A blow to the back of the head is dangerous enough, but when doctors have examined such head injuries, they’ve found a strange phenomenon. A hit to the back of the head cause breaks in the front of the skull.

It’s called a contrecoup injuries, although it usually refers to brain injuries. But it’s the same phenomenon: For example, a person who has fallen down and hit the left side of their head against the floor will often have damage to the brain on both the left side of their head and the right side of their head, ranging from a slight swelling to extensive tissue damage. Sometimes the damage to the opposite side of the brain is more extensive than the side that was hit.

Doctors are not entirely certain why this happens. Some believe the brain gets knocked by one side of the skull and floats across the skull to hit the other side. Others think that the blow to one side of the head causes fluid to rush in and protect the brain, only to force the brain against the opposite side of the head. Because the brain is soft and mobile, it’s understandable that it might damage itself by moving against the hardened skull.

Sometimes the skull itself is what breaks, and these injuries are far more puzzling. In a collision between the skull and the brain, the skull might injure the brain, but the brain is unlikely to injure the skull. Sometimes the brain isn’t even involved. One doctor reported a contrecoup injury caused by a man stumbling and hitting his left cheek. The next day he felt light-headed and went to see a doctor. His left cheek was swollen, but had no broken bones. He did, however, have a clear fracture on the right side of his head, just below his temple.

The most common type of contrecoup skull fracture occurs when the occipital bone (the bowl-like part of the skull that juts out above the back of your neck) gets hit. The part of the skull diagonally across from it, just above the eyes, can break apart. If the blow lands on the right part of the occipital bone, the left side of the front of the skull is damaged, and a blow to the left part of the occipital bone causes a shattering of the right front of the skull. The most famous medical example of this comes from the report from the doctor conducting the autopsy on Abraham Lincoln. He found that the bullet to the back of the president’s head had caused the front of his skull to shatter, and believed that the transmitted force of the brain caused the skull fracture.


How can a hit to one side of a structure to cause the other side of the structure to fracture? Contrecoup injuries caused by a hit to the back of the skull are probably, in part, due to the fragility of the bones in the front of the skull. It’s possible that it is the transmitted force of the brain. It might be the pressure of the fluid around the brain. Or it may be the structure of the skull itself, and how force travels through it. Practical experiments are impossible, and so we still don’t really know.

[Sources: National Museum of Health and Medicine, The Mechanism of Contre-Coup Injury]

Image: Roland Unger.