Biomecanica

2 BIOMECHANICS OF CLOSED HEAD INJURY
A. J. McLean and Robert W. G. Anderson

This chapter discusses ways in which the brain is thought to be injured by a blunt impact to the head. The impacting object is assumed to be unlikely to penetrate the skull in the manner of a bullet, for example. The chapter is also concentrated on injuries to the brain, rather than lacerations and abrasions to thescalp or fractures of the skull. Obviously, if the skull is fractured and displaced inwards, then the part of the brain underlying the fracture will be injured. However, the brain can be very severely injured without the skull being fractured by the impact to the head (Gennarelli, 1980). Other intracranial injuries, such as subdural hematomas, are referred to briefly in relation to theories ofmechanisms of primary injury to the brain. Secondary complications of head injury also affect the brain but they are not considered in this chapter.

moving one) but an impact will also produce contact effects on the head, such as skull deformation or fracture, with an associated risk of injury to the brain. However, in practice it appears that injury to the human brain is almost always the result ofan impact to the head, or to a protective helmet, rather than an impulse transmitted through the neck (Tarriere, 1981; McLean, 1995). An impact to a given location on the head can be characterized by the impact velocity and the physical properties of the struck or striking object.
2.1.2 IMPACT VELOCITY

2.1 Impact to the head
2.1.1 IMPACT AND IMPULSE

Closed head injury is, in the greatmajority of cases, a consequence of an impact to the head. However, there are references in the literature to the production of diffuse axonal injury in ‘non-impact’ experiments in which the head of an animal was accelerated in a manner that minimized the direct contact effects of an impact to the head (Gennarelli and Thibault, 1982; Adams, Graham and Gennarelli, 1981). There are also reports ofbrain injury resulting from acceleration of the upper torso of an animal without any direct impact to the head (Ommaya, Hirsch and Martinez, 1966). These reports are discussed later in this chapter. For the present, the reader’s attention is drawn to the distinction between an impact to the head and an impulse transmitted to the head through the neck. Both an impact and an impulse, as described above,can accelerate a stationary head (or decelerate a

Some forensic pathology research literature implies that the type of brain injury differs according to whether the head is stationary and is struck by a moving object, or is moving and strikes a stationary object (Yanagida, Fujiwara and Mizoi, 1989). This distinction can be of considerable legal significance in cases of assault in which thevictim sustains a head injury which could have been caused either by a blow to the head or from striking the head in the resulting fall. However, as Holbourn (1943) observed, the moving head typically strikes an object that is considerably more massive than the head, whereas the stationary head is more often hit by objects which are of similar mass to the head or even lighter, such as a club. Inphysical terms the difference between the head moving or being stationary on impact is solely in the frame of reference. Most readers will have experienced the paradoxical sensation of not knowing which train is moving when the train alongside theirs in the station starts to move. There is no physical difference between the forces involved in a stationary head being hit or a moving head striking afixed object, given that other factors such as the velocity of the impact and the characteristics of the object contacted by the head are the same. Throughout this chapter the terms ‘struck’

Head Injury. Edited by Peter Reilly and Ross Bullock. Published in 1997 by Chapman & Hall, London. ISBN 0 412 58540 5

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BIOMECHANICS OF CLOSED HEAD INJURY

and ‘striking’ will therefore be used...