Surgical Anatomy and Posterior Approach to the Thoracic and Thoracolumbar Spine




Overview


Several surgical approaches have been developed to remove thoracic vertebral lesions in an anterior to posterolateral direction. The choice of surgical approach is decided depending on the disease entity, level of the lesion, laterality, multiplicity, presence of instability, and the necessity of reconstruction. The posterior approach provides a less extensive exposure of the vertebral body than the anterior approach, but the involved anatomy is familiar to spine surgeons, and it can be applied to patients with anesthetic risk.


Advantages of the posterior approach over the anterior approach are numerous :




  • The spinal cord is identified earlier.



  • Disease in the posterior elements can be treated.



  • The spinal column can be simultaneously stabilized with posterior instrumentation devices.



  • Imbalances in the sagittal plane (listhesis), coronal plane (scoliosis), and axial plane (rotation) can be more easily corrected with a posterior approach.



  • Thoracotomy is avoided.



With increasing bone removal, from the transpedicular approach to the costotransversectomy and finally to the lateral extracavitary/lateral parascapular approach, the operative trajectory becomes more lateral to visualize the affected vertebrae better. In this chapter, an overall review of the posterior or posterolateral approach to the thoracic spine will be presented with the pertinent anatomic assessment.




Muscular Anatomy


The most superficial muscle of the dorsal spine is the trapezius muscle ( Fig. 34-1 ), which originates along the external occipital protuberance and each spinous process from C1 to T12. The insertion of the trapezius is the lateral third of the clavicle, the acromion, and the scapular spine. This muscle provides the stabilization and abduction of the shoulder. Immediately deep to the trapezius muscle on the upper thoracic level lie the rhomboid major, rhomboid minor, and levator scapulae muscles (see Fig. 34-1 ). The rhomboid muscles originate from the spinous processes of the cervical and thoracic spine and insert at the ventral edge of the scapula. The levator scapulae muscles connect the scapula to the upper cervical vertebrae.




Figure 34-1


Superficial layer muscles: trapezius, rhomboids, levator scapulae, and latissimus dorsi muscles.


The serratus posterior superior muscle is another muscle that fixes the cervicothoracic junction area spinous processes to the lateral part of the rib cage ( Fig. 34-2 ). In the exposure of the cervicothoracic junction, the spinous process insertions of these muscles are taken down as a single group for lateral retraction. As these muscles are taken down, the scapula is released from its attachments to the spinous processes and rotates anterolaterally out of the operation field.




Figure 34-2


Deep-layer muscles. Erector spinae and transversospinalis muscles.


On the lower portion of the back, the latissimus dorsi muscle spans over the body. It originates from the spinous processes of the six lower thoracic vertebrae, lumbar and sacral vertebrae and ilium, inserting onto the humerus (see Fig. 34-1 ).


In the deeper part of the back are two large groups of muscles: the erector spinae (sacrospinalis) and transversospinalis muscles. The erector spinae consists of three separate groups of muscles that run from the sacrum and iliac crest to the ribs or transverse process of the vertebrae: the iliocostalis (lateral), longissimus (middle), and spinalis (medial; see Fig. 34-2 ). The iliocostalis muscle inserts into the angles of the ribs and into the cervical transverse processes from C4 through C6. The longissimus thoracis muscles insert into the thoracic transverse processes and nearby parts of the ribs between T2 and T12. The spinalis muscle is largely aponeurotic and extends from the upper lumbar to the lower cervical spinous processes.


The transversospinalis muscle group passes obliquely cephalad from the transverse processes to the spinous processes immediately deep to the erector spinae muscle. These muscles fall into three layers. The most superficial layer, the semispinalis muscle, arises from the tips of the transverse process and inserts into the tips of the spinous processes. The semispinalis capitis passes from the upper six thoracic transverse processes and lower four cervical articular processes to the occipital bone between the superior and inferior nuchal lines. The semispinalis cervicis muscle starts from the upper thoracic and lower cervical transverse process and attaches to the spinous processes of C2 through C5.


The semispinalis thoracis muscle runs from the transverse processes of the lower six thoracic vertebrae onto the spinous processes of the upper thoracic and the last two cervical vertebrae. The intermediate layer, the multifidus, arises from the sacrum, posterior sacroiliac ligament, accessory processes of the lumbar spine, and articular processes of the thoracic spine and inserts to the spinous processes of the vertebrae up to C2. The deepest muscles of this group, the rotators, are small muscles that bridge from the transverse processes to the lamina of the vertebra directly above.




Posterior Thoracic Cage


The head of a rib articulates with the adjacent parts of its own vertebral body, the vertebra above, and the intervertebral disk between them ( Fig. 34-3 ). The exceptions to this general rule are the first, eleventh, and twelfth ribs, which articulate only with their own vertebral body. On the vertebral body from the second to the tenth level, each rib head has two synovial joints with a vertebral body and intervening radiate ligament enforcing the joint. These are two independent joint surfaces, separated by the posterolateral position of the intervertebral disk. The inferior articular surface is numbered the same way as the rib and has a height slightly greater than the pedicle, and its posterior limit corresponds to the point of insertion of the pedicle. Its height represents about one third of the height of the body. In contrast, the superior facet represents only half the height of the inferior facet.




Figure 34-3


Thoracic bony structures from the inside of the thoracic cavity. Rib head contacts intervertebral disk on the superior portion of the pedicle. At lower thoracic levels, rib head contacts on the midportion of the pedicle.


The third synovial joint is the costotransverse joint, which is strengthened by superior and lateral costotransverse ligaments ( Fig. 34-4 ). The superior costotransverse ligament joins the neck of the rib to the transverse process immediately above. The ribs are also attached to one another through the intercostal musculature, which originates medially on each superior rib and inserts laterally on its immediately inferior rib. This strip of muscles contains the intercostal nerve, artery, and vein. Most frequently, the intercostal vein is most cephalad, with the intercostal artery close to it but caudad ( Fig. 34-5 ). The intercostal nerve is frequently found separate from these structures and is located most caudad of the three. Immediately ventral to the intercostal bundle and intercostal muscles lies the pleura.




Figure 34-4


Rib head contact with vertebral body via two joints, the costotransverse and costovertebral joints. The costotransverse joint is wrapped with the lateral costotransverse ligament; the costovertebral joint is surrounded by the radiate ligament.



Figure 34-5


Retromediastinal structures in left thoracic cavity. The aorta is seen to run along the anterolateral surface of the thoracic vertebral body, and the sympathetic chain is laid over the rib heads.




Posterior Mediastinal Space and Neurovascular Structure


Some neurovascular structures are related to the spine in the posterior mediastinal space. Between T4 and T7, the aorta has a close relationship with the left lateral surface of the vertebral bodies. It then moves medially to occupy a more anterior position, and at the level of the diaphragm, it is strictly prevertebral. The segmental arteries arise from the posterior surface of the thoracic aorta and run horizontally, following the concavity of the vertebral body. At the level of the foramen, they bifurcate into a radiculomedullary and intercostal branch. The principal medullary artery, the artery of Adamkiewicz, is located on the left side in about 60% of patients and originates mostly between T9 and T11.


In the upper thoracic region, the first two intercostal spaces are supplied by branches of the costocervical trunk through the highest intercostal artery. Because the aorta is displaced downward and to the left, the upper four intercostal arteries ascend to reach intercostal spaces three through six. They stretch obliquely across each vertebral body from caudad to cephalad in direct apposition to the periosteum of the vertebral body and are located deep to the azygos and hemiazygos vein, the thoracic duct, and the sympathetic trunk. On the left side, the superior hemiazygos vein occupies a position lateral to the aorta and receives collateral branches down to the sixth or seventh interspace.


The azygos vein is lateral to the esophagus on the right side and runs inferiorly to join the superior vein cava at the fourth interspace. At the point where it turns medially, it may receive some branches, which may be divided if necessary ( Fig. 34-6 ).




Figure 34-6


Retromediastinal structures in right thoracic cavity. The azygos vein runs cephalocaudal to the right sympathetic chain and contributes to the formation of the greater splanchnic nerve.


The sympathetic chain runs vertically and lies atop the heads of the ribs at the anterior edge of the foramina. From the intercostal nerves the chain receives the rami communicantes. A section of a few of these will be of no functional consequence as long as the major chain is preserved. From the inferior thoracic ganglia are derived larger trunks that constitute the splanchnic nerves, and these should be spared.




Lateral Extracavitary Approach


The lateral extracavitary approach (LECA) is an extension of the costotransversectomy. The more extensive rib resection provides a more ventral and wider operative view across the midline. LECA is indicated for the removal of extradural mass lesions anterior and lateral to the spinal cord or cauda equina, followed by anterior vertebral fusion. It can be applied for the management of thoracic disk herniation, upper lumbar disk herniation, trauma, tumors, and inflammatory diseases that involve up to three and sometimes four vertebral levels. LECA may not be applicable above the T4 level because of the scapula and below the L4 level because of the iliac crest.


Positioning and Incision


A midline vertical incision (three levels above and three levels below) is made with a gently curved lateral portion (12 to 14 cm). This incision offers access to both the posterior midline and anterior vertebral body through the lateral approach ( Fig. 34-7 ).




Figure 34-7


L-shaped skin incision on the thoracic and lumbar spine.


Muscle Dissection


Skin and subcutaneous tissue are incised and reflected to the extended incision side. The thoracodorsal fascia is then dissected from the midline and is incised along the horizontal skin incision line. The thoracolumbar fascia appears silver to white. When it is dissected and retracted, the lateral branch of the dorsal ramus of the spinal nerve is seen to run over the surface of the muscle layer ( Fig. 34-8 ).




Figure 34-8


Exposure of thoracolumbar fascia and erector spinae muscles. The lateral branch of the dorsal ramus of the spinal nerve is tied with string.


Trapezius or latissimus dorsi muscle is divided with the attached fascia, depending on the level of the lesion. The entire skin, subcutaneous tissue, muscle, and fascia flap are then elevated and retracted laterally. A plane is defined at the lateral aspect of the erector spinae group, and these muscles are elevated as a layer off the ribs and are retracted medially ( Fig. 34-9 ).




Figure 34-9


The erector spinae muscle group is elevated and retracted medially to expose the posterolateral surface of the spinal canal.


Rib Resection


All muscles and attached ligaments are cleaned from the ribs, and the rib is transected 7 to 10 cm lateral to the costovertebral junction. The endothoracic fascia and pleura are separated using blunt dissection. On identification of the neurovascular bundle, the intercostal nerve is separated from the vessels. The transverse processes and associated intertransversarii muscles are removed.


The superior costotransverse ligaments and radiate ligament, post costotransverse ligament, are incised with a scalpel. After the costovertebral joint is opened, the rib head is elevated out of the field. It is important to remove the rib and transverse process at the articulation to ensure full exposure ( Fig. 34-10 ).




Figure 34-10


The intercostal bundle—vein, artery, and nerve—under the lower margin of the rib ( inside view ).


Identification of the Neural Foramen


Each intercostal nerve is then traced into its respective foramen ( Fig. 34-11 ). A ligature is placed around the nerve, which is cut 3.0 cm distal to the dorsal root ganglia, and the nerve is retracted to the dorsal side. The retracted nerve roots cause the spinal cord retraction, which enables the surgeon to view the vertebral body across the midline.




Figure 34-11


The exposure of the neural foramen following the intercostal nerve. The trace of the nerve can identify the neural foramen and epidural fat.


The parietal pleura is dissected off the vertebral bodies using a Cobb elevator. If the rami communicantes that connect the nerve root and sympathetic ganglion are divided, the vertebral body can be exposed easily to the ventral tip. The sympathetic chain is contained within a fascial compartment formed by fusion of the mediastinal and prevertebral fascia over the costovertebral articulation. Displacing the sympathetic chain anterolaterally via subperiosteal dissection reveals the anterolateral surface of the vertebral body, pedicle, and foramen. The segmental arteries are dissected off the vertebral bodies and are divided between clips. The foraminal margins above and below the lesion are defined with a blunt nerve hook. Care is taken not to dissect into the spinal canal. After identification of the foramen, the pedicle is removed using a combination of rongeurs and thin foot-plated punches. The table is then rotated 15 to 20 degrees away from the surgeon to maximize visualization of the spinal canal.


Removal of the pedicle provides the lateral view of spinal cord. To facilitate exposure of the dorsal cord, the ipsilateral facet complex and lamina can be removed. When the epidural space is opened, epidural venous plexus bleeding is severe.


Corpectomy or Disketomy


The annuli adjacent to the vertebral bodies to be removed are incised with a No. 15 blade, and a punch is used to create a seat for a drill bit ( Figs. 34-12 and 34-13 ). With use of a brace and bit, the disk material and end plates are drilled out about three fourths of the way across the vertebral body, thus ensuring that the surgeon is across the spinal canal. The posterior intervertebral disk space, about 1 cm ventral to the canal, is the portion to be drilled out. The intervening vertebral bone is removed using a rongeur or a high-speed drill to go deep through the vertebra. At this point, there should be at least 1 to 2 cm of bone left anteriorly and dorsal shelf posteriorly. Careful dissection of the dura–bone interface can be helpful to break up adhesions and to define spicules, which may be stuck to the sac. The backward-angled curette is used to remove the posterior cortex from the ventral dura. The posterior cortex can be removed in a single piece by working primarily at the junctions of the intervertebral disks, and the posterior cortex removal can be extended across the spinal canal ( Fig. 34-14 ).




Figure 34-12


Diskectomy is performed during the retraction of the dorsal rami of the spinal nerve. The string is tied on the spinal nerve to be retracted.



Figure 34-13


After partial removal of the disk, the intervening vertebral body is resected. The distal end of the dorsal root ganglion is cut.



Figure 34-14


After the vertebral body is removed, the ventral dura is decompressed.


Vertebral Body Reconstruction


Following decompression, troughs are cut into the intact bone adjacent to the corpectomy site to seat the interbody strut graft. If posterior instrumentation is needed, this is done before the impaction of the interbody graft. When posterior instrumentation is performed, realignment of even severe deformities can be tried without risk of cord injury.


The bone graft is prepared 10% to 15% longer than the defect to be spanned. The ends are trimmed to 45-degree angles with the shorter length the leading edge. The graft should be positioned at least 1 cm away from the dural sac to prevent cord compression if cull correction is not maintained.


Wound Closure


The paravertebral muscle layer and the thoracodorsal fascia are tightly closed, and the midline muscles are reapproximated using nonabsorbable sutures. If the pleurae have been entered inadvertently, an attempt to primarily close the defect should be made. If primary closure is impossible, a #32 chest tube should be placed in the pleural cavity.




Transcostovertebral Approach


Posterior surgical approaches to the thoracic spine include the transpedicular, transfacetal, costotransversectomy, and lateral extracavitary approaches. Transpedicular and transfacetal approaches are included in the transcostovertebral approach.


Midline incision and usual posterior exposure are done. The transverse process of the involved level is resected en bloc to uncover the costotransverse junction and to provide access to the costovertebral joint ( Fig. 34-15 ).




Figure 34-15


Transverse process resection with pedicle. The facet joint is drilled away with the superior portion of the pedicle and the entire transverse process. The underlying rib head is cut away partially.


The lateral portion of the facet joint and the superior half of the pedicle are removed with a drill. The thoracic pedicle can be identified by following the superior facet. The pedicle is the landmark for the inferior margin of the disk. After removing the facet and pedicle, the surgeon can reach the costovertebral joint ( Fig. 34-16 ), which consists of the lateral end of the disk, rib head, and lateral aspects of the pedicle. From the center of the joint, the drilling is continued outward circumferentially to include immediately adjacent structures, such as the posterior cortex of the rib head and lateral end plates above and below the annulus ( Fig. 34-17 ). This maneuver exposes the lateral and anterior aspects of the spinal cord.




Figure 34-16


After the facet and pedicle are removed, the costovertebral joint is visible. Dotted lines show the disk space.

Jul 11, 2019 | Posted by in NEUROSURGERY | Comments Off on Surgical Anatomy and Posterior Approach to the Thoracic and Thoracolumbar Spine
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