Mechanism of whiplash injury

OBJECTIVE: To propose a different hypothesis of whiplash injury mechanism based on a series of experimental studies summarized in this communication. DESIGN: A series of biomechanical studies simulating whiplash trauma using isolated human cadaveric spine specimens. BACKGROUND: Whiplash injuries are on the rise as reported in several recent studies, due primarily to the increased traffic density. Although the symptoms associated with whiplash have been described, our understanding of the injury mechanism remains poor. The prevailing view of neck hyper-extension causing the injury has not been supported by recent experimental studies. METHODS: Eight fresh human cadaveric cervical spine specimens were prepared and traumatized to varying degrees under controlled conditions using a bench-top model of whiplash trauma. Before and after each trauma, the specimen was studied by functional radiography and flexibility test to document changes in the anatomic alignment and biomechanical properties at each level indicating injuries sustained. At the end of all testing, CT-scans, MRI and cryomicrotome images were obtained. During each trauma, relative motions of all intervertebral joints were recorded with a high speed movie camera. Elongations of the vertebral artery and several capsular ligaments were also monitored during the trauma using specially designed transducers. RESULTS: The hyper-extension hypothesis of injury mechanism was not supported by these studies. We found a distinct bi-phasic kinematic response of the cervical 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 spine were extended, and the head reached its maximum extension. The occurrence of anterior injuries in the lower levels in the first phase was confirmed by functional radiography, flexibility tests and imaging modalities. 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 during the S-shape phase of whiplash. CONCLUSION: We propose, based upon our experimental findings, that the lower cervical spine is injured in hyperextension when the spine forms an S-shaped curve. Further, this 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 provide truer understanding of whiplash trauma and may help in improving the diagnosis, treatment, and prevention of these injuries.