Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Clinical Anatomy

Understanding the structure of the cervical spine and the pathoanatomy of characteristic patterns of injury is essential for initial, effective management. Simply stated, the cervical spine supports the head and protects the neural elements; but motion about the cervical spine can be quite complex.

Many agree that distinct regional differences exist between the upper cervical spine and the lower cervical spine. In this section, we will briefly explore some of these important characteristics.

The occiput and the first two vertebrae make up the upper cervical spine. The atlas (C1) is a bony ring that articulates with the occipital condyles. The major function of the atlanto-occipital joint is motion in the sagittal plane. In fact, 40% of all cervical flexion and extension occurs above the axis (C2). In contrast, only 5° to 10° of lateral bending occurs at the atlanto-occipital joint.

The axis (C2) has a true vertebral body, from which the odontoid process, or dens, projects. The major stabilizing force at this joint is the transverse atlantal ligament (TAL). The ligament crosses posterior to the dens and attaches to C1 on both sides; this prevents anterior translation of the atlas on the axis. This specialized osseoligamentous anatomy allows C1 to rotate on C2 in a highly unconstrained manner, providing 60% of all cervical rotation.

The lower cervical spine consists of the C3 through C7 vertebrae. These joints account for the remaining arc of neck motion, including flexion, extension, lateral bending, and rotation. Two contiguous vertebrae and supporting soft tissues make up a motion segment, and these motion segments can be separated into an anterior and a posterior column. Stability of a cervical motion segment is derived mainly from the anterior column elements.

The vertebral bodies and intervertebral discs provide the majority of resistance to compression. In contrast, it is the surrounding paraspinal musculature and ligaments that resist shear forces.

Of course, the coronal orientation of the articular processes of the facet joints in the cervical spine provides additional resistance to anterior translation.

Distraction and tensile forces are resisted by the annulus fibrosus and longitudinal ligaments. Secondary support structures include the supraspinous and interspinous ligaments, the ligamentum flavum, and the facet capsules.

The spinal cord lies within, and is protected by, the upper and lower spinal columns. From the foramen magnum to the cervicothoracic junction, the osseoligamentous structures of the cervical spine provide a protective space described as the cervical spinal canal. This spinal canal occupies a funnel shape from cephalad to caudal.

In fact, the cord occupies less than half the canal’s cross-sectional area at the level of the atlas, but this space reduces significantly in the lower cervical spine. Almost 75% of the cross-sectional area of the canal is occupied by the larger spinal cord between C4 and C7.

The dimensions of the lower cervical spine remain relatively consistent among most patients, but some important, distinct differences exist. For instance, the diameter of the midsagittal cord averages between 8 and 9 mm; however, a range of 14 to 23 mm exists for the vertebral canal at thecorresponding level.

Criteria for radiographic stenosis are anteroposterior dimensions measuring less than 13 mm on a lateral radiograph. Additionally, a Pavlov ratio of less than 0.8 has been used to describe stenosis.

Even without radiographic criteria for a stenotic canal, the potential for increased risk at the time of injury exists secondary to anatomical variation.

In summary, if the space available for the spinal cord is reduced because of a narrow canal, an important risk factor and predictor of acute neurological dysfunction is present. When the diameter of the midsagittal cervical spinal canal is 10 mm or less, cord compression can be anticipated. Cervical canal stenosis has been implicated as a risk factor for the burner phenomenon  in college football players.

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