Helmet Protection against Traumatic Brain Injury: A Physics Perspective

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Helmet Protection against Traumatic Brain Injury
 A Physics Perspective
Eric Blackman University of Rochester
Acknowledgments -DSSG program,Institute for
Defense Analyses -Melina Hale (Chicago)
-Sarah Lisanby (Columbia) -Willy Moss (LLNL)
-Michael King (LLNL)
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Closed Traumatic Brain Injury (TBI)

• physical injury to the brain without skull
  fracture
• concussions (non-local midbrain brainstem
  frontal lobe)
• diffuse axonal injury (shear damage of axons,
  white matter grey matter linkage)
• contusions (general bruising)
• subdural hematoma (bridging vein damage)

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Sources of TBI without skull fracture

• head impacts (ITBI)
• skull comes to a stop, brain crashes into skull
• blast overpressure (OTBI)
• brain deforms from pressure and shear waves even
  without bulk acceleration (skull flexure--more
  later)
• possible alternate route via thorax to brain
• blasts propel victim so ITBI OTBI must be
  common (B07)

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Cost of TBI (USA)

• Human costs
• Civilian 2x106 cases/yr 50 auto 25 sports
  (McArthur 04)
• 20 deaths per 100,000 20 billion/yr treatment
• Military
• before 2006 estimated 3 of soldiers have TBI
  (60 of hospital injured soldiers)
• 0.6 of all solidiers serious TBI
• New screenings 2006-2009 18 of all troops
  have TBI (Fts. Carson, Hood, Bragg, Cp.
  Pendelton..),1.5 of all troops unfit to return.
• 2.7 million (Blimes 07) per 25 yr post-TBI
  lifetime gt2 billion/year for treatment
• Workforce / mission / security costs

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Why is TBI a Research Frontier? Modern
equipment has reduced fatalities, leaving
survivors with prevalent secondary injuries.
Many aspects of TBI science are therefore nascent

• medical screening and correlation with trauma
• macho culture TBI not always understood as
  physical
• PTSD vs TBI
• treatment
• physiology and biology of injury
• mechanisms of injury (impact vs. blast)
• physics of protection

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Head Impacts

• Gravity or explosion converts gravitational
  potential energy or chemical energy into bulk
  kinetic energy
• Rapid deceleration upon impact implies large
  force
• During impact, kinetic energy is converted into
  deformation energy
• Brain damage from energy dissipated in brain
  rather than helmet or skull
• tissue stress (force per unit area) threshold for
  injury
• duration of force threshold for injury
• Functional forms not well understood

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TBI from Impacts

• As head impacts, brain keeps moving it is
  coupled to skull by cerebral spinal fluid (CSF)
• Brain crashes into skull displacing fluid
  stresses brain tissue both by compression and
  shear
• Protecting skull from fracture is insufficient to
  protect brain from crashing into skull
• Need to
• reduce head acceleration (reduces maximum force
  incurred by brain-skull crash)
• reduce energy absorbed by brain (reduces energy
  available to sustain a distorted brain for
  extended period)

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Role of Helmets for ITBI

• Hard shell protects the skull at point of impact
  and absorbs, redistributes, and releases energy
  rapidly
• Hard shell alone is no good
• Need cushioning to reduce head impact
  acceleration and thus force on brain
• Cushioning standard must be more stringent to
  protect against brain injury than skull fracture
• subtleties in helmet/skull/brain force coupling

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Origin of TBI/Blunt Impact Standards
Ono et al. 1980 (human cadaver and scaled monkey
data)
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Impact Acceleration Profile
Peak force for short time
Lower force over longer time
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Widely used Injury Measures
1. Peak g
2.
3.

• SI and HIC empirically accommodate acceleration
  and duration from cadaver and animal injury data
  
• Can create human injury probability graph
• e.g. Head HIC gt 1000 (sec), 16 risk of life
  threatening TBI (Prasad Mertz 1985) scaled
  monkey data auto industry.

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Classification of TBI Severity (Hayes et al. 07)
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HIC15 AIS4 Injury Risk (Prasad Mertz 85, data
compilation)
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How are HIC and SI used?

• NHTSA uses HIC 1000. (supposedly 1 chance of
  fatality 30MPH collision for restrained driver)
• NOCSAE uses SI1200 ( JHTC) but for NFL does
  not fully protect against TBI should be 140
  based on data
• NO SI/HIC standard for military Helmets peak g
  standard only and its NO GOOD.
• Slobodnik (1980) need lt150G at 1.5 meters drop
• special forces helmets standard is 150G at 1.5
  feet(!)
• Free falls of 3 feet for a 5kg head form
  including PAGST or ACH helmets give 300G
  (McEntire et al.05)

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Current Military Helmets Fail
Wayne State Head Tolerance (WSHTC) Gujdardan et
al. 1966
Combat Helmets for 4.5 ft drop
McEntire et al. 05

• Japan Head Tolerance (JHTC, 25 p of concussion
  HIC1100) Ono et al. (1980,scaled monkey data)

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(Pellman et al. 03)
Viano et al 2007
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Innovative use of Accelerometers (Duma et al. 05
(03 season) college football)
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Military helmets 4.5 foot drop
30 risk curve like JHTC (uses scaled monkey
data)

• Actual gt70
• NFL risk curve

Blast simulations (w/injury) (Moss, King,
Blackman 09)
augmented from Pellman et al (03,06)
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ITBI protection standards AND measures are
flawed

• measures HIC, SI based only on (limited)
  experimental data body mass and impact angles
  not included, have little theoretical foundation,
  not even the best indicators...
• even if measures were correct standards in
  military and NFL are inadequate
• ITBI measures are useless for BTBI (later)

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JHTC
Woodpeckers dont get TBI HIC relies on fixed
brain mass and surface area

• Gibson 06

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Role of Body Mass and Impact Angle on Injury
Thresholds (Blackman 2009)
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Physical Quantities that TBI measures should
correlate

• External

• linear force (mass X linear acceleration)
• total energy and energy input rate
• torque (moment of inertia X rotational
  acceleration)
• Internal
• brain tissue stress or pressure maximum
• brain tissue rate of elastic energy change
  (localized)

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Numerically Simulating Impact Head ModelsTBI
Thresholds based on Internal Forces

• Zhang et al. 04 reproduced NFL collisions with
  Wayne State Head Model
• WSHM gray matter (cell) white (fibrous) shear
  moduli 20 larger for white white is 2-D
  isotropic, grey is 3-D isotropic brain stem
  shear mod 40 higher than cerebrum etc..
• Data on these properties differ, but code can
  incorporate whatever the data require

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TBI internal measures from simulations

• Zhang et al 04 reproduce videoed impacts with
  head form collisions, then use experimental data
  as input for numerical simulations to calculate
  internal stresses
• Maximum stress at core (diencephalon, upper brain
  stem)
• rate of maximum strain ( rate of elastic energy
  change) and peak stress are best correlators with
  injury

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Coup Contrecoup pressures

• Liyiang et al (04)

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(Pellman et al. 03)
Viano et al 2007
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NFL MTBI measures (King 03 Zhang et al. 04
Newman 05 )
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P versus peak Linear Acceleration
P versus HIC
Football HIC no better than Peak Acceleration or
Impact Power as measure
P versus Impact Power
Newman 05 King 03
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NFL Data/Analysis With Drop Tests Suggest Better
Impact TBI Measures can be Identified

• Newman 2000 e.g. best external correlators
• impact power
• peak linear acceleration
• King et al 05 Liang et al. 04 e.g. best
  internal correlators are
• Rate of maximum elastic energy change
• Maximum stress
• goal of TBI protection design should be to
  robustly understand how external forces correlate
  with internal stresses and injury

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• Left helmet has reduced concussions by 31
  relative to right helmet in high school football
  (Collins et al. 2006)

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Military data are needed

• Unreliability of head indices and different
  nature of impacts in football, auto, and combat
• Mass matters, neck strength matters
• Overpressure injury involves different
  mechanisms than impact injury
• Blast produces pressure impact injury

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Blast Injury Protection

• frontier of helmet design
• Sources of Injury
• Primary (overpressure)
• Secondary (shrapnel)
• Tertiary (impact)

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Basic Blast Physics
Static over-pressure at fixed position
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Basic Blast Physics
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Blast Wave Injury
(Moss King, personal comm.)
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Outdated Bowen Curves No TBI threshold
200 atm
1 atm
100 atm
1 atm

• Bowen et al 68

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Initial Simulations of Blast vs. Impact
Moss, King, Blackman (2009)
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The Head in the LLNL Sims.
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(2.3 kg C4)
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Model for Impact

• HIC 1090
• peak g 194 g
• impact duration 2.1 ms

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Snapshot of Impact vs Blast Pressures
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• Reducing wave speed in brain by reducing bulk
  modulus produces deeper penetration of pressure
  extremes as stress gradients are slower to relax

• Brain pressure peaks then followed by after
  shocks long after blast front passes

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Role of Current Helmets for Blast

• without pads, underwash amplfies pressure
  under helmet
• but with pads, head is more strongly coupled to
  skull
• need to optimize for blast impact

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Moss et al. 09 .
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Initial Lessons From Blast-Head Interaction
Simulations

• Skull flexure NOT acceleration is primary
  mechanism of OTBI through skull
• need helmet that prevents skull flexure a rigid
  shell foam that damps the stress waves
• underwash in current helmets exacerbates injury
• current helmet cushioning protects against
  underwash but does not damp skull flexure
• need to optimize OTBI and ITBI protection
• skull may not be only pathway

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Future Studies

• BLAST overpressure (OTBI)
• Add more realistic head model
• maybe skull is not path of maximum coupling to
  brain
• blast inside enclosure (vehicle)
• IMPACT (ITBI)
• Include body attached to head for the impact and
  vary impact with angle to extract effective mass
• revise HIC to include impact angle body mass
• For BOTH
• Correlate specific external forces with specific
  internal stresses
• Run impact simulations for pre-injured brain from
  overpressure
• Correlate specific brain damage with medical
  symptoms
• Optimize helmets

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Properties of Simulated Skull/Brain/Helmet Foam

• Skull Youngs modulus9Gpa, density 1.7g/cc
• brain density 1.04 g/cc, instantaneous shear
  modulus 37.5kPa quasi static shear
  modulus6.95kPa, decay factor 700 /s bulk
  modulus2.19Gpa (water)
• helmet foam bulk mod1.3Mpa, quasi static shear
  mod20.1 kPa instantaneous shear mod2 MPa,
  decay factor 100/s, density 0.136g.

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Present Helmets Ineffective Against TBI but Can
be Improved

• Blast causes impact injury and overpressure
  injury
• Need better TBI measures and standards
• Short Term cushioning inserts for that reduce
  head form acceleration by factor gt 3 from 5 ft.
  drop
• Longer Term Present TBI helmet measures are
  imprecise
• Mass is not included in current impact TBI
  measures
• need combat specific data (accelerometers,
  pressure gauges) to correlate with clinical TBI
  for blast and impact
• numerical simulations of blast and impact with
  head model
• correlate external forces with internal brain
  stresses and symptoms
• NFL/NCAA Football lead way for use of technology
  to improve helmet design and TBI monitoring from
  impacts
• Overpressure safety threshold needed NOT based
  on accel. cushioning helps but worst injury may
  not be via skull
• Progress from blast impact simulations (w/Moss
  King, LLNL)

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Present Combat Helmets Ineffective Against TBI
but Can be Improved

• Blast causes both impact injury and overpressure
  injury
• Need better TBI measures and tougher standards
• Short Term cushioning inserts for that reduce
  headform acceleration by factor gt 3 from 5 ft.
  drop
• Longer Term Present TBI helmet measures are
  imprecise
• Mass is not included in current impact TBI
  measures
• need combat specific data (accelerometers,
  pressure gauges) to correlate with clinical TBI
  for blast and impact
• Need numerical simulations of blast and impact
  with head model correlate external forces with
  internal brain stresses and symptoms
• NFL/NCAA Football lead way for use of technology
  to improve helmet design and TBI monitoring from
  impacts
• Overpressure safety threshold needed NOT based
  on bulk acceleration cushioning helps BUT is
  worsts injury via skull?
• Progress and promise from blast impact
  simulations (w/ Moss King LLNL)

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Conclusions/Recommendations

• Many costs of TBI gt1.1 billion/yr for mil.
  medical care
• Can make substantial improvements
• Blast is a 2 stage injurer overpressure and
  impact
• Both need to be addressed
• Improving impact TBI measures and standards
• Quick fix use 1/3 blunt acceleration standard
  for MICH helmets and get new inserts for ALL
  combat helmets.
• Pad the inside of vehicles
• Longer term Field data from accelerometers
  coupled with numerical simulations to correlate
  external and interal forces
• Directed funding specifically to improve impact
  injury TBI measures and standards directed
  toward identifying the optimal cushioning inserts

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Conc./Recommendations (cont)

• Effective mass often greater than head mass
  derive from theory and field data lowers current
  thresholds for injury
• HIP better than widely used HIC
• Test bivariate correlation HIP and Amax with
  injury and correlate with internal stresses from
  simulations
• Improving overpressure TBI measures standards
• Need field data pressure gauges in helmet and
  on body
• overpressure TBI thresholds from correlating
  external forcing with internal stress in models
  and comparison to field data
• overpressure measures are NOT based on bulk
  acceleration so HIC etc are useless.
• Same cushioning from impact will help but need to
  how blast overpressure is transmitted to brain
  simple models can help

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Conc./Recommendations (cont)

• aerodynamic issues, passive vs. active blast
  mitigation
• neck and tethering
• blast experiments for TBI
• Great promise from numerical simulations of blast
  impact Ultimately correlate external forces
  with internal brain tissue stresses and medical
  symptoms
• tackling the most pertinent questions can reduce
  complexity

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Example Blast Simulation
MKB 09
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Pellman et al. 03 (Viano, Pellman,
Witherall..03-07)
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HIC and SI are flawed

• No dependence on mass
• Not the best indicator of impact injury even when
  mass is constant
• Not quantitatively derived from first
  principles
• based on empirical formulae that fit un-refereed
  scaled monkey data (JHTC 80) and cadaver skulls
  (WSHTC, 61)
• Doesnt distinguish specific injury type well
• What about neck strength? (note HIC ?V4/ d1.5)
  Chou Nyquist(74)
• HIC and SI are useless for overpressure blast
  injury

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Use of Accelerometers in NCAA

• 6 accelerometers measure all head motion store
  data send data to sideline laptop
• IMPACT or other software to clinically diagnose
  MTBI
• Correlate data with incidence of TBI
• Input into head model to identify internal brain
  tissue stress
• Ironically VT study was of limited use because
  the helmets are too GOOD (only 1 concussion from
  3000 impacts monitored)
• Need to adopt for military use in combo with
  pressure measurements for blast correlation

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Helmets and ITBI standards

• Current combat helmets no TBI protection
  standard
• NOCSAE (sports) yes, NHTSA (motor vehicle)
  (fatality TBI only)
• Special forces MICH helmets have a required blunt
  impact standard but it insufficient
• ACH and PASGT (infantry helmets) with padding
  kit meet MICH standard and thus also
  insufficient.
• What is reasonable standard? (fundamental
  question)
• Bare Minimum Slobodnik (1980) 150G peak
  acceleration at 1.5 meters drop needed from
  reconstructing crashed helicopter injuries
• MICH 150G at 1.5 feet (FAILS)