Cadaver Lab - Spinal Cord Anatomy

Muscular Reflection and Exposure of the Vertebral Column

• The initial phase of the laminectomy involves the bilateral removal of the erector spinae and transversus spinalis muscle groups.

• The dissection spans from vertebral level $T_4$ to level $S_3$.

• A horizontal incision is made using a scalpel at the level of $T_4$ to facilitate muscle removal.

• The superior edge of the cut muscles is elevated, and the entire muscle group is reflected in an inferior direction.

• As the reflection progresses inferiorly, the muscles must be detached from the vertebral column to ensure the laminae are cleanly exposed.

• Residual muscle fragments are removed from the laminae using scraping motions with a chisel.

Thoracic Laminectomy Procedure and Techniques

• The laminectomy procedure commences in the thoracic region, specifically targeting the vertebrae from $T_6$ to $T_{12}$.

• A chisel or power saw is utilized to cut the laminae on both the left and right sides of the spinous processes.

• To ensure wide exposure of the vertebral canal, the cuts must be executed at the lateral end of the laminae.

• The cutting instrument (chisel or saw) should be held at an angle of $45^{\circ}$ to the vertical plane to maximize the resulting exposure of the vertebral canal.

• Surgical caution is required to avoid cutting through the transverse processes, which would risk entering the thoracic cavity.

• Ligamentous management during thoracic removal:     - A scalpel is used to sever the interspinous ligament between $T_6$ and $T_7$.     - A separate cut is made in the interspinous ligament between $T_{12}$ and $L_1$.     - The interspinous ligaments between levels $T_7$ and $T_{12}$ are preserved to hold the intervening spinous processes together as a single unit upon removal.

• A chisel is used to pry the unit consisting of the spinous processes and their associated laminae out of the vertebral column.

• Care must be taken during prying to avoid damaging underlying nervous and meningeal structures. When performed correctly, the dura mater remains intact within the vertebral canal along with the spinal cord.

Ligamentous Connections of the Vertebral Column

• Observations made on the deep surface of the removed spinal specimen:     - Ligamenta Flava: These connect the laminae of adjacent vertebrae.     - Interspinous Ligaments: These connect the spinous processes of adjacent vertebrae.     - Supraspinous Ligament: This ligament connects the tips of all spinous processes from the sacrum up to the $C_7$ vertebral level.     - Ligamentum Nuchae: At the $C_7$ level, the supraspinous ligament blends with the ligamentum nuchae, which extends superiorly to the external occipital protuberance.

Lumbar and Sacral Laminectomy Refinements

• The laminectomy procedure is continued inferiorly through the lumbar and sacral regions down to vertebral level $S_3$.

• Constant direct observation of the exposed canal is necessary to guide correct cutting.

• Specific regional challenges:     - In the lower lumbar levels, the canal may sit at a significant depth due to the natural curvature of the vertebral column.     - In the lower lumbar and sacral regions, the vertebral canal curves sharply in a posterior (superficial) direction, necessitating increased caution.

• Sacral Incision: A V-shaped incision is made on the posterior surface of the sacrum. The inferior point of this wedge-shaped cut must terminate at vertebral level $S_3$.

• Safety Warning: When cutting the sacrum, do not drive the chisel or power saw too deeply, as the tool could penetrate the rectum located anteriorly.

• The spinous processes and laminae are pried out as a unit and placed in a tissue container, leaving the posterior surface of the dura mater visible from levels $T_6$ to $S_2$.

Examination of the Spinal Meninges and Spaces

• Epidural (Extradural) Space:     - This space is exposed once the laminectomy is complete.     - It contains adipose tissue (fat) and the posterior internal vertebral venous plexus.     - Identification may be difficult due to the embalming process; blunt dissection is required to remove these elements and clarify the space.

• Dura Mater (External Meningeal Layer):     - This is the outermost protective layer.     - The dural sac terminates inferiorly at vertebral level $S_2$.     - In the thoracic region, the dura is lifted with forceps and a small midline incision is made using scissors, then extended inferiorly to level $S_2$.

• Arachnoid Mater (Middle Meningeal Layer):     - Identified as very thin and delicate compared to the dura mater.     - The arachnoid is retracted laterally during the dural incision to prevent damage.     - Subarachnoid Space: Located deep to the arachnoid mater. In living individuals, this space contains Crebrospinal Fluid (CSF). In a cadaver, the CSF is absent as it drains during the embalming process.

• Pia Mater (Internal Meningeal Layer):     - This is the thinnest layer and completely invests the spinal cord.     - It lies directly on the surface of the spinal cord and cannot be dissected away from the nervous tissue.

Gross Anatomy of the Spinal Cord and Specialized Structures

• Lumbar Enlargement: Located at the lower thoracic vertebral levels. This region corresponds to spinal cord segments $L_2$ through $S_3$, which provide nerve supply to the lower limbs.

• Conus Medullaris (Medullary Cone): The tapered, inferior end of the spinal cord proper. It is located between vertebral levels $L_1$ and $L_2$.

• Cauda Equina: Named for its resemblance to a horse's tail, this is the collection of anterior and posterior nerve roots that surround the conus medullaris and descend within the dural sac below the end of the spinal cord.

• Filum Terminale:     - Filum Terminale Internum: A delicate filament of pia mater that arises from the tip of the conus medullaris and extends to vertebral level $S_2$.     - Filum Terminale Externum (Coccygeal Ligament): Formed when the filum terminale internum becomes encircled by the lower end of the dural sac at level $S_2$. It passes through the sacral hiatus to attach to the cocyx.

Nerve Root Arrangement and Internal Stabilization

• Denticulate Ligaments: These are two lateral extensions of the pia mater (one on each side) that anchor the spinal cord to the inner surface of the dura mater.     - Each ligament possesses $21$ teeth or attachments points.     - They provide lateral stable anchoring for the spinal cord.

• Nerve Roots:     - Posterior (Dorsal) Roots: Situated on the posterior side of the denticulate ligament. They are formed by the merging of several posterior rootlets.     - Anterior (Ventral) Roots: Situated on the anterior side of the denticulate ligament. They are formed by the merging of anterior rootlets.

• Vasculature: Small blood vessels course along the anterior and posterior roots. These are branches of the posterior intercostal, lumbar, or vertebral arteries depending on the level. They enter via the Intervertebral (IV) foramen to supply the spinal cord.

Anatomy of the Spinal Nerve and IV Foramen

• In the thoracic region, a spinal nerve is exposed by removing surrounding tissue.

• Safety Technique: A probe is placed into the IV foramen posterior to the nervous tissue to protect the nerve while overlying tissue is removed.

• Dorsal Root Ganglion (Spinal Ganglion): The posterior wall of the IV foramen is removed with bone cutters to reveal this structure. It contains the sensory cell bodies of the spinal nerves.

• Spinal Nerve Formation: The point lateral to the dorsal root ganglion where the anterior and posterior roots merge.

• Rami: The spinal nerve divides into two primary branches:     - Posterior Ramus: Supplies the deep muscles of the back and the overlying skin.     - Anterior Ramus: Responsible for the innervation of the anterolateral trunk and the limbs.