RADIOLOGIC EVALUATION OF ILIOSACRAL SCREW PLACEMENT

Study Design. This study analyzed anteroposterior, lateral, inlet, and outlet radiographic representations of different iliosacral screw ori entations and evaluated anatomic features of the superior aspects of t he sacral alae. Objectives. The purpose of this anatomical and radiolo gic study waste assess the value of anteroposterior, inlet, outlet, an d lateral views with regard to the planning of iliosacral screw placem ent, to determine if screws penetrating the sacral surfaces and forami na can be detected during or after operation, and to evaluate the anat omy of the superior aspects of the sacral ala quantitatively. Summary of Background Data. Direct iliosacral screw fixation has recently beco me 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 o f pelvic radiographs and the position of a screw may result in incorre ct screw placement during surgery or misdiagnosis of postoperative neu rologic complications. The morphology of the sacrum is complex. No pre vious data relative to the superior aspect of the sacral alae are avai lable. Methods. Four bony pelves were used to model the different ilio sacral 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 fro m 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 tw o linear parameters. Results. The results showed that a misdirected dr ill bit penetrating the anterior aspect of the ala is best appreciated by the inlet view. A misdirected drill bit penetrating the superior a spect of the afa or the S1 foramen is best represented in the outlet v iew. The average angle between the coronal plane of the S1 vertebra an d the anterior aspect of the ala was 27.1 degrees; between the superio r aspect of the S1 vertebral body and superior edge of the ala, 36.9 d egrees; and between the superior aspect of the S1 vertebral body and p osterosuperior edge of the ala, 24.5 degrees. Conclusions. The inlet v iew shows the orientation of screws relative to the coronal plane and extraosseus screws extending anterior to the ala, whereas the outlet v iew elucidates the placement of screws relative to the transverse plan e and extraosseus screw tips extending into the sacral foramina or sup erior to the ala. Evaluation of preoperative pelvic computed tomograph y scans may be helpful in understanding the unique morphology of each individual patient and enhancing the safety of iliosacral screw placem ent.