Pathological studies on the ossification of the posterior longitudinal ligament (opll)

Ossification of the posterior longitudinal ligament (OPLL), which causes marked spinal cord compression, occurs more frequently in Japan than in other parts of the world. To investigate the mechanism of this ossification, we examined two cases of mixed type of OPLL inducing severe neurological signs due to spinal cord compression and three cases of segmental OPLL type without neurological signs. Ossification of the ligament begins at the upper end of posterior margin of the vertebral body and distends mainly downward along the posterior longitudinal ligament (PLL) and finally replaces the PLL completely. Ossification consists of compact bones with lamellar structures. In the upper end of the posterior margin of the vertebral body, where the PLL fuses with the cortical bone of vertebral body, ossification always has a direct contact with cortical bone and in this portion proliferation of cartilaginous tissue and calcification were observed. Proliferation of cartilaginous tissue plays a key role in the formation of OPLL and occurs mainly by an enchondral ossification. In three among twenty one cases of cervical vertebrae examined as the control, small foci of ossification were observed which might be related to the initial phase of OPLL. Spinal cords of case 1 and 2 showed marked indentation and flattening with the loss of nerve cells, severe fibrous gliosis of the gray matter and demyelination of the anterior column. Venous congestion due to compression and vulnerability of gray matter to anoxia is important for the histological changes of spinal cord.