The Biomechanics Behind Whiplash Associated Disorder
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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 from a motor vehicle collision. The prognosis of WAD tends to be unknown and unpredictable, there can be two types of cases, acute or chronic.  Acute cases normally have a full recovery whereas chronic cases lead to long term pain as well as future disability.  Most whiplash associated disorders are minor soft tissue injuries lacking evidence for fractures.
The Quebec Task Force produced a mechanism used to classify patients suffering from whiplash associated disorder, based on the severity of both signs and symptoms. There are different grades ranging from I to IV, these are described below.
· Grade I: Patient experiences 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.
The mechanism of injury in whiplash associated disorder occurs in 3 stages. In the first stage, both the upper and lower spine experience flexion, causing a loss of cervical lordosis.  In stage two, extension of the lower vertebrae gradually leads to extension in the upper vertebrae which causes the cervical spine to adopt an S-shaped curve. The lower vertebrae is subjected to an extension moment, whereas at the upper levels a flexion moment occurs.  Finally, in stage three the neck is completely extended with a shearing force causing compression of facet joint capsules. 
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 accelerating forward causing the seat to interact with them, this occurs in the seatback causing the thorax to drive anteriorly.  With forward acceleration of the thorax, the inertia of the body causes deformation of the seatback, it then deflects towards the rear of the car. In time, inertial loads from the torso are overcome by the seatback, leading it to rebound forward.  This drives the person’s body forward and into the seatbelt. During the primary phase of the impact, the head-neck complex is subjected to inertial loading. The specific loading that occurs will have a structural response on the cervical spine, but this would be influenced by the characteristics of the input acceleration applied at the cervicothoracic junction as well as the interaction of the head and its spinal components.)
Crash dummies, human cadavers, human volunteers and computer models have all been used to study the kinematics and kinetics of whiplash associated disorder. These studies have demonstrated that the cervical spine is subjected to a horizontal shear or retraction during the initial stages of rear-end collisions.  The studies showed different tissues and joints that could be damaged/injured during impact.
- Anterior Longitudinal Ligaments and Discs
Starting with anterior longitudinal ligaments, cadaveric studies of whiplash injuries have found tears as well as rim lesions of the anterior anulus fibrosus.  The strains that occur in the anulus fibrosus of lower cervical discs have been found to exceed physiological limits. These findings suggest a potential pathologic mechanism for tears of the anterior disc or of the anterior longitudinal ligament being a source of nociception following whiplash injury.
- Dorsal Root Ganglion
The dorsal ganglia can become damaged from a whiplash injury. They are at risk for injury because of the rapid changes that occur in the canal pressure, which are caused due to the rapid head motions from the impact. This knowledge offers a potential mechanism of how neck and shoulder pain may result from nerve root trauma caused due to whiplash.
- Vertebral Artery
The incidence of cervical arterial dissections was found to be significantly greater in those who experienced whiplash compared to healthy controls, in a retrospective analysis of 500 whiplash patients. It is thought that vertebral artery injury may occur from an intimal tear. These occur most often at the C1-2, which is the principal location of cervical axial rotation.  Vertebral artery injury has been suggest to be caused by cervical spine extension combined with axial rotation occurring beyond the physiological limit.  Previous biomechanical work, provides evidence for this non-physiological coupled neck motion causing vertebral artery injury due to elongation during vehicle collisions. 
In response to the impact induced lengthening caused by reflex neck muscle activation, direct injury to muscles may occur.  Previous biomechanical studies have found muscle fascicle strains occurring in the sternocleidomastoid muscle and the semispinalis capitis muscle in human subjects The strains were larger than those shown in the injury data, suggesting that the acute phase of whiplash injury may be the cause of muscle lesions rather than the chronic phase.
Biomechanics are important in both assessing and understanding treatment procedures that can be done. By appreciating the biomechanics behind whiplash associated disorder we can analyze better treatments as well as improve techniques that are more specific to how injuries are caused. By looking at the biomechanics we can interpret the lesions caused on the different structures around the neck and prepare better plans as well as researching further on how to prevent such lesions.
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