Risk Factors of Screw Malposition in Robot-Assisted Cortical Bone Trajectory

Abstract

Study design. Retrospective single-center study using prospectively collected data. Objective. To describe the incidence of and identify risk factors for intraoperative screw malposition secondary to skive or shift during robot-assisted cortical bone trajectory (RA-CBT) insertion. Summary of Background Data. RA-CBT screw malposition occurs through two distinct modes, skive or shift. Skive occurs when a downward force applied to the cannula, drill, tap, or screw, causes the instrument to deflect relative to its bony landmark. Shift is a change in position of the robot-assisted system relative to the patient after registration. Methods. A consecutive series of patients older than 18 years who underwent RA-CBT screw placement between January 2019 and July 2022 were enrolled. Baseline demographic and surgical data, Hounsfield Units (HU) at L1, and vertebral shape related to screw planning were collected. Skive or shift was recorded in the operating room on a data collection form. Results. Of 1344 CBT screws in 256 patients, malposition was recognized intraoperatively in 33 screws (2.4%) in 27 patients (10.5%); 19 via skive in 17 and 14 via shift in 10 patients. These patients had higher BMI than patients without malposition (33.0 kg/m2 vs 30.5 kg/m2, P=0.037). Patients with skive had higher HU (178.2 vs 145.2, P=0.035), compared to patients with shift (139.2 vs 145.2, P=0.935) and patients without screw malposition. More than half of screw malposition was observed at the UIV. At the UIV, if the screw’s overlap to the bone surface at the insertion point was decreased, skive was more likely (57% vs 87%, P<0.001). No patients were returned to the operating room for screw revision. Conclusions. Intraoperative screw malposition occurred in 2.4% of RA-CBT. High BMI was associated with screw malposition, regardless of etiology. Skive was associated with high HU and decreased screw overlap to bone surface at the insertion point.