Imaging in cavus has two main functions:
- to assess the severity and geometry of the deformity
- to identify an underlying neurological diagnosis
Assess the deformity
The main view is a standing lateral radiograph. This will show the apex of the cavus. The calcaneal pitch should be measured: >30° is abnormal. The angle between the axis of thefirst metatarsal and the axis of the talar neck (Meary’s angle) is normally 0-5 °. Paulos (1980) found an average of 20° in cavus patients, and Aktas 17.8°.
A standing hindfoot alignment view (Paulos 1980, Saltzman + el-Khoury 1995) gives a good indication of the alignment of the hindfoot: normally the axis of the calcaneum should be about 5deg valgus to the axis of the tibia. In cavus the calcaneal axis may be neutral or varus. By positioning the foot with the first ray on and off a block a "radiographic Coleman block test" can give additional information on hindfoot flexibility.
A dorsoplantar view will give a measure of adduction. The axis of the first metatarsal and the axis of the talar neck should be in line.
These three views taken together are our stndard "deformity series".
Identify an underlying neurological diagnosis
A plain radiograph of the spine will show scoliosis and may show a defect in the posterior elements or a spur at a diastematomyelia. MRI of the spine and/or head will show defects in formation of the cord or brain, tumours or tethered cord.
Ward (1998) found that MRI of the spine in children with lower limb abnormalities was unlikely to show dysraphism in the absence of neurological signs. Schwend (1995) reviewed MRIs performed on patients with scoliosis and again found that these were unrewarding in the absence of neurological signs. Therefore, a selective approach to MRI of the neuraxis seems indicated.