Two parallel linear densities on lateral radiographs of the lumbosacral spine: neither iliopectineal lines nor basis ossis sacri.

Two parallel radiodense lines are constantly seen on lateral radiographs of the lumbosacral spine. Whether they originate from the pelvis or sacrum is unclear. Using metallic wire markers, we performed a radiographic investigation on bony skeletons to define their origins and anatomical locations. Our study demonstrates that each arises from the sharp, well-defined bony ridge which forms the anterosuperior boundary of the auricular surface for articulation with the sacrum on either side. Each ridge lies along the medial border of the iliac fossa and represents an increase in compact bone in response to weight transmission. These lines have useful applications for they provide fixed radiographic landmarks on the bony pelvis for certain angular measurements in orthopaedics.