Whiplash injury is defined as neck pain persisting greater than 12 weeks following a whiplash mechanism of injury. Usually, but not always, this involves an automobile collision. Whiplash injury is a common source of complaints among personal injury patients. Its significance is usually debated in litigation. However, If one considers the actual mechanism and mechanics of injury its significance becomes more obvious.
Incidence of whiplash injury
Whiplash injury is common and has a high prevalence in some types of accidents. The estimated likelihood of chronic neck pain is 15%-60% after an automobile accident. The likelihood goes up with accident severity. In high-speed motor vehicle accidents chronic neck pain can be seen in 80% of victims.
The mechanism
The whiplash injury mechanism has been well studied. The mechanism involves a straightening of the neck, followed by extension of the mid-neck and flexion of the upper neck, followed by complete extension of the neck followed by complete flexion of the neck. The entire process takes about 200 msec. Because the neck is accelerating in an arc, the G forces felt on the neck are greater than the G forces imparted on the vehicle.
Whiplash injury and G-forces
The mechanics of this injury explain a lot about it. Lower G forces imparted on a vehicle impart higher G forces on the neck due to the arc like motion of the neck during the injury. This motion results in a far greater acceleration of the neck than the acceleration of the vehicle. As a result, the G forces felt on the neck are 2-6 times greater than the G forces felt on the vehicle. It stands to reason then that defense expert calculations of delta v or vehicular G forces tell only part of the story. Multiply the defense expert’s G force calculations by 2-6 and you have a more accurate picture.
Another thing to consider is that while the head itself can withstand high G’s, the rest of the spine cannot. The junction of the head and the spine, at C1, is meant for flexion. It can absorb flexion and extension forces becouse it is designed to. 50% of the flexion and extension of the entire cervical spine comes from the head-C1 junction. When defense experts note the G forces imparted on the head by sneezing or head butting a soccer ball, they are demonstrating the normal motion of the head-C1 junction. However, the rest of the spine cannot handle high flexion and extension G force loads like the head can at C1. Since 50% of flexion comes from the head-C1 joint, then 50% of flexion comes from the whole rest of the spine. That means that C2/3, C3/4, C4/5, C5/6, C6/7 and C7/T1 together can flex only as much as the head-C1 joint can by itself. This explains a lot. First, head-C1 whiplash injuries are incredibly rare, since there is normally so much ability to flex and extend. Secondly, C2/3 whiplash injury is common since whiplash motion causes upper neck flexion and extension. Lastly, C5/6 and C6/7 injury is also common, since whiplash motion causes lower neck extension and flexion. The parts of the cervical spine that are not designed to have high flexion and extension (C2/3, C5/6 and C6/7 mainly) tend to injure when placed under these loads, while the segment that has high normal flexion and extension ability (the head-C1 junction) doesn’t tend to injure under whiplash loads.
The cervical facets: The weak link in whiplash
There is also an interplay between the mechanics of injury and the physiology of the cervical spine that leads to certain types of injuries. The cervical facet joints have a lower threshold for failure than the cervical discs. A chronic injury of the facet capsule or facet components can be caused by impact forces generating 3.5G’s. This force is not hard to generate; it can be imparted on an occupant’s head at speeds of 8 MPH. If the neck is twisted, imparting a pre-torque, half the force may be required.
Cervical facet injuries usually do not show up on MRI. If severe enough, there will be a fracture or dislocation. This severity is very uncommon and physicians usually have to rely on diagnostic injections to come to a diagnosis.
Disc herniations and whiplash
In contrast to cervical facet joints, discs require much more force in order to injure. The commonly injured C5/6 disc requires 6.5-10 G’s before physiological disc limits are reached. This leads to several conclusions. Firstly, cervical facet injuries are much more common than disc herniations after whiplash injury. Secondly, because the threshold for disc injury is higher, patients with post whiplash disc herniations on MRI can be assumed to have concomitant cervical facet injuries.
Disc herniations can be confused with degenerative findings. Gradient echo MRI protocols added to an MRI can help come to an accurate diagnosis.
Conclusion
Whiplash injury can be hard to grasp, unless one understands the mechanics of the injury. Physics, biomechanics and physiology provide a good explanation for the characteristics of whiplash injury. Now, we understand why a low-speed impact can injure the neck, why the head can withstand G forces that the neck can’t and why so many whiplash patients have chronic neck pain yet a normal MRI. Science clearly favors the injured patient in litigation. Defense expert arguments, measured against the existing science, seem incomprehensible and are at best disingenuous.
John Lawn, MD
The views expressed are the personal views of the author and do not represent the views of The Brain, Spine and Joint Group, its managers, affiliates, partners, employees or its clients. Furthermore, the information provided by the author is not intended to be expert or legal advice.
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