Whiplash is a neck injury where, because loads and displacements develop in the neck, they have become a main interest in the study of whiplash injury mechanisms. Human subject studies, however, have only reported the peak speed of the head relative to a fixed reference frame.
Since head acceleration is often the result of head-restraint impact, these peak values might be conducive to whiplash injury and do not reflect loads developed in the tissues of the neck. The kinematics of the head have been computed relative to the axis between the C7-T1 vertebra to supply a better indication of the dynamics. The first peak in the acceleration trace is the result of acceleration of the torso relative to the still-stationary head.
This peak is the larger and later acceleration peak, which is governed by the impact between the head restraint and the head. When head acceleration is measured, a result that highlights the importance of quantifying the relative rather than absolute dynamics of the head, the first negative peak isn’t observed.
Cause of Whiplash Injuries
Cadaveric, animal, and human subject experiments have led investigators to propose various anatomical sites for whiplash injury, including the cervical facet joints, facet capsular ligaments, vertebral arteries, dorsal root ganglia, craniovertebral junction, and cervical muscles. Muscle injury could be responsible for some symptoms seen in patients, however, of the anatomical sites listed above, only the facet joints have been linked to chronic whiplash pain. The facet joints have therefore become the focus of research directed at understanding the mechanical basis for chronic whiplash injuries.
In a study conducted by researchers and colleagues (1996), chronic whiplash pain was relieved in about 60% of whiplash patients by anesthetizing medial branches of the cervical dorsal rami. Articular branches from these nerves run through the capsular tissues and presumably originate in the capsular tissue from mechanoreceptors and nociceptors. Possible injury sites inside the facet joints include fractures of the bony elements, bruising of the rectal folds (menisci), or ruptures or tears of the capsular ligament. Bony fractures and aspect hemarthroses are not commonly observed in whiplash patients and are therefore related to loading. Bruising of the folds is common after fatal chest or head injury, and motions of the cervical vertebra consistent with this type of injury have been documented during the severe loading associated with whiplash injury in human subjects.
Cineradiography used to inspect the intervertebral motion of subjects exposed to simulated influences has shown that the C5 vertebra rotates about a stage than during voluntary expansion moves. This pattern of movement resulted in compression of the facet joints posteriorly through impact-induced motion than during movement and resulted in increased distraction of the vertebral bodies anteriorly. These researchers proposed that the posterior synovial fold might be pinched by posterior compression of the facet joints though this motion that was altered, detected in only four of six subjects. This proposed mechanism of injury is promising, and followup research that quantifies the loads applied to the meniscus during whiplash exposures and the loads necessary to injure the meniscus is necessary to confirm whether this proposed injury mechanism occurs at the loads generated during collisions that produce whiplash injury.
Tears or ruptures to the cervical facet joint capsular ligaments have also been observed under severe loading conditions, and excess capsular ligament strain was proposed as a possible mechanism for whiplash injury under minor- to moderate-loading conditions. The facet joint capsules contain fine, unmyelinated nerves that likely have nociceptive function, and distending these ligaments by injection of contrast media has generated whiplash-like pain patterns in normal individuals. With subsequent loading to failure, the technology strain in the capsular ligaments under both whiplash-like heaps and cadaveric motion sections have recently been quantified. Maximum strains in the facet joint capsular ligaments under loads were, on average, half. In two of the 13 specimens the maximum strains observed in the ligament under whiplash-like heaps were larger than those detected in their initial failure.
This finding suggested that the neck loads that develop during an automobile accident could injure some individual’s facet capsular ligaments. Further work is needed to determine if these ruptures generate pain and whether the subcatastrophic failures identified with ruptures inside the capsular ligaments in the response of the tissues correlate.
Whiplash injury has proved a difficult injury to research because its pathoanatomy has remained poorly understood. The facet joints have been isolated as a site of chronic pain in a large proportion of a population.
Human subject testing has provided the essential kinematic and kinetic response information needed to conduct tests of those cells, and tissue evaluations have led to a possible mechanical explanation for whiplash injury. Additional research is required to complete the connection between the whiplash symptoms experienced by some people and an automobile accident. Better understanding of the etiology of whiplash injury will lead to both improved care and improved methods of injury prevention.
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By Dr. Alex Jimenez
Additional Topics: Automobile Accident Injuries
Whiplash, among other automobile accident injuries, are frequently reported by victims of an auto collision, regardless of the severity and grade of the accident. The sheer force of an impact can cause damage or injury to the cervical spine, as well as to the rest of the spine. Whiplash is generally the result of an abrupt, back-and-forth jolt of the head and neck in any direction. Fortunately, a variety of treatments are available to treat automobile accident injuries.