Anatomy
The Bones
The vertebral column contains two curves. Both concave anteriorly, one in the thoracic region and one in the lumbar region. Flexion of the spine rids the column of both these curves and allows you to proceed with the procedure.
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During flexion, the inferior articular processes of the top lying vertebra slide upwards. In consequence the inter-laminar foramen enlarges and becomes diamond shaped allowing for an easier puncture.
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The Ligaments
As the needle punctures the skin it will need to transverse three ligaments before it can access the spinal canal. The ligaments will be described below in the order that they are encountered by the penetrating needle.
1. Supraspinous Ligament
This is a tough ligament which runs vertically over the apices of the spines of the lumbar and thoracic vertebrae and continues higher up in the cervical region as the ligamentum nuchae.
2. Interspinous Ligament
This ligament is a very thin one and runs between spines of adjacent vertebrae. The ligament attaches along the length of the spinous processes uniting the upper border of the lower one to the lower border of the more cranial one. Anteriorly the ligament blends in with the ligamentum flavum and posteriorly it blends in with the fibers of the supraspinous ligament. It takes the shape of a rectangle.
3. Ligamentum Flavum
The ligamentum flavum is composed of mainly elastic fibers and is yellow in color, and in name. Flavum is Latin for yellow. It runs from an anterior-inferior surface of a cranial lamina to the posterior-superior surface of the lamina below. Laterally the ligament blends with the capsule of the joint between the articular processes and then passes over medially where it will meet it's counterpart from the opposite side. Veins will sometimes pass through this ligament and can be punctured accidentally during the lumbar puncture.
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a = Ligamentum Flavum b = Supraspinous Ligament c = Interspinous ligament
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The Foramina
1. Interlaminar foramen - most commonly used for Lumbar Puncture
In the lumbar region at rest, the interlaminar foramen is small and triangular in shape. The sides of the triangle can be thought of as the medial borders of the inferior articular processes of the top lying vertebra. the base of the triangle can be thought of as the top border of the lamina of the lower lying vertebra. (see picture at top of page). The interlaminar foramen can be expanded in area by flexion of the spine.
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Intervertebral foramen
Where two vertebrae lie, one on top of the other, a foramen forms. The superior notch of the inferior lamina and the inferior notch of the superior lamina form the lower and upper borders of the intervertebral foramen. The boundaries are delineated by the intervertebral disc anteriorly the capsule posteriorly and the pedicles of neighboring vertebrae superiorly and inferiorly. This foramen is rarely used for the Lumbar Puncture, but can be useful when the traditional puncture site is not practical because of extraordinary circumstances.
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The Meninges and The Spaces
A. Extradural space
This space lies outside the meninges, between the meninges and the periosteum lining the borders of the vertebral canal. This space extends from the base of the skull to the end of the sacrum and is occupied mostly by small veins, fatty tissues and nerves in transit between the spinal cord and the periphery.
1. Dura mater
The dura mater of the spine, for it has a cranial counterpart, is also known as the theca. The dura mater is the toughest of the three meninges and it ends at the lower limit of the 2nd sacral vertebra. It lies most exteriorly of the three.
A. The Subdural space
This is really only a potential space between the dura and the arachnoid mater. During a lumbar puncture procedure the dura and arachnoid are usually punctured simultaneously since they lie in such close contact.
2. Arachnoid mater
The arachnoid mater, as mentioned above, lies in close union with the dura in the lumbar area. It is the middle of the three meninges and also ends at the 2nd sacral vertebra. It is relatively avascular.
B. The Subarachnoid or Intradural space
This space, the space of interest for the lumbar puncture procedure, lies between the arachnoid and pia meninges and is traversed throughout by fibers joining the meninges called trabeculae as well as the emerging spinal nerves. The spinal cord finishes at the 1st or 2nd Lumbar vertebra and below this the subarachnoid space takes on a circular appearance. It's diameter is roughly 15mm.
3. Pia mater
This highly vascular membrane lies closest to the spinal cord. It consists of two layers and gives emerging nerve roots their initial cover.