Two cases of continuous type of OPLL and one case of segmental type of OPLL were autopsied and histologically studied. Characteristic findings were as follows:
1. In OPLL, the ossification initiates near the posterior border and edge of the vertebral body, and thickens toward the superficial layer of the posterior longitudinal ligament. In the axial ossifying point of the continuous type of OPLL, various directions of ossification, including axial direction and from superficial to deep direction, are recognized.
2. Degeneration and posterior protrusion of the intervertebral disc are more evident in the segmental type of OPLL than in the continuous type of OPLL. However, other factors such as age and history of disc herniation should be considered.
3. Endochondral ossification process is predominant in the ossifying point, and membranous ossification process is focally noticed.