Cervical Disc Injuries

Background

Acute cervical spine injury has been associated with sports such as football, gymnastics, rugby, ice hockey, and diving. Athletes with cervical disc injury may present with neck pain, radicular pain, quadriparesis, or quadriplegia secondary to myelopathy.

Cervical disc injury includes 2 entities. The more common form involves annular tears with herniation of the nucleus pulposus (ie, soft disc herniation). The second type of disc injury is an annular tear without herniation of the nucleus pulposus (ie, internal disc disruption).

When considering the term cervical disc injury, it is important to recognize the natural history of cervical degenerative disease. It is this process that may insidiously predispose one to cervical disc injury of either an acute or chronic nature. Cervical disc injuries can be treated conservatively or by surgery, depending on the clinical presentation.

Return to play is an important but controversial issue following successful treatment of cervical disc injury. No accepted universal guidelines regarding return to play exist.

This article intends to outline the etiopathology, evaluation, and treatment of cervical disc disease. Available guidelines for return to play following cervical disc injury are also presented.

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In a study of asymptomatic individuals younger than 40 years, the incidence of cervical disc herniation was 10%, the incidence of disc degeneration was 25%, and the incidence of foraminal stenosis was 4%.[1] In another study, the incidence of cervical focal disc protrusions in asymptomatic volunteers was 50% and of annular tears at one or more levels was 37%.[2]

PreviousNextFunctional Anatomy

Seven cervical vertebrae articulate with one another anteriorly via the interbody joint with an intervening intervertebral disc and 2 uncovertebral joints. Laterally, they articulate via the paired posterolaterally placed zygapophyseal (facet) joints.

Each cervical vertebra forms a ring with the vertebral body anteriorly, the pedicles laterally, and the laminae posteriorly. The ring is known as the spinal or neural canal. As the vertebrae stack upon one another, the connection of the spinal foramina is known as the spinal canal. Through the spinal canal runs the spinal cord, nerve roots, vessels, and meninges (membranous covering of the spinal cord and nerve roots).

The cervical spinal nerves take origin from the spinal cord as the anterior and posterior rootlets. The posterior rootlet has a segmental brain that lies on its most lateral extent at the inner portion of the intervertebral foramen. The posterior and anterior rootlets join to form a spinal nerve, which is only approximately 2 cm long and lies within the intervertebral foramen. The spinal nerve divides into a posterior and anterior ramus at the outlet of the intervertebral foramen. The spinal nerves exit the intervertebral foramen above the numbered cervical vertebrae, and the thoracic and lumbar nerves exit the intervertebral below the numbered vertebra. Consequently, the eighth cervical nerve exits between the C7 and T1 segment

The symptoms related to pathology at each of the intervertebral disc segments have been well described and are not elaborated on in this article. Note that during dynamic range of motion (ROM), the intervertebral foramen, which houses the exiting cervical nerves, becomes very dynamic. In flexion, the intervertebral foramen enlarges in patency, and it decreases with extension. In rotation, the ipsilateral side becomes smaller, and the contralateral side becomes larger. The extreme changes of the foramina are magnified when motions are coupled with flexion and extension.

Distinctiveness of the cervical disc

The predominance of literature has addressed the lumbar spine; much of the lumbar spine literature has been extrapolated and applied to the cervical spine. Bogduk, using microdissection, systematically evaluated 59 human cadaveric intervertebral discs.[3] The orientation, location, and attachments of each strip bundle of collagen were recorded photographically and in sketches. He concluded that the cervical annular fibrosus did not consist of concentric laminae of collagen fibers, as noted in lumbar discs. Rather, the annulus forms a crescentic mass. The primary thickness is anterior and tapers laterally toward the uncinate processes. Posterolaterally, it is essentially deficient; posteriorly, it is represented by a thin layer of paramedian vertically oriented fibers. The anterior crescentic mass is likened to an interosseus ligament more so than a ring of concentric fibers surrounding the nucleus pulposus.[4]

PreviousNextSport Specific Biomechanics

Cervical spine injury is commonly associated with axial loading with the neck in flexion. In flexion of the neck to 30°, the normal lordosis of the cervical spine is obliterated and axial loading of the head is dissipated through a straight spine.[5] Examples of axial loading in players include a football player striking his opponent with the crown of his helmet, an ice hockey player striking his head on the board while doing a push or check, a diver striking the ground with his head after diving in shallow water, and a gymnast accidentally landing head down while performing a somersault on a trampoline.

The effects of axial loading of the cervical spine include fracture of vertebrae, cervical disc herniations, ligament rupture, facet fracture, and dislocations. The neurologic deficits are greater in athletes with congenital spinal stenosis.[6, 7]

New guidelines in athletic sports have decreased the incidence of spinal cord injury. For example, permanent cervical quadriplegia has decreased significantly in high school and college level football, secondary to changes in the rules involving tackling. The Guidelines of NCCA Football rules committee banned spear tackling in football.[8] In 1977, The American Academy of Pediatrics published a statement banning the use of trampolines in schools because of the high incidence of quadriplegia associated with this apparatus.[9] The Canadian Committee on the prevention of spinal injury due to hockey recommends rules against boarding and crosschecking and on education to avoid spearing and impact with boards.[10] Similar guidelines for diving prohibit diving in water that is less shallow than twice one's height.[5]

Disc herniation resorption

Absorption of a cervical herniated disc has been appreciated. Mochida followed the regression of cervical disc herniation by using MRI. He noted that acutely, active resorption of herniated material occurred. The MRI findings did suggest that part of the resorbed material may have consisted of hemorrhagic substance. Mochida noted that extruded material exposed to the epidural space was resorbed more quickly than subligamentous herniation probably because of increased exposure to the immune system.[11]

Resorption of herniated disc material should not be confused with repair. Injured or degenerative disc material does not repair itself to a significant extent. In review of intervertebral segment physiology and metabolic turnover, Nachemson drew some remarkable conclusions.[12] He cited that diffusion of solutes can take place through the central portion of the endplates, as well as through the annulus fibrosus. There are also vascular contacts between the marrow spaces, the vertebral body, and the hyaline cartilaginous endplates. These vascular contacts are significantly less in discs that show advanced degenerative changes. He also cited that the area between the nucleus and annulus posteriorly is proportionally less than the area of the anterior margins, lending itself to possible nutrient deficiency and hastened fibrotic infiltration.

The surface area for diffusion is smaller posteriorly. Combining the relative diffusion limitations posteriorly and the mechanics of posterolateral disc herniation, it becomes rather apparent why a possible pattern of failure exists in this region.[12]

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Cervical Facet Syndrome

Background

Neck pain is common in the general population and even more common in a chronic pain management practice. Very few reliable epidemiologic studies regarding the prevalence of neck pain exist; however, a Finnish study[1] and a Norwegian study[2] estimated the prevalence of neck pain in the general population to be approximately 34%. Furthermore, the prevalence of chronic neck pain, defined as lasting 6 months or longer, is estimated at approximately 14%.[1, 2]

In 1933, Ghormley coined the term facet syndrome to describe a constellation of symptoms associated with degenerative changes of the lumbar spine.[3] Relatively recently, the term cervical facet syndrome has appeared in the literature and implies axial pain presumably secondary to involvement of the posterior elements of the cervical spine.

Cervical facet syndrome.

Many pain generators are located in the cervical spine, including the intervertebral discs, facet joints, ligaments, muscles, and nerve roots. The facet joints have been found to be a possible source of neck pain, and the diagnosis of cervical facet syndrome is often one of exclusion or not considered at all.

Clinical features that are often, but not always, associated with cervical facet pain include tenderness to palpation over the facet joints or paraspinal muscles, pain with cervical extension or rotation, and absent neurologic abnormalities.[4] Imaging studies usually are not helpful, with the exception of ruling out other sources of pain, such as fractures or tumors. Signs of cervical spondylosis, narrowing of the intervertebral foramina, osteophytes, and other degenerative changes are equally prevalent in people with and without neck pain.[5]

For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center and Osteoporosis Center. Also, see eMedicineHealth's patient education articles Whiplash, Shoulder and Neck Pain, and Neck Strain, and Chronic Pain.

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Aprill and Bogduk estimated the prevalence of cervical facet joint pain by reviewing the records of patients who had presented with neck pain for at least 6 months secondary to some type of injury.[6] These patients underwent discography, facet joint nerve blocks, or both at the request of the referring physicians.

A total of 318 patients were investigated, and 26% of the patients had at least one symptomatic facet joint. However, only 126 patients of the original study group had their facet joints investigated, and 65% of these patients had painful facet joints.[6] Furthermore, 62% of the patients who underwent both discography and facet joint nerve blocks had painful facet joints. This study indicated that the prevalence of cervical facet joint pain may be as low as 26% or as high as 65%, depending on how aggressively it is sought.[6]

A large study by Manchikanti et al involved 500 patients with chronic, nonspecific spine pain. The prevalence of facet joint pain was determined using controlled comparative local anesthetic blocks with 1% lidocaine followed by 0.25% bupivacaine.[7] This study indicated that the prevalence of cervical facet joint pain was 55%.

It seems apparent that the cervical facet joints may be a common source of neck pain; however, there are other pain generators in the cervical spine, such as the intervertebral discs, that may be involved as well. To evaluate the contribution of the disc to neck pain, a sample of 56 patients were selected from the previous study population. This group consisted of patients who had undergone both discography and facet joint nerve blocks at the same segment of the cervical spine as part of the diagnostic process.[8]

The results demonstrated that 41% of this group had a painful disc and facet joint at the same segment, and an additional 23% had a painful facet joint but not a painful disc at the same segment.[8] Therefore, most of the sample had a painful facet joint, but there was often a painful disc at the same level. This finding is not surprising when one considers how the facet joints and discs are intimately involved in motion of the cervical spine.

Cervical facet joint pain is a common sequela of whiplash injury. Barnsley and Lord et al studied the prevalence of chronic cervical facet joint pain after whiplash injury using double-blind, controlled, diagnostic blocks of the facet joints.[9] The joints were blocked randomly with either a short-acting or long-acting anesthetic, and, if complete pain relief was obtained, the joint was blocked with the other agent 2 weeks later. Of the 38 patients who completed the trial, 27 obtained complete relief from both anesthetics and longer relief from the longer acting agent.[9] Therefore, the prevalence of this sample is 54%, making cervical facet joint pain the most common cause of chronic neck pain after whiplash injury in this population.

Lord and Barnsley et al subsequently studied the prevalence of chronic cervical facet joint pain after whiplash injury using a double-blind, placebo-controlled protocol.[10] The sample consisted of 68 consecutive patients referred for neck pain secondary to a motor vehicle accident and longer than 3 months in duration. Those individuals with a predominant headache underwent a third occipital nerve block and were removed from the study if they received pain relief.[10] The third occipital nerve has a cutaneous branch and a branch to the C2-C3 facet joint; therefore, patients with pain from this segment could not participate in the placebo study because they would feel the effects of the local anesthetic. The remaining 41 patients underwent diagnostic blocks with either a short-acting or a long-acting local anesthetic, followed by a second block with either normal saline or the other anesthetic, followed by a third block with the remaining agent.

The investigators reported the positive responders experienced complete relief with each anesthetic and no relief with the normal saline. The prevalence of cervical facet joint pain after whiplash injury was found to be 60%, and the most common levels were C2-C3 and C5-C6.

PreviousNextFunctional Anatomy

The cervical spine is made up of the first 7 vertebrae and functions to provide mobility and stability to the head, while connecting it to the relative immobile thoracic spine (see the image below). The first 2 vertebral bodies are quite different from the rest of the cervical spine. The atlas, or C1, articulates superiorly with the occiput and inferiorly with the axis, or C2.

Cervical vertebra.

The atlas is ring-shaped and does not have a body, unlike the rest of the vertebrae. The body has become part of C2, and it is called the odontoid process, or dens. The atlas is made up of an anterior arch, a posterior arch, 2 lateral masses, and 2 transverse processes. The transverse foramen, through which the vertebral artery passes, is enclosed by the transverse process. On each lateral mass is a superior and inferior facet (zygapophyseal) joint. The superior articular facets are kidney-shaped, concave, and face upward and inward. These superior facets articulate with the occipital condyles, which face downward and outward. The relatively flat inferior articular facets face downward and inward to articulate with the superior facets of the axis.

The axis has a large vertebral body, which contains the fused remnant of the C1 body, the dens. The dens articulates with the anterior arch of the atlas via its anterior articular facet and is held in place by the transverse ligament. The axis is composed of a vertebral body, heavy pedicles, laminae, and transverse processes, which serve as attachment points for muscles. The axis articulates with the atlas by its superior articular facets, which are convex and face upward and outward.

The remaining cervical vertebrae, C3-C7, are similar to each other, but they are very different from C1 and C2. They each have a vertebral body, which is concave on its superior surface and convex on its inferior surface. On the superior surfaces of the bodies are raised processes or hooks called uncinate processes, which articulate with depressed areas on the inferior aspect of the superior vertebral bodies called the echancrure or anvil. These uncovertebral joints are most noticeable near the pedicles and are usually referred to as the joints of Luschka.[11] These joints are believed to be the result of degenerative changes in the annulus, which leads to fissuring in the annulus and the creation of the joint.[12] The spinous processes of C3-C5 are usually bifid, in comparison to the spinous processes of C6 and C7, which are usually tapered.

The facet joints in the cervical spine are diarthrodial synovial joints with fibrous capsules. The joint capsules in the lower cervical spine are more lax compared with other areas of the spine to allow for gliding movements of the facets. The joints are inclined at 45° from the horizontal plane and angled 85° from the sagittal plane. This alignment helps to prevent excessive anterior translation and is important in weight bearing.[13]

The fibrous capsules are innervated by mechanoreceptors (types I, II, and III), and free nerve endings have been found in the subsynovial loose areolar and dense capsular tissues.[14] In fact, there are more mechanoreceptors in the cervical spine than in the lumbar spine.[15] This neural input from the facets may be important for proprioception and pain sensation and may modulate protective muscular reflexes that are important in preventing joint instability and degeneration.

The facet joints in the cervical spine are innervated by both the anterior and dorsal rami. The occipitoatlantal (OA) joint and atlantoaxial (AA) joint are innervated by the ventral rami of the first and second cervical spinal nerves. Two branches of the dorsal ramus of the third cervical spinal nerve innervate the C2-C3 facet joint, a communicating branch and a medial branch known as the third occipital nerve.

The remaining cervical facets, C3-C4 to C7-T1, are supplied by the dorsal rami medial branches that arise one level cephalad and caudad to the joint.[16, 17] Therefore, each joint from C3-C4 to C7-T1 is innervated by the medial branches above and below. These medial branches send off articular branches to the facet joints as they wrap around the waists of the articular pillars.

Intervertebral discs are located between each vertebral body caudad to the axis. The discs are composed of 4 parts, including the nucleus pulposus in the middle, the annulus fibrosis surrounding the nucleus, and 2 end plates that are attached to the adjacent vertebral bodies. The discs are involved in cervical spine motion, stability, and weight bearing. The annular fibers are composed of collagenous sheets called lamellae, which are oriented 65-70° from the vertical and alternate in direction with each successive sheet. Therefore, the annular fibers are prone to injury with rotation forces because only one half of the lamellae are oriented to withstand the force in this direction.[15] The middle and outer one third of the annulus is innervated by nociceptors, and phospholipase A2 has been found in the disc and may be an inflammatory mediator.[18, 19, 20]

Several ligaments of the cervical spine, which provide stability and proprioceptive feedback, are worth mentioning.[21, 22] The transverse ligament, the major portion of the cruciate ligament, arises from tubercles on the atlas and stretches across its anterior ring while holding the dens against the anterior arch. A synovial cavity is located between the dens and the transverse process. This ligament allows for rotation of the atlas on the dens and is responsible for stabilizing the cervical spine during flexion, extension, and lateral bending. The transverse ligament is the most important ligament in preventing abnormal anterior translation.[23]

The alar ligaments run from the lateral aspects of the dens to the ipsilateral medial occipital condyles and to the ipsilateral atlas. The alar ligaments limit axial rotation and side bending. If the alar ligaments are damaged, as in a whiplash injury, the joint complex becomes hypermobile, which can lead to kinking of the vertebral arteries and stimulation of the nociceptors and mechanoreceptors. This may be associated with the typical complaints of patients with whiplash injuries such as headache, neck pain, and dizziness. The alar ligaments prevent excessive lateral and rotational motions, while allowing flexion and extension.

The anterior longitudinal ligament (ALL) and the posterior longitudinal ligament (PLL) are the major stabilizers of the intervertebral joints. Both ligaments are found throughout the entire length of the spine; however, the anterior longitudinal ligament is closely adhered to the discs in comparison to the posterior longitudinal ligament, and it is not well developed in the cervical spine. The anterior longitudinal ligament becomes the anterior atlantooccipital membrane at the level of the atlas, whereas the posterior longitudinal ligament merges with the tectorial membrane. Both ligaments continue onto the occiput. The posterior longitudinal ligament prevents excessive flexion and distraction.[24]

The supraspinous ligament, interspinous ligament, and ligamentum flavum maintain stability between the vertebral arches. The supraspinous ligament runs along the tips of the spinous processes, the interspinous ligament runs between the spinous processes, and the ligamentum flavum runs from the anterior surface of the cephalad vertebra to the posterior surface of the caudad vertebra. The interspinous ligament and especially the ligamentum flavum control for excessive flexion and anterior translation.[24, 25, 26] The ligamentum flavum also connects to and reinforces the facet joint capsules on the ventral aspect. The ligamentum nuchae is the cephalad continuation of the supraspinous ligament and has a prominent role in stabilizing the cervical spine.

PreviousNextSport Specific Biomechanics

The patterns of motion of C2–C7 are determined by the orientation of the facet joints, the intervertebral discs, and the uncovertebral joints. The orientation of the facet joints lead to coupling of rotation and lateral flexion. For example, as the vertebral bodies laterally flex to the left, they also rotate to the left (the spinous processes move to the right). The degree of rotation that is coupled with lateral flexion decreases in the more caudal motion segments, possibly due to the difference in facet orientation in the caudal segments, which may contribute to unilateral facet joint dislocations in the lower cervical spine.[27]

The height of the articular process increases with caudal progression, which determines the quality of flexion and extension and allows more gliding motion in the cephalad segments.[28] Horizontal translation of a vertebral body more than 3.5 mm as measured on a lateral radiograph during flexion and extension is considered to be the upper limit of normal motion.[29]

The orientation of the facet joints alone does not determine the pattern of motion. In the lumbar spine, the pattern of motion does not change after the facets are removed, which implies that the discs and ligaments determine the pattern of motion.[30] Also, because of the orientation of the annular fibers in the disc, there is very little rotation in the lumbar spine.[31] However, it is known that there is a great deal of rotation in the cervical spine. Therefore, the discs do not seem to be the primary determinant of motion in the cervical spine.

The joints of Luschka are suggested to be involved primarily in rotation and may aid in the coupling of rotation and lateral flexion.[32] Another purpose of the joints of Luschka may be to protect the disc from injury as it ages and loses its water content. This may explain why these joints are not present at birth but develop later in childhood.[33]

The orientation of the OA joints allow for substantial flexion and extension (13°), less lateral flexion (8°) and rotation (10°), and minimal translation (1 mm).[34, 35] The AA joints allow for axial rotation of 65°, which is 40-50% of the total cervical spine rotation, negligible lateral flexion, 10° of flexion and extension, and lateral translation of 4 mm.[34, 36] This degree of axial rotation can cause kinking of the vertebral arteries that run in the transverse foramina of C6 to the atlas. The contralateral artery begins to kink at 30° and the ipsilateral artery at 45°.[37] Consequences include nausea, vomiting, visual problems, vertigo, and stroke.[38]

With axial rotation of the atlas on the axis, there is a coupled movement of vertical translation of the atlas, so that it is at its lowest position at the extremes of right and left rotation and at its highest position at neutral. This coupling of translation with rotation is secondary to the orientation of the facets.[4] The instantaneous axis of rotation (IAR) is a term used to describe the motion of one vertebral body in relation to the vertebral body below.

The IAR has been estimated at the OA joint,[36] the AA joint,[34] and in the cervical spine from C2-C3 to C6-C7.[39] In the middle and lower cervical spine, the IAR has been measured for each segment from C2-C3 to C6-C7 in asymptomatic people.[40] In a subsequent study, the IARs were measured in persons with neck pain, who had not received a diagnosis after examination and imaging of the cervical spine.[39] Abnormal IARs were found in 46% of the patients, and an additional 26% had marginal findings. However, the location of the abnormal motion segments did not correlate with the findings on discography or facet joint blocks.

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Cervical Radiculopathy

Background

Cervical radiculopathy is a dysfunction of a nerve root of the cervical spine. The seventh (C7; 60%) and sixth (C6; 25%) cervical nerve roots are the most commonly affected.[1, 2, 3, 4, 5, 6, 7]

Sagittal magnetic resonance image of the cervical spine. This image reveals a C6-C7 herniated nucleus pulposus. Axial magnetic resonance image of the cervical spine. This image reveals a C6-C7 herniated nucleus pulposus.

In the younger population, cervical radiculopathy is a result of a disc herniation or an acute injury causing foraminal impingement of an exiting nerve.[8] Disc herniation accounts for 20-25% of the cases of cervical radiculopathy. In the older patient, cervical radiculopathy is often a result of foraminal narrowing from osteophyte formation, decreased disc height, degenerative changes of the uncovertebral joints anteriorly and of the facet joints posteriorly.

Factors associated with increased risk include heavy manual labor requiring the lifting of more than 25 pounds, smoking, and driving or operating vibrating equipment. Other, less frequent causes include tumors of the spine, an expanding cervical synovial cyst, synovial chondromatosis in the cervical facet joint, giant cell arteritis of the cervical radicular vessels, and spinal infections.[9, 10] The purpose of this article is to provide information on the presentation, evaluation, differential diagnosis, and treatment of cervical radiculopathy.

For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education articles Shoulder and Neck Pain and Neck Strain.

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Cervical radiculopathy occurs at a much lower frequency than radiculopathy of the lumbar spine. The annual incidence is approximately 85 cases per 100,000 population.

PreviousNextFunctional Anatomy

Seven cervical vertebrae and 8 cervical nerve roots exist. The C1-2 (atlantoaxial) joint forms the upper cervical segment.[1, 3, 11, 12] This joint allows for 50% of all cervical rotation. The occipitoatlantal joint is responsible for 50% of flexion and extension. Below the C2-C3 level, lateral bending of the cervical spine is coupled with rotation in the same direction. This is due to the 45° inclination of the cervical facet joints.

The vertebral bodies of C3-C7 are similar in appearance and function. They articulate via the zygapophyseal or facet joints posteriorly. On the lateral aspect of the vertebral bodies are sharply defined margins, which articulate with the facet above. These articulations are called uncovertebral joints, or the joints of Luschka. These joints can develop osteophytic spurs, which can narrow the intervertebral foramina.

Intervertebral discs are located between the vertebral bodies of C2-C7. The discs are composed of an outer annular fibrosis and an inner nucleus pulposus and serve as force dissipators, transmitting compressive loads throughout a range of motion (ROM). The intervertebral discs are thicker anteriorly and therefore contribute to normal cervical lordosis.

The foramina are largest at C2-C3 and progressively decrease in size to the C6-C7 level. The nerve root occupies 25-33% of the foraminal space. The neural foramen is bordered anteromedially by the uncovertebral joints, posterolaterally by facet joints, superiorly by the pedicle of the vertebra above, and inferiorly by the pedicle of the lower vertebra. Medially, the foramina are formed by the edge of the end plates and the intervertebral discs. The nerve roots exit above their correspondingly numbered vertebral body from C2-C7. C1 exits between the occiput and atlas, and C8 exits below the C7 vertebral body. Degenerative changes of the structures that form the foramina can cause nerve root compression. This compression can occur from osteophyte formation, disc herniation, or a combination of the 2.

PreviousNextSport-Specific Biomechanics

Cervical radiculopathy in athletes can occur from several mechanisms. These injuries can occur from an extension, lateral bending, or rotation mechanism, which closes the neural foramen and results in ipsilateral nerve root injury. Conversely, a traction injury can occur with a sudden flexion or extension, coupled with lateral bending away from the affected nerve root.

Additionally, cervical disc herniations can occur with a sudden load with the neck in either flexion or extension. In elderly persons with osteophyte formation, repetitive neck extension and rotation in certain sports, such as swimming or tennis, may result in a more insidious injury.

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Cervical Discogenic Pain Syndrome

Background

Cervical intervertebral disc disease accounts for 36% of all spinal intervertebral disc disease, second only to lumbar disc disease, which accounts for 62% of all spinal intervertebral disc disease. Cervical problems tend to be less debilitating than lumbar problems, and they do not cause individuals to miss work as often as lumbar spine problems do.[1, 2]

One of 5 visits to an orthopedic practice is for cervical discogenic pain (CDP), with C5-6 and C6-7 accounting for approximately 75% of visits. C7 is the most common nerve root involved.[3] Cervical discogenic pain syndrome (CDPS) presents with proximal symptoms first, and, later, it can progress to brachialgia.

For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education articles Shoulder and Neck Pain and Neck Strain.

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Cervical intervertebral disc disease accounts for 36% of all spinal intervertebral disc disease. This condition is somewhat more common in women. Although acute attacks may start at a very young age with episodes of acute torticollis or "wry neck," the incidence peaks when persons are aged 45-50 years (see image below).

Appearance of torticollis as a result of sternomastoid fibrosis in a young child.

Of all sports-related injuries, 2-3% are spinal injuries and the majority of these happened during unsupervised activities such as football, soccer, wrestling, diving, surfing, skiing and sand lot games.[4] The majority of the available literature, however, is found for football and this group is the most likely to sustain cervical trauma.

Statistical estimates of the incidence of cervical injury for football players varies ranging from 1 quadriplegic injury per 7,000 to 1 injury per 58,000.[5] Another review reported that since 1977, there has been an annual incidence of fewer than 10 cases of permanent injury to the cervical spinal cord among football players.[6] In 1976, the National Collegiate Athletic Association football rules committee disallowed the technique of spear tackling or the technique of using the helmet to butt or ram an opponent. This resulted in a remarkable decrease in the incidence of catastrophic neck injuries over the next 9 years.[7]

PreviousNextFunctional Anatomy

The cervical spine permits a wide range of motion (ROM) of the head in relation to the trunk. A degree of stability and flexibility is required to control the motion and dissipate the forces applied to the spine. Great differences in anatomy and function exist between the occiput-C1, the C1-2 (upper complex), and C3-C7 (lower complex) levels. Eight motion segments occur between the occiput and T1. No disc exists between C1 and C2; therefore, the first intervertebral disc is between C2 and C3.

The intervertebral disc consists of an outer annulus fibrosus and an inner nucleus pulposus. The intervertebral disc is thicker anteriorly, contributing to the normal cervical lordosis. The C6-7 disc is the thickest disc of the cervical spine. The nucleus pulposus and the inner one half of the annulus fibrosus are avascular and receive nutrition through diffusion, compression, dehydration, and imbibition of fluids.[8]

The annulus fibrosus, particularly the outer one third, has been found to be innervated by the sinuvertebral nerve and the vertebral nerve. The sinuvertebral nerve arises from the ventral ramus (somatic root), whereas the vertebral nerve (autonomic root) is derived primarily from the sympathetic nervous system. However, the vertebral nerve has connections with the cervical ventral rami, which suggests the possibility of the vertebral nerve also conveying somatic afferents from the disc.[9, 10, 11]

The nociceptors and mechanoreceptors in the annulus fibrosus mediate pain transmission from structural disruption of the intervertebral disc itself or from the chemically mediated inflammatory effect of phospholipase A2.[10, 12] Pacinian corpuscles and Golgi tendon organs present in the posterolateral region of the outer one third of the annulus transmit proprioceptive information from the intervertebral disc.[8, 12, 13, 14, 15]

The adult cervical disc has a crescentic shape anteriorly, with the apex of the crescent at the uncovertebral joints on each side. The posterior annulus has multiple vertical fissures allowing for a very degenerative appearance during discography and on gross examination. In addition, the nucleus of the cervical disc tends to be poorly centralized when compared with the lumbar disc. In the lumbar disc, the nucleus tends to be well localized in the center of the disc, and the posterior annulus tends to remain relatively intact when compared with the cervical disc. Annular fissures in the lumbar disc tend to be circumferential and/or radial in nature.

PreviousNextSport-Specific Biomechanics

Biomechanics is the study of the changes in the anatomic structures occurring during body movements. The movements of the cervical spine include flexion and extension in the sagittal plane, lateral flexion in the coronal plane, and rotation in the horizontal plane. Lateral flexion and rotation occur as coupled movements. Other movements of the cervical spine include protrusion (ie, the head is moved as far forward as possible with the neck outstretched and maintaining forward-facing position) and retraction (ie, the head is moved as far backward as possible and maintaining a forward-facing position).

Fifty percent of rotation of the cervical spine occurs in the upper cervical complex with the atlas rotating ipsilaterally around the odontoid. Protrusion causes upper cervical spine extension and lower cervical spine flexion, whereas retraction causes upper cervical spine flexion and lower cervical spine extension. At the occiput-C1 and C1-2 levels, ROM is greater with the protruded and retracted position than with full-length flexion and full-length extension positions.[16] See the image below.

Three-dimensional computed tomography scan of C1.

The annular fibers are made up of collagenous lamellae with alternating directions of inclination oriented 35° from the horizontal. The annulus is more susceptible to injury with rotation and translation movements due to resistance offered only by the lamella oriented in the direction of movement. In the cervical spine, as in the lumbar spine, the intervertebral disc dissipates the transmission of compressive loads throughout the ROM by slowing the rate at which these forces are transmitted through the spine. By diverting the load via temporarily stretching the annular fibers, the disc protects the vertebra from taking the entire load at once.

In asymmetric loading, the nucleus pulposus migrates toward the area with less load. Thus, in flexion movements of the cervical spine, anterior offset loading of the intervertebral disc occurs, in which the nucleus pulposus moves posteriorly and the posterior annular wall is stretched. In addition, the cervical lordosis reduces, the vertebral canal lengthens, and the intervertebral foramina open.[2]

In extension movements of the cervical spine, posterior offset loading of the intervertebral disc occurs, in which the nucleus moves anteriorly and the anterior annular wall is stretched. Shortening of the vertebral canal and closing of the intervertebral foramen also occur.[2] In lateral flexion and rotation (coupling movement) of the cervical spine, there is offset loading of the intervertebral disc on the side of flexion and rotation, with nuclear material moving to the opposite side (site of the convexity), and the posterolateral annular wall is stretched.[2]

The intervertebral foramina house the exiting cervical nerves. The largest cervical spine foramen is at the C2-3 level, and the smallest foramen is at the C6-7 level.[17] The cervical foramina become very dynamic during cervical spine ROM. The intervertebral foramina enlarge with flexion and decrease with extension. In rotation, the ipsilateral side becomes smaller, and the contralateral side enlarges. The extreme changes of the foramina occur with coupled movements (ie, flexion-rotation and extension-rotation-lateral flexion).[18]

In addition to the above biomechanical concerns, cervical spinal stenosis has been evaluated with regard to catastrophic cervical sports injuries. The Torg/Pavlov ratio (measured by dividing the sagittal diameter of the spinal canal by the sagittal diameter of the vertebral body) when less than 0.8 was thought to subject the football player to high risk of cervical cord injury due to suspected cervical stenosis (see image below). However, subsequent studies found that this ratio may be erroneously low in players that have wide vertebral bodies. A study by Cantu suggested that functional stenosis as documented by myelogram or magnetic resonance imaging (MRI) may be a more appropriate measure of stenosis.[6]

Lateral cervical spine plain radiograph illustrating the Torg/Pavlov ratio. Classification of athletic cervical spine injuries

A review by Bailes and Maroon classified athletes with cervical injuries into 3 types[4] :

Type I injuries were those that caused permanent spinal cord damage, including conditions such as anterior cord syndrome, Brown-Sequard syndrome, central cord syndrome, and mixed incomplete syndrome.Type II injuries were classified as those that occur transiently after athletic trauma with normal neurologic examination and normal radiologic evaluation. Type II injuries included spinal concussion neurapraxia, and "burning hands" syndrome. The burning hands syndrome was described as suspected injury to the spinothalamic and corticospinal tracts, resulting in arm and hand weakness with burning dysesthesias.[19] This is distinct from the burner or stinger injury that is a common cervical injury in football players and is thought to be due to traction on the upper trunk of the brachial plexus. In this condition, athletes typically have a burning, dysesthetic pain that begins in the shoulder region and radiates unilaterally into the arm and hand, with C5-C6 distribution numbness or weakness. Type III injuries were classified in athletes with only radiologic abnormalities but without neurologic deficit. These included congenital spinal stenosis, acquired spinal stenosis, herniated cervical disc, an unstable fracture, fracture/dislocation, ligamentous injury, and spear-tackler’s spine. Spear tackler’s spine was described by Torg et al described athletes that were at high risk for quadriplegic injury. These athletes had developmental cervical canal stenosis, reversal of the cervical lordosis, preexisting posttraumatic cervical radiographic abnormalities, and documentation of using spear-tackling techniques. PreviousProceed to Clinical Presentation , Cervical Discogenic Pain Syndrome

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Cervical Spine Acute Bony Injuries in Sports Medicine

Background

Cervical spine fractures lead to substantial morbidity and mortality. Neck injury in athletes can quickly end or change the future of an athlete. Failure to properly recognize and provide early care in cervical spine fracture cases may lead to devastating complications.[1, 2, 3, 4]

A C3 spinous fracture is depicted in the image below.

Lateral view of a C3 spinous fracture.

For patient education resources, see the Back, Ribs, Neck, and Head Center, as well as Neck Strain, Vertebral Compression Fracture, and Whiplash.

NextEpidemiologyFrequencyUnited States

The incidence of all spinal injuries in the United States has been reported at approximately 10,000 cases per year. Nearly 200,000 people in the United States have a history of spinal injuries. These statistics do not differentiate between injuries with fracture and injuries without fracture.[5, 6, 7]

Sports-related activities represent 10-15% of these injuries, and spinal injuries represent 2-3% of all sports-related injuries. Certain sports (eg, American football, diving, gymnastics, skiing, wrestling, rugby, hang gliding, surfing, equestrian events) are more frequently associated with the risk of spinal trauma.[2, 3, 4, 6, 7, 8, 9, 10, 11, 12]

The most common spinal injuries cited in the literature are injuries secondary to contact sports such as football. Nearly 1.2 million high school athletes and 200,000 college and professional athletes participate in football. The National Football Head and Neck Injury Registry contains data on cervical spine injuries as a result of participation in football. A trend can be seen over time, as equipment and helmets improved. The incidence of cervical spine injuries increased until 1976. In that year, antispearing rules were established to prevent the athlete from using the helmet as driving force in tackles. Direct collision created higher axial loads than the neck could withstand, leading to high injury rates. This rule, along with better coaching of blocking and tackling techniques, has resulted in a significant decrease in the number of spinal injuries.[10]

Diving is often cited as another significant cause of cervical spine injuries. Injuries resulting from diving are often associated with devastating outcomes. Diving rules (eg, depth of starting areas) and proper technique have lowered the probability of injury during supervised athletic events. However, unsupervised swimming and diving into shallow water present significant risks. Public awareness of this problem has led to the development of special awareness programs, but the risk of injury remains high.

PreviousNextFunctional Anatomy

The human spine serves to provide structural support and bony protection of the spinal cord. The cervical spine consists of 7 bony vertebrae separated by flexible intervertebral discs. They are joined together by an intricate network of ligaments, which helps form the normal lordotic curve of the cervical neck.[13]

The spinal column can be divided into 2 separate columns based on function and injury patterns. The anterior column consists of the bodies of the vertebrae, intervertebral discs, and the anterior and posterior longitudinal ligaments. The function of the vertebral body is to support weight. The posterior column contains the spinal canal and consists of the pedicles, laminae, articulating facets, and transverse and spinous processes. These structures form the vertebral arch, which encloses the vertebral foramen and protects the neural tissues.

The arch is formed by bilateral pedicles that are oriented posteriorly and join 2 laminae. The spinous process arises posteriorly from the vertebral arch. The cervical transverse processes and 4 articular processes also arise from the arch. The cervical transverse processes are unique to the vertebral column with an oval foramen transversarium. The vertebral arteries pass through these foramina. The posterior column also includes a group of ligaments including the supraspinous, infraspinous, interspinous, and nuchal ligaments.

The first 2 cervical vertebrae are atypical in form and function. The next 5 vertebrae are all similar in structure and function. The atlas, C1, is a ring-shaped bone that supports the skull. Two concave, superior articular facets articulate with the occipital condyles. The atlas does not have a body or spinous process. The atlas has an anterior and posterior arch, each with a tubercle and lateral mass. The axis, C2, is the strongest cervical vertebrae. The atlas rotates on 2 large articulating surfaces. The odontoid process (dens) projects superiorly from the C2 body and is the bony structure that the atlas rotates on. The odontoid process is held in place by the transverse ligament of the atlas.

PreviousNextSport-Specific Biomechanics

Contact sports, falls, and diving in sports may lead to vertebral stress and fractures. Sports that involving tackling can increase exposure to mechanisms causing fractures.

PreviousProceed to Clinical Presentation , Cervical Spine Acute Bony Injuries in Sports Medicine

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Cervical Spine Sprain/Strain Injuries

Background

The most frequent cervical injuries in athletes are probably acute strains and sprains of the musculature of the neck, as well as soft-tissue contusions.

A strain refers to an injury to a muscle, occurring when a muscle-tendon unit is stretched or overloaded. Cervical muscles that are commonly strained include the sternocleidomastoid (SCM), the trapezius, the rhomboids, the erector spinae, the scalenes, and the levator scapulae.

A sprain refers to a ligamentous injury, and the diagnosis of cervical sprain implies that the ligamentous and capsular structures connecting the cervical facet joints and vertebrae have been damaged. Practically, a cervical sprain may be difficult to differentiate from a strain, and the 2 injuries often occur simultaneously. Pain referred to the muscle can arise from any source that is modulated by the dorsal rami.

Numerous epidemiologic studies have been completed in the hopes of identifying the injury risk patterns that are associated with specific sports. Many athletes are reluctant to report minor injuries, and because the overwhelming numbers of sports-related spinal injuries are self-limited and resolve before being reported, the accuracy of these studies has been challenged. The mainstay of prevention and treatment of cervical spine injuries is maintaining good strength and flexibility through conditioning.

For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education articles Neck Strain, Sprains and Strains, Muscle Strain, and Whiplash.

Related Medscape Reference topics include the following:

Atlantoaxial Injury and DysfunctionCervical Disc DiseaseCervical Spine Injuries in SportsNextEpidemiologyFrequencyUnited States

Cervical spine injuries occur in an estimated 10-15% of football players, most commonly in defensive ends, linemen, and linebackers.[1, 2] The reinjury rate in high school football players following all cervical spine injuries is reported at 17.2%. Football players with 2 previous injuries are reported as having an 87% risk of reinjury. Wrestlers with no history of injuries to the neck have a 20% chance of sustaining a neck injury in a given year[3] ; however, wrestlers who have had a previous neck injury have an approximate 50% chance of recurrence.

Sporting accidents are second only to motor vehicle accidents as the leading cause of emergency department visits involving neck injuries, and as more people participate in athletic activity, the incidence of cervical injuries can be expected to rise as well.[4, 5]

PreviousNextFunctional Anatomy

The spinal cord is protected by the cervical spine, which provides support for the head and allows for a significant amount of range of motion (ROM).[6, 7] Seven cervical vertebrae, stacked vertically, comprise the skeletal portion of the spine. Each vertebra (except C1 and C2) has a common body anteriorly and a ring of bone formed by the laminae and pedicles posteriorly. This protective ring of bone forms the spinal canal, which surrounds and protects the spinal cord. The tissues that surround the cord and the spinal fluid fill the remaining space. The C1 vertebra, or atlas, is ring-shaped, has large lateral masses, and attaches to the occipital condyles of the skull, providing support. See the images below.

Bony framework of head and neck. Cervical vertebrae, the atlas and the axis. Cervical vertebrae. Atlantooccipital junction.

The transverse ligament lies anteriorly between the 2 lateral masses of C1 and just posteriorly to the odontoid process of the C2 vertebra, or axis (see the image below). Projecting upward from the body of C2, the odontoid process is contained between the anterior arch of C1 and the transverse ligament. Displacement of C1 and C2 may be associated with rupture of this ligament, which may result in a spinal cord injury.

Internal craniocervical ligaments.

The remaining cervical vertebrae (C3-C7) are similar in function and appearance. The ovoid vertebral bodies are wider than they are tall. The bilateral raised uncinate processes located posterolaterally correspond to similar beveled surfaces on the inferior aspect of the superior vertebral body. These joints of Luschka, also known as uncovertebral joints, are not present in the embryologic development of the cervical spine but arise as a result of the degenerative and adaptive changes of annular tissue to stresses and loads.

The cervical zygapophyseal joints are synovial in nature. Their articular surfaces are covered with hyaline cartilage, and their fibrous capsules are lined with synovium. The orientation of the cervical zygapophyseal joints allows them to play a weight-bearing role and to provide resistance to anterior translation. Because the C2-C3 facet sits between the upper and lower parts of the cervical spine that move differently, it is considered transitional anatomically and biomechanically.[7]

The lower cervical spine flexes and extends, and the atlantoaxial joint moves in rotation. During lateral bending, the spinous processes move to the convexity of the curve (spinous processes move to the right during left lateral bending) in the middle and lower cervical regions. Coupled lateral bending occurs in the opposite direction to the applied axial rotation above the C2-C3 level. Lateral bending from C2-C3 distally is always coupled with rotation in the same direction because of the approximate 45° inclination of the cervical zygapophyseal joints. The obliquity of the articular surfaces in the frontal plane determines the relative amount of side bending or rotation that occurs. The more vertical the joint surface, the more side bending is coupled; the more horizontal the joint surface, the more rotation is coupled.

Regressive changes occur in cervical zygapophyseal menisci with age. The meniscus retracts and narrows between childhood and the fourth decade of life. The meniscus helps increase the contact surface area when articular facets come together, thus helping to transmit some of the load.

The many articulations between the cervical vertebrae make the extensive ROM in the cervical spine possible. However, this large ROM comes at the cost of stability. Cervical spine stability is provided by a combination of the zygapophyseal joints and numerous ligaments and muscles. Extension, flexion, lateral bending, and rotation are permitted by the orientation of the zygapophyseal joints and ligaments. Positioning of the head makes combinations of these motions necessary. In a young person, cervical flexion and extension is about 100°. Bilateral rotation is about 80°, with approximately 50% of this range occurring between C1 and C2. The range of lateral bending is about 30-50°. Older individuals usually have reduced end ROMs because cervical mobility usually decreases with age.

Intervertebral discs are located in between each of the cervical vertebrae from C2-C7. These discs consist of a water-containing central portion, the nucleus pulposus, and a tough fibrous outer ring, the annulus fibrosis. The discs are subject to prolonged and repetitive loading from muscle forces acting across them and from the weight of the head. With their viscous central portion, the intervertebral discs are able to transmit these forces between the end plates of adjacent vertebral bodies. These biconvex discs conform to the concavity of the vertebral bodies, and they also contribute to normal cervical lordosis because they are thicker anteriorly. Only the outer one third to one half of the annulus fibrosis in adults receives a vascular supply. The rest of the annulus and the whole nucleus pulposus are avascular.

The annular fibers consist of 10-20 circumferential collagenous lamellae. The fibers within each lamella are oriented 35° from the horizontal, although the direction of inclination alternates with each lamella. As a result, rotation and translation are more likely to damage the annulus because resistance can be offered only by half of the lamellae whose fibers are oriented in the direction of motion.

The functions of a ligament are: (1) to provide stability to the joint, (2) to absorb energy during trauma, and (3) to act as a joint position transducer during physiologic motions. Ligaments, along with the paracervical muscles in the cervical spine, prevent motion between vertebrae that might injure the spinal cord or nerve roots. The cervical spine ligaments have numerous and complex interrelationships (see the images below).

External craniocervical ligaments. Internal craniocervical ligaments.

Running vertically along the anterior and posterior aspects of the vertebral bodies, the anterior and posterior longitudinal ligaments attach to the discs as well. The tightly attached posterior longitudinal ligament is thick in its central portion, which helps prevent a disc herniation from pressing directly on the cord posteriorly. The interspinous ligaments are also located posteriorly but are not as well developed in the cervical region.

The ligamentum flavum, a yellowish elastic membrane, overlies the space between the laminae of adjacent vertebrae and the neural arches. The posterior location of the ligamentum flavum helps to restrain hyperflexion. The ligamentum flavum becomes shortened and thicker in hyperextension and elongated and thinner in hyperflexion. During hyperextension, it may protrude into the cervical canal as much as 3.5 mm. Impingement on the spinal cord during extension is normally prevented by the elastic properties of the ligament; however, hypertrophy of the ligamentum flavum or loss of elasticity through degeneration may lead to canal narrowing or cord impingement.

The capsular ligaments, oriented approximately orthogonal to the articular facets, provide maximal mechanical efficiency in resisting distraction of the facets but relatively poor resistance to shear. The posterior longitudinal ligament limits flexion and distraction, the tectorial membrane limits flexion and extension, and the supraspinous and interspinous ligaments limit flexion and anterior horizontal displacement.

The main function of the alar ligaments is to restrain rotation. The alar ligaments originate from the posterolateral aspect of the dens of C2 and insert on the medial surfaces of the occipital condyles. When a single alar ligament is cut, axial rotation increases significantly to both sides; thus, both ligaments are required to be intact for restraining motion. Alar ligaments are stretched the most when the head is rotated and flexed together, and the ligaments are relaxed during extension. The anterior aspect of the transverse ligament acts as the pivot about which C1 (ie, the atlas) rotates.

Holding the odontoid process of C2 against the anterior ring of the atlas, the transverse ligament functions as a restraining band on the dens. Flexion and anterior displacement of the atlas is restrained by its orientation. The facet joint capsules are strong fibrous structures that contribute to posterior stability.

A muscle injury or reaction of some degree is associated with almost every cervical injury. The musculature of the neck is vulnerable to the same types of injuries that affect muscles elsewhere in the body. The role of the muscles is to stabilize the spine, carry loads, and produce motion. The action of the intervertebral muscle forces is to restore the intervertebral motions of an injured spine to its intact values.

The capital flexor muscles include the following:

Longus capitisRectus capitis anterior and lateralSuprahyoid and hyoid muscles

The capital extensor muscles include the following:

Splenius capitisSemispinalis capitisLongissimus capitisObliquus capitis inferior and superiorRectus capitis posterior major and minor

The cervical flexor muscles include the following:

Anterior scaleneMiddle scaleneSCM

The cervical extensor muscles include the following:

Semispinalis cervicisLongissimus cervicisSplenius cervicis

Because the bulk of the flexor muscle groups are at the C4-C5 level and the main mass of the extensor muscle groups overlies the C6-T1 levels as well as the atlantoaxial area, these muscle groups are likely sites of major stresses. The muscle groups that laterally flex and rotate the cervical spine include the following:

Rectus capitis lateralisObliquus capitis inferior and superiorIntertransversariiMultifidiIliocostalis cervicisLongus colliLevator scapulaeLongissimus capitisSplenius cervicisSplenius capitisSCMScalene muscles

The images below illustrate several views of muscles of the neck.

Lateral view of the muscles of the neck. Anterior view of the muscles of the neck. Infrahyoid and suprahyoid muscles. Scalene and prevertebral muscles.

Related Medscape Reference topics include the following:

Cervical Spine Injuries in SportsCervical Sprain and StrainDisk Herniation ImagingPreviousNextSport-Specific Biomechanics

When contact is made with the head or body, deceleration injuries occur, and sudden flexion and extension of the neck can result. This type of injury is likely to occur in contact or collision sports such as football, soccer, rugby, or lacrosse.

The posterior neck muscles may be strained when resisting flexion forces, and/or the anterior neck muscles may be strained when resisting hyperextension. Microtears or strains in these muscles are caused by the sudden muscular contractions that try to decelerate the applied force. Forced twisting, which is common in wrestling, can also cause a cervical strain. The twisting injury usually happens in the wrestler who is pinned on the mat, and a flexion-extension injury is more likely to happen during the takedown. Deceleration and rotational forces can also cause microtears or stretching of the small intertransverse and interspinous ligaments as well as the joint capsules.

In cervical sprains, the immediate soft-tissue trauma occurs in the structures in and around the facet joints. This trauma occurs with varying severity, including multiple tears in ligamentous tissue with focal hematomas and hemorrhages. A fibrous tissue contraction is the net final effect of the repair of these strained capsular and ligamentous tissues so that restriction of motion and stiffening of the neck may eventually result. The short capsular ligaments of the Luschka interbody joints lack the normal laxity of the capsular structures surrounding the facet joints. Because of their anatomic position, the articulating surfaces of these vertebral body joints are particularly susceptible to injury from axial compression when the head is in a laterally tilted or neutral position.

The cervical spine can absorb much of the imparted energy of collisions by dissipation through the normal lordotic curve of the cervical spine, the paravertebral musculature, and the intervertebral discs. However, when the neck is flexed about 30°, the forces applied to the top of the head are directed to a straight-segmented column because the normal lordotic curve is flattened. The cervical spine is then less able to dissipate the exerted forces in this situation, leading to fracture(s) and possible spinal cord injury. This proposed mechanism is supported by biomechanical studies that replicate it. In individuals with straight cervical spines, less energy is needed to fail under an axial load than in those with a normal lordotic curve; this finding underlines the importance of the cervical musculature in maintaining proper lordosis.

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