Basilar impression

1. Upward displacement of foramen magnum margins and cervical spine into the posterior fossa
   1. may be associated with:
      1. Down’s
      2. Klippel Feil
      3. Chiari
      4. Syringomyelia
      5. Rheumatoid arthritis
2. Same clinical syndrome as that is seen in Paget’s, osteogenesis imperfecta, rickets
3. Important measurements in basilar impression
   1. McRae’s line: tip of clivus (basion) to opisthion (posterior margin of foramen magnum) (this is essentially a measurement across the foramen magnum)
      1. Should be > 19 mm and no part of odontoid should be above that line
      2. Remember: Hal McRae played Baseball (basion)
   2. McGregor’s line: posterior margin of hard palate to most caudal point of occiput
      1. No more than 4.5 mm of the dens should be above this line
      2. Remember: McGregor makes helmets for the Occiput
   3. Chamberlain’s line: posterior portion of hard palate to opisthion
      1. No more than 3 mm of the dens should be above this line