The posterior approach to the lumbar plexus, first described by Chayen et al. in 1976, is called the psoas compartment block. The knowledge gained since that time now shows that such a type of delineable compartment „sandwiched“ between the psoas muscle with its distribution of branching nerves does not exist as such; but rather an „unorderly arrangement“ of fibres of the lumbar plexus (including the lateral femoral cutaneous nerve and the femoral nerve in particular) runs between the layers of the psoas muscle, but also caudad between the psoas muscle and the quadratus lumborum muscle. Analogous to the brachial plexus block near the clavicle, the three nerves of the lumbar plexus that are important for neural supply to the lower extremity (femoral nerve, lateral femoral cutaneous nerve, obturator nerve) are located very closely together. This means that a single injection at this site is sufficient to anaesthetise all three nerves completely. Moreover, partial anaesthesia of the lumbosacral trunk can occasionally be expected. However, this effect does not usually mean that one can forego the additional block of the sciatic nerve.
Ultrasound-guided psoas compartment block
The psoas block has established itself in only a few anaesthesia departments as a routine procedure. The primary reason for this appears to be the fear of complications such as damage to the kidneys or intraabdominal organs. On the positive side is the all-round effectiveness of this block, which we see demonstrated almost every day, in the lumbar plexus area. The psoas block can with justification be designated as the true „3-in-1-block“.