Sacral plexus

The sacral plexus is formed by anterior rami of L4 to S4 and its branches innervate the pelvis, perineum and lower limb.

Gross anatomy

The sacral plexus forms on the anterior belly of the piriformis muscle and is formed by the lumbosacral trunk (L4-5) of the lumbar plexus, which enters the pelvis coursing medially to psoas major and unites with the ventral rami of the S1 to S4 nerve roots, which emerge from the anterior foramina of the sacrum. It forms a triangular shape with an inferiorly pointing apex. 

The are a total of twelve branches⁵. The anterior sacral rami give off six branches and then divide into the anterior and posterior divisiondivisions, each giving offwhich provide a further three branches each (see below). 

Branches

Anterior rami

• nerve to piriformis (S1-S2)
• perforating cutaneous nerve (S2-S3)
• posterior femoral cutaneous nerve (S1-S3)
• parasympathetic pelvic splanchnic nerves (S2-S4)
• pudendal nerve (S2-S4)
• perineal branch (S4)

See: mnemonic.

Anterior division of anterior rami

• nerve to quadratus femoris and inferior gemellus muscles (L4-S1)
• nerve to internal obturator and superior gemellus muscles (L5-S2)
• tibial portion (L4-S3) of the sciatic nerve (L4-S3)

See: mnemonic.

Posterior division of anterior rami

• superior gluteal nerve (L4-S1)
• inferior gluteal nerve (L5-S2)
• common peroneal portion (L4-S2) of the sciatic nerve (L4-S3)

See: mnemonic.

Relations

• medial 
  • anterior sacral foramina
  • sympathetic trunk
• posterior
  • piriformis
• anterior
  • parietal pelvic fascia
  • lateral sacral arteries and veins
  • common iliac vessels on top of lumbrosacral trunk
  • internal iliac vessels
  • ureter (in front of internal iliac vessels) lies well anterior to upper part of plexus 

Variant anatomy

• common peroneal and tibial parts of the sciatic nerve fail to unite and course into the lower limb separately
• additional caudal nerve root contribution to the sacral plexus ⁶