Chapter 139

• 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

###### Figure 139-2. Lumbosacral Plexus and Muscle Innervation

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