at times, one will see either no discernible instability or “posterior instability” with os odontoideum moving posteriorly into spinal canal during neck extension.
degree of C1-C2 instability on XR does not correlate with presence of myelopathy; sagittal diameter of spinal canal at C1-C2 level < 13 mm does correlate with myelopathy detected on clinical examination.
Classification - 2 anatomic types:
Orthotopic - ossicle that moves with anterior arch of C1.
(unstable - but cord damage is rare because AP diameter of neural canal is greatest at C2 level and because bilateral pedicular fractures permit spinal canal to decompress itself with forward displacement of C2 body)