Structural skeletal abnormalities include the following: Atlas assimilation (congenital fusion of the atlas and occipital bone) Atlas hypoplasia Basilar invagination Congenital Klippel-Feil malformation (eg, with Turner syndrome or Noonan syndrome), often associated with atlanto-occipital anomalies Os odontoideum (anomalous bone that replaces all or part of the odontoid process) Platybasia, often with basilar invagination, Chiari malformations (descent of the cerebellar tonsils or vermis into the cervical spinal canal), and other abnormalities Systemic disorders that affect skeletal growth and development and involve the craniocervical junction include the following: Achondroplasia (impaired epiphyseal bone growth, resulting in shortened, malformed bones) sometimes causes the foramen magnum to narrow or fuse with the atlas and thus may compress the spinal cord or brain stem.