Position and anatomical landmarks
The patient lies in the prone position on an X-ray permeable table with a pillow under the abdomen to flatten the lumbar lordosis. The skin of the lower back and gluteus region is thoroughly disinfected.
The C-arm is positioned in such a way that the direction of the radiation beam in the transverse plane is approximately 20° oblique, such that the facet joints are projected away and the vertebral column is clearly visible. For the angle in the sagittal plane, the C-arm is rotated on its axis. As a result, the transverse process changes location relative to the vertebral body. The direction of the radiation beam must be such that the axis of the transverse process lies slightly above the middle of the corpus vertebrae. Usually, an SMK-C15 cannula is used for this procedure. An injection point is marked just caudally to the transverse process and somewhat medially to the lateral border of the vertebral body.
Figure. Schematic drawing of the lumbosacral innervation.6 *Connections to the dural nerve plexus
After local anaesthetisation of the skin, the needle is advanced in tunnel vision. The general rules of this technique must be observed; in other words, corrections to the direction of the needle must be made while the needle is in the superficial layers, and the depth of the needle must be checked regularly on the lateral views. No contact should be made with the transverse process. The needle is advanced until contact is made with the vertebral body. On the lateral view, the tip of the needle lies somewhat ventrally to the posterior side of the vertebral body. Thereafter, contrast agent (0.5 ml) is injected. On the AP view, this usually results in a very compact shadow. On the lateral view, the contrast spreads anteriorly over the vertebral body. In case of intravascular dispersal, a minimal change in position is usually sufficient. Finally, 1 ml lidocaine (2%) is injected. Pain reduction is recorded 30 minutes after the procedure. The test block is positive if 50% pain reduction is achieved.