120 - A New Technique of S2 Alar Iliac Screw Insertion Using a Custom-made S...

#120 A New Technique of S2 Alar Iliac Screw Insertion Using a Custom-made Surgical Guide

Image-Guidance, Navigation, Robotics

Poster Presented by: Y. Nagamoto

Author(s):

Y. Nagamoto (1)
M. Iwasaki (2)
H. Tobimatsu (1)
H. Aono (1)

(1) Osaka National Hospital, Orthopaedic Surgery, Osaka, Japan
(2) Osaka University Graduate School of Medicine, Department of Orthopaedics, Suita, Japan

Abstract

Introduction: Adult deformity patients requiring long fusion to the sacrum usually require additional anchors into the ilium. Therefore, several augmentation methods have been advocated for this fusion and S2 alar iliac screw (S2AIS) is the most recent emerging augmentation technique. The main advantages of S2AIS over previous techniques include biomechanical strength, less frequent skin protrusion, and easiness to connect adjacent pedicle screws. On the other hand, insertion of S2AIS is not easy without any guiding devices. To avoid the complications during screw insertion, we developed a new insertion technique using a custom-made surgical guide.

Material and Methods: 7 patients who had S2AIS placed as part of a long fusion were retrospectively compared to the preoperative three-dimensional surgical plannings. The patients comprised 4 adult spine deformities (4 female; mean age, 74.2 yr) and a 3 muscular scolioses (2 male and 1 female; mean age, 15.3 yr). First, all patients underwent preoperative CT (computed tomography) and then 3-dimensional pelvic surface models were made from the CT data. Second, the screw placement was planned 3-dimensionally on the pelvic models in a virtual space. Third, a custom-made surgical guide, which was a mold shaped to fit the spinous process of sacrum and bilateral posterior superior iliac spine, was designed to make screw holes as planned on the PC. Finally, the guide was made from resin using a 3-dimensional modeling device. In the operation, the guide was firmly press-fitted to the surface of the pelvis. Drilling and tapping were carried out through the guide. Probing was carefully done before screw insertion using the navigation system. Postoperatively, the accuracy of screw placement was evaluated by superimposing 3-dimensionally postoperative screw insertion on preoperative planning. The error of the insertion point and coronal (α°) and sagittal (β°) screw trajectory were measured.

Results: 14 screws were used for this study. All screws were placed appropriately within cancellous bone except 1 minor lateral violation of the ilium cortex. The average displacements of insertion point were 4.5±2.7mm. The average difference of α° and β° was 5.5±4.7°and 2.3±2.1°, respectively.

Discussion and Conclusion: Unlike the pedicle screw passes through the hollow structure of the soft cancellous bone, the S2AIS technique needs to penetrate the hard subchondral bone twice by blind procedure. For the above reason, we have thought the technique was comparatively difficult and some more accurate guiding devices must be needed alternative to the image intensifier. In this study, the S2AIS could easily and safely inserted with the aid of the guide. As the pelvis is larger anatomy and has more distinct anatomical points than the spinal vertebrae, S2AIS is thought to be the best indication of this technique. However, long-term results of S2AIS are currently unknown and further study is needed to fully discern the clinical and biomechanical advantage of S2AIS.

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