The area of pain in patients with piriformis syndrome. (Reprinted with permission from Philip Peng Educational Series)
Piriformis syndrome, an etiology related to prolonged or excessive contraction of the piriformis muscle and its close relationship to the sciatic nerve and the inferior gluteal artery, is associated with pain in the buttocks, hip, and lower limb (Fig. 8.1). Pain aggravated on sitting, external tenderness near the greater sciatic notch, pain on any maneuver that increases piriformis muscle tension, and limitation of straight leg raising are features commonly observed in this syndrome. Injections of local anesthetics, steroids, and botulinum toxin into this muscle using anatomic landmarks, electrical stimulation, fluoroscopy, and ultrasound for guidance are often performed for diagnostic and therapeutic purposes.
The piriformis functions as an external rotator of the lower limb in the erect position, an abductor when supine, and a weak hip flexor when walking. It originates from the ventral surface of the S2 to S4 vertebrae. Running laterally anterior to the sacroiliac joint, this muscle exits the pelvis through the greater sciatic foramen. At this point the muscle becomes tendinous inserting into the upper border of the greater trochanter as a round tendon. All neurovascular structures exiting the pelvis to the buttock pass through the greater sciatic foramen (Fig. 8.2). The superior gluteal nerve and artery pass superior to the piriformis. Inferior to the piriformis lie the inferior gluteal artery and nerve, the internal pudendal artery, the pudendal nerve, obturator internus nerve, posterior femoral cutaneous nerve, quadratus lumborum nerve, and sciatic nerve.
The anatomical relationship between the piriformis muscle and sciatic nerve is variable. Most commonly, the sciatic nerve passes below the piriformis muscle (83%). There are five other variations as shown in Fig. 8.3.The close relationship of the piriformis muscle to the sciatic nerve explains why patients experiencing piriformis syndrome may also experience symptoms of sciatic nerve irritation.
Piriformis syndrome is an uncommon cause of pain occurring in the back, buttock, or hip. Typically, pain is felt in the region of the sacroiliac joint, greater sciatic notch, and piriformis muscle with radiation down the lower limb similar to sciatica. The pain is exacerbated by walking, stooping, or lifting. On physical examination there may be gluteal atrophy and tenderness on palpation and pain on stretching of the piriformis muscle.
A few clinical tests can be helpful (Figs. 8.4 and 8.5). Often, it is a diagnosis of exclusion with clinical assessment and investigations necessary to rule out pathology of the lumbar spine, hip, and sacroiliac joint.
Patient position: Prone
Probe: Curvilinear (2–6 MHz)
First, locate the posterior superior iliac spine (PSIS). (approximated by the dimple of Venus). Then, place the ultrasound probe over the gluteal region with the medial part of the probe over PSIS (left upper figure position A in Fig. 8.6). The sonograph showed the iliac crest and the gluteus maximus (G Max) muscle (left lower figure).
The probe is moved in the caudal direction to the greater sciatic notch (left upper diagram position B in Fig. 8.6) when the curved hyperechoic shadow of the ischium appears (right upper sonogram). The scan can be optimized by aligning the probe with the long axis of the piriformis and tilting the probe in medial orientation.
It is important to optimize the scan by titling so as to identify the superficial and deep border of the piriformis (Pi) and the sciatic nerve (arrowheads). The latter is located deep to the piriformis medial to the ischium. By applying the Doppler (right lower sonogram), the branch of inferior gluteal artery can be seen accompanying the sciatic nerve.
Needle: 22 or 25 G, 3.5- or 5-inch needle (preferably echogenic)
Drugs: 1–2 cc of one of the following:
Local anesthetic (0.25% bupivacaine)
Local anesthetic with methylprednisolone 40 mg
Botulinum 50–100 units
The target is the muscle belly of the piriformis. The needle is inserted in-plane with the entry point 2 cm away from the medial edge of the probe (Fig. 8.7).
To enhance the visualization of the needle, one can insert the needle 2 cm away from the medial edge of the probe or tilt the probe more medially to allow a shallower angle of the needle.
Start scanning from the iliac crest and move the probe slowly in the caudal direction to avoid confusion with muscles in the lesser sciatic notch.
When tilting the probe medially to optimize the scan, quite often the authors will also place the probe on the lateral aspect of gluteal regional and “push” the probe medially as well as the medial tilt action.
Observe the slight curvature of the ischium deep to the lateral part of the piriformis. Straightening of the ischium suggests US scan is at the level of the ischial spine and the sacrospinous ligament – this is the boundary between the greater and lesser sciatic notches and caudal to the inferior border of the piriformis.
Involvement of the piriformis muscle in sciatic neuropathy has been supported by evidence from computed tomography (CT), magnetic resonance imaging (MRI), scintigraphy, and ultrasound (US). Proximity of the piriformis muscle to the sciatic nerve, inferior gluteal artery, and vein makes landmark- or fluoroscopy-based infiltration risky. Electrical stimulation of the muscle (in the absence of contraction of muscles innervated by the sciatic nerve) suggests that the needle tip is in the muscle belly, but there is significant variation in the needle-to-nerve distance for presence or absence of muscle twitches to be regarded as a reliable indicator of needle tip position. A study in cadavers showed that only 30% of fluoroscopy-guided injections were accurate with majority of incorrect injections in the gluteus maximus. Ultrasound allows visualization of the muscle and the surrounding neurovascular structures, thereby enhancing both accuracy and safety. Comparison of local anesthetic against a combination of local anesthetic with steroids as injectates to treat piriformis syndrome indicates that the combination does not confer any analgesic benefit over local anesthetic alone for a period of 3 months following the injections.
Obturator Internus Muscle
The obturator internus (OI) is one of the short external rotators of the thigh at the hip (Fig. 8.2). The other muscles that have the same action include the piriformis, superior and inferior gemelli, and quadratus femoris. Pathology of this muscle can present with gluteal pain and symptoms similar to piriformis syndrome. Important anatomical relationships include the Alcock’s canal medially that contains the pudendal neurovascular bundle and is formed by splitting of the muscle’s fascia into a medial and lateral layer – this canal underlies the plane of levator ani. Pathology in (tears or tendinitis) or around this muscle (bursitis) results in gluteal pain that is retrotrochanteric. This is often referred to as the “deep gluteal syndrome.” Acute strain, chronic overuse, myofascial pain, contusions, tendinopathy, calcific tendinitis, or bursitis may be responsible for pain originating in the OI.
OI is partly a hip muscle and partly an intrapelvic muscle. It originates from within the pelvis along the obturator foramen and membrane, exits through the lesser sciatic foramen by curving around the posterior ischium, and courses laterally deep to the sciatic nerve toward its common insertion with the superior and inferior gemelli as the tricipital tendon on the medial greater trochanter (Fig. 8.2). The OI may be subjected to stress during its sharply angulated exit from the pelvis, as evidenced by the presence of a cartilaginous region on the posterior ischium over which the OI tendon glides. The OI bursa is located medially between the OI and posterior ischial cartilage and reduces frictional stresses in this region.
Pain is present in the mid-buttock region and tenderness on palpation of the interval between the piriformis muscle and the ischial tuberosity can be elicited. Patients often report pain on sitting – apart from the OI, the hamstring-ischial bursa complex is another source for this pain.
Physical examination to elicit pain from OI requires maneuvers similar to those for stressing the piriformis muscle. Pain in innervation territory of the pudendal and/or sciatic nerves due to dynamic compression of the nerve by the OI muscle and tendon is an unusual presentation. Injecting the OI sheath, muscle, or bursa can relieve the pain depending on the site of the pathology.