Pathology of ossification of the posterior longitudinal ligament and ligamentum flavum

Ossification takes place in the posterior longitudinal ligament and ligamen tum flavum of the spine under certain conditions of unknown nature. Ossific ation of these ligaments has the following characteristics: (1) ectopic bon e formation occurring within the spinal ligaments; (2) ossification accompa nies ligamentous tissue hyperplasia and cell proliferation; (3) before ossi fication, fibrocartilaginous cell proliferation, calcification and tissue r esorption with vascular ingrowth take place sequentially; (4) ossification of the ligament has a specific site of predilection and often occurs in com bination with senile ankylosing vertebral hyperostosis (Forestier's disease ) or diffuse idiopathic skeletal hyperostosis; and (5) ossification and sym ptom development are remarkably more frequent in the Japanese population, R ecent studies revealed that bone morphogenetic proteins and transforming gr owth factor-beta played an important role in the matrix hyperplasia and oss ification of the spinal ligament, and metabolic and genetic aberration ofte n characterized patients suffering from this disorder.