Introduction: Atlantoaxial instability has been reported to occur in 14-24% of patients with Down syndrome.Os odontoideum represents one possible etiology for symptomatic neurologic dysfunction due to atlantoaxial instability.Occipital cervical fusion and C1-2 fusion have been validated as successful treatment strategies after considering various nuances of the O-C1 joint, degree of subluxation and rotation, and need for distraction.Various fixation methods at C2 have been used to achieve solid arthrodesis,including pedicle screws, laminar screws, and transarticular screws.Anatomical studies in os odontoideum patients indicate that up to 34% of patients harbor C2 pedicles that are less than 5 mm, precluding safe cannulation of the C2 pedicles without increasing the risk for vertebral artery injury.Similarly,C2 laminae smaller than 5.5 mm are not suitable for screw placementExperienced surgeons have shown that axis pedicle diameters of less than 6mm have a two-fold higher incidence of cortical breach.As a result, alternative methods like C2 laminar screws have been implemented with some success,though less biomechanically stiff compared to traditional pedicle screw constructs
Methods: We present a 21 year-old girl with Down syndrome who presented with progressive tetraparesis secondary to atlantoaxial instability related to os odontoideum. Preoperative imaging revealed severe spinal cord compression from C1 posterior arch subluxation with C2 pedicles measuring(R)4.3mm and(L)3.8mm in maximal axial dimension. C2 laminae measured(R)3.6 mm and (L)3.9 mm in maximal axial dimension.
Results: Intraoperative CT-guided navigation was used for C2 pedicle screw placement utilizing four in situ fiducials on bilateral C2-C3 laminae.In situ fiducials allowed for multiple re-registrations throughout the case as anatomy shifted with retractor repositioning and planning screw trajectories.Though margin for error was minimal,the C2 pedicles were successfully cannulated with (R)4mm and(L)3.5mm screws.
Conclusions: This case demonstrates the invaluable utility of navigation-based systems that allow for real-time feedback in high risk cases.
Patient Care: This can lead to improvement in successful pedicle screw placement in patients with anomalous, small, or difficult anatomy, reducing attendant risks syndrome
Learning Objectives: -Anatomy and pathology of Atlantoaxial instability in Down syndrome.
-Treatment options for Atlantoaxial instability.
-Learning technical nuances of intra operative CT guided navigation and placement of hardware in such high risk cases.
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4. Kuroki, H et al. Posterior occipito-axial fixation applied C2 laminar screws for pediatric atlantoaxial instability caused by Down syndrome: Report of 2 cases. International Journal of Spine Surgery 6 (2012) 210-215