BI without Chiari malformation. Tip of odontoid tends to be above CL, McR, and WCCLin. Brainstem compression is due to odontoid process invagination. 85% can be reduced with traction.
Treatment: transoral surgery is recommended, usually accompanied by posterior fusion.
BI+Chiari malformation. Odontoid tip tends to be above CL, but not McRor WCCL. Brainstem compression is due to reduced p-fossa volume. Only 15% can be reduced with traction. Foramen magnum decompression is appropriate.