Dr. Ferris comments:

The information below is very analytical in nature. I have bolded and underlined the key highlights of the below literature.  The studies sited below confirm what I continually discuss with my patient’s and what I see clinically…that there is little to no correlation of car damage and actual patient injury. As discussed below, there are so many different factors that can affect the amount of potential injury, such as position of head at impact, head restraint position, car impact point (rear-end, front-end, corner, etc.), gender, pre-existing joint degeneration, and many others. Many factors are beyond our control, but I like to educate my patients on the ones that they can control, which include:

• Head Restraint Position- the top of the restraint should be level with the top of the head and within 3 inches of the back of the head
• Seat belt- Using the lap and shoulder belt is still the most effective means of reducing serious injury
• If you notice you are about to have a collision…
  • Sit fully back against seat and head restraint
  • Shrug shoulders and contract neck muscles firmly to limit neck motion
  • Look straight ahead with head back slightly
  • Apply brake firmly (if already stopped)
  • Place hands flat on steering wheel

If you have any questions, visit our website and send us email or give us a call at 317-585-9111. We would be happy to answer your questions or setup a complimentary consultation with one of our doctors to assess your conditions.

Cervical Spine Injury Mechanism of Whiplash

A great deal of information has been published about low speed rear-end collisions and potential mechanisms of injury. Lord et al¹ have shown that the neck/facet 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.

Yang et al²reported that compression of the cervical spine temporarily weakens the cervical ligaments, making them susceptible to injury from extension during whiplash.

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.

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.

Cervical Spine Compression

This study confirmed what other studies have shown about compression—that during the early phases of the collision, 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. This loss of strength increases the potential for injury.

S-Shaped Curve

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.”

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, rather than sliding over each other smoothly, are jammed into each other. 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.

Effect of Muscular Tension

This current study confirms that when an occupant has advanced awareness of an impending collision, the resultant 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.

This study also determined that involuntary muscle reflex that occurs even without advanced awareness of a collision was not significant enough to reduce neck ligament damage. “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.”

Effect of Seat Stiffness

Which is worse: a rigid seat back or an elastic seat back?

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:

• “The interpretation of these variations in terms of neck moment, shear and axial compression forces reveal that the axial compression force applied to the cervical spine is approximately 150 N [33.8 pounds of force] with the rigid seat around 100 ms in the early phase of impact, which is about twice greater than the standard seat.”

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:

• “The sheer force…is 241 N [54 pounds] with the standard seat around 110 ms when the rebound of the torso has occurred, which is roughly 1.6 times greater than the value of 152 N [34.2 pounds] with the rigid seat.”

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., top of head rest even with top of head 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.

Effect of Posture and Head Position

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:

1. The S-shaped curve phenomena becomes more pronounced in the flexion position, putting more stress on the lower segments of the cervical spine.
2. The axial compression that occurs at 100 ms is worse in the flexion position.

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.”

Women and Whiplash

“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.

1. Lord SM, Barnsley L, Wallis BJ, et al. Percutaneous radio-frequency neurotomy for chronic cervical zygapophysial joint pain. New England Journal of Medicine 1996;335(23):1721-1726.
2. Yang KH, Begeman PC, Muser M, et al. On the role of cervical facet joints in rear end impact neck injury mechanisms. Society of Automotive Engineers 1997;SAE 970497.
3. Grauer JN, Panjabi MM, Cholewicki J, Nibu K, Dvorak J. Whiplash produces an s-shaped curvature of the neck with hyperextension at lower levels. Spine 1997;22:2489-2494.
4. Ono K, Kaneoka K, Wittek A, Kajzer J. Cervical injury mechanism based on the analysis of human cervical vertebral motion and head-neck-torso kinematics during low speed rear impacts. Society of Automotive Engineers, 41st STAPP Car Crash Conference Proceedings 1997; SAE 973340.
5. Tennyson SA, King AI. A biodynamic model of the human spinal column. Proceedings of the SAE Mathematical Modeling Biodynamic Response to Impact. Society of Automotive Engineers, 31-44, 1976.
6. Matsumoto M, Fujimara Y, Suzuki N, Ono T, et al. Relationship between cervical curvature and disc degeneration in asymptomatic subjects. Journal of Eastern Japan Association of Orthopaedics and Traumatology 1977;9:1-4.

All papers from the Society of Automotive Engineers (SAE) are available directly from that organization. Visit their web site at www.sae.org.