What is the pathophysiology of piriformis syndrome?

Updated: Sep 16, 2020
  • Author: Milton J Klein, DO, MBA; Chief Editor: Ryan O Stephenson, DO  more...
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Answer

The piriformis muscle is flat, pyramid-shaped, and oblique. [6] This muscle originates to the anterior of the S2-S4 vertebrae, the sacrotuberous ligament, and the upper margin of the greater sciatic foramen. Passing through the greater sciatic notch, the muscle inserts on the superior surface of the greater trochanter of the femur. With the hip extended, the piriformis muscle is the primary external rotator; however, with the hip flexed, the muscle becomes a hip abductor. Branches from the L5, S1, and S2 nerve roots innervate the piriformis muscle, as demonstrated in the image below. Due to this innervation, a lower lumbar radiculopathy may cause secondary irritation of the piriformis muscle, which may complicate diagnosis and hinder patient progress.

A cadaver study by Iwanaga et al indicated that the superior gluteal nerve is another important source of piriformis muscle innervation. Of 20 sides from 10 cadavers studied, the investigators found that in 80%, the piriformis muscle was innervated by 2-3 nerves. Nerves originated from the superior gluteal nerve on 14 sides (70%); from the inferior gluteal nerve on one side (5%); from the L5 ventral ramus on one side (5%); from the S1 ventral ramus on 17 sides (85%); and from the S2 ventral ramus on 14 sides (70%). [7]

Nerve irritation in the herniated disk occurs at t Nerve irritation in the herniated disk occurs at the root (sciatic radiculitis). In piriformis syndrome, the irritation extends to the full thickness of the nerve (sciatic neuritis).

Many developmental variations of the relationship between the sciatic nerve in the pelvis and piriformis muscle have been observed. [8, 9, 10] In approximately 20% of the population, the muscle belly is split, with 1 or more parts of the sciatic nerve dividing the muscle belly itself. In 10% of the population, the tibial/peroneal divisions are not enclosed in a common sheath. Usually, the peroneal portion splits the piriformis muscle belly, although in rare cases, the tibial division does so.

In a study of 200 pairs of sacral roots (100 patients, none of whom had piriformis syndrome) by Russell et al, T1-weighted magnetic resonance imaging (MRI) scans revealed that 199 of the S1 nerve roots (99.5%) were positioned above the piriformis muscle, while 150 of the S2 nerve roots (75%) traversed the muscle and 50 of them (25%) were located above it. The images also showed that 194 S3 nerve roots (97%) traversed the muscle and that 190 S4 nerve roots (95%) were below it. The piriformis muscles had an average size of 1.9 cm; in 19% of the study's patients, the muscle was asymmetrical by more than 3 mm. [11]

Involvement of the superior gluteal nerve usually is not seen in cases of piriformis syndrome. This nerve leaves the sciatic nerve trunk and passes through the canal above the piriformis muscle.

Blunt injury may cause hematoma formation and subsequent scarring between the sciatic nerve and short external rotators. Nerve injury can occur with prolonged pressure on the nerve or vasa nervorum.


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