Posterior longitudinal ligament

At a glance

English

Posterior longitudinal ligament

Latin (TA2)

Ligamentum longitudinale posterius

System

Ligament

Region

Neck

Side

Single, on the midline

About the posterior longitudinal ligament

Two longitudinal ligaments run the length of the vertebral column, one in front of the vertebral bodies and one behind them. The posterior longitudinal ligament is the one behind, which means it lies inside the vertebral canal, forming the front wall of the canal immediately in front of the spinal cord and its dura. Above, it continues upwards as the tectorial membrane over the dens to the base of the skull; below, it reaches the sacrum and may extend onto the coccyx.

Its shape is distinctive. Over each vertebral body it is narrow and loosely attached, leaving room beside it for the basivertebral veins that leave the back of the body. Over each intervertebral disc it widens and binds firmly to the annulus fibrosus, which in the thorax and lower back gives it a toothed outline with scalloped edges in between. It is thicker in the thoracic column than in the neck or the lower back. The fibres are glossy and densely packed; the superficial ones span three or four vertebrae, and the deep ones join only neighbouring bodies.

Mechanically it is a modest restraint on bending forwards (flexion), far weaker than the anterior ligament. It also helps hold the disc material in place behind. Because it is a narrow strip in the midline, the posterolateral corners of each disc are left without cover, which is why a prolapsed lumbar disc usually bulges to one side and presses on a nerve root rather than straight back onto the cord.

The ligament is richly supplied with pain fibres and with vasomotor nerves, so it can itself be a source of back pain and responds to injury with increased blood flow. In ossification of the posterior longitudinal ligament, a condition most often seen in the cervical spine, the ligament turns slowly to bone and can narrow the canal enough to compress the spinal cord.

Attachments and function

Attaches from

The posterior surface of the body of the axis, continuous above with the tectorial membrane.

Attaches to

The posterior surface of the sacrum, sometimes reaching the coccyx, after attaching to each disc on the way.

Function

Weakly resists flexion of the vertebral column and helps keep the discs from bulging backwards into the canal.

Nerve supply

The meningeal (sinuvertebral) branches of the spinal nerves, which carry pain and vasomotor fibres.

Blood supply

Branches of the segmental spinal arteries entering through the intervertebral foramina.