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Lab 3 - Laminectomy and Ventral Body Wall

Pre-lab Exercise: Spinal Cord and Ventral Body Wall

Primary Lab Objectives and Goals:


  1. Retract epaxial back muscles laterally; review the relationships between the superficial hypaxial muscles and the 3 layers of the deeper epaxial muscles.

  2. Perform laminectomies between vertebral levels T8-L4. This process will expose the spinal cord and also help you understand the relationship between the spinal cord, dural sacs, and specific parts of the vertebrae.

  3. Examine the spinal meninges, spinal cord, and spinal nerves. Understand how the spinal cord is anchored and how the spinal nerves exit the vertebral canal and dural sac.
  4. Examine the surface anatomy of the body wall and the intercostal spaces. This will be a chance to understand a common pathway for the ventral rami of the spinal nerves as well as blood flow in the body wall.
  1. Examine female pectoral anatomy with a particular emphasis of anatomy and lymphatics of the breast.

  2. Identify and reflect the superficial thoracic muscles.  These are upper limb muscles (outgrowths of the body wall). They connect the upper limbs to the thorax.

NOTE: Steps 5 and 6 should be completed if at all possible.  But they can wait until the beginning of the next lab as well.

 

Procedure:

1. PREPARE VERTEBRAL COLUMN.

  • Using a scapel, blunt probe, and your finger, mobilize and reflect erector spinae laterally as far as possible.
  • Remove the intermediate and deep epaxial musculature (e.g. the multifidi, semispinalis thoracis, intertransverarii, interspinales) and expose the vertebral arches and transverse processes from about T8-L4.
  • The expaxial muscles in the region T8-L4 should be completely cleared form the bone in order to complete the laminectomy.

 

2. LAMINECTOMY.

  1. Use a chisel and hammer to cut through the laminae on each side of the exposed vertebrae from about T8 to L4.
  2. Note - It is very common to inadvertently take off the transverse processes of the vertebrae when you are doing the laminectomy. Be sure to clear away as much of the muscle as you can. Then identify the transverse processes, which are projecting laterally or posterolaterally from the sides of the vertebra. You should place your chisel at the junction of the transverse process and the lamina. Angle the chisel slightly medially and give it a substantial (but not too substantial) whack with the hammer. This is a challenging process and we have provided a video here to give you some pointers.

  3. Once you have broken through the lamina on each side, cut through the ligaments (ligamenta flava and interspinous ligament) that connect the arches and spinous processes above T8 and below L4.
  4. Remove the freed segment(s) of vertebrae, exposing the dural sac of the spinal cord.

step2 figure

 

 3. DURAL SAC

  1. Examine the dural sac in place, following it out into the intervertebral foramen.
  2. Using sharp-pointed scissors carefully cut a longitudinal slit through the dura and arachnoid mater. The dura and arachnoid matter usually cannot be separated, although there is a potential space between them.

    You have entered the subarachnoid space where you will see the spinal cord and dorsal and ventral roots. This is the region where cerebrospinal fluid circulates.

  3. Identify the denticulate ligaments (see figure below) running through the subarachnoid space

    FUNCTIONAL ANATOMY:
    The denticulate ligaments are lateral tooth-shaped extensions of pia mater that penetrate the arachnoid and fuse to the dura mater to help anchor the spinal cord within the vertebral canal.

Atlas Images:

 

 

  1. Study the way in which the dorsal and ventral roots of each spinal nerve are formed by the fusion of multiple rootlets from the spinal cord.
  2. Note that their dural and arachnoid covering extends outward into the intervertebral foramen as far as the point where the two roots themselves fuse to form a mixed spinal nerve.
  3. Try to locate the dorsal root ganglion in the intervertebral foramen, just medial to where the two roots join to form the mixed spinal nerve.

    Note: You have now exposed most of the constituents of a typical spinal nerve, from the dorsal ramus back to the component rootlets that coalesce to from the dorsal and ventral roots.

 

 

  1. Study the way in which the cauda equina is formed from a series of rootlets.
  2. Determine where the spinal cord ends, and locate the filum terminale, a central section of pia matter running from the conus medullaris (the end) of the spinal cord into the sacrum.
  3. Transect a portion of the spinal cord in the T8 to T11 region and remove it so you can see the cross-section.
  4. Identify the white and grey matter and the ventral and dorsal horns.

Atlas Images:

 

 

4. BODY WALL AND INTERCOSTAL SPACE

  1. Reflect the scapula and attached serratus anterior laterally, exposing the posterior (dorsal) ends of the intercostal spaces.
  2. If necessary, dissect away iliocostalis and longissimus on one side to expose the ribs.
  3. At this end of the intercostal space, the internal intercostal muscle becomes a ligamentous internal intercostal membrane. The ventral ramus of the spinal nerve therefore can be seen here by cutting through the external intercostal muscle and the underlying membrane of the internal intercostal.

     

    Note: the intercostal nerve and vessels lie between the internal intercostal and the innermost intercostal muscles. The main branches are tucked into a groove on the anterior inferior surface of each rib. Note the relationship between the vein, artery, and nerve in the intercostal space. Proceed carefully here and use a blunt probe so that you do not accidentally sever these structures.

  4. Expose the ventral ramus and trace it laterally, identifying its accompanying posterior intercostal vessels.

    The ventral ramus cannot easily be traced medially toward the spinal cord, because it emerges deep to the ligaments which join the rib to the vertebral transverse process.

step 3 figure

Atlas Images:

 

Surface Anatomy Exercise

At this point, you may choose to do this optional surface anatomy exercise regarding the bony landmarks of the thorax and the the anatomy of heart auscultation.

 

 

5. FEMALE PECTORAL ANATOMY: On female cadavers only (students with male cadavers should work with another group to see this dissection).

  1. Working as a team, roll the cadaver over so that it is supine.
  2. The mammary glands are large modified sweat glands whose main substance lies in the superficial fascia of the pectoral region.
  3. Make an incision through the nipple and observe dense bands of connective tissue separating lobules filled with fat.
  4. Identify the mammary glands, lactiferous ducts and sinuses, and suspensory ligaments.

In the aged (non-cycling) female breast, duct and glandular tissue is not easily observed.

Atlas Images:

 

 

6. GENERAL PECTORAL ANATOMY

  1. If you did not already do this in step 5 above, working as a team, roll the cadaver over so that it is supine.
  2. Make a midline incision through the skin from the top of the sternum to the xiphoid process.
 

 

  1. Reflect the skin and superficial fascia to both sides.
  2. Identify the pectoralis major muscle and its attachements. Both the pectoralis major and the underlying pectoralis minor move the upper limb.
  3. Note – Pectoralis major is innervated by the medial and lateral pectoral nerves, named for where they branch off the cords of the brachial plexus. Pectoralis minor is innervated by only the medial pectoral nerve.
  4. Transect (cut) the pectoralis major along the attachments to the sternum and clavicle. Reflect it laterally.
  5. Look for the medial pectoral nerve running between pectoralis major and pectoralis minor. Look for the lateral pectoral nerve on the deep surface of pectoralis major.
  6. Identify the pectoralis minor and its attachments. Transect this muscle along its attachments to the ribs and reflect laterally.
  7. After reflecting pectoralis major and minor, the external intercostal muscles are now exposed in the intercostal spaces.

Atlas Images:

  1. Near the midline the external intercostal muscle becomes membranous, exposing the internal intercostal fibers. Examine the muscle fibers of the internal intercostals running nearly at right angles to those of the external intercostals.
  2. Along the lateral thorax, cut through the external and internal intercostal muscles to expose the 3rd layer of intercostal muscles, the innermost intercostal muscle.  Attempt to locate the intercostal nerve and vessels between the 2nd and 3rd layers of muscles, just below the rib, in the intercostal groove.

 

 Readiness Assessment.

  1. To perform the laminectomy you will cut between which two bony processes on the vertebral arch?
  2. What is the first layer of the meninges that you will encounter once you enter the vertebral canal?
  3. What forms the cauda equina?

 

 Clinical Notes

1. Herniated Disc
The jelly-like nucleus pulposus (remnant of the notochord) may protrude through a ruptured annulus fibrosis of the intervertebral disk. When this protrusuion occurs posterolaterally, the nucleus pulposis is likely to impinge on the spinal nerves as tehy are exiting the vertebral canal. Pressure on a spinal nerve may cause pain radiating along its cutaneous distribution. The most common place for a herniated disc to occur in humans is between L5 and S1. Why might this be, and where might you expect the pain to radiate?

2. Spinal Tap
A spinal tap usually is performed between vertebrae L3 and L4. This region of the spinal canal lies inferior to the terminal end of the spinal cord (conus medullaris). By performing the spinal tap between L3 and L4, the clinician avoids piercing the spinal cord with the needle, which cause permanent nerve damage.

spinalTap

3. Lymphatic Drainage of the Breast
Most of the lymphatic drainage of the breast enters the axillary lymph nodes. However, the drainage from the medial half may enter thoracic nodes, which are difficult to remove surgically. Tumors in the medial half of the breast have a worse prognosis than those of the lateral breast.

 

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Updated 9/23/18 - Zeininger