PROCEDURE
Introductory Video Guides (review before lab)
Foundation Dissection Skills
1. PRE-WORK (before lab) review the organization of the human vertebral column from your atlas or on an articulated skeleton in the lab.
a) Curvatures: Take a moment to review the general features of the vertebral column. Note the primary (thoracic and sacral) and secondary (cervical and lumbar) curvatures to the spine. These curvatures become more pronounced with age. (Netter162)
b) Cervical vertebra: In the cervical region the vertebrae are smaller in size with a bifid spinous processes and prominent 'holes' in the transverse processes, the transverse foramina. These foramina form a path for the vertebral arteries as they course up to the skull and brain. (Netter026) (Netter027)
c) Thoracic vertebra: Identify the larger body and neural arch (pedicles and laminae) which form a 'padlock-like' arrangement. Extending from the neural arch are the laterally positioned transverse processes and posteriorly positioned spinous process. (Netter163)
d) Lumbar vertebra: Lumbar vertebrae have a larger body to support the weight of the upper body and lack rib joint articulations. Also, intervertebral disks increase in size to match the larger vertebral bodies. (Netter164)
e) Zygapophysial joints (also known as 'facet joints' or intervertebral synovial joints): The neural arch of each vertebra articulates with another by articular facets located on the superior and inferior articular processes arising from the neural arch. (Netter163)
f) Intervertebral cartilaginous joints: The body of each vertebra forms a fibrocartilaginous joint with the other vertebral bodies, with the fibrocartilage known as an intervertebral disk. Each disc is made up of a gelatinous mass, the nucleus pulposus, which is surrounded by a tough fibrous layer, the annulus fibrosus. (Netter164)
g) Rib joints: In the thorax, ribs form articulations with the body of each vertebra (costovertebral joint) and with each transverse process (costotransverse joints). (Netter193).
h) Intervertebral foramen: Identify the intervertebral foramina, the openings where spinal nerves emerge. A foramen is formed in part by the superior vertebral notch of the vertebra below lining up with the inferior vertebral notch of the vertebra above. The anterolateral region of the notch is formed by part of the vertebral body and intervertebral disk. (Netter164)
2. Identify the attachments
of the spinalis muscle
to the spinous processes in the lower back region (between approximately T9 and
L5) observed in the previous lab session.
(Netter181)
(Photo1006)
3. Transect the spinalis muscle attachments
to the vertebra by making a deep incision immediately lateral and parallel to
the spinous processes spanning from approximately T9 to L5.
(Figure108)
(Photo1016)
This cut should be made deep enough to contact the lamina at the base of the spinous process. The neural arch will protect the spinal cord, so there is no risk of cutting into the cord while transecting the spinalis muscle attachments.
These cuts are approximate, plus/minus a vertebral level will not impact the end result of the dissection.
4. Force your fingers into the cut and
retract the erector spinae muscle group laterally such that you can free any
remaining muscle attachments to the spinous processes from T9 to L5.
(Photo1017)
This will also separate the spinous attachments of the deeper transversospinalis group of muscles, though we cannot see those. The transversospinalis muscles will be reflected together with the erector spinae muscles as a single muscle block.
Underneath the erector spinae muscles (Netter181) lie elements of the transversospinalis group of deep back muscles, which are also attached to spinous processes (Netter182). As the name implies, these muscles connect between the spinous and transverse processes on the vertebra.
5. At approximately the level of L5 make a
deep transverse cut through the erector spinae and
transversospinalis groups of muscles laterally as far as the transverse process
of L5
(approximately 1-2" to each side of the midline).
(Figure109)
(Photo1018)
(Photo1019)
This transverse incision should start at the depth of the vertical incision made parallel to the spinous processes in the previous steps.
6. At the lateral end of the previous cut
(near the end of the transverse processes), make a shallow incision superiorly through
the back muscles paralleling the vertebral column.
(Figure110)
(Photo1020)
This incision should be no deeper than the depth of the cut you made paralleling the spinous processes. It is important that the lateral incision be similar to the midline incisions, as a deep incision along this lateral aspect may easily inadvertently penetrate into the abdominal cavity from the posterior side.
The lateral incision does not need to be perfect, as the next step involves a gradual reflection of the muscles where any residual attachments can be transected as they are observed.
This cut isolates a left and right rectangular block of the back muscles with incisions on the medial side, lateral side, and inferior side of the blocks.
7. Force your fingers deep to this muscle
block and tear/cut through any attachments to the vertebra so that you can
reflect the block of muscles superiorly.
(Figure110)
(Photo1021)
As you reflect the deep muscle block use your fingers or scissors as needed to cut any attachments as you reflect the muscle block.
This reflects both the erector spinae muscles and transversospinalis muscles, without providing a view of the transversospinalis group.
We will not be studying the transversospinalis group in any detail, but take a moment to review your atlas on their basic organization.
8. Continue reflecting the muscle block
superiorly forming a 'trench' down to the vertebral laminae between L5 and
approximately
T9.
(Figure110)
(Photo1022)
9. Remove any
remaining
soft tissue attached to the vertebra
by cutting (scalpel) or scraping any remaining muscle away from bone with the point of
a chisel or scalpel handle.
(Photo1023)
(Photo1024)
CAUTION: When using a scalpel handle as a scraping tool ensure that it does not have a blade attached.
Remove soft tissue so you can see the flat laminae and the upright spinous processes of vertebra T10 to L5. Removal does not need to be perfect, but ensuring no large masses of soft tissue remain will make cutting with your Stryker saw in later steps more efficient.
10. Examine the exposed portions of the
vertebral column and palpate for the bony points and ligaments.
(Netter167)
(Netter168)
(Photo1025)
(Photo1026)
Spinous processes - palpate along the exposed sides of the spinous processes to feel for the bony regions that are the processes themselves.
Interspinous ligament - all of the spinous processes are linked by a sheet-like interspinous ligament (filling the gap between spinous processes). Palpate to feel the movable ligament sheet between the spinous processes.
Supraspinous ligament - this is the free posterior edge of the interspinous ligament that is thickened and spans the tips of the spinous processes along the entire spinal column.
In the neck, the supraspinous ligament becomes the ligamentum nuchae. The ligaments holding the spinous processes to each other (the supraspinous and interspinous ligaments) are strong. These become tight in flexion serving to prevent hyperflexion of the vertebral column.
The supraspinous ligament and ligamentum nuchae are also midline anchoring points for the more superficial muscles of the back.
Articular processes - Run your finger superior to inferior along the base of the exposed trench approximately 2-3cm from the midline. The series of 'bumps' are the articular processes of the lumbar vertebrae.
Lamina - The bone between the spinous and articular processes (approximately a finger tip wide) forms the flattened sheet of the lamina of the vertebral arch.
11.
Remove the spinous processes from vertebra T10 to L4
by positioning the Stryker saw parallel to the vertebral column and cutting through the
base of the spinous processes.
(Figure111)
(Photo1027)
(Photo1028)
T10 and L4 are approximate, plus or minus a level will not affect the final dissection and an additional level can be opened later if needed.
12.
Using a scalpel or scissors cut through the ligaments holding the row of spinous
processes at each end (between T9/10 and between L4/L5) and lift the row of
spinous processes out.
(Photo1029)
13. Using a Stryker saw positioned at a ~20
degree angle, cut through the
laminae of one
vertebra (approximately L2 or L3) on
both the left and right sides.
(Figure112)
(Photo1030)
Find the correct position for the Stryker cut by running your fingers along the laminae of the vertebral column, feeling for the superior to inferior row of 'bumps' formed by the articular and mammillary processes of the vertebrae. (Netter164)
The Stryker cut should occur just on the medial side of the row of bony processes.
14. Drive a chisel into the
Stryker cut
and lever the bone span of the posterior neural arch out so the arch can be
completely removed.
(Figure113)
(Photo1031) (Photo1032) (Photo1033)
A combination of Stryker cutting, chiseling, and levering out the bone segment will help you preserve the spinal cord lying below.
Removing the posterior neural arch exposes a lumbar segment of the vertebral canal. If you look into the 'hole' you should see small amounts of fat/blood that are part of the epidural space and a sheet of tissue that is the dura. (Photo1033)
This space, containing fat and the venous plexus, is the epidural space (i.e. outside the dura). Epidural analgesia or anesthesia injections are made into this space through either direct injection or via an inserted catheter.
15. Move to the next vertebra inferiorly
and cut through the laminae of that vertebra (as above).
(Figure113)
(Photo1034)
Use the positions of the previous cut or the articular processes to help keep the cuts roughly parallel to the previous cuts.
16.
Drive a chisel into the
Stryker cut
and lever the bone span of this next posterior neural arch out.
(Photo1035)
Examine the anterior side of the removed arch for remnants of the ligamentum flavum.
In the living this ligament has a light yellow coloration, however the ligament loses all color during embalming.
The ligamentum flavum connects laminae to each other. This ligament spans the gap between superior and inferior laminae along the entire length of the vertebral column. (Netter168)
17. Continue removing posterior neural
arches working both inferiorly and superiorly from your starting point until
arches T10 to L4 are removed.
(Photo1036)
Once complete this laminectomy exposes the lower thoracic and lumbar regions of the vertebral canal. The levels are approximate, plus/minus a vertebral level will not affect the final outcome.
18.
Clean any veins or fat from the epidural space and observe the external layer of
the meninges of the spinal cord (the dura mater).
(Netter174)
(Photo1036)
19. Grasp the dura mater at the midline in the lumbar
region of the laminectomy and make a small craniocaudal slit in the
dura mater,
(Netter174)
(Photo1037)
This slit cuts through the dura and the arachnoid membrane, which adheres tightly to the deep surface of the dura, to open the subarachnoid space. In the living, the subarachnoid space contains cerebrospinal fluid. Numerous, thin, spider-web-like arachnoid trabeculae pass through the space to attach to the pia mater.
20. Extend the incision in the dura along the entire
length of the laminectomy from T10 to L4 slitting the dura mater to
expose the arachnoid and subarachnoid space.
(Netter174)
(Photo1038)
Observe the robust nature of the dura mater. The meninges completely encase the spinal cord and spinal nerve roots like a 'sack'.
The region inferior to the ending of the spinal cord (L1-2), containing only floating nerve roots, is the lumbar cistern. This space contains cerebrospinal fluid and is continuous with the subarachnoid space of the spinal cord and brain.
Look on the deep side of the dura mater for fragments of arachnoid membrane and delicate filaments (trabeculae) that span across to the pia covering of the spinal cord and nerve roots.
The arachnoid membrane adhesion to the dura is tight and typically only the thin arachnoid trabeculae fibers spanning across the subarachnoid space to the pia can be seen.
21. Examine within the lumbar
cistern to identify the bundle of nerve rootlets called the cauda equina.
(Figure115)
(Netter174) (Netter170)
(Photo1039)
The dorsal and ventral roots exit the vertebral canal at intervertebral foramina corresponding to specific spinal cord segments. But, since the spinal cord is shorter than the vertebral column, the segments of the spinal cord do not all line up with their named vertebral levels. (Netter170)
The cervical and upper thoracic spinal levels match up reasonably well with their vertebral counterparts. However, the lower thoracic and particularly the lumbosacral levels must have elongated nerve roots in order to reach their target intervertebral foramina in the adult (i.e. the sacral spinal segments physically line up with T12-L1 vertebral levels although their axons go inferiorly to exit the spinal canal at the sacral foramina). (Netter170)
In order for the spinal segment nerve roots to find their target intervertebral foramina, the roots become elongated developmentally. These are confined in the lumbar cistern where they form a bundle called the cauda equina ("horse’s tail").
22. Follow the nerve roots of the cauda equina superiorly
to find their connection to the spinal cord.
(Figure115)
(Netter174) (Netter170) (Photo1011)
23. Examine the dorsal surface of the spinal cord for the
emerging rootlets and the convergence of these
into dorsal spinal roots.
(Netter174) (Photo1011)
The dorsal nerve root is formed from subgroups of rootlets at each spinal segment along the dorsolateral sulcus of the spinal cord.
In the cervical and thoracic regions, roots are short and almost horizontal (Photo1012). In the lumbosacral region, they are longer and almost vertical directed into the cauda equina. (Photo1011)
24. Follow the spinal cord inferiorly to where it ends at
L1-2 in a short cone-like taper called the conus medullaris.
(Figure115)
(Netter170)
(Netter169)
(Photo1011)
25. Gently separate the nerve roots at the base of the
spinal cord to find the filum terminale emerging from the end tip of the
conus medullaris.
(Figure115)
(Photo1040)
Dorsal and ventral nerve roots emerge from the spinal cord along the dorsolateral and ventrolateral margins of the spinal cord. The filum terminale is the only structure emerging directly at the midline.
The filum terminale is a thickening of connective tissue derived from the pia mater. It extends from the tip of the conus medullaris and runs caudally down the vertebral canal to anchor at the sacrum and coccyx.
26. Examine the dorsal surface of the spinal cord where you
may see a shiny surface formed by the single cell layer of the pia mater.
(Netter174)
It is not always easy to see due to the thin, delicate nature of the pia mater and variability in donor preservation.
If well preserved, you may be able to insert the point of a
sharp probe against the surface of the spinal cord and slowly raise the probe to
lift part of the pia. With care you may see the pia mater as it lifts appear
Arising from the pia mater laterally on the spinal cord are
small shelf-like triangular extensions of pia anchoring into the arachnoid.
These are the denticulate ligaments that, although delicate, provide lateral stability to the
spinal cord position in the vertebral canal.
27. Identify the approximate location of an intervertebral
foramen by observing the nerve roots laterally en-route to an intervertebral
foramen.
(Netter174)
If you look from inside the lumbar cistern you will see the nerve roots converge to a 'hole' in the wall of the dura.
If you reflect the dura and look at the outside you will observe the continuation of the roots in a meninges wrapped bundle towards their target intervertebral foramen.
This bundle proceeds into the intervertebral foramen where the dorsal root ganglion will be located.
Just distal to the dorsal root ganglion is the spinal nerve proper where ventral roots join with the dorsal roots emerging from the dorsal root ganglion. (Netter175)
D
V
Note, in a lab where you have used the Stryker saws remember to return your battery to one of the charger stations such that you will have a fully charged battery next time you need the instrument.
Post-Lab Daily Clinical Review Cases
Each lab has several multiple choice practice cases to reinforce understanding of the material within the lab and associated lectures. These are optional to review at home for consolidating and testing understanding.
CHECKLIST
Skeletal Structures
Vertebral column
Vertebra
Spinous process
Laminae
Transverse
process
Pedicles
Superior articular process & facet
Inferior articular
process & facet
Intervertebral disks
Intervertebral
foramen
Spinal curvatures
Joints
Costovertebral
joint
Costotransverse joint
Zygapophysial (facet) joint
Soft Structures
Ligaments
Interspinous
ligament
Supraspinous ligament
Epidural space
Meninges
Dura mater
Spinal cord
Conus medullaris
Filum terminale
Lumbar cistern
Cauda equina
Nerve rootlets