8 Radiological Investigations
Radiological investigations of various anomalies of the craniovertebral junction (CVJ) were previously based on the assessment and measurement of several different lines and angles, drawn on plain roentgenograms (craniometry). With the availability of cross-sectional imaging techniques like computed tomography (CT) and magnetic resonance imaging (MRI), assessment of pathologies of the CVJ and their effects on the cervicomedullary neural structures has become more refined.
Magnetic Resonance Imaging of the Craniovertebral Junction
MRI, with its multiplanar capabilities and high soft tissue contrast resolution, has become the mainstay in the radiological evaluation of the CVJ. It is especially useful because it reveals the cord, soft tissue, ligamentous, and vascular anatomy in detail. Sagittal T1-weighted MR images are of greatest value in the assessment of the CVJ ( Fig. 8.1 ).
MRI must be done in all patients presenting with neurological symptoms. Signal abnormalities due to compressive effects on the cervicomedullary junction, impingement on the lower cranial nerves, and compromised adjacent vertebral arteries can be assessed using MRI.1 Dynamic MRI also can detect cases of cord compression that are not seen in the neutral position and thus is diagnostic in all cases of mobile atlantoaxial instability.2 Hypoperfusion in the cerebellum due to vertebrobasilar insufficiency in CVJ anomalies has been documented,3 and MR angiography can help assess the site and nature of the occlusion.
Computed Tomography of the Craniovertebral Junction
CT can be used to assess structural bony abnormalities, differentiate congenital clefts from fractures, and as an adjunct to MRI in assessing arthropathies. Axial high-resolution, thin-section CT shows the bony anatomy in detail. Sagittal and coronal images ( Fig. 8.2 ), as well as three-dimensional reconstructions, are useful in assessing the relations of bony structures with respect to each other. CT myelography with the use of intrathecal water-soluble contrast medium to delineate the dural sheath is not often indicated except in conditions where there is a contraindication for the use of MRI.
Craniometry: The Essential Lines and Angles
Various lines and angles can be used to assess the relationship of bony components of the CVJ to one another. Some of the more useful lines employed in the measurement of the CVJ are outlined here.
Chamberlain Line
This line joins the posterior margin of the hard palate to the opisthion (the posterior lip of the foramen magnum) on a lateral radiograph of the skull. The anterior arch of the atlas vertebra lies below this line. In normal individuals, the tip of the dens should be no more than 5 mm above this line ( Fig. 8.3a,b ).
McGregor Line
The McGregor line, which joins the posterior margin of the hard palate to the undersurface of the occiput, can be obtained from a lateral radiograph. In normal individuals, the anterior arch of the atlas vertebra lies below the line, and the tip of the dens should be no more than 7 mm above the line. The mean position of the odontoid peg when assessed on sagittal MR images has been found to lie 1.2 mm below the Chamberlain line and 0.9 mm below the McGregor line.4
Welcher Basal Angle
The Welcher basal angle is formed at the junction of the nasion–tuberculum and the tuberculum–basion tangents. The basion is the anterior lip of the foramen magnum. The angle is normally less than 140°. In platybasia, the angle measures greater than 140° ( Fig. 8.3a,b ).
Wackenheim Clivus Canal Line
The Wackenheim clivus canal line is created by extrapolating the line of the posterior surface of the clivus ( Fig. 8.3c,d ). Normally, this line is tangential to the posterior margin of the dens or intersects the posterior one third of the dens.
Clivus Canal Angle
The clivus canal angle is formed at the junction of the line created by extrapolating the line of the posterior surface of the clivus (Wackenheim clivus canal line) and the posterior vertebral body line ( Fig. 8.3c,d ). In normal individuals, this angle is greater than 150°.
Atlanto-occipital Joint Axis Angle (Schmidt Angle)
This angle is measured in the anteroposterior view or on a coronal CT/MRI. It is formed at the junction of the lines traversing the atlanto-occipital joints. An angle measuring less than 124° is seen in occipital condyle hypoplasia ( Fig. 8.4 ).
Digastric Line (Fischgold Line)
This line is measured on frontal projections and joins the origins of the digastric muscles at the medial base of the mastoids. The tip of the dens lies below the digastric line ( Fig. 8.4 ).
Bimastoid Line
The bimastoid line lies caudad to the digastric line and joins the tips of the mastoid processes. Normally, the tip of the dens lies up to 10 mm above this line.
Klaus Height Index
The Klaus height index is determined by drawing a Twining line from the internal occipital protuberance to the tuberculum sellae as a baseline. From this a perpendicular line is drawn to the tip of the dens. This index measures 40 mm. If the index is 34 mm or less, a diagnosis of basilar invagination is made. Goel et al.5 measured the Klaus height index on MRI and found it to be much more accurate than the conventional measurements based on plain x-rays. The tentorium could be clearly identified on MRI and the distance of the tip of the odontoid from the line of the tentorium indicated the height of the posterior cranial fossa.
Modified Klaus Height Index
This measures the height of the posterior fossa. The distance between the Twining line and the plane of the foramen magnum is measured. Normally, this value is at least 30 mm.
McRae Line
The McRae line is represented drawn from the basion to the opisthion. Normally, it measures 40 mm.
Spinolamellar Line
The spinolamellar line is drawn from the interoccipital ridge above and downward along the fused spinous process portions of C2–C3 below. This curvilinear line should intersect the fused posterior arch of the atlas. If the atlas lies anterior to this line, posterior compression of the spinal cord is usually present.
Foramen Magnum–Basion Angle
This angle lies between a line connecting the posterior arm of the plane of the foramen magnum and the point at which it crosses the basion with an anterior arm to the nasion. Normally, it is 163 ± 3.4°. It is abnormal if the value is more than 180°.
Francesconi Invagination Index
The Francesconi invagination index is measured by subtracting the occipitoforaminal distance from the occipitobasal distance. The occipitoforaminal distance is measured by drawing a perpendicular from the Twining line to the plane of the foramen magnum. The occipitobasal distance is measured by drawing a perpendicular from the Twining line to the McGregor line. Normally, the Francesconi invagination index is 3 to 10 mm. In basilar invagination, this distance is more than 10 mm.
Goel’s Omega Angle
A line is drawn along the hard palate. Another line drawn parallel to this passes through the center of the base of the axis. A third line is drawn from the center of the base of the axis along the tip of the odontoid process. The angle between the second and third lines is the modified omega angle5 ( Fig. 8.5 ).
Distance between the Odontoid Tip and the Pontomedullary Junction
The distance of the tip of the odontoid from the pontomedullary junction, as measured on MRI, was observed by Goel et al.5 to be a useful index in defining the reduction of the posterior cranial fossa bone size.
Kulkarni and Goel’s Vertical Atlantoaxial Instability Index
The vertical atlantoaxial instability index, as described by Goel and Kulkarni,6 measures the vertical relationship of the atlas and axis ( Fig. 8.6 ). A horizontal line is drawn through the lower end plate of the axis. A second line is drawn parallel to this and tangential to the lower border of the anterior arch of the atlas. A third line is drawn parallel to these lines and tangential to the superior margin of the dens. The shortest distance between the first two lines (x) is divided by the shortest distance between the first and third lines (y). Depending on the severity, the vertical instability is graded from 1 (mild) to 3 (severe).
Anomalies of the Craniovertebral Junction
Congenital anomalies ( Table 8.1 ) and acquired conditions ( Table 8.2 ) affecting the CVJ, including genetic and developmental ones, have been tabulated.
Congenital Anomalies Affecting the Craniovertebral Junction
Congenital anomalies of the CVJ may occur separately, or they may be associated with conditions such as Down syndrome and achondroplasia.7
Anomalies of the Occiput
Most anomalies of the occiput are associated with a decrease in the height of the skull base.
Platybasia
The kyphotic notch in the base of the skull is of interest to anatomists, anthropologists, and radiologists. Platybasia refers to a decrease in the basal kyphosis or flattening of the base of the skull. Objectively, platybasia is diagnosed when the Welcher basal angle exceeds 140°. Platybasia usually occurs in conjunction with basilar invagination ( Fig. 8.7 ). The two are not synonymous, however, and it may occur in isolation, in which case there are no neurological signs or symptoms.
Basilar Invagination and Basilar Impression
These are occipital dysplasias with upward displacement of the margins of the foramen magnum into the posterior fossa.8,9 The medial rims curve upward, whereas the more lateral or paracondylar occipital bones curve downward. There is a reduced posterior fossa volume with an irregularly shaped foramen magnum, a short vertical clivus, and a high odontoid position.10 Basilar invagination ( Fig. 8.7 ) is a primary growth disturbance with dissociated development of the skull and premature fusion of the sutures leading to occipital hypoplasia. It can also be defined as prolapse of the cervical spine into the base of the skull, as suggested by von Torklus and Gehle.11 Syringohydromyelia and Chiari I malformations, as well as atlantoaxial dislocations ( Fig. 8.8 ), may be associated with basilar invagination. Basilar impression is a secondary or acquired condition resulting from softening of the occipital bone. It is seen in conditions such as rickets, osteomalacia, and Paget disease and results in an actual “impression” of the skull base. Basilar impression may result in severe compression of the brainstem.
Related posts:
2 Embryology and Development of the Craniovertebral Junction
4 Quantitative Anatomy of the Lateral Masses of the Atlas and Axis Vertebrae
3 The Vertebral Artery in Relation to the C1–C2 Complex
5 Biomechanics of the Craniovertebral Junction
6 Biomechanics of the Unstable Craniovertebral Junction
7 Biomechanical Considerations in Craniovertebral Stabilization
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