The document summarizes research on low-speed rear-end collisions and whiplash injuries. It finds that fewer than 3% of 1997 vehicles had head restraints in proper position to prevent whiplash injuries. Only 1 in 20 1999 vehicles had good head restraint design, though an improvement over 1997. Most people do not adjust head restraints, leaving them in the down position where they provide little protection. Proper head restraint positioning is important to avoid hyperextension of the neck during rear impacts. The phases of rear-end collisions can cause straightening and extension of the spine beyond normal ranges and injure ligaments.

1. Low Speed Rear-End Collisions Mechanism of Injury © Copyright 2002, James Publishing, Inc.

4. The Phases of a Rear-End Collision 1. Normal Position 2. Spine Straightens 3. Head Extension 4. Rebound

10. Normal Cervical Spine

12. Motion During a Collision 0 milliseconds At the moment of impact, the car seat just begins to move and the occupant has not yet been accelerated forward.

13. Motion During a Collision 50 milliseconds As the car seatback pushes the torso forward, the spine moves forward, resulting in a straightening of the thoracic and cervical spine. Head remains stationary Seatback pushes torso forward

14. Motion During a Collision This difference in motion between the neck and torso results in an S-shaped curve, where nearly all of the bending in the cervical spine takes place in the lower cervical spine. This rapid bending in just a few joints can result in ligament damage in the lower spine. 75 milliseconds At this point in the collision, the car seat is rapidly pushing the occupant's torso forward, while the head remains stationary due to inertia. Head remains stationary Seatback pushes torso forward

15. 75 milliseconds 50 milliseconds Spine Straightens S-Shaped Curve

16. Motion During a Collision 150 milliseconds At about 150 milliseconds, the torso has pulled so far forward on the lower neck that the head is forced backwards over the head restraint. Depending on the severity of the collision, the ligaments in the front portion of the spine can be injured during this phase of the collision. Head rotates back Seatback pushes torso forward

17. Motion During a Collision 200 milliseconds Finally, the head and torso are thrown forward by the force of the car seat. Head thrown forward Force from car seat

18. The Cervical Facet Joints Vertebral Body Facet Joints Intervertebral Disc Spinous Process Facets Normal Gliding Motion

19. Abnormal Facet Motion Pinching of facet Torso movement forward Torso moving forward Stretching of ligaments and disc Facet Joint Reflectors Stretch of facet capsule

21. Nerves of the Facet Joint The pain from inflamed facet joints is transmitted by the medial branch of the dorsal ramus. Stimulation of the facet nerves often results in referred pain. Facet Joints Medial Branch Dorsal Ramus Spinous Process Spinal Cord

22. Referred Pain Patterns "... the prevalence of cervical zygapophysial joint pain was 60%." The most common facets to be injured were at C2/C3 and C5/C6. Wallis BJ, Lord SM, Bogduk N. Resolution of psychological distress of whiplash patients following treatment by radiofrequency neurotomy: a randomised, double-blind, placebo-controlled trial. Pain 1997;73:15-22. C2/3, C3 C3/4, C4/5, C4 C6/7, C6, C7 C2/3, C3/4, C3 C4/5, C5/6, C4, C5 C4/5, C5/6, C4 C7/T1, C7