1. Vehicle Passenger Safety: Exploring Whiplash Protection Systems

2. Injury Mechanics
Analysis of the forces and motions that occur during events that cause injury (e.g., auto accidents, sports), often in comparison to human tissue injury tolerance levels.

3. Head and Neck Injury in Automotive Accidents
Rear-end collision:
“Whiplash injury”
Inertial response: the head and neck are not directly hit, but relative motions occur because of inertia.
We want to know the stresses and strains in anatomical tissues to determine how and where injury occurs.

4. Physics: Newton’s Laws
1st Law: A body at rest stays at rest, or a body in motion stays in motion, unless acted upon by an external force. (Inertia)
2nd Law: F=ma (force = mass * acceleration)
3rd Law: To every action there is an equal and opposite reaction

5. Rear-end Collision
Stationary car is hit from behind; the force causes a forward acceleration (2nd Law).
The occupant’s head remains in its original position while the car and body move forward (1st Law).
The head hits the head restraint, which reacts with a force (3rd Law); the head rebounds forward.

6. Head and Neck Motion: Computer reconstruction of a human subject undergoing a 5 mph rear-end collision in the lab
Figure from Vasavada et al., Spine, 2007, based on human experimental data from MEA Forensic Engineers and Scientists, Richmond, BC, Canada

7. Head and Neck Motion
Initial rearward movement of the head relative to the trunk: translation and rotation.
Results in an “S-shaped” curve of the spine (not physiologic).
(Panjabi et al., Spine, 1997)

8. What might be injured?
Ligaments
Bones (vertebrae)
Discs

9. What might be injured?
Over 25 Pairs of Muscles

10. What do we need to know?
Forces and motions that are imposed on the head-neck system.
Stress and strain on internal structures (bones, ligaments, discs, muscles).
Comparison of collision-induced stresses/strains to those which cause injury to tissues (injury tolerance levels)
How can we measure these values?

11. We need to use models A representation of a system Physical
Mathematical

12. (Class generates a list)
What kinds of models can be used to study head-neck injury in auto accidents?
(Class generates a list)

13. Models that have been used
Cadavers
Live Human Volunteers
Animals (Live or Cadaver)
Physical Models (“Crash-test dummies”)
Computer Models

14. What are the advantage and disadvantages?
Cadavers
Live Human Volunteers
Animals (Live or Cadaver)
Physical Models (“Crash-test dummies”)
Computer Models
(Group Discussion)

15. Cadavers Advantages Disadvantages Represent the human anatomy
Can measure some “internal” variables
Disadvantages
No active muscle
Difficult to obtain
Difficult to work with

16. Live Human Volunteers Advantages Disadvantages
Active muscles producing forces
Disadvantages
Ethical considerations (limited to low-speed collisions)
Cannot measure certain “internal” variables
Subject awareness (repeatability; may be different from a real collision)

17. Animals (Live or Cadaver)
Advantages
Can expose to higher-speed impacts than live humans
Disadvantages
Ethical considerations (e.g., distress)
Anatomy is different
Cannot control their behavioral response

18. Physical Models (Dummies)
Advantages
Expose to high-speed impacts
Consistent (repeatable)
Can measure “internal” variables
Disadvantages
Mechanical properties different from humans
No active muscles

19. Computer Models Advantages Disadvantages
Calculate tissue loads and strains
Simulate muscle activity
Vary parameters to test different conditions (subject size, impact conditions, muscle response)
Predictions about high-speed conditions
Disadvantages
Difficult to validate all assumptions

20. What have we learned? Studies have quantified:
Motion of the head and neck: variations with impact and occupant properties (e.g., size, gender, impact velocity).
Stresses and strains in some anatomical structures.
Stress and strain levels that cause injury in some anatomical structures.

21. What have we learned?
We still don’t know the exact mechanism of injury and how that leads to long-term pain.
General consensus: Reducing head movement can decrease the incidence and severity of whiplash injury.

22. Head Restraints Latest standards (since 2005)
Top of head restraint at least 800 mm above the hip joint.
Distance between back of head and head restraint (backset) at least 55 mm.

23. Adjustable Head Restraints
Designed to provide optimal protection for a large variation in occupant size.
Adjustability for greater comfort and visibility.
Over half of the driving public do not adjust head restraints correctly, so they are not effective in preventing whiplash! (O’Neill et al., American Journal of Public Health, 1972)

24. Active Head Restraints
Designed to protect against injury even when not positioned properly initially.
During a collision, the restraint moves forward, limiting rearward head movement.
Found to reduce injury risk by 75%.
(Viano and Olsen, Journal of Trauma, 2001)

25. Active Head Restraints

26. Problem Statement
Design an active head restraint that will protect a model of the head and neck to minimize injury in rear-end collisions

27. Head-Neck Model Head (ball): mass is related to occupant size.
Neck (spring): stiffness (inversely proportional to spring length) is related to neck anatomy and muscle activation.

28. Anthropometry
Study of human size

29. Anthropometry
Family of “Anthropometric Test Dummies (ATDs)”

30. Anthropometry Head mass differences (based on head circumference3):
Average male to average female: ~15%
Female to child: ~30%
Male to child: ~45%
Large male to small female: ~45%

31. Anthropometry Neck stiffness differences
(based on neck circumference2):
Average male to average female: ~50%
Female to child: ~50%
Male to child: ~100%
Large male to small female: ~100%

32. Model parameters Head mass: Add weight (washers) to the ball
Neck stiffness: Spring stiffness is inversely related to its length
 shorter springs represent stiffer necks

33. Simulation of Whiplash
Normally: the car is initially stationary, hit from behind, and accelerates forward.
Our approach: the car is moving backward and decelerates.
 Either way, the head moves backward!

34. Today’s Task
Quantify the head and neck motions that occur in models representing different sized occupants