Chapter 139. Lumbosacral Plexus

Denis Jochum, MD

The lumbar plexus is usually formed by the ventral rami of the L1–L3 and part of the ventral branch of L4
The sacral plexus is formed by the lumbosacral trunk (L4, L5), the first sacral ventral ramus, part of the ventral branch of S2, and a small portion of the ventral branch of S3
Classically, the ventral branch of L4 is the junction between the two plexi; it gives off a branch to the femoral nerve, one to the obturator nerve, and the lumbosacral trunk that becomes part of the sciatic nerve
There are several varieties of lumbosacral plexus:
- The most common is a “prefixed” plexus, including all or part of T12
- A “normal” plexus
- A “postfixed” plexus
- Or even a plexus spread in both directions:
  - These anatomical variations can affect regional anesthesia practice because of variations in motor and sensory distributions
The division into two planes of the lumbosacral plexus is less clear than for the brachial plexus
The two terminal branches of the lumbar plexus are the femoral nerve and obturator nerve, which correspond, respectively, to the dorsal and the ventral layers
For the sacral plexus, the tibial nerve is the ventral layer and the common peroneal nerve the dorsal layer
The layout of the main nerves of the lumbosacral plexus requires the use of combined blocks for anesthesia of the lower limb