The course of the n. ischiadicus in relation to the piriformis syndrome

Piriformis muscle

Origin: Sacrum (facies pelvina ventrally)

Insertion: Trochanter major

Actions: Hip > ABD, EXT, EXT ROT

It is well known that the when the sciatic nerve that normally passes under the piriformis muscle is compressed, then radiating pain in the buttock, posterior thigh and lower leg appears - and this is known as the piriformis syndrome. What can cause this syndrome?

1. Inactive gluteal muscles

I have already posted an article on the lower cross syndrome. Based on this syndrome, usually, gluteal muscles become week - and it is these muscles that play an important role in the hip extension and adduction. When these muscles are weak, the piriformis muscle is "forced" to work harder and become tight, compressing the sciatic nerve. Besides, overuse injury resulting in piriformis syndrome can result from activities performed in the sitting position that involves strenuous use of the legs as in rowing/sculling and bicycling.

2. Hypomobile sacroiliac joints

Another purported cause for piriformis syndrome is stiffness, or hypomobility, of the sacroiliac joints. The resulting compensatory changes in gait would then result in shearing of one of the origins of the piriformis, and possibly some of the gluteal muscles as well, resulting in piriformis malfunction.

3. Foot overpronation

Piriformis syndrome can also be caused by overpronation of the foot. When a foot overpronates it causes the knee to turn medially, causing the piriformis to activate to prevent over-rotating the hip. This causes the piriformis to become overused and therefore tight, eventually leading to piriformis syndrome.

4. Falling injury

Piriformis syndrome may also be associated with falling injury.

Finally, it is interesting to know the course of the n. ischiadicus in relation to the piriformis muscle (after Rauber/Koprch).

The n. isciadicus leaves the lesser pelvis through the foramen infrapiriforme in almost 85% of cases. However, it is possible (in about 15% of cases) that the fibular division and sometimes the n. cutaneous femoris porterior pass through the piriformis muscle and may become compressed at that location. In only about 0.5% of cases the fibular part of the n. isciadicus leaves the lesser pelvis above the piriformis muscle.

Reference: Schuenke M, Schutle E, Schumacher U. Thieme Atlas of Anatomy: General Anatomy of the Musculoskeletal System. New York: Thieme Medical Publishers, Inc.; 2006. p. 493.