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. That skull fractures commonly occur when individuals fall on the back of the head becomes obvious when one realizes that a free-fall of 6 ft for a head weighing 10 lb gives an available energy of 60 ft lb, well within the range necessary to produce a linear fracture of the skull if it impacts an unyielding surface. The velocity of the head at the time of impact is approximately 20 ft/s or 13.5 mph.3
If a head strikes or is struck by a deformable object, not all the energy possessed by either the object or head will be available for deformation of the skull. At impact, the object will tend to indent and deform so as to wrap itself around the head. Thus, the energy delivered is no longer in a localized focus but is dispersed over a considerable area, reducing the possibility of a skull fracture. Linear or comminuted fractures of the skull produced by impaction of a head and a relatively soft and flexible object, such as the instrument panel of a motor vehicle, require kinetic energy levels at impact of between 268 and 581 ft lbs.4 Impact velocities are from 43 ft/s (29 mph) to 65 ft/s (45 mph). In one test, a human head impacting at 577 ft lb of energy did not fracture.4 The fractures produced with a head impacting an unyielding surface (in which 33.3 to 75 ft lb of energy is needed to fracture the skull) are essentially identical to fractures produced with heads impacting a yielding surface and requiring 268 to 581 ft lb to fracture.4 Thus, the magnitude of energy necessary to produce a skull fracture is approximately 33.3 to 75 ft lb, with other energy being utilized to deform and dent objects it impacts.
One point that has been stressed by numerous authors and should be repeated is that there is no absolute correlation between the severity of brain injury and the production of a linear skull fracture. Skull fractures can occur without any significant or detectable brain injury or any impairment of consciousness. Conversely, death may result from extensive brain injury without a skull fracture.
Simple linear fractures are typically seen in low-velocity impacts with a large area of contact between the head and impacting object. A fall to the pavement is the best example. With increased velocity and, thus, greater force, one may have a series of complete or incomplete circular fractures encircling the impact point (Figure 6.2). These fractures result from failure of the external surface of the bone at the edge of the inbent area, due to extreme inbending at the time of impact.2 If the velocity and energy of impact are increased even more, one gets stellate fractures, where there is depression of the bone at the point of impact. The severe inbending about the impact site produces fractures on the inner surface that radiate out from the site of the blow.2 Fractures resulting from the outbending of the bone at a distance from the point of impact, and arising in the outer surface of the skull, extend toward the point of impact and join with the fractures radiating outward from the point of impact. Circular fractures may occur at the junction of the inbending bone on its external surface (Figure 6.2). The concentric or circular fracture lines may be incomplete in that they stop at a linear fracture, indicating that the linear fracture preceded the concentric fracture. The opposite may also occur with the linear fractures stopping at the concentric fracture lines, which indicates that the latter preceded the former.
A depressed skull fracture occurs when the skull is struck with an object having a relatively large amount of kinetic energy but a small surface area, or when an object with a large amount of kinetic energy impacts only a small area of the skull. The scalp does not significantly affect the nature of the injuries to the skull. Large deformations occurring at a distance from the point of impact are no longer present.2,3 At the point of impact, there is a depressed fracture, possibly with fragmentation. The fractures are due to failure of the inner surface of the skull secondary to the inbending. An example of this type of fracture is the circular depressed fracture of a hammer blow (Figure 6.3). Here there are no linear fractures radiating to or from the circular depression in the skull. If there is insufficient energy to produce fractures of both the outer and inner tables of the skull, there will be a
depressed fracture in the outer table, with the inner table intact. The fracture of the outer table is almost always larger than the fracture of the inner table. Most depressed fractures are compound in that there is an associated laceration of the scalp. Epilepsy is a complication in a small percentage of depressed fractures.
Blows in different areas of the head can have different effects. A blow to the top of the head tends to produce a cranial vault fracture that might or might not extend into the temporal region or base of the skull. A blow in the occipital region generally produces a linear fracture of the posterior fossa; a blow to the temporo-parietal region fractures through the temporal bone
to the base of the skull; and a mid frontal blow produces fractures running to the orbits and sometimes into the maxilla.3
Basilar skull fractures are quite common in forensic medicine. The base of the skull, by virtue of its construction and irregular shape, is weak. Almost any diffuse impact to the vertex of the skull will produce basilar fractures. Basal skull fractures can occur from blows anywhere along the circumference of the skull below the cranial vault. They can run anterior-posteriorly, pos-terior-anteriorly, side to side and any combination of these three. Basal skull fractures may be missed on X-rays of the skull. With a basal fracture, intracranial passage of a nasogastric tube or nasophrayngeal airway can occur.5
Hinge fractures are transverse fractures of the base of the skull that completely bisect the base of the skull, creating a "hinge." The authors divide them into three categories (Figure 6.4(A)). Type I run in the coronal plane, extending from the lateral end of one petrous ridge, through the sella turcica, to the lateral end of the contralateral petrous ridge. Type II run from front to the contralateral back, passing through the sella turcica. Type III run from side to side in the
coronal plane but do not pass through the sella turcica. Type I hinge fractures are the most common form of transverse fractures of the base of the skull. They have traditionally been ascribed to impacts on the side of the head and, less commonly, to impacts on the tip of the chin. In the latter instance, one would expect a laceration of the tip of the chin, though not necessarily fracture of the mandible.
Ring fractures are circular fractures of the base of the skull that surround the foramen magnum. Typically, they run from the sella turcica partly down the petrous ridges, before turning posteriorly, and then medially, joining in the posterior fossa, enclosing the foramen magnum (Figure 6.4(B)) They may be due to impacts on the top of the head that drive the skull downward onto the vertebral column, falls on the buttocks that drive the spine into the base of the skull, and impacts to the tip of the chin. In ring fractures from impacts on the tip of the chin, almost invariably there is a laceration of the chin. Even though the force of impact is transmitted through the mandible to the base of the skull, in most instances, fractures of the mandible are not present. Experiments have revealed that more force is needed to fracture the mandible than to produce a basal fracture.6
Humphry et al. reviewed 86 cases of basal skull fracture.7 They found no correlation between the site of impact and generation of a hinge or ring fracture. Hinge and ring fractures of the base of the skull can be produced by impacts anywhere on the circumference of the head.
In skulls in which the sutures are not completely fused, suture lines represent areas of weakness and fractures may travel along them (diastatic fractures). Rarely, in infants and young children, diastatic fractures can be produced by severe cerebral edema. Thus, the authors had a case of an 18-month-old male admitted to a burn unit with burns over most of his body. He was never conscious and died a week after admission. At autopsy, there was separation of the coronal, sagittal, and lambdoidal sutures due to the edema (Figure 6.5).
Contre-coup fractures of the anterior cranial fossae are isolated fractures of the anterior cranial fossae associated with contre-coup injuries of the brain, with the impact point on the opposite side of the skull. In a study of 171 deaths from cranio-cerebral trauma due to falls, Hein and Schulz found that contre-coup fractures of the anterior cranial fossae occurred in 12% of the cases.8 All cases had fractures at the impact site, which was the occipital region. If one considered only falls where there was occipital impact, then 24% of the cases had contre-coup fractures.
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