Vehicle Passenger Safety: Exploring Whiplash Protection Systems

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.

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.

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

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.

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

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)

What might be injured? Ligaments Bones (vertebrae) Discs

What might be injured? Over 25 Pairs of Muscles

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?

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

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

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

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

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

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)

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

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

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

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.

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.

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.

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)

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)

Active Head Restraints

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

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.

Anthropometry Study of human size

Anthropometry Family of “Anthropometric Test Dummies (ATDs)”

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%

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%

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

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!

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