• Atsushi Nakano, Sachio Hayama, Takashi Fujishiro, Yoshiharu Nakaya, Takuya Obo, Toma Yano, Ichiro Baba, and Masashi Neo.
• Department of Orthopedic Surgery, Osaka Medical College, Osaka, Japan. Electronic address: ort095@osaka-med.ac.jp.
• World Neurosurg. 2021 Jun 1; 150: e491-e499.

ObjectiveAlthough the spontaneous regression of pseudotumors after posterior fusion has been reported, the predictive factors remain unclear. We examined the radiological features that predict for the regression of retro-odontoid pseudotumors after posterior fusion, with a specific focus on cyst formation.MethodsWe included 28 patients with a diagnosis of retro-odontoid pseudotumor using preoperative magnetic resonance imaging. The radiographic parameters and pseudotumor thickness were measured pre- and postoperatively. The regression rate for each pseudotumor was calculated. The presence of a cyst around the retro-odontoid pseudotumor was investigated. If present, the cyst thickness was measured. To elucidate the predictors for the postoperative regression of pseudotumors, the patients were divided into 2 cohorts: the regression group with a regression rate >40% and the no-regression group with a regression rate of <40%. Multivariate logistic regression analysis, including the demographic data and preoperative radiographic parameters as independent variables, was performed.ResultsThe mean pseudotumor size had decreased significantly from 8.8 ± 3.6 mm preoperatively to 5.3 ± 2.0 mm postoperatively (P < 0.0001). The mean regression rate was 35.9% during a magnetic resonance imaging follow-up period of 8.6 months (range, 6-12 months). Cystic lesions were noted in 10 patients (35.7%) preoperatively. The mean cyst size was 4.7 ± 1.9 mm. All cysts were located dorsal to the pseudotumors and were involved at the maximum spinal compression levels. Nevertheless, all the cysts had disappeared postoperatively. Multivariate logistic regression analysis revealed that the pseudotumor regression group had had a significantly greater proportion of cysts (57.1% vs. 14.3%; odds ratio, 11.7; P = 0.013).ConclusionsThe presence of cystic lesions protruding from retro-odontoid pseudotumors might serve as a predictive factor for the spontaneous regression of pseudotumors after posterior fusion.Copyright © 2021. Published by Elsevier Inc.

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