<p><b>OBJECTIVES</b>To investigate the safe distance from the tip of the cannulated screw to the apex of the femoral head, and to avoid cutting out of the cannulated screws from the femoral head.</p><p><b>METHODS</b>From November 2007 to April 2008, the placement configuration of the cannulated screws in the femoral head on the anteroposterior (AP) and lateral view was investigated. And the relation between the three-dimensional configuration and the two-dimensional perpendicular view of the femoral head to establish a solid geometry formula was analyzed. According to the configuration, the distances from the tips of different cannulated screws to the apex of the femoral head to confirm the screws placement within the femoral head was measured.</p><p><b>RESULTS</b>The actual risk of cutting out of the cannulated screws varied according to the different placement of the cannulated screws in the femoral head, even if the screw tips were within the femoral head on the AP and lateral radiograph. The mean diameter of femoral head was 49.8 mm. If the cannulated screw is in the center of femoral head on the lateral view, the cannulated screw would not cut out as long as it was in the femoral head on the AP view. When the angle was 22.5°on the lateral view, and under 22.5°on the AP view, the distance from the screw tip to the apex of the femoral head would exceeded 2.2 mm. If the angle > 45°on the AP view, the distance would exceed 9.6 mm. When the angle was 45°on the lateral view, and under 22.5°on the AP view, the distance would exceed 8.2 mm. When the angle > 45°on the AP view, the distance would exceed 17.7 mm. When the angle was 67.5°on the lateral view, the distance would exceed 23.1 mm on AP view.</p><p><b>CONCLUSIONS</b>If the cannulated screw is in the center of femoral head on the lateral view, the cannulated screw won't cut out as long as it is in the femoral head on the AP view. The angle is larger on the AP and lateral view (especially on the lateral view), and the distance is longer.</p>