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ANATOMY FOR DIAGNOSTIC IMAGING
The sacroiliac joints (see Fig. 3. 7) between the auricular surfaces (so called because of their ear shape) of the sacrum and ilium on each side are true synovial joints with cartilage-covered articular surfaces and a synovial capsule. The articular surfaces have several elevations and depressions that fit into each other and contribute to the stability of the joint. The fibrous capsule is thickened posteriorly with the dense sacroiliac ligaments, which are the strongest ligaments in the body. Accessory ligaments include the iliolumbar ligament from the transverse process of the fifth lumbar vertebra to the iliac crest, and the sacrotuber¬ ous and sacrospinous ligaments from the sacrum to the ischial tuberosity and ischial spine, respectively. Only a small amount of rotatory movement is allowed at this joint, with some increase in the range of movement during pregnancy. Radiological features of the joints of the vertebral column Plain radiographs The distance from the anterior border of the odontoid process to the posterior border of the anterior arch of the atlas - the atlantoaxial distance - is used as a measure of the integrity of the atlantoaxial joint. Normal measurements are up to 3 mm in the adult and 5 mm in the child the latter reflects the presence of unossified cartilage in the young spine. The sacroiliac joints lie obliquely at an angle of approximately 25° to the sagittal plane and are best radiographed by oblique views. Alternatively, by placing the patient in the prone position, the diverging beams passing at approximately 25° pass through both joints simultaneously. The plane of the sacroiliac joints is perpendicular to the plane of the facet joint of the same side and so these support each other structurally. As a result of this relationship, AP oblique views show best the facet joint nearest the film and the sacroiliac joint furthest from the film. Facet (apophyseal joint) arthrography This may be performed to visualize these joints or as a prelude to the administration of anaesthetic or steroid or other agents, which are injected by a posterior oblique approach under fluoroscopic control. On AP views the joints are seen as smooth, oval synovial cavities. On a lateral view the joint cavity is S-shaped. In spondylolysis adjacent facet joints may be seen, on arthrography, to communicate via an abnormal tract. Computed tomography The facet joints are well seen on axial CT. The articular process anterior to the joint is that of the vertebra below, and that posterior to the joint is that of the vertebra above.
The sacroiliac joints and the dense sacroiliac ligament are also seen on axial views. Reconstruction in other planes is useful in addition to axial views of the atlanto-occipital and atlantoaxial joints. Magnetic resonance imaging Using MRI, the facet joints are seen on axial views, where the joint space is seen as a region of high signal intensity between the low-intensity cortical bones of the articular processes. Disease processes Some disease processes, for example rheumatoid arthritis, affect synovial rather than cartilaginous joints. In the spine this is manifest as involvement of the joints about the atlas and axis (this can lead to erosion of the dens, which is surrounded by synovial joints), the facet joints and the sacroiliac joints. Of the joints between the vertebral bodies, only the neurocentral joints of the cervical vertebrae are involved. LIGAMENTS OF THE VERTEBRAL COLUMN (Fig. 3. 13) The anterior longitudinal ligament extends from the basilar part of the occipital bone along the anterior surface of the vertebral bodies and intervertebral discs as far as the upper sacrum. It is firmly attached to the discs and less firmly to the anterior surface of the vertebral bodies. The posterior longitudinal ligament passes along the posterior surface of the vertebral bodies from the body of the axis to the sacrum. It is firmly attached to the intervertebral discs but separated from the posterior surface of the vertebral bodies by the emerging basivertebral veins. The posterior longitudinal ligament continues superiorly as the membrana tectoria from the posterior aspect of the body of the axis to the anterior margin of the foramen magnum. The supraspinous ligament is attached to the tips of the spinous processes from the seventh cervical vertebra to the sacrum. Above level C7 it is represented by the liga¬ mentum nuchae, which is a fibrous septum lying in the midline sagittal plane that extends from the spines of the cervical vertebrae to the external occipital protuberance and the external occipital crest. Adjoining laminae are connected by ligamenta flava, which pass from the anterior surface of one lamina to the posterior surface of the lamina below. The yellow colour that gives them their name is due to their significant content of elastic tissue. They are the only markedly elastic ligament in man, and can stretch on flexion without forming folds on extension that could impinge on dura. Relatively weak ligaments connect adjacent transverse processes - the intertransverse ligaments - and adjacent spinous processes - the interspinous ligaments.
