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Shaw Chiropractic
A Medical-Legal Newsletter for Personal Injury Attorneys
by Dr. Steven W. Shaw

Whiplash Kinematics and Biomechanics
Despite an increasing knowledge base developing from the whiplash literature I am still frequently asked to explain how an occupant can have a whiplash syndrome in low impact trauma. The supposition is that if the neck hyper-extension does not occur then injury cannot take place. This newsletter will explain the most accepted theory of whiplash biomechanics.
The following is a composite of all of the relevant literature, relying heavily on the work of Ono et al. and Siegmund et al which has been compiled by Dr. Arthur Croft. Bear in mind, of course, that special risk variables can significantly alter the kinematic response to cervical acceleration deceleration (CAD) trauma. This model is based on an ideally positioned occupant, in a relatively good state of health, looking straight forward, wearing restraint belts, using a head restraint, and seated in a standard car seat and in a car struck squarely from the rear (180°) with good bumper alignment (i.e., no over-ride or under-ride, and no offset), free runout (i.e., no second collisions). The model is also applicable to speed changes of about 2.5 mph to 6 mph.

Phase 1 (0-50 msec)
After impact, the car seat pushes forward against the occupant. Most seat backs will yield elastically from a typical starting inclination of 20-25° to about 30-35°. The head's inertia holds it initially in space. At about 25msec after impact, the head and T1 begin to accelerate, the head slightly faster than T1. The thoracic and cervical spines begin to straighten, resulting in an upward motion of the torso, which is enhanced by the ramping up of the torso against the seat back. This results in high compressive forces in the neck. Head rotation begins about 50-70 msec after impact

Phase 2 (50-100 msec)
During this phase the neck assumes the s-shaped configuration, with hyperextension occurring in the lower cervical spine (chiefly C5-6), while flexion occurs in the upper spine. Due to compression of the spine, spinal ligaments become slack, thereby reducing the stiffness of the spine up to 70%. Maximal compression occurs around 50 msec, followed quickly by an increasing shear. The s-shaped curve is associated with the head lag period and the spine is subjected to high shear force. All of this happens prior to the head strike against the head restraint at about 90-120 msec--if it is in good enough position. This is when peak head horizontal acceleration (about 2.5x the acceleration of the vehicle) and peak head over-speed (about 2 times the speed change of the vehicle) occurs. By 80-110 msec the head is beginning to move relative to the earth, but by this time up to two inches of retraction has occurred between the head center of mass and T1. Most of the injury probably occurs during this phase.

Phase 3 (100-150 msec)
Torso extension reaches a maximum around 130 msec and extension of the head begins. Ramping motion reaches its maximum at about 150 msec, coinciding with cervical spine extension. Depending on the occupant's starting position, relative position of the head restraint, interaction with the seat back, and stature, injury mechanisms can continue through this phase.

Phase 4 (150-300 msec)
The head/neck extension angle become maximum at about 250 msec, but this is largely dependent upon the occupant's starting position, relative position of the head restraint, interaction with the seat back, stature, and the acceleration level. Accordingly, some potential for injury exists in this phase as well. The general sequence of events is portrayed in the figure. Here the s-shaped curve configuration is illustrated, with hyperextension of the lower segments and flexion of the upper segments. This is probably the chief point of injury and occurs before the head strikes the head restraint.

As you can see, the majority of injury potential occurs in Phase 2. This is when segmental hyperextension and hyperflexion occur simultaneously and at different segmental levels. Interestingly, this occurs before head impact.