Fractures

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

Causes of Fracture

• The causes of fracture fit into two distinct categories:
  1. Fracture of trauma
     • Breakage of normal healthy bone due to excessive stress pressure of short duration.
  2. Pathological fracture
     • Breakage of bone weakened by some underlying metabolic, inflammatory or neoplastic condition.

Description of a Fracture

• There are various terms to describe a fracture's appearance.
• Separation of the ends of the fracture may be complete or incomplete.
• When there is no penetration of the overlying skin, a the fracture is described as closed.
• When the sharp ends penetrate the overlying skin, the fracture is compound.
  • In this scenario there is the danger of introducing infection.
• Comminuted describes a fracture where there are multiple small fragments of bone at the site of breakage.
• Where the edges of the fracture are impacted into each other, the fracture is said to be compressed.
• When one side of the fracture is depressed below the plane of the other, the term depressed is used.
  • This occurs in the flat bones of the skull.
• Microfractures are fractures that are only visible on histological section as cracks in the bone.
  • Grossly, there might be evidence of some haemorrhage in the area.

Fracture Repair

Fracture repair (Courtesy of BioMed Archive)

• On breakage, there is rupture of the periosteal, cortical and medullary vessels, causing:
  • A blood clot in the breakage area.
    • Fibrin is the important component.
  • Local necrosis of tissue supplied by these vessels.
    • This lowers the local pH.
• The fate of the blood clot depends upon its location.
  • The periosteal portion is lysed and disappears;
  • The medullary portion is removed by macrophages.
• The necrotic material is removed by phagocytosis.
  • Necrotic bone marrow is removed by macrophages.
    • This is a fairly rapid process.
  • Osteoclasts remove necrotic bone.
    • This is a slow process.
• On the periosteal side, the periosteum proliferates into the clot.
  • Forms a fibrous collar around the bone called the soft callus.
• The cells in the inner aspect of the soft callus, particularly those near the fracture fragments, differentiate into osteoblasts.
  

  Fracture callus (Courtesy of BioMed Archive)
  • Grow across the divide between the two fragments, laying down coarse woven bone.
• The woven bone laid is known as the hard callus.
  • This periosteal coarse bone is of utmost importance in repair.
    • It is responsible for much of the strength of the fracture repair.
  • This is replaced over a period of time by mature compact bone.
    • Aligns itself according to the stresses applied to it.
• Periosteal cells that are further away from the fracture fragments differentiate into cartilage-producing cells.
  • Produce a cone of cartilage between the two fragments.
    • As the local pH changes to more alkaline conditions, this cartilage undergoes calcification, with invasion by blood vessels and osteoblasts.
      • The cartilage is replaced by bone - endochondral ossification.
• On the medullary side, the endosteum proliferates and invades the clot, laying down bone.
  • This bone may totally occlude the medulla and is later remodelled to reconstitute a patent lumen.

Complications

• There are several possible complications that may arise in the repair of bone.

1. Inadequate immobilisation of the fractured ends will lead to incomplete repair by callus formation.
   • An intervening mass of fibrocartilage remains, forming a false joint.
     • In some cases the false joint can even appear to form a synovial lining.
   • If the fractured ends are sufficiently far apart, no substantial callus forms.
     • The intervening space is taken up by connective tissue organisation.
2. Failure to align the fractured ends in proper apposition to one another will produce excessive callus.
   • This takes a longer time to be remodelled by the adult compact bone.
3. Comminution delays healing due to persistent irritation.
4. Infection delays healing due to the effects of the toxins on theproliferating cells.
   • May give rise to a systemic infection affecting the rest of the body.