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Psoas block

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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.

 

Anatomical landmarks

Spinous process of the 4th lumbar vertebra, posterior superior iliac spine

 psoas1

 

Block technique

The patient is placed in the flexed lateral recumbent position, similar to that used in spinal anaesthesia, with the legs bent and with the leg to be blocked uppermost. The landmarks include the spinous process of the 4th lumbar vertebra (L4) and the posterior superior iliac spine, which are marked. The puncture site is located along the connecting line between the landmarks at the transition from the medial to the lateral third. Following skin disinfection and infiltration of the puncture channel, the electronic stimulation needle is advanced with a strictly sagittal orientation. In the event of bone contact with the transverse process of the 5th lumbar vertebra (L5), the puncture direction should be corrected in a cranial direction so as to pass over the transverse process. The femoral nerve is reached by continuing to advance the needle by 1 – 2 cm. Contractions of the femoral quadriceps muscle show that the needle is in the direct vicinity of the nerve. Once a threshold current of 0.2 -0.3 mA is reached, a test dosage of the local anaesthetic is injected. This is done in order to rule out an intravascular or intraspinal position of the needle. If no adverse effect is noted after 1 to 2 minutes, then the remainder of the dosage may be administered.

psoas2

An alternative puncture technique similarly starts by locating the spinous process of the L4; the puncture is at the same level, but now at distance of approx. 4 cm to the lateral. During puncture, the aim is to achieve contact with the transverse process L 4. Once this is accomplished, the direction of puncture must be corrected caudad under the transverse process. In both cases, the femoral nerve is reached at approximately the same place, but approached at different angles. The latter method sets up a more favourable situation for the catheter technique.