Fluoroscopic views for safe insertion of lag screws into the posterior column of the acetabulum

Abstract

Background: Percutaneous lag screw fixation is an alternative treatment for non-displaced or minimally displaced posterior column fractures. This study aims to explore new fluoroscopic views of the acetabulum for safe percutaneous insertion of posterior column lag screws.

Methods: Axial computed tomography (CT) scans were taken of sixteen embalmed adult cadavers. The axial CT images at the level of the middle height of the acetabulum were selected. The angle (angle α) between the posterior cortex of the posterior column (PCPC) and the line intersecting the axial plane and the coronal plane, and the angle (angle β) between the medial wall and the line intersecting the axial plane and the sagittal plane were identified and measured. Tangential views of the PCPC and medial wall were obtained by referencing the measured angles. A lag screw was inserted into the posterior columns of the sixteen pelvic specimens under fluoroscopic guidance using an iliac oblique view and the two tangential views. CT scans were performed to evaluate the lag screw position. Axial CT images of 52 volunteers were obtained and the angles α and β were measured following the same methods used for the cadaveric specimens.

Results: The angles α and β for the specimens were 29.3 ± 2.8.1 and 8.1 ± 1.4 degrees, respectively. On the tangential view of the PCPC, the posterior cortex appears as a nearly straight line between the lesser and greater sciatic notches. On the tangential view of the medial wall, the medial wall appears as a distinct straight line. Using these radiographic images, the lag screws were inserted into the posterior columns of bony pelvic specimens. Screw placement was confirmed by CT, and found to be fully intraosseous in all cases without any cortical breaches. The angles α and β were 30.4 ± 4.1 and 9.2 ± 1.9 degrees for male volunteers and 28.5 ± 3.7 and 7.7 ± 1.8 degrees for female volunteers, significant difference in these angles between cadaveric specimens and human volunteers.

Conclusion: The tangential views of both the PCPC and medial wall can be obtained following the aforementioned methods The oblique iliac view and the two tangential views enable safe insertion of posterior column lag screws.

Figure 1

On the axial computed tomography (CT) image at the middle height level of the acetabulum, Line A represents the posterior cortex of the posterior column. The angle between Line A and the line intersecting the axial plane and the coronal plane is labelled angel α. Line B represents the medial wall of the acetabulum. The angle between Line B and the line intersecting the axial plane and the sagittal plane is labelled angle β.

Figure 2

On the axial CT images, Line C represents the line running through the anterior and posterior points of the lateral brim of the acetabulum. The angle between Lines A and C is labelled angle γ.

Figure 3

The diagrams illustrate the position of the C-arm unit and the male patient to obtain the tangential views of the posterior cortex of the posterior column (A) and the tangential view of the medial wall of the acetabulum (B).

Figure 4

On the tangential view of the posterior cortex of the posterior column, the posterior cortex appears as a nearly straight-line segment between the lesser and greater sciatic notches (Line segment A, the red curly brace).

Figure 5

On the tangential view of the medial wall of the acetabulum, the medial wall appears as a distinct straight line.

Figure 6

The oblique coronal reconstructed CT images shows that the full length of the lag screw is in the bony corridor of the posterior column, and the shortest distance between the posterior cortex and lag screw was at the levels of the greater and lesser sciatic notches.