Bones Fractures - Pathology

()Map MUSCULOSKELETAL SYSTEM (Map) BONES DEGENERATIVE

Fracture classification

• Traumatic - normal bone broken by excessive force
• Pathologic - abnormal bone broken by minimal or no trauma
  • E.g. due to osteomyelitis, bone neoplasia

• Closed - overlying skin and soft tissue is intact
• Compound - overlying skin and soft tissue are perforated
• Comminuted - bone is shattered at fracture site
• Compresses - the ends of the fracture are impacted into each other
• Avulsed - due to pull of a ligament
• Transverse
• Spiral

Fracture repair

Recent healing fracture (Image sourced from Bristol Biomed Image Archive with permission)

• Fracture
  • Ruptured blood vessels -> haemorrhage and clot formation, some ischaemic necrosis
    • Dead marrow liquefies and is phagocytosed
    • Dead bone is removed by osteoclasts
      • Continues long after bone has been united by callus
  • Periosteum tears, fragments displaced
• -> Haematoma formation, necrosis of any isolated fragments
• -> Mesenchymal cells proliferate in haematoma - granulation tissue
  • Invading cells: endothelial, fibroblasts, osteoprogenitor
• -> Loose collagenous tissue (primary callus)
• -> Mesenchymal cells differentiate to osteoblasts and chondroblasts
• -> Woven bone (secondary callus)
  • External callus - from periosteum
    • Periosteal growth eventually bridges the gap between the fracture ends
    • Blood supply is outstripped
    • Cartilage is produced instead of osteoid
    • Blood vessels invade cartilage -> endochondral ossification
    • Osteoid becomes ossified
  • Internal callus - from endosteum
    • Rarely forms cartilage
    • May occlude the medullary cavity
  • Osteoid remodelled by osteoclastic resorption
• -> Mature lamellar bone

• Early reactive fracture repair may be mistaken for osteosarcoma on biopsy

Complications of fracture repair

• Malnutrition
• Lack of adequate blood supply (leads to hypoxia)
  • Leads to excess cartilage in callus
  • Healing can still occur since this can turn to bone
• Excess movement
  • Leads to excess fibrous tissue in callus adn formation of false joint (below)
• Presence of necrotic bone (may form a sequestrum)
• Poor alignment
• Bacterial infection

Pseudoarthrosis

• False joint
• Non-osseus union
• From infection, poor imobilisation, avascularity, extensive tearing damage to periosteum
• Composed of fibrous tissue
• If mobility continues -> hyaline cartilage forms -> neoarthrosis (formation of new joint)
• In Osteodystrophia fibrosa, osteomalacia and rickets - callus forms but does not mineralise

Head and neck of femur fracture

• Vascularisation from joint capsule -> if complete separation -> bone and marrow undergo avascular necrosis
• Fibrovascular tissue and osteogenic cells invade -> deposit new bone
• Resorption of articular cartilage and subchondral bone -> osteoarthropathy commonly develops

Sore shins

• Dorsal metacarpal disease of racehorses
• In 2-3 year olds
• Excessive compressive stress on dorsal cortex of third metacarpal -> microfractures
• Hyperaemic overlying periosteum - -> new bone production to cover the lesion

Also see Repair in the Bones - Pathology