Placement of half-pins for supra-acetabular external fixation: an anatomic study

An alternative location for placement of half-pins during pelvic external fixation is the dense supra-acetabular bone in the region of the anterior-inferior iliac spine. Although these fixators have gained popularity, to the authors' knowledge there are no studies evaluating the potential anatomic risks of placement of half-pins in this area; no safe corridors have been defined. Additionally, pins are placed near the hip capsule and no studies exist defining the superior extent of the hip capsule which potentially may be violated by placing half-pins in this location. The purposes of the current study were to evaluate the neurovascular risks and accuracy of fluoroscopically guided percutaneous placement of supra-acetabular half-pins, and to evaluate the anatomic superior extent of the hip capsule. Ten fresh frozen cadaveric pelves were used. A 5-mm half-pin was placed in the supra-acetabular bone under fluoroscopic guidance. Iliofemoral dissection was done and the proximity of the half-pin to local neurovascular risks was measured with a caliper. The hip capsule was exposed and the superior extent of the hip capsule was measured. Intraosseous pin placement was evaluated by direct observation. Nine pins were completely in bone, one had partially exited posteriorly and laterally. The lateral femoral cutaneous nerve was at risk with a mean distance of 10 mm (range, 2-25 mm) from the half-pins. The femoral nerve and femoral artery were not at risk. The average superior extent of the hip capsule was 16 mm above the joint (range, 11-20 mm). Half-pins can be placed accurately and safely in the supra-acetabular region using percutaneous techniques, appropriate soft tissue sleeves, and fluoroscopic guidance. Insertion of pins at least 2 cm above the hip is recommended to avoid potential hip capsule penetration.