Ossification of the posterior longitudinal ligament of the cervical spine: histopathological findings around the calcification and ossification front

Object. The authors studied the histological and immunohistochemical features of ossified posterior longitudinal ligament (PLL) of the cervical spine, especially in the calcification and ossification front. Methods. Samples of en bloc ossified PLL plaque obtained in 31 patients were stained with H & E and immuno-histochemically prepared for collagens (types I and II), vascular endothelial growth factor (VEGF), transforming growth factor (TGF)–β, and bone morphogenetic protein (BMP)–2, and by terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate nick-end labeling method for apoptosis. Results. Enchondral ossification was evident between the ligamentous enthesis and deep layer of the ligament, with irregularly disorganized arrangement of elastic fibers in association with advancement of the degenerative process. In the ossification front, many hypertrophic metaplastic chondrocytes were noted in the ossifying plaque immediately contiguous to the ligament fibers, together with a considerable degree of neovascularization. Both TGFβ and BMP-2 were highly expressed in metaplastic hypertrophic chondrocytes in the ossification front, and BMP-2 was also expressed in fibroblastic cells near the ossified PLL plaque. Expression of type I collagen was significant in the matrix of the ossified PLL lesion, whereas that of type II was marked in metaplastic chondrocytes in the ossification front. Apoptotic hypertrophic chondrocytes were observed mainly in the fibrocartilaginous area near the calcification front. Conclusions. The enchondral ossification process in the ossified PLL was closely associated with degenerative changes of elastic fibers and cartilaginous cartilage formation, together with the appearance of metaplastic hypertrophic cartilage cells and neovascularization. The authors also found that VEGF-positive metaplastic chondrocytes in the ossification front and different expression patterns of collagens probably play some role in the extension of the ossified PLL from the ossification front.