The parasacral region is covered by the gluteus maximus muscle.

Removal of the gluteus medius muscle reveals the iliac bone that is an important marker of the parasacral parallel shift technique.

Removal of the gluteus maximus muscle reveals the emergence of the sciatic nerve from the sacral plexus at the lower border of the piriformis muscle.

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.

Bendtsen TF, S

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.

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.

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.

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.