Anomalies of the Dens (Syn.  Odontoid Process) 
Anomalies (congenital, acquired or traumatic) of the dens are not uncommon; they often result in atlanto-axial instability and may require surgical intervention.  Hypoplasia of the dens may lead to invagination into the basilar part of the occipital bone.

Odontoid Fractures
Anderson and D’Alonzo classified odontoid fractures according to their location – this classification also gave an indication as to the prognosis.  
 
Type 1 fractures are mechanically stable and represent avulsion of the alar ligament(s).  

Type 2 fractures are located at the base of the odontoid.  These are the commonest type of fracture and have the worst natural history.  Mechanical instability and non-union are the possible sequelae.  

Type 3 fractures arise through the cancellous bone of the vertebral body and have a favorable prognosis if they can be reduced.