Radiologic evaluation of iliosacral screw placement

This study analyzed anteroposterior, lateral, inlet, and outlet radiographic representations of different iliosacral screw orientations and evaluated anatomic features of the superior aspects of the sacral alae.

The purpose of this anatomical and radiologic study was to assess the value of anteroposterior, inlet, outlet, and lateral views with regard to the planning of iliosacral screw placement, to determine if screws penetrating the sacral surfaces and foramina can be detected during or after operation, and to evaluate the anatomy of the superior aspects of the sacral ala quantitatively.

Direct iliosacral screw fixation has recently become popular because it provides stable fixation using reasonably small implants and is biomechanically equal or superior to other techniques of internal fixation. However, misinterpretation of the relationship of pelvic radiographs and the position of a screw may result in incorrect screw placement during surgery or misdiagnosis of postoperative neurologic complications. The morphology of the sacrum is complex. No previous data relative to the superior aspect of the sacral alae are available.

Four bony pelves were used to model the different iliosacral screw orientations possible during iliosacral reconstruction. A drill bit was inserted laterally from the posterior ilium through the sacroiliac joint and into the S1 vertebra. Radiographs were taken from anteroposterior, lateral, inlet, and outlet views for evaluation of placement. Twenty-two dry sacra were obtained for anatomic evaluation of the superior aspects of the sacral alae. All symmetrical structures were measured bilaterally. Measurements included three angular and two linear parameters.

The results showed that a misdirected drill bit penetrating the anterior aspect of the ala is best appreciated by the inlet view. A misdirected drill bit penetrating the superior aspect of the ala or the S1 foramen is best represented in the outlet view. The average angle between the coronal plane of the S1 vertebra and the anterior aspect of the ala was 27.1 degrees; between the superior aspect of the S1 vertebral body and superior edge of the ala, 36.9 degrees; and between the superior aspect of the S1 vertebral body and posterosuperior edge of the ala, 24.5 degrees.

The inlet view shows the orientation of screws relative to the coronal plane and extraosseus screws extending anterior to the ala, whereas the outlet view elucidates the placement of screws relative to the transverse plane and extraosseus screw tips extending into the sacral foramina or superior to the ala. Evaluation of preoperative pelvic computed tomography scans may be helpful in understanding the unique morphology of each individual patient and enhancing the safety of iliosacral screw placement.