Case Report Retroodontoid Pseudotumor Related to Development of Myelopathy Secondary to Atlantoaxial Instability on Os Odontoideum M. Hamard , S. P. Martin , and S. Boudabbous Department of Imaging and Medical Information Sciences, Division of Radiology, Geneva University Hospitals, Geneva, Switzerland Correspondence should be addressed to M. Hamard; marion.hamard@hcuge.ch Received 31 May 2018; Revised 10 September 2018; Accepted 16 September 2018; Published 30 September 2018 Academic Editor: Atsushi Komemushi Copyright © 2018 M. Hamard et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Retroodontoid pseudotumor (ROP) is a nonneoplasic lesion of unknown etiology, commonly associated with infammatory conditions, and the term of pannus is usually used. Less frequently, ROP formation can develop with other noninfammatory entities, with atlantoaxial instability as most accepted pathophysiological mechanism for posttraumatic or degenerative ROP. As it can clinically and radiologically mimic a malignant tumor, it is paramount for the radiologist to know this entity. Magnetic resonance imaging is the modality of choice to reveal the possible severe complication of ROP in the form of a compressive myelopathy of the upper cervical cord. Te purpose of the surgical treatment is the regression or complete disappearance of ROP, with posterior decompression by laminectomy and posterior C1-C2 or occipitocervical fxation. We present the case of an elderly patient with retroodontoid sof tissue mass secondary to a chronic atlantoaxial instability on os odontoideum, an extremely rare cause of ROP. Te patient developed a posttraumatic cervical myelopathy related to the decompensation of this C1-C2 instability responsible for the formation of a compressive ROP. We will overview the retroodontoid pseudotumor and its diferential diagnosis. 1. Introduction Retroodontoid pseudotumor (ROP) is an entity that can mimic malignant tumors and is from uncertain etiology. Some consider it as a low-grade fbrosarcoma. ROP are mainly related to infectious processes and less frequently to infammatory disorders. ROP might have neurological complications due to mass efect on the spinal cord. 2. Case Description A 77-year-old female patient was admitted in our institution following a ground-level fall due to relatively sudden grade 3-4 right hemiparesis with lower limb predominance. Tis patient was not known for any systemic disease, no rheumatoid arthritis, or other joint-related generalized disease. An initial enhanced CT was performed for the suspicion of an ischemic stroke. Te exam revealed a smooth and well-corticated bone ossicle measuring 14 mm and located superiorly to the odontoid process corresponding to an os odontoideum (Figures 1(a)–1(c)). Te ossicle was associated with an atlantoaxial subluxation and with the posterior wall of 14 mm on spinal canal (Figures 1(e) and 1(f), white lines) that has increased since a previous CT 8 years ago. Te late enhanced phase showed an intracanal hyperattenuated but no enhancing pseudomass situated just posterior to the ossicle (Figure 1(d)). A complementary cervical enhanced MRI with admin- istration of Gadolinium confrmed a well-corticated ossicle and demonstrated a tissular retroodontoid process (Figures 2(a) and 2(b)). Te tissue component showed a low signal on T1- and T2-weighted images and no enhancement (Fig- ure 2(g)), compatible with a ROP. Te main diagnosis was a noninfammatory ROP developed on atlantoaxial instability, secondary to an os odontoideum. Te main diferential diagnosis was pseudoarthrosis of an old fracture of the dens of axis. Infammatory arthritis such as gout, rheumatoid, or psoriatic arthritis was suggested as diferential diagnosis, but less likely because of the negative history of those diseases. Hindawi Case Reports in Radiology Volume 2018, Article ID 1658129, 10 pages https://doi.org/10.1155/2018/1658129