Everyone knows that lumbar punctures are about positioning. Sure, but what if they curled up like a ball and you keep hitting bone.
My wife is an anesthesia resident and she has done an insane number of spinal blocks, which are effectively lumbar punctures. And, I was discussing how I was struggling with LPs.
This is what she taught me within 2 minutes and instantly every LP afterwards has felt effortless:
When you insert the spinal needle and you almost instantly hit bone, that is a good thing. This means you are hitting the spinous process. The reason it’s a good thing is because it means you are midline. If you are midline all you have to do is adjust your angle in the direction of the spine and you will go straight into the dura.
Imagine if you hit the bone instantly here. This means either you just need to aim a little lower at the same angle or you adjust your angle. That is the only way you can hit bone early.
However, if you are hitting bone late, then you are likely hitting the lamina. Look at image B below. This means that you are not midline. So you have 2 options, adjust your angle perpendicular to the plane of the spine (paramedian approach) or remove the needle and reinsert midline.
While you can certainly get a successful LP through a paramedian approach, you will be just poking blindly if you don’t know hat you are doing. In this situation, it is better to remove the needle and feel your landmarks and then attempt again.
This is what it looks like if you are hitting bone late.
tl;dr:
1) early bone means midline, adjust angle in plane of spine and should have a straight shot
2) late bone means your angle is too tilted or you are not midline (more likely). Feel again and insert needle midline if you want to do it midline or adjust the angel perpendicular to the spine for a paramedian approach.