S2 Dissection Summary

Part 1

• • Carefully relieve the attachments of the rhomboid major & minor mm. from the inferior portion of the ligamentum nuchae and the spinous processes of vertebrae C7-T5.

  • Bilaterally reflect the superficial back musculature (trapezius mm. and rhomboid mm.) from medial to lateral to prepare for the laminectomy (removal of the laminae of the vertebrae to open the vertebral canal).

  • 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.

  • 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.

  • 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.

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

Part 2

• • 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.

Part 3

• • 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.

  • 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.

  • 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.

  • 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.

Part 4

• • 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.

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