Sciatic nerve, as the largest branch of the sacral plexus and the thickest nerve of the body, forms from the union of ventral branches of L₄-S₃. Then it leaves the pelvis via the greater sciatic foramen below the piriformis and descends between the greater trochanter and ischial tuberosity. Afterwards, it divides into the tibial and the common proneal nerves, most frequently at the level of the upper angle of the popliteal fossa.
Bifurcation into its two major divisions may occur, anywhere, between the sacral plexus and the upper angle of the popliteal fossa.
However, concurrent occurrence of these variations: dividing of the sciatic nerve into two terminal branches in pelvis, existence of piriformis with completely separated upper and lower parts, the common proneal nerve passing through the two parts of piriformis in which one part of inferior gluteal nerve fibers and tibial nerve passing under the lower part of this muscle in company with the other part of inferior gluteal nerve, is a rare and very important phenomenon.
This phenomenon may be of great importance in view of both entrapment of these members between two parts of piriformis which can lead to "piriformis syndrome", and being next to the muscular injection site of the gluteal region. Furthermore, it might be of major significance for medical specialists in particular for anatomists and surgeons to reduce the postoperative complications.