Second Sacral Alar Screw Fixation: Anatomic Study of Three-Dimensional Computed Tomography and Case Report

OBJECTIVE: S2 alar screw would be an alternative choice without breaking the sacroiliac joint. The aim of this study was to measure radiographic parameters for optimal placement of posterior S2 alar screw for instrumentation and fusion. METHODS: Three-dimensional computed tomography scans of the pelvis of 60 normal adults were used to map the S2 alar screw. Entry point was typically chosen lateral and superior to the S2 dorsal foramen. Ideal S2 alar screw trajectories were explored by rotating the three-dimensional pelvis, while ensuring trajectories were of maximum length and width. After identification of an optimal trajectory, related linear anatomic parameters and sagittal and transverse angles were determined. RESULTS: Ideal S2 alar screw trajectories were identified in each computed tomography scan. According to this morphometric study, trajectories for female patients were more lateral in the transverse plane (female 33.73 +/- 5.99 degrees vs. male 29.82 +/- 4.11 degrees, P < 0.001). Maximal length of trajectory in male patients was significantly longer than in female patients (female 40.82 +/- 4.29 mm vs. male 43.42 +/- 4.02 mm, P = 0.001). Fourteen S2 alar screws were used in 7 patients with high-grade spondylolisthesis, scoliosis, or nonunion at lumbosacral site. No complications occurred during S2 alar screw placement. One S2 screw failed owing to severe local osteoporosis. No patient developed local pain or wound-related problems. CONCLUSIONS: S2 alar screw is an alternative sacral fixation point to provide additional biomechanical stability of lumbosacral constructs. A trajectory with maximum length through the S2 ala can be determined using three-dimensional computed tomography.