Forensics 101: Bullet Wounds in Bone—The Skull

In a previous Forensics 101 post, we looked at how kerfs—the grooves and notches made by tools on bone—can help scientists identify the method of death in a murder investigation. But the rise of gun crime in North America has made the forensics of wound ballistics increasingly important. There are two different types of damage in this kind of wound—soft tissue and bone. In this post we’re going to strictly look at bone damage, concentrating on the skull and its very characteristic fracture patterns.

Unlike blunt force trauma, gunshot wounds often cause both an entrance and an exit wound. Investigators need to be familiar with how bone behaves in both circumstances to reconstruct the order of events and be able to piece together the details of the fatal shot. Different variables that affect the type of damage done to the bone include the velocity of the bullet (which depends on the type of gun used and the distance between the shooter and the victim), the size/caliber of the bullet, and the angle of impact.

If a bullet penetrates the skull perpendicular to the surface, a round defect is formed, often with outward radiating fractures extending from the bullet hole. The force of the bullet’s entrance increases the intracranial pressure inside the skull, causing the pieces of bone between the radiating fractures to push outwards. These ‘heaving fractures’ can be differentiated from blunt force trauma fractures because the bone sits above the plane of the skull instead of below it. The energy transfer from the bullet to the bone can be so efficient that the radiating fractures can travel through the bone to the far side of the skull faster than the bullet can traverse the brain and exit. This fact can be crucial in determining the order of fractures since a new exit fracture cannot cross an existing entrance fracture.

When a bullet strikes the skull tangentially, a characteristic ‘keyhole’ is formed—a defect that is circular at one end with tangential fractures radiating outwards in parallel, allowing the bone between them to lever out.

Exit wounds often tend to be much larger than entrance wounds for a number of reasons: the bullet is misshapen or ‘mushroomed’ from the initial bone strike, the bullet may no longer be moving along a straight trajectory, or the projectile may be tumbling end-over-end. Often large chunks of bone may be completely detached from the skull following the bullet’s exit. Sometimes, however, the bullet’s energy is spent following the initial strike; when this occurs, the bullet does not exit the skull and can be recovered later during autopsy.

Contrary to popular belief, the size of the bullet wound does not directly correlate to an exact bullet caliber because factors such as bullet shape, jacket material, stability of the bullet’s flight path and whether any other targets have been hit tangentially can affect the force with which the bullet strikes the bone.

Photo credit: Ann H. Ross, The University of Tennessee and Gérald Quatrehomme et al, Florida Atlantic University

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