The study of plate-screw fixation in the posterior wall of acetabulum using computed tomography images

The article aims to delineate the width of posterior column, the thickness of posterior wall, and safe angles for screw placement in the posterior wall to avoid intraarticular screw penetration.

The computed tomography (CT) images of 32 cadaveric adult bony hemipelvic specimens were initially obtained for the purpose of the study. Each specimen was sectioned at 1-cm intervals, and each plane of the crosssection was perpendicular to the surface of posterior column. By analyzing the CT images postprocessed with multiplanar reconstruction, the width of posterior column, the thickness of posterior wall, and the modified safe angle for screw placement in the posterior wall were measured and recorded. To validate the data obtained from the cadaveric model, this method was applied on 30 adult volunteers. The corresponding data were recorded and compared with those acquired from the cadaveric bony hemipelvis. In efforts to approve the usefulness of this method in practice, we have performed screw insertions in another 10 acetabular specimens and two operative cases by using the data from analyzing the CT images.

The width of posterior column, the thickness of posterior wall, and the safe angles for screw insertion in the posterior wall were measured and recorded in both specimens and volunteers. Comparison of the corresponding data was made between specimens and volunteers, and no significant difference was found in the same gender and side (p > 0.05). The corresponding width of posterior column, thickness of posterior wall, and safe angles for screw placement was found to be statistically different between males and females in both specimens and volunteers (p < 0.05). In specimens group, the safe angles for the entry points 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 cm medial to the lateral acetabular brim were 49.23 degrees +/- 11.54 degrees, 42.48 degrees +/- 8.97 degrees, 29.53 degrees +/- 7.86 degrees, 23.68 degrees +/- 6.20 degrees, 18.42 degrees +/- 5.41 degrees and 15.91 degrees +/- 4.37 degrees in males and the corresponding angles for the entry points 0.5, 1.0, 1.5, 2.0, and 2.5 cm medial to the lateral acetabular brim were 45.02 degrees +/- 8.82 degree, 35.98 degrees +/- 7.60 degrees, 23.77 degrees +/- 6.29 degrees, 19.96 degrees +/- 4.36 degrees, and 14.68 degrees +/- 3.48 degrees in females, respectively. CT images of 10 acetabular specimens and two cases with posterior wall fractures show all screws were inserted into the posterior wall without penetration into the joint space.

The oblique multiplanar reconstruction images perpendicular to the surface of posterior column were selected to describe the safe angle for screw insertion into posterior wall, which can provide consistent results in both specimens and volunteers. The method can be applied in practice both on acetabular specimens and operative cases and is helpful to make individual perioperative planning for safer fixation of posterior wall fracture.