Pathomechanism of Thoracic Disk Herniation: A Postmortem Histological Study

Abstract

Introduction Kokubun et al histologically demonstrated that herniated masses in all cases of cervical disk herniation contained cartilaginous endplate (CEP) fragments (Spine 1996). They also revealed that the CEP-type of herniation was the predominant type of herniation in the cervical spine resulting from horizontal and vertical cleft formations of the disks at the final stage of the degeneration. On the other hand, it is generally understood that herniated tissues in the lumbar spine can consist of nucleus pulposus and anulus fibrosus, and that lumbar disk herniation does not necessarily happen at the final stage of disk degeneration. However, to our knowledge, the histological features of the herniated masses in thoracic disk herniation have not yet been clarified. In the present study, intervertebral disks from autopsy cases were examined histologically as to elucidate the pathomechanism of thoracic disk herniation. Materials and Methods Specimens of 240 intervertebral disks from T1-T2 to T12-L1 were obtained at autopsy from 20 individuals, 16 men and 4 women. Only spines without tumorous or infectious abnormalities were collected. The age at the time of death ranged from 38 to 93 years (average, 63 years). A 5-mm thick midsagittal slab was cut-off from each specimen, then fixed in 10% buffered formalin, decalcified in 4% edetic acid, and embedded in paraffin. The sections were sagittally cut and stained with hematoxylin and eosin, toluidine blue, and elastica-Masson and were examined with light microscopy. Results Thoracic disk degeneration A horizontal cleft traversing the fibers of the disks was observed in 92% of the disks examined. Total 21%, 34%, and 37% had a horizontal cleft that was less than one-third, one-third to two-third, and more than two-third of the anteroposterior diameter of each disk, respectively. One or more vertical clefts extending to the CEP were observed in 38% of the disks examined. Specimens showing discontinuous separation of fiber bundles that might be artifacts were excluded. A portion of the CEP was avulsing, reflected or detached from itself or the vertebral body (CEP fragmentation) in 23% of the disks examined. Gaps without interposing tissue, such as fibrocartilage or granulation, were excluded. Disk herniation penetrating the deep layer of the posterior longitudinal ligament was observed in 55% (11) of the autopsy cases and 9% (21) of the disks examined. The youngest age with disk herniation was 38 years whereas the oldest was 93 years. Multiple herniations were observed in five autopsy cases. The frequencies of the above abnormalities were analyzed relative to decades and disk levels. Abnormal features of horizontal cleft, vertical cleft, and CEP fragmentation were most frequently observed in the ninth decade. The frequency of disk herniation decreased with aging. The frequency of horizontal cleft was high at T1-2, T2-3, or T5-6. The frequency of vertical cleft as well as CEP fragmentation was high at T7-8, T8-9, T9-10, or T10-11. The frequency of disk herniation was nearly the same at each disk level except T4-5. Histology of the disk herniation The 21 disk herniations were histologically classified into two types. Six were CEP type in which herniated masses contained CEP fragmentation, observed frequently at T7-8, T8-9, T9-10, or T10-11. Remaining 15 herniations were nucleus pulposus (NP) type in which herniated masses consisted of nucleus pulposus and anulus fibrosus; these were observed with nearly the same frequency at each disk level. In all cases of CEP type herniations, all of the abnormal features of horizontal cleft, vertical cleft and CEP fragmentation were observed. On the contrary, in cases of NP type herniations, horizontal cleft, vertical cleft and CEP fragmentation were observed only in 60%, 47%, and 20%, respectively. Conclusion Thoracic disk herniation was histologically observed in 55% of the autopsy cases and in 9% of the disks, using midsagittal sections. Two types of disk herniation were observed; CEP type herniation containing cartilaginous endplate fragments occurs in the disks at their final stage of degeneration. NP type herniation consisting of nucleus pulposus and anulus fibrosus occurs in the disks at their early stage of degeneration. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared