Optimal placement of iliosacral screws: 3D computed tomography simulation

Purpose of the study Percutaneous iliosacral screws are used advantageously to fix unstable pelv ic girdle avoiding the morbidity of open access for conventional screw fixa tion. The insertion technique must be precise due to the risk of injury to the lumbosacral nerve trunk, the cauda equina roots, and the first sacral n erve. We undertook a study of the implantation site of iliosacral screws lo oking for a means of standardizing the drilling procedure on the basis of 3 D computed tomography (CT) data. Material and methods A CT series with 3D reconstruction was performed on 11 pelvis bones. We ret ained pelvis parameters and characterized the axis and narrow zone of the s acral wing. The insertion routes of 6.5 mm cancelous bone screws were simul ated: two iliosacral routes fixing S1, and two iliosacroiliac routes fixing S1 and S2. The values of the pelvic parameters and the positions of the sc rews were compared with the Spearman correlation test and graphic regressio n. Results The pelvic incidence was a mean 47 degrees. The length of the sacral wing w as a mean 73 mm. The narrow zone of the wing was 47 mm from the lateral ili ac fossa. In the narrow zone, the wing section showed an oval shape: 22 mm largest diameter, 11 mm smallest diameter. The wing was oriented 84 degrees in the paracoronal plane perpendicular to the plane of the sacral plate, 6 7 degrees in the para-axial plane parallel to the sacral plate, and 37 degr ees in the sagittal plane of the subject. The length of the upper S1 screw was a mean 80 mm. This upper screw was inclined 89 degrees in the para-coro nal plant, 61 degrees in the para-axial plane and 28 degrees in the sagitta l plane. The length of the lower S1 screw as a mean 80 mm. This lower screw was inclined 74 degrees in the para-coronal plane, 91 degrees in the para- axial plane and 110 degrees in the sagittal plane. The fixation screws coul d be inserted in 12 out of 22 cases. Correlations were found with height of the subject, length of the wing and the screw, and screw inclination. The inclination of the upper S1 screw in the para-coronal plane was correlated with the larger diameter of the sacral wing. Discussion The pelvis parameters measured were comparable with data in the literature. The very small dimensions of the narrow zone dictate a very precise drilli ng for the narrow zone. This narrow zone determines the inclination of the screw insertion. In the sagittal plane the standard deviation was very larg e making it impossible to interpret the data. The route of the upper screw runs obliquely forward in the plane parallel to the sacral plate. The lower screw runs upwardly in the plane perpendicular to the sacral plate. It doe s not appear possible to insert fixation screws in a routine procedure. Pre operative assessment would be necessary before percutaneous insertion. Conclusion The 3D CT reconstructions of the sacral wing can be used to determine the p recise optimal position of the two iliosacral screws. The principle orienta tions can be deducted from the plane of the sacral plate. Approximate indic ations can help reduce operative time and exposure to irradiation (patient and surgeon). Percutaneous iliosacroiliac screw fixation cannot be proposed for all patients.