Helmet Protection against Traumatic Brain Injury: A Physics Perspective - PowerPoint PPT Presentation

1 / 77
About This Presentation
Title:

Helmet Protection against Traumatic Brain Injury: A Physics Perspective

Description:

Helmet Protection against Traumatic Brain Injury: A Physics Perspective – PowerPoint PPT presentation

Number of Views:336
Avg rating:3.0/5.0
Slides: 78
Provided by: sdss9
Category:

less

Transcript and Presenter's Notes

Title: Helmet Protection against Traumatic Brain Injury: A Physics Perspective


1
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)
2
  • javascriptvoid(0)

2
3
3
4
4
5
5
6
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)

6
7
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)

8
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

9
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

9
10
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

11
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)

12
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

13
Origin of TBI/Blunt Impact Standards
Ono et al. 1980 (human cadaver and scaled monkey
data)
14
14
15
Impact Acceleration Profile
Peak force for short time
Lower force over longer time
16
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.

17
Classification of TBI Severity (Hayes et al. 07)
17
18
HIC15 AIS4 Injury Risk (Prasad Mertz 85, data
compilation)
19
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)

20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
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)

24
(Pellman et al. 03)
Viano et al 2007
25
Innovative use of Accelerometers (Duma et al. 05
(03 season) college football)
26
(No Transcript)
27

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)
28
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)

28
29
JHTC
Woodpeckers dont get TBI HIC relies on fixed
brain mass and surface area
  • Gibson 06

30
(No Transcript)
31
Role of Body Mass and Impact Angle on Injury
Thresholds (Blackman 2009)
31
32
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)

33
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

34
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

35
Coup Contrecoup pressures
  • Liyiang et al (04)

36
(Pellman et al. 03)
Viano et al 2007
37
NFL MTBI measures (King 03 Zhang et al. 04
Newman 05 )
38
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
39
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

40
  • Left helmet has reduced concussions by 31
    relative to right helmet in high school football
    (Collins et al. 2006)

41
(No Transcript)
42
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

43
Blast Injury Protection
  • frontier of helmet design
  • Sources of Injury
  • Primary (overpressure)
  • Secondary (shrapnel)
  • Tertiary (impact)

43
44
Basic Blast Physics
Static over-pressure at fixed position
45
Basic Blast Physics
46
Blast Wave Injury
(Moss King, personal comm.)
47
Outdated Bowen Curves No TBI threshold
200 atm
1 atm
100 atm
1 atm
  • Bowen et al 68

48
Initial Simulations of Blast vs. Impact
Moss, King, Blackman (2009)
49
The Head in the LLNL Sims.
50
(2.3 kg C4)
50
51
Model for Impact
  • HIC 1090
  • peak g 194 g
  • impact duration 2.1 ms

51
52
Snapshot of Impact vs Blast Pressures
52
53
(No Transcript)
54
  • 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

55
(No Transcript)
56
(No Transcript)
57
(No Transcript)
58
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

59
Moss et al. 09 .
60
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

61
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

62
END
62
63
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.

63
64
64
65
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)

66
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)

67
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

68
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

69
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

70
(No Transcript)
71
Example Blast Simulation
MKB 09
72
72
73
Pellman et al. 03 (Viano, Pellman,
Witherall..03-07)
74
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

75
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

76
(No Transcript)
77
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)
Write a Comment
User Comments (0)
About PowerShow.com