Neck injury during whiplash increased with head turned postures
Head-turned postures increase the risk of cervical facet capsule injury during whiplash
From: Spine. 2008 Jul 1;33(15):1643-9
Injury to the cervical facet capsular ligaments is a potential mechanism for chronic neck pain after acute whiplash injury. Distending the facet capsule by injecting contrast media has produced whiplash like pain patterns in normal individuals, and anesthetic blocks have isolated the cervical facet joints as the source of pain in about half of a chronic whiplash population. More recently, in vivo animal models of facet capsule loading have shown that group III and IV afferents (thought to mediate pain) from the facet capsule have a graded electrical response to mechanical loading of the facet joint in the goat and have suggested that a capsular ligament strain threshold exists above which allodynia pain in response to a normally nonnoxious stimulus is produced. These data support a facet capsule based mechanism for whiplash injury, but do not establish whether human capsular ligaments are injured in the low speed rear end collisions to which many whiplash injuries are attributed.
Whiplash patients who had their head turned at impact have more severe and persistent symptoms than patients who were facing forward. These findings have prompted biomechanical studies using human cadaveric necks to investigate why a head turned posture increases injury potential. Dynamic rear impact tests of prerotated ligamentous spines (occiput-T1) produce increased neck flexibility (interpreted as injury) in extension, lateral bending and axial rotation. Though concentrated in the lower cervical spine, these injuries were not isolated to particular spinal ligaments. Detailed measurements of the strain field in the facet capsule have also shown that a head-turned posture generates higher capsular strains than a neutral head posture, but the quasi static loads applied during those tests were limited to pure neck flexion/extension moments and did not include the axial compression or posterior shear present during whiplash loading. Thus the question of how a head turned posture combined with multiaxial whiplash loads affects facet capsular ligament strain has yet to be answered.
The goals were to use human cadaveric motion segments to: (1) quantify the intervertebral kinematics and facet capsule strains under whiplash like loads in the presence of an initial axial rotation, and (2) compare the capsule strains generated by these combined loads to the previously published strains needed to injure these ligaments in isolated shear failure. The overall hypothesis was that capsular strains during this simulated whiplash exposure are similar to those needed to injure the capsular ligament.
Axial pretorque and the resulting axial rotation of the intervertebral joint have a large effect on the maximum principal strain in the cervical facet joint capsule when combined with compression, shear, and extension loads simulating a low speed rear end automobile impact. Peak strains in the capsule with an ipsilateral pretorque were double the previously reported peak strains without a pretorque but similar to the previously reported strains to cause partial failures in these specimens. These findings potentially explain the increased severity and persistence of whiplash neck symptoms in patients who had their head turned at impact.
Previous findings suggest that the facet capsules located on the side of the neck towards which a vehicle occupant’s head is turned are most likely to be injured in a rear end crash, although they could find no clinical or epidemiological data to support or refute this proposition.
The quasi static loading rates used in the current flexibility tests and previously published failure tests were similar, but nonetheless lower than those present during actual whiplash exposures. Quasi static loading rates have been shown to affect the magnitude of the load at failure, but maximum principal capsular strain and displacement to failure are not significantly affected by loading rate. Thus aside from the unwinding effect, the capsular strains reported here are expected to be similar to those present during dynamic whiplash events.
During the multiaxial tests, 2 of 13 specimens exceeded the strain needed to cause partial failure of the capsule. Although they could not discount the possibility that other specimens experienced a partial failure during the whiplash like exposures, the potential for 15% of specimens to exceed a threshold for partial failure is consistent with earlier quasi static work and more recent dynamic work. Similar levels of capsule strains have produced behavioral and electrophysiological evidence of short and long term pain in animals, although both animal experiments strained the dorsal aspect of the capsule rather than the lateral aspect studied here. This 15% risk of partial failure in the capsule is similar to the 12% risk of whiplash exposed individuals suffering chronic symptoms ( over 6 months), though considerable work remains to determine whether these similar risk values are related or coincidental.
Two other specimens exceeded the strain needed to cause gross failure of the capsule. There was no evidence of gross failure during their tests and thus this finding likely highlights limitations in their technique. They previously assumed that failures occurred in the element with the highest maximum principal strain, yet in this study they compare whiplash and failure strains quadrant-by-quadrant rather than element-by-element. Regional differences in the ligament could also result in different mechanical tolerances at different locations within a quadrant or element. Moreover, the failure tests were conducted along the anteroposterior axis of the facet joint, whereas the whiplash tests exposed the joint to compound three-dimensional displacements. This means that different ligament fibers may have borne the loads during the whiplash and failure tests. Thus even though their technique provides more detailed strain field information than other recently published techniques, even finer techniques perhaps looking at region specific or fiber specific strains are needed to capture regional differences and properly characterize the capsular ligament’s full three dimensional behavior during whiplash.
The high strain caused by pretorque alone raises the question of why facet capsular ligaments in these joints are not injured when rotating one’s head maximally to the side. Aside from the large rotations taken up at the atlantoaxial joint, one reason may lie in the regional differences described above. The facet capsule likely develops the necessary shape, slack, and tolerance to accommodate voluntary head rotations. The superposition of vertebral retraction during whiplash loading may then shift peak strain to fibers in the capsule that are normally not highly strained during voluntary rotation or combined loading scenarios. Alternatively, the small increase in strain produced by the whiplash loads may be sufficient to injure ligament fibers that are near their limit as a result of a prerotation. Further exploration of this phenomenon will require a more detailed characterization of the dynamic, full field strains in the facet capsule, and definition of the overall and regional tolerances of the facet capsular ligament and its microstructural components.
In summary, they examined the intervertebral kinematics and facet capsule strains under whiplash like loads in the presence of an initial axial rotation. We found that an axial rotation doubles the maximum principal strain in the capsular ligament compared to the neutral posture. We also found that capsular strains during the simulated whiplash exposure with the head turned were not significantly different from maximum principal strain associated with partial failure of the capsule. Thus these findings support the overall hypothesis that excessive capsular strains experienced by some individuals during some whiplash conditions may be responsible for painful capsular whiplash neck injury.
