Physical modelling of Infant Head Impacts

322 Pages·2015·5.85 MB·English

by  Jonathan Hughes

Biomechanics of Skull Fracture and Intracranial Injury in Young
Children as a Consequence of a Low Height Fall

By

Jonathon Hughes

A thesis is submitted to Cardiff University for the degree of Doctor of Philosophy

School of Engineering and School of Medicine
Cardiff University
2014
Declaration

This work has not previously been accepted in substance for any degree and is not
concurrently submitted in candidature for any degree.

Signed……………………………………(candidate) Date ………………………..

This thesis is being submitted in partial fulfilment of the requirements for the
degree of PhD

Signed……………………………………(candidate) Date ………………………..

This thesis is the result of my own independent work/investigation, except where
otherwise stated. Other sources are acknowledged by explicit references.

Signed……………………………………(candidate) Date ………………………..

I hereby give consent for my thesis, if accepted, to be available for the
photocopying and for the inter-library loan, and for the title and summary to be
made available to outside organisations.

Signed……………………………………(candidate) Date ………………………..

I hereby give consent for my thesis, if accepted, to be available for the
photocopying and for the inter-library loans after expiry of a bar on access
previously approved by the Graduate Development Committee

Signed……………………………………(candidate) Date ……………………….
Acknowledgments

First off I would like to state that this research and thesis has by no means been a
sole effort, where numerous people have made contributions from small to large.

Initially I would like to thank the staff in the emergency department and in the
medical records department at the University Hospital of Wales Cardiff. The staff
always happy to help researchers even though they are incredibility busy with
their work.

To the research team working with Professor Kemp and Dr Maguire, many thanks
for the support in helping throughout the project. A special thank you to
statisticians Lazlo Trefan and Daniel Farewell who aided with numerous technical
discussions.

To my supervisors, both medical and engineering. working between two research
teams can be challenging. However in this research area it is paramount that a
multidiscipline approach is adopted in order to find a unified solution. I am
grateful to both Dr Jones and Dr Theobald for giving me this opportunity and for
their continued support throughout the project. I am also thankful to both
Professor Kemp and Dr Maguire, who have been instrumental in helping me
through many aspects of this project, particularly for their support whilst
presenting in the United States.

Finally to my family and friends who have to endured numerous stressful
moments. I extremely thankful for all the support throughout this project and I
know I would not have made it to the finish line without it.
Abstract

Background
A challenge for clinicians when presented with a significant head injury in a young
child and a postulated fall height is to determine the plausibility of such an injury.
Previous authors have aimed to determine the head injuries that can result from a
low height fall, however due to a lack of clarity it is difficult to determine a fall
height at which certain head injuries including skull fracture and intra cranial
injury (ICI) becomes more likely. Biomechanical thresholds aimed at young
children exist for skull fracture and adult thresholds for subdural haemorrhage,
however they have not been assessed against the injuries seen in a clinical setting.
Consequently this study investigated low height falls in a paediatric clinical setting
to determine differentiating variables and characteristics in the mechanism of
head injury between children with a minor head injury and those with a skull
fracture and / or ICI. The primary aim of which was to determine a fall height
threshold for skull fracture and / ICI in young children. Following this,
biomechanical methods were used to include, the development of an accurate
anthropomorphic testing device (ATD) and a finite element model of an infant
head, to investigate the differentiating variables and ultimately the clinical fall
height threshold.

Method
A case control study of children ≤ 48 months of age who had a minor head injury
and those with a skull fracture and / or ICI, to identify variables and characteristics
of falls that influenced injury severity. Children were ascertained from those who
attended the University Hospital of Wales Cardiff from a low height fall. The
clinical characteristics and biomechanical variables evaluated included the
mechanism of injury, surface of impact, site of impact and fall height (taking into
consideration height of object and centre of gravity of the child’s body and head
mass). Categorical variables were assessed using a Chi Square test and continuous
variables using Student t-test or the non parametric equivalent. A modified logistic
regression was used to evaluate the likelihood of sustaining a skull fracture and
/or ICI based on fall height.
Initially to investigate the differentiating variables a biofidelic infant headform was
designed via image processing and segmentation of computed tomography (CT)
datasets and manufactured using materials with similar properties to the bone and
soft tissues of the head. The headform impact response was initially validated
against infant cadaver data and then it was subject to tests classed as sub-injurious
based on the clinical data collected from the hospital. The headform was dropped
at impact angles of 90o, 75o and 60o at three velocities (2.4m/s, 3m/s, 3.4m/s)
corresponding to three heights (0.3m, 0.45m, 0.6m), onto four domestic surfaces
(carpet, carpet & underlay, laminate and wood) using two skin friction surrogates
(latex, polyamide). A Student t-test was used to measure the affect of the
coefficient of static friction and a three factorial ANOVA to measure the affect of
impact velocity, surface type and angle of impact had on kinematic variables (peak
g, HIC, rotational acceleration, change in rotational velocity and duration of
impact).

Finally to investigate the differentiating variables a finite element (FE) model of an
infant head was developed, again through image processing of infant head CT
datasets. The FE model consisted of the scalp, sutures, cranial bones, dura
membranes, cerebral spinal fluid, bridging veins and the brain and the impact
response was also initially validated against infant cadaver data. Post validation a
parametric test across four different scenarios (0.3m impact onto the occipital,
frontal, vertex and parietal areas of the head) was conducted to assess the affect
material properties have on impacted response of the model. Finally the FE was
used to assess the affect height (0.3m, 0.6m, 1.2m) and anatomical site of impact
have on the impact response of the head, including kinematic variables and
material response variables.

Results
Identified cases included 416 children with a minor head injury and 47 with a skull
fracture and / or ICI. The mean fall height for minor head injuries was significantly
lower than for a fall causing skull fracture and / or ICI (P<0.001). Utilising the height
of centre of gravity of the head, no skull fracture and / or ICI was sustained in children
who fell <0.6m (2ft). Skull fractures and / or ICI were more likely in children ≤12
months (P<0.001), following impacts to the temporal/parietal or occipital region of the
head (P<0.01), and impacts onto wood (P<0.05).

All tests using the biofidelic headform were conducted with impact velocities
corresponding to fall heights ≤0.6m, where an increase in impact velocity, increase
in surface stiffness and a decrease in impact angle significantly affected both
rotational and translation kinematic variables (P<0.05). Peak rotational
accelerations at 90 degrees were 11, 363 rad/s2 on wood at an impact velocity
corresponding to a height of 0.6m and significantly increased to 16,980 rad/s2
with a 30 degree decrease in impact angle (P<0.001). However head injury
criterion (HIC) decreased for wood at impact velocity corresponding to 0.6m from
245 to 121 for a 30degree decrease in impact angle (P<0.001).

The parametric test using the finite element model indicated that the skull stiffness
has the greatest affect on the dynamic response of the head, an increase in the skull
stiffness of 7% increased HIC by 26%. Height and anatomical site of impact
affected kinematic and material response variables. The mean value of peak G and
HIC at the clinical defined threshold of 0.6m fall height was 85g and 284g,
respectively. An increase in fall height to The stiffest parts of the head were the
frontal areas and the least stiff were impacts focal to the sutures. Impacts focal to
sutures indicated high stress zones on adjacent bones, for example an impact to
the vertex indicated high stress zones on the left and right parietal bones.
The greatest strain on the connectors used to model the bridging veins was at the
most focal impact point, the vertex. For a 1.2m fall the greatest peak stretch ratio
for a vertex impact was 1.31.

Conclusion
A threshold above which skull fracture and / or ICI of 0.6m was proposed. The
corresponding mean values for peak g and HIC using the finite element models at a
0.6m fall corresponded well with current biomechanical thresholds for skull
fracture, particularly the current National Highway Transport Safety
Administration standard. This study highlights the importance of developing
threshold specific to young children that are both clinically and biomechanically
relevant. A clinical finding was that head injury severity was influence by
anatomical site of impact. This was supported by the biomechanical analysis where
skull fracture risk and strain on the bridging veins were both influenced by site of
impact. The high stress on adjacent bones from a single impact focal to the sutures,
suggest the potential for fracture on multiple cranial bones from a single point of
impact. Whilst further research is required to validate fracture patterns, it
highlights the potential for a bi-parietal fracture from a vertex impact.
Glossary
Abusive Head Trauma (AHT)- An injury to the head where the mechanism is non
accidental.
Acceleration -Rate of change of velocity over time.1
Bilateral Skull Fracture – Fracture of multiple cranial bones on the right and left
aspects of head2.
Bridging vein –Veins that bleed from the cerebral hemispheres into the super
sagittal sinus3.
Bulk Modulus – Material property, ratio of pressure to volumetric strain.1
Cerebral Oedema –Accumulation of fluid on the brain3.
Comminuted Skull Fracture – Fracture in which bone is broken or crushed in a
number of places2.
Contracoup – Injury opposite the site of impact beneath the skull4.
Coup injury – Injury direct beneath the skull at the site of impact.4
Diffuse Axonal Injury (DAI) – An injury that results in widespread lesions to the
white matter as the result of shearing of axons due to rotation5.
Depressed Skull Fracture - A break in a cranial bone with an inward depression2.
Diastatic fracture – A fracture that involves widening of sutures2.
Epidural Haematoma (EDH) - A bleed in the space between the meningeal layer
and the skull3.
Extra axial haemorrhage –A bleed outside the brain3.
Falx cerebi – Dura layer separates the left and right cerebral hemispheres3.
Glasgow Coma Scale (GCS) – Scale to measure the level of neurological
dysfunction in three separate components; motor, verbal and eye opening
responses.6
Intracranial Injury (ICI) An injury inside the cranial bones3.
Linear Skull Fracture – Single fracture of a single cranial bone2.
Shear Modulus – Material property, ratio of shear stress to shear strain1.
Strain- Elongation relative to its original length1.
Stress – Force per unit area1.
Subarachnoid Haematoma (SAH)- A bleed beneath the arachnoid meningeal
layer3.
Subdural Haematoma (SDH) - A bleed in the space between the dura and
arachnoid meningeal layer3.
Tentorium Cerebellum – Dura matter that separates the cerebellum from the
occipital lobe3.
Traumatic Brain Injury – A nondegenerative, noncongenital insult to
the brain from an external mechanical force, possibly leading to permanent or
temporary impairment of cognitive, physical, and psychosocial functions, with an
associated diminished or altered state of consciousness7.
Velocity – Rate of change of distance with time1.
Viscoelastic –Property of material that has both elastic and viscous
characteristics1.
Young Modulus – Material property, ratio of stress to strain1.

References
1. Rees DW. Basic solid mechanics: Macmillan; 1997.
2. Bilo RA, Robben SG, van Rijn RR. Forensic aspects of paediatric fractures:
Springer; 2010.
3. Standring S. Gray’s anatomy. The anatomical basis of clinical practice.
2008;3.
4. Drew LB, Drew WE. The contrecoup-coup phenomenon. Neurocritical care.
2004;1(3):385-390.
5. Smith DH, Meaney DF, Shull WH. Diffuse axonal injury in head trauma. The
Journal of head trauma rehabilitation. 2003;18(4):307-316.
6. Teasdale G, Jennett B. Assessment of coma and impaired consciousness: a
practical scale. The Lancet. 1974;304(7872):81-84.
7. Dawodu A. Traumatic Brain Injury (TBI)-Definition, Epidemiology,
Pathophysiology. Medscape. 2013.
Contents Page
1 Chapter 1 – Introduction ........................................................................................ 21
1.1 Introduction ...................................................................................................................... 22
1.1.1 Aims and objectives .................................................................................................. 24
1.1.2 Thesis overview.......................................................................................................... 26
1.2 References ......................................................................................................................... 29
2 Chapter 2 – Literature Review .............................................................................. 32
2.1 Epidemiology of Head Injuries Children ............................................................... 33
2.2 Head Injury Severity ...................................................................................................... 33
2.3 Biomechanics of Head Injury ..................................................................................... 34
2.3.1 Mechanisms of Injury ............................................................................................... 35
2.3.2 Head Injury Thresholds ........................................................................................... 37
2.3.2.1 Translational Accelerations ......................................................................... 37
2.3.2.2 Rotation Accelerations ................................................................................... 40
2.3.3 Material Properties ................................................................................................... 41
2.3.3.1 Skull ....................................................................................................................... 42
2.3.3.2 Suture .................................................................................................................... 44
2.3.3.3 Scalp ...................................................................................................................... 45
2.3.3.4 Cerebrospinal Fluid (CSF) ............................................................................. 46
2.3.3.5 Brain ...................................................................................................................... 46
2.3.3.6 Dura Matter ........................................................................................................ 48
2.3.3.7 Bridging Veins ................................................................................................... 49
2.4 Key Area to Address From the Literature ............................................................. 51
2.5 References ......................................................................................................................... 54
3 Chapter 3 - Clinical Assessment of Head Injuries in Young Children ....... 59
3.1 Introduction ...................................................................................................................... 60
3.1.1 Background .................................................................................................................. 60
3.1.1.1 Low height fall ................................................................................................... 61
3.1.1.2 Definition of a low height fall ....................................................................... 61
3.1.1.3 Biomechanical variables ................................................................................ 62
3.1.1.4 Evaluations/Quality of history provided ................................................ 63
3.1.1.5 Injury severity ................................................................................................... 63
3.1.2 Aims and Objectives .................................................................................................. 70
3.2 Method ................................................................................................................................ 71