Radiographic Analysis of the Cervical Spine in Patients With Retro-Odontoid Pseudotumors

Study Design. A retrospective review of 10 consecutive patients with a noninflammatory retro-odontoid pseudotumor. Objective. To examine the radiographic characteristics in patients with a retro-odontoid pseudotumor and to evaluate the efficacy of posterior fusion. Summary of Background Data. A retro-odontoid pseudotumor, a reactive fibrocartilaginous mass, is known to develop after chronic atlantoaxial instability; however, one-third of the reported cases showed no overt atlantoaxial instability. The pathomechanism for such "atypical" cases remains unclear, although altered cervical motion secondary to ossification of the anterior longitudinal ligament (OALL) or severe spondylosis has been implicated. Methods. We reviewed the charts and radiographs of 10 patients with a retro-odontoid pseudotumor who underwent surgery. Preoperative radiographs were evaluated for atlas-dens interval (ADI), presence of OALL, range of motion, and segmental motion adjacent to the atlantoaxial joint. Computed tomography was evaluated for degenerative changes of zygapophysial joints. Results. There were 6 men and 4 women. Atlantoaxial instability (ADI >4 mm) was observed in 2 patients. ADI was less than 3 mm in 5 patients. Frequent association of OALL (6 patients) and marked decrease in C2 to C7 range of motion (mean, 17.6; range, 3-36) were noted. Ankylosis of O-C1 was observed in 4 patients and C2 to C3 in 6. Severe degenerative change of C2 to C3 zygapophysial joint was observed in 4 patients. The patients underwent occipito-cervical fusion (9 patients) or direct removal of the pseudotumor (1 patient). Postoperative magnetic resonance imaging invariably demonstrated the mass regression. Conclusion. Retro-odontoid pseudotumors were not always associated with radiographic atlantoaxial instability. Our data indicate that extensive OALL and ankylosis of the adjacent segments are risk factors for the formation of the pseudotumor. Retro-odontoid pseudotumors may develop as an "adjacent segment disease" after altered biomechanics of the cervical spine, especially those in the adjacent segments. Posterior fusion was effective even in cases without radiographic atlantoaxial instability.