The cephalad two cervical vertebrae and the associated ligaments are regarded as part of the craniovertebral complex and have distinctive anatomical and biomechanical properties5,6,13,20,32,34. In children, approximately 70 per cent of all osseous injuries to the cervical spine occur between the first and third cervical vertebrae16. The factors that contribute to these injuries include ligamentous laxity, poorly developed cervical muscles, incompletely developed vertebrae that are wedge-shaped, horizontally oriented facet joints, and a relatively large head14,26.
The biomechanics of the occipito-atlanto-axial area are complex. Approximately 55 per cent of the entire rotation of the cervical spine takes place at the atlanto-axial joint5, axial rotation is coupled with lateral bending to the opposite side23, and the alar and capsular (first and second cervical) ligaments play important roles in limiting both movements under normal conditions. If excessive axial torque (as occurs during trauma, for example) is applied to this joint complex and the resultant angular rotation exceeds 63 degrees3,13,21, then there may be rupture of the alar and capsular ligaments and dislocation of the first and second cervical facet joint13. As a result of the dislocation, the diameter of the spinal canal may be reduced to ten millimeters or less, causing compression of the spinal cord3,21. Nevertheless, injury to the spinal cord in survivors is rare2,14,32. The main long-term problems arise from loss of mobility and tilting of the head or torticollis, which can result in facial asymmetry3,10.
Early recognition of atlanto-axial rotatory dislocation and rotatory subluxation is essential in order for closed reduction to be successful10. An attempt at closed reduction was delayed by months and was not effective in either of our patients. Closed reduction for both atlanto-axial rotatory dislocation and atlanto-axial rotatory subluxation is successful only when the problem has been diagnosed and treated immediately after the traumatic event, and only then will posture and mobility of the head9,17 return to normal.
To our knowledge, no report in the literature has described the pathological anatomy at the atlanto-axial joint in either chronic atlanto-axial rotatory dislocation or rotatory subluxation. A number of hypotheses concerning the mechanism of fixation have been made over the years; they include damage to the synovial membrane3 and rupture of the transverse ligament of the atlas11. In our two patients, abundant fibrous tissue and a segment of the transverse ligament of the atlas were found between the joint surfaces of the lateral masses of the first and second cervical vertebrae, and there was cross union of the atlas and axis; these findings have not been previously described, to our knowledge. In addition, the capsular ligaments on the side of the dislocation were completely disrupted in both patients.
Important lessons are to be learned from our experience. As with other injuries or fractures of a joint, failure of early closed reduction probably indicates that either soft tissue or a fragment of bone is interposed between the joint surfaces1. Irreducible rotatory injuries at the first and second cervical joint should not be treated differently from other dislocations. If early reduction is impossible, prolonged traction is not the answer. If computed tomography or magnetic resonance imaging suggests that there is tissue in the joint, this is more evidence that open reduction is probably necessary. We suggest that, if closed manipulation fails, early open exploration of the joint allows the best chance of reduction and maintenance of normal posture and mobility. When radiographic investigations indicate the likelihood of cross union of the atlas and axis and the decision is made to attempt to correct the deformity, posterior arthrodesis alone is not effective. Even with extensive bilateral mobilization of the atlanto-axial joints, relocation may be extremely difficult. The use of an osteotome to divide the base of the odontoid process or to open the atlanto-axial joints can potentially cause cross union of the atlas and axis or fusion of the atlanto-axial joints. Although we accept this as a potential problem, our justification of the procedure is that the head is placed in the correct anatomical alignment, which prevents the development of facial asymmetry.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
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