Whiplash Crash Sequence

Whiplash Crash Sequence: What Happens When You Are Hit From Behind?

Upon impact, the target vehicle begins to move forward into the occupant, making contact chiefly through the seat back. In accordance with Newton's 1st law of motion, the occupant's inertia resists this motion. As the seat back continues to move forward, the occupant must yield. Initially, the thoracic curve is flattened by the seat back. This results in a vertical compressive force which is transmitted through the spine. So far, we have not been able to determine to what degree-if any-the lumbar spine also flattens. As the vertical compressive force (-z) continues up the spine, some rise of the torso also occurs. This is called ramping and is halted after 1-3 inches of vertical displacement, usually because of the restraining effect of the seat belt and the weight of the torso. Meanwhile, as the torso now is undergoing both a -z acceleration vertically and a forward x linear acceleration, the head-also acting in accordance with Newton's 1st law of motion-resists this motion. As the vertical force extends upwards into the neck it initiates flexion of the upper cervical motion segments and hyperextension of the lower segments. Compression then quickly gives way to tension as the upward moving head and now downward moving torso attempt to disengage. Also, as the torso moves forward with respect to the head, significant amounts of horizontal shear force occur in the neck roughly parallel to the facet joint line. As this is initiated under conditions of compression, the overall stiffness of the neck may be diminished as a result of ligamentous slack, offering less resistance to shear.

As the torso continues to move forward, the neck begins to pull the head along with it. This has the effect of further flexing the upper cervical spine and hyperextending the lower cervical spine (primarily the C5-6 segments) and the spine assumes an S-shaped configuration The head also is induced to extend along with the neck as the head takes up the backset distance during the head lag phase. Depending on specific head restraint geometry (occupant's position relative to the restraint), head restraint contact will usually occur in about 100 msec (i.e., one tenth of a second), at which time head translational acceleration will peak. Any stored energy in the seat back from its deflection (usually about 5-15 degrees) will be released as the occupant begins to move forward into the re-entry phase. This effectively increases the torso and head speed (overspeed) over that of the torso.

As this change of phase occurs, the direction of horizontal shear reverses rapidly and the rearward bending moment (My) quickly gives way to a forward bending moment (-My). Depending on the initial position of the occupant with respect to safety harnesses, the lap and shoulder portions will eventually restrain the forward moving torso which will effectively aggravate the neck's bending moment since the forward moving head's inertia again is behaving in accordance with Newton's 1st law of motion-this time in the forward (x) direction with some angular momentum and acceleration.

Thus, it is likely that in many instances injury occurs in the initial phase as a result of head lag, compression/tension, and shear effects along joint lines. Lower cervical hyperextension during the S-shaped phase is also likely to be associated with injury. Global hyperextension of the neck can occur, depending on head restraint geometry, but we and other researchers have produced injuries in volunteers well within the normal physiological range of motion of the neck. It is also possible that further injury can occur during the forward phase, and this is somewhat more likely in females (and small males) because of their lower inertia and higher resulting acceleration. It should be pointed out that second collisions may also aggravate this second phase by imposing additional decelerative effects and accentuating neck bending moments and shear forces.

For the last 15 years (5 of which in Ft Myers) Dr. A.J. Rubano has focused a large percentage of his practice and resources to providing quality chiropractic care to victims of motor vehicle crashes.  Dr. Rubano and his staff of chiropractors, chiropractic assistants, and massage therapist use a variety of therapies, exercises, and sate of the art technology including the DRX9000 non surgical spinal decompression system and cold laser therapy to help people whose lives have been interrupted by an accident start feeling better fast.  If you have been involved in a crash of any kind, even if it seemed minor, don't hesitate to call our office to have your questions answered and get your injuries the care you deserve.  In most cases, crash victims have little or no out of pocket cost.