J Biomech 2005 (Jun); 38 (6): 1313—1323
Stemper BD, Yoganandan N, Pintar FA
Department of Neurosurgery,
Medical College of Wisconsin,
9200 West Wisconsin Avenue,
VA Medical Center,
Milwaukee, WI 53226, USA
Although considerable biomechanical investigations have been conducted to understand the response of the cervical spine under whiplash (rear impact-induced postero-anterior loading to the thorax), studies delineating the effects of initial spinal curvature are limited. This study advanced the hypothesis that abnormal curvatures (straight or kyphotic) of the cervical column affect spinal kinematics during whiplash loading. Specifically, compared to the normal lordotic curvature, abnormal curvatures altered facet joint ligament elongations. The quantifications of these elongations were accomplished using a validated mathematical model of the human head-neck complex that simulated three curvatures.
The model was validated using companion experiments conducted in our laboratory that provided facet joint kinematics as a function of cervical spinal level. Regional facet joint ligament elongations were investigated as a function of whiplash loading in the four local anatomic regions of each joint. Under the normal posture, greatest elongations occurred in the dorsal anatomic region at the C2-C3 level and in the lateral anatomic region from C3-C4 to C6-C7 levels. Abnormal postures increased elongation magnitudes in these regions by up to 70%. Excessive ligament elongations induce laxity to the facet joint, particularly at the local regions of the anatomy in the abnormal kyphotic posture. Increased laxity may predispose the cervical spine to accelerated degenerative changes over time and lead to instability. Results from the present study, while providing quantified level- and region-specific kinematic data, concur with clinical findings that abnormal spinal curvatures enhance the likelihood of whiplash injury and may have long-term clinical and biomechanical implications.
Key Points about this article from Dan Murphy, D.C.
Thanks to the American Chiropractor for permission to reproduce this information!
The whiplash experiments in this study were collisions at less than 6 miles/hour.
1. This study showed that pre-whiplash accident straight or kyphotic curvatures of the cervical spine increase injury to the facet joint ligaments.
2. Specifically, straight or kyphotic curvatures of the cervical spine increased cervical facet capsular ligament stretch by up to 70%.
3. Increased capsular ligament stretch induces laxity to the facet joint, particularly at regions of kyphotic posture.
4. Increased laxity predisposes the cervical spine to accelerated degenerative changes, over time, and leads to instability. [Very Important]
5. Straight cervical spines and cervical kyphosis affect injury mechanisms and lead to increased acute and chronic disorders.
6. Pre-whiplash impact alignment of the cervical spine significantly affects injury mechanisms and severity of injury.
7. There is increased whiplash injury severity for patients with pre-existing abnormal cervical postures.
8. Chronic illness is associated with abnormal spinal posture. [Important]
9. The first phase of whiplash injury is a non-physiologic S-curvature characterized by flexion in upper and extension in lower cervical segments, and this is when the primary whiplash injury occurs.
10. During the initial whiplash non-physiologic S-curvature phase, the cervical facet joints are injured due to the stretch of the capsular ligaments.
11. Subcatastrophic facet capsular injuries can initiate a pain response.
12. Pre-accident abnormal kyphotic posture increases likelihood of reaching the subcatastrophic threshold for injury.
13. In a 5-year study of 146 patients, those with cervical kyphosis, but with no degenerative changes after whiplash injury, had a significantly higher incidence of disc degeneration in the lower cervical spine (C5-C6) than those with normal curvatures at the time of injury.
14. Disc degeneration at a single level alters segmental loading patterns, which accelerates degenerative changes at adjacent levels.
15. Cervical spondylosis at one level pre-disposes the levels above to early instability and decreased load carrying capacity.
16. Straight or kyphotic curvatures of the cervical spine influence ligament elongations during whiplash trauma, and also influence the long-term prognosis of the injury.
17. Head restraints have limited (5-20%) ability to decrease neck injuries in rear impacts, because greatest ligament stretch occurs during the S-curvature, before the head contacts the head restraint.
18. Capsular ligament stretch from whiplash, particularly in kyphotic postures, leads to motor unit laxity and to spinal degeneration over time.
A 1978 graduate of Western States Chiropractic College, Dr. Dan Murphy is on the faculty of Life Chiropractic College West, and is the Vice President of the International Chiropractic Association. For more information, visit www.danmurphydc.com.