WHAT IS OUR CURRENT KNOWLEDGE OF WHIPLASH AND BRAIN TRAUMA DURING LOW IMPACT REAR END CRASHES
Over the years a number of mathematical models have been developed to help us better understand some of the features of whiplash injury. These studies continue today. A finite element model (FEM) of the human neck (with geometry modeled from the MRI scan of a 50th percentile male spine) has been validated against cadaver tests of 15 mph rear impact crashes from Duke University. Both solid (bone) and soft elements (nucleus and anulus using linear viscoelastic material properties) were modeled based on existing literature. Collision with a pre-deployed airbag was also modeled. The model correlated well with the experimental data in the rear impact crashes and clearly demonstrated the head lag (retraction) seen in human volunteer crash tests. The model also correlated well with airbag tests. During the rear impact tests (with FEM simulations), upward motion of T1 was noted with compression of the spine. This is due either to the ramping up of the torso or straightening of the thoracic spine. Compression led to loosening of the ligaments of the neck at about 40 msec after impact. During compression, the neck becomes less stiff, diminishing its resistance to shear forces. Up to 27% capsular stretch was observed. With FEA, the more complex we make our models, the more processing time is required to solve the simulations. In one of the most sophisticated FEA head/neck models of today, an IBM supercomputer, with five processors, requires 60 hours to process only 50 msec of data.
A multibody model developed at TNO Netherlands is the Mathematical Dynamic Model (MADYMO). Research on MADYMO is ongoing, although there do not appear to be any strong human subject validations for rear impact simulations. Brain injuries are also modeled using mathematical models.
Although such work is done less frequently these days, from the 1960s to the 1980s several researchers experimented with primates in whiplash crash simulations. Much was learned concerning the types of soft tissue lesions that could be produced, most of which were not visible using conventional x-ray techniques. Researchers have measured the subcortical EEG in rhesus monkeys exposed to simulated whiplash trauma. They found abnormal hippocampal spiking and subclinical epilepsy-an interesting finding in view of the association between memory and the hippocampus.
Researchers have more recently subjected pigs to controlled whiplash experiments, measuring pressure changes within the spinal canal which result from changes in canal volume as the neck moves in extension and flexion. The head angular accelerations and displacements were consistent with a moderate to moderate-to-severe CAD injury (peak head acceleration of ~25 g; peak displacement of ~75 deg.). None of the animals displayed any obvious neurological abnormality afterward, but minimal capsular bleeding in the cervical ganglia was discovered. Using Evans dye, they determined that many nerve cells within the spinal ganglia (mostly from C4-C7) had lost their normal blood-nerve barrier and conjectured that these changes could be sufficient to cause a similar loss and rebuilding of the afferent synaptic connections within the laminae of the posterior horn of the cord, and that this could contribute to the symptoms of whiplash in patients weeks after trauma. These experiments set the stage for the development of the Neck Injury Criterion (NIC). Note: The Spine Research Institute of San Diego is not engaged in animal research of any kind.
There is a great deal of research currently available utilizing cadavers or, as they are called in this field of research, post mortem human subjects (PMHS), or even less sympathetically, post mortem test objects (PMTO). The value of using PMHS is that there is no risk to human volunteers. Moreover, unlike human volunteers, we can dissect the PMHS to identify what types of injuries might have occurred during testing. We can also attach accelerometers and other instruments, as well as photoreflective targets directly to the subjects which can provide information not attainable with live human subjects.
There are, of course, a number of drawbacks and limitations as well. Nevertheless, a good deal of our current knowledge in this field was initially discovered using this kind of testing which might involve whole specimens, isolated spinal segments, or even isolated facet joints alone.
Anthropometric test devices (ATDs)
ATD, a.k.a. crash test dummies, have been used for many years as surrogates or stand-ins for humans in tests that are deemed too dangerous for human test subjects. The most familiar of these ATDs to most Americans is the Hybrid III dummy which is currently the designated model used in FMVSS crash tests. While it does serve as a useful surrogate in these higher speed (30-35 mph) frontal crash tests, it does not have sufficient neck flexibility or compliance for use in low speed rear impact crash tests-it is said to lack biofidelity. For example, because the Hybrid III does not have an articulated thoracic spine, it cannot experience the flattening of the kyphotic thoracic curve that results in spinal compression and upward motion seen in human volunteers. Its cervical spine is also too stiff to simulate a relaxed human cervical spine. Thus, there has been a need for a biofidelic rear impact dummy (RID) ATD to use in the development of more effective automotive safety systems.