in its center until advanced age. In this cartilage, rudiments of the lower epiphyseal lamella of the atlas and the upper epiphyseal lamella of the axis may sometimes be found. The apex of the odontoid process has a separate centre that appears in the second and joins about the twelfth year; this is the upper epiphyseal lamella of the atlas. In addition to these, there is a secondary centre for a thin epiphyseal plate on the undersurface of the body of the bone. Clinical significance Fracture of dens Fractures of the dens, not to be confused with Hangman's fractures, are classified into three categories according to the Anderson Alonso system: Type I fracture - Extends through the tip of the dens. This type is usually stable. Type II fracture - Extends through the base of the dens. It is the most commonly encountered fracture for this region of the axis. This type is unstable and has a high rate of non-union. Type III fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surgery. Additional images See also References External links Netter, Frank. Atlas of Human Anatomy , "High Cervical Spine: C1-C2" Bones of the thorax