Head Injury and Helmet Testing Flashcards
Three categories of head injury
- Superficial Injuries (prevented by helmet)
- Skull fractures (prevented by helmet)
- Closed head injuries
Closed head injuries
- can occur without skull fractures
- caused by high acceleration of the head which can not be prevented by strong materials alone
Concussion
Traumatically induced transient
disturbance of brain function
Coup Injury
Brain impacts the skull at the site of impact
Countercoup Injury
Brain rebounds and hits the opposite
side of the skull
- Posterior skull is less smooth, increasing potential damage to the cerebellum in contrecoup injuries
Cavitation
Gas bubbles form in cerebrospinal
fluid due to rapid brain movement
- Bubbles collapse upon rebound, releasing energy
- Contributes to contrecoup injury
- injuries range form mild concussion to permanent disability or death
Human brain acceleration tolerance
- Accelerations greater than 400 Gs exceed the human brain’s tolerance
- The brain can withstand 400Gs if the duration is less than 1 millisecond
- Tolerance is based on a relationship between force and time
- Graph demonstrates the nonlinear relationship between acceleration and duration in terms of brain tolerance
HIC index
Values above 1000 are considered intolerable for humans
- The formula accounts for acceleration duration and peak acceleration
Criticism and usage
- HIC only considers linear acceleration, while angular acceleration may contribute to diffuse axonal injury
- Despite limitations, HIC is a widely used head impact severity measure.
Gambit Model of rotational acceleration
- Frontal plane motions cause impairment most readily, but higher rotational accelerations in the transverse and sagittal planes can also induce damage
- Ventricular system and membranes partitioning brain regions help dampen rotational strain
- Model combines linear (>250Gs) and angular accelerations (>25000rad/s/s)
- GAMBIT values below 1 are tolerable
Limitations of GAMBIT Model
Does not factor in timing or direction of acceleration factors
Mild Traumatic Brain Injury
- A complex pathophysiologic process induced by mechanical loading of the brain.
- Common symptoms include temporary impairment of neurological functions
mTBI vs concussion
While some consider concussion to be the same as mTBI, a person can have a mTBI without experiencing symptoms of concussion
Long-term/repeated risk of mTBI
- Repeated mTBI may result in chronic degenerative brain damage, known as Chronic Traumatic Encephalopathy (CTE)
- CTE is now found in individuals without a diagnosed concussion
CTE and tau proteins
- High levels of Tau proteins in cerebral fluid are a key indicator of CTE.
- Currently, no way to measure Tau levels in living humans
CTE prevention and recovery
CTE is preventable with minimized exposure to TBI and sufficient recovery time between injuries
Helmet Testing: drop test
Common method used to measure head accelerations and assess helmet effectiveness
- aims to keep accelerations below a HIC value of 1000
Head forms used in testing
- Head forms simulate the mass and moments of inertia of a human head
- Equipped with accelerometers to measure accelerations during impact
DROP TEST: testing setup
- The head form is attached to a sled that is dropped from various heights onto a rigid object
- The height of the drop determines the velocity of impact
DROP TESTING: Simulation of real world impacts
- The drop test simulates impacts from various scenarios like sports, motor vehicle accidents, and crime scenes
- Recent advancement include non-rigid head forms that better simulate skull deformation and absorb more energy, potentially improving injury prediction
Motorcycle Helmet Drop Test
- While peak acceleration is below 400 Gs, the acceleration exceeds 200 Gs for several milliseconds, likely causing a concussion and a HIC value greater than 1000.
- The helmet is designed to reduce fatal or severe injuries in a crash, not to prevent concussions or less severe injuries.
Shear Thickening Fluids
- STFs are stress-responsive fluids with highly concentrated suspensions of colloids (tiny dispersed particles) in a liquid
- Exhibit non-Newtonian behavior - their viscosity increases with applied force.
How do STFs work
- Under normal conditions, STFs flow easily
- When impacted, the fluid thickens and absorbs energy, providing enhanced protection
- Unlike traditional helmet foams (e.g., polyurethane), STFs do not fail catastrophically under extreme impacts
Why is rheology important?
- Rheology is the study of how materials flow and deform under force.
- Helps engineers fine-tune STFs for optimal energy absorption.
STF vs Traditional Helmet material
- Fowler et al. (2015) demonstrated that STF material outperforms traditional 33mm foam padding (Riddell football helmet) in energy absorption above 15 Joules of impact.
- Peak impact forces increased with traditional padding, from ~3500N (15J)
to over 10,000N (30J), entering concussion range. - STF, even at thinner thicknesses (8mm), showed lower forces compared to traditional foam, with a more noticeable difference at higher impact velocities
(30 Joules).
Helmet Design for water sports
- Scheer (2015) designed a helmet shell to reduce drag and impact forces in high-speed water sports (e.g., wakeboarding).
- The shell featured channels and fins to reduce linear acceleration, while dimples reduced drag, increasing contact time and reducing impact force.
- Accelerometers measured linear and angular accelerations at various entry angles when the head form was dropped into water.
- Results showed a 17% average reduction in acceleration across all angles, with a peak reduction of 37% for crown impacts.
