Thoracic Spine (T2-12)

The thoracic spine (T2-12) has a unique advantage over other spinal segments in that the nerve roots in this region can be sacrificed with minimal morbidity. In performing a multilevel spondylectomy, increasing the number of nerve roots and segmental vessels that are sacrificed increases the risk of ischemic injury to the cord. The exact tolerance of the human spinal cord for ischemia has not yet been defined, and there is probably a fair amount of variability between patients. Kato et al. have described a canine model, in which they set out to determine the number of segmental vessels that can be ligated before ischemia occurs.² They found that the interruption of bilateral segmental arteries of four or more consecutive levels, including the level of the Adamkiewicz artery, risks producing ischemic spinal cord dysfunction.

Tumors that are located ventrally, in the vertebral body, will need to undergo a preparatory stage from a dorsal approach. The approach is a standard midline approach with a subperiosteal exposure and exposure of the level of interest and two to three levels above and below index level(s) (Fig. 107-2A). Laterally, the exposure is carried out over the ribs for at least 6 cm on either side. Preoperatively, the MRI and CT are examined critically to understand exactly how far into the dorsal elements the tumor might extend. If the tumor extends into the pedicles and transverse process, this portion of the vertebral segment cannot be violated and must be dissected such that it can remain in continuity with the specimen³ (Fig. 107-2B). The dorsal elements that are uninvolved with tumor at the index level(s) are removed, as are the dorsal elements above and below this level (Fig. 107-2C). Because the spinal column will be completely destabilized after the resection, instrumentation is placed at this part of the operation. The ribs at the index level(s) and the level above and below are cut 6 cm lateral to the transverse processes and, if not involved in tumor, are removed and retained as graft material. Care is taken not to injure the neurovascular bundle or pleura ventral to the rib.

FIGURE 107-2 A, Preoperative sagittal T2-weighted MRI showing rostral and caudal planned osteotomies (red). B, Preoperative axial T2-weighted MRI showing planned laminar and rib osteotomies (red). The planned en bloc specimen is outlined in orangeC, Posterior exposure and removal of posterior elements.D, Dissection plane (red) between vertebral body and segmental vessels. E, Delivery of en bloc specimen. F, En bloc pathology specimen. G, Reconstruction.

The segmental vessels are traced from the rib to the foramen. A dissection plane is opened, keeping the segmental vessels ventral, opening the fatty plane between the vessel and the vertebral body. Staying in this plane with the dissection will allow the inferior vena cava and the aorta to remain ventral and away from the vertebral body and tumor. The radicular branches coming off the segmental vessels that supply the spinal roots and spinal cord are ligated and cut. Staying in this plane, keeping the segmental vessels ventral in the dissection plane, will eventually allow the dissection to be carried ventral the vertebral body and dorsal to the aorta and vena cava. As this dissection plane is developed from both the left and right sides of the patient, the aorta, vena cava, and esophagus will be kept out of the field and protected from the spine (Fig. 107-2D). A silastic sheet can be placed in this dissection plane, protecting these vascular structures and the pleura. Attention is then turned to the spinal canal. The nerve roots of the index level(s) are ligated proximal to the dorsal root ganglia and are cut. Ligating the roots proximal to the dorsal root ganglia minimizes the chance of developing long-term chronic pain. The epidural plane is then defined, and if tumor capsule enters this plane, the epidural space is dissected, preserving the tumor capsule. Tomita saws are placed encircling the spinal column, one rostral and one caudal to the tumor. The final assembly of the instrumentation is completed, and by using the Tomita saws, the osteotomies are completed. The specimen is now free, completing the preparatory stage. The en bloc specimen is now gently rotated away from the spinal cord and delivered out through the space created by removing the ribs (Figs. 107-2E and F). The ventral vertebral defect is reconstructed, and the grafting and closure are then performed (Fig. 107-2G).

Tumors bounded by pleura, in which the pleura will need to remain with the specimen to obtain a negative margin, will need to be prepared in a slightly different fashion. Rather than approaching the tumor from an entirely dorsal approach, a ventral approach is added. The first steps of the preparatory stage are the same as those in the entirely dorsal approach. In contrast to the entirely dorsal approach, however, the ventral aspect of the spine will need to be dissected from a ventral approach to leave the pleura intact over the specimen (Fig. 107-3). The ribs need not be dissected as far laterally as is done in the entirely dorsal approach, and rather than resecting the ribs, the ribs need to be sectioned only beyond the lateral extent of the tumor. The sectioning of the pleura, osteotomies through the disc space, and then the delivery of the tumor will take place through a ventral exposure. The ventral exposure will take the form of either a thoracotomy or a thoracoscopic approach. After the exposure is completed, the segmental vessels coming off the aorta and going to the vena cava are identified, ligated, and cut. The pleura is cut in such a way as to create a margin for the tumor. The rostral and caudal osteotomies are completed, and the tumor specimen is delivered. The ventral spinal reconstruction is then completed, and the thoracotomy is closed.

FIGURE 107-3 Pleural margin.

The ability to sacrifice the thoracic nerve roots with little consequence allows the surgeon to approach the spine and perform the preparatory and delivery stages of an en bloc resection through an entirely dorsal approach. The need to maintain the pleura as a margin will dictate a second approach to achieve the desired resection. Because the nerve roots in the cervical and lumbar spine cannot be sacrificed without significant morbidity, in general, these segments of the spine will need at least two approaches to complete both stages of the en bloc resection.

Cervical Spine (C2-T1)

In the cervical spine, an en bloc resection of a tumor will only be realistic if the tumor is not involved in both vertebral arteries, and if the vertebral artery that remains after the tumor removal is able to provide an adequate blood supply to the posterior fossa. This will need to be determined in the preoperative period. If a cervical tumor involves one of the vertebral arteries (Fig. 107-4A) and would therefore require sacrifice of that vertebral artery, then it must be determined preoperatively whether the contralateral vertebral artery will be sufficient to provide the necessary vascular supply.⁴ This determination can be made by estimating which artery is dominant. It the nondominant artery is involved in the tumor, no further investigation is usually required. If the dominant artery is involved in the tumor or dominance is not clear, then ancillary testing can be helpful. A conventional cerebral angiogram is useful in determining the patency of the circle of Willis to determine whether the anterior circulation could possibly provide the blood supply to the posterior circulation. In addition, a vertebral artery balloon occlusion test can be performed to predict functionally whether a patient will tolerate the sacrifice. Even if it is determined preoperatively that a patient may tolerate unilateral sacrifice of a vertebral artery, this must be undertaken with caution, as vasospasm, embolization, or thrombosis of the remaining vertebral artery could lead to disaster.

FIGURE 107-4 A, Preoperative sagittal T2-weighted MRI. B, Intraoperative photograph demonstrating circumferential identification (white vessel loops) of the left vertebral artery rostral and caudal to the tumor. C, En bloc pathology specimen.

After it has been determined whether a vertebral artery will need to be sacrificed and that the patient will most likely tolerate the loss of the vertebral artery, the consequences of the nerve root sacrifice must be weighed by having the patient understand the postoperative disability that he or she will endure. If this is acceptable to the patient, then proceeding with the resection is a possibility. As in the thoracic spine, the preparatory stage begins on the side of the spinal canal opposite the tumor. In general, ventrally located tumors will first be approached dorsally, and the tumor will be delivered away from the ventral aspect of the spinal cord.

During the dorsal approach, the dorsal elements of the spine are removed, exposing the normal thecal sac. A tumor that is ventrally situated and extends laterally to involve a vertebral artery and nerve roots will require that the artery and the nerve roots are sectioned during the preparatory stage. The nerve roots to be sacrificed are identified within the spinal canal and are ligated and sectioned proximal to the dorsal root ganglion. The lateral masses rostral and caudal to the tumor are carefully resected with a high-speed diamond bur. As the lateral masses are resected, the foramina for the nerve roots will be encountered. Ventral to the nerve roots lies the vertebral artery. As the vertebral artery is dissected, great care must be taken, as there is an exuberant venous plexus surrounding the vertebral artery. Bleeding from this plexus can be carefully controlled with thrombin and powdered hemostatic gelatin (Gelfoam). The vertebral artery is identified rostral and caudal to the tumor. It is circumferentially identified and then ligated and sectioned (Fig. 107-4B). Prior to ligation, a test occlusion of the vertebral artery can be made with aneurysm clips while monitoring brainstem auditory evoked potentials and the electroencephalogram. If any changes are recorded during the test occlusion, then the plan for vertebral artery sacrifice must be aborted unless a vertebral artery bypass is contemplated.

After the nerve roots and vertebral artery have been controlled, attention is turned toward the contralateral side where the vertebral artery is to be preserved. On the side where the artery is to be preserved, the artery can be skeletonized and identified to protect it during the delivery stage of the tumor. Skeletonization of this artery does carry the risk of causing vasospasm, and if this is the only remaining blood supply to the posterior fossa, this can be catastrophic. To minimize the chance of vasospasm, the vertebral artery should remain encased in the foramen transversarium if possible, and the sagittal osteotomies, lateral to the tumor, should be performed medial to the vertebral artery and foramen transversarium. At this point, the spinal instrumentation is placed, and bone graft is placed for the arthrodesis.

The ventral approach to the cervical spine will then be used to complete the preparatory stage and to deliver the tumor. A large ventral cervical exposure is performed. For high cervical tumors, a high cervical approach needs to be used, or a transoral transmandibular circumglossal approach should be considered. The temptation of a transoral transmandibular approach must be tempered by the potential consequence of the patient not being able to receive postoperative adjuvant therapy in the form of high-dose radiation therapy. Postoperative high-dose radiation therapy might not be an option, as this increases the risk of mandibular pseudarthrosis and pharyngeal dehiscence.

After the ventral cervical approach has been completed, the osteotomies rostral and caudal to the tumor need to be completed. These osteotomies need to be taken dorsally, all the way through the posterior longitudinal ligament. After these osteotomies have been completed, the specimen should be mobile, and the tumor is removed en bloc (Fig. 107-4C). The operation is then completed with a ventral reconstruction and arthrodesis.

Lumbar Spine (L1-5)

The lumbar spine is very similar to the cervical spine in regard to the challenges and limitations of nerve root sacrifice, but the lumbar spine has the advantage that it does not have the added complication of the vertebral artery. A ventrally located lumbar tumor will be approached in two stages: The preparatory stage will be a standard dorsal approach (Fig. 107-5A), and the delivery stage will be a ventral approach.⁵ For L5 lesions, this will be a midline retroperitoneal or transperitoneal approach (Fig. 107-5B). For lesions above L5, this will usually be a lateral retroperitoneal approach, usually from the side that the tumor presents itself. If a tumor is located ventrally but on both sides of the vertebral body, then a midline approach for lesions rostral to L5 is considered, but this will entail significant mobilization of the aorta and inferior vena cava.

FIGURE 107-5 A, Illustration of the dorsal approach for en bloc resection of L5. B, Illustration of the ventral approach for en bloc resection of L5.