Pathology of ossification of the posterior longitudinal ligament and ligamentum flavum.

Ossification takes place in the posterior longitudinal ligament and ligamentum flavum of the spine under certain conditions of unknown nature. Ossification of these ligaments has the following characteristics: (1) ectopic bone formation occurring within the spinal ligaments; (2) ossification accompanies ligamentous tissue hyperplasia and cell proliferation; (3) before ossification, fibrocartilaginous cell proliferation, calcification and tissue resorption with vascular ingrowth take place sequentially; (4) ossification of the ligament has a specific site of predilection and often occurs in combination with senile ankylosing vertebral hyperostosis (Forestier's disease) or diffuse idiopathic skeletal hyperostosis; and (5) ossification and symptom development are remarkably more frequent in the Japanese population. Recent studies revealed that bone morphogenetic proteins and transforming growth factor-beta played an important role in the matrix hyperplasia and ossification of the spinal ligament, and metabolic and genetic aberration often characterized patients suffering from this disorder.