Thoracic ossification of the human ligamentum flavum: Histopathological and immunohistochemical findings around the ossified lesion

Object. The object of this study was to histopathologically and immunohistochemically characterize ossification of the ligamentum flavum (OLF) in samples of the thoracic spine harvested en bloc during surgery and to enhance the understanding of the ossifying process, particularly calcification and ossification. Methods. Samples of OLF plaque were obtained en bloc from 43 patients who underwent posterior decompression. The histopathological findings were correlated with radiological subtypes using computed tomography. The expression of type I and type II collagens, vascular endothelial growth factor (VEGF), transforming growth factor (TGF)β, and bone morphogenetic protein (BMP)-2 was investigated. Results. Surgical decompression using the posterior floating and en bloc resection technique resulted in neurological improvement in 40 of 43 patients. Progression of the OLF lesion longitudinally and medially was associated with significant degeneration of elastic fibers, fiber bundle derangement, decrements in fiber diameter, and fragmentation. Calcification and ossification paralleled the degeneration of the elastic fibers, extended more medially, and fused in the central area. Expression of BMP-2, TGFβ, and VEGF was significant in chondrocytes in the calcified cartilage and fibrocartilage layers, especially around the calcified front. Conclusions. Histopathologically, the progress of calcification and ossification was closely associated with the degeneration of elastic fibers and with significant expression of BMP-2, TGFβ, and VEGF in the ossification front.