Draw the intercrestal line between the tops of the iliac crests which intersects the midline at a 90 degree angle.

A third line parallel to midline is marked through the posterior superior iliac spine in a cephalad direction to intersect the intercrestal line. The distance between the two intersections is divided into thirds, and the site for needle insertion will be 1 cm cephalad from the junction of the middle and lateral thirds, approximating the location of the L₄ transverse process.

Approach and Technique

1.    After sterile skin preparation and injection of a local anesthetic skin wheal, a 4 to 6 in stimulating needle is advanced perpendicular to the skin.
   
2.    Prepuncture ultrasound scanning may help practitioners confirm the location and depth of the lumbar transverse processes prior to performing LPB.

The distance from skin to lumbar transverse process is highly variable and dependent on body mass index (BMI) and gender.

However, the distance from transverse process to the lumbar plexus anteriorly is independent of BMI or gender and is fairly constant at approximately 2 to 3 cm.

Elicitation of a quadriceps contraction signifies correct needle tip placement within the lumbar plexus.

LUMBAR PLEXUS BLOCK

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