Approach Considerations
There are three main techniques commonly used for piriformis injection, differentiated according to the modality employed for localization, as follows:
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Injection under fluoroscopic guidance
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Injection under ultrasonographic (US) guidance
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Injection under electromyographic (EMG) guidance
Computed tomography (CT) is rarely used to guide piriformis injection because it exposes the patient to unnecessary radiation. If a fluoroscope is not available, CT may be used as a substitute; the technique is essentially the same as with fluoroscopy.
In any of the techniques described below, 100 U of onabotulinumtoxinA may be infiltrated instead of the combination of triamcinolone with bupivacaine or ropivacaine. A 50 U/mL dilution with preservative-free normal saline is recommended.
Fluoroscopy-Guided Piriformis Injection
For piriformis injection under fluoroscopic guidance, the patient is first placed in a comfortable prone position, with the C-arm positioned to provide an anteroposterior view of the affected side. [10] The sacrum and the greater trochanter are identified and used as medial and lateral bony landmarks, respectively (see the image below).
Fluoroscopy-guided piriformis injection. Greater trochanter and lateral border of sacrum are identified fluoroscopically and used as markers for needle insertion.
The skin is prepared with three consecutive povidone-iodine scrubs. A skin marker may be used to draw a line from the posterior inferior iliac spine to the greater trochanter. The estimated skin entry site is at the midpoint of this line.
Local anesthesia, both superficial and deep, is provided through local infiltration of 1% lidocaine buffered with sodium bicarbonate. The needle is advance to a bony end point and then withdrawn. Proper needle placement is confirmed by injecting a contrast solution, which should delineate the contour of the piriformis (see the image below).
Fluoroscopy-guided piriformis injection. Anteroposterior fluoroscopic image of piriformis after contrast administration.
Once needle placement is confirmed, a diagnostic or therapeutic block may be carried out. Diagnostic blocks are performed with 1 mL of 1% lidocaine and 3 mL of 0.5% ropivacaine or bupivacaine. After a diagnostic block, the patient may be examined to evaluate for pain and hip function. Therapeutic blocks are performed with 3 mL of 0.5% ropivacaine or bupivacaine combined with 40 mg of triamcinolone.
Ultrasound-Guided Piriformis Injection
For piriformis injection under US guidance, a linear ultrasound probe is positioned with its lateral side medial to the greater trochanter and its medial side lateral to the ischial tuberosity. [8, 11] In this position, the sciatic nerve is identified as an oval honeycombed structure with mixed echogenicity. The sciatic nerve is then followed cephalad until it courses beneath the piriformis and deviates medially toward the sacrum. (See the image below.)
Ultrasound-guided piriformis injection. Greater trochanter and lateral border of sacrum are identified. Ultrasound probe is directed parallel to piriformis. Needle is inserted in lateral third of piriformis.
Once the relation between the sciatic nerve and the piriformis has been determined, an entry point is identified on the piriformis lateral to this intersection. At this point, the linear probe is positioned parallel to the piriformis fibers in such a way that the needle is in an in-plane orientation. The gluteus maximus and the piriformis are visualized and demarcated by a sheath that appears as a hyperechoic band (see the image below).
The needle is then advanced under US visualization, traversing the gluteus maximus and piercing the piriformis.
Electromyography-Guided Piriformis Injection
For piriformis injection under EMG guidance, insert a disposable injectable monopolar needle electrode (23-25 gauge; 2-5 in.) is inserted at the midpoint of a line drawn from the posterior superior iliac spine to the greater trochanter. The needle should touch bone and then be withdrawn slightly.
Piriformis recruitment is tested by means of external rotation of the thigh. Once needle placement is confirmed, 40 mg of triamcinolone, together with 2-3 mL of 0.5% ropivacaine or bupivacaine, is infiltrated into the muscle.
Complications
Potential complications after piriformis injection include the following:
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Infection or abscess at the injection site
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Sciatic neuropathy due to direct injection into the nerve
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Bleeding or hematoma formation at the injection site [12]
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Temporary leg weakness and numbness if the local anesthetic causes a nerve block - To address this possibility, the patient should receive a neurologic evaluation before driving home
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Testing for piriformis pain. Involved hip is flexed, passively adducted, and internally rotated with patient standing.
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Testing for piriformis pain. Involved hip is flexed, passively adducted, and internally rotated with patient reclining.
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Testing for piriformis pain. Involved hip is flexed, passively adducted, and internally rotated with patient reclining.
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Freiberg test. Forceful internal rotation of thigh on affected side elicits pain.
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Beatty maneuver. Patient lies on uninvolved side and abducts involved thigh upward, which elicits pain.
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Fluoroscopy-guided piriformis injection. Anteroposterior fluoroscopic image of piriformis after contrast administration.
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Ultrasound-guided piriformis injection. Greater trochanter and lateral border of sacrum are identified. Ultrasound probe is directed parallel to piriformis. Needle is inserted in lateral third of piriformis.
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Ultrasonogram identifies sciatic nerve, gluteus, and piriformis.
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Fluoroscopy-guided piriformis injection. Greater trochanter and lateral border of sacrum are identified fluoroscopically and used as markers for needle insertion.
