The Biomechanics Behind Whiplash Associated Disorder

What is it

Whiplash Associated Disorder (WAD) is a term that is used to describe injuries in and around the neck obtained from deceleration-acceleration movements that occur abruptly, such as motor vehicle accident. ⁽¹⁾ The prognosis of WAD tend to be unknown and unpredictable, there can be two types of cases, acute or chronic. ⁽²⁾ Acute cases normally have a full recovery whereas chronic cases lead to long term pain as well as future disability. ⁽²⁾  Most whiplash associated disorders are minor soft tissue injuries lacking evidence for fractures. ⁽³⁾

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Quebec Classification

The Quebec Task Force is a mechanism used to classify patients suffering from Whiplash associated disorder, based on the severity of both signs and symptoms. ⁽⁴⁾ There are different grades raging form I to IV, these are described below.

·     Grade I: Patient complains of neck pain stiffness or tenderness.

·     Grade II: Patient exhibits musculoskeletal signs which include decreased range of motion and point tenderness.

·     Grade III: Patient shows neurologic signs which may include sensory deficits as well as muscle weakness.

·     Grade IV: Patient shows neck dislocation or fracture. ^{(1) (4)}

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Pathophysiology

Whiplash associated disorder occurs in 3 stages. In the first stage there is a flexural deformation of the neck, where both the upper and lower spine experience flexion, causing a loss of cervical lordosis. ⁽⁶⁾ In stage two the cervical spine assumes an S-shaped curve while it the lower vertebrae begin to extend which gradually causes the upper vertebrae to extend. ⁽⁵⁾ Finally, in stage three the neck is completely extended with an intense sheering force causing compression of facet joint capsules. ^{(5) (6)}

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Kinematics During Rear Impact

We must be able to understand the biomechanics during rear impact to be able to assess the severity of injury it can cause. From the perspective of the driver the impact vehicle is rapidly accelerated forward causing the seat to interact with them, this occurs in the seatback causing the thorax to drive anteriorly. ⁽⁷⁾ As the thorax is accelerated forward the inertia of the body deforms the seatback which deflects it towards the rear of the car. In time the seatback overcomes inertial loads from the torso and rebounds forward. ⁽⁷⁾ This drives the person’s body forward and into the seatbelt. This causes the head-neck complex to sustain inertial loading during the initial stages of the impact. ⁽⁸⁾ The specific loading that occurs will have a structural response on the cervical spine, but this would be dependent on the input acceleration applied at the cervicothoracic junction as well as the interaction of the head and its spinal components. ⁽⁷⁾⁽⁸⁾