S1 Nerve Root Blocks: Technique

The technique for blocking the S1 nerve root is governed by the different anatomy of the sacrum and its foramina. In a patient lying prone, the sacrum is typically inclined so that the posterior and anterior sacral foramina are not coincident along postero-anterior views. If desired, and if C-arm fluoroscopy is available, the X-ray tube can be tilted in a cephalo-caudad direction along the length of the patient to bring the posterior and anterior sacral foramina into view in a coincident pattern, but this is not essential.

The S1 nerve roots course medial to the S1 pedicle before leaving the sacrum through the S1 anterior sacral foramen which lies below and lateral to the pedicle. The target point for an S1 block lies at the inferior medial corner of the pedicle and access to this point is obtained through the posterior sacral foramen (Fig. 6). On postero-anterior screening, what should be visualized is the S1 pedicle.

Figure 6.

Fig. 6. Stages in the execution of a left S1 selective nerve root block. (a) Posterior and (b) oblique view of a needle in correct position on the target point. (c) Posterior and (d) oblique view after injection of 1.0 ml of contrast medium.

A 25-G or 22-G spinal needle should be inserted through the skin behind the sacrum slightly lateral and below the target point on the S1 pedicle so that the needle passes towards the target point with a slight medial and cephalad orientation. The objective is to have the tip of the needle rest on the medial end of the caudal surface of the pedicle behind the anterior wall of the sacrum. To achieve this position, the needle must pass through the posterior sacral foramen but must not leave the sacrum through the anterior sacral foramen.

If the posterior sacral foramen can be visualized its margin can be negotiated under direct vision. If the posterior sacral foramen cannot be visualized it can none the less be negotiated by 'feel'. By aiming the needle cephalad of the target point, after penetrating the skin, erector spinae aponeurosis and multifidus muscle, the tip will strike the dorsal surface of the sacrum above the S1 posterior sacral foramen. Thereafter, to enter the foramen, the needle need only be readjusted progressively caudad so that it essentially 'walks' down the superior wall of the foramen which is formed by the S1 pedicle. Success in this maneuver will be indicated by progressive increases in the depth of penetration of the needle until it arrives at the target point.

Passage through the anterior sacral foramen is avoided by maintaining contact with the S1 pedicle and by maintaining a medial orientation of the needle so that it is inclined towards the sacral canal. Passage through the anterior sacral foramen will be indicated by loss of resistance, in which case the needle should be withdrawn and replaced in contact with the pedicle.

Once the needle is in position, contrast medium and subsequent agents can be injected following the same protocol as for lumbar nerve root blocks.