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The procedure often must be altered to suit the situation. Be careful to evaluate for adequate space between the spinous processes. If not sufficient initially, the space will often increase significantly with the addition of a second bolster. In this situation, it helps to check the approach, before prep, in the lateral fluoro position, to be certain that the selected skin site will allow a direct path to the target.

Occasionally patients require no needle angulation at all to reach the target, which can affect skin entry site. Again, previewing in the lateral plane will help.

If one is unexpectedly contacting bone and the AP and lateral views do not clearly show which bone is obstructing the path, craniocaudal tube tilt down the barrel of the needle is usually illuminating.

Larger patients may necessitate the use of a 6-inch needle. At this length, 25-gauge needles are usually quite difficult to steer; thus, a 6-inch 22-gauge needle is a better choice for large patients.

Occasionally, as the needle is advanced with the air-release technique, a bounce-back is followed by a partial air release after the next advancement. While this is unsatisfying, this invariably signals entry into the epidural space, and further advancement ("just to be sure") may only send the needle into the thecal sac. Alternatively, if a partial or full air release occurs before expected, the position may be off-center and posterior to the ligamentum flavum, a space that can accept air insufflation (Fig. 9).

FIGURE 9 (A) Initial injection of contrast collects posteriorly due to superficial position of the needle tip, which has not yet entered the epidural space. (B) Anteroposterior view at the same time demonstrates unilateral contrast flow to be expected if the tip is posterior to the ligamentum flavum. (C) After further advancement, epidural location is confirmed.

FIGURE 9 (A) Initial injection of contrast collects posteriorly due to superficial position of the needle tip, which has not yet entered the epidural space. (B) Anteroposterior view at the same time demonstrates unilateral contrast flow to be expected if the tip is posterior to the ligamentum flavum. (C) After further advancement, epidural location is confirmed.

After each set of four needle advances and tests, reinsertion of the stylet is advisable to clear the needle tip.

Many ESI patients with degenerative disc disease will have close apposition of their spinous processes. Sometimes, this phenomenon will preclude passage of a needle between the spinous processes at the desired level. An alternative approach to the posterior epidural space, the paramedian approach, almost always proves successful. For this approach, tilt the tube 10° oblique from the true AP plane. This will profile a space between the spinous process and the ipsilateral lamina, shaped like an upside-down horseshoe (Fig. 10). One then adjusts cranio-caudal tilt of the fluoroscopy tube to place the apex of the horseshoe at the level of the disc. The "horseshoe" is a tunnel for passage of the spinal needle into the posterior epidural space. One should attempt to pass the needle near the margin of the spinous process, rather than more laterally. Once the needle is at the posterior margin of the facet joint on the lateral view, proceed using lateral fluoroscopy as with a standard interlaminar approach. If one looks in the straight anteroposterior plane, the needle will be nearly midline when it reaches the epidural space.

Finally, if the patient is allergic to iodinated contrast, gadolinium will suffice, especially if a digital subtraction run is employed.

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