Gehweiler classification of atlas fractures. A type 1 atlas fracture is isolated fracture of the anterior arch; type 2 fracture is isolated, predominately bilateral, fracture of posterior atlas ring; type 3 fracture is combined injury of anterior and posterior arch of the atlas, the “classic Jefferson-fracture.” In stable injuries, the transverse atlantal ligament is intact (type 3a); in type 3b these fractures are associated with lesion of transverse atlantal ligament and are classified as unstable (type 3b). Type 4 fractures are fractures of lateral mass, and type 5 fractures are isolated fractures of C1 transverse process.