The largest interspace is L5/S1 in most patients. A paramedian approach is easier than the midline approach for the most part. The spinous processes are sometimes elongated and the interspinous ligament is calcified in some. Occasionally one side of the interlaminar space is smaller than the other due to a more inferior extending lamina on one side. Sharp needles are easier to pass through hypertrophied and calcified ligaments than blunts. The sitting position is the easiest to identify the anatomy by palpation compared to lateral position. Prone position placement is more difficult than sitting or lateral positioning due to the interlaminar windows being smaller in the neutral or slightly hyperextended position of lying prone. If a lateral position or prone position placement is necessary, placement in 20 degrees reverse Trendelenburg will make identification of the subarachnoid space easier due to producing an increased hydrostatic pressure (the hydrostatic pressures in the lateral or prone positions without reverse T may be very low in some patients requiring significant time before efflux is seen from the needle). Another trick is to attach a syringe and aspirate on advancement beyond the skin. Note: the interlaminar ligament is frequently hypertrophied at the same level as a disc herniation making it sometimes more of a challenge to penetrate. Finally, in issues of critical spinal stenosis, there may not be any CSF at all flowing distally beyond the level of the stenosis. Therefore, if a spinal block is placed at L5S1, there may be no spinal fluid at all obtained, resulting in multiple attempts and penetrations at that level, with an increased risk of nerve injury. If an injection is somehow performed at a level without CSF due to critical stenosis above, it results in a very patchy block that is limited to a few inferior dermatomes. Pronounced lumbar flexion can increase the cross-sectional area at a level of critical spinal stenosis, and thereby increase the flow of CSF across this level.
