The neck is a very complex structure. Many medical researchers and healthcare providers have noted that the neck’s structure is more vulnerable and subject to injury than any other portion of the spine. The neck is placed between the thoracic (mid-back) 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 structures.
Physical laws dictate that rear-end collisions produce a sudden acceleration which is transmitted through the car seat to the occupant’s body. The head, being a mass at rest, remains at rest until acted upon by the rear-ending force. The great flexibility of the neck, with the weight of the head, approximately 8-12 pounds, resting upon its otherwise free end, results in a forceful hyper-extension of the neck as the body is accelerated forward. When the head hits the top of the seat or headrest, this impact, plus the reflex contraction of the neck muscles, start the head in a forward motion. The head continues forward 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 hold the head and neck on the body. This type of injury tends to stretch and tear the soft tissue that limit extension (going backward) and flexion (going forward) of the neck.
Insurance adjusters are trained to argue that the nature and extent of traumatic injuries rests 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 restraints, 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. Let’s examine a few variables.
Road Surface Conditions: The acceleration of a car struck from behind can be measured 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 collision, 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 musculoskeletal injury will likely be more severe.
Velocity: 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, hyper extending the cervical spine. At two-tenths (2/10) of a second, the head is launched forward from its pre-stretched position and is stopped by the ligaments, steering wheel, windshield, or the chin hitting 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.
To get a better idea of how much force and acceleration were generated in a rear-end collision, an engineer or accident reconstructionist would ask:
- What distance did the struck vehicle move 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?
- 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.1
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. 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 variable to consider is the distance between the occupant’s head and the restraint at the time of impact. This distance can be affected by the posture of the occupant and by the degree of seat back inclination. The bigger this distance, the less effective the head restraint.
Age: Range of motion in the cervical spine decreases with age, just as the supporting soft tissue become less elastic. Strength of the neck musculature also diminishes with age. Over the adult life span, cervical range of motion is reduced, cervical muscle reflexes slow, and voluntary strength capability diminishes by twenty-five percent (25%). This loss of flexibility and strength significantly increases the potential for serious injury.
Rotated Head: The likelihood of a severe injury is greater when uneven loads are applied to the spine. This can occur when a vehicle is struck in the left-rear corner, as it is turning, or 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 neck’s ability to extend backwards is decreased by fifty percent (50%). This results in increased forces on the joints on one side and different forces on the other. The spaces in the spine through which the nerves pass are smaller when the head is turned, making the nerves more vulnerable.
1 Ian MacNab, “Acceleration Extension Injuries of the Cervical Spine,” Spine, Vol. II, 2nd ed., 1982, p. 654.
