An Evaluation of Lateral Atlanto-Axial Displacement in Injuries of the Cervical Spine

Anteroposterior roentgenograms of the cervical spine obtained following trauma frequently show displacement of the lateral articular masses of the atlas with respect to those of the axis. There has been a tendency to discount this finding on the grounds that it can be produced by normal movement of the head and that it is often seen in the absence of injury. We have not encountered in the literature any study correlating the various types of lateral displacement and the frequency of their occurrence in injuries of the cervical spine. This investigation was undertaken because it was our impression that these displacements are more significant than has been suspected. Three separate types of motion are possible in the normal atlanto-axial joint: (a) With flexion of the neck, a slight degree of separation can be demonstrated between the dens and anterior arch of the atlas (3). This motion is not pertinent to our discussion and will not be considered further. (b) Rotation of the atlas about the dens is the main function of the atlanto-axial joint. During our present study it became apparent that we did not understand the exact anatomic relationship of the various bony parts during rotation. This will be the subject of further investigation. It is clear, however, that as a result of this movement the rotated articular mass is foreshortened in the anteroposterior projection and gives the appearance of medial displacement (Fig. 1). The amount of over-riding that can be produced varies from individual to individual, depending on the laxity of the joint capsule and ligaments. (c) Lateral movement of the atlas on the axis also takes place and, while sometimes difficult to demonstrate, is produced most easily by abduction of the head (2) (Fig. 2). Coutts (1), quoting Poirier and Charpey, stated that as much as 4 mm. lateral motion was possible. In addition to confirming these observations, Paul and Moir (4) noted lateral asymmetry in an appreciable number of normal individuals even when the head was in neutral position. A group of 200 consecutive patients who had suffered injury to the neck was studied. Only those for whom adequate anteroposterior roentgenograms of the atlas and axis were available, in addition to lateral views of the cervical spine, were included. Most of these roentgenograms were obtained very soon after injury and admission to the hospital. While many patients had more than one examination, only the first were used for this study. Several patients with congenital anomalies were eliminated because accurate analysis of their roentgenograms was impossible. Observations To describe asymmetry or over-riding of the articular surfaces of the lateral masses of the atlas and axis as seen in the anteroposterior projection, the term offset has been employed. The following types of offset were seen.