An Unusual Association of Os Odontoideum Compressive Synovial Cyst and Multiple Sclerosis: Case Report and Review of the Literature

Os odontoideum is a rare atlantoaxial abnormality that affects the odontoid process. It has been proposed to be a congenital anomaly due to lack of chondral tissue or failed ossification along the growth line of the proatlas and the first spinal sclerotome, which are the precursors for the future odontoid process [1]. Some authors have argued against this theory and believe that os odontoideum is likely a result of previous trauma with nonunion of the healed fracture, hence, mimicking a type-II odontoid fracture [2]. This was based on frequent radiological findings of non-union above the superior articular facets of the axis, a location that is above the synchondrosis line. Nevertheless, the ultimate result is an isolated rostral ossicle that can move with the anterior arc of C1 (orthotopic) or functionally fused to the basion (dystopic) [3].