The second type of injury that can be incurred is to the skull. In general, whenever a head is either struck with or strikes an object having a broad flat surface area, the skull at the point of impact flattens out to conform to the
The second type of injury that can be incurred is to the skull. In general, whenever a head is either struck with or strikes an object having a broad flat surface area, the skull at the point of impact flattens out to conform to the shape of the surface against which it impacts. As the skull is flattened and bent inward, adjacent, but more distant areas, are bent outward by a wave of deformation consisting of the central area of inbending and the peripheral outbending (Figure 6.1 )2,3 This outbending can occur at a considerable distance from the point of impact. Where the skull curves sharply, the extent of inbending and outbending is not so great as in less-curved areas. If a fracture of the skull occurs, the fracture does not begin at the point of impact, but at the point of outbending. Linear fractures begin on the external surface of the skull by the forces produced by the outbending of the bone. After inbending, the skull attempts to return to its normal configuration. As the inbent portion of the skull does so, the fracture line extends from its originating site toward the area of impact, as well as in the opposite direction. The fracture line may or may not reach the point of impact and could actually continue through it.
In any fall or blow to the head, the degree of deformation of the skull, the generation of a fracture and the extent of any fracture produced is dependent on a number of factors:
Figure 6.1 Indenting of skull at point of impact with outward bending at periphery.
Figure 6.1 Indenting of skull at point of impact with outward bending at periphery.
• The thickness of the scalp
• The configuration and thickness of the skull
• The elasticity of the bone at the point of impact
• The shape, weight, and consistency of the object impacting or impacted by the head
• The velocity at which either the blow was delivered or the head strikes the object
The amount of energy required for production of a single linear fracture from a low-velocity blow or fall depends on whether the head strikes a hard unyielding surface or a relatively soft yielding surface. With a yielding surface, a large proportion of the impacting energy is transferred to the surface by way of the deformation of the surface, thus decreasing the amount of energy available to cause head injury. In the case of a hard unyielding surface, e.g., a steel plate, in which there is essentially no energy transferred to the impacted surface, it takes approximately 33.3-75 ft lb of energy to produce a single linear fracture.3,4 This energy is absorbed in 0.0012 s. The first 0.0006 s is used in deforming and compressing the scalp and the residual 0.0006 s is used in deforming the bone.3 The amount of energy necessary to produce multiple linear fractures or stellate fractures is almost identical to that needed to produce a single linear fracture, with only a slight increase required.3 In fact, the same amount of force necessary to produce a single linear fracture could produce a stellate fracture in another area of the skull. 