Morbus de Anquin (Spinous engagement syndrome)
Morbus de Anquin is a type of congenital stenosis in which the spinal process of S1 is absent and the lamina of S1 has a large medial cleft. There may be a residual island of bone in the area of the cleft. This malformation is associated with a downward hooklike elongation of the spinous process of L5. The patient assumes an increased lordotic posture during standing or walking. The tip of the hooklike spinous process of L5 presses directly on the ligamentum, bridging the spina bifida occulta of S1 or on the rudimentary bony island in its central portion, thus reducing the midsagittal diameter of the upper sacral canal. This condition results in radicular pain during standing or walking and is relieved by sitting.