The typical cervical vertebrae (C3–C6) possess the same structural parts as all other true vertebrae, plus some unique and distinctive physical features (Figure 5-34). The spinous processes are bifid to allow for better ligamentous and muscular attachment. Each transverse process from C6 upward contains the transverse fora- men, allowing for the passage of the vertebral artery. The body of the typical cervical vertebra has anterior and posterior surfaces that are small, oval, and wide transversely. The anterior and pos- terior surfaces are flat and of equal height. The posterior lateral aspect of the superior margin of the vertebral bodies is lipped, forming the uncinate processes, which serve to strengthen and stabilize the region. The uncovertebral articulations (joints of Von Luschka) are pseudojoints that have a synovial membrane with synovial fluid but no joint capsule (Figure 5-35). They serve as tracts that guide the motion of coupled rotation and lateral
flexion. They begin to develop at 6 years of age and are complete by 18 years of age.
The articular facets are teardrop-shaped, with the superior facet facing up and posteriorly and the inferior facet facing down and anteriorly, placing the joint space at a 45-degree angle midway between the coronal and transverse planes (Figure 5-36). The disc height–to–body height ratio is greatest (2:5) in the cervical spine, therefore allowing for the greatest possible ROM (Figure 5-37).
The short and rounded pedicles of cervical vertebrae are directed posterolaterally. The superior and inferior vertebral notches in each pedicle are the same depth. The laminae are long, narrow, slender, and sloping. The intervertebral foramina in this region are larger than in the lumbar or thoracic areas and are triangular in shape.
The C7 vertebra (vertebra prominence) is considered the atypi- cal segment of the lower cervical spine. It demonstrates anatomic characteristics of both the cervical vertebra and the thoracic ver- tebra. It has a spinous process that is quite long and slender, with a tubercle on its end. The inferior articular processes are similar to those in the thoracic spine, and the superior processes match those of the typical cervical vertebra. C7 has no uncinate processes and no transverse foramen. The transverse processes are large,
L R
Figure 5-32 Right lateral flexion of the upper cervical spine (solid
arrow) with translation of the atlas (broken arrow) toward the right.
Uncinate processes Body Pedicle Superior facet Inferior facet Articular process Lamina Spinous process Vertebral foramen Transverse foramen Transverse process
Figure 5-34 Structure of a typical cervical vertebra.
Figure 5-33 Atlas rotation produces a wider lateral mass and narrower appearance of the atlanto-odontal interspace on the side of posterior rotation. This may lead to a false impression of a lateral flexion or translational malpo- sition of the atlas on the anterior-to-posterior open-mouth radiograph.
From above AP open- mouth view Open- mouth view Left rotation of atlas Right rotation of atlas Neutral
Figure 5-35 The uncinate processes limit pure lateral flexion to only a few degrees while serving as guides to couple lateral flexion with rotation.
Joints of Von Luschka
broad, and blunt. The transverse processes may become enlarged or develop cervical ribs, with the potential to create thoracic outlet compromise (Figure 5-38).
Cervical Curve
The cervical spine forms a lordotic curve that develops secondary to the response of upright posture. The functions of the cervical curve and the anterior-to-posterior (A-P) curves throughout the spine are to add resiliency to the spine in response to axial com- pression forces and to balance the center of gravity of the skull over the spine. The center of gravity for the skull lies anterior to the foramen magnum (Figure 5-39).
The facet and disc planes in large part determine the degree of potential lordosis. Congenital diversity in pillar height and facet
angulation therefore leads to significant variation in the degree of cervical lordosis present in the population. In addition, degenera- tive changes or stress responses in these structures may change the “normal” lordosis.
There are a number of opinions as to what the normal cervical curve should be and how it should be measured.6,8-14 There is also
significant debate on what constitutes an abnormal curve and what biomechanical consequences, if any, will result from alteration in the cervical lordotic curve. A reduced cervical curve (hypolordosis) has the potential to shift more weight onto the vertebral bodies and discs and increase muscular effort as the posterior neck muscles work to maintain head position and spinal stability. An increased cervical curve (hyperlordosis) will potentially increase the compres- sive load on the facets and posterior elements (Figure 5-40). 45�
Figure 5-36 The cervical facet planes, demonstrating a 45-degree angle to the horizontal plane.
2:5
4 10
3 3
Figure 5-37 The location of the nucleus pulposus and the disc height–to–body height ratio in the cervical spine.
Lamina Spinous process Articular process Superior articular facet Transverse process Pedicle Vertebral body
Figure 5-38 The structure of the C7 vertebra (vertebral prominence).
Center skull mass Gravity
W
External occipital protuberance Atlas center mass Capitis muscle effort C4 center mass C5 center mass T1 center mass Resultant vector
Figure 5-39 The center of gravity for the skull. If the cervical curve changes, the center of gravity shifts.
Various methods for radiographically measuring lordosis have been suggested. The most common method involves direct mea- surement of the curve by forming an angle between a line extend- ing through the center of C1, with a line drawn along the inferior endplate of C7 (Figure 5-41). Although the cervical lordosis apparently extends to the T1–2 motion segment, measurements commonly use the C7 level as the lowest point reliably viewed on a lateral cervical x-ray film. Another method presented by Jochumsen12 proposes classifying the cervical curve by measuring
the distance from the anterior body of C5 to a line running from the anterior arch of the atlas to the anterior superior aspect of the body of C7 (Figure 5-42). There is some agreement that the cervi- cal curve midpoint is the C5 vertebra (C4–5 interspace).
The proposed optimal curve for the cervical spine can be extrap- olated from the mechanical principle that states the strongest
and most resilient curve is an arc that has a radius of curvature equal to the cord across the arc (Figure 5-43). The length of the radius, and hence the cord, should equal approximately 7 inches or 17 cm. As the radius increases, the curve increases (flattens, as in hypolordosis) and vice versa.
Range and Pattern of Motion of the lower Cervical Spine
The lower cervical spine exhibits its greatest flexibility during flexion and extension movements (Table 5-2; see Figure 5-24). Lateral flexion exhibits slightly greater movement than rotation. Both rotation and lateral flexion decrease significantly at the thoracocervical junction.
Flexion and Extension. Movement averages approximately 15 degrees of combined flexion and extension per segment and is greatest at the C5–6 motion segment.15 Flexion and extension
1 1 2 3 4 5 6 7 1 2 3 4 5 6 7 T1 T2 2 3 4 5 6 7 A B C
Figure 5-40 The cervical curve extending from C1 to T2. A, Normal.
B, Hypolordosis with a kyphosis involving the middle segments. C, Alordotic.
1 45� 30� 2 3 4 5 6 7
Figure 5-41 The angle of the cervical curve should be about 30 to 45 degrees when measured between lines drawn through C1 and C7.
1 2 3 4 5 mm measurement 6 7
Figure 5-42 Jochumsen’s measuring procedure for determining the adequacy of the cervical curve.
Radius 60� 60� 60� Radius Radius � Chord Arc Chord
Figure 5-43 Diagram demonstrating the relationship formed when a chord equals the radius of an arc.
occur around an axis located in the subjacent vertebra and com- bine sagittal plane rotation with sagittal plane translation (Figure 5-44). This pattern of combined segmental angular tipping and gliding develops a stairstep effect, which is noted on flexion and extension radiographs.
With flexion, the articular joint surfaces slide apart, produc- ing stretching of the facet joints and posterior disc and anterior disc approximation and compression. With extension, the oppo- site occurs. The disc is subjected to compression on the concave side and tension on the convex side. The side of the disc sub- jected to tension retracts and the side subjected to compression bulges.5 The net effect of these two opposing forces is to limit
shifting of the nucleus pulposus during movements of flexion and extension and lateral flexion (Figure 5-45). Krag and colleagues16
implanted small metal markers within the lumbar and thoracic IVDs and confirmed the bulging and retraction of the discs dur- ing lumbar segmental flexion movements. However, they did note some minor posterior migration of the nucleus that was not iden- tified by previous mathematical models. This phenomenon has not been investigated for the cervical spine.
The coupled translation that occurs with flexion and extension has been measured at approximately 2 mm per segment, with an upper range of 2.7 mm.17 Translational movements do not occur
evenly throughout the cervical spine.15 For every degree of sagit-
tal plane rotation, more translation occurs in the upper cervical segments than in the lower cervical segments. This leads to a flat- ter arch of movement in the upper cervical spine (Figure 5-46). Accounting for radiographic magnification, White and Panjabi5
C2
C7
Figure 5-46 With active flexion and extension movements, more translation takes place in the upper segments than the lower segments, leading to a flatter arc of movement.
TABLE 5-2 Segmental Range of Motion for the Lower Cervical Spine*
Vertebra Combined flexion and Extension one-Side lateral flexion one-Side Axial Rotation
C2–3 C3–4 C4–5 C5–6 C6–7 C7–T1 5 to 16 (10) degrees 7 to 26 (15) degrees 13 to 29 (20) degrees 13 to 29 (20) degrees 6 to 26 (17) degrees 4 to 7 (9) degrees 11 to 20 (10) degrees 9 to 15 (11) degrees 0 to 16 (11) degrees 0 to 16 (8) degrees 0 to 17 (7) degrees 0 to 17 (4) degrees 0 to 10 (3) degrees 3 to 10 (7) degrees 1 to 12 (7) degrees 2 to 12 (7) degrees 2 to 10 (6) degrees 0 to 7 (2) degrees *Numbers in parentheses indicate averages.
Modified from White AA, Panjabi MM: Clinical biomechanics of the spine, ed 2, Philadelphia, 1990, JB Lippincott.
IAR IAR
A B
Figure 5-44 Sagittal plane movement of a cervical motion segment in flexion (A) and extension (B), locating the instantaneous axis of rota- tion and the stair-stepping appearance that occurs with combined tipping and gliding movements.
Compression Tension
Instantaneous axis of rotation
Figure 5-45 Representation of changes in the disc with flexion, as well as extension, or lateral flexion movements.
have recommended 3.5 mm as the upper end of normal trans- lational movement in the lower cervical segments. Translation beyond 3.5 mm suggests end range segmental instability.
Lateral Flexion. Lateral flexion averages approximately 10 degrees to each side in the midcervical segments, with decreasing flexibility in the caudal segments. The IAR for lateral flexion has not been determined. Speculation places the axis in the center of the subjacent vertebral body (Figure 5-47).
Lateral flexion in the lower cervical spine is coupled with rotation in the transverse plane. The coupling is such that lat- eral flexion and rotation occur to the same side. This leads to posterior vertebral body rotation on the side of lateral flexion, thereby causing the spinous processes to deviate to the convexity of the curve (Figure 5-48). The degree of coupled axial rotation decreases in a caudal direction.14 At the second cervical verte-
bra there are 2 degrees of coupled rotation for every 3 degrees of lateral bending, and at the seventh cervical vertebra there is only 1 degree of coupled rotation for every 7.5 degrees of lateral bending.
During lateral flexion the facets on the side of lateral flexion (concave side) slide together as the inferior facet slides inferome- dially because of the coupled rotation. On the opposite side, the facets distract and the inferior facet slides superiorly. The IVD approximates on the side of lateral flexion and distracts on the opposite side.
Rotation. ROMs for segmental axial rotation on average are slightly less than those for lateral flexion, with a similar tendency for decreased movement in the lower cervical segments, espe- cially at the C7–T1 motion segments. The axis of rotation is also somewhat speculative and has been placed by Lysell14 in the ante-
rior subjacent vertebral body (see Figure 5-47).
Rotational movements in the lower cervical spine demonstrate the same coupling as described for lateral flexion. In other words, left or right axial rotation is coupled with lateral flexion to the same side. This leads to a pattern of motion in which, on the side of cervical rotation (posterior body rotation), the inferior facet of the superior vertebra glides posteroinferiorly as the contralateral glides anterosuperiorly (Figure 5-49).
Cervical Kinetics
Nonsegmental muscles produce integrated global movement of the cervical spine as a result of the head’s moving in relation to the trunk. Concentric and eccentric muscle activity is combined, with eccentric activity predominating during flexion, extension, and lateral flexion. Concentric muscle activity refers to the devel- opment of sufficient muscle tension to overcome a resistance, causing the muscle to visibly shorten and the body part to move. However, eccentric muscle activity occurs when a given resistance overcomes the muscle tension, causing the muscle to actually lengthen. Relaxation of a muscle against the force of gravity, cre- ating a deceleration of the moving body part, is an example of eccentric muscle activity.15
The segmental (intrinsic) muscles function to coordinate and integrate segmental motion. The intrinsic muscles act as involun- tary integrators of overall movement. Movements of the head ini- tiate normal movements of the cervical spine, but with conscious E
E
F ?
F Flexion and
Extension BendingLateral RotationAxial
R L
L�R
R L
Figure 5-47 The theoretic locations for the instantaneous axis of rotation for each plane of movement in the lower cervical spine. (From White AA, Panjabi MM: Clinical biomechanics of the spine, ed 2, Philadelphia, 1990, JB Lippincott.)
A B
Figure 5-48 A, Left lateral flexion coupled with physiologic left rotation. B, Movement of the facet surfaces with left lateral flexion and coupled left rotation in the lower cervical spine.
Figure 5-49 Illustration of segmental left rotation coupled with normal left lateral flexion.
effort, movement may be initiated at lower segmental levels. They operate by the same concentric and eccentric principles as the larger nonsegmental muscles.
Flexion is initiated by anterior cervical muscles and controlled or limited by eccentric activity of the semispinalis, longissimus, and splenius muscle groups. Flexion is further limited by the elastic limits of myofascial tissue, nuchal ligament, joint capsule, PLL, ligamentum flavum, posterior IVD, anterior vertebral bod- ies, and the chin hitting the chest.
Posterior cervical muscles controlled or limited by the eccen- tric activity of the sternocleidomastoid (SCM), scaleni, and lon- gus coli muscle groups initiate extension. Extension is further limited by the elastic limits of the myofascial tissue, anterior IVD, ALL, joint capsule, posterior vertebral bodies, and articu- lar pillars.
Lateral flexion is initiated by ipsilateral contraction and con- trolled or limited by the contralateral eccentric activity of the sple- nius capitis, semispinalis cervices, and longus coli muscle groups. Lateral flexion is further limited by the elastic limits of some myo- fascial tissue, contralateral joint capsule, periarticular ligaments, flaval ligament, IVD, ipsilateral joint capsule, and ipsilateral articular pillars.
Rotation is initiated by concentric contraction of the ipsilat- eral splenius capitis and cervicis, longissimus cervicis, and contral- ateral semispinalis muscles. Eccentric muscle contraction occurs simultaneously to guide and break movements and involves action of the contralateral splenius capitis, cervicis, longissimus cervicis, and ipsilateral semispinalis and scaleni muscles. Movement is fur- ther limited by capsular and periarticular ligaments and segmental muscles.