Lumbar puncture (LP, spinal tap) is a relatively common procedure in neonatal ICU's. It can be done as part of a sepsis workup - looking for infection. Less commonly it may be done as a way of treating hydrocephalus (excess fluid in the ventricles of the brain) until a shunt can be placed or until a temporary condition causing hydrocephalus resolves. The most common temporary cause of hydrocephalus is interventricular hemorrhage (IVH, Bleeding in the brain). It's been years since we've had a baby with a bleed that severe, though.
More than one physician or nurse practitioner has commented that successful lumbar punctures depend more on the skill of the person positioning the baby than on the skill of the person wielding the needle.
There's more than a little truth in that, but the person doing the tap needs to be able to tell the holder how to adjust the position for best access. The more accurate their directions, the better the position.
There are two basic positions for LP. Sitting and lying on the side. Most of the time, we position the baby on his side with the lower part of the spine curved as much as is practical given the baby's condition and equipment being used. I generally place one hand on the baby's upper back and the other on the back of the upper thighs extending onto the diaper area.
The diaper is pulled down to expose the lower spine, but not so far as to risk fecal contamination. The nurse practitioner or physician cleans the site and places a sterile drape over the lower back. The lumbar area of the spine needs to be curled in order to open the spaces between the vertebrae. This is accomplished by curling the baby's spine from the bottom, keeping the upper part of the spine as straight as possible so as not to interfere with breathing. When I position a baby this way, it is very rare for a nurse practitioner or physician to fail to get a successful tap.
The seated position can be used for less fragile babies and the principle is the same, but hand position is different. The baby needs to be leaning forward with the head supported and lower spine flexed. One hand is placed on each side. I generally have 2 fingers of each hand on the upper back, thumbs under the chin, and pinkies under the knees. My hand size limits the size of babies I can position this way. It's trickier for the holder to learn this position and to keep the baby immobilized while the LP is done.