Posterior Longitudinal Ligament¶

Definition¶

The posterior longitudinal ligament (PLL) is a narrow fibrous band that runs along the posterior surface of the vertebral bodies within the vertebral canal, from the body of the axis (C2) to the sacrum. It is continuous superiorly with the tectorial membrane at the craniocervical junction.

Anatomy¶

Structure¶

Width: widest in the cervical region and narrowest in the lumbar region — this progressive narrowing contributes to the higher incidence of posterolateral disc herniations in the lumbar spine
Shape: has a characteristic serrated or denticulate appearance, being wider over the discs (where it blends with the annulus fibrosus) and narrower over the vertebral body midline
Depth: two layers — a superficial layer spanning several segments and a deep layer bridging adjacent vertebrae

Attachments and Relations¶

Superior: continuous with the tectorial membrane, which attaches to the clivus
Inferior: tapers to the sacrum
Anterior: lies against the posterior aspect of the vertebral bodies and intervertebral discs
Posterior: separated from the dura mater by the epidural space containing the internal vertebral venous plexus (Batson plexus) and epidural fat

Function¶

Resists hyperflexion and posterior disc herniation
Provides posterior vertebral body stability
Weakest in the lumbar region, which is one reason lumbar disc herniations are most common

Clinical Pearl

The PLL is wider over the disc and narrower over the vertebral body. Because of this, lumbar disc herniations most commonly occur in a posterolateral direction — to the side of the PLL — where the ligament is thinnest. Central herniations must breach the full width of the PLL and are less common but can be more clinically significant.

Imaging Findings¶

Radiography¶

Not directly visualized
Indirect signs of PLL injury include retrolisthesis and widened posterior disc space

CT¶

Ossification of the PLL (OPLL) is the major pathologic finding — a hyperdense mass posterior to the vertebral bodies within the spinal canal
Most common in the cervical spine (C2–C5)
Higher prevalence in East Asian populations (2–4% in Japan)
Four morphologic types: continuous, segmental, mixed, and circumscribed
Can cause significant spinal canal stenosis and myelopathy

MRI¶

Finding	Appearance
Normal PLL	Thin, low-signal line posterior to vertebral bodies on sagittal T1/T2
PLL disruption	Discontinuity with adjacent edema (high T2/STIR signal)
OPLL	Low signal on all sequences; may cause cord compression with T2 cord signal change
Disc herniation	Disc material extends beyond the PLL margin

Key MRI Finding

On MRI, OPLL appears as a thick low-signal band posterior to the vertebral bodies. When the cord is compressed, look for intramedullary T2 hyperintensity indicating myelopathy. CT is superior to MRI for characterizing the morphology and extent of ossification.

Key Points¶

The PLL runs along the posterior vertebral bodies within the spinal canal from C2 to the sacrum
It is widest in the cervical region and narrowest in the lumbar region
The serrated shape (wider at discs, narrower at bodies) explains why most lumbar herniations are posterolateral
OPLL is most common in the cervical spine and in East Asian populations
CT is best for characterizing OPLL morphology; MRI is best for evaluating cord compression

References¶

Aryal V, Black AC, Jimenez A. Anatomy, Back, Posterior Longitudinal Ligament. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. PMID: 32809526. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560691/
Salaud C, Ploteau S, Hamel O, Armstrong O, Hamel A. Morphometric study of the posterior longitudinal ligament at the lumbar spine. Surg Radiol Anat. 2018;40(5):563-569. PMID: 29288395. https://pubmed.ncbi.nlm.nih.gov/29288395/
Choi BW, Song KJ, Chang H. Ossification of the posterior longitudinal ligament: a review of literature. Asian Spine J. 2011;5(4):267-276. PMID: 22164324. https://pubmed.ncbi.nlm.nih.gov/22164324/
Inamasu J, Guiot BH, Sachs DC. Ossification of the posterior longitudinal ligament: an update on its biology, epidemiology, and natural history. Neurosurgery. 2006;58(6):1027-1039. PMID: 16723881. https://pubmed.ncbi.nlm.nih.gov/16723881/
Abiola R, Rubery P, Mesfin A. Ossification of the Posterior Longitudinal Ligament: Etiology, Diagnosis, and Outcomes of Nonoperative and Operative Management. Global Spine J. 2016;6(2):195-204. PMID: 26933622. https://pubmed.ncbi.nlm.nih.gov/26933622/
Kawaguchi Y, Matsumoto M, Iwasaki M, et al. New classification system for ossification of the posterior longitudinal ligament using CT images. J Orthop Sci. 2014;19(4):530-536. PMID: 24817494. https://pubmed.ncbi.nlm.nih.gov/24817494/