Posterior Longitudinal Ligament: Anatomy, Pain, and OPLL

The posterior longitudinal ligament is a long, narrow band of connective tissue that runs along the back surface of the vertebral bodies inside the spinal canal, stretching from the base of the skull down to the sacrum. Its primary job is to resist excessive forward bending of the spine and help keep the intervertebral discs from bulging backward into the spinal cord. Despite being one of the thinnest structures in the spine, it plays an outsized role in spinal stability, pain signaling, and a condition called ossification of the posterior longitudinal ligament (OPLL) that can quietly compress the spinal cord over years.

Where It Sits and What It Does

The posterior longitudinal ligament (PLL) sits just behind the vertebral bodies, directly in front of the spinal cord and its surrounding membranes. It attaches firmly to the intervertebral discs and more loosely to the vertebral bodies themselves, with small blood vessels running through the gaps between the bone and the ligament. This positioning makes it one of the spine’s front-line barriers against disc material pushing into the spinal canal.

The ligament contributes to both stability and flexibility of the spine.¹ It is widest in the upper spine and tapers dramatically as it descends. A morphometric study found that the average width of the PLL was about 7.8 mm at the first lumbar vertebra but only around 1.9 mm at the fifth lumbar vertebra, covering roughly 21% of the vertebral body width at the top of the lumbar spine and just 3% at the bottom.² That sharp narrowing at the lower lumbar levels is one reason the lower back is so vulnerable to disc herniations: the ligament is thinnest precisely where mechanical loads are highest.

In the cervical spine (neck region), the PLL and its counterpart on the front side, the anterior longitudinal ligament, have similar tensile properties. One study found a trend toward slightly less stiffness but more energy absorption at failure in the anterior ligament compared with the posterior one, though these differences were not statistically significant.³ In practical terms, the two ligaments share the mechanical workload of keeping the cervical spine stable.

How It Develops

The PLL forms early in fetal development. An embryological study of human fetuses found it could be detected as early as 10 weeks of gestation, making it one of the earlier spinal ligaments to appear.⁴ By the time the spine is fully formed, the PLL is a continuous structure from the skull base to the sacrum, though its thickness and width vary considerably by region.

The Nerve Supply and Its Role in Back Pain

The PLL is not just a passive strap of tissue. It is richly supplied with nerve fibers, which is part of why damage to it or the structures around it can be so painful. The nerve supply comes primarily from the sinuvertebral nerve, a recurrent branch that loops back into the spinal canal after branching off from the spinal nerve. This nerve splits into ascending and descending branches that connect with branches from neighboring vertebral levels, forming a network that spreads across the back of the disc and the PLL itself.⁵

This dense innervation serves at least two purposes. The nerve fibers likely help regulate posture and movement by providing the brain with information about spinal position and load. But they also explain why the PLL is implicated in discogenic pain, the deep, hard-to-localize back pain that originates from damaged discs. The sinuvertebral nerve has a sympathetic component in the lumbar spine, which has attracted considerable clinical interest because it opens the door to targeted pain procedures.⁶ Understanding the anatomy of this nerve has led to diagnostic and therapeutic approaches for chronic disc-related pain, including nerve blocks and ablation procedures aimed at interrupting the pain signals traveling through it.

The Connection to Disc Herniation

When a disc herniates, what has essentially happened is that the soft interior of the disc has pushed through the tough outer ring and past the PLL into the spinal canal. The PLL acts as one of the final barriers preventing this. Lumbar disc herniation occurs when the PLL can no longer contain the disc material within the intervertebral space.⁷

The PLL’s dramatic narrowing at L4-L5 and L5-S1 (the two lowest lumbar disc levels) leaves less ligament covering the back of those discs, and microanatomic studies have confirmed that the PLL is thinner at its attachment to the disc’s outer ring at these levels.⁸ It is no coincidence that L4-L5 and L5-S1 are the most common sites for lumbar disc herniations. The combination of heavy mechanical loading and a thinned-out PLL creates a structural weak point.

In some surgical procedures for disc herniation, the PLL is opened or partially removed to access and remove the protruding disc material. Whether to repair or suture the PLL afterward is an active area of surgical interest, with at least one study suggesting that repairing it after discectomy can help maintain spinal canal dimensions.⁹

Ossification of the Posterior Longitudinal Ligament

The most clinically significant pathology of the PLL is ossification, a process in which the ligament gradually turns to bone. OPLL was first described in an 1838 autopsy report that found cervical spinal cord compression from a bony PLL. The condition received little attention until a Japanese report in 1960 brought it into mainstream clinical awareness. Since then, it has become one of the more studied causes of spinal cord compression, particularly in East Asian populations.

The ossification happens through a combination of two bone-formation processes: endochondral ossification (where cartilage forms first and is then replaced by bone) and membranous ossification (where bone forms directly within tissue). Several genes appear to drive these processes in the PLL, and analysis of implicated genetic loci has identified specific genes that may promote OPLL through each pathway.¹⁰ The result is a growing mass of bone where flexible ligament used to be, progressively narrowing the spinal canal and pressing on the spinal cord.

Who Gets OPLL

OPLL has a well-documented ethnic gradient. It affects roughly 0.8–3.0% of aging Asian populations and about 0.1–1.7% of aging European Caucasian populations.¹¹ A large population-based study in Japan found a radiographic prevalence of about 1.9%, with significantly higher rates in men than in women.¹²

The picture is more complex in Black populations, where far less research has been done. A study of over 3,200 Black patients found an overall prevalence of about 1.5% for thoracic OPLL, with an interesting reversal of the sex pattern: women had significantly higher rates than men (2.4% versus 0.8%). The average age of affected patients was also significantly higher, around 65 compared with about 58 in unaffected patients.¹³ The different sex ratio and the thoracic (rather than cervical) location suggest that OPLL may behave differently across ethnic groups, not just in frequency but in where and how it develops.

Genetics and Risk Factors

There is a strong familial component to OPLL. Despite an unclear inheritance pattern, linkage analysis has identified multiple candidate genes. These include genes for collagen, nucleotide pyrophosphatase, transforming growth factors, and the vitamin D receptor. Abnormal expression of bone morphogenetic proteins and certain interleukins involved in bone development has also been found in OPLL patients.¹⁴

One genetic mapping study found significant linkage near the HLA locus on chromosome 6 and identified specific variants in the collagen gene that differed between OPLL patients and controls. Certain haplotype combinations were significantly more common in affected individuals, while a protective haplotype was significantly less common.¹⁵ The genetic architecture is clearly polygenic, with multiple genes each making small contributions to risk.

Metabolic conditions have long been suspected as risk factors. A large database study initially found that hyperlipidemia, diabetes, and obesity were all associated with higher OPLL rates. However, after matching patients by age, sex, and overall health burden, those associations disappeared, suggesting the link was confounded by demographics rather than being a direct causal relationship.¹⁶ This is a useful reminder that observational associations between OPLL and metabolic disease may simply reflect the fact that both become more common with age.

Symptoms and How They Progress

Many people with OPLL have no symptoms at all, at least initially. The disease can begin silently and may be discovered incidentally on imaging done for another reason. Some patients progress slowly to develop myelopathy, a constellation of symptoms caused by spinal cord compression. These can include difficulty with fine motor tasks in the hands, clumsiness when walking, numbness or tingling in the arms or legs, and in severe cases, problems with bladder or bowel control.¹⁷

The cervical spine is the most commonly affected region, and cervical OPLL is the most clinically significant because of the spinal cord’s proximity. Some patients also develop radiculopathy, where a nerve root is compressed, causing pain, weakness, or numbness in a specific arm or leg distribution.¹⁸

Imaging and Classification

Standard X-rays can detect OPLL, but they miss a meaningful proportion of cases. A comparison study found that in about 29% of cases, the ossified lesions were either too small or too unclear to show up on lateral X-rays, while three-dimensional CT imaging caught them all. CT also revealed the side-to-side extent of the ossification, information that plain X-rays simply cannot provide. In patients who also had MRI, spinal cord compression was typically found at the upper or lower edges of the ossified lesions seen on CT.¹⁹

Over the decades, researchers have developed a bewildering number of classification systems for OPLL. A systematic review identified over a dozen, based on X-ray, CT, three-dimensional CT, and MRI. The most widely used system classifies OPLL into continuous, segmental, mixed, and localized types, and this system accounted for over 90% of the classification usage in the literature the review examined.²⁰ From a practical standpoint, what matters most to surgeons is how much of the spinal canal the ossification occupies and where the cord compression is worst, since these factors drive treatment decisions.

When Surgery Is Considered

When symptoms are mild and stable, conservative management with periodic monitoring is considered appropriate. But this watchful approach carries risk. A study using a national database found that patients with cervical OPLL who were managed without surgery had dramatically higher rates of spinal cord injury compared with matched controls without OPLL. The adjusted hazard ratio was about 32 for hospitalized spinal cord injury and over 100 for disabling spinal cord injury.²¹ Those numbers are sobering and help explain why the question of when to intervene remains controversial: the conservative path is not as safe as it might seem.

Patients with OPLL are also vulnerable to spinal cord injury from minor trauma that would be harmless to someone without the condition. An already-narrowed spinal canal leaves no buffer zone, so even a low-speed fall or fender bender can cause acute cord injury.²² This heightened vulnerability to everyday mishaps is something patients with known OPLL should be counseled about.

Once symptoms of myelopathy are present and progressing, surgery to decompress the spinal cord becomes the standard recommendation.²³ The two main surgical approaches are anterior (from the front of the neck) and posterior (from the back). Each has trade-offs that depend heavily on the degree of canal narrowing.

Anterior Versus Posterior Surgery

The choice between going through the front or back of the spine for OPLL surgery is one of the more debated decisions in spine surgery. Two separate systematic reviews and meta-analyses have examined this question, and their findings largely agree. The anterior approach tends to produce better functional recovery overall and is particularly advantageous when the ossification occupies more than about half the canal diameter.²⁴ When the canal is less than half occupied, functional outcomes are similar between the two approaches.²⁵

The trade-off is that the anterior approach, which involves removing the vertebral body and the ossified ligament directly, carries higher rates of surgical complications. The posterior approach, which works by making more room for the cord to drift away from the compression, is a less invasive operation with fewer complications. So the practical decision often comes down to severity: larger ossifications that severely compress the cord benefit enough from the anterior approach to justify the added risk, while smaller ones can be treated with the safer posterior route.

Complications of OPLL Surgery

The most feared complication of anterior surgery for OPLL is a tear of the dura, the membrane that surrounds the spinal cord and contains cerebrospinal fluid. Because the ossified ligament often adheres tightly to the dura, removing it can tear the membrane. In one surgical series, intraoperative cerebrospinal fluid leaks occurred in about 6% of anterior corpectomy cases, with dural defects ranging from a few millimeters to about 15 mm. All were successfully repaired without requiring reoperation.²⁶

When direct suture repair is not possible because of ragged tears or tight working space, surgeons have developed workarounds involving artificial dural patches, collagen sponges, and fibrin glue. In one series of seven patients with dural tears during anterior OPLL surgery, fluid collections (pseudomeningoceles) developed in over half, but all eventually resolved without further intervention.²⁷ These complications, while manageable in experienced hands, are one of the reasons the posterior approach is preferred when the clinical situation allows it.

OPLL After a Diagnosis but Before Symptoms

Perhaps the most difficult clinical scenario is the patient who gets imaging for an unrelated complaint and is told they have OPLL but no neurological symptoms. This is not uncommon, especially in populations with higher prevalence rates. The optimal path for these patients has never been established in a controlled trial. Some experts advocate early surgery to prevent the potentially catastrophic spinal cord injury that could come from a minor fall. Others argue that operating on an asymptomatic person exposes them to surgical risks for a problem that may never cause symptoms during their lifetime.

The data on spinal cord injury risk in conservatively managed OPLL patients adds weight to the argument for vigilance, though not necessarily for immediate surgery. What most clinicians agree on is that patients with known OPLL should avoid high-risk activities, take precautions against falls, and have regular clinical follow-up with periodic imaging to monitor progression. The speed at which ossification grows varies widely between individuals, and some people’s OPLL remains stable for decades while others progress relentlessly.²⁸

Why the Lower Lumbar PLL Gets the Least Protection

Returning to the ligament’s basic anatomy, the dramatic thinning of the PLL at the lower lumbar spine deserves some practical attention. At L1, the PLL covers about a fifth of the vertebral body’s width. By L5, it covers barely 3%.²⁹ This means the lowest two discs, which bear the most compressive load during sitting, bending, and lifting, have the least ligamentous backup. The width decrease can happen gradually from L1 to L5 or abruptly from L4 onward, and microanatomic analysis has confirmed that the PLL’s attachment to the disc’s outer ring is thinner at these levels as well.

This structural reality has implications beyond disc herniation. During spinal surgery at the lower lumbar levels, the PLL provides less of a safety margin between surgical instruments and the dural sac. Surgeons operating at L4-L5 or L5-S1 need to account for the fact that the PLL at these levels is sometimes barely more than a filament. It also means that when disc degeneration weakens the outer ring of the disc, there is less reinforcement from the PLL to compensate, which may partly explain why recurrent herniations are more common at lower levels even after successful initial surgery.