Basilar impression

Basilar impression occurs when parts of the cervical (neck) vertebrae enter the hole in the skull base (foramen magnum).

Basilar invagination is the same problem, except that it’s caused by softening in the bones at the skull base.

Cranial settling occurs when the bone at the top of the neck on which the skull rotates (dens) extends into the foramen magnum.

see Basilar invagination