Bipartite Atlas with Os Odontoideum with Block Cervical Vertebrae: A Case Report with Emphasis on the "Overlooked" C1 Lateral Masses.

Though rare, a variety of bipartite atlas has been described. However, little has been mentioned about the hypoplastic C1 lateral masses in these cases. An unusual case of bipartite atlas, with hypoplastic lateral masses, os odontoideum and block vertebrae has been described with the surgical challenges encountered. The emphasis here is to study the C1 lateral masses and attempting C1-2 fusion before resorting to occipito-cervical fusion. A 42-year-old lady presented with progressive spastic quadriparesis. Radiology revealed blocked C2-5 vertebrae with os odontoideum with atlanto-axial dislocation with bifid anterior arch of atlas, hypoplastic C1 lateral masses (ill formed postero-inferior part) and posterior arch. Magnetic resonance imaging showed cervicomedullary compression. Reduction was achieved by opening the C1-2 joints posteriorly. Though bilateral C1-2 lateral mass fusion was attempted, we succeeded only on one side necessitating O-C2 fusion on the other. Patient improved neurologically, though neck movements were restricted. Partially formed (peg shaped) lateral mass of C1 suggests its development from more than one sclerotome. With normal superior facets of C1 lateral mass, an attempt should always be made at fusing the C1-2 joint rather than O-C2 in such cases to preserve some neck movements.