Suboccipital Triangle and Spinal Cord

Atlas References

Netter: 170, 174–178, 184
McMinn: 94–99, 104, 105
Gray's Atlas: 20–32, 36–42, 399, 400

Suboccipital Triangle

o
Make a midline skin incision from the spinous process of the 7th cervical vertebra (C7) to the external occipital protuberance ( Fig. 3.1 ).
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At the level of the external occipital protuberance, make a horizontal skin incision connecting the right and left mastoid processes.
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Reflect the skin and the subcutaneous fat as one layer.
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Beneath the subcutaneous tissue, a connective tissue layer covers the upper portion of the trapezius muscle ( Fig. 3.2 ); dissect away this layer and expose the trapezius ( Fig. 3.3 ).
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While cleaning out the fascia that overlies the most cephalic portion of the trapezius muscle (see Fig. 2.10 ), look for the greater occipital nerve ( Fig. 3.4 ).
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Also at this location, locate the occipital artery and preserve it as the trapezius is reflected.

Anatomy Note

The greater occipital nerve usually can be found about 1 inch (2.5 cm) from the midline of the neck and 1 inch inferior to the superior nuchal line, as this nerve pierces the trapezius muscle (see Fig. 2.11, Fig. 2.12 ).

Dissection Tip

Usually, the deep fascia over the trapezius muscle, below the superior nuchal line, is thick and difficult to cut until the C7 level. About 1 cm lateral to the midline, the 3rd occipital nerve (medial branch of dorsal ramus of C3 spinal nerve) is seen along the nuchal ligament (ligamentum nuchae) intermingled with the deep fascia over this area (see Figs. 3.3 and 3.4 ). Generally, the 3rd occipital nerve is very small and often is cut during routine dissections.

o
Identify the greater occipital nerve and the 3rd occipital nerve , and preserve them after reflecting the trapezius away from its cranial and cervical attachments. Carefully reflect the trapezius muscle from its cranial and cervical attachments ( Figs. 3.5 to 3.7 ).

Dissection Tip

The trapezius is thin at its cephalic and cervical attachments (see Fig. 3.6 ). Be careful during its reflection. While reflecting the trapezius up from its distal end, keep the scalpel blade facing downward toward the vertebrae, not upward, in order to prevent damage to the muscle.

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The small amount of connective tissue between the trapezius and splenius capitis muscles can be removed.
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At the lateral border of the splenius capitis, identify the lesser occipital nerve (see Fig. 3.7 ).
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Identify the splenius capitis and splenius cervicis muscles; divide them at their origins from the spines of the cervical and upper thoracic vertebrae and reflect them laterally, exposing the semispinalis capitis muscle ( Figs. 3.8 through 3.10 ).
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Cut the attachment of the semispinalis capitis muscle from the skull and the ligamentum nuchae to reveal the suboccipital triangle. Preserve the greater occipital and lesser occipital nerves as the semispinalis muscle is reflected.

Dissection Tip

Make one small cut on the semispinalis capitis muscle medial to the greater and lesser occipital nerves ( Fig. 3.11 ). In this way, the semispinalis capitis muscle easily can be reflected laterally without disturbing the course of the nerves ( Fig. 3.12 ).

Fig. 3.11, Make an incision into the semispinalis capitis muscle near the midline down to approximately the T1 level. Preserve the greater and 3rd occipital nerves.

Fig. 3.12, Reflected trapezius, splenius capitis, and semispinalis capitis muscles.

o
Trace the occipital nerves down through the underlying fat and connective tissue, clearing away the connective tissue using the “separating scissors technique” ( Fig. 3.13 ).

Dissection Tip

Even after the semispinalis capitis muscle is reflected, generally the boundaries or contents of the suboccipital triangle are not immediately apparent and are hidden by overlying connective tissue. Remove these tissues carefully ( Figs. 3.14 and 3.15 ).

Fig. 3.14, Dissection of suboccipital triangle demonstrating the rectus capitis posterior major muscle and greater and 3rd occipital nerves. Remove all fat and connective tissue.

Fig. 3.15, Dissection of suboccipital triangle revealing surrounding muscles, including the rectus capitis posterior major and minor and semispinalis capitis muscles.

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Place the tip of your finger into the suboccipital triangle to locate the posterior arch of the atlas.
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After removal of the connective tissue, identify the muscles that form the sides of the suboccipital triangle: inferior capitis oblique, superior capitis oblique, and rectus capitis posterior major ( Fig. 3.16 ) ( Plate 3.1 ).

Anatomy Note

The rectus capitis posterior major is found superficial to the rectus capitis posterior minor. Although the rectus capitis posterior minor is considered a “suboccipital” muscle, it does not contribute to the margins of the suboccipital triangle.

Dissection Tip

Use fine scissors and take your time to expose and clearly demonstrate the vertebral artery. Besides the connective tissue over the vertebral artery, a rich venous plexus is also present. Clean away the connective tissue and venous plexus. You will see only the posterior wall of the vertebral artery.

o
Expose the posterior arch of the atlas, and identify a tough connective tissue layer joining the posterior arch of the atlas to the skull, the posterior atlanto-occipital membrane. Clean the loose connective tissue away from the arch and expose the vertebral artery, which lies in the sulcus of the posterior arch of the atlas. Between the sulcus of the posterior arch and the vertebral artery, identify the suboccipital nerve, which innervates the suboccipital muscles and overlying semispinalis capitis muscle ( Fig. 3.17 ).
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Using a scalpel, cut the rectus capitis posterior major and inferior capitis oblique muscles from the spinous process of the axis, and reflect them laterally ( Fig. 3.18 ).
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Note the exit of the greater occipital nerve emerging from even deeper tough connective tissue ( Fig. 3.19 ).
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Remove this connective tissue surrounding the greater occipital nerve, and expose its exit from the dura mater ( Fig. 3.20 ).
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Retract the rectus capitis posterior minor superiorly, and expose the dorsal root ganglion of the 2nd cervical (C2) spinal nerve ( Fig. 3.21 ).

Laminectomy

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In preparation for the laminectomy, reflect or cut away the muscles of the back from the spines and transverse processes of the vertebrae as completely as possible using a scalpel, scissors, and chisel ( Fig. 3.22 ).
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Clean away as much as possible all of the intrinsic muscles of the back from the laminae of the lower cervical, thoracic, and upper lumbar regions ( Fig. 3.23 ).
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With a bone saw or chisel and mallet, cut through the laminae longitudinally from the lower cervical to the lower lumbar regions ( Figs. 3.24 through Fig. 3.26 ).
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Angle the saw or chisel laterally.

Dissection Tip

If using a chisel and mallet, perform the surface fracturing technique. Short tapping blows of the mallet are used until the lamina is felt to fracture. Do not allow the chisel to be driven too deeply, or injury to the spinal cord or its rootlets will occur.

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Direct the blade of the saw or chisel anteromedially to avoid cutting the spinal nerves (see Fig. 3.25 ) ( Plate 3.2 ).
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Complete the dissection bilaterally (see Fig. 3.26 ).
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Using toothed forceps, gently lift spinous process–lamina unit away from spinal canal, revealing dura mater, dorsal root ganglia, and spinal cord ( Fig. 3.27 ).
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Remove the spinous process–lamina unit from the lumbar region, exposing dura mater ( Fig. 3.28 ).

Dissection Tip

To facilitate the process, several laminae can be removed together as a block (see Figs. 3.26 to 3.28 ). After excision of the vertebral laminae and spinous processes, the vertebral canal may not be exposed widely enough for clear visualization of its contents. If this is the case, remove additional bone as necessary with bone rongeurs or with a mallet and chisel. Exercise particular care in the regions of the intervertebral foramina to avoid cutting or tearing away the spinal nerve rootlets. Sharply pointed edges of bone may be present in the dissection field after the laminectomy is completed. Identify any sharp spicules and remove them to avoid injuries to your hands.

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Remove the spinous processes and the laminae en bloc from the spinal canal ( Fig. 3.29 ).
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Note the contents of the spinal canal: the epidural fat and the vertebral venous plexus of Batson ( Fig. 3.30 ).
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Remove the fat and the venous plexus from the epidural space to expose the dura mater. Lift a part of the dura in the midline, and make a small, slit-like incision with scissors or a scalpel ( Fig. 3.31 ).
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Continue the midline incision throughout the entire length of the dura mater ( Fig. 3.32 ).
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If possible, avoid incising the underlying arachnoid layer by lifting and maintaining tension on the dura mater. Reflect the dura laterally to expose the contents of the dural sac. If the incision in the dura is made successfully, the arachnoid will appear as a thin, almost transparent layer (see Fig. 3.32 ).

Dissection Tip

Sometimes, arachnoid calcifications (plaques) may be present ( Fig. 3.33 ). These are usually incidental findings and have been attributed to trauma, myelography, subarachnoid hemorrhage (see Fig. 3.32 ), and spinal anesthesia.

o
Identify the spinal cord and its covering of pia mater. Observe the beginning of the filum terminale at its origin from the conus medullaris, the terminal part of the spinal cord. Note the cluster of nerve roots on either side of the conus medullaris, the cauda equina ( Fig. 3.34 ; see also Figs. 3.31, 3.32, and 3.38 ).
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Transect and remove the spinal cord with its dural covering ( Fig. 3.35 ).
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Look for the tooth-like denticulate ligaments on each side of the spinal cord, lifting the cord carefully with forceps for inspection. These projections occur at the level of each vertebra, from the occipital bone to the last thoracic spinal nerve. The denticulate ligaments pierce the arachnoid mater to attach to the dura mater ( Figs. 3.36 and 3.37 ).
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Identify the posterior longitudinal ligament on the vertebral bodies after the removal of the spinal cord and dura mater ( Fig. 3.38 ).