Sacral Anatomy [107, 111]
The sacrum is normally composed of five modified vertebrae which are fused together. It is a triangular bone mass extending from the inferior vertebral column and containing the sacral and coccygeal nerves.
The skin in the sacral region is usually thick, and the subcutaneous tissue varies in thickness according to the habitus of the individual. It tends to be thinner than that found in the adjacent gluteal and lumbar regions. Situated deep below the superficial fascia are two layers of fibrous connective tissue (the thoracolumbar fascia and the tendon of the erector spinae muscle group of the deep back muscles). Deeper still are to be found a few muscle fibers of the erector spinae muscle group and many fibers of the inferior portion of the multifidus muscle layer. The left- and right-sided muscle groups are situated in a depression, whose medial and lateral walls are formed by the median sacral crest (spinous process) and the lateral sacral crest, respectively. The thickness of the tendon-muscle mass is approximately two centimeters in the region of the second sacral foramen, one centimeter near the third foramen, and 5 centimeters near the fourth foramen. Deep below the muscle mass, there is the periosteum covering the posterior surface of the bone. Components of the sacroiliac, sacrotu-berous and sacrospinous ligaments are situated superiorly and laterally.
The dorsal surface of the sacrum is convex and irregular, with ridges and grooves. In the mid-line, there is the median sacral crest, consisting of three or four tubercles (rudimentary spinous process). At the inferior pole, the sacral hiatus is due to the failure of the fusion of the laminae of the fifth sacral vertebra. Laterally, the sacral crest just lateral to the sacral grooves comprises a row of four small tubercles representing the fusion of the articular processes. It forms the medial aspect of the posterior foramina. The lateral foramina correspond to the fusion of the transverse processes and are the site of insertion of the gluteus maximus muscle. The dorsal sacral foramina transmit the small dorsal rami of the sacral spinal nerves from the sacral canal to the deep back muscle compartment. The foramina are closed by a thin membrane. Small bony projections may be formed on the medial aspects of the foramina and are associated with muscle attachment points.
The posterior foraminae become smaller from top to bottom. They appear to be equidistant in the vertical plane from the midline. Within the sacral foramina, there is abundant adipose tissue, particularly at the level of the third and fourth posterior foramina enclosing the nerve roots. Each nerve root follows an oblique course from top to bottom and from internally to externally. Each nerve root contains afferent and efferent parts of the somatic and parasympathic branches. A thin branch of each sacral root joins the skin surface and provides the buttock with topographic sensitivity. A foraminal arterial branch is always present lateral to the ventral nerve root, close to the inferolateral edge of each posterior sacral foramen. A veinous plexus is generally observed near the midline. The second foramen is nearly half filled by its nerve root along with its ganglion, which partially plugs the foramen. The third and fourth nerve roots occupy a relatively smaller proportion of their respective foramina.
The sacral foramina can be considered as a cylindrical space into which the sacral neuromodulation electrode is introduced. The upper sacrum tends to more curved than its lower part, especially in males. Needles can be inserted into the foramina as far as the anterior part of the foramen in order to reach the sacral root, at a wide range of angles in both the vertical and horizontal planes. For the third sacral cylinder, the angle is approximately 60 to 70 degrees to the posterior surface of the sacrum.
The ventral surface is concave in the vertical plan. There are four transverse lines on the surface which represent the original division of the bone into five separate vertebral bodies. Immediately anterior to the bone of the sacrum in the midline, there is the periosteum and continuation of the anterior longitudinal ligament. The piriformis muscle is attached to the sloping surfaces of the anterior foramina. Then, a layer contains portions of the pelvic nerve, components of the hypogastric plexuses, and blood vessels before the posterior pelvic viscera (rectum and lower sigmoid colon).
Localization of Sacral Foramen
and midline. There are three sets of landmarks to confirm location of the S3 foramen, which is the elective foramen for neuromodulation.
- The S3 foramen is found by palpating the upper edge of the greater sciatic notch, 2 cm just lateral to the sacrum [thon, wju, 1991].
- Another technique  estimates the location of the S3 foramina approximately 2 cm from the midline, and 9 cm above the sacrococcygeal junction cephalad from the tip of the coccyx, identified by a knuckle-like protuberance at the apex of the sacrum (equidistant between apex of the sacrum and coccyx). However, this technique is sometimes difficult, especially in obese patients.
- The sacral crest, the region where the sacrum approaches the horizontal plane, corresponds to S4. From this point, the sacral spine curves downward to S3, which is located 2 cm above the S4 landmark.
The use of fluoroscopy is certainly the best approach for a quick and effective electrode placement, especially in overweight patients. Even though the sacral foramina may not be visualized fluoroscopically, interrelationships between fluoroscopically identifiable landmarks may be used to deduce their location. On anteroposterior radiographs of the pelvis, the interrupted line across the inferior aspect of the sacroiliac joint corresponds with the S3 foramen. On lateral views of the sacrum, the S3 foramen corresponds to the midpoint between the base of the sacrum and the tip of the coccyx.
Electrodes are generally placed in the third sacral foramen (S3). The electrode leads are subsequently attached to an implantable pulse generator. Patients undergoing sacral nerve stimulation must complete three phases of therapy.
Phase 1 or the ''acute phase'' involves a percutaneous test stimulation where a temporary electrode is placed in the S3 foramen and connected to an external pulse generator.
Phase 2 or the ''sub-chronic phase'' follows on from the acute phase. It involves monitoring and adjusting the external pulse generator to identify the optimal comfort level of stimulation and to evaluate therapy.
Phases 1 and 2 are dependent on an external generator and are considered as peripheral nerve evaluation (PNE).
During phase 3 or the ''chronic phase'', a permanent device is implanted.
Temporary sacral nerve stimulation is the first step. It comprises the temporary application of neuromodulation as a diagnostic test to determine the best location for the implant and to control the integrity of the sacral root. This stage is important and may generate a number of technical difficulties . Operators should always follow the manufacturer's instructions.
The physician can verify electrode placement with anatomical or radiological landmarks (# chapter), and by analyzing motor or sensitive responses. There is considerable disagreement whether PNE can be performed outside of a hospital setting, with or without fluoroscopy, and with or without muscle responses . However, neurologic patients may develop severe dysautonomia during electric sacral nerve stimulation [135, 136].
The testing hardware consists of a needle, test lead, test stimulator, interconnect cabling and a ground pad. A 20-gauge foramen needle with a beveled tip is used to gain access to the sacral nerve for placing the test stimulation lead. The stainless steel needle is depth-marked along its length (9 or 12 cm) and electrically isolated along its central part. The portion near the hub is exposed to allow connection to the test stimulator. By stimulating through the unisolated tip of the needle, the physician can determine the correct sacral nerve stimulation site for the test stimulation lead. For PNE, the test lead is a fluoro-polymer-coated, coiled, 11-stranded straight wire. An exposed metal tip at the distal end serves as an electrode. The lead contains its own stylet, which is removed once the correct position has been found. The external test stimulator is used both for patient screening and for intraoperative usage in determining lead placement thresholds. This provides output characteristics that are similar to those of the implantable neurostimulator and can be operated in either monopolar or bipolar modes. Amplitude control is accessible to the patient when it is being used as a screening device. The physician can set amplitude limits to ensure patient safety and the validity of the test. Finally, the ground pad (stuck to the patient's skin) provides the positive polarity in the electrical circuit during the test stimulation and the evaluation.
Anatomical orientation is much easier in a prone position. The patient must be comfortable, and a local anesthesia is administered to infiltrate the skin and subcutaneous tissues (particularly the periosteum).
Once the needle is in place, it is possible to determine nerve responses. Variations in neural anatomy may induce S3 motor responses whereas stimulation is given at S2 or S4. Consequently, the levels must be defined functionally as well as anatomically. In general, two levels of sacral nerve sites are tested to locate the optimum response. In most patients, stimulation of the S3 sacral nerve yields optimal results.
Typical S3 responses include the following: contraction of the levator ani muscles, causing a ''bellows'' contraction of the perineum (deepening and flattening of the buttock groove); plantar flexion of the big toe (and sometimes other toes) due to sciatic nerve stimulation and paresthesia in the rectum, perineum, scrotum or vagina.
Stimulation of S2 causes the following: rotation of the leg or hip, plantar flexion of the entire foot, contraction of the calf, contraction of the superficial pelvic floor, and a pulling sensation in the genital area and in the leg.
Stimulation of S4 causes activation of the posterior levator ani muscles, no motor response in the lower extremities, and pulling sensations in the rectum only.
The lead is then threaded through the needle cannula, and the foramen needle and lead stylet are removed. When the electrode is in place, the appropriate response is reconfirmed and the lead is coiled under the skin. Anterior/posterior and cross-table lateral X-rays of the sacral region provide documentation of the lead's position. This X-ray can serve as a reference for positioning during the implantation phase.
Duration of the ''Sub-Chronic Phase'':
As in the standard test stimulation procedure, the equipment is set up for 3 to 7 days of evaluation. At the end of the evaluation, the percutaneous lead extension is removed. If test stimulation is successful, a permanent device is implanted.
The original technique for implanting a long-term sacral neuromodulator was described by Schmidt et al. . The manufacturer's recommendations should be followed.
Initially, neurostimulators were used for pain control (Itrel II®, MEDTRONIC). Then a specific neurostimulator (InterStim®, MEDTRONIC) was developed with the same technology. The chronic lead has four electrodes and a larger stimulation zone than the temporary test stimulation lead. Control equipment are used to adjust stimulation parameters (generally amplitude at 0.1 volts, rate at 10 to 14 pulses per second (Hz), and pulse width at 210 microseconds).
The patient is given a general anesthetic without long-acting muscle relaxants, which could block the motor responses needed to verify the effects of stimulation. During the implant procedure, the sacral region, buttocks and feet should also be visible to allow observation of motor responses. The patient is positioned facedown with slight hip flexion. Prior to incising, some surgeons provide a local anesthetic to prevent postoperative pain.
To implant and anchor the lead, the sacral foramen must first be located and fluoroscopy is recommended at the beginning of the procedure to help lead placement. Then a 5 cm midline or paramedial incision over the selected foramen is made. The skin and adipose tissue are dissected down to the glistening, white, fibrous lumbodorsal fascia. The fascia is then divided approximately 1.5 cm lateral from the midline, parallel to the spine over the appropriate foramen. The distal end of the lead is inserted into the foramen. Beginning at the distal tip of the lead, the four electrodes are numbered from zero to three; the numbered connector contacts correspond to these electrodes. Each of the four metal contacts should be tested and the lead repositioned to obtain the desired response. The optimal nerve responses are identified, the distal end of the lead is anchored to the lumbodorsal fascia and the lead is connected to the extension. The neuro-stimulator is placed in a subcutaneous pocket in the upper buttock. The neurostimulator may also be placed in a pocket in the abdomen (particularly in very thin patients). Postoperatively, the lead and neurostimulator placements are usually documented with X-rays.
Minimally Invasive Surgery:
Recently, Spinelli  reported a new technique of sacral nerve stimulation, characterized by a percutaneous approach to the sacral nerves resulting in minimal invasiveness of the procedure and the ability to have the patient awake during electrode placement. Under local anesthesia, it is possible to place a definitive quadripolar lead during the percutaneous test, which could reduce the risk of an inconclusive stimulation response. If test stimulation is successful, the pulse generator can be implanted under local anesthesia. Nevertheless, long-term evaluation of this technique is mandatory.
Unilateral or Bilateral Stimulation?
Since the original technique described by Tanagho and Schmidt, the unilateral sacral foramen electrode has been the gold standard for sacral neuromodulation . Indeed, bilateral is not superior to unilateral sacral neuromodulation . In rare cases, bilateral chronic sacral neuromodulation may prove necessary  particularly when unilateral percutaneous nerve evaluation fails .
It seems that bilateral stimulation does not increase the excitatory response but increases bladder inhibition at a lower stimulation intensity. Some authors have reported success with bilateral stimulation but the risk of complications is increased  and life of the device is significantly shorter.
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