Photo 1. Superficial back musculature, reflected

3.) Using sharp dissection, incise on either side of the spinous processes. 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. Procedural: planned incisions

4.) 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

5.) Scrape remaining musculature from the laminae with a chisel, to allow clean and precise cuts for the laminectomy. Use care when clearing the posterior aspect of the sacrum as the bone in this area is delicate.

Photo 5. Laminae, prepped for laminectomy

6.) Observe supraspinous ligaments connecting the apices (posterior tips) of adjacent spinous processes of vertebrae, and interspinous ligaments adding support between adjacent spinous processes.

Note: A variety of ligaments stabilize the vertebral column. Providing direct support to the bodies of the vertebrae and intervertebral discs are the anterior & posterior longitudinal ligaments. The posterior ligamentous complex -- consisting of the supraspinous & interspinous ligaments, ligamenta flava, and facet joint capsules -- support the laminae, spinous processes, and facet joints.

Note: Supraspinous ligaments are typically only present from C7 to L4, and are frequently sparse in areas. Supraspinous ligaments tend to connect 3-4 serial spinous processes, and they tend to blend with interspinous ligaments. Above C7, the ligamentum nuchae connects the apices of adjacent spinous processes. Below L4, supraspinous ligaments are functionally replaced by posterior thoracolumbar aponeurosis.

Note: 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. The interspinous ligaments differ with respect to region of the spine. Among cervical vertebrae, interspinous ligaments are typically absent. Among thoracic vertebrae, interspinous ligaments are gracile. Among lumbar vertebrae, interspinous ligaments are robust and often paired. These ligaments are more visible post-laminectomy.

Photo 6. Interspinous and supraspinous ligaments

Perform a laminectomy

Find these structures:

• Ligamenta flava

7.) Perform a laminectomy to 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, a chisel and mallet, or bone shears (once inside the vertebral canal). Locate the vertebra 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). After all cuts are completed, remove the posterior wall of the vertebral canal. Examine the anterior surface of the posterior wall of the vertebral column to see the ligamenta flava.

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

Note:

• Be sure to wear the appropriate personal protective equipment (PPE) for any type of bone cutting. This includes protective eyewear. When using an autopsy saw, a combination of protective eyewear and face mask must 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 of spinal nerves.

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

Photo 7. Vertebral column

Photo 8. Procedural: planned cuts

Photo 9. Procedural: planned cuts

Note: Do NOT use the autopsy saw to remove the posterior sacrum. The bone is very thin here, and removal can be completed by careful removal with bone shears, chisels, and hemostats.

Note: At the sacrum, do not cut too far laterally, as dorsal primary rami of spinal nn. exit the posterior sacral foramina in this area.

Photo 10. Procedural: planned cuts

Note: Predominantly composed of yellow elastic tissue, ligamenta flava (sing. = ligamentum flavum; Latin = yellow ligament) are part of the posterior ligamentous complex. Ligamenta flava span the distances between adjacent laminae within the vertebral canal. Much like the interspinous ligaments, ligamenta flava become increasingly more robust in the more inferior portions of the vertebral column.

Photo 11. Ligamenta flava

Note: 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.

Identify features of meninges, spinal cord, and spinal nerve root

Find these structures:

• Epidural fat

• Posterior internal vertebral venous plexus

• Dura mater

• Filum terminale externum/Coccygeal ligament

8) If present, examine the epidural fat and posterior internal vertebral plexus. These structures are superficial to the dura mater. 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: There are four vertebral venous plexuses (anterior & posterior external, anterior &  posterior internal). External vertebral venous plexuses are found outside of the vertebral canal, whereas internal vertebral venous plexuses are located within the canal.These plexuses anastomose, and they are difficult to distinguish by any means other than location. Anastomoses of the vertebral plexuses occur in and around the intervertebral foramina.

Photo 12. Epidural fat and posterior internal vertebral venous plexus

Note: The nervous system is anatomically divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, whereas the PNS consists of twelves pairs of cranial and thirty-one pairs of spinal nerves (and their components), and portions of autonomics.

Note: The CNS (and proximal portions of the PNS) are surrounded by the meninges: three layers of connective tissues which provide physical protection and support to structures contained within. The meninges consist of (superficial-to-deep): dura mater, arachnoid mater, and pia mater.

Note: The dura mater is composed of dense irregular connective tissue, and is the outermost covering of the CNS. The spinal dura differs from the cranial dura in that the spinal dura is a single layer, whereas the cranial dura is a bilayer.

Photo 13. Dura mater, intact

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 termination of the dural sac. 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 14. Filum terminale externum

Find these structures:

• Arachnoid mater

• Subarachnoid space

• 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.

• Posterior spinal vv.

9.) Make a shallow midline incision using scissors through the dura mater. Stay in the midsagittal plane, and avoid incising too laterally to protect elements of spinal nn. Observe the arachnoid mater. This will appear as thin wisps between the dura mater and pia mater. Observe the pia mater. It is translucent, and is very difficult to separate from the spinal cord.

Photo 15. Procedural: incising the dura mater

Photo 16. Procedural: dura mater, reflected

Note: In vivo, the bulk of the arachnoid mater adheres superficially to the dura mater, with extensions of the arachnoid anchoring these layers to the pia mater on the surface of the CNS (the space amongst these extensions, superficial to the pia mater, is known as the subarachnoid space). Cerebrospinal fluid (CSF) is found in the subarachnoid space. Often as a result of embalming, the entirety of the arachnoid mater may delaminate from the dura, and be found on the pia mater.

Note: A potential space, the subdural space, exists between the dura mater and the arachnoid mater. Trauma, hemorrhagic disorders, vascular malformations, anticoagulant therapy, or iatrogenic injury may lead to hemorrhages that fill actual spaces, or open potential spaces (Gordon et al. 2014). If a bleed is in the vertebral canal outside of the dura mater, it is an epidural (or extradural) hemorrhage. If a bleed is between the dura mater and arachnoid mater, it is a subdural hemorrhage. If a bleed is within the subarachnoid space, it is a subarachnoid hemorrhage. Blood within the substance of the spinal cord is known as hematomyelia.

Photo 17. Arachnoid mater

Note: Spinal nerves exit the spinal cord at regular intervals as dorsal and ventral rootlets, which coalesce into dorsal and ventral roots, which join to form the trunk of the spinal nerve. The dorsal root is most obvious as it is associated with a mass of cell bodies known as the dorsal root ganglion (DRG). Dorsal roots (and rootlets) are entirely afferent (sensory), whereas ventral roots (and rootlets) are efferent (motor). The trunk of the spinal nerve exits the vertebral canal laterally via an intervertebral foramen and almost immediately bifurcates into a dorsal primary ramus (DPR) and a ventral primary ramus (VPR). These rami may further bifurcate along their courses.

Note: From your view, dorsal rootlets should be evident at each level of the spinal cord, extending inferolaterally into dorsal roots.

Note: The level of the spinal cord from which spinal nerve arise (via rootlets, then roots) does not necessarily align laterally with commensurately numbered intervertebral foramina. For instance, spinal n. C1 exits the vertebral canal between the occipital bone of the cranium and the first cervical vertebra (C1). Each successively numbered cervical spinal nerve (C2-C8) will exit the vertebral canal via the intervertebral foramen of the cervical vertebra above it (i.e. nerve C2 exits the canal through the intervertebral foramen between vertebrae C1 & C2). Spinal nerves of the thoracic and lumbar regions of the spinal cord exit the vertebral canal via intervertebral foramina below their commensurately numbered vertebrae (i.e. T1 nerve exits below the T1 vertebra, T12 n. exits below T12 vertebra, etc.).

Photo 18. Dorsal rootlets and roots

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

• 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.

• 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 19. Pia mater

Photo 20. Filum terminale internum

Photo 21. Denticulate ligaments

Note: The cervical enlargement of the spinal cord extends from C4-T1 segments, and lumbosacral enlargement extends from T11-S1 segments. These enlargements reflect an increased number of lower motor neurons servicing the upper (cervical) and lower (lumbosacral) limbs.

Photo 22. Cervical and lumbosacral enlargements

Note: 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. are found on the posterior portion of the spinal cord. The veins are typically more obvious than the arteries.

Photo 24. Posterior spinal arteries and veins

Find these structures:

• Anterior spinal a

• Anterior spinal vv.

• Posterior longitudinal ligament

• Intervertebral discs

• Grey matter

• White matter

• Ventral horns

• Dorsal horns

• Lateral horns (intermediolateral nucleus = IML)

10.) In the thoracic region, posteriorly elevate 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. 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. 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. If distinguishable, further investigate the grey matter to distinguish the ventral, dorsal, and lateral (containing the IML) horns.

Photo 25. Procedural: reflection of spinal cord

Photo 26. Anterior spinal artery and veins

Note: Gross tissue of the CNS is referred to as either white or grey matter. White matter is predominantly comprised of axon tracts and commissures. Grey matter is predominantly comprised of cell bodies, dendrites, and capillaries.

Photo 27. Grey and white matter

Note: Grey matter of the spine is organized into horns: dorsal, ventral, and lateral. 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 division of the autonomic nervous system), but is also present at levels S2-S4 (associated with the parasympathetic division of the autonomic nervous system).

Photo 28. Horns of the grey matter

11.) With the spinal cord reflected, the anterior portion of the dura mater is visible. Remove dura mater 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.

Note: The posterior longitudinal ligament (PLL) supports the stacked vertebral bodies and intervertebral discs posteriorly. The PLL extends from the axis (C2) to the sacrum, and fuses with each anulus fibrosus of the intervertebral discs. The PLL is the most anterior feature of the vertebral canal.

Note: Intervertebral discs are found between vertebrae, and consist of two parts: the anulus fibrosus and the nucleus pulposus. The anulus fibrosus consists of collagen and fibrocartilage, and surrounds the nucleus pulposus. The nucleus pulposus, a remnant of the notochord, is an association of hydrated ground substance and collagen. As an individual ages, the nucleus pulposus becomes more homogenized with the anulus fibrosus.

Note: Intervertebral discs are separated by bone superiorly and inferiorly by articular (hyaline) cartilage. These types of midline joints consisting of fibrocartilage sandwiched between articular cartilage are known as symphyses.  

Photo 29. Posterior longitudinal ligament and intervertebral discs

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

Find these structures:

• Dorsal root ganglion

• Trunk of spinal n.

• Dorsal primary ramus of spinal n.

• Ventral primary ramus of spinal n.

12.) 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 processes and facet joints of the vertebrae) must 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

13.) Continue dissecting laterally to see the trunk, dorsal primary ramus (DPR), and ventral primary ramus (VPR) of the spinal n.

Note: The trunk is very short and almost immediately bifurcates into the dorsal and ventral primary rami.

Note: Ventral primary rami (VPR) and dorsal primary rami (DPR) are mixed divisions of a spinal nerves. Not visible at this time are the rami communicantes (branches to and from the VPR and sympathetic trunk). VPR serve anterior and lateral aspects of the trunk and limbs, whereas DPR serve deep muscles of the back and skin of the back. VPR are larger than DPR and typically form named nerves.

Photo 31. Spinal nerve