The atlantoaxial joint, provided with a relatively wide range of motion, is secured and supported chiefly by soft tissue such as ligaments and articular capsule with little bony support. Thus, relaxation or rupture of the soft tissue structures
due
to
rheumatoid inflammation or trauma may easily cause instability or subluxation. Resulting in compression of the spinal cord and various degree of neurologic symptoms.
Although the degree of atlantoaxial instability can be assessed conveniently by flexion/extension lateral radiogram, it is difficult to recognize the cause of instability and quantify the cord compression during flexion/extension motion. This
study
has
been carried out to prospectively evaluate the usefulness of flexion/extension MRI as a qualitative and quantitative diagnostic toll of atlantoaxial instability.
We reviewed 17 patients with atlantoaxial instability who were taken MRI examination and treated at St. Mary's Hospital, Catholic University Medical College between June, 1991 and December, 1993 and the following results were obtained :
1. Seventeen patients consisted of 5 men and 12 women aged 21 to 68 years (mean, 43 years)
2. Of 17 patients, 11 were rheumatoid arthritis ; 4, trauma ; one Os odontoideum ; one dens nonunion.
3. The 11 patients with atlantoaxial instability of rheumatoid etiology could be classified into three groups according to the cause of the instability and cord compression as revealed by flexion/extension axial MRI : The first group (n=6)
characterized by compression of the cord in flexion with relief in extension, was caused by lax transverse ligaments. In the second group (n=2), despite the instability from erosion of the dens, there was no direct cord compression because the
transverse ligaments were intact. The third group (n=3) showed retrodental granulation tissue in addition to erosions in the dens and/ or ligaments laxity with resultant cord compression.
4. In the traumatic group, there were four patients with instability from lax or ruptured transeverse ligaments and one case with cord compression due to instability accompanied by nonunion of the dens.
5. In the great majority of the patients, the degree of cord compression was proportional to atlantodental interval(ADI) both on flexion/extension lateral radiogram and MRI. However, in group Ii rheumatoid arthritis, there was no cord
compression
on
MRI despite of the widening of ADI on radiogram. On the other hand, in group III rheumatoid arthritis, a marked cord compression was noted due to retrodental granulation tissues in the absence of significant ADI change.
In conclusion, flexon/extension sagittal and axial MRI was useful for a qualitative and quantitative evaluation of atlantoaxial instability by noting the degree and cause of instability and cord compression on cervical motion.
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