Three-dimensional image-guided placement of S2 alar screws to adjunct or salvage lumbosacral fixation

Abstract

Background context: Achieving fusion across the lumbosacral junction is challenging because of the unfavorable biomechanics associated with ending a fusion at this level. Bicortical placement of S1 pedicle screws can increase the construct stability at the lumbosacral junction; however, construct failure and pseudoarthrosis can still result. Iliac screws have been shown to increase the stiffness of lumbosacral constructs, but disadvantages include difficulty in connecting the iliac screw to adjacent sacral screws, painful screw loosening or prominence requiring removal, and the inability to place the screws in some patients with previous iliac crest autograft harvest.

Purpose: The purpose of the study is to describe a technique of S2 alar screw placement using three-dimensional image guidance.

Study design/setting: The study design is a retrospective analysis.

Patient sample: Twenty patients undergoing lumbosacral fusion had 32 screws placed using this technique.

Outcome measures: An independent radiologist graded screw placement and lumbosacral fusion on thin-cut postoperative computed tomographic (CT) scans.

Methods: Image guidance in this study was accomplished with the Medtronic Stealth Station Treon (Medtronic Inc., Littleton, MA, USA) used in conjunction with the O-ARM (Medtronic Inc.). Indications for placement of S2 alar screws included the following: to adjunct S1 pedicle screws in multilevel fusion cases; as an adjunct or alternative to S1 pedicle screws in pseudoarthrosis revision cases in which the S1 screws had loosened; as an alternative to S1 pedicle screws in cases where medial trajectory of an S1 pedicle screw was difficult to obtain because of a low-set lumbosacral junction; and a combination of the above. The entry point of the screw was typically chosen lateral and superior to the S2 dorsal foramen with the trajectory directed anterior, inferior, and lateral. Attempt was made to place the screw with the tip purchasing, but not penetrating through, the triangular area of cortical bone that can be found at the anterior, inferior, and lateral boundary of the sacral ala. An independent radiologist graded the placement of the screws on the intraoperative CT scan obtained with the O-ARM or on postoperative CT scans. Lumbosacral fusion was assessed on postoperative CT scans obtained at follow-up.

Results: No complications occurred in this study as a result of S2 alar screw placement or image guidance. Five screws did penetrate the anterior cortex of the sacrum, with no clinical consequence. At the time of abstract submission, 16 patients were able to have follow-up CT scans, 15 of which were graded as solid fusion at the lumbosacral junction by the grading radiologist.

Conclusions: Three-dimensional image guidance allows for safe placement of large S2 sacral alar screws that can provide additional biomechanical stability to lumbosacral constructs or serve as an alternate point of sacral fixation when S1 pedicle screws cannot be salvaged or placed in a medial trajectory.