MECHANISM OF WHIPLASH INJURY

Objective. To propose a different hypothesis of whiplash injury mechan ism based on a series of experimental studies summarized in this commu nication. Design. A series of biomechanical studies simulating whiplas h trauma using isolated human cadaveric spine specimens. Background. W hiplash injuries are on the rise as reported in several recent studies , due primarily to the increased traffic density, Although the symptom s associated with whiplash have been described, our understanding of t he injury mechanism remains poor. The prevailing view of neck hyper-ex tension causing the injury has not been supported by recent experiment al studies, Methods. Eight fresh human cadaveric cervical spine specim ens were prepared and traumatized to varying degrees under controlled conditions using a bench-top model of whiplash trauma. Before and afte r each trauma, the specimen was studied by functional radiography and flexibility test to document changes in the anatomic alignment and bio mechanical properties at each level indicating injuries sustained. At the end of all testing, CT-scans, MRI and cryomicrotome images were ob tained. During each trauma, relative motions of all intervertebral joi nts were recorded with a high speed movie camera. Elongations of the v ertebral artery and several capsular ligaments were also monitored dur ing the trauma using specially designed transducers. Results. The hype r-extension hypothesis of injury mechanism was not supported by these studies. We found a distinct bi-phasic kinematic response of the cervi cal spine to whiplash trauma, In the first phase, the spine formed an S-shaped curve with flexion at the upper levels and hyper-extension at the lower levels. In the second phase, all levels of the cervical spi ne were extended, and the head reached its maximum extension, The occu rrence of anterior injuries in the lower levels in the first phase was confirmed by functional radiography, flexibility tests and imaging mo dalities. The largest dynamic elongation of the capsular ligaments was observed at C6-C7 level during the initial S-shaped phase of whiplash . Similarly, the maximum elongation of the vertebral artery occurred d uring the S-shape phase of whiplash. Conclusion. We propose, based upo n our experimental findings, that the lower cervical spine is injured in hyperextension when the spine forms an S-shaped curve. Further, thi s occurs in the first whiplash phase before the neck is fully extended , At higher trauma accelerations, there is a tendency for the injuries to occur at the upper levels of the cervical spine. Our findings prov ide truer understanding of whiplash trauma and may help in improving t he diagnosis, treatment, and prevention of these injuries.