Spinal cord morphology and pathology in ossification of the posterior longitudinal ligament.

We analysed nine autopsy cases of ossification of the posterior longitudinal ligament (OPLL) to elucidate the relationship between morphology and pathology of the spinal cord. The cross-sectional shape of the spinal cord at the most severely affected segment was classified into two categories: boomerang (convex lateral surfaces and concave anterior surface) and triangular (angular lateral surfaces and flat anterior surface). In the cases with a boomerang shape, even when the compression was severe, major pathological changes were restricted to the grey matter and the white matter was relatively well preserved. No secondary descending degeneration of the lateral columns was seen, and ascending degeneration of the posterior column was restricted to the fasciculus cuneatus whose fibres were derived from the affected segments. In the cases with a triangular shape, pathological changes were more severe, both white matter and grey matter were involved, and only the anterior columns were free of pathological changes. There were severe pathological changes over more than one segment, and both descending degeneration of the lateral pyramidal tracts and ascending degeneration of the posterior column, including the fasciculus gracilis, were observed. The transverse area of the spinal cord was > 60% of normal in most of the cases with a boomerang shape, but it was reduced to < 60% of normal in more than one segment in the cases with a triangular shape. The compression ratio of the spinal cord (sagittal diameter/transverse diameter x 100%) was not related to pathological changes. In conclusion, a triangular-shaped spinal cord with transverse area of < 60% of normal in more than one segment appeared to be associated with severe and irreversible pathological changes in cases of OPLL.