
    • Bacteria

Aetiology

  • Bugs vary with age
    • Staphylococcus aureus most common
    • Unknown in 1/3
  • Community vs Hospital-acquired
  • Neonates
    • Hospital-acquired 60%
      • E coli & Candida
      • S aureus
      • Group B Streptococcus
      • Typically premature or unwell
    • Community-acquired
      • S aureus
      • Group B Streptococcus
      • Usually well
      • 2/3 Osteomyelitis
  • Infants & Children < 3 years
    • Staphylococcus aureus > Haemophilus Influenzae
    • Streptococcus pyogenes
    • Streptococcus pneumoniae
    • E coli
    • Proteus
    • Pseudomonas
  • Children >3 years
    • Like adults
  • Adults
    • Staphylococcus aureus > Streptococcus (pyogenes & pneumoniae) > Gram negative (E coli, Proteus, Klebsiella, Pseudomonas)
    • IVDU – Gram negative
    • Sexually active – Neisseria gonorrhoeae
    • Prosthetic joint – Staph epidermidis

Pathology

Septic Arthritis, gram stain of staphylococcus aureus, the most common cause of septic arthritis
Gram stain of Staphylococcus aureus, the most common cause of septic arthritis. Image by Dr Graham Beards, Wikimedia Commons, CC BY-SA 4.0.
  • Synovium oedematous & hyperaemic
  • Increased cloudy synovial fluid
  • 2/7
    • Frank pus
    • Cartilage destruction
      • Starts at areas of joint contact
  • Synovial membrane replaced by granulation tissue
  • Adhesions wall off pockets of pus
  • Physis destroyed if intra-capsular
  • Fibrous ankylosis
  • Joint dislocation
  • AVN femoral head
  • Cartilage Destruction
    • First glycosaminoglycans of cartilage matrix, later collagen itself
      • Enzymes
        • From WC / Bugs / Cartilage itself
          • Lysosomal enzymes (PG)
          • Collagenases (collagen)
          • Metalloproteases (proteins)
    • Staphylokinase activates plasmin
    • Pressure
      • Unloading/ overloading with immobilisation » degrades cartilage
      • AVN
      • Dislocation
    • Mechanical
      • Immobilisation » lack of cartilage nutrition
    • End result = Cartilage Fragmentation

Clinical Features

  • Infant
    • History prior infection
      • Eg. Umbilical sepsis
    • Irritability
    • Failure to thrive
    • Low fever ~ Beware
    • Most often affect Hip
    • Joint warm, swollen, & tender
    • Decreased active ROM
    • Decreased passive ROM
    • Muscle spasms
    • Intra-articular pressure
      • Hip abducted 45°/ Flexion 15°/ External Rotation 15°
      • Knee flexion 20°
  • Child
    • As above, but more localised
    • Most often affect Knee
    • 10% involve more than one joint
  • Adult
    • 50% have pre-existing arthritis
    • 30% have Hx of trauma
    • RA may have multiple joint involvement

Investigations

Septic Arthritis, radiograph of the left hip in septic arthritis caused by melioidosis
Radiograph of the left hip in septic arthritis caused by melioidosis. Image by N. P. Weerasinghe, H. M. M. Herath and T. M. U. Liyanage, Wikimedia Commons, CC BY 4.0.

ESR

  • Usually > 100
  • Not reliable in
    • First 48/24
    • Neonate
    • Steroids
  • Takes weeks to drop
    • Lags behind resolution

WCC

  • Often normal
  • Neutrophils +++
  • PMN leukocytosis

Blood Culture

  • Positive 60%

Aspiration

  • Essential investigation Knee/ Elbow / GHJ
  • ASAP
  • Except Neonate Hip
    • Drain ASAP
    • As aspiration difficult & need GA
  • MCS & Cell count

Synovial Fluid

  • Yellow / Grey / Opaque
  • WCC > 50 000 x 10e6/L (usually > 100 000/mm3)
  • Neutrophil usually > 75%
  • Glucose < BSL
  • Protein up usually 6-8g/dL
  • ± Crystals as acid pH decreases solubility
  • Bugs in 30%
  • Culture positive 60%

XR

  • Soft tissue swelling
  • Narrow joint space if > 1/52
  • Later subchondral osteoporosis
  • End-stage bone destruction seen

Te99 Scan

  • Can differentiate from OM
  • Excellent screen

USS

  • Zarwin Radiology 1993
    • 96 patients with painful hips
    • None of 40 infected hips had normal USS

Neonate Hip

  • Wide joint space ± Subluxed on XR
  • Primary OM in metaphysis
  • Metaphyseal rarefaction
  • Periosteal new bone formation
  • ± Erosion of proximal epiphysis
  • Fluid in joint on USS
  • Te Scan differentiate focal metaphyseal OM
    • Also cold if head dead

Differential Diagnoses

Infants & Child

  • Acute OM
    • Usually less ↓ ROM
    • Can get symptomatic effusion
    • 70% both
  • Transient Synovitis
    • More ROM
    • Afebrile
    • ESR normal
  • JRA
    • Gradual onset
    • Less systemic features
    • More ROM
  • Perthes
  • Trauma
  • Haemophilia
  • Cellulitis

Adults

  • Gout
    • Especially in knee
    • Peri-articular punched-out lesions
  • Pseudogout
    • Meniscal & articular calcium
  • RA

Management

Antibiotics

  • Start after M/C/S
  • Start broad-spectrum bacteriocidal
    • Gram stain as guide
    • Flucloxacillin & Gentamicin in Adults
    • Flucloxacillin & Ceftriaxone 40mg/kg/dose q24h IV in Children
  • IVAB until systemic toxicity & local swelling subside & CRP normal
    • ~ 2/52
  • Then 6/52 oral AB

Drainage

  • Surgical emergency
  • Arthrotomy or Arthroscopy
    • Small capsulectomy
    • Washout pus
    • ± Synovectomy
  • Close skin only
  • Splint joint in position of function
    • Pain relief
  • Mobilise as pain allows
    • Traditional to immobilise joint to relieve pain in acute phase of disease
    • Salter et al 1981 advocates early passive ROM (via CPM) based on rabbit studies which show that this
    • Improves cartilage nutrition
    • Stimulates chondrocytes to synthesize matrix components
    • Prevents adhesions
    • Helps clear lysosomal enzymes & purulent exudate

Prognosis

  • Worse if
    • Treatment delayed (most important)
    • Hip
    • Associated OM
    • Staphylococcus aureus or multiple organism
    • Neonate as diagnosis delayed & usually hip

Outcomes of Septic Arthritis (of Hip)

  • Normal
  • Growth disturbance
    • Coxa magna
    • Coxa breva
    • LLD
  • Subluxation
  • Dislocation
  • Destroyed epiphysis
    • Secondary Osteoarthritis
  • Ankylosis
  • “Tom Smith arthritis”
    • Arthritis of the hip following neonatal septic arthritis
    • From umbilical manipulation?
    • Due to delayed diagnosis

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.