Anatomy

At typical cervical levels, the ventral and dorsal roots of the spinal nerves traverse laterally and caudally in the vertebral canal to form the spinal nerve in the intervertebral foramen. The foramen is oriented obliquely anteriorly and laterally. Its roof and floor are formed by the pedicles of consecutive vertebrae. Its posterolateral wall is formed largely by the superior articular process of the lower vertebra and in part by the inferior articular process of the upper vertebra and the capsule of the zygapophyseal joint formed between the two articular processes. The anteromedial wall is formed by the caudad portion of the upper vertebral body, the uncinate process of the lower vertebra, and the posterolateral corner of the intervertebral disk. Immediately lateral to the external opening of the foramen, the vertebral artery rises closely anterior to the articular pillars of the zygapophyseal joint.

The spinal nerve, in its dural sleeve, lies in the caudad half of the foramen. The cephalad half is occupied by epiradicular veins. The ventral ramus of the spinal nerve arises just lateral to the intervertebral foramen and passes anteriorly and laterally onto the transverse process. Radicular and spinal medullary arteries arise from the vertebral artery and the ascending cervical artery; radicular arteries supply the spinal nerve itself, whereas spinal medullary arteries continue medially to join the anterior and or posterior spinal arteries, which provide critical perfusion to the spinal cord itself.

Position

The procedure can be performed with the patient lying in a supine, oblique, or lateral decubitus position, depending on the operator’s preference and the patient’s comfort. The position must allow adequate visualization of the cervical intervertebral foramina in the anteroposterior (AP), lateral, and oblique planes (Fig. 49-1A). The important first step is to obtain a correct oblique view of the target foramen (Fig. 49-1B). In this view the foramen is maximally wide transversely, and the anterior wall of the superior articular process projects onto the silhouette of the lamina. If these criteria are not satisfied, the inclination of the fluoroscope must be adjusted until they are. The correct oblique view is essential because in less oblique views, which may nevertheless show a foramen, the vertebral artery lies along the course of the needle. Older C-arm fluoroscopy units often restrict the degree of rotation of the side opposite the unit to less than 45 degrees, which can prevent adequate visualization of the cervical intervertebral foramina on the patient’s right side when the C-arm is positioned from the patient’s left. The 60 degrees of anterior oblique angulation often needed for good visualization can be achieved simply by placing a foam cushion under the patient’s right side, thereby tilting him or her to the left, or by tilting the surface of the table to the patient’s left.

Figure 49-1. Cervical transforaminal and selective nerve root injection. A, Proper patient positioning and fluoroscopic C-arm angulation. B, Needle placement along axis of intervertebral foramen under fluoroscopic guidance. C, Axial view of final needle positions for transforaminal and selective nerve root injection, demonstrating the angle of the foramen and adjacent structures. D, Final needle positions for transforaminal injection within intervertebral foramen and selective nerve root injection just lateral to the foramen. E, Right anterior oblique radiograph demonstrating a needle in position along the posterior aspect of the right C6-C7 intervertebral foramen. Inset: Mid-portion of image with bony structures labeled: C6, C6 vertebral body; C7, C7 vertebral body; Ped, pedicle; La, lamina; SpP, spinous process; SAP, superior articular process; IAP, inferior articular process. F, Final anteroposterior radiograph after cervical transforaminal injection, with the needle in its final position and radiographic contrast outlining the spinal nerve (arrowheads) and extending into the lateral epidural space (arrows).

Needle Puncture

A 25-gauge, 2.5- to 3.5-inch needle is passed into the neck through a skin puncture at a point overlying the posterior half of the target foramen. Some experts advocate the use of a blunt-tipped needle to reduce the likelihood of penetration into an arterial structure. The needle tip should always lie over the anterior half of the superior articular process lest it be inserted prematurely and too far into the foramen. Once the needle has reached the superior articular process, its depth is noted. Subsequent insertion should not be more than a few millimeters beyond this depth. The needle is then repositioned to enter the foramen tangential to its posterior wall, opposite the equator of the foramen (Fig. 49-1C through E). Cephalad to this level, the needle may encounter veins; caudad to this level, the needle may encounter the spinal nerve and its arteries. The needle must stay in contact with the posterior wall lest it encounter the vertebral artery.

Under an AP fluoroscopic view, the tip of the needle is finally adjusted so that it lies opposite the midline of the articular pillars in the sagittal plane. Insertion beyond this depth increases the likelihood of puncturing the dural sleeve or thecal sac. The final needle position is checked and radiographically recorded on an oblique view, which documents needle placement against the posterior wall of the foramen, and on an AP view (Fig. 49-1F), which documents the depth of needle insertion.

To ensure that the final needle position does not change with attempts to connect and disconnect syringes directly to the needle, a short length of sterile connecting tubing is attached to the needle and further injections carried out through the distal end of this tubing. Under direct, real-time fluoroscopy, a small volume of nonionic contrast medium (≤1.0 mL) is injected. The solution should outline the proximal end of the spinal nerve and spread centrally toward the epidural space (see Fig. 49-1F).

Once the target nerve has been correctly outlined, a small volume of a short-acting local anesthetic (1% lidocaine, 0.5 to 1.5 mL) is injected to block the target nerve and render the subsequent injection of corticosteroid less painful. While ensuring that the needle has not changed position, the procedure is completed by injecting a small dose of corticosteroid (betamethasone 3 to 6 mg or triamcinolone 20 to 40 mg). Although the size of the particles in available depot steroid solutions varies widely (methylprednisolone > triamcinolone > betamethasone), all of these preparations have particles of sufficient size to block the end-arterioles supplying the brain or spinal cord, if injected directly into an artery supplying one of these structures. Much attention has been given to use of the nonparticulate steroid dexamethasone, with experts recommending use of 4 mg (1 mL of a 4 mg/mL solution). Although early evidence from animal models suggests that dexamethasone is not harmful when injected into the cerebral circulation, the safety and efficacy of this soluble steroid have not been carefully examined in humans.

Anatomy

At lumbar levels, the ventral and dorsal roots of the spinal nerves traverse laterally and caudally in the vertebral canal to form the spinal nerve in their respective intervertebral foramina. The foramina are oriented laterally. The foraminal roof and floor are formed by the pedicles of consecutive vertebrae. The posterolateral wall is formed largely by the superior articular process of the lower vertebra and in part by the inferior articular process of the upper vertebra and the capsule of the zygapophyseal joint formed between the two articular processes. The anteromedial wall is formed by the caudad end of the upper vertebral body and the posterolateral corner of the intervertebral disk.

The spinal nerve, in its dural sleeve, traverses obliquely through the foramen. In the cephalad half of the foramen, the dorsal root ganglion lies just deep to the pedicle of the cephalad vertebra; this region is also occupied by epiradicular veins. As the root traverses inferolaterally through the foramen, it divides into a ventral ramus and a dorsal ramus. The ventral ramus of the spinal nerve passes anteriorly and laterally adjacent to the transverse process of the caudad vertebra bounding the foramen. Radicular arteries arise from the abdominal aorta and its branches and accompany the spinal nerve and its roots to the spinal cord. As in the cervical region, the location and size of the radicular arteries are variable, and the importance of recognizing their presence for carrying out this block safely and effectively must be emphasized.

Position

The procedure is typically performed with the patient in the prone position, with a pillow under the abdomen above the iliac crests and the pelvis tilted anteriorly (Fig. 49-2A). The first step is to obtain a 10- to 20-degree oblique view of the target foramen that allows the needle to pass into the lateral aspect of the intervertebral foramen. This is most difficult for the LS-Sl level, where the iliac crest blocks entry to the foramen when the oblique angle is too extreme.

Figure 49-2. Lumbar transforaminal and selective nerve root injection. A, Proper patient positioning and fluoroscopic C-arm angulation. B, Needle placement under fluoroscopic guidance. C, Axial view of final needle positions for transforaminal and selective nerve root injection. D, Lateral view of final needle positions for transforaminal and selective nerve root injection. E, Final anteroposterior radiograph after contrast injection and selective nerve root injection (left L5), with the needle tip along the superior surface of the nerve root (arrow) and contrast outlining the spinal nerve and extending into the lateral epidural space (arrowheads).

Needle Puncture

Through a skin puncture point overlying the superior portion of the target foramen, just caudad to the pars interarticularis (the junction of the transverse process with the lamina or just caudad to the most proximal portion of the transverse process), a 25-gauge, 2.5- to 3.5-inch needle is passed into the back (Fig. 49-2B). Some experts advocate the use of blunt-tipped needles to reduce the likelihood of penetration into an arterial structure. The needle tip should always lie over the posterior aspect of the intervertebral foramen (Fig. 49-2C). Once the needle has reached the pars interarticularis, its depth should be noted; the radiographic image orientation is then switched to the lateral projection (Fig. 49-2D). Subsequent insertion is carried out using the lateral projection, observing the needle as it enters the foramen. The needle is advanced slowly; further insertion is halted if the patient reports a paresthesia or the needle reaches the mid-portion of the foramen in the AP dimension.

The final needle position is checked and recorded on an AP view, which documents the medial extent of the needle’s advancement. To ensure that the final needle position does not change with attempts to connect and disconnect syringes directly to the needle, a short length of sterile connecting tubing is attached to the needle and further injections carried out through the distal end of this tubing. Under direct, real-time fluoroscopy in the AP view, a small volume of nonionic contrast medium (≤1.0 mL) is injected. The solution should outline the proximal end of the exiting nerve root and spread centrally underneath the pedicle toward the epidural space (Fig. 49-2E).

Once the target nerve has been correctly outlined, a small volume of a short-acting local anesthetic (l% lidocaine, 0.5 to 1.5 mL) is injected to anesthetize the target nerve and render the subsequent injection of corticosteroid painless. While ensuring that the needle has not changed position, the procedure is completed by injecting a small dose of corticosteroid (betamethasone 3 to 6 mg or triamcinolone 20 to 40 mg). Although the size of the particles in available depot steroid solutions varies widely (methylprednisolone > triamcinolone > betamethasone), all of these preparations have particles of sufficient size to block the end-arterioles supplying the spinal cord, if injected directly into an artery supplying this structure. Although much attention has been given to use of the nonparticulate steroid dexamethasone for cervical transforaminal injection, with experts recommending use of 4 mg (1 mL of a 4 mg/mL solution), the incidence of intra-arterial injection is much lower in the lumbar region than at cervical levels, and thus less emphasis has been placed on use on nonparticulate steroids at lumbar levels. Although early evidence from animal models suggests that dexamethasone is not harmful when injected into the cerebral circulation, the safety and efficacy of soluble steroid have not been carefully examined in humans.