660 Intraoperative O-C2 and O-EA Angular Measurements for Achieving Optimal Alignment During Posterior Fossa Decompression and Occipitocervical Fusion for Complex Chiari Malformation

INTRODUCTION: Excess flexion or extension during occipitocervical fusion (OCF) can lead to postoperative complications such as dysphagia and neck pain. Achieving neutral craniocervical alignment intraoperatively can be challenging in patients with Chiari malformation (CM) with craniocervical instability (CCI) undergoing posterior fossa decompression (PFD) and OCF because the head must be flexed for optimal PFD and subsequently extended to neutral for OCF. METHODS: We retrospectively reviewed patients with CM 1 and 1.5 who underwent PFD and OCF. Patient demographics, preoperative presentation, XR morphometrics, operative details, postoperative complications, and clinical outcomes were analyzed. RESULTS: Out of 39 patients (mean age 24.6 years, range 3-51, 76.9% female), 37 patients did not require revision surgery after PFD and OCF. In this non-revision cohort, O-C2a (13.5 ± 10.4° vs. 17.5 ± 10.1°, P = .047) and nPAS (10.9 ± 3.4 mm vs. 13.1 ± 4.8 mm, P = .007) increased significantly after surgery. In 2 patients who required revision due to postoperative dysphagia, these measurements acutely decreased after initial surgery (mean difference -16.6° in O-C2a and -12.8° in O-EAa), and revision surgery restored the morphometrics back to baseline. Applying these observations, we now intraoperatively measure these fluoroscopic morphometrics and ensure neutral craniocervical alignment using the Mayfield head clamp locking mechanism prior to hardware placement. CONCLUSIONS: Utilizing fluoroscopic morphometrics pre- and intra-operatively can guide surgeons in appropriate positioning during combined PFD and OCF. With neutral rotatory fixation, single-axis flexion/extension of the head during each stage of the surgery with the Mayfield head clamp locking mechanism facilitates positioning in conjunction with these fluoroscopic-based measurements.