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  • 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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