Fig 1.
Axial pelvic CT image at the level of the maximal diameter of the femoral head showing the angle (α) between the posterior wall of the acetabulum and the sagittal plane.
Line OA, parallel to the line connecting the pubic symphysis and the sacral promontory, represents the sagittal reference line. Line OB denotes the tangential line to the posterior acetabular wall.
Fig 2.
External view of a right hemipelvis cadaveric specimen showing a 1.5 mm, 6 cm-long Kirschner wire positioned along the posterior surface of the acetabular posterior column at the level of the greater sciatic notch.
Fig 3.
Schematic diagram of the right acetabular axial CT view at the level of the maximal diameter of the femoral head.
Points A, B, and C define the acetabular posterior wall. Three standard 45° imaging planes are illustrated: obturator oblique view 1 passes through point B, obturator oblique view 2 passes through point A, and the iliac oblique view passes through point A. All three views form a 45° angle with the horizontal plane. Obturator oblique view 1 and the iliac oblique view intersect perpendicularly at point O. A α° obturator oblique view, forming a 30° angle with the horizontal plane, passes through the line connecting points A and B. Points A, O, and B define a radiographic blind zone.
Fig 4.
Under 45° obturator oblique view fluoroscopy, the Kirschner wire projects as the posterior ilioischial line.
This line corresponds superiorly to the outer cortex of the ilium and inferiorly to the outer margin of the ischium. The image of the acetabular posterior wall is located posterior to the midpoint of the posterior ilioischial line. The region posterior to the posterior ilioischial line at this level is defined as the maximum danger zone (D).
Fig 5.
Under α° obturator oblique view fluoroscopy, the Kirschner wire projects as the posterior ilioischial line.
This line overlaps superiorly with the outer cortex of the ilium, inferiorly with the outer margin of the ischium, and centrally aligns with the acetabular posterior wall. At this angle, the posterior ilioischial line is tangent to the posterior surface of the posterior column.
Fig 6.
Under 45° obturator oblique view fluoroscopy, the tip of the 2.5 mm Kirschner wire is located within the maximum danger zone.
Fig 7.
Under α° obturator oblique view fluoroscopy, the tip of the 2.5 mm Kirschner wire is positioned posterior to the posterior ilioischial line.
Fig 8.
The tip of the 2.5 mm Kirschner wire is located outside the bony corridor, having penetrated the posterior surface of the posterior column.
Fig 9.
Screw Penetration of the Posterior Acetabular Wall: A Case of Fluoroscopic Under-Detection. (A) Intraoperative 45° obturator oblique fluoroscopic view showing no apparent breach of the posterior acetabular wall.
(B, C) Postoperative CT images demonstrating penetration of the posterior acetabular wall by the screw.