Asymmetric Safety Corridors for Free-Hand S2-Alar-Iliac Screw Placement: Quantifying Direction-Specific Tolerance...

Area of Science:

• Spinal surgery
• Orthopedic biomechanics
• Surgical navigation

Background:

• Free-hand S2-alar-iliac (S2AI) screw placement accuracy is limited.
• Optimal trajectories are patient-specific but difficult to reproduce.
• Quantifying safety margins is essential for improving surgical outcomes.

Purpose of the Study:

• To quantify direction-specific safety margins for S2AI screw placement.
• To assess the relationship between pelvic parameters and screw trajectory deviations.
• To provide guidance for intraoperative adjustments in free-hand S2AI screw fixation.

Main Methods:

• Retrospective analysis of CT scans from patients undergoing S2AI screw fixation.
• Measurement of pelvic parameters: pelvic tilt (PT), sacral slope (SS), and pelvic incidence (PI).
• CT-based planning to determine optimal screw angles; assessment of postoperative trajectories and cortical violations using GEE and ROC analysis.

Main Results:

• Significant inverse correlations found between PT/PI and optimal screw angles (p < 0.001).
• Lateral and medial deviations strongly associated with cortical violations (OR 2.33 and 2.10, p < 0.001).
• Asymmetric safety margins identified: 1.5° lateral, 8.1° medial, 18.5° inferior.

Conclusions:

• S2AI screw placement involves tolerance-based adjustments around optimal trajectories.
• Direction-dependent and asymmetric safety margins exist, with narrow lateral tolerance.
• Findings offer practical guidance for intraoperative trajectory modifications in free-hand S2AI screw fixation.