A great deal of new information has been published in the last few months about low speed rear-end collisions and potential mechanisms of injury. Lord et al1 have shown that the cervical zygapophysial joints are at particular risk of injury during whiplash-type accidents, and that treatment of these lesions has a positive effect on pain and psychological symptoms (Soft-Tissue Review Volume 3, Number 1). Yang et al2 reported that compression of the cervical spine temporarily weakens the cervical ligaments, making them susceptible to injury from extension during whiplash (Soft-Tissue Review Volume 2, Number 9). And Grauer et al3 recently published a study that showed that the cervical spine undergoes a "S-shaped curve" during whiplash motion that results in excessive hyperextension of the lower cervical spine (Soft-Tissue Review Volume 2, Number 11).
Now a new study has just been published that supports these studies and provides additional insights into the complexity of whiplash kinematics. In this study, researchers examined the mechanics of simulated rear-end collisions with high-speed video and cineradiography—a technique that permits analysis of the motion of each vertebral segment. The test collisions were at very low speeds—4 to 8 km/hr (2.5 to 5 mph). The researchers compared the test collision movements with the normal extension motion of the subjects.
The results of this thorough study are summarized below.
For decades, researchers spent most of their time investigating the sagittal (or front and back) motions of occupants during whiplash-type collisions. Compression of the cervical spine (or axial movement) as a potential mechanism of whiplash injury is getting increased attention in the literature.
This study confirmed what other studies have shown about compression—that during the early phases of the collision (from 50 to about 150 milliseconds), the axial forces on the cervical spine are in the range of 33 pounds. According to Yang et al, a compressive force of 40 pounds results in a 73% reduction in ligament stiffness at C5-6, so even a 33-pound force could result in a significant loss of ligament stiffness. This loss of strength increases the potential for injury.
Researchers have been investigating the biomechanics of whiplash for over 40 years. One of the major limitations with these studies was the fact that only the gross motions of the head and neck could be investigated—researchers could see the total extension of the entire head-neck complex, but not the motion of each individual motion segment. Since the total head-neck extension was well within the normal range, engineers assumed that whiplash injuries could not be caused by over-extension.
The latest literature, however, has been able to look at each individual segment of the spine, and has found that the spine does not undergo smooth, even extension during whiplash but that the spine is subjected to an S-shaped curve during the early phase of the collision.4
This is a relatively new finding in the literature, and one that was independently documented by Grauer et al.3 Grauer reported that the whiplash motion was not simply extension, but a complex combination of compression, flexion of the upper cervical spine, and excessive extension of the lower cervical spine. Their study, however, was conducted on cadaver spines, and so there were some questions of whether these findings would also apply to living occupants.
Apparently they do, as this current study by Ono et al4 reports the same phenomena:
"A subject's torso shows the ramping-up motion by the inclined seatback during rear-end impact. As the head remains in its original position due to inertia in the initial phase of impact, an axial compression force is apt to be applied to the cervical spine, which in turn moves upward and the flexion occurs at about the same time. The lower vertebral segments (C6, C5 and C4) are extended and rotated earlier than the upper vertebral segments. Those motions are beyond the normal physiological range of motion. It is found that by comparing the motions during crash with the normal extension motions of the same subject that the rotational angle pattern is reversed by the pattern of the normal state around 100 ms. The lower the vertebral segment, the larger the rotational angle becomes. That is, the rotational angle between the fifth and sixth vertebral segments is the largest of all. This is a non-physiological motion of the vertebral segments." [Emphasis added.]
Normally, the facets slide over each other, allowing smooth, equal movement of the motion segments. When the spine is compressed, however, the mechanics of facet movement changes dramatically. Researchers have found that the Instantaneous Axis of Rotation (IAR)—or the point that the vertebrae rotate around—actually moves.
The result of this abnormal motion? The facets of the vertebrae (see arrows), rather than sliding over each other smoothly, are jammed into each other, as shown in the illustration on the right. Such abnormal motions are believed to result in joint injury—a lesion that would not be detectable with modern imaging techniques, but one that could cause chronic pain.
A number of researchers have speculated that advance warning of an impending collision has a protective effect on impact severity, as the occupant has an opportunity to tense the musculature of the neck before impact.
This current study also evaluated the role of muscle tension on the kinematics of the neck, and found that, indeed, tensing of the muscles resulted in a 30-40% reduction in total head extension. The researchers, however, did not study individual motion segment movement during the tense muscle collisions.
Some researchers have questioned whether or not the reflex tensing of the neck musculature can have a protective effect in collisions without advanced warning of impact. This current study also evaluated EMG readings in all test collisions, and states:
"The average start time of the neck flexors discharge was measured here to be 79 ms. Since there is about 70-100 ms delay between the EMG onset and the time when muscle force can reach maximum, and the head angle reached its maximum at 200-250 ms after the start of an impact,5 we conclude that muscle effect on kinematics of the head-neck complex was insignificant when the neck muscles were relaxed before impact."
Which is worse: a rigid seat back or an elastic seat back?
Over the last few years, engineers have debated the role of seat rigidity in low speed impacts. This current study also examined the effect of seat stiffness in collisions, and found that there is a complex relationship between body kinematics and seat characteristics.
To understand the role of seats, we have to remember the hazardous conditions that rear-end impacts create: compression of the cervical spine and sharp rotation of head and neck. The current study found that both types of seat backs create hazards, but in different ways:
1. Rigid seats create a sharp ramping effect on the body. In a rigid seat, the occupant's body cannot move straight backwards, and so it must move up the seat. Every study published on low speed impacts has found that some degree of ramping occurs. The more rigid the seat, the sharper the ramping. As the authors state:
As we saw earlier, compression can have a dramatic effect on ligament strength. In the above quote, the researchers found the compression with a stiff seat could amount to about 34 pounds of force, in a collision of just 5 mph.
2. Elastic seats allow too much bounce, causing rapid rebound of the occupant's torso. At approximately 100 ms, the torso has compressed the elastic seat to its greatest amount, and the seat then springs forward, accelerating the torso with it. The head is moving backwards at the same instant, creating a large difference in speed between the torso and the head. This can result in very large shear forces on the spine, as the authors state:
In summary, then, both types of seats put occupants at risk of injury, but in different ways. If the vehicle is equipped with good head restraints that are properly positioned (i.e., high enough and within 3 inches of the back of the head), the chance of injury will be dramatically reduced from such motions. Unfortunately, other studies have found that only 10% of head restraints are properly adjusted.
Cars are designed to protect occupants from injuries such as rear-end collisions, but engineers assume certain things that make cars less safe than they ideally should be. For instance, modern cars are designed for the 50th percentile person—or someone of average stature. If you are taller or shorter than the 50th percentile, the car will not protect you as much as if you are of "average" height.
Engineers also assume that occupants sit in the seats in the way they are intended—with the head restraint properly adjusted, looking straight forward, with back straight, exactly in the middle of the seat, with both hands on the steering wheel. Obviously, most people in the real world are not sitting like this. They are monitoring traffic, talking to someone next to them, changing the radio station, et cetera.
Researchers have identified out of body position and posture as a potential risk factor for injuries from low speed collisions, and this study has provided some new information on this topic.
The authors studied the effects of flexion, neutral, and extension head position before impact on the outcome of the collision. Not surprisingly, they report that neutral or extension pre-collision head position is safer than a flexion (kyphotic) position, for two reasons:
The position of the head is so important, the authors write, "In this regard, more attention should be paid to the cervical spine alignment than any other parameter affecting the occupant's seating position such as seat stiffness and seatback inclination angle, when considering parameters for the evaluation of neck injuries."
These findings may provide some insight into why women seem to suffer more whiplash injuries than do men.
"Matsumoto et al6 in a recent study conducted on the relationship between cervical curvature and disc degeneration using 495 subjects reported that the lordosis position accounts for 35% or so of the cause of such injuries among female occupants younger than 40, while kyphosis...accounts for 65% or so...Based on our experimental study, it can be pointed out that the rotational angle of the cervical vertebrae becomes obviously larger at the kyphosis position. This may explain the higher minor impact neck injury incidence for occupants with the kyphosis position." 4
In other words, pre-existing disc degeneration and/or kyphosis may put women at a higher risk of injury in low speed impacts.
All papers from the Society of Automotive Engineers (SAE) are available directly from that organization. Visit their web site at www.sae.org.
