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 Osteomelitis
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

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
  - Immobilization » 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 localized
- 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

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
Mobilize as pain allows
- Traditional to immobilize 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