Osteomyelitis Quiz on Pathogenesis and Pathology

Study Notes

Lecture by Dr. Ahmed Roshdi, PhD/MD, Professor of Pathology, Sohag University
Topics covered: Introduction, Pathology of osteomyelitis, Pott's disease of vertebra, Healing of bone fracture

A long bone has:
- Spongy bone (trabecular)
- Compact bone (cortex)
- Medullary cavity (containing bone marrow)
- Epiphysis (end of the bone)
- Diaphysis (shaft of the bone)
- Metaphysis (region between epiphysis and diaphysis)
- Epiphyseal line (a remnant of growth plate)
- Periosteum (outer covering of bone)
- Endosteum (inner lining of bone)
- Nutrient vessels, foramina.
Bone is composed of woven bone and lamellar bone.
Cells in normal bone include osteoblasts (making bone), osteocytes (mature bone cells), and osteoclasts (bone resorption).

Diseases of bone include:
- Inflammatory (osteomyelitis)
- Dystrophies
- Tumours

Definition: Acute suppurative inflammation of bone and bone marrow, commonly in children.
Causative organisms: Most often Staphylococcus aureus, other bacteria like streptococci, Escherichia coli, Pseudomonas.
Types:
- Acute hematogenous OM: blood-born infection
- Acute non-hematogenous OM: direct spread of infection to bone.
Pathological features of acute hematogenous OM:
- Incidence: commonly affects children
- Route of infection: hematogenous route;
- Site: metaphysis of long bone (often site of minor trauma, as blood supply is slow),
- Pathogenesis:
  - Starts as a focus at the metaphysis of femur or tibia.
  - Infection spreads to medullary canal (bone marrow), periosteum (subperiosteal abscess) and joint space (less frequent).
  - Ischemia of bone due to inflammatory thrombosis of blood vessels.
  - Compression of blood vessels by inflammatory exudate and subperiosteal abscess.
  - Necrotic bone forms fragments (sequestra).
  - Periosteum stimulated to deposit new bone trabeculae (involucrum)
  - Pus discharged through sinus tracts (cloaca).
- Microscopic findings: fragmented bone trabeculae (sequestrum), inflammatory exudate, neutrophils, pus cells, and macrophages.
Complications: Toxemia, Septicemia, Septic thrombophlebitis, septic arthritis, chronic suppurative osteomyelitis, altered growth rate (due to involvement of epiphyseal plate), secondary amyloidosis, malignancy (e.g., squamous cell carcinoma) in sinus tract that becomes lined by squamous epithelium.

Definition: Acute suppurative inflammation of bone, commonly in adults.
Sites: Skull and long bones are common.
Etiology: Direct spread from an adjacent septic focus (e.g., sinusitis, otitis media, mastoiditis) or compound fracture of bone.
Pathology: Similar to acute hematogenous osteomyelitis, but affects diaphysis of long bones and does not typically show subperiosteal abscess formation.

Definition: Chronic, specific inflammation of bone (secondary TB).
Causative organism: Mycobacterium tuberculosis bacilli (human or bovine).
Route of infection:
- Hematogenous (bloodstream spread from TB in lung or intestine)
- Direct extension from adjacent infected joint or soft tissue.
Site: Vertebrae (Pott's disease) and ends of long bones.
Pathological features:
- Tuberculous infection first reaches Haversian canals and bone marrow.
- Inflammation expands with caseation necrosis.
- Bone destruction, replaced by caseous material.
- Multiple sinuses discharging caseous material through soft tissues and skin.
- Common involvement of joint spaces and intervertebral discs.
- Minimal or absent periosteal new bone formation.
- Microscopic findings: granulomatous inflammation (epithelioid cells, lymphocytes, Langhan's giant cells with caseation necrosis)

Definition: Secondary tuberculous infection affecting vertebrae.
Rout of infection: hematogenous spread from TB of the lung, intestine, or elsewhere.
Sites: Commonly affects lower thoracic and upper lumbar vertebrae. Vertebral bodies and intervertebral discs are destroyed; transverse processes and spines are spared.
Pathological features:
- Deformity: due to destruction of vertebral bone and collapse of vertebral bodies resulting in kyphosis and scoliosis.
- Cold abscess: caseous material collects under prevertebral fascia and spreads in various directions according to site of lesion. Appearance of abscess locations vary depending on affected vertebral level.
- Paraplegia: Compression on spinal cord (by inflammatory edema or cold abscess) or by collapse of vertebra

Definition: A regenerative process to restore damaged bone to its integrity.
Steps of healing:
1. Hematoma and inflammation: Due to tear of blood vessels, acute inflammation with scanty exudate. Macrophages and osteoclasts remove inflammatory products.
2. Soft callus and provisional callus: Granulation tissue formation (soft callus) followed by collagen and fibrocartilage deposition (provisional callus or woven bone).
3. Lamellar bone formation: Woven bone gradually removed by osteoclasts and replaced by osteoid matrix and collagen bundles, which then acquire lamellar arrangement and become calcified.
4. Remodeling of bone: excess bone cortex (external callus) or within medullary canal (internal callus) removed by osteoclasts. Bone marrow regeneration occurs.
Causes of delayed or improper healing:
- Faulty immobilization
- Infection at fracture site
- Impaired blood supply
- Interposition of soft tissues
- Nutritional disturbances, old age.