OPLL - ossification of the posterior longitudinal ligament.

It is a spinal ligament which runs down the back (posterior) of the spine inside the vertebral column.

Essentially, the ligament starts to ossify or convert to bony material. Eventually in some cases, the bone formed will actually produce bone marrow. The ossification can be fairly localised or it can run the length of the whole ligament.

There are multiple dangers of the condition. The bony growth takes up additional space in the spinal canal and as it grows and takes up more space, it can eventually cause compression of the spinal cord. Usually a laminectomy and removal of the bony tissue is the only option if this happens and the symptoms become too serious to leave it. Worse though is the risk that the presence of this hard, bony material next to the spinal cord means that even a very minor bump or trauma can cause the bony material to slice through the spinal cord. There is obviously no return from that kind of injury, where the cord is cut, there will be some degree of permanent paralysis depending on the extent and location of the damage.

Even if neither of these things happen, it can cause pain, limited movement and so on. Some people are not symptomatic even when there is significant ossification and compression, others are symptomatic with very little ligament change.

Hope that helps to explain.