Skeletal Infections

Skeletal Infections Overview - Germs Get Going

• Definition: Microbial invasion of bone (osteomyelitis), joints (septic arthritis), or soft tissues.
• Common Culprits:
  • Staphylococcus aureus (most common overall).
  • Streptococci (neonates, cellulitis).
  • Salmonella (sickle cell disease).
  • Pseudomonas aeruginosa (IV drug users, puncture wounds).
  • Mycobacterium tuberculosis (spinal - Pott's disease).
• Routes of Infection:
  • Hematogenous spread (most common in children).
  • Direct inoculation (trauma, surgery).
  • Contiguous spread (adjacent soft tissue infection).

⭐ Staphylococcus aureus is the most common organism causing acute osteomyelitis across all age groups, except in specific conditions like sickle cell anemia (Salmonella) or IV drug use (Pseudomonas).

Osteomyelitis Imaging - Bone's Battleground

• X-ray (XR): Initial. Soft tissue swelling (early). Periosteal reaction, lytic lesions, sclerosis (10-21 days).
  • Chronic: Sequestrum (necrotic bone), involucrum (new bone shell), cloaca (opening).
• MRI: Most sensitive & specific for early diagnosis & extent.
  • T1: ↓ signal in marrow.
  • T2/STIR: ↑ signal (edema).
  • Post-contrast: Enhancing phlegmon/abscess.
• CT: Defines cortical destruction, sequestra, gas.
• 📌 SICK bones: Sequestrum, Involucrum, Cloaca, Know it's chronic!

⭐ MRI is the imaging modality of choice for detecting osteomyelitis in its early stages (within 24-48 hrs) and for delineating soft tissue involvement.

Septic Arthritis Imaging - Synovial Showdown

• Early Signs (X-ray may be normal for 7-10 days):
  • Soft tissue swelling
  • Joint space widening (effusion)
• Later Signs (X-ray):
  • Juxta-articular osteoporosis
  • Joint space narrowing (cartilage destruction)
  • Subchondral bone erosions (marginal or central)
  • Periosteal reaction
• Ultrasound (USG):
  • Detects joint effusion (earliest sign)
  • Guides aspiration
  • Synovial thickening, hyperemia (Doppler)
• MRI:
  • Most sensitive & specific
  • Shows synovial enhancement, marrow edema, abscess

⭐ MRI is the gold standard for early diagnosis and assessing extent, especially for sacroiliitis or sternoclavicular joint involvement. Shows marrow edema, synovial enhancement, and fluid collections effectively.

Spinal Infections - Spinal Sabotage

• Spondylodiscitis: Infection of intervertebral disc (IVD) & adjacent vertebral bodies.
  • Pyogenic: Staphylococcus aureus (most common); rapid, destructive.
  • Granulomatous: Tuberculosis (Pott's spine); insidious, slow.
• Pott's Spine (TB Spondylitis):
  • Thoracic spine most affected.
  • Key signs: Paradiscal vertebral destruction, anterior wedging, cold abscess, gibbus deformity.
  • Risk: Neurological deficits due to cord compression.

⭐ MRI is the gold standard for early diagnosis, assessing extent (e.g., epidural abscess), and guiding management of spinal infections.

Specific Infections & Mimics - Unique Unrest

• Tuberculosis (Pott's Spine): Thoracolumbar. Anterior vertebral body destruction, paraspinal (cold) abscess, gibbus. Disc spared early. Rice bodies.
• Brucellosis: Spondylodiscitis, sacroiliitis (unilateral common). Pedro Pons' sign (lumbar osteophyte). Disc often preserved.
• Fungal (Maduromycosis): "Dot-in-circle" sign. Chronic, foot.
• Hydatid Disease: Expansile lytic lesions, daughter cysts.
• Mimics: LCH (vertebra plana), SAPHO.

⭐ TB spine: disc typically spared early vs. pyogenic (rapid disc destruction).

High‑Yield Points - ⚡ Biggest Takeaways

• Staph. aureus: most common in pyogenic osteomyelitis.
• X-ray signs (periosteal reaction) in acute osteomyelitis are late (10-14 days).
• MRI: most sensitive for early osteomyelitis detection and extent.
• Brodie's abscess: chronic, localized osteomyelitis; lytic lesion, sclerotic rim.
• Chronic osteomyelitis: characterized by sequestrum, involucrum, cloaca.
• TB spondylitis (Pott's spine): thoracolumbar, vertebral destruction, cold abscess.
• Neonatal osteomyelitis: often epiphyseal, may cause septic arthritis.