Percutaneous fixation of acetabular fractures: computer-assisted determination of safe zones, angles and lengths for screw insertion

Introduction

Percutaneous retrograde screw fixation for acetabular fractures is a demanding procedure due to the complex anatomy of the pelvis and the varying narrow safe bony corridors. Limited information is available on optimal screw placement and the geometry of safe zones for screw insertion in the pelvis.

Methods

Three-dimensional reconstructions of 50 consecutive CT scans of polytrauma patients (35 males, 15 females) were used to introduce three virtual CAD bolts (representing screws) into the anterior column (superior ramus of the pubic bone), posterior column (the ischial bone) and the supraacetabular region, as performed during percutaneous screw fixation. The three-dimensional (3D) position of these screws was evaluated with a computer software (MIMICS) after virtual optimal insertion. The 3D position, the narrowest zone and the distance to the hip joint of the two columns and the supraacetabular region were defined.

Results

The mean maximal screw length for the three virtual screws measured between 107.4 and 148?±?18.7?mm. The narrowest zone of the pelvic bone (superior pubic ramus) had a width of 9.2?±?2.4?mm. The average distances between the bolts and the hip joint were 3.9 and 19.4?±?7.4?mm. For the anterior column (superior pubic ramus) screw, the mean lateral angle to the sagittal midline plane was 39.0?±?3.2° and the mean posterior angle to the transversal midline plane was 15.1?±?4.0°. The mean supraacetabular screw angles measured 22.4?±?3.4° (medial), 35.3?±?4.6° (cranial) and the mean angles for the ischial screw were 12.0?±?5.4° (posterior) and 18.4?±?4.0° (lateral).

Conclusions

The zones for safe screw positioning are very narrow, making percutaneous screw fixation of the acetabulum a challenging procedure. The predefined angles for the most frequently positioned percutaneous screws may aid in preoperative planning, decrease operative and radiation times and help to increase safe insertion of screws.