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 two magenta stars mark the interspaces between the sacrum and TP L5 and between TP L5 and L4.
