Pathology
Figure 1 shows a posterior view of the pelvis and gluteal region with most of the muscles removed. In this area, nerves may get compressed against bones, ligaments or muscles. Compression against bone occurs where the nerves travel right next to the greater sciatic notch. Pressure against ligaments occurs where they travel past the sacrospinous ligament. The most common form of nerve irritation in this region occurs from pressure by the piriformis muscle.

In Figure 2, the sciatic nerve travels from the anterior aspect of the sacrum inferior to the piriformis muscle and then down the back of the lower extremity. In classic piriformis syndrome, the nerve will get compressed between the piriformis muscle and the sacrospinous ligament - a rigid, taut tissue. If the nerve is compressed against the ligament there may be considerable irritation of its outer connective-tissue layers. Sometimes it’s the fibrous bands of the piriformis muscle that compress the sciatic nerve. In either case, even a low level of pressure left on the nerve for a long period of time can create symptoms.

A further complication in identifying piriformis syndrome is the anatomical variation in the sciatic-nerve location, occurring in about 15 percent of the population, which is distinguishable only through MRI or exploratory surgery. There are two divisions of the sciatic nerve: the tibial and peroneal. They will most often pass together inferior to the piriformis muscle, as illustrated in Figure 2; however, in some cases they may split as they go past the piriformis muscle. The peroneal division may go through the piriformis muscle while the tibial division goes inferior to it (see Figure 3). In other cases the peroneal portion will go superior to the piriformis muscle, while the tibial division is inferior to it. In a small percentage of cases - perhaps less than 1 percent of the population - both divisions of the sciatic nerve may pass directly through the piriformis muscle.        

As mentioned earlier, the sciatic is not the only nerve that may get compressed in this region. Pressure may be placed on the superior gluteal nerve between the piriformis muscle and the greater sciatic notch (see Figure 2). The piriformis muscle may also compress the inferior gluteal nerve, either with fibrous bands in the muscle or with pressure against the sacrospinous ligament.

The pressure on nerves in piriformis syndrome is usually from a hypertonic piriformis muscle, but it may also occur from external pressure, such as sitting on a wallet. There are also reports of piriformis syndrome occurring from a direct blow to the buttock area. As a result of the blunt trauma, adhesions may develop between the piriformis muscle, the sciatic nerve and the roof of the greater sciatic notch.

Myofascial trigger points in the piriformis or other gluteal muscles may play an important role in piriformis syndrome. Piriformis trigger points will often perpetuate muscle tightness, leading directly to nerve compression. Trigger points in the gluteus minimus are known to reproduce “sciatica-like” symptoms and may easily be confused with nerve entrapment by the piriformis muscle. Furthermore, sacroiliac joint dysfunction may perpetuate trigger points in the piriformis muscle and increase the likelihood of nerve compression. A sudden load placed on the sacroiliac region or the piriformis muscle - from a fall on the stairs, for example - is often the initial cause of perpetual trigger-point problems. The constant hypertonicity may then lead to nerve compression.