Posterior division of lumbosacral trunk

The posterior division of the lumbosacral trunk refers to the posterior component of fibers within the lumbosacral trunk that contribute to the formation of the posterior (dorsal) division of the sacral plexus.

Anatomical Context

The lumbosacral trunk is formed primarily by the ventral ramus of the L5 nerve root with a contributing branch from the L4 nerve root. It emerges at the medial margin of the psoas major muscle, descends over the ala (wing) of the sacrum in close contact with the sacroiliac joint, and crosses the pelvic brim to join the S1–S3 ventral rami on the anterior surface of the piriformis muscle, thereby forming the sacral plexus.

Anterior and Posterior Divisions

Once the lumbosacral trunk merges with the sacral nerve roots to form the sacral plexus, the contributing fibers split into anterior (ventral) and posterior (dorsal) divisions:

• The posterior division of the lumbosacral trunk carries fibers from L4 and L5 that unite with the posterior divisions of S1 and S2 to form the common peroneal (fibular) nerve — the smaller of the two components of the sciatic nerve. The posterior divisions also contribute to the superior gluteal nerve (L4, L5, S1) and the inferior gluteal nerve (L5, S1, S2).

• The anterior division of the lumbosacral trunk contributes fibers that join the anterior divisions of S1–S3 to form the tibial nerve — the larger component of the sciatic nerve.