Biomechanics of Whiplash
After an automobile accident, neck and back pain is a common consequence of whiplash injury. Unfortunately, the pain a person feels after whiplash is a source of contention because critics do not recognize that an injury to the neck and back can occur in a whiplash accident. Specifically, although clinical indications of whiplash are widely identified, the understanding of the underlying injury mechanism is poor.1 In an effort to dispel this controversy, and to gain information, scientist have studied the motion of the body (kinematics) during traumatic force, such as a car accident, through the use of biomechanical experimentation.
A review of literature on the biomechanics of whiplash reveals data that substantiates the basis for the common whiplash symptoms (i.e., neck pain, headaches, back pain). In 2002, Narayan Yoganandan, and others, published an analysis of Biomechanics of Whiplash Injuries using an experimental model.2 The results of this study demonstrated that the varying movements, during a simulated auto collision, between the head, neck, and spine, showed that they could be correlated with headaches and neck pain. “The [movement] of the soft tissue related structures explain the mechanism of these common whiplash associated disorders.”3
Why do accident victims suffer so much pain from a low speed accident with relatively little damage to the automobile? The reasons became clear to orthopedists Stephen Forman and Arthur Croft, who examined the biomechanics of whiplash resulting from a rear-end collision. Specifically, “[w]hat was discovered was that at 8 mph in a rear impact collision the head and neck were exposed to accelerated forces up to 21/2 times that of the vehicle itself, and at higher-speed collision this acceleration force may increase 4 to 10 times.”1 This explains both why drivers in the striking car do not endure injuries of the same intensity as the victims, and why the victims suffer injuries of great magnitude even though the striking car was operating at a low speed.
The following sequence of events the victim’s body tolerates during a rear-collision were recorded by Foreman and Croft in their book Whiplash Injuries – The Cervical Acceleration/Deceleration Syndrome.2
Initial Phase: (0-100 milliseconds) After 100 milliseconds from impact the shoulder, caught up in the vehicle’s acceleration, also accelerates. At this point the head is not accelerating and instead of moving forward is moving rearward and is moving into extension (pulling or dragging force of a limb) and not flexion (bent). This results in shear stress and axial stretch in the cervical spine. If there is a head rest, the extension of the head will continue until it hits the head rest. If there is no head rests, then the head could extend until there is no more limit to the muscles and ligaments.
Phase I: (100-200 milliseconds) The torso is forced backward into the seat back. There is an abrupt upward movement of the torso while the head and neck remain fixed and there is a straightening of the cervical, thoracic (middle segment of the vertebra column), and possibly the lumber (lower back) spines. Because of this, the cervical (upper back) spine becomes compressed and at the end of this phase the head and neck begin to extend. This also may be a time when there are ruptures of the disc.
Phase II: (200-300 milliseconds) It is during this phase that the temporomandibular joint (TMJ) can be injured. The seat back will now return to its previous position and at this point the torso begins its forward movement and acceleration. The head is still moving in the opposite direct (extension), and the vehicle is at its peak acceleration. This all adds to the shear strain of the lower cervical spine. There is some slack in the lap belt and shoulder restraint system because the torso rises and moves rearward in the seat. This will cause the foot to be drawn away from the brake pedal which may cause the vehicle to accelerate. By the end of this phase both the torso and vehicle will be at their peak acceleration, while the head is just beginning is acceleration phase.
Phase III: (300-400 milliseconds) During this phase the head and the torso are at peak acceleration, while the vehicle’s acceleration is decreasing. If there is any slack left in the restraint system, then this will permit some more forward movement of the torso, pelvis, and head, increasing the potential for injury. Foreman and Croft feel that at this phase there will be reapplied pressure to the break (if it was lost during Phase II), and therefore increase the deceleration process, causing greater flexion rotation forces in the neck and cervical spine.
Phase IV: (400-600 milliseconds) The vehicle is no longer accelerating, while the head and neck are at full deceleration. The torso may be abruptly restrained from the shoulder harness, but the head continues to decelerate unrestrained in a forward arc and this acute and violent bending may be responsible for much of the ligament and muscular soft tissue injury.
1M. M. Panjabi, et al., Biomechanics of Whiplash Injury, Der Orthopade (Springer-Verlag 1998).
2Narayan Yoganandam, et al., Biomechanical Analysis of Whiplash Injuries, 34 Accident Analysis & Prevention 663-71 (Sept. 2002).