Which of the following type of odontoid fracture is considered unstable?

Odontoid fractures usually result from flexion forces. Odontoid fractures are classified as type I, II, or III. Type I fracture involves the tip only. Type II fracture passes through the base of the odontoid process. Type III fracture passes through the body of C2. Types II and III are considered unstable and should be externally immobilized by a halo vest or fused surgically. Surgery often is undeaken for widely displaced fractures (poor chance of fusing) and for those that fail external immobilization. Type I fractures usually fuse with external immobilization only. Ref: Schwaz's principle of surgery 9th edition, chapter 42.