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L3-4 Interspace

The first anatomical landmarks that must be identified are the spinous processes in the midline of the back. These bony landmarks provide a “Y” axis for locating the desired interspace. Next, an “X” axis is needed so that the correct level for the LP can be identified. This axis is provided by palpating the iliac crest on each side and drawing an imagery line between them. That line will intersect the midline at the upper border of the L4 spinous process. So the L3-4 interspace is just above this bony landmark and the L4-5 interspace is just below it. Either interspace is acceptable for the spinal tap.
The end of the spinal cord (the conus medullaris) is usually located at the inferior aspect of the L1 vertebrae so either the L3-4 or L4-5 interspace are well below the spinal cord and acceptable to use for the spinal tap.

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Bone Anatomy of Interspace

The interspace is a small hole surrounded by bone. The spinal needle must be aimed at the center of that hole for a successful tap. If the needle is directed to one side or the other then it will hit the articular processes. The needle will also hit bone if it is not parallel to the spinous process that is above and below the interspace. An important anatomical fact to remember is that the spinous process is not in the same plane as the vertebral body. At the L3-4 level, the spinous process is at a 10-15 degree angle from the plane of the vertebral body. In the sagittal plane, the needle must be aimed at the correct angle in order to avoid hitting bone above or below the interspace.
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Three Important Points for a Successful LP

1 The interspace can be opened up to its maximum extent by having the patient in a fully flexed position- knees and neck flexed to the chest.

2. The spinal needle must be introduced and advanced parallel to the spinous processes. The spinous processes are at angle so the needle needs to be at about a 10-15 degree angle from the plane of the vertebral body. If the operator aims the needle at the patient’s umbilicus the needle should be at the correct angle.

3. Make sure the patient’s spinal column is straight. Have the patient lay on a firm but comfortable surface with the shoulders and the pelvis perpendicular to the mat.

Sagittal Tissue Block- Skin to Subarachnoid Space

The needle must first penetrate the epidermis and dermis then the subcutaneous fat. Connecting the tips the of spinous processes is the supraspinous ligament. The needle must pass through this ligament and the intraspinous ligament that is attached to the borders of adjacent spinous processes. If the needle is exactly in the midline it will pass between the paired ligamentum flavum which spans the intralaminar space between adjacent vertebrae. Often the needle is not exactly in the midline so it will have to penetrate either the right or left ligamentum flavum into the epidural space. The ligamentum flavum is most easily penetrated when it is stretched by the patient being in a flexed position. The epidural space contains fat. At the midline the epidural space in adults is about 7mm with it being a lot less on either side. The dura is the next structure the needle must pass through with the subarachnoid space being just beneath it. At the L3-4 or L4-5 interspace level there are nerve roots but no spinal cord as the conus medullaris is usually at or just above the L1-2 interspace.
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Depth of Needle

For this particular specimen, which is from an average sized adult, the needle would have to be inserted approximately 2 inches in order to reach the subarachnoid space.  If the patient is larger or heavier, there will be more distance between the skin and the subarachnoid space.
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Anatomy of a Bloody Tap

In doing the spinal tap, the tip of the needle should be just into the subarachnoid space but not too far into it. If the needle is advanced too far, it could nick a small blood vessel associated with one of the nerve roots of the cauda equina or if advanced even further it could go through the ventral dura and nick one of the epidural veins. Piercing a blood vessel as described above is thought to be the cause of a bloody tap.
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