In thirty-six cases with lumlar diseases, CT studies of the shape of the lateral recess (L5) were performed. A series of CT scans at L5 lateral recess showed the narrowest level of it and the L5 neve root condition in it. Three types of the shape were classified, which were triangular type, trefoil type and flat trefoil type. Especially the flat trefoil type was associated with LCS. We concluded that the narrowest level of the lateral recess of LCS was the entrance of the nerve root, which was trapped there by the configuration of the recess with the degenerative soft tissue, on the other hand, the lateral recess of OA was not narrowest at the entrance level, but narrowest at the pedicle level. The difference in the shape contributed to the pathogenesis of symptoms of the LCS.
