Whiplash Injury

The Cervical Facet Capsule and Its Role in Whiplash Injury

Since about 1995, the facet joint of the cervical spine has been the center of attention in the study of whiplash injuries. Lord et al1 stated that as many as 60% of whiplash patients' pain could be attributed to the facets. A number of many respected engineers from around the world have demonstrated that the cervical spine undergoes abnormal motion during low speed crashes, and this abnormal motion may be responsible for chronic whiplash symptoms.

One important study from 1998 found that the cervical facet joints seemed to "pinch" one another during the whiplash motion. This detailed study provided strong engineering evidence that the facet was indeed a possible culprit in whiplash pain.

Now, a new study from Duke University adds to this detailed engineering analysis of facet motion. This study's goal was to analyze the amount of load that the cervical facet joints could withstand before failing, and to determine if the facet strength was altered by the starting position of the joints—as may be found when an occupant has his or her head turned at the moment of the collision. The authors were interested in this, because a turned head has consistently been reported in the literature as a risk factor for increased symptoms after a rear end automobile collision.

The researchers performed the experiments with human cadaver cervical joints from the lower cervical spine (C4/C5 and C5/C6). The right facet capsule was prepared with carefully placed markers, so that the researchers could analyze the strains that occurred to the joint during loading.

There were four different types of tests performed:

vertebrae

By carefully examining the amount of strain that the joint experienced, the researchers hoped to determine whether bending the joint—similar to motion experienced during a rear end collision—could result in excessive strain to the joint.

After analysis the researchers found that the twisted vertebrae (those exposed to a pretorque) experienced much greater strains than did the pure flexion-extension motion with no pretorque. The greatest strains were those experienced by a facet with contralateral torque, or those joints that were stretched before the bending motion occurred.
In an occupant with a turned head at the time of impact, contralateral torque would occur opposite the direction of the turned head. For instance, if the occupant's head is turned to the left, the right facet would be stretched, and would experience greater strain during the bending motion.

The researchers then compared these loads to the amount of load required to tear the joint capsule. This is their conclusion:

"Axial pretorque increased initial capsular strains for all loading scenarios. In fact, for flexion after a contralateral pretorque, the maximum strains were nearly twice those of the neutral specimens. These results demonstrate that a pretwist of the head and neck increases strain magnitudes in the facet capsule and indicate that pretwist may provide a mechanical basis for increased injury risk or nociceptive stimulation."

"Capsular strains observed during bending, for all configurations, were less than those required to produce catastrophic injury of the joint. These results show that the facet capsular ligament is not ruptured during the vertebral motions imposed in this study. It is therefore unlikely that vehicle occupants undergo gross failure of the capsular ligament resulting from low-speed, rear-end collisions. However, when subcatastrophic ligament failures are considered, the distinction is less clear. Capsular strains were significantly less in bending for the neutral and ipsilateral pretorque groups than the subcatastrophic ligament failures. The contralateral facet strain was less but not significantly less than the strain to subcatastrophic failure. This finding indicates that some portion of the automobile occupant population may develop strains during bending with a pretorque that are not different in magnitude from those required to cause subcatastrophic injury of the ligament. Assuming a gaussian distribution for both the strain during bending and the strain to subcatastrophic failure, approximately 1% of specimens would develop strains during bending that were greater than those of subcatastrophic failure. Nociceptive activation may also be possible at these strain levels, thereby leading to pain development in the absence of observable joint failure. Although it has been shown that stretching the facet capsule results in increased discharge of the sensory nerve fibers of the facet capsule and joint pain the magnitudes of strains required to stimulate nerve fibers is unknown."

  1. Lord SM, Barnsley L, Wallis BJ, Bogduk N. Chronic cervical zygapophysial joint pain after whiplash: a placebo-controlled prevalence study. Spine 1996;21:1737-1744.>
  2. Winkelstein BA, Nightengale RW, Richardson WJ, Myers BS. The cervical facet capsule and its role in whiplash injury: a biomechanical investigation. Spine 2000;25(10):1238-1246.
  3. Yoganandan N, Pintar FA. Facet joint local component kinetics in whiplash trauma. Advances in Bioengineering. New York: American Society of Mechanical Engineers, 1997;36:221-222.

 

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