5 Modelling of the Brain for Injury Simulation and Prevention
99
Recently, sports-related concussions have attracted substantial media coverages
and become a prominent public health problem, which Harrison [32] characterised
it as a ‘concussion crisis’. Several decades before concussion became a ‘crisis’,
some consumer protection and regulatory agencies, such as the Snell Foundation,
Consumer Product Safety Commission (CPSC), and National Operating Committee
on Standards for Athletic Equipment (NOCSAE), have started prescribing minimal
helmet performance criteria or thresholds, despite the fact that the exact head/brain
injury mechanism has not been totally understood. These performance criteria
are, in general, based on a linear acceleration mechanism that was derived at
Wayne State University (WSU) from correlating the presence of skull fracture
and peak resultant linear acceleration using a stress-coat technique and lab-made
accelerometers [25, 27]. Nearly simultaneously, researchers at Oxford University
proposed a rotational acceleration mechanism due to the fact that brain tissues
are very compliant in shear [35]. Because most helmets are designed to meet the
linear acceleration standards prescribed by Snell, CPSC, and NOCSAE, their main
function would be to reduce the linear acceleration. Consequently, the inevitable
question is why the helmet is somehow effective in reducing head and brain injury
[49] even though most researchers believe that rotation is the main culprit for TBI?
Was it really true that wearing a football helmet reduced the risk of TBI by 20%
only when 10 most popular helmets used in the USA were tested (the American
Academy of Neurology [5])?
To partially address these questions, Zhang et al. [110] conducted a series of
mini-sled experiments to simulate American football-relevant impacts using the
head and neck complex of a Hybrid III dummy that is instrumented for measuring
both linear and angular accelerations. Compared to the non-helmeted head, the
authors found that the peak linear acceleration was reduced by an average of 29.5%
and 22.1%, respectively, when mounting a VSR4 (Riddell Elyria, OH) and a BIKE
(Bike Athletic Company, Atlanta, GA, USA) helmets to the dummy head in all
conditions tested. However, reduction in the peak angular acceleration was only
5.6%. Additionally, angular acceleration was positively correlated with that of the
linear acceleration in both helmeted and non-helmeted tests. These experimental
results were not surprising, because physics dictated that the magnitude of angular
acceleration induced by linear impact is proportional to that of the magnitude of
linear acceleration and the perpendicular distance of the impact vector to the centre
of gravity (CG) of the struck object. For impacts delivered directly to the CG, there
wouldn’t be any angular acceleration. This is the reason why rotational acceleration
was only slightly lower when the impact was aimed near the CG of the helmeted
dummy head, despite that linear acceleration was knowingly reduced.
Because global head kinematic data (e.g. linear and angular accelerations) can
be measured and measurements collected can provide valuable data to estimate the
extent of player exposure, these kinematic data have been used to estimate the risk of
sustaining a TBI. As human cannot be instrumented like a crash dummy, concussion
observed in NFL football games provided a unique ‘human laboratory’ for studying
mechanisms of concussion and human tolerances [72, 73, 77, 97]. In this series of
NFL studies that initially involved 28 players (22 concussed and 6 with no injury),
99
Recently, sports-related concussions have attracted substantial media coverages
and become a prominent public health problem, which Harrison [32] characterised
it as a ‘concussion crisis’. Several decades before concussion became a ‘crisis’,
some consumer protection and regulatory agencies, such as the Snell Foundation,
Consumer Product Safety Commission (CPSC), and National Operating Committee
on Standards for Athletic Equipment (NOCSAE), have started prescribing minimal
helmet performance criteria or thresholds, despite the fact that the exact head/brain
injury mechanism has not been totally understood. These performance criteria
are, in general, based on a linear acceleration mechanism that was derived at
Wayne State University (WSU) from correlating the presence of skull fracture
and peak resultant linear acceleration using a stress-coat technique and lab-made
accelerometers [25, 27]. Nearly simultaneously, researchers at Oxford University
proposed a rotational acceleration mechanism due to the fact that brain tissues
are very compliant in shear [35]. Because most helmets are designed to meet the
linear acceleration standards prescribed by Snell, CPSC, and NOCSAE, their main
function would be to reduce the linear acceleration. Consequently, the inevitable
question is why the helmet is somehow effective in reducing head and brain injury
[49] even though most researchers believe that rotation is the main culprit for TBI?
Was it really true that wearing a football helmet reduced the risk of TBI by 20%
only when 10 most popular helmets used in the USA were tested (the American
Academy of Neurology [5])?
To partially address these questions, Zhang et al. [110] conducted a series of
mini-sled experiments to simulate American football-relevant impacts using the
head and neck complex of a Hybrid III dummy that is instrumented for measuring
both linear and angular accelerations. Compared to the non-helmeted head, the
authors found that the peak linear acceleration was reduced by an average of 29.5%
and 22.1%, respectively, when mounting a VSR4 (Riddell Elyria, OH) and a BIKE
(Bike Athletic Company, Atlanta, GA, USA) helmets to the dummy head in all
conditions tested. However, reduction in the peak angular acceleration was only
5.6%. Additionally, angular acceleration was positively correlated with that of the
linear acceleration in both helmeted and non-helmeted tests. These experimental
results were not surprising, because physics dictated that the magnitude of angular
acceleration induced by linear impact is proportional to that of the magnitude of
linear acceleration and the perpendicular distance of the impact vector to the centre
of gravity (CG) of the struck object. For impacts delivered directly to the CG, there
wouldn’t be any angular acceleration. This is the reason why rotational acceleration
was only slightly lower when the impact was aimed near the CG of the helmeted
dummy head, despite that linear acceleration was knowingly reduced.
Because global head kinematic data (e.g. linear and angular accelerations) can
be measured and measurements collected can provide valuable data to estimate the
extent of player exposure, these kinematic data have been used to estimate the risk of
sustaining a TBI. As human cannot be instrumented like a crash dummy, concussion
observed in NFL football games provided a unique ‘human laboratory’ for studying
mechanisms of concussion and human tolerances [72, 73, 77, 97]. In this series of
NFL studies that initially involved 28 players (22 concussed and 6 with no injury),