Agenesis of the posterior arch of the atlas and complex alterations of the craniovertebral junction: A case report

The craniovertebral junction is a unique part of the somite-derived axial skeleton. The absence or hypoplasia of the posterior arch of C1 is frequently associated with compensatory hypertrophy of the anterior arch of C1 and of the spinous process of C2. Here, we report a patient with agenesis of the posterior arch of C1 without neurologic deficits. Our patient presented with complex alterations of the craniovertebral junction that involved interactions between the condyles, clivus, atlas, and epistropheus. To our knowledge, dislocation of the odontoid process above the Chamberlain line, including cranial migration of the anterior arch of C1, has not been reported in the literature.