Whiplash Bio-mechanics
The first factor to consider is that Whiplash injuries (hyper-extension, hyper-flexion injuries) occur in an extremely short period of time. Most of these injuries occur in about one-quarter of a second. This means that the occupants of a vehicle which is struck from behind do not have time to react to the accident. Keep this time-frame in mind as we cover some of the actions that occur.
Vehicle Impact
Let us start with the onset of a standard rear-end collision. Let us consider what happens from the perspective of the driver of the vehicle that was hit from behind. On initial impact where the vehicle in the rear hits the car in front; The force of impact begins to move the front vehicle forward. Since the seat of the car is attached to its frame, the driver’s seat moves forward with the car. But the driver is not attached to the frame of the car, and he/she continues to remain in a fixed position; this is due to inertia. Physics defines inertia as “the tendency of a body to resist acceleration.” Keep in mind, all of this is occurring within milliseconds.
Then, within a faction of a second, the car seat is pushed into the driver’s lower and mid back. This rapid forward acceleration also pushes the lower part of the drivers neck forward (lower cervical spine). This has the effect of straightening out the normal curve in the driver’s neck (the lordotic curve) and the curve in the driver’s mid back (the kypotic curve). This creates an abnormal S shaped curve in their cervical spine (neck).
A considerable amount of damage can be done during this phase (vehicle impact).
o Normally, neck motion is the result of multiple vertebral joints, each of which contribute only a few degrees of motion to an action. Therefore movements like neck extension are made up of the summation of multiple vertebra each adding small degrees of motion to produce the total action.
o When the neck is in this abnormal S-shaped position, the joints of the neck (facet joints) are forced past what is consider their normal physiological range-of-motion limit. This excessive motion causes damage to the area around the spinal joints (facet joints). This damage can include: facet capsule ligament tearing, bony impingements, and intra-articular (within the joint) hemorrhages. The degree of joint damage depends on the severity of collision.
Chronic whiplash and whiplash-associated disorders: An evidence-based approach Journal of the American Academy of Orthopedic Surgeons October 2007;15(10):596-606 Schofferman J, Bogduk N, Slosar P.
Hyper-Extension Phase
This next high-speed, forward motion, jerks the driver’s head back. In many cases the head moves right back over the headrest. This often occurs since most people keep their headrest too low to be effective, or it can occur due to poor head rest design. If the impact of the accident is severe enough, a considerable amount of soft-tissue and joint damage can occur in the front of the neck as the head is thrown back.
Common areas of damage as the head is thrown back:
Joints
o Facet joints are the most commonly injured joints in the neck.
§ These synovial facet joints support weight and control movement between each individual vertebrae.
§ The facet joints are the most common source of chronic pain neck pain after a whiplash injury.
American Academy Orthopedic Surgery 2007; 15:596-606).
Chronic cervical zygapophysial joint pain after whiplash: a placebo-controlled prevalence study. Spine 1996;21(15):1737-1745
Ligaments
o ALL (Anterior Longitudinal Ligament)
§ This ligament runs down the front of the vertebral bodies and prevents excessive extension. Damage to this ligament causes instability in the neck (cervical spine), and can be a cause of chronic neck pain after a whiplash injury. This occurs due to force being transmitted through the posterior spinal structures (facet joints).
Ivancic PC, Pearson AM, Panjabi MM, Ito S. Injury of the anterior longitudinal ligament during whipash simulation. European Spine Journal 2004;13:61-68.
Cholewicki J, Panjabi MM, Nibu K, Macius ME. Spinal ligament transducer based on a hall effect sensor. Journal of Biomechanics 1997;30(3):291-293.
o Facet capsules
§ The facet capsules in the neck are often injured due to the severity of muscle contractions and vertebra motions during whiplash accidents.
An anatomical investigation of the human cervical facet capsule, quantifying muscle insertion area J. Anat. (2001) 198, pp. 455–461
Anterior Muscles and Nerves Injured During Hyper-Extension
· This deep muscle runs along the front of your neck between the top vertebra in your neck (C1) and your mid-upper chest (T3) and is commonly injured in whiplash injuries.
· This muscle (Longus Colli) contains a high density of muscle spindlefibers. Muscles that contain a higher level of these fibers are used for fine motor control, muscle tone, and positional sense. Injuries to these muscles affect a broad range of motor functions.
Fiber composition and fiber transformations in neck muscles of patients with dysfunction of the cervical spine. Journal of Orthopaedic Research 1995;13:240-249
· This superficial muscle overlaps the SCM (sternocleidomastoid muscle).This muscle arises from the connective tissue (fascia) on the upper parts of the neck, chest (pectoralis major), and extends out over the shoulder (deltoid) and down over the collar bone (clavicle).
· The Platysma is supplied by the facial nerve (CN-VII). Injury to this muscle can create trigger points which cause a prickling type of feeling across the face and upper chest.
· These are a group of three muscles on the lateral side of the neck (anterior, medial, and posterior scalenes).
· A network of nerves (brachial plexus) and a major artery (subclavian artery) pass through the anterior and medial scalene muscles.
· This is an area that is commonly injured during whiplash accidents and can cause either neurological or vascular problems from the neck, right down to the hands.
o SCM (Sternocleidomastoid) muscle
· This superficial muscle is located on the lateral anterior side of the neck. It is involved in flexion and rotation of the neck
· Two major nerves (lesser occipital nerve and greater auricular nerve) pass by the SCM. Compression of these nerves generate symptoms ofoccipital neuralgia.
· Occipital neuralgia is a medical condition characterized by chronic pain in the upper neck, back of the head, and behind the eyes. This is sometimes known as C2 neuralgia or Arnold’s neuralgia.
· The Spinal Accessory Nerve provides input (motor innervations) to theSCM and Trapezius muscles. Compression of this nerve can result in limited range-of-motion and decreased strength of the SCM andTrapezius muscles. This will affect shoulder and neck strength.
Hyper-Flexion Phase
Next, the car seat springs forward causing the driver’s whole torso to move forward at a high velocity. Physics defines velocity as “rapidity or speed of motion; swiftness.” Keep in mind as your seat moves forward your head is still moving back. This action is similar to the final moment before a spring releases.
Then, almost instantaneously your torso and head are flung forward. This forward motion is so strong that if the driver does not have their seat belt on they could, depending on force of impact, be thrown right out of their seat into the steering wheel or even through the window. This strong forward action causes the driver’s whole spine to flex forward often past their physiological limits. This action can cause a considerable amount of posterior neck, mid-back, shoulder, and even low back damage.
Common areas of damage as the head is thrown forward:
Joints
o Facet joints - These joints are the most commonly injured joints in the neck.
Posterior Ligaments
o Supraspinous ligament - strong fibrous cord, which connects together the spinous processes.
Posterior Muscles and Nerves
· Group of back muscles that runs beside the vertebrae in an almost vertical direction.
· These neck extensors are commonly injured during whiplash accidents.
o Rectus Capitis Posterior Minor muscle
· Individuals with chronic neck pain exhibit muscle atrophy of this muscle as seen of MRI. This can be caused by trauma or compression of the first cervical branches of the spinal nerves (C1 dorsal ramus). This compression occurs because of entrapment within the rectus capitis posterior major muscle.
· Atrophy of this muscle and C1 injury causes symptoms such as: suboccipital headaches with radiation of pain behind the eyes, dizziness, and benign positional vertigo (BPV). With BPV you will feel a sudden sensation of movement, or spinning, when you move your head or hold it in certain positions.
NEUROGENIC ATROPHY OF SUBOCCIPITAL MUSCLES AFTER A CERVICAL INJURY: A Case Study American Journal of Physical Medicine & Rehabilitation, Volume 77(6) November/December 1998, pp 545-549 Andary, Michael T. MD; Hallgren, Richard C. PhD; Greenman, Philip E. DO; Rechtien, James J. DO, PhD
o Suboccipital Triangle – This is an area in the neck at the base of the skull surrounded by the following three muscles
· Superior and Inferior obliques
o All of the suboccipital muscles are extremely important as they contain very high levels of muscle spindle fibers. Muscle spindles provide postural information to the central nervous system. Damage to these structure in experimental animals cause gait disturbances and ataxia (an inability to coordinate voluntary muscle movements)
o The Suboccipital nerve supplies input to the muscles of the suboccipital triangle. Compression of this nerve can occur at the superior oblique muscle.
· The greater occipital nerve is located directly under the Semispinalis Capitis. Compression of this nerve is one of the causes of cervicogenic headaches, these are referred to as occipital neuralgias.
§ Occipital neuralgia is a medical condition characterized by chronic painin the upper neck, back of the head, and behind the eyes. This is sometimes known as C2 neuralgia or Arnold’s neuralgia.
§ Research has shown that about 85% of patients with whiplash injuries have trigger points in the Semispinalis Capitis muscle.
A Distinct Pattern of Myofascial Findings in Patients After Whiplash Injury Archives of Physical Medicine and Rehabilitation Volume 89, Issue 7, July 2008, Pages 1290-1293
· This neck extensor is commonly injured in whiplash injuries.
Luo Z, Goldsmith W. Reaction of a human head/neck/torso system to shock. Journal of Biomechanics 1991;24(7):499-510
o Transversospinalis (spinalis cervicis, cervical multifidus, rotatores cervical)
· The spinalis cervicis muscle is not present in everyone (inconsistent muscle). Orginates from a ligament in the lower neck (ligamentum nuchae).
· The is some research showing the cervical multifidus msucle can cause increased loading of the joint capsule surrounding the joint in the neck (collision-induced loading of facet capsular ligaments).
· These deep posterior muscles are often injured during whiplash accidents.
Are cervical multifidus muscles active during whiplash and startle? An initial experimental study Gunter P Siegmund, Jean-Sébastien Blouin, Mark G Carpenter, John R Brault, and J Timothy Inglis BMC Musculoskelet Disord. 2008; 9: 80. Published online 2008 June 5. doi: 10.1186/1471-2474-9-80
o Trapezius (upper fibers)
· The third occipital nerve travels under the trapezius muscle until it pierces this muscle and ends up in the lower part of the head (occiput).Compression of this nerve causes occipital neuralgias.
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A whiplash can usually be sustained in a rear car accident, a movement of the head forwards, backwards or sideways. Whiplash injury claims for compensation can range from £1,000 or so to around £86,500, so the availability of damages depends on a variety of factors.
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