Median (third) occipital condyle causing atlantoaxial instability and myelopathy.

Dr. Nicandro Figueiredo – Avenida das Flores 941 / 1 floor / Instituto Neurológico e da Coluna Vertebral 78043-172 Cuiaba MT Brasil. E-mail: [email protected] Abnormalities of the craniovertebral junction are of great interest not only to anatomists but also to clinicians, because many of these malformations can produce neurological symptoms and death. Median occipital condyle is presented in postmortem anatomic studies, but there is a lack of clinical study regarding this anomaly in the literature. Considering the segmental complexities involved in the phyletic and developmental establishment of the normal human craniovertebral articulations, the ocasional ocurrence of anomalous separations, fusions, and intercalated ossicles should not be surprising. One of these manifestations of variant segmentation is the third condyle (basilar tubercle). This structure occurs as a projection on basion (anterior central point) of the foramen magnum. Some incidences are expressed as a simple rounded tubercle, but in more developed cases there is actually an articular facet that receives the tip of the odontoid process forming a true diarthrosis. In a series of 600 skulls, some suggestions of craniovertebral malformations, including a third condyle, was present in 14 per cent. The mamalian apical odontoid element is the phylogenetic equivalent of the proatlas of reptiles. In most members of this vertebrate class this proatlas fuses to the occiput of the skull, whereas in mammals it becomes fused to the atlantal contribution of the odontoid. We therefore report a case of a patient with median occipital condyle, associated to transverse ligament laxity and atlantoaxial instability (AAI), who had mild symptoms few months before a sudden tetraplegia.