Biomechanics of a Cervical Hyper-extension-Hyper-flexion Injury (Whiplash)

By Richard H. Adler, Attorney at Law

The cervical spine is a very complex structure. An understanding of the mechanisms involved in hyperextension/flexion injuries requires the patient’s doctor to be familiar with the anatomy of the neck, how acceleration forces impact the delicate balance of the interconnecting structures in the neck, and how certain physical factors affect the acceleration rate, which consequently affect the degree of severity of injury.

The neck’s structure is:

more subject to injury than any other portion of the vertebral column. It is vulnerably placed between the dorsal spine, which is relatively immobile, and the skull, a weight that must be balanced on the cervical spine and held in place by the supporting capsular, ligamentous, cartilaginous and muscular structure.
Ruth Jackson, M.D., The Cervical Syndrome, 4th ed., Thomas Books, Springfield, Illinois, 1977, pg. 5.

A rear-end collision is an elementary case-in-point of Newton’s physics principals:

Mass in motion remains in motion until acted upon by some eternal force.
The head, being a mass at rest, remains at rest until acted upon by the force of the rear-end collision. The collision produces a sudden acceleration force acting on the struck vehicle which is transmitted through the car seat to the occupant’s body.

As the body is accelerated forward, there is a forceful hyperextension of the neck. The hyperextension is further aggravated by the significant flexibility of the cervical spine upon which rests the head, weighing approximately eight to twelve pounds.

As the cervical spine hyperextends, the head snaps back, striking the back of the seat or headrest. The reflex contraction of the neck muscles, with the sudden impact to the back of the head, suddenly throws the head forward. The head continues forward, hyperflexing the cervical spine, until it is acted upon by some external force, such as contact with the steering wheel, the windshield, or the restraining action of the soft tissue structures which support the head and neck.

This type of hyperextension/flexion injury stretches and tears the supporting capsular, ligamentous, cartilaginous, and muscular tissues of the cervical spine.

A hyperextension/flexion injury most often impacts the cervical spine between the C4-C7 vertebrae. According to Dr. Ruth Jackson, the C4-C5 region receives the greatest stress and strain upon hyperextension, and the C5-C6 area sustains the greatest stress during the hyperflexion stage. (Ruth Jackson, M.D., The Cervical Syndrome, 4th ed., Thomas Brooks, Springfield, Illinois, 1977, pg. 40.) Researchers McKenzie and J.F. Williams, “The Dynamic Behavior of the Head and Cervical Spine During `Whiplash,'” J. Biomechanics, 1971, vol. IV, no. VI, pgs. 470-477.) Yet other researchers, such as Clemens and Burow, studied cadavers subjected to whiplash and found that most injuries occurred at C5-C6 and C6-C7. (H.J. Clemens and K. Burrow, “Experimental Investigation on Injury Mechanisms of the Cervical Spine at Frontal and Rear-Front Vehicle Impacts,” Proceedings, 16th Stapp Car Crash Conference, SAE, Detroit, 1972.)

The insurance industry commonly predicts the nature and extent of soft tissue injuries solely on the extent of damage to the rear-ended vehicle. This practice is misleading and inaccurate. It fails to take into account significant variables such as road surface conditions, degree of velocity and acceleration of the vehicles, size and weight of the vehicles, position of head restrains, age of the occupants, element of surprise, and position of the body and head at the time of impact. Each of these factors affects the degree of severity of an injury.

ACCELERATION: ROAD SURFACE CONDITIONS: The acceleration of a car struck from behind can be measured in a simplistic manner by utilizing a physics mathematical formula to determine the amount of G-forces produced during a collision. However, G-forces by themselves do not measure the true acceleration. A number of other variables may be involved in any particular accident, affecting the degree of force exerted on the occupant’s body. For example, the ability of a car to roll or slide after impact will directly affect acceleration. If the brakes are on at the time of impact, it will accelerate less; however, if the car is on ice, it will accelerate rapidly and the corresponding soft tissue injury will be greater. (Ian MacNab, “Acceleration Extension Injuries of the Cervical Spine,” The Spine, vol. 11, 1975, Ch. 10). The same is true if the car is moving forward at the time it is struck.

ACCELERATION: VELOCITY: As previously discussed, when a vehicle is rear-ended it is accelerated forward. About one-tenth (1/10) of a second later, as the car slows, the torso begins to accelerate, hyperextending the cervical spine. At two-tenths (2/10) of a second, the head is launched forward from it pre-stretched position and is stopped by the ligaments, steering wheel, windshield, or the chin lifting the chest. It is a well-established principle that sudden acceleration caused by a rear-end impact exerts even greater G-forces on the head and the cervical spine than on the struck vehicle. Croft calculates that the forces on the head and neck are 2 to 2.5 times greater (Whiplash Injuries: The Cervical Acceleration/Deceleration Syndrome, 1988), and Ewing estimates that they are up to five times greater. (C.L. Ewing, et al; “Living Human Dynamic Response to G Impact Acceleration: II. Accelerations Measured on the Head and Neck,” Proceedings, 13th Stapp Car Crash Conference, SAE, Detroit, 1968.)

To get a better idea of how much force and acceleration were generated in a rear-end collision, you may want to inquire as to the following:

Distance the vehicle moved after impact.

Were road conditions wet, dry, or icy?

Was the driver braking at the time of impact?

Were items inside the vehicle thrown about?

Did the impact knock off the occupant’s glasses or hat?

Based upon the experimental results of researchers Severy and Mathewson, a crude graph of estimated acceleration forces was constructed.

This graph is based upon data obtained in rear-end impact collisions of like-sized vehicles, during which the struck vehicle’s brakes were not applied. This graph indicates that a 10 m.p.h. rear-end collision results in approximately 5-Gs of acceleration force. This 5-G force is the equivalent of catching a two-hundred (200) pound sack of cement tossed from a first-story window. A 25 m.p.h. rear-end collision results in approximately 10-Gs of acceleration force. In most cases, however, it is impractical to attempt to calculate the G-forces generated in a rear-end collision, simply because the velocity of the striking vehicle is seldom known due to the large mechanical variables that affect the creation of this force.

ACCELERATION: SIZE OF VEHICLE: The relative size of colliding vehicles is also an important variable in determining the extent of injury. For example, a streetcar traveling at a speed of 3 m.p.h. will produce the same amount of damage and acceleration force as a compact car traveling at 40 m.p.h. (Ian MacNab, “Acceleration Extension Injuries of the Cervical Spine,” The Spine, vol II, 2nd ed., 1982, p. 654.)

ACCELERATION: HEAD RESTRAINTS: Head restraints are designed to limit the backward displacement of the head during the acceleration phase of whiplash. Head restraints should be adjusted so that the center is level with the ears. This is about the center point of gravity for the head. However, during the acceleration phase of the whiplash, the torso is forced backward against the seat back and at the same time, may undergo some upward vertical displacement as well, depending upon the degree of inclination of the seat back and the amount of friction between seat back and driver. This phenomenon is known as ramping.

Another important parameter regarding head restraints is the distance at the time of impact between the occupant’s head and the restraint. This distance can be affected by the posture of the occupant and by the degree of seat back inclination. An increase in this distance results in a proportionate decrease in the effectiveness of the head restraint.

ACCELERATION: AGE: Range of motion in the cervical spine decreases with age, along with a concurrent decrease in the elasticity of the supporting tissues. Strength of the neck musculature also diminishes with age. Over the adult life span, cervical range of motion is reduced by an average of nearly forty percent (40%), cervical muscle reflexes slow by twenty-three percent (23%), and voluntary strength capability diminishes by twenty-five percent (25%). This loss of flexibility and strength significantly increases the potential for serious injury. (D.R. Foust, et al, “Cervical Range of Motion and Dynamic Response and Strength of Cervical Muscles,” Proceedings, 17th Stapp Car Crash Conference, SAE Detroit, 1973, p. 285.)

ROTATED HEAD: The likelihood of a severe injury is greater when non-symmetrical loads are applied to the spine. This can occur when a vehicle is struck in the left-rear corner as it is turning left. This type of collision may also occur when the occupant’s head is turned to the side while gazing out a window or talking to another occupant. When the head is rotated 45°, the spine’s extension capability is decreased by fifty percent (50%). This results in an increased compressive load at the facet joint and articular pillar on the ipsilateral side, and an increased tensile load at the facet joint on the contralateral side. The intervertebral foramen is also smaller on the side of rotation and lateral flexion, thereby making the spinal nerve vulnerable to injury.

CONCLUSION: To appreciate acceleration forces and how they affect soft tissue injuries, the health care provider needs to take into account various physical factor involved in the accident. Hopefully, this article provides some food for thought which can be used in your evaluation and treatment of patients with soft tissue injuries.

Very truly yours,

Richard H. Adler
Attorney at Law

Keywords Car Accident Spinal Injury