Sacral Plexus
On this page
Direct answer
The sacral plexus is formed by the ventral rami of the fourth and fifth lumbar and the first to third (often fourth) sacral nerves, joined by the lumbosacral trunk. It lies on the posterior wall of the pelvis, over the anterior surface of piriformis, behind the internal iliac vessels and the ureter. Its largest branch, the sciatic nerve, supplies the back of the thigh and, through its divisions, the entire leg and foot apart from the medial strip served by the saphenous nerve.
What you must remember
- Roots and position: lumbosacral trunk (L4–L5) plus S1–S3 (often S4); lies on piriformis within the pelvis, covered anteriorly by parietal pelvic fascia, the internal iliac vessels and the ureter.
- Main branches: sciatic nerve (L4–S3), the largest and thickest nerve in the body; superior gluteal nerve (L4–S1, posterior divisions); inferior gluteal nerve (L5–S2, posterior divisions); pudendal nerve (S2–S4, anterior divisions).
- Other branches: nerve to quadratus femoris (L4–S1, also supplies inferior gemellus); nerve to obturator internus (L5–S2, also supplies superior gemellus); posterior cutaneous nerve of the thigh (S1–S3); perforating cutaneous nerve (S2–S3); direct muscular twigs to piriformis, and branches from S2–S4 to pelvic viscera, levator ani and coccygeus.
- Exit from the pelvis: almost all branches leave through the greater sciatic foramen; the pudendal nerve re-enters through the lesser sciatic foramen with the internal pudendal vessels to reach the perineum.
- Superior gluteal nerve: leaves above piriformis with the superior gluteal vessels; supplies gluteus medius, gluteus minimus and tensor fasciae latae; its injury produces a positive Trendelenburg sign.
- Inferior gluteal nerve: leaves below piriformis; supplies only gluteus maximus.
- Clinical importance: misplaced intramuscular gluteal injections endanger the sciatic and gluteal nerves, so the upper and lateral quadrant of the buttock is used; a pudendal nerve block (guided by the ischial spine) relieves pain in the second stage of labour; pelvic tumours or prolonged labour on the pelvis may compress the plexus.
Common confusion
Students often merge the superior and inferior gluteal nerves: the superior gluteal nerve is the only structure passing above piriformis and supplies the abductors (medius, minimus, tensor fasciae latae), while the inferior gluteal nerve passes below piriformis and supplies only gluteus maximus. Do not confuse the pudendal nerve (perineum, S2–S4) with the posterior cutaneous nerve of the thigh (skin of the back of the thigh), as both exit below piriformis.
Exam-focused takeaway
Theory and MCQ papers repeatedly test which structures pass above and below piriformis, the root values of the gluteal and pudendal nerves, and the muscles supplied by the nerves to obturator internus and quadratus femoris. In viva, be ready to explain the safe site for gluteal injection, the anatomy of a pudendal block at the ischial spine, and why sacral plexus injuries disturb bladder, bowel and sexual function.
Frequently asked questions
What are the roots and position of the sacral plexus?
Ventral rami of L4–S3 (often S4), joined by the lumbosacral trunk. It lies on the anterior surface of piriformis on the posterior pelvic wall, behind the internal iliac vessels and ureter.
Which is the largest branch and what does it supply?
The sciatic nerve (L4–S3), the thickest nerve in the body. It supplies the hamstrings and, as its tibial and common peroneal divisions, every muscle below the knee.
What is the only nerve that leaves the greater sciatic foramen above piriformis?
The superior gluteal nerve, accompanied by the superior gluteal vessels. It supplies gluteus medius, gluteus minimus and tensor fasciae latae.
How is a pudendal nerve block performed?
The ischial spine is palpated transvaginally or transrectally, and local anaesthetic is injected just above and medial to it, around the pudendal nerve (S2–S4) as it crosses the sacrospinous ligament. It is used for pain relief in the second stage of labour and minor perineal surgery.
Why must gluteal injections be given in the upper outer quadrant?
The sciatic nerve and inferior gluteal vessels pass below piriformis in the lower medial part of the buttock. The upper outer quadrant, above and lateral to this, keeps the needle away from them.