Biomechanics 101

If you are afraid of physics, don’t worry, we are with you.

Two Types of Motion in Head Injuries:

Translation: These are impacts that involve rectilinear movement with essentially no complex movements or twisting about the axis of the head or body. These would be like a flat fall from some height or like sitting in a swivel chair and moving directly backward and striking head on vertical wall. Observations indicate that such impacts are less likely to produce brain injury than if there is some rotational component present. That is not to say no brain injury can result, only that the threshold of force is greater than for rotational scenarios.

Rotational Falls: These occur when the head is moving in an arc with respect to the body or lower structures. A typical example is a backward fall from a standing position from a push or a fall backward from a stool. In rotational falls or impacts, the head is obviously moving with respect to a rigid surface and when it comes to rest within 2-10 msec (average 4-5 as measured in experimental situations), the brain may undergo complex movements inside the skull resulting in strains on axons and anatomic structures in the brain such as bridging veins that may cause them to be injured. There are various measures of the parameters of rotational impacts that include G forces, rotational acceleration and rotational velocity. There are estimated injury thresholds for these forces in a variety of situations both in living experimental animals and in biomechanical analyses in man.

Two Types of Force Application:

Static Loading: This is a steady force applied to the head or some other structure. Force may be applied slowly, for example, as in an elevator door closing (Ommaya Goldsmith and Thibault, 2002) or as in childbirth.

 
Dynamic Loading: Generally, this is an impulsive force such as a blow or fall impact to the head. These may be complex.

Impulsive loading (for head injuries): This is movement of the head induced by applied motion at a different position on the body, as theorized in shaking or as found in car collisions.
 
Impact: This does not imply how the dynamic loading occurred: blow, fall against something, slam, etc. Blunt force impact is a nonspecific term meaning that the impact did not have an edge to it. It could be accidental or inflicted, but it is a broad surface type of impact ,however it was acquired.

Shake vs. Impact?

Pediatricians and other proponents of the Shaken Baby Syndrome tell us that shaking a baby can kill it. Although we advise no one to ever shake a baby, it is scientifically not known, whether shaking alone, of an otherwise healthy baby, can produce brain injury and death. The one exception appears to be spinal and spinal cord-lower brain stem injury due to forceful shaking in very young babies (Geddes). Very few cases of spinal injury have been published in admitted cases of shaking, and information about those are so scant that meaningful biomechanical or scientific analysis is limited. There may, however, be circumstances where shaking may exacerbate existing processes occurring in underlying brain pathology (i.e. chronic subdural hematomas).

 

What We Know:

  • Pediatricians say shaking alone can kill an otherwise healthy child.
  • The highest velocity demonstrated for a human shaking a baby (model) is 11 G’s.
  • Impact creates a force 50 -100 times greater than shaking alone.
  • A 3-4 foot fall produces G force greater than can be created by human shaking.

 

Children More Fragile Then Adults:

  •  Adult resistance levels for fractures are 11 times greater than that of neonate.
  • Skull deformation (unique to the infant skull because it is not a closed bone structure) is much more damaging than a shake.
  • The skulls of children over the age of a year approach the characteristics of adult skulls.
  • Adult skulls are more rigid, more resilient thus their brains are enclosed in a more protective helmet. No one has ever told you not to touch an adult on his or her “soft spot.”
  • 26-46% of babies have intracranial hemorrhages from birth.
  • The infant’s bone structure allows for skull deformations.
     

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    Citations

    1. Duhaime, A.C., Gennarelli, T., Tibualt, L.E. Bruce, D.A., Margulies, S.S., and Wiser R. (1987). The Shaken Baby Syndrome: A clinical, pathological, and biomechanical study. Journal of Neurosurgery 66:409-415.

    2. Ommaya, Goldsmith, and Thibault. Biomechanics and neuropathology of adult and pediatric head injury. British Journal of Neurosurgery 2002;16(3): 220-242.