Traumatic Brain Injury (TBI) is often considered the signature injury of the ongoing military conflicts in Afghanistan and Iraq. However, there are many more non-combat victims of the devastating effects of TBI, which can affect anyone, anytime, anywhere.
The very young and very old are both highly vulnerable to significant head trauma from otherwise innocuous household short falls. According to the Centers for Disease Control (CDC), accidental falls are the number one cause of injury and number two cause of fatality in infants and young children. While teens and adolescents may believe they are superhuman at times, their young brains, which continue to develop into their mid-twenties, require extra protection due to the risky behaviors, which coincide with a peak in TBI fatalities. While the fully-formed adult skull may serve well as a helmet to protect the brain from milder impacts, there are many potentially injurious events that affect this group including automobile and motorcycle accidents, blunt force trauma, and impacts due to amateur and professional sports, including football, hockey, soccer, horse-riding, etc. Finally, in our golden years, where the brain atrophies, thereby increasing the risk of deformation, in combination with anti-coagulant (blood-thinning) therapies for various medical conditions can place elders at heightened risk of injury due to minor falls.
The brain is the most complex thing in the world. There are one million nerve cells in a section of brain the size of a grain of rice, it is impossible to comprehend the billions and billions of neuronal pathways in an average brain. In fact, there are more neural connections in each brain than there are stars in the sky. Unlike most cells in the body, which die and are replaced, neurons, which are formed in the fetus, are designed to last for a lifetime.
There are two primary mechanisms associated with traumatic brain injury – impact loading and impulse loading.
Impact loading is caused by a direct blow to the head, resulting in extracranial focal injuries, such as contusions, lacerations and hematomas (bruises), skull fractures with or without penetrating injuries. The shock waves from a blunt force trauma may propel through the skull and brain causing underlying focal brain injuries, such as subdural, subarachnoid, intracerebral hemorrhages. The primary traumatic effects of an impact involve neural or vascular elements of the brain, which can be affected by delayed effects, such as deafferentiation or secondary events such as ischemia, swellings, cerebral edema and increased intracranial pressure. Impact TBIs produce significant injuries in 40.4% of events, the most common example of which is a fall.
Impulse or inertial loading of the brain produces diffuse injuries, which are cause by sudden movement of the head, the most common cause of which is a motor vehicle accident. Impulse loading typically presents with classical symptoms of cerebral concussion, which may lead to loss of consciousness in only 10% of mild events. Inertial loading at the surface of the brain can cause subdural hemorrhage due to bridging vein rupture at the supra-sagittal sinus, whereas if affecting the neural structures deeper within the brain can produce diffuse axonal injury (DAI). A DAI causes a primary defect in the axonal membrane, producing an ionic shift, leading to depolarization of the axons and altered transmission of the neural networks with widespread neurological dysfunction. DAI cannot typically be resolved surgically, although 23.9% of the more severe cases result in fatality.
Unlike a broken leg or amputated arm, a brain injury and its effects are difficult to visualize. Thus, in such cases, a biomechanist may be called upon to evaluate the circumstances of the injury and calculate the likelihood and severity of injury as a function of forces that were imposed. Such analyses often involve biomechanical reconstructions, where the biomechanist will recreate the circumstances of the alleged injury-causing event, as accurately as possible. Biofidelic mannequins may be used to represent the injured party. Sensors installed in the mannequin are used to measure force acting on and within the body, results of which may then be compared to known injury thresholds.