Mechanism of spider-web or mosaic fracture

Spider-Web or Mosaic Fracture of Skull

This is basically a depressed comminuted fracture with or without a central depression, from which linear or fissured fractures radiating outward with successive circular fractures connecting them.

This may have been caused by a slow moving (not like a bullet) high kinetic energy blunt weapon with a larger surface area. (They can also occur as a result of a high kinetic energy falls. In that case, they may not have a depressed centre.)

At the point of impact the 'inbending' (local deformation) of the skull exceeds the elastic limit of the bone causing linear fractures of the inner table due to tensile stresses. They extends outward from the centre of impact. At the same time linear fracture occur in the periphery (not the immediate periphery) due to 'outbending' (general deformation), which extends toward the centre of impact and join the fractures described above.

If the energy is adequate the immediate periphery of the 'inbended' area fails and circular fracture occurs. Depending on the amount of kinetic energy there may be multiple successive circular fractures at various distances from the point of impact. In contrast to the linear fractures they start in the outer table.

You can see from this diagram that when there is 'inbending' of the skull at the point of impact the inner table is stretched and the outer table is compressed. At the immediate periphery the bone is 'outbended' and the outer table is stretched. That is why the fracture starts at the inner table at the centre and at the outer table at the immediate periphery.

According to this mechanism the representation of the size of the weapon on the depressed comminuted fracture need not be exactly the same. The size of the fracture may be slightly larger than that of the weapon.

Next: how to describe a skull fracture

Priyanjith Perera

14 November 2012