The interspace between the sacrum and the transverse process (TP) of L5 is the lumbosacral osteofibrotic tunnel bounded by the ala sacrum inferiorly, the TP L5 cranially, the fifth lumbar neural foramen medially, the lumbosacral ligament anteriorly, and the intertransverse ligament posteriorly.

The lumbosacral ligament typically extends obliquely inferolateral from the TP L5 to the ala sacrum.

When local anaesthetic is injected into the psoas compartment it spreads behind the muscle up to the level of lumbar vertebra L4 and covers the terminal nerves of the lumbar plexus and the lumbosacral trunk of the sacral plexus.

When the needle penetrates the lumbosacral ligament it enters the psoas compartment which is the fascial space posterior to the psoas major.

The psoas compartment contains the terminal nerves from the lumbar plexus and the lumbossacral trunk that is the sacral plexus component from L4 and L5.

Place the patient in the lateral decubitus position.

Place the low-frequency 6-2 MHz) curved array probe across the iliac crest and parallel shift it medially along the iliac crest until the sacral bone comes into view. In this position the probe is rotated until both the upper margin of the sacral bone and the transverse process of the L5 vertebral body and interspace between the two bony structures come into view.

The intertransverse and lumbosacral ligaments are visualized (see next page).

The needle is inserted with a steep out of plane approach perpendicular to the skin surface and advanced until it penetrates the lumbosacral ligament and a loss-of-resistance is obtained. Electrical stimulation is superfluous.

20 mL of local anaesthetic is injected.

The single-injection suprasacral parallel shift (SSPS) lumbosacral plexus block is indicated for:

– preoperative analgesia before hip fracture surgery as an alternative to femoral nerve block

– postoperative analgesia after major hip or proximal thigh surgery

First the needle pierces the intertransverse ligament between the posterior upper margin of the sacrum and the posterior lower margin of the transverse process L5

Then the lumbosacral ligament that extends from the anterior lower margin of transverse process of L5 and the lateral part of the ala sacra.

The psoas compartment can be accessed by inserting the needle between the upper margin of the sacrum and the transverse process (TP) of L5 or between the TP’s L4 and L5.

If the needle is inserted more cranial the psoas major is strongly adherent to the neuraxis and the needle tip enters the fascial space between the anterior and posterior lamina of the psoas major.

The lumbar plexus and its terminal branches are contained partly inside the fascial space between the large anterior and the thin posterior lamina of the psoas major. In the lumbar paravertebral region, the two layers make a sort of a pocket towards the neural foramina of the vertebral column.

Below the transverse process (TP) of vertebral body L5 the psoas major deviates antero-laterally and creates a space – the socalled “psoas compartment” between the ala sacra and the psoas major. The terminal nerves of the lumbar plexus exit the psoas major at the level of the TP L5 and descend inside the psoas compartment together with the lumbosacral trunk (L4-L5) that is the most cranial component of the sacral plexus and supply the major part of the sacral innervation to the hip joint.

In the pelvis the lateral femoral cutaneous nerve and the femoral nerve from the lumbar plexus runs laterally across the iliacus underneath the transversalis fascia.

The obturator nerve and the lumbosacral trunk run across the ala sacra medial to the psoas major inside the psoas compartment.

Place the patient in the lateral decubitus position.

Place a low-frequency curved array probe in the axial plane in the flank just cranial to the iliac crest.

The muscles of the abdominal wall are visualized (see next page). The probe is moved dorsal until the quadratus lumborum is seen medial to the aponeurosis of the transversus abdominis.

With a slight caudad tilt of the probe the body and transverse process (TP) of vertebral body L4 can be seen surrounded by the “shamrock muscles” (see next page).

The lumbar plexus is visualized anterior to the TP (see second next page).

The needle is inserted from the back of the patient with in-plane technique and the needle tip is guided by ultrasound and electrical nerve stimulation to the target lumbar plexus (see third next page). Inject 20-30 mL of local anaesthetic.