The lateral entry point S2 alar-iliac (L-S2AI) screw: a preoperative computed tomography analysis of adult spinal deformity patients

Purpose To radiographically compare lateral entry point S2-alar-iliac (L-S2AI) screw with conventional S2AI (C-S2AI) and conventional iliac screw (CIS) lengths and trajectories. Methods Twenty-five preoperative CT scans of consecutive patients undergoing adult spinal deformity realignment surgery over a random 2-year period were analysed. Maximum in-bone length, caudal and lateral trajectories of CIS, C-S2AI, and L-S2AI screws were measured and compared using One-way ANOVA with Tukey's post hoc tests. Multivariate logistic regression was performed to identify predictors of high screw length discrepancy between C-S2AI and L-S2AI. Results Potential screw length was longest for CIS, followed by L-S2AI, then C-S2AI (114.5 +/- 8.3 mm vs 101.4 +/- 9.6 mm vs 80.6 +/- 5.9 mm, respectively) in all patients (p < 0.001). Actual screw lengths found both CIS and L-S2AI to be longer than C-S2AI (95.3 +/- 8.5 mm and 93.4 +/- 7.5 mm vs 82.1 +/- 7.3 mm; p = 0.008 and 0.003). Potential lateral angulation was smallest for CIS, followed by L-S2AI, then C-S2AI (21.9 +/- 7.0 degrees vs 31.9 +/- 7.1 degrees vs 40.9 +/- 6.7 degrees, respectively) in all patients (p < 0.001). L-S2AI and C-S2AI had the same caudal angulation (24.9 +/- 6.8 degrees), which was smaller than CIS (30.8 +/- 5.8 degrees) in all patients (p < 0.001). Univariate, but not multivariate analysis, revealed that lumbar lordosis > 40 degrees (OR 7.2, p = 0.041), diagnosis of degenerative spondylolisthesis (OR 10.5, p = 0.017), and > 7 instrumented levels (OR 2.6, p = 0.049) were significantly associated with high screw discrepancies. Conclusion The L-S2AI screw combines advantages of CIS and C-S2AI screws, which includes increased screw length, reduced lateral angulation, a low-profile screw head, ease of connection to proximal hardware, and the biomechanical advantage of a quadcortical purchase.