Posterior Longitudinal Ligament

The posterior longitudinal ligament is a longitudinal band of dense connective tissue that runs along the posterior surfaces of the vertebral bodies and intervertebral discs within the vertebral canal. It extends from the axis superiorly to the sacrum inferiorly and forms part of the anterior boundary of the vertebral canal. Its principal functions are to reinforce the posterior aspects of the intervertebral discs and to help limit excessive flexion of the vertebral column.^[1][2]

The posterior longitudinal ligament is considerably narrower than the anterior longitudinal ligament, particularly across the posterior surfaces of the vertebral bodies. It broadens over the intervertebral discs, producing a characteristic scalloped appearance. This arrangement is important clinically because the ligament reinforces the central posterior part of each disc more effectively than its posterolateral portions.

Superiorly, the ligament continues beyond the body of the axis as the tectorial membrane, which extends through the craniovertebral junction to the occipital bone. Inferiorly, the posterior longitudinal ligament becomes progressively narrower and continues into the sacral canal.

Location and Extent

The posterior longitudinal ligament lies inside the vertebral canal, immediately posterior to the vertebral bodies and intervertebral discs. It is therefore positioned anterior to the dura mater and epidural contents.

Its principal extent is from the body of C2 to the sacrum. Above C2, it continues as the tectorial membrane, which passes posterior to the dens and its associated ligaments before attaching to the internal surface of the occipital bone.

Along its course, the ligament follows the posterior contour of successive vertebral bodies and discs rather than maintaining a uniform width.

Attachments

The posterior longitudinal ligament is attached to the posterior surfaces of the vertebral bodies and intervertebral discs, but the strength of these attachments is not uniform.

• Intervertebral discs: The ligament is firmly attached to the posterior portions of the annuli fibrosi.
• Vertebral margins: Strong attachment occurs near the superior and inferior margins of the vertebral bodies.
• Central vertebral bodies: Attachment is relatively loose compared with that at the discs and vertebral margins.
• Superior continuation: Continues above C2 as the tectorial membrane.
• Inferior continuation: Extends into the sacral canal and becomes progressively narrower.

The relatively loose attachment over the central posterior surfaces of the vertebral bodies leaves space for the internal vertebral venous plexus and associated connective tissue between the ligament and bone.

Structure

The posterior longitudinal ligament consists predominantly of longitudinally arranged collagen fibers. Like the anterior longitudinal ligament, it contains fibers of different lengths rather than a single set of fibers extending throughout its entire length.

Superficial fibers generally span several vertebral levels, while deeper fibers extend across shorter distances between neighboring vertebrae and discs. The deeper fibers are particularly closely associated with the annulus fibrosus.

The ligament is broader at the intervertebral disc levels and narrower over the middle portions of the vertebral bodies. This alternating width gives it a denticulated or scalloped outline when viewed from within the vertebral canal.

Relationship to the Intervertebral Discs

The posterior longitudinal ligament is firmly attached to the posterior surfaces of the intervertebral discs. Its fibers blend with the outer posterior layers of the annulus fibrosus and reinforce the disc along the midline.

At each disc level, the ligament expands laterally compared with its width over the adjacent vertebral bodies. However, this expansion does not provide equally strong reinforcement across the entire posterior circumference of the disc.

The posterolateral portions of the annulus lie lateral to the strongest central reinforcement provided by the ligament. This anatomical arrangement is one factor relevant to the common posterolateral direction of clinically significant lumbar disc herniations.^[2][3]

Relationship to the Vertebral Bodies

Over the posterior surfaces of the vertebral bodies, the posterior longitudinal ligament is narrower and less firmly attached than it is at the intervertebral discs.

Between the ligament and the posterior surface of a vertebral body are connective tissue, small vessels, and components of the internal vertebral venous plexus. These structures occupy the epidural space anterior to the dural sac.

The ligament therefore does not adhere uniformly to the entire posterior surface of each vertebral body.

Relationship to the Vertebral Canal

The posterior longitudinal ligament forms part of the anterior boundary of the vertebral canal. Immediately anterior to it are the vertebral bodies and intervertebral discs, while posterior to it lies the epidural space and, more posteriorly, the dura mater surrounding the spinal cord or cauda equina.

Direction	Relationship
Anterior	Vertebral bodies and intervertebral discs
Posterior	Epidural space and dura mater
Lateral	Internal vertebral venous plexuses and epidural connective tissue
Superior	Continues as the tectorial membrane above C2
Inferior	Continues into the sacral canal

Because of this location, pathological thickening, ossification, or displacement of structures immediately anterior to the ligament can affect the space available within the vertebral canal.

Tectorial Membrane

At the upper cervical spine, the posterior longitudinal ligament continues superiorly as the tectorial membrane. This broad, strong membrane extends from the posterior surface of the body of the axis toward the cranial base.

The tectorial membrane passes posterior to the dens and the deeper ligaments of the atlanto-axial region and attaches to the internal surface of the basilar part of the occipital bone.^[1][3]

This continuation integrates the posterior longitudinal ligament with the ligamentous system of the craniovertebral junction while preserving the distinction between the ligament within the vertebral column and its specialized superior continuation.

Regional Anatomy

The posterior longitudinal ligament extends through the cervical, thoracic, lumbar, and sacral regions, but its relative width and mechanical relationships change along the vertebral column.

Cervical Region

The ligament is relatively broad in the cervical region. It covers a greater proportion of the posterior surfaces of the cervical vertebral bodies and discs than it does in the lumbar region.

At the upper end of the cervical spine, it becomes continuous with the tectorial membrane. Its location directly anterior to the cervical epidural space places it between the cervical vertebral bodies and the dural sac.

Thoracic Region

In the thoracic region, the ligament continues along the posterior surfaces of the vertebral bodies and discs. It remains broader over the discs than over the vertebral bodies.

The thoracic vertebral canal contains the spinal cord throughout this region, so lesions arising from the posterior vertebral bodies or discs can have important relationships to the cord and its coverings.

Lumbar Region

The posterior longitudinal ligament becomes relatively narrow in the lumbar region. Its lateral extensions over the discs provide less extensive coverage of the posterolateral annulus than the broad anterior longitudinal ligament provides anteriorly.

This arrangement is anatomically relevant to lumbar disc herniation. Disc material commonly becomes displaced posterolaterally, where reinforcement by the posterior longitudinal ligament is less substantial than in the central posterior region.

Sacral Region

Inferiorly, the posterior longitudinal ligament continues into the sacral canal. It becomes narrower and less prominent as the vertebral bodies become fused within the sacrum.

Function

The posterior longitudinal ligament contributes to stability of the vertebral column and reinforcement of the posterior intervertebral discs.

• Limits flexion: Becomes tensioned as the vertebral column flexes and the posterior aspects of adjacent vertebral bodies separate.
• Reinforces intervertebral discs: Provides fibrous support to the posterior annulus fibrosus.
• Stabilizes motion segments: Connects successive vertebral bodies and discs.
• Restrains posterior disc displacement: Provides a mechanical barrier along the central posterior aspect of the discs.
• Distributes tensile forces: Its fibers span different numbers of vertebral levels, allowing forces to be shared across adjacent segments.

These functions occur together with those of the intervertebral discs, facet joints, ligamenta flava, interspinous and supraspinous ligaments, and surrounding muscles.

Behavior During Spinal Movement

Flexion

During flexion, the anterior parts of adjacent vertebral bodies approximate while their posterior margins separate. The posterior longitudinal ligament is stretched and contributes to limiting excessive flexion.

The posterior annular fibers, facet joint capsules, ligamenta flava, interspinous ligaments, and supraspinous ligament are also placed under varying degrees of tension during flexion.

Extension

During extension, the posterior aspects of adjacent vertebral bodies approximate and the posterior longitudinal ligament becomes relatively less tense. The anterior longitudinal ligament provides a more important ligamentous restraint to excessive extension.

Rotation and Lateral Flexion

Rotation and lateral flexion produce asymmetric loading of the posterior longitudinal ligament and other spinal structures. The ligament contributes to overall segmental stability, but the available range of these movements depends strongly on facet orientation, disc mechanics, and regional vertebral anatomy.

Comparison with the Anterior Longitudinal Ligament

Feature	Posterior Longitudinal Ligament	Anterior Longitudinal Ligament
Position	Posterior surfaces of vertebral bodies within vertebral canal	Anterior surfaces of vertebral bodies
Relative width	Narrower, especially over vertebral bodies	Broad and strong
Disc attachment	Strong	Strong
Primary movement restrained	Flexion	Extension
Relationship to vertebral canal	Forms part of anterior boundary	Outside vertebral canal
Superior continuation	Tectorial membrane	Continuous with anterior craniovertebral ligamentous structures

The difference in width between the two ligaments is particularly important. The anterior longitudinal ligament forms a broad sheet across the anterior vertebral column, whereas the posterior longitudinal ligament provides more limited coverage of the posterior surfaces of the discs.

Relationship to Disc Herniation

The posterior longitudinal ligament is closely related to the anatomy of intervertebral disc herniation. A central posterior displacement of disc material encounters the ligament directly, while posterolateral displacement occurs closer to its thinner lateral margins.

In the lumbar spine, the combination of annular structure and relatively limited posterolateral ligamentous reinforcement contributes to the frequent posterolateral direction of disc herniation. Displaced material can then enter the lateral recess or vertebral canal and come into relation with spinal nerve roots.

The posterior longitudinal ligament should not be regarded as the sole determinant of herniation direction. Disc degeneration, annular disruption, loading, vertebral anatomy, and other mechanical factors also influence the pattern of displacement.

Innervation

The posterior longitudinal ligament receives sensory innervation associated particularly with the recurrent meningeal nerves, also known as sinuvertebral nerves. These nerves re-enter the vertebral canal through the intervertebral foramina and supply structures within and around the canal, including the posterior longitudinal ligament and outer portions of the intervertebral discs.^[1]

Sympathetic fibers also contribute to the innervation of tissues in this region. The presence of sensory nerve endings provides an anatomical basis for the ligament to participate in nociceptive and proprioceptive signaling when mechanically or pathologically affected.

Blood Supply

The posterior longitudinal ligament receives small vascular branches from the segmental arterial networks supplying the vertebral bodies, vertebral canal, meninges, and surrounding tissues.

Its close relationship to the internal vertebral venous plexus should not be confused with its arterial supply. The venous plexus lies within the epidural space and provides an important valveless venous pathway along the vertebral column.

Clinical Significance

Posterolateral Disc Herniation

The relatively narrow configuration of the posterior longitudinal ligament is clinically important in the lumbar spine. The ligament provides stronger reinforcement near the midline than farther laterally, leaving the posterolateral portions of the discs less directly reinforced.

A posterolateral lumbar disc herniation may extend into the lateral recess and affect a traversing spinal nerve root. The exact neural relationship depends on the disc level and direction of displacement.

Ossification of the Posterior Longitudinal Ligament

Ossification of the posterior longitudinal ligament (OPLL) is a pathological process in which portions of the ligament become ossified. It most commonly affects the cervical spine, although thoracic and other levels can also be involved.

Because the ligament lies immediately anterior to the spinal cord and dural sac, substantial ossification can reduce the dimensions of the vertebral canal. The anatomical consequences depend on the location and extent of the ossified tissue and the available canal space.

Spinal Trauma

The posterior longitudinal ligament can be disrupted in traumatic injuries involving excessive translation, distraction, or deformation of a spinal motion segment. Its condition is evaluated together with the intervertebral discs, vertebral bodies, facet joints, and posterior ligamentous structures.

Ligament disruption may be demonstrated on MRI or inferred from abnormal vertebral alignment and associated structural injuries. Its significance depends on the complete pattern of spinal damage.

Surgical Anatomy

The posterior longitudinal ligament is encountered during procedures involving the posterior surfaces of vertebral bodies and intervertebral discs, particularly anterior decompressive approaches to the cervical spine.

Because the epidural space, dura mater, and neural structures lie posterior to the ligament, its relationship to these structures is important during removal of disc material or decompression of the vertebral canal.

The posterior longitudinal ligament forms a continuous internal reinforcement along the posterior surfaces of the vertebral bodies and intervertebral discs. Its firm disc attachments, scalloped configuration, continuation as the tectorial membrane, and position immediately anterior to the epidural space make it an important structure for understanding spinal stability, disc herniation, vertebral canal anatomy, and disorders such as ossification of the posterior longitudinal ligament.