• Jung Hyun Park, Eugene Lee, Joon Woo Lee, Yusuhn Kang, Joong Mo Ahn, Jin S Yeom, and Heung Sik Kang.
• Department of Radiology, Seoul National University Bundang Hospital, Seong Nam, Gyeongi-do, Republic of Korea.
• Spine. 2017 Dec 1; 42 (23): 1763-1771.

Study DesignCase series study.ObjectiveThe aim was to investigate the incidence of retro-odontoid pseudotumor in patients with atlantoaxial instability (AAI) and evaluate pseudotumor regression after posterior fixation.Summary Of Background DataThe incidence of retro-odontoid pseudotumor in atlantoaxial instability patients remains uncertain. Moreover, the regression of retro-odontoid pseudotumor after posterior fixation in patients with various underlying diseases needs to be further investigated.MethodsFrom July 2004 to August 2015, 175 patients with AAI underwent posterior fixation operations at our institution. After excluding 11 patients (previous operation, n = 4; history of tumor, n = 7), the final study population comprised 164 patients. The final study population was categorized according to their underlying diseases (rheumatoid arthritis [RA], os odontoideum, atlanto-occipital assimilation, dens fracture, AAI of unknown cause, etc.) and age (adult and pediatric groups). The incidence of retro-odontoid pseudotumor in each group was analyzed. Pre- and postoperative magnetic resonance or computed tomography images were reviewed to assess its regression following surgery.ResultsOf the 164 patients included, 38 had retro-odontoid pseudotumor (23.2%). Three were diagnosed with RA and the rest were non-RA patients including os odontoideum (n = 12), dens fracture (n = 6), atlanto-occipital assimilation (n = 4), Morquio syndrome (n = 1), and AAI of unknown cause (n = 12). Pseudotumor size regressed in all 38 patients after atlantoaxial posterior fixation. There was a statistically significant decrease in pseudotumor size (the length between the anterior border of the odontoid process to the posterior border of the pseudotumor) from a mean length of 17.7 to 14.9 mm (P < 0.001).ConclusionsThe patients had various underlying diseases and the overall incidence of retro-odontoid pseudotumor in patients with symptomatic AAI was 23.2% at our institution during the past 11 years. All patients who underwent posterior fixation for AAI showed a statistically significant decrease in pseudotumor size.Level Of Evidence4.

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