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Intervertebral joints: want to learn more about it?

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Intervertebral joints

Intervertebral disc joint, Zygapophyseal joint
Intervertebral joints (Articuli intervertebrales)

The intervertebral joints connect directly adjacent vertebrae of the vertebral column. Each intervertebral joint is a complex of three separate joints; an intervertebral disc joint (intervertebral symphysis) and two zygapophyseal (facet) joints.

An intervertebral disc joint is the articulation of two contiguous vertebral bodies and the intervening intervertebral disc. It is classified as a secondary cartilaginous joint or symphysis (fibrocartilage composition) and amphiarthrosis (permits minimal movement). A zygapophyseal joint (facet joint) is a synovial joint that connects the articular processes of neighboring vertebrae. Both intervertebral disc and zygapophyseal joints extend between the levels of the axis (C2) and sacrum (S1).

The main function of the intervertebral joints is to unite the vertebrae into a single, semirigid, sufficiently mobile yet supportive structure for the entire human body. The intervertebral discs account for the presence of the vertebral column curvatures and permit its movements, to a greater or lesser degree. In turn, the zygapophyseal joints guide the movements. The intervertebral joints are stabilized by several accessory ligaments, which unite the vertebral bodies, arches and processes.

This article will describe the anatomy and function of the intervertebral joints.

Key facts about the intervertebral joints
Type Intervertebral disc joint: Cartilaginous joint; symphysis
Zygapophyseal joint
: Synovial plane joint, nonaxial, uniplanar
Articular surfaces Intervertebral disc joint: Articular surfaces on vertebral bodies
Zygapophyseal joint
: Articular surfaces on articular processes
Ligaments Longitudinal ligaments (anterior, posterior), ligamenta flava, interspinous ligaments, intertransverse ligaments, supraspinous ligaments, nuchal ligament (cervical spine only)
Innervation Intervertebral disc joint: sinuvertebral nerve
Zygapophyseal joint
: sinuvertebral nerve, medial branch of the posterior ramus of spinal nerve
Blood supply Segmental arteries
Movements Flexion, extension, lateral flexion and axial rotation of the vertebral column

Intervertebral symphysis

Articular surfaces

Intervertebral disc joints (intervertebral symphyses) extend between C2 and S1 vertebral levels. They are formed by adjacent vertebral bodies and the intervening intervertebral disc. There are no intervertebral discs at C0-C1 or C1-C2 vertebral levels.

The superior and inferior vertebral body joint surfaces are quite textured. Their central parts are particularly irregular, becoming slightly smoother at the periphery. The irregularity of the joint surfaces are further increased by the vascular foramina which perforate them. These foramina transmit the basivertebral veins and intraosseous nutrient arteries, which supply and drain blood to/from the vertebral bodies.

In the vertical plane, the shape of the joint surfaces ranges from relatively flat to sellar (concave) with elevated margins, resembling lips. These elevated margins (uncinate processes) are especially pronounced in the cervical vertebrae, where they form the uncovertebral joints. In the horizontal plane, the articular surfaces appear convex anteriorly and concave posteriorly, to facilitate the formation of the vertebral foramen through which the spinal nerves pass. Cervical vertebrae are an exception because their joint surfaces can be almost flat posteriorly.

Vertebral end-plates overlie the discal surfaces of the vertebral bodies. They are less than 1 mm thick and non-uniform, being thinnest in the centre. Vertebral end-plates consist of both bone and hyaline cartilage in early life. The cartilage mineralizes and is replaced by bone in adulthood. The functions of the vertebral end-plate are shock absorption and strengthening the intervertebral disc joint.

The articular surfaces of directly adjacent vertebral bodies are separated by fibrocartilaginous intervertebral discs. They adhere to both the vertebral end-plates and the bony vertebral rim. The attachment to the vertebral bodies is via ring apophyses. Intervertebral discs are wedge-like, being particularly thick anteriorly. Each intervertebral disc is composed of a nucleus pulposus and an anulus fibrosus.

The nucleus pulposus forms the inner core of the intervertebral disc. It is soft, gelatinous and highly moveable. These characteristics facilitate movement of the intervertebral disc and joint, especially in the cervical region. The peripheral anulus fibrosus surrounds the nucleus pulposus. It has a lamellar structure, being composed of collagen and fibrocartillage. This makes it stiffer and less mobile than the nucleus pulposus. The function of the anulus fibrosus is to encase the nucleus pulposus and interconnect the vertebrae, reinforcing the vertebral column.

Ligaments and joint capsule

Intervertebral disc joints are not enclosed by a joint capsule. Their stability is provided by the anterior and posterior longitudinal ligaments.

The anterior longitudinal ligament is the stronger of the two longitudinal ligaments. It extends from the base of the occipital bone to the pelvic surface of the upper sacrum. Along the way, it attaches to the anterior aspects of the vertebral bodies, vertebral end plates and anulus fibrosus of the intervertebral discs. The role of the anterior longitudinal ligament is to reinforce the vertebral column anteriorly.

The posterior longitudinal ligament lies in the vertebral canal of the vertebral column, anterior to the spinal cord. It extends from the second cervical vertebra until the first sacral vertebra. Along the way, it attaches to the posterior aspects of the intervertebral discs, vertebral end plates and the margins of the vertebral bodies. The role of this ligament is to reinforce the vertebral column posteriorly.

Zygapophyseal joint

Articular surfaces

The zygapophyseal, or facet joint, is an articulation between the superior and inferior articular processes of adjacent vertebral bodies. Being a synovial joint, the joint surfaces are lined with hyaline cartilage.

The articular surfaces of the cervical articular processes are flat and ovoid in shape. The corresponding articular surfaces forming each zygapophyseal joint are orientated almost oppositely in the coronal plane; the superior ones face superoposteriorly while the inferior ones project anteriorly. Both are orientated at approximately 45° to the transverse plane.

The articular surfaces of the thoracic articular processes are thin and almost flat. The corresponding articular surfaces start projecting more medially in the coronal plane; the superior ones are directed posteriorly and superolaterally, while the inferior project anteriorly and superomedially. Both are positioned 60° to the transverse plane and 20° to the frontal plane.

The articular surfaces of the corresponding lumbar articular processes differ. The superior ones are vertical and concave, being oriented posteromedially in the coronal plane. The inferior ones are vertically convex and face anterolaterally. Both are orientated at 90° to the transverse plane and 45° to the frontal plane.

Ligaments and joint capsule

The zygapophyseal joint is surrounded by a thin and loose fibrous joint capsule. It extends between the margins of the corresponding articular processes that form each zygapophyseal joint. The joint capsule is especially loose in the cervical zygapophyseal joints. This aspect eases the movement between adjacent vertebrae and increases the range of movement of the cervical spine. The fibrous capsule is lined by a synovial membrane that secretes viscous synovial fluid, which acts as a lubricant. The articulating surfaces of the zygapophyseal joint are lined by hyaline cartilage.

The lumbar zygapophyseal joints are distinct compared to the rest. Their anterior portions of the joints have been entirely replaced by ligamenta flava. In addition, they contain intra-articular subscapular fat and menisci. The menisci help to improve joint congruity by projecting into the irregularities of the articular surfaces.

The zygapophyseal joints are stabilised by four ligaments:

  • Ligamenta flava
  • Interspinous ligaments
  • Supraspinous ligaments
  • Intertransverse ligaments

Ligamenta flava connect the laminae of neighbouring vertebral arches. This composite ligament is located between the levels of the first cervical and fifth lumbar vertebrae. There are two ligamenta flava at each level, one left and one right. The anterior attachments of each ligamentum flavum is the ipsilateral zygapophyseal joint capsule. From here, they extend posteriorly between the laminae of the vertebral arches. They fuse together at the beginning of the spinous process.

The elasticity of the ligamenta flava is important during spinal flexion and extension. During flexion, they accommodate a limited amount of distancing of the vertebral laminae. During extension, the ligaments recoil and return the laminae to the normal position. In addition, the ligamenta flava continuously compress the intervertebral discs. This supports the zygapophyseal joints and the vertebral column in the neutral position.

Interspinous ligaments interconnect the spinous processes of adjacent vertebrae. They extend between the ligamenta flava anteriorly and the supraspinous ligament posteriorly. The function of the interspinous ligaments is to limit forward flexion of the vertebral column.

Supraspinous ligaments join the tips of neighboring spinous processes. They extend from the seventh cervical vertebra until the third or fourth lumbar vertebra. The main function of the supraspinous ligament is to resist separation of the spinous processes during forward flexion.

Intertransverse ligaments extend between transverse processes of contiguous vertebrae. They are significant mostly in the lumbar spine, contributing to its stability. They are recruited during lateral flexion of the spine, offering resistance to the opposite side.

Innervation

The intervertebral disc and zygapophyseal joints are innervated by the sinuvertebral nerve, which originates from the anterior ramus of spinal nerve. Along its course, the sinuvertebral nerve receives sympathetic contributions from the gray communicating branch of spinal nerve. In contrast to the intervertebral disc joint, the zygapophyseal joint is additionally innervated by the medial branch of the posterior ramus of spinal nerve.

Blood supply

Intervertebral joints receive arterial blood from the segmental arteries of the vertebral column; costocervical trunk and vertebral, ascending cervical, posterior intercostal, lumbar, iliolumbar and lateral sacral arteries. These segmental arteries send out posterior spinal branches to the zygapophyseal joints and the most peripheral portion of the nucleus fibrosus, supplying it with blood. They also send out nutrient, metaphyseal and peripheral arteries to the articular surfaces of the vertebral bodies. From here, nutrients and oxygen diffuse to the avascular intervertebral discs.

Venous blood from the intervertebral joints drains into the spinal and basivertebral veins. These subsequently open into the vertebral venous plexuses, which then empty into the intervertebral veins.

Movements

The structural classification of the intervertebral disc joint is a fused, fibrocartilaginous symphysis. However, functionally, it is considered an amphiarthrosis which permits a limited amount of movement. The soft and deformable nature of the intervertebral disc allows it to translate, tilt, rock and compress, increasing the range of motion of the vertebral column.

The zygapophyseal joint is structurally classified as a plane synovial joint. Functionally, it behaves as a uniplanar diarthosis, allowing nonaxial movements in one plane (translation). However, zygapophyseal joints facilitate a multiplanar and multidirectional movement of the spine through the different orientations of the articular processes in each region of the vertebral column; rotation and lateral flexion in the cervical spine, lateral flexion in the thoracic spine, forward flexion and extension in the lumbar spine. Therefore, the intervertebral disc joints are the limiting factor in the range of motion of the spine, while the zygapophyseal joints direct the movement.

Combined together, the intervertebral joints allow a multiaxial movement of the vertebral column in three degrees of freedom (average maximum glenohumeral active RoM is shown in brackets):

  • Cervical spine
    • (Anterior) flexion (25°) - Extension (85°)
    • Bilateral flexion (40°)
    • Bilateral axial rotation (50°)
  • Thoracic spine
    • (Anterior) flexion (30-40°) - Extension (20-30°)
    • Bilateral flexion (20-25°)
    • Bilateral axial rotation (35°)
  • Lumbar spine
    • (Anterior) flexion (55°) - Extension (30°)
    • Bilateral flexion (20-30°)
    • Bilateral axial rotation (1-2°)

During flexion of the spine, the superior vertebral body slides anteriorly, tilting forward and compressing the anterior portions of the intervertebral discs. This simultaneously causes the inferior articular surface of the superior vertebra to move superiorly and anteriorly relative to the superior articular surface of the inferior vertebra, similar to a seesaw movement. The result is a posterior widening of the superior zygapophyseal joint. The lumbar spine is capable of a higher RoM in flexion due to the 45° orientation of the articular process within the sagittal plane. As flexion and extension also take place in the sagittal plane, there is less restraint of the zygapophyseal joints and the vertebral bodies can slide more anteriorly.

Flexion is limited by tension of the posterior longitudinal ligament, ligamenta flava, nuchal ligament (cervical spine only), joint capsule of the zygapophyseal joint, thoracic cage (thoracic spine only) and the tone of the deep, or intrinsic muscles of the back, located paravertebrally.

During extension of the spine, the opposite occurs. The superior vertebral body slides posteriorly, tilts backwards and compresses the posterior portion of the intervertebral discs. The inferior articular surface moves posteriorly and inferiorly, widening the anterior part of the superior zygapophyseal joint. Extension decreases significantly in the thoracic and lumbar spines compared to the cervical region because the articular process impacts on the large spinous processes of the vertebrae spanning these regions. Extension is limited by tension of the anterior longitudinal ligament, impaction of the posterior vertebral processes and tone of the anterior neck (cervical spine only) and anterior abdominal muscles (thoracic spine only).

When the vertebral column is flexed laterally to the right, the right sided (ipsilateral) articular processes extend, while the left sided (contralateral) articular processes flex at the zygapophyseal joints. The right inferior articular process of the superior vertebra glides inferiorly and posteriorly relative to the superior articular process of the inferior vertebra. Simultaneously, the right side of the intervertebral disc is compressed, while the left side is stretched.

During contralateral flexion of the vertebral column to the left, the opposite occurs; the left zygapophyseal joints extend and the right ones flex. The vertebral column is incapable of pure lateral flexion, coupling this movement with a slight concomitent axial rotation. Lumbar spine is an exception due to its very little capability of axial rotation. Lateral flexion is greatest in the cervical region because the articular processes are oriented obliquely, 45° between the transverse and frontal planes. Hence, they can couple lateral flexion (frontal plane) with axial rotation (transverse plane) more freely.

Lateral flexion is limited by the joint capsule of the zygapophyseal joints, compression of the intervertebral discs, impaction of the articular processes and tension of the contralateral trunk muscles, ligamenta flava and intertransverse ligaments (thoracic, lumbar spines only). In the thoracic region of the vertebral column, lateral flexion is accompanied by a change in shape of the thoracic cage.

During axial rotation, the inferior process of the superior vertebra slides laterally and externally relative to the superior process of the inferior vertebra. This causes the vertebral bodies to rotate relative to one another around a shared axis. The intervening intervertebral disc is simultaneously twisted, an action that pulls the vertebrae closer together. The effect is similar to repeatedly squeezing a washcloth. Pure rotation is impossible in the vertebral column and it is always coupled with a small degree of lateral flexion.

Rotation is extremely limited in the lumbar spine due to the closed articular space of the articular processes. The small gap between them becomes impacted very quickly by the rotation of the articular processes. The limitation is also due to the orientation of the lumbar articular surface in the sagittal plane. Rotation is limited by impaction of articular processes from the opposite sides, as well as tension of the supraspinous and interspinous ligaments, ligamenta flava and opposing trunk muscles, for instance the abdominal obliques. In the lumbar spine, axial rotation of the vertebrae causes rotation of the articulated ribs.

The closed packed position of the intervertebral joints is full extension, while the open packed (resting) position is half-way between flexion and extension. The joints’ capsular pattern is extension, and equally limited lateral flexion and rotation. Additional accessory (arthrokinematic) movements are possible at the intervertebral joints as a result of manual manipulation. Traction and lateral sliding can occur at the cervical intervertebral joints. Rocking, apposition, separation and distancing is possible at the thoracic intervertebral joints. Finally, rotation and anterior gliding are accessory movements that can take place in the lumbar spine.

Muscles acting on the intervertebral joint

The neck and trunk muscles are responsible for producing movement at the intervertebral joints.

Muscles acting on the intervertebral joint
Flexion Sternocleidomastoid, longus capitis, longus colli, scalenus anterior, abdominal oblique, rectus abdominis muscles
Extension Trapezius, splenius capitis, splenius cervicis, longissimus, semispinalis multifidus, iliocostalis, spinalis, rotatores, interspinales, quadratus lumborum
Lateral flexion Trapezius, scalenus medius, scalenus posterior, sternocleidomastoid, longus colli, levator scapulae, splenius capitis, splenius cervicis, iliocostalis, longissimus, splenius thoracis, semispinalis cervicis, semispinalis thoracis, multifidus thoracis, multifidus lumborum, intertransversarii, quadratus lumborum
Axial rotation Scalenus anterior, sternocleidomastoid, longus capitis, longus colli, splenius capitis, splenius cervicis, longissimus capitis, semispinalis, multifidus thoracis, multifidus lumborum, rotatores, levatores costarum, abdominal oblique, transversus abdominis

The prime flexors moving the vertebral column at the intervertebral joints are the cervical prevertebral muscles (longus capitis, longus colli). Further inferiorly in the thoracic and lumbar regions, they are helped by scalenus anterior, sternocleidomastoid and the anterolateral abdominal muscles (abdominal obliques and rectus abdominis). Bilateral tension of these muscles flexes the vertebral column and opposes the action of the extensors. When these antagonistic muscles work simultaneously, an upright posture is maintained.

The principal extensors of the intervertebral joints are the posterior cervical muscles (splenius capitis, splenius cervicis), which are helped by the suboccipital and trapezius muscles. In the lumbar and thoracic regions, the deep (intrinsic) muscle groups of the back acting on the spine are erector spinae, transversospinales and interspinales. Bilateral contraction extends and hyperextends the vertebral column. They oppose the flexors and are considered the main postural muscles.

The above mentioned muscles are also capable of lateral flexion as a result of unilateral contraction of the respective muscles. Simultaneously, the contralateral muscles will relax, causing the spine to flex ipsilaterally. These muscles are also capable of axial rotation because they extend superiorly and either anterior or posterior relative to the rotation axis of the intervertebral joints. Therefore, contraction of the muscle fibers will rotate the vertebrae relative to each other.

For more details about the muscles acting on the intervertebral joints, take a look at the following study unit.

Intervertebral joints: want to learn more about it?

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