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Whiplash Injuries Are Worse When the Head is Turned
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Posted on: 07/24/2008
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Whiplash is defined as a sudden extension of the cervical spine (backward movement of the neck) and flexion (forward movement of the neck). Rear-end or side-impact motor vehicle collisions are the number one cause of whiplash with injury. Soft tissues affected can include the muscles, ligaments, tendons, joints, and discs of the cervical spine (neck).
Chronic pain may result from a whiplash injury. This is especially true if the head was turned to the side at the point of impact. Scientists are trying to figure out just what causes this lingering pain for some people.
Animal studies point to a strain on the capsular ligament around the facet joint. There is an electrical response in the facet joint when a mechanical load is applied to the joint. This finding suggests that the capsular ligament has a threshold for pain. This means that pain occurs when the load (strain) placed upon it is above a certain level.
In this study, researchers take a look at human capsular ligaments in cadavers (bodies preserved for study after death). The main interest was in finding out why a head-turned posture during a low-speed crash increases the risk for chronic whiplash pain.
Mechanical testing and motion analysis were performed on 13 cervical spine segments. Special 3-D markers were used on the facet capsular ligament to estimate the amount of strain placed on the ligament. Loads were tested similar to a rear-end impact at eight kilometers per hour (about five miles per hour).
The maximum principal strain (MPS) was measured for each load up to the point of capsular ligament failure. The effects of vertebral motion such as compression, posterior shear, and rotation were also measured. They looked at the distribution of strain in four separate locations of the capsule (top, bottom, front, back).
Most of the strain occurred in the superior (top) and anterior (front) portions of the capsule. The capsule affected was on the same side of the neck toward which the face was turned. The strain on the capsule with the head turned was twice what it was with the head face forward at the point of impact.
When shear forces were added, strain on the same area of the capsule increased even more. Even before shear loads were applied, there was a large increase in strain on the capsule with the head turned. During the tests, 15 per cent of the specimens failed under the strain.
With the head in just the right position and with just the right amount of force, the ligament tensile strength may be overcome by the load. The result is injury to the ligamentous capsule. When the face is turned to the right, the right capsular facet is affected. When the face is turned to the left, the left side is injured.
The authors are still wondering why don't the high strains placed on the facet capsule during normal head turning cause injury. They suggest that maybe the capsule's shape and give (slack) during normal motion is enough to allow natural head turning without maximal strain. Perhaps the added strain of a whiplash-load is just enough to overload the ligament causing tissue failure.
It's likely that there are other explanations as well. The capsular ligament may have different mechanical tolerances at different locations within the ligament. So for example, multiaxial loads may affect the top front portion of the capsule more than the lower bottom half.
The authors suggest ways to continue testing capsular strain during whiplash injury. This first study only begins to scratch the surface of this topic. Other types of load must be examined. Results may differ between males and females. Results may vary with different speeds. And it will be important to figure out why some specimens failed but others didn't.
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References:
Gunter P. Siegmund, PhD, et al. Head-Turned Postures Increase the Risk of Cervical Facet Capsule Injury During Whiplash. In Spine. July 1, 2008. Vol. 33. No. 15. Pp. 1643-1649.
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