Biomechanics of Cervical Hyperextension/flexion Injuries
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.
Many medical researchers have noted that 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.
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. 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 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 musculoskeletal injury most likely will be greater.
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, 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.
To get a better idea of how much force and acceleration were generated in a rear-end collision, it is recommended that the following questions get asked:
- 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?
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,” Spine, vol II, 2nd ed., 1982, p. 654.)
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.
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, 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 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 45E, 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.
To appreciate acceleration forces and how they affect musculoskeletal injuries, the health care provider needs to take into account various physical factor involved in the collision. Hopefully, this article provides some food for thought which can be used in your evaluation and treatment of patients with soft tissue injuries.
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