gms | German Medical Science

Objective: An Os odontoideum is defined as an independent ossicle of variable size with smooth circumferential cortical margins separated from the foreshortened odontoid peg. However the pathophysiology and treatment for this deformity remains controversial. It has been widely accepted that symptomatic os odontoideum due to spinal cord compression has to be addressed by decompression and fusion. On the other hand most authors propose conservative treatment for stable os odontoideum without spinal cord compression. For patients in between these extremes a dynamic MRI might implicate a more differential selection for surgery.

Method: In five consecutive patients a functional MRI was performed using an open MRI in the dorsal decubitus position. Flexion was achieved by positioning a pad underneath the head. Extension was achieved by positioning a pad underneath the shoulders and the upper thoracic spine. In one case an additional upright MRI was performed. The primary endpoint was to compare the extent of flexion and extension shown on conventional radiographies and the extent shown on functional MRI images.

Results: In four of five cases the mobility shown on the dynamic MRI was comparable to the dynamic conventional radiographies showing intermittent or permanent medullary compression in three cases. In the fifth case an additional upright MRI achieved the same amount of flexion and extension as on the dynamic radiography allowing to exclude any medullary compression. The patients with medullary compression were fused using C1-lateral mass screws coupled by C2 pars interarticularis screws (Harms procedure) and decompressed by C1 laminectomy.

Conclusions: Dynamic MRI imaging is not always able to reproduce the same amount of flexion and extension as shown on dynamic conventional radiographies. However intermittent medullary compression can only be seen on dynamic MRI studies. When the full range of flexion and extension is not achieved in an open MRI in the dorsal decubitus position an upright MRI might achieve a more complete range of mobility.