Case study 81 - Ossification of the Posterior Longitudinal Ligament (OPLL) Ossification of the posterior longitudinal ligament, also referred to as OPLL, is a spinal condition where the posterior longitudinal ligament becomes calcified and less flexible. The posterior longitudinal ligament runs the entire length of the spine from the neck to the end of the spine and stabilizes the spinal column bones. The exact cause of OPLL is unknown, but it typically occurs when the soft tissues on the spine