Mechanics of Whiplash

Initially, in a rear-end impact, the torso of the victim translates backward while the head and neck remain stationary and the vehicle moves forward underneath. This is followed by the abrupt upward movement of the torso as the thoracic kyphotic curvature is straightened.² This sets the head into vertical acceleration that straightens and compresses the cervical spine. This is followed by a series of abnormal distortions of the neck. First there is an initial flexion of the upper cervical spine, followed by extension of the lower cervical segments.² This induces an S-shaped distortion in the entire cervical spine.^3–6 This is then followed by extension of all levels of the cervical spine.⁵ Yang and King⁷ also found significant posterior shear deformation present with large facet capsular stretch. They considered this to be a major source of pain. Following the extension phase, the head is cantilevered forward into flexion with fanning of the spinous processes.⁸

Structures Injured during the Four Phases of Whiplash

When a vehicle is struck from behind, the occupant’s torso is accelerated while the unrestrained head and neck lag behind.⁸ As the head and neck are forced into extension, the anterior cervical muscles are stretched while contracting in an effort to prevent hyperextension. Most frequently injured by this phase are the “anterior strap muscles,” including the sternocleidomastoid and scalene muscles.² The splenius capitus muscles are frequently injured, especially if the head and neck are rotated at the time of impact. Having the head turned at time of impact increases the risk of injury.⁸

Structures injured during the second phase of whiplash from a rear-end collision are those vulnerable to shear strain. Penning⁹ speculates that the primary mechanism of injury in whiplash is actually hypertranslation of the head backward. He notes that it is the overstretching of the ligaments of the upper cervical spine, especially of the atlantoaxial segment (including the alar ligament), that leads to disorders of proprioceptive information. When ligaments and joint capsules are stretched, the axial traction permits the joints to separate and then subsequently compress with jamming and altered alignment. Ligamentous injury occurs when the cervical muscles become stretched to the point that the ligaments are called into play to stabilize the spine.² When the ligaments become stretched, further injury to the discs and articular capsules can ensue. Disc injury usually consists of a disruption of the annulus fibrosus viewed on radiographs as a widening of the posterior disc space, a narrowing of the anterior disc space, and often a concomitant anterior hypermobile subluxation caused by disruption of the posterior elements.⁸ During phase 3, acceleration is diminished, with the head and torso thrown forward, straining the superficial posterior cervical muscles, including the upper trapezius. During phase 4, if the body is restrained by a seatbelt, the head will continue to move forward until it strikes the chest or an external object. This is the phase when the upper cervical posterior muscular and ligamentous elements of the cervical spine, including the suboccipital muscles, become injured. Croft⁸ maintains that it is the upper cervical spine that sustains the greatest injury from whiplash because it tends to be the biomechanical pivot point sustaining the greatest whiplike action.

The biomechanical injuries seen clinically from the whiplash mechanism tend to follow a characteristic pattern. Ligamentous sprain at the C4-C5 to C5-C6 segments creates a hypermobility that may account for the degenerate changes commonly seen in the mid cervical region as a sequelae to whiplash injury. Hyperextension injuries caused by rear-end collisions frequently strain the anterior strap muscles (scalene and sternocleidomastoideus). If the head is rotated and tilted to one side, the torsional effect causes greater damage on one side than on the other. Forceful hyperextension injuries may produce traction on the anterior longitudinal ligament, which sprains the fibers attached to the intervertebral disc. An avulsion fracture may occur as a piece of bone is torn from the inferior margin of the vertebral body.¹⁰ Rupture of the underlying annulus may occur, with displacement of nuclear material.¹¹ Compressive forces on the posterior structures may produce avulsions of the capsular ligaments, as infolding or creasing of the interlaminar ligaments and damage to the articular cartilage as the posterior joints are jammed together.¹² Extension with compression can produce a crushing of the posterior elements of the vertebra as in a roll over crash.

Hyperflexion injuries caused by head-on collisions may tear or stretch the nuchal ligaments, the capsular ligaments of the Luschka and posterior facet joints, the interspinous ligaments, and the other posterior ligaments of the neck.

Dislocation of the posterior facets with or without cord injury may occur, and in severe cases,¹² fracture of the posterior elements of the vertebrae as they are forced apart can occur. Compression fracture of the vertebral bodies can also occur and may not be visualized in early radiographs, becoming evident when more compression and healing have occurred.⁶

Whiplash injuries to neurological structures include contusion of the brain and spinal cord.¹³ Damage to the cortex and cerebellum may occur from a contracoup as the brain hits the inner table of the skull on the opposite side, as well as from a direct blow to the skull. Damage to the spinal cord is produced by a combination of hyperextension and backward shearing forces. Trauma to the cord may also occur as the result of edema as well as transection. If the head is rotated at the time of impact, the shearing force may fracture a vertebral arch or posterior facet.⁶ The lateral masses of the atlas and axis may suffer compression fractures, and a transverse process fracture may occur on the side of rotation. Ligaments on the contralateral side from rotation may be torn, causing dislocation of the atlas or axis.⁶ More commonly seen are rotational subluxations of the atlas or the axis. Described by Jacobson and Adler¹⁴ in 1956 as a pathological fixation in a position within the normal range of motion, this condition was described in detail by Coutts¹⁵ in 1934. Wortzman and Dewar¹⁶ report that rotational fixation is usually of a moderate nature, such as occurs in a flexion-extension injury in a rear-end collision. (See Chapter 8.) Current imaging using cervical roentgenography and conventional or computed tomography are used to show dislocations, subluxations, and fractures.¹⁷ Magnetic resonance imaging (MRI) is more commonly used to assess different types of soft-tissue lesions related to whiplash injuries. Dynamic imaging may show functional disturbances. Flexion extension views, high resolution static MRI, and especially dynamic MRI warrant more widespread use.¹⁸ (See Chapter 5.)

Injuries caused by side collision may produce strain of the lateral neck muscles and tearing of the alar and atlantoaxial ligaments and upper joint capsules. If severe, a wedging of the lateral aspect of a vertebral body and its associated lateral mass can occur.¹⁹

Direct traumatic insult to the nerve roots can produce neuropathy²⁰ in addition to inflammation in the dural sleeves and perineural tissues, which may result in fibrosis. Adhesions between the dural sleeves and the adjacent capsular structure may prevent normal motion of nerve roots. Irritation of the cervical sympathetics gives rise to a variety of symptoms.²¹ Sympathetic ganglion damage as well as damage to sympathetic fibers in the spinal cord is thought to be responsible for these symptoms. Cervical sympathetic nerve irritation may occur by reflex stimulation as well as by direct trauma. Because of their close proximity to the vertebral arteries, the cervical sympathetics are particularly vulnerable to injury. The vertebral arteries and the encircling sympathetic nerves within the transverse foramen may be subjected to trauma as they are pulled backward against the posterior wall of their bony rings, or by subluxation or fracture of the adjacent bony structure, or injury to adjacent soft tissues.

Grieve²² compares the mechanism of whiplash injury to multiple sprained ankles in the neck, with the added complications of nerve root and plexus traction injury, meningeal irritation, tearing of ligaments and muscle fibers, and trauma to blood vessels and lymphatics. He notes that the overall effect is an upset of sensitive structure and delicately balanced function.