Greater and Lesser Sciatic Foramina
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Direct answer
The greater and lesser sciatic foramina are the two openings between the pelvis and the gluteal region, formed by the greater and lesser sciatic notches converted into foramina by the sacrotuberous and sacrospinous ligaments. The greater sciatic foramen transmits nearly all the vessels and nerves leaving the pelvis for the lower limb — grouped above and below piriformis — while the lesser sciatic foramen transmits the tendon of obturator internus, its nerve, and the pudendal nerve with the internal pudendal vessels into the perineum, earning it the name gateway to the perineum.
What you must remember
- Greater sciatic foramen boundaries: greater sciatic notch, anterior border of the sacrum, sacrospinous ligament (inferior boundary) and sacrotuberous ligament (posteroinferior boundary).
- Above piriformis: the superior gluteal nerve and superior gluteal vessels only — the single most tested fact of the region.
- Below piriformis: from lateral to medial — sciatic nerve (with the nerve to quadratus femoris deep to it), posterior cutaneous nerve of the thigh, inferior gluteal nerve and vessels, and most medially the nerve to obturator internus, the pudendal nerve and internal pudendal vessels.
- Lesser sciatic foramen boundaries: lesser sciatic notch bounded by the sacrospinous ligament above and the sacrotuberous ligament below and behind.
- Lesser sciatic foramen contents: tendon of obturator internus with its nerve, the pudendal nerve and the internal pudendal vessels — structures crossing from the greater foramen around the sacrospinous ligament into the perineum.
- Ligament attachments: sacrospinous ligament runs from the ischial spine to the lower sacrum and coccyx; sacrotuberous ligament from the ischial tuberosity to the sacrum, coccyx and posterior iliac spines; their crossing orientation converts the two sciatic notches into foramina.
- Clinical importance: pudendal nerve block is given at the ischial spine, which is palpable through the lateral vaginal wall; the spine is also a landmark for assessing station in labour and for intramuscular injection boundaries; the obturator internus tendon bursa at the lesser foramen may cause gluteal pain.
Common confusion
Students pass the pudendal structures "through the lesser sciatic foramen" from the start — in fact they first leave via the greater foramen below piriformis, hook around the sacrospinous ligament or ischial spine, and then enter the lesser foramen. Also, remember the superior gluteal vessels and nerve alone pass above piriformis; everything else in the greater foramen lies below it.
Exam-focused takeaway
Expect one compulsory viva or MCQ on the structures above and below piriformis, and on the contents of the lesser sciatic foramen. Be ready to draw the two ligaments on a hip bone, explain why the lesser foramen is the gateway to the perineum, and describe the pudendal block at the ischial spine.
Frequently asked questions
What forms the boundaries of the greater sciatic foramen?
The greater sciatic notch of the hip bone, the anterior border of the sacrum, the sacrospinous ligament below and in front, and the sacrotuberous ligament below and behind. Piriformis fills and subdivides the foramen.
Which structures pass above piriformis?
Only the superior gluteal nerve and the superior gluteal vessels. They run between gluteus medius and minimus, supplying the abductors of the hip.
What passes through the lesser sciatic foramen?
The tendon of obturator internus with the nerve to obturator internus, the pudendal nerve and the internal pudendal vessels. It is the route from the gluteal region into the perineum.
Why is the pudendal nerve block given at the ischial spine?
The pudendal nerve crosses the sacrospinous ligament at its attachment to the ischial spine, which is palpable transvaginally. Local anaesthetic injected there blocks S2–S4 supply to the perineum for labour and perineal surgery.
How do the sacrotuberous and sacrospinous ligaments differ?
The sacrotuberous ligament is longer and more vertical, from the ischial tuberosity to the sacrum and coccyx; the sacrospinous ligament is shorter and more horizontal, from the ischial spine to the lower sacrum and coccyx. Together they convert the sciatic notches into the greater and lesser sciatic foramina.