Percutaneous fluoroscopically assisted iliosacral screw insertion has become one of the most popular methods of stabilisation of the posterior aspect of the vertically unstable pelvis. Screw malpositioning rates range from 0 to 10 per cent. Screw malplacement can cause injury to the iliac and gluteal vessels, L4 to S1 nerve roots and sympathetic chain.

We performed two radiographic studies on dry human bones to seek safe radiographic landmarks for insertion of iliosacral screws.

Part 1. Two parallel linear densities are always present on lateral plain radiographs of the lumbosacral spine and pelvis. Using wire markers on pelvic bones, we accurately define the origin of these pelvic lines. Steel wires of different lengths were placed along the iliopectineal and arcuate lines of the pelvis. The shorter wire stopped at the anterior limit of the sacro-iliac joint. The longer wire extended further along the entire course of the medial border of the ilium to the iliac crest posteriorly. We demonstrate that each ‘ pelvic line ‘ represents the sharp bony ridge that forms the anterosuperior limit for insertion of iliosacral screws.

Part 2. In a second experimenton dry pelvis, we inserted balloons filled with radio-opaque contrast medium into the spinal canal of the sacrum and exiting through the anterior and posterior sacral foramina on either side. Plain lateral radiographs and CT scan with reformatted images were obtained. We present a previously undescribed radiological sign on plain lateral radiographs of the lumbosacral spine. The inferior and posterior boundaries of the acorn sign are delineated.