Intraoperative Posterior Migration of the Odontoid Fragment During Posterior C1–C2 Screw–Rod Fixation for Acute Type II Dens Fractures: A Technical Case Series

Abstract Posterior C1–C2 screw–rod fixation is a well-established treatment for unstable type II odontoid fractures. Although these constructs allow controlled reduction in atlantoaxial pathology, their effect on the mobile odontoid fragment during acute type II dens fracture fixation remains insufficiently described. We describe a series of patients in whom intraoperative posterior migration of the odontoid fragment was observed during rod seating and report the technical modification used to reduce this displacement. Among 28 patients with acute type II odontoid fractures treated surgically during the study period, 5 patients demonstrated intraoperative posterior odontoid fragment migration during posterior C1 lateral mass–C2 pars screw fixation and were included in this technical case series. Intraoperative lateral fluoroscopy was used to document odontoid fragment position after screw placement, after straight rod seating, and after rod contouring and tightening. Posterior fragment displacement was measured and compared descriptively. Mean posterior odontoid fragment displacement increased from 1.0 ± 0.82 mm before rod placement to a maximum of 4.0 ± 0.82 mm after straight rod seating in extension. After loosening the construct and contouring the rods to better match native C1–C2 sagittal alignment, posterior displacement decreased to 1.0 ± 0.82 mm. Alignment was maintained in all patients at 3-month follow-up, with improvement in mean Nurick grade from 4.2 ± 0.84 to 2.4 ± 0.55. In this small technical case series, straight rod seating was temporally associated with posterior migration of the mobile odontoid fragment during posterior C1–C2 fixation for acute type II odontoid fractures. Careful rod contouring, staged tightening, and continuous lateral fluoroscopic monitoring may help minimize unintended translational forces and allow early recognition of potential canal compromise.