LabLink

Session 2: Deep Back & Spinal Cord

Page: Preparing the back for deep dissection

Find these structures:

Check structures as you find them on the donor.

• • Laminae

  • Spinous processes

  • Supraspinous ligaments

  • Interspinous ligaments

1.) On each side of the spinous processes, move all of the superficial back musculature dissected in Lab One laterally.

Photo 1. Superficial back musculature, reflected

2.) Using sharp dissection, perform two mirroring parasagittal incisions as close as possible to the midline. Begin superiorly with two incisions just lateral to the external occipital protuberance, continue lateral to the spinous processes of all vertebrae, and end the incisions on either side of the tip of the coccyx.

Photo 2. Parasagittal incisions

Pull and reflect the serratus posterior mm., splenius mm., erector spinae mm., and deep back (transversospinalis) mm., and thoracolumbar aponeurosis laterally 8-10 cm (approximately the width of a hand). This deep musculature may be removed if the muscles will not stay in a reflected position.

Photo 3. Deep back musculature

Photo 4. Posterior laminae

Scrape remaining musculature from the laminae with a chisel, which will allow clean and precise cuts for the laminectomy.

Use care when clearing the posterior aspect of the sacrum. This area is more delicate.

Photo 5. Laminae, prepped for laminectomy

Observe the supraspinous ligaments connecting the apices (posterior tips) of adjacent spinous processes. Supraspinous ligaments are typically only present from C7 to the sacrum. In the cervical region, the thick nuchal ligament connects the apices of adjacent spinous processes.

Interspinous ligaments connect one spinous process to another from the root of the spinous process to the apex, and are best visualized from a lateral view. These ligaments may be more visible post-laminectomy.

Photo 6. Interspinous ligaments and supraspinous ligaments

Page: Open the vertebral canal by removing laminae and spinous processes.

Find these structures:

Check structures as you find them on the donor.

• • Ligamenta flava

3.) Perform a laminectomy, and remove the posterior wall of the vertebral canal (vertebral laminae and spinous processes) and sacral hiatus. You may opt to use either an autopsy saw, or a chisel and mallet.

Locate the spina prominens (C7). This will help with orientation for the rest of the procedure. Plan to remove sections of posterior wall of the vertebral canal in many sections.

Begin with a section of approximately five thoracic vertebrae (T1-T5).

Photo 7. Posterior vertebral column

Make two cuts per vertebra. Cuts should be on the lateral-most extent of the lamina before the transition to the transverse processes at an angle. Please refer to the photos and notes below to better understand this procedure.

Photo 8. Laminae incisions

Photo 9. Laminae incisions

Notes:

• • Be sure to wear the appropriate personal protective equipment (PPE) for any type of bone cutting. This would include protective eyewear. When using an autopsy saw, a combination of protective eyewear and face mask should be worn.

  • Be very deliberate with cuts, and frequently refer to articulated skeletal specimens in the laboratory before and during this stage of the dissection.

  • As you cut, take care not to cut too deeply, which may endanger the roots, trunks, or rami of the spinal nerves.

  • If done properly, this portion of the dissection will require the greatest amount of your lab time and effort.

After all cuts are completed, remove the posterior wall of the vertebral column.

Examine the anterior surface of the posterior wall of the vertebral column to see the ligamenta flava.

Photo 11. Ligamenta flava

You may choose to widen the window into the vertebral canal by trimming any remaining laminae with bone shears. Be sure to allow enough room to view the spinal cord and meninges in their entirety.

Page: Identify features of meninges, spinal cord, and spinal nerve roots.

Find these structures:

Check structures as you find them on the donor.

• • Epidural fat

  • Posterior internal vertebral venous plexus

  • Dura mater

  • Filum terminale externum/Coccygeal ligament

4.) Examine the epidural fat and posterior internal vertebral plexus, if present. These structures are superficial to the dura mater.

Note: During a laminectomy, these structures are sometimes removed with the laminae. If this occurs, observe these features on another donor or prosection.

Note: There are four vertebral venous plexuses (anterior & posterior external, anterior &  posterior internal). These plexuses anastomose and are often difficult to distinguish by any means other than location. Anastomoses occur in and around the intervertebral foramina.

Photo 12. Epidural fat and posterior internal vertebral venous plexus

After identification of these structures, remove the epidural fat and posterior internal vertebral venous plexus to reveal the dura mater.

Observe where the dura mater terminates: approximately at the level of the second sacral vertebra (S2). The filum terminale externum/coccygeal ligament should extend inferiorly from this level, through the sacral hiatus, to the tip of the coccyx.

Note: The filum terminale has two parts: internum and externum. The filum terminale internum is an extension of the pia mater after the conus medullaris (L2) which extends approximately to the coccyx. Its function is to anchor the spinal cord to the coccyx. The filum terminale externum is the dural part of the filum terminale, and is often referred to as the coccygeal ligament. After the dural sac terminates at S2, the externum envelopes the internum to attach to the coccyx. The filum terminale externum serves to anchor the dural sac to the coccyx.

Photo 13. Dura mater, intact

Photo 14. Filum terminale externum

Find these structures:

Check structures as you find them on the donor.

• • Arachnoid mater

  • Pia mater

  • Dorsal rootlets and roots (of spinal nn.)

  • Ventral rootlets and roots (of spinal nn.)

  • Filum terminale internum

  • Denticulate ligaments

  • Cervical enlargement

  • Lumbosacral enlargement

  • Conus medullaris

  • Cauda equina

  • Posterior spinal aa. & vv.

5.) Make a shallow midline incision using scissors through the dura mater. Stay in the midsagittal plane, and avoid incising too laterally to protect rootlets and roots of spinal nn.

Photo 15. Midsagittal incision

Photo 16. Dura mater, reflected

Observe the arachnoid mater. This will often appear as thin wisps between the dura mater and pia mater.

Photo 17. Arachnoid mater

Dorsal rootlets should be evident at each spinal cord level, extending inferolaterally into dorsal roots.

Photo 18. Dorsal rootlets and roots

Observe the pia mater. It is translucent on the spinal cord, and it is very difficult to separate from the cord.

Photo 19. Pia mater

The pia mater has several specializations that should be observed here:

• • Filum terminale internum - can be observed at the tip of the conus medullaris (~L2), traveling inferiorly through the dural sac, and enveloped by the filum terminale externum from S2 approximately to the coccyx. The filum terminale internum is opaque white compared to the surrounding cauda equina.

Photo 20. Filum terminale internum

• • Denticulate ligaments - paired, extensions of pia mater that are located between the dorsal and ventral rootlets and roots of spinal nn., and also attach to the dural sac. There are 20-22 denticulate ligaments. These structures are opaque white compared to the rootlets and roots of spinal nn.

Photo 21. Denticulate ligaments

Observe the various structures and landmarks of the spinal cord. The cervical enlargement of the spinal cord extends from C4-T1 segments, and lumbosacral enlargement extends from T11-S1 segments.

Photo 22. Cervical and lumbosacral enlargements

The conus medullaris is the inferior end of the spinal cord and is typically located between T12-L3. It tapers to a point and is surrounded by the cauda equina - roots of spinal nn. (lumbar and sacral).

Photo 23. Conus medullaris and cauda equina

Note the small, paired posterior spinal aa. and three posterior spinal vv. on the lateral portions of the spinal cord. The veins are typically more obvious than the arteries.

Photo 24. Posterior spinal arteries and veins

Find these structures:

Check structures as you find them on the donor.

• • Anterior spinal a. & vv.

  • Posterior longitudinal ligament

  • Intervertebral discs

  • Grey matter

  • White matter

  • Ventral horns

  • Dorsal horns

  • Lateral horns (contain intermediolateral nucleus = IML)

6.) In the thoracic region, posteriorly lift the spinal cord and make a transverse cut through the spinal cord using scissors. Continue to gently lift the spinal cord (without damaging rootlets) until a view of the anterior portion of the cord is possible.

Photo 25. Anterior spinal cord

This will allow a view of the anterior spinal a. and vv. Unlike the posterior spinal aa., the anterior spinal a. is unpaired and is located on the midline of the spinal cord. There are typically three anterior spinal vv.

Photo 26. Anterior spinal artery and veins

Look at the cut edge of the spinal cord. Try to distinguish between the grey and white matter. This is often difficult to distinguish in embalmed cadavers.

Photo 27. Grey and white matter

If distinguishable, further investigate the grey matter to distinguish the ventral, dorsal, and lateral (containing the IML) horns.

The ventral horn is the largest of the horns.

The lateral horn is most obvious at T1-L2 spinal cord levels (associated with the sympathetic nervous system), but is also present at levels S2-S4 (associated with the parasympathetic nervous system).

Photo 28. Horns of the grey matter

With the spinal cord reflected, the anterior portion of the dura mater is visible. Remove dura to locate the the posterior longitudinal ligament, which is visible on the posterior portion of the vertebral bodies and intervertebral discs. The intervertebral discs are deep to the posterior longitudinal ligament between the vertebral bodies.

Photo 29. Posterior longitudinal ligament and intervertebral discs

Page: Dissect and locate dorsal root ganglion, trunk of the spinal n., and primary rami.

Find these structures:

Check structures as you find them on the donor.

• • Dorsal root ganglion

  • Trunk of spinal n.

  • Dorsal primary ramus of spinal n.

  • Ventral primary ramus of spinal n.

At one (or more) spinal cord level, expose the dorsal root ganglion, trunk of the spinal n., and dorsal and ventral primary rami of the spinal n.

This is completed by following the dorsal root through the dural root sheath.

Additional bone (transverse process of the vertebrae) may need to be removed using bone shears to identify structures located within and lateral to the intervertebral foramen.

At the level of the intervertebral foramen, the dorsal root ganglion will be visible as a bulge on the dorsal root.

Photo 30. Spinal nerve

Continue dissecting laterally to see the trunk of the spinal n. The trunk is very short and almost immediately branches into the dorsal and ventral primary rami.

Photo 31. Spinal nerve