Transcript Neck muscle contributions to head stability and

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?
Bones
(vertebrae)
Discs
Ligaments
What might be injured?
• Over 25 Pairs of Muscles
What do we need to know?
1. Forces and motions that are imposed on the
head-neck system.
2. Stress and strain on internal structures
(bones, ligaments, discs, muscles).
3. 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
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
– Represent the human anatomy
– Can measure some “internal” variables
• Disadvantages
– No active muscle
– Difficult to obtain
– Difficult to work with
Live Human Volunteers
• Advantages
– 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
– 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