There has been a significant amount of progress in the understanding of posterior hip anatomy and sciatic nerve kinematics and it is now well recognised that the sciatic nerve can be entrapped at many other locations (in the subgluteal space) rather than just at the piriformis alone. Therefore, the term ‘deep gluteal syndrome’ is now preferred to describe the presence of pain in the buttock caused from extra-pelvic entrapment of the sciatic nerve instead of the ‘piriformis syndrome’ [
50].
The deep gluteal space is the cellular and fatty tissue located between the middle and deep gluteal aponeurotic layers [
51]. Its border is defined by the following structures: superiorly, inferior margin of the sciatic notch; inferiorly, proximal origin of the hamstrings at ischial tuberosity; posteriorly, the gluteus maximus; anteriorly, posterior acetabular column, hip joint capsule and proximal femur; laterally, lateral lip of linea aspera and gluteal tuberosity; medially, sacrotuberous ligament and falciform fascia. The sciatic nerve enters the pelvis at the sciatic notch on the inferior surface, in the majority (83%) [
52], to the piriformis muscle and sacrotuberous ligament. There are anatomical variations of the course of the sciatic nerve in relation to the piriformis muscle. There are six variations originally described by Beaton and Anson [
53]: a) the sciatic nerve passes along the inferior surface to the piriformis, b) the sciatic nerve divides into two with the peroneal branch piercing through piriformis, c) the sciatic nerve splits with the peroneal branch running over the superior surface and the tibial branch inferior, d) the sciatic nerve pierces through piriformis, and e) the sciatic nerve passes over the superior surface of piriformis. Anomalous variation has been thought to predispose to entrapment of the sciatic nerve, but a recent review by Smoll [
52] suggests this is not as significant as originally thought. However, it is important to note anatomical variation during operative management [
52]. As the nerve courses distally towards the ischium and hamstring origin, it passes posterior to the gemelli-obturator internus complex and quadratrus femoris and can be entrapped at any location throughout its course.