OPLL, or ossification of the posterior longitudinal ligament, is a condition in which a flexible ligament running along the back of the spinal column gradually turns to bone. This thickened, rigid mass takes up space inside the spinal canal and can press on the spinal cord, causing pain, numbness, and difficulty with movement. It most commonly affects the cervical spine (the neck region) and is found in 4 to 9 percent of people in East Asian populations, compared to roughly 2 to 2.5 percent in Western populations.
Where It Happens in the Spine
The posterior longitudinal ligament is a thin band of connective tissue that runs nearly the entire length of the spine, from the second vertebra in the neck all the way down to the base. It sits just behind the vertebral bodies, directly next to the spinal cord, and its job is to stabilize the spinal column. Because this ligament already lives in such tight quarters, even a small amount of abnormal bone growth can crowd the spinal cord. The cervical spine is the most common site, though OPLL can also develop in the thoracic (mid-back) region at lower rates.
Types of OPLL
A widely used Japanese classification system divides OPLL into four types based on its shape and location on imaging:
- Segmental: One or more separate patches of bone growth behind individual vertebral bodies. This is the most common pattern, found in about 34 percent of cases.
- Mixed: A combination of continuous and segmental patterns, accounting for roughly 32 percent of cases.
- Continuous: A long, unbroken strip of ossification spanning several vertebrae, seen in about 26 percent of cases.
- Circumscribed: A small, localized mass that forms mainly behind a disc space. This is the least common type, at about 7 percent.
The type matters because it influences how much of the spinal canal is affected and which surgical approach works best.
Who Gets It and Why
OPLL has a strong genetic component. Studies in both Asian and Caucasian families have identified multiple susceptibility genes tied to bone and cartilage formation. Several of these genes are involved in collagen production (particularly the COL11A2 gene, which is linked to cartilage) and signaling pathways that regulate bone growth. In simple terms, the cells in the ligament receive faulty signals that tell them to start producing bone where they shouldn’t.
The ethnic difference in prevalence is striking. CT-based screening studies found cervical OPLL in 6.3 percent of Japanese, 8.2 percent of South Korean, and 4.1 percent of Chinese populations. In the United States, Asian Americans had a prevalence of about 5 to 6 percent, compared to roughly 1.3 percent in Caucasian Americans, 2.1 percent in African Americans, and 1.9 percent in Hispanic Americans. These numbers suggest a genetic susceptibility layered on top of environmental or metabolic factors that researchers are still working to fully understand.
Symptoms and How They Develop
Many people with OPLL have no symptoms at all, especially in the early stages. The ossified ligament can sit quietly inside the spinal canal for years. Symptoms typically appear when the bone mass grows large enough to compress the spinal cord, a condition called myelopathy.
Common symptoms include neck pain, numbness or tingling in the hands and arms, clumsiness with fine motor tasks like buttoning a shirt, and difficulty walking or a feeling of unsteadiness. In more advanced cases, numbness can extend to the lower extremities. Pain and numbness in the limbs tend to be more prominent than low back pain or chest tightness. Notably, compression from an ossified ligament often causes more pain than compression from ordinary age-related disc degeneration or spondylosis. Some patients find that their daily limitations come more from pain than from actual muscle weakness.
How OPLL Grows Over Time
OPLL is a progressive condition, meaning the ossified mass tends to keep growing. In patients who have been tracked over time, the average rate of lengthening along the spine is about 1.6 millimeters per year. It grows faster in the first couple of years after detection, averaging around 2.2 mm per year, then slows to roughly 1 mm per year in subsequent years. The bone also thickens gradually, pushing further into the spinal canal at an average of about 0.14 to 0.20 mm per year.
Growth is not uniform across the neck. The C5 vertebral level shows the fastest rate of thickening, followed by C4. The C7 level tends to progress more slowly. This matters because the mid-cervical spine is already a relatively narrow part of the canal, so even modest growth there can tip a person from asymptomatic to symptomatic.
Diagnosis: CT Is the Gold Standard
CT scanning is the most reliable way to diagnose OPLL. It clearly shows the ossified bone, including small deposits that plain X-rays can miss due to overlapping bony structures in the neck. CT also reveals the exact shape, size, and location of the mass, which is essential for classifying the type and planning treatment.
MRI is useful for a different reason: it shows the soft tissues, including the spinal cord itself, so doctors can assess whether the cord is being compressed and whether there are signs of damage. However, MRI alone can be misleading because it has difficulty distinguishing ossified ligament from a thickened but non-ossified ligament or a herniated disc. A suspected case on MRI almost always needs CT confirmation. In practice, most patients end up getting both scans.
Treatment Options
When OPLL is mild and not causing significant symptoms, monitoring with periodic imaging is a reasonable approach. Physical therapy and pain management can help maintain function. But once myelopathy develops, meaning the spinal cord is being compressed enough to cause neurological symptoms, surgery is generally the path forward.
There are two broad surgical approaches. Posterior surgery works from the back of the neck to create more room for the spinal cord by removing or reshaping the bony arches of the vertebrae. The two main techniques are laminoplasty (opening the bony arch like a hinge to widen the canal) and laminectomy with fusion (removing the arch entirely and stabilizing the spine with hardware). Both produce comparable improvements in nerve function and pain scores, so the choice often depends on the surgeon’s assessment of the individual case.
Anterior surgery approaches from the front of the neck to directly remove the ossified mass. This is a more technically demanding operation, but it can produce better neurological outcomes in specific situations, particularly when the ossification fills 50 percent or more of the spinal canal. Surgeons use a measurement called the K-line, a straight line drawn between the midpoints of the spinal canal at C2 and C7 on a side-view X-ray. If the OPLL crosses that line (called “K-line negative”), it means the mass is bulging significantly into the canal, and anterior surgery tends to deliver better results than posterior decompression alone.
Surgical Risks
Surgery for OPLL carries a complication rate of roughly 30 percent, which is higher than many other spinal procedures. The most common complication is C5 palsy, a temporary weakness in the deltoid or bicep muscles of the arm, occurring in about 7.6 percent of cases. Dural tears (small rips in the membrane surrounding the spinal cord) and surgical site infections each occur in about 3 percent of cases. Epidural hematoma, a collection of blood near the spinal cord, happens in about 1.5 percent. Most of these complications are manageable, but they underscore why surgery is typically reserved for people with clear neurological decline rather than those with mild or stable symptoms.