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Fracture Education

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Biomechanical differences between adult and child

  • Why do children's bones bend before they break?

    The osteoid density of a child's bone is less than an adult's. Juvenile bone is more porous than adult bone because the Haversian canals occupy a much greater part of the bone (Figure 4). This is the principal reason a child's bone can bend more than an adult's bone.

     

    Figure 4: Children's bones are more porous than adults. The Haversian canals occupy a larger space in bone of a child.

     

    Fracture patterns

    The ability to bend before breaking leads to unique fracture patterns in children including:

    Buckle injury

    Failure of a child's bone in compression results in a "buckle" injury, also known as a "torus" injury. These most commonly occur in the distal metaphysis, where porosity is greatest.

     

     
       

     

    Plastic bowing (or deformation)

    Long bones may bend without breaking the cortex. Children's bones can be bent to 45 degrees before the cortex is disrupted and a greenstick or a complete fracture occurs. However if the bending force is released the bone may only partially return to its pre-bent position, resulting in plastic bowing.

    The bones most commonly affected by plastic bowing are the ulna and fibula. In the ulna, plastic bowing may indicate a Monteggia fracture-dislocation. Because there is no obvious fracture of the ulna, the injury is frequently not diagnosed, resulting in delayed diagnosis and long-term morbidity.

    Plastic deformation of ulna 

     

    Monteggia fracture-dislocation
     
     
      
     
     

     

    Greenstick fracture

    A greenstick fracture occurs when there is sufficient energy to start a fracture but insufficient energy to complete it. The cortex fails on the tension side and the cortex on the compression side bends but remains intact. The degree of deformity is variable and sometimes complete reduction can only be achieved by deliberately completing the fracture i.e. fracturing the concave cortex, which is bent but not broken.

     

     

     

    Complete fractures

    In children, the outer layer surrounding the bone, the periosteum, is much stronger than in adults but is loosely attached. Muscles arise from, and are inserted onto, the periosteum rather than the bone itself and this permits coordinated growth of the bone and the soft tissues. Complete fractures are rarely comminuted in children and are often less severely displaced than in adults. The periosteum is usually torn on the convex side of a fracture but the periosteum on the concave (compression) side may remain intact. The intact periosteum is able to act as a hinge, which restricts the degree of displacement and provides the key to closed reduction and position of immobilisation in a cast.


     

     
      
    The direction of the deforming force governs the type and direction of the complete fracture line:

     
    • A spiral fracture is the result of twisting forces and will always have a partially intact periosteal hinge

     
     
    • A transverse fracture results from angulation and usually has an intact periosteal hinge on the compression side. Most transverse fractures can be reduced by increasing the deformity to about 90 degrees so that the ends can be "unbuttoned" out of the surrounding muscle and soft tissues and then straightening the bone whilst maintaining longitudinal traction. A plaster cast with three point moulding is designed to hold this sort of fracture.
     
     
       
     ▪ An oblique fracture is due to axial overload with sheering forces. The displacement and stability depend largely on the degree of soft tissue injury, especially the degree of tearing and stripping of the periosteum. These fractures are reduced by longitudinal traction and can be difficult to hold in a cast. Some require an intramedullary nail to maintain the length of the bone.

     
       
    ▪ A "butterfly" fracture results from a combination of axial loading with angulation. These are not common in children and a combination of three point pressure and distraction may be required to hold the fracture aligned.

     
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