Biomechanics of concussion

The human brain is the continuous subject of extensive investigation aimed at understanding its behavior and function. Despite a clear evidence that mechanical factors play an important role in regulating brain activity, current research efforts focus mainly on the biochemical or electrophysiological activity of the brain. Here, we show that classical mechanical concepts including deformations, stretch, strain, strain rate, pressure, and stress play a crucial role in modulating both brain form and brain function. This opinion piece synthesizes expertise in applied mathematics, solid and fluid mechanics, biomechanics, experimentation, material sciences, neuropathology, and neurosurgery to address today’s open questions at the forefront of neuromechanics. We critically review the current literature and discuss challenges related to neurodevelopment, cerebral edema, lissencephaly, polymicrogyria, hydrocephaly, craniectomy, spinal cord injury, tumor growth, traumatic brain injury, and shaken baby syndrome. The multi-disciplinary analysis of these various phenomena and pathologies presents new opportunities and suggests that mechanical modeling is a central tool to bridge the scales by synthesizing information from the molecular via the cellular and tissue all the way to the organ level.

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Mechanics of the brain: perspectives, challenges, and opportunities

OBJECTIVE: Brain responses from concussive impacts in National Football League football games were simulated by finite element analysis using a detailed anatomic model of the brain and head accelerations from laboratory reconstructions of game impacts. This study compares brain responses with physician determined signs and symptoms of concussion to investigate tissue-level injury mechanisms. METHODS: The Wayne State University Head Injury Model (Version 2001) was used because it has fine anatomic detail of the cranium and brain with more than 300,000 elements. It has 15 different material properties for brain and surrounding tissues. The model includes viscoelastic gray and white brain matter, membranes, ventricles, cranium and facial bones, soft tissues, and slip interface conditions between the brain and dura. The cranium of the finite element model was loaded by translational and rotational accelerations measured in Hybrid III dummies from 28 laboratory reconstructions of NFL impacts involving 22 concussions. Brain responses were determined using a nonlinear, finite element code to simulate the large deformation response of white and gray matter. Strain responses occurring early (during impact) and mid-late (after impact) were compared with the signs and symptoms of concussion. RESULTS: Strain concentration "hot spots" migrate through the brain with time. In 9 of 22 concussions, the early strain "hot spots" occur in the temporal lobe adjacent to the impact and migrate to the far temporal lobe after head acceleration. In all cases, the largest strains occur later in the fornix, midbrain, and corpus callosum. They significantly correlated with removal from play, cognitive and memory problems, and loss of consciousness. Dizziness correlated with early strain in the orbital-frontal cortex and temporal lobe. The strain migration helps explain coup-contrecoup injuries. CONCLUSION: Finite element modeling showed the largest brain deformations occurred after the primary head acceleration. Midbrain strain correlated with memory and cognitive problems and removal from play after concussion. Concussion injuries happen during the rapid displacement and rotation of the cranium, after peak head acceleration and momentum transfer in helmet impacts. Language: en

Concussion in professional football: brain responses by finite element analysis: part 9.

OBJECTIVE: Impacts causing concussion in professional football were simulated in laboratory tests to determine collision mechanics. This study focuses on the biomechanics of concussion in the struck player. METHODS: Twenty-five helmet impacts were reconstructed using Hybrid III dummies. Head impact velocity, direction, and helmet kinematics-matched game video. Translational and rotational accelerations were measured in both players’ heads; 6-axis upper neck responses were measured in all striking and five struck players. Head kinematics and biomechanics were determined for concussed players. Head displacement, rotation, and neck loads were determined because finite element analysis showed maximum strains occurring in the midbrain after the high impact forces. A model was developed of the helmet impact to study the influence of neck strength and other parameters on head responses. RESULTS: The impact response of the concussed player’s head includes peak accelerations of 94 28 g and 6432 1813 r/s 2 , and velocity changes of 7.2 1.8 m/s and 34.8 15.2 r/s. Near the end of impact (10 ms), head movement is only 20.2 6.8 mm and 6.9 2.5 degrees. After impact, there is rapid head displacement involving a fourfold increase to 87.6 21.2 mm and 29.9 9.5 degrees with neck tension and bending at 20 ms. Impacts to the front of the helmet, the source of the majority of National Football League concussions, cause rotation primarily around the z axis (superior-inferior axis) because the force is forward of the neck centerline. This twists the head to the right or left an average of 17.6 12.7 degrees, causing a moment of 17.7 3.3 Nm and neck tension of 1704 432 N at 20 ms. The head injury criterion correlates with concussion risk and is proportional to ∆V 4 /d 1.5 for half-sine acceleration. Stronger necks reduce head acceleration, ∆V, and displacement. Even relatively small reductions in ∆V have a large effect on head injury criterion that may reduce concussion risks because changes in ∆V change head injury criterion through the 4th power. CONCLUSION: This study addresses head responses causing concussion in National Football League players. Although efforts are underway to reduce impact acceleration through helmet padding, further study is needed of head kinematics after impact and their contribution to concussion, including rapid head displacement, z-axis rotation, and neck tension up to the time of maximum strain in the midbrain. Neck strength influences head ∆V and head injury criterion and may help explain different concussion risks in professional and youth athletes, women, and children.

Concussion in professional football: biomechanics of the struck player—part 14

Study design Best evidence synthesis. Objective To undertake a best evidence synthesis on the burden and determinants of whiplash-associated disorders (WAD) after traffic collisions. Summary of background data Previous best evidence synthesis on WAD has noted a lack of evidence regarding incidence of and risk factors for WAD. Therefore there was a warrant of a reanalyze of this body of research. Methods A systematic search of Medline was conducted. The reviewers looked for studies on neck pain and its associated disorders published 1980-2006. Each relevant study was independently and critically reviewed by rotating pairs of reviewers. Data from studies judged to have acceptable internal validity (scientifically admissible) were abstracted into evidence tables, and provide the body of the best evidence synthesis. Results The authors found 32 scientifically admissible studies related to the burden and determinants of WAD. In the Western world, visits to emergency rooms due to WAD have increased over the past 30 years. The annual cumulative incidence of WAD differed substantially between countries. They found that occupant seat position and collision impact direction were associated with WAD in one study. Eliminating insurance payments for pain and suffering were associated with a lower incidence of WAD injury claims in one study. Younger ages and being a female were both associated with filing claims or seeking care for WAD, although the evidence is not consistent. Preliminary evidence suggested that headrests/car seats, aimed to limiting head extension during rear-end collisions had a preventive effect on reporting WAD, especially in females. Conclusion WAD after traffic collisions affects many people. Despite many years of research, the evidence regarding risk factors for WAD is sparse but seems to include personal, societal, and environmental factors. More research including, well-defined studies with accurate denominators for calculating risk, and better consideration of confounding factors, are needed.

The burden and determinants of neck pain in whiplash-associated disorders after traffic collisions: results of the Bone and Joint Decade 2000-2010 Task Force on Neck Pain and Its Associated Disorders.

In this study a mathematical model, based on Navier Stokes equations, was developed and validated against experimental data. This model predicts the pressure changes in the spinal canal as a function of the volume change inside the canal during neck bending in the x-z (sagittal) plane. Another aim of the study was to investigate pressure phenomena and ganglion injuries at static neck extension loading and dynamic neck extension trauma with a head-restraint present. Experiments on pigs were conducted. Preliminary results indicate that ganglion injuries, as well as pressure transients inside the spinal canal, seem to correlate to the phase shift when the neck passes an s-shape (or maximal retraction) during the rearward motion of the head. That is, when the upper neck quickly changes from a flexion to an extension shape. Static loading of the neck resulted in no signs of injuries to the ganglia. A possible candidate for a neck injury criterion is presented, based on the relative acceleration between the top and the bottom of the cervical spine. A tolerance level based on the pig tests is also discussed.

https://publications.lib.chalmers.se/records/fulltext/181305/local_181305.pdf

A new neck injury criterion candidate-based on injury findings in the cervical spinal ganglia after experimental neck extension trauma

impact-velocities (<20 km/h) often cause pain in the
neck region as well as a number of other neurological
symptoms, most of which can be related to the nerve
paths that pass through the cervical intervertebral
foramina.
When the neck is flexed or extended in the sagittal
plane the length of the cervical spinal canal alters but
the cross-sectional area of the canal remains almost
constant. During flexion-extension motion of the
cervical spine, the size of the inner volume of the
spinal canal will change. Since the tissues inside the
canal can be considered incompressible, an alteration
will take place of either the amount of cerebro spinal
fluid or the amount of blood in the veinplexa of the
epidural space, or both. This requires fluid
transportation through the intervertebral foramina as
well as along the spinal canal. During a whiplash
extension motion, the flow velocity can be expected to
rise far above physiologically normal levels and
pressure gradients can thus be expected to occur. In turn,
the soft tissues inside and around the cervical spine and
particularly in the intervertebral foramina will sustain
mechanical strain and stress.
Anaesthetised pigs were exposed to a swift
extension-flexion motion of the neck while the
pressure inside the spinal canal and the skull was
measured. Pressure pulses of magnitudes up to 150
mmHg (20 kPa) were observed during the motion. The
magnitude of pressure is for each moment dependent on
the position of the neck, the velocity and the
acceleration of the motion.
Plasma membrane dysfunction was indicated by the
results from light microscopic analyses of the cervical
and the three upper thoracic spinal ganglia revealing
the staining of nerve cells and satellite cells by protein
complexed to the Evans Blue dye.

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Pressure Effects in the Spinal Canal During Whiplash Extension Motion - A Possible Cause of Injury to the Cervical Spinal Ganglia

Strain relief from the cerebral ventricles during head impact: experimental studies on natural protection of the brain.

Whiplash associated disorders, occurring in car accidents, are an increasing problem worldwide. According to real-life data from police records, the struck car's velocity change (delta V) and occupant gender are two of the most important factors related to Abbreviated Injury Scale (AIS) 1 neck injuries. In this study, a new rear-impact ranking of cars based on 4432 police reported accidents is presented. The ranking concerns the relative neck injury risk and compensates for the influences of car weight and gender. Moreover, some important factors influencing the risk of AIS 1 neck injury are proposed. These include: the stiffness, damping and yielding characteristics of the seat back, the muscle response of the occupant, and the delta V of the struck car and acceleration pulse. Using a mathematical model it is shown that the influence from these factors can be explained by a recently proposed neck injury criterion (NIC). This criterion is based on the neck motion at the passage of full neck retraction. The NIC, based on a number of volunteer tests, is analysed and validated. The consequence of injury outcome of an observed overall seat back stiffening is also discussed. In conclusion, for delta V below 20 km/h, real-life data show that the geometry of the head restraint is of minor importance. A seat back with low yielding limit or soft performance may be preferable. Moreover, the new NIC seems to be a good predictor of real-life neck injuries.

https://publications.lib.chalmers.se/records/fulltext/181668/local_181668.pdf

Prediction of neck injuries in rear impacts based on accident data and simulations

An ordinary rigid bumper system and a compliant bumper system for pedestrian protection developed by the NHTSA, US Department of Transportation, were compared in an experimental study of leg injuries in car-pedestrian accidents. Human leg specimens were struck in 20 experiments with a production car front using the two bumper types. Impacts were made with an ordinary front configuration with the bumpers at the 45 cm level and a 12.5 cm lower front configuration with the bumpers at the 32.5 cm level. The impact velocity was 30-32 km/h. Serious leg injuries were noted with both front configurations and bumper types. The compliant bumper seemed to cause less serious injuries than the rigid one, and the lower front configuration seemed to cause less serious injuries than the ordinary one. A lower bumper level than today's standard and a compliant bumper type is recommended in combination to reduce the risk of serious leg injuries in car-pedestrian accidents. For the covering abstract of the conference see HS-036 716. (Author/TRRL)

Bertil Aldman

Papers

A new neck injury criterion candidate-based on injury findings in the cervical spinal ganglia after experimental neck extension trauma

Pressure Effects in the Spinal Canal During Whiplash Extension Motion - A Possible Cause of Injury to the Cervical Spinal Ganglia

Strain relief from the cerebral ventricles during head impact: experimental studies on natural protection of the brain.

Prediction of neck injuries in rear impacts based on accident data and simulations

Experimental Study of a Compliant Bumper System