A Rare Developmental Abnormality of the Atlas
THE case forming the basis of this report is that of a white female about 35 years of age whose car, waiting for a light to turn, was bumped into by a street car, causing her head to jerk backward with some degree of violence.
The first x-ray examination in this case was made a few days after the accident, and an interpretation of the films by the attending surgeon as a fracture of the atlas, precipitated legal proceedings.
Our first-sight impression on the examination of the x-ray films in this case, was that they did present a fracture of the atlas. On further and more detailed examination, however, certain points presented themselves which are entirely incompatible with fracture of this bone at the points where fracture, at first sight, appears to be. In this case the posterior portions of the posterior arch of the atlas are separated from the anterior portions of the bone by a space of about one-half inch, in which space no bone at all appears, neither the normal anterior portions of the posterior arch of the atlas nor any fragments of them. The most posterior portion of the atlas though apparently displaced backward is actually in its normal position—not displaced at all. This fact is easily demonstrated by actual measurements on the films of normal necks of the same approximate size. What has become of the bone which normally bridges these gaps of one-half inch between the right and left aspects of the posterior arch and the right and left lateral masses? It is nowhere to be seen in the films of this case.
To answer this question correctly we must recall certain facts concerning the development and ossification of the atlas. Gray's “Anatomy” records the fact that this bone develops by an inconstant number of centers, the variation being from two to five. Piersol's “Human Anatomy,” 1930 Edition, page 131, records the facts that: “Sometimes the union of the posterior arches does not occur. The anterior nucleus may be absent, and the front arch may show a median suture or be represented by ligament or cartilage. In one instance the anterior arch was wholly wanting, the lateral masses being fastened to the dens by ligament.” Another anatomist, Morris, makes record of the fact that ossification of this bone is sometimes incomplete in adult life, and this, of course, means that in such cases the bone cannot be completely demonstrated on the x-ray film. It also means that any lack of ossification can be clearly demonstrated.
The conclusion reached in this case is that it presents a developmental abnormality in the form of incomplete ossification of those parts of the ring of the atlas which connect its most posterior part with its lateral masses on either side. To be more accurate, it presents a complete lack of ossification of the parts mentioned. These parts of the atlas, however, are, of course, present in the form of tough semi-rigid hyaline cartilage containing no lime salts.