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Anatomic study of human ilia.To determine the anatomic dimensions influencing intrailiac anchor placement and design.The development of the Galveston intrailiac implant anchor markedly improved results of spinal instrumentation to the pelvis. Better quantification of the passage dimensions is needed for the design of anatomically specific implants.The length and outer diameter of two intrailiac anchor passages were determined from randomly selected, disease-free pairs of ilia from 129 white skeletons belonging to the Hamann–Todd Osteological Collection (Cleveland, OH, Museum of Natural History).Path B, an anchor passage running from the posterior superior iliac spine to the anterior inferior iliac spine, was significantly longer than path A, proceeding from the posterior superior iliac spine to the superior rim of the acetabulum, in adult and teenage males and females. For example, in adult females path B measured 141.2 ± 7.5 mm versus 124.9 ± 7.1 mm, P = <0.0001. The absolute minimum path A length available for an intrailiac post or screw anchor, following the required posterior superior iliac spine removal, was 80 mm in adults and male teenagers and 70 mm in female teenagers. For path B the hip joint was not at risk, and a safe anchor passage of at least 90 mm in teenagers and 110 mm in adults was available. The minimum thickness of the paths occurred directly above the sciatic notch. It was significantly larger in path A than path B, 20.2 ± 2.4 mm versus 14.4 ± 3.2 mm, P < 0.0001.Of the two supra sciatic intrailiac paths studied, the path passing from the posterior superior iliac spine toward the anterior inferior iliac spine provided a longer and potentially safer anchor site. These studies suggest techniques for accessing longer anchor paths and the need for larger diameter anchors in order to achieve optimum intra iliac anchor stiffness and strength.