BACKGROUND
Retro-odontoid pseudotumor is a fibrous or pannus mass behind the odontoid process, mainly caused by atlantoaxial instability, degeneration, or abnormal deposits behind the dens. Most scholars agree immobilization of the upper cervical spine can solve this problem; the mass can absorb spontaneously. However, retro-odontoid pseudotumor cases without atlantoaxial instability are limited, and it remains controversial whether the mass should be removed.
CASE REPORT
A 67-year-old man had intermittent neck pain for 10 years and numbness and weakness of the extremities for 1 month. Primary diagnosis was intra-spinal neoplasm (C1), cervical spondylotic myelopathy, and subaxial cervical stenosis. After C3-C7 laminoplasty, we removed the posterior arch of the atlas, and the mass was resected via the lateral side of the spinal cord using a curette and clamp, followed by C1-C2 fixation and bilateral lateral atlantoaxial joint fusion in single stage.
The intra-spinal neoplasm was debunked, and a diagnosis of a retro-odontoid pseudotumor was made based on pathology. Postoperative radiography showed sufficient decompression of C3-C7 and sufficient grafted bone in the lateral atlantoaxial joints. The pseudotumor was completely resected, and no compression was seen on MRI. The symptoms resolved immediately after the procedure. The Japanese Orthopaedic Association score was 16, and 1-year follow-up showed no tumor recurrence.
CONCLUSIONS
The retro-odontoid pseudotumor should be removed if there is obvious compression of the spinal cord. Lateral atlantoaxial joint bone grafting is recommended because of the removal of the grafting bed (posterior C1 arch) for posterior fusion. C1-C2 fixation and fusion could reserve the motion of C0-C1.

Keywords: Atlanto-Axial Joint; Hypertrophy; Inflammation; Osteoarthritis, Spine; Osteotomy