This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Spinal puncture may be performed either for diagnostic purposes, for relief of accumulations of subarachnoid fluid, or for the purpose of producing spinal anaesthesia.
The lumbar region is usually selected and the puncture made in the median line and either above or below the spine of the fourth lumbar vertebra. A line passing from the highest point of the crest of one ilium to that of the opposite side passes through the lower part of the spine of the fourth lumbar vertebra. The puncture should always be made below the upper border of the second lumbar vertebra, because the spinal cord extends down to that point (Fig. 490). The lumbar spines are nearly or quite horizontal and do not incline downward as do those of the cervical and dorsal regions. The patient should bend the body forward, as by so doing the space between the vertebrae posteriorly is increased.
Fig. 490. - Lumbar puncture of the spine.
The needle used should be from 6.25 cm. (2 1/2 in.) to 10 cm. (4 in.) long, according to the age and size of the patient. It should be introduced in the median line and pushed upward. In its entrance it pierces the muscles, then the ligamentum subflavum, which passes from one lamina to the other, and finally the dura mater and arachnoid. Failure is liable to occur either from the patient straightening the spine when the puncture is made or from failure to enter the spinal membranes owing to pushing the dura in front of the cannula. A needle enters more readily and surely than does a small trocar with its cannula. The shoulder formed by the cannula, particularly if not well made, is apt to push the tough dura ahead of it instead of puncturing.
The laminae pass from the transverse and articular processes to the spinous processes. On each side of the median line the erector spinae muscles form thick masses and the spinous processes lie in the groove between them. Hence, in doing a laminectomy, the depth at which the laminae lie is apt to be found much greater than is expected. An incision is first made directly on the spinous processes and continued down on each side to the laminae. With a chisel-like periosteal elevator the attachments of the muscles and periosteum are detached from the base of the spinous processes and laminae as far out as the transverse processes. The bleeding from the muscles is controlled by packing. The laminae may be divided with a saw inclined inward or the supraspinous, interspinous, and subflava ligaments may be divided, the spinous processes cut close to their base and removed, and finally the laminae removed with bone forceps. When the laminae are removed the dura mater is found separated from the bone by fat and connective tissue. The veins here encountered may bleed freely but cease on pressure being made. If necessary the dura may be opened, in which case the portion of the body toward the head may be lowered to prevent too great loss of cerebrospinal fluid. The roots of the spinal nerves will be found passing out laterally and should if possible be avoided. If the posterior or sensory root is divided it has the same tendency to re-unite as do sensory nerves elsewhere, but division of the anterior root causes permanent motor paralysis. The dura and other structures are then sutured without drainage.