CRANIOCERVICAL JUNCTION: C1 (atlas) + C2 (axis) + occiput

Variants of CVJ: precondylar tubercles, third occipital condyle, ossification of ligament of odontoid process

Craniometry:

• LATERAL VIEW
  1. Chamberlain line
     = line between posterior edge of hard palate + posterior margin of foramen magnum (= opisthion)
     • tip of odontoid process usually lies below / tangent to Chamberlain line by >3 mm
     • tip of odontoid process may lie up to 1 ± 6.6 mm above the Chamberlain line
  2. McGregor line
     = line between posterior edge of hard palate + most caudal portion of occipital squamosal surface
     • Substitute to Chamberlain line if opisthion not visible
     • tip of odontoid <4.5–5.0 mm above this line
  3. Wackenheim clivus baseline
     = BASILAR LINE = CLIVAL LINE = line along clivus
     • usually falls tangent to posterior aspect of tip of odontoid process
  4. Craniovertebral angle = clivus-canal angle
     = angle formed by line along posterior surface of axis body and odontoid process + basilar line
     • ranges from 150° in flexion to 180° in extension
     • ventral spinal cord compression may occur at <150°
  5. Welcher basal angle
     = intersection of nasion-tuberculum line and of tuberculum-basion line (along clivus)
     • angle averages 132° (should be <140–145°)
  6. McRae line
     = line between anterior lip (= basion) to posterior lip (= opisthion) of foramen magnum
     • tip of odontoid below this line = NO basilar invagination; if poorly seen → Chamberlain line
     • ANTEROPOSTERIOR VIEW (= “open-mouth” / odontoid view)
  7. Atlanto-occipital joint axis angle
     = formed by lines drawn parallel to atlantooccipital joints
     • lines intersect at center of odontoid process
     • average angle of 125° (range, 124° to 127°)
       
       
       
       
  8. Digastric line
     = line between incisurae mastoideae (origin of digastric muscles)
     • tip of odontoid below this line
  9. Bimastoid line
     = line connecting the tips of both mastoid processes
     • tip of odontoid <10 mm above this line

Normal dimensions for adults:

[posterior axial line = vertical line drawn along posterior aspect of the subdental body of C2]

• Basion-dens interval (in 95%) <12 mm
• Basion-posterior axial line interval (in 98%)
  posterior to dens <12 mm
  anterior to dens <4 mm
• Prevertebral soft tissues at C2 <6 mm
• Anterior atlanto-dens interval <2 mm
• Lateral atlanto-dens interval (side-to-side) <3 mm
• Atlanto-occipital articulation <2 mm
• Atlantoaxial articulation <3 mm

(for children <5 mm)