Os odontoideum is seen separated from the odontoid and in relationship to the axis (orthotopic type os, arrow 1). Dynamic x-rays in extension were used to depict the degree of abnormal motion between C1 and C2 (odontoid hypoplasia and axial synchondrosis arrow2). There is anterior instability, with the os odontoideum subluxing forward in relation to the body of C2.