The Mechanism of Skull Fracture

Deformation patterns of the skull due to blows of low velocity, as well as the mechanism of production of linear skull fracture, have been previously described (1–5). When the dry skull is coated inside and out with “stresscoat” brittle lacquer, and then subjected to a blow, the lacquer cracks in the areas of greatest tensile deformation. The cracks appear on the outside of the skull in the regions in which the bone bends outward and on the internal surface where the bone bends inward. A region of inbending—generally circular, oval, or star-shaped—always surrounds the point of application of a blow, no matter where it is struck. Where the skull curves sharply, however, the extent of the inbending is not so great as in a less curved region. By means of the “stresscoat” technic, it has been shown that outbending of bone may occur at a considerable distance from the point of application of the blow. In some specimens a contrecoup type of outbending has been observed approximately diagonally opposite the point of impact. A study of the “stresscoat” pattern of deformations of the skull and comparison with clinical fractures indicate that most linear fractures are initiated on the outer surface of the skull by tearing-apart forces due to outbending of the bone. The evidence shows that a linear fracture due to a blunt blow insufficient to cause a depression is formed as follows: At impact, the area around the point of application of the blow is inbended. Simultaneously there is an outbending of the bone peripheral to the inbended area. This outbending is selective and may be localized to a certain part of the skull, where a linear fracture is initiated due to the resultant tearing-apart forces. The fracture then extends toward the point of impact and in the opposite direction. Extension is directly toward the region of impact rather than to one or the other side, because, although initially this area is in compression (bent in), it rebounds immediately after the energy of the blow has been absorbed and becomes an area of tensile stress. A fracture which has started at a considerable distance will, of course, propagate in a direction normal to the greatest tensile stress. Therefore it must travel toward the center of the area of impact. If insufficient energy was expended in the blow, the fracture may remain limited and not reach the point of impact. The present study deals with the determination of areas of primary, secondary, and tertiary stress levels in the skull as a result of blows in various locations and the correlation of these findings with an analysis of experimental and clinical skull fractures. In the study of threshold deformations, it was found that, as the energy of the blow was increased, the “stresscoat”-covered skulls presented more extensive patterns of cracks in the lacquer as a result of more severe deformations. The appearance of the first cracks following an impact at a given point denotes the area of primary stress level. When more energy is used to produce additional cracks in the lacquer, the resultant patterns indicate the area of secondary stress level.