Low Speed Rear-End Collisions

Mechanism of InjuryMechanism of Injury

© Copyright 2002, James Publishing, Inc.

• "Institute researchers evaluated head restraint geometry in more than 200 passenger vehicles, all 1997 models. Measuring the geometry of all seat options in each vehicle model they could find in dealer showrooms, the researchers found fewer than 3 percent with good geometry.“

Insurance Institute for Highway Safety, Status Report, April 12, 1997;32(4):4.

• "The head restraints in about a third of all 1999 passenger vehicles are poorly designed. Only 1 in 20 of the 1999 cars, pickups, and utility vehicles earns a good rating for head restraint design. Disappointing as these findings are, they represent an improvement since 1997 models were evaluated. Then more than half of all passenger vehicles were equipped with head restraints rated poor.“

Insurance Institute for Highway Safety, News Release, June 3, 1999.

• "Most people don't adjust their head restraints, leaving them in the 'down' position. In most cases, this means the restraints provide little or no protection from whiplash injuries. But even in the unadjusted position, the head restraints still meet the federal requirements that have been in effect for 30+ years.“

Insurance Institute for Highway Safety, Status Report, April 8, 2001;36(4):4-6.

Viano and Gargan documented the head restraint position of 1,915 vehicles at an intersection. They found that only 10% of the occupants had the head restraint in the proper position to avoid hyperextension. Only ¼ of the adjustable head restraints were in the “up” position.

Viano DC, Gargan MF. Headrest position during normal driving: implication to neck injury risk in rear crashes. Accident Analysis and Prevention 1996;28(6):665-674.

Proper HeadRestraint Positioning

Improper HeadRestraint Positioning

The Phases of a Rear-End Collision1. Normal Position 2. Spine Straightens

3. Head Extension 4. Rebound

Car seat begins to move forward

Occupant remains stationary, due to inertia

No occupant forces

0 milliseconds

Car seat pushes into occupant’s torso

Torso is accelerated forward with the seat

Head is still stationary, due to inertia

100 milliseconds

150 milliseconds Torso fully accelerated

by the car seat Lower neck is pulled

forward by the rapidly moving torso

Causing the head to rotate backward over the head restraint

Head is still moving backwards

Car seat begins to spring forward

Torso is again accelerated forward

175 milliseconds

Head and torso are accelerated forward ahead of the car seat, resulting in flexion of the spine

300 milliseconds

Normal Cervical Spine

Normal, smooth curvature of the spine

Each intervertebral joint contributes evenly to the motion

Normal Cervical Spine Extension

Motion During a Collision

0 millisecondsAt the moment of impact, the car seat just begins to move and the occupant has not yet been accelerated forward.

Motion During a Collision

50 millisecondsAs 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

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 millisecondsAt 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

75 milliseconds50 milliseconds

Spine Straightens S-Shaped Curve

Motion During a Collision

150 millisecondsAt 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

Motion During a Collision

200 millisecondsFinally, the head and torso are thrown forward by the force of the car seat.

Head thrown forward

Force from car seat

The Cervical Facet Joints

Vertebral BodyFacet Joints

Intervertebral DiscSpinous Process

Facets

Normal Gliding Motion

Abnormal Facet Motion

Pinching of facet

Torso movement forward

Torso moving forward

Stretching of ligaments and disc

Facet Joint

Reflectors

Stretch of facet capsule

Areas of InjuryThe rapid motion of the neck during a crash can result in a number of injuries - many of which are impossible to see on x-rays or MRI. Here are some of the injuries that have been shown after whiplash crashes.

1. Rim Lesions

2. Endplate avulsions

3. Tears of the anterior longitudinal ligament

4. Uncinate process

5. Articular subchondral fractures

6. Articular pillar

7. Articular process

8. Ligament Tear

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

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

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