Summary of Key Points
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Anterior decompression and fusion of the thoracic and lumbar spine can be technically challenging and typically requires a thoracotomy or retroperitoneal approach involving the great vessels and mediastinal or retroperitoneal organs. These approaches should be undertaken by experienced surgeons, working at mature centers, who are familiar with postoperative management.
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Approximately two thirds of spinal metastases are found anteriorly in the vertebral body and pedicles. Laminectomy alone does not address the compression from anteriorly located oncologic pathology and can further destabilize the spine. Anterior and anterolateral approaches allow for direct decompression and restoration of spinal stability while minimizing the risks associated with manipulation of the spinal cord.
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Synthetic constructs should be considered for anterior spinal reconstruction in patients with an expected survival of less than 18 months. Solid bony fusion is unnecessary in this time span and will be inhibited by adjuvant chemotherapy and radiation.
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Autogenous bone graft is appropriate for reconstruction in patients with nonmalignant disease or malignant disease with a relatively longer survival period. Tricortical iliac crest graft can be used for up to two-level corpectomies, but more extensive decompression may necessitate a humeral or fibular graft.
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The standard for anterior spinal instrumentation are dual-rod systems or plates, as they provide significantly more support than single-rod constructs, particularly against forces of axial rotation. However, there are times when a single-rod system is necessary, such as when the adjacent vertebral bodies are too small or have been partially destroyed by the tumor.
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The primary goals of surgical management of spinal infection are decompression of the neural elements, aggressive debridement, and collection of a sample to allow targeted antibiotic therapy. An anterior approach is generally supported for cases of extensive vertebral body destruction. Supplementary posterior instrumentation may be required for patients with more than two involved levels, severe segmental kyphosis, or involvement at the lumbosacral junction.
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The most common indication for anterior or anterolateral approach in trauma is a thoracic or lumbar burst fracture when neural decompression is needed. Anterior approaches may also be considered for patients with severe segmental kyphosis, evidence of fracture malunion, or persistent pain despite conservative management.
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Thoracoscopic techniques have demonstrated promise to reduce morbidity for anterior and anterolateral access to the spine; however, caution should be exercised because the techniques are less familiar to most spine surgeons than open approaches, and the steep learning curve reduces their practicality for surgeons who do not perform these techniques on a routine basis.
Anterior spinal decompression was described by Royle as early as 1928. This approach remained essentially unused until 1956, when Hodgson and Stock reported the use of anterior decompression in the treatment of tuberculous lesions. A progressive increase in the use of anterior and anterolateral approaches for spinal decompression to treat various spinal lesions such as tuberculosis, pyogenic osteomyelitis, kyphotic deformities, neoplasms (primary and metastatic), and burst fractures has been subsequently observed.
This chapter describes the principles of anterior and anterolateral thoracic and lumbar decompression for spinal pathology. This will focus primarily on decompression for spinal neoplasms. The indications, preoperative considerations, and steps in surgical management (exposure, decompression, reconstruction, fusion, and instrumentation) are described in comprehensive detail. This discussion then exends to include other pathologic conditions amenable to anterior or anterolateral approaches, such as spinal infection, thoracolumbar and lumbar trauma, and thoracic intervertebral disc herniation. Finally, we will review the current developments in the field of thoracoscopic access techniques for anterior and anterolateral approaches.
Spinal Tumors
The surgical management of patients with spinal tumors and associated spinal cord compression has shifted from a laminectomy approach to anterior approaches with anterior decompression. Because most spinal tumors are located anteriorly, a laminectomy can limit the degree of anterior resection and can exacerbate existing spinal instability associated with tumors that have destroyed spinal body segments. Several authors have reported that the outcomes of anterior decompression of spinal tumors with spinal cord compression are significantly better in most cases when compared with radiation therapy (RT) alone or in conjunction with laminectomy. Patchell and colleagues demonstrated the superiority of combined surgery and radiation in their randomized study. Ambulation as their primary end point was significantly better in the combined surgery and radiation cohort. The results of studies on anterior decompression suggest that it is an effective method to preserve and improve neurologic function in patients with neural compromise from primary and metastatic tumors of the thoracic and lumbar spine.
Indications
The principal indications for anterior decompressive surgery in patients with anterior spinal tumors are (1) progressive neurologic deficits, (2) pathologic fracture or impending spinal instability, and (3) mechanical or compressive pain. In rare circumstances, resection of a lesion to make the diagnosis is required. A computed tomography (CT)–guided needle biopsy should be undertaken if the lesion has characteristics of a primary tumor. The issue of pain can be controversial; if the etiology of the pain is from compression of the neural tissues or mechanical instability, justification for decompression and fixation can be made. The authors have intervened surgically on terminal patients with a life expectancy of 3 to 4 months for whom conservative measures have failed to provide sufficient pain relief. Life expectancy is therefore an important factor to consider, but one must appreciate the collective limitations in arriving at an exact figure.
The radiosensitivity of the tumor is an important consideration when planning overall management. Radiosensitive tumors (e.g., lymphoma, myeloma, Ewing sarcoma, neuroblastoma) can be treated with radiation therapy initially, if the cord compression is the result of epidural tumor alone. Surgical decompression should be the initial treatment when a significant degree of compression can be attributed to bony or ligamentous fragments or spinal deformity, as a result of destruction by tumor. In cases of failed radiation therapy with persistent or recurrent spinal cord compression, surgical intervention is also recommended. However, the increased complications associated with operating on a previously radiated site favors surgery before radiation. Surgical decompression can be considered for radiation-resistant tumors such as melanoma or renal cell carcinoma and intermediate radiosensitive tumors such as those of the lung, breast, or prostate. The rate of clinical progression provides the surgeon with valuable information when deciding the optimal timing of intervention. Rapid progression of symptoms is best managed with surgery because the effects of radiation can initially be associated with swelling. The medical status of the patient and ability to tolerate the surgery are also taken into consideration. The issue of radiation sensitivity of tumors is changing. The field of radiation treatment, much like the surgical field, has seen significant advances. Stereotactic delivery of radiation and image modulation are just two examples.
Preoperative Assessment
Initially, plain spine radiographs (anteroposterior and lateral) are used to determine the level and extent of tumor involvement. The spinal alignment can also be observed from these films. A CT scan without intravenous contrast at the appropriate spinal levels allows better definition of the degree of bony destruction of the spinal column. Although magnetic resonance imaging (MRI) is less precise than CT in outlining bony destruction, it provides the most precise means for illustrating the site and degree of spinal cord compression by tumor or soft tissues. Myelography and postmyelography CT scans can be used when MRI is contraindicated or unavailable. CT angiography is becoming more useful for determining the degree of vascularity of the tumor and delineating the normal surrounding vascular anatomy as the technology continues to develop.
To avoid complications from intraoperative and postoperative instability of the spinal column, it is important to assess the spinal stability before performing a vertebral decompression. Stability can be considered in terms of the three-column theory, after the extent of bony destruction produced by tumor has been determined from imaging. Single-column involvement can be considered relatively stable. The additive destabilizing effects of decompression, however, must factor into the decision-making process.
Anterior column and middle column involvement is the most common finding in symptomatic patients with spinal tumors and is frequently associated with some degree of vertebral body collapse and bony retropulsion into the spinal canal. If these conditions are treated by corpectomy, with a strut graft used for fusion, stability of the spinal column can be achieved.
Careful review of the degree of involvement of the dorsal elements is required. The need for augmentation of anterior fixation with posterior instrumentation will be based on the integrity of the laminae, lateral masses, pars interarticularis, and facets. Corpectomy and vertebral replacement techniques can result in persistent posterior element instability if overdistraction and opening of the facets are achieved. This instability can prevent subsequent fusion and result in failure of the anterior fixation. The ease with which distraction can be obtained following decompression can also provide the surgeon with information regarding the degree of posterior instability. Intraoperative radiographs can be taken to ensure the proper amount of distraction. In cases when the posterior instability is deemed significant, posterior stabilization should be undertaken. Discretion of the surgeon will be used to decide if and when the posterior stabilization procedure is required.
When planning surgery for spinal tumors, an assessment of stability must also take into account angulation and alignment. Higher failure rates will be observed with a poorly aligned construct. It is also important to consider the nature of the tumor in terms of its capacity to infiltrate and destroy bony tissue and its response to RT or chemotherapy. No algorithm or guidelines are available for defining the optimal approach to spinal tumors because of the highly associated variability; instead the surgeon must consider all of the preoperative factors including tumor characteristics, anatomy, surgeon comfort, patient comorbidities, and goals of the operation when deciding the optimal approach.
Preoperative Angiography and Embolization
Angiography, with a view to embolization, is recommended for patients with known vascular tumors (e.g., melanoma, renal cell carcinoma, metastatic thyroid tumor, primary giant cell tumor) or where imaging suggests a relatively vascular tumor ( Figure 74-1 ). If these tumors are amenable to embolization, it should be performed no more than 48 hours before surgery. Waiting too long after embolization may result in recanalization of the embolized vasculature. The expertise of the interventional neuroradiologist will influence greatly the impact of presurgical embolization by reducing intraoperative blood loss. Additional information concerning the vascular supply of the spinal cord can also be obtained.
Surgical Management
Intraoperative Monitoring and Anesthetic Management
The authors recommend electrophysiologic monitoring, if available, including somatosensory and motor evoked potentials, at all vertebral levels involving the spinal cord. Electromyography (EMG) monitoring is also useful in the lumbar region, if segmental pedicular fixation is contemplated. Monitoring setup time and cost are definite drawbacks to this type of technology, although its use is generally supported in the literature. In approaches of the upper and middle thoracic spine, a double-lumen endotracheal tube allows the lung in the operative field to be deflated, improving the surgical exposure. In lesions of the lower thoracic and thoracolumbar regions, the lung can be retracted easily. Invasive arterial pressure monitoring and central venous pressure (CVP) monitoring and access are recommended in order to address any blood loss during the surgery.
Positioning
Patients are positioned in the full lateral decubitus position ( Fig. 74-2 ) with an axillary roll placed under the dependent axilla to prevent neurovascular compromise. A mild flexion in the hip will assist mobilization of the psoas muscle should the exposure require it.
Incision and Exposure
The side of approach and the level of the spine that is involved are important factors in determining where to make the incision. To maximize resection, the decision to perform a right-sided or left-sided skin incision and approach should be determined by the side of the spine with greater tumor involvement. If neither side is predominantly involved by tumor, the spine is most often approached from the right side, at or above the T5 vertebral segment, to avoid the arch of the aorta. Below T5, the spine is generally approached from the left side to minimize retraction on the liver. Careful review of the axial images in order to appreciate the corridor of access between the great vessels and the lateral aspect of the spine is essential. This information may, in fact, push a surgeon to the other side and to tolerate the liver retraction.
Lesions involving the cervicothoracic (C7-T1), upper thoracic (T1-5), lower thoracic (T6-11), thoracolumbar (T12 and L1), and lumbar and sacral (L2 to sacrum) segments require specific approaches ( Table 74-1 ) and considerations.
| Spinal Segment | Surgical Approach |
|---|---|
| Cervicothoracic C7-T2 (see Fig. 74-11 ) | Extended ventral cervical (division of strap muscles) |
| Transsternal | |
| Cervicosternotomy (“trapdoor” approach) | |
| Upper thoracic T2-5 (see Fig. 74-10 ) | High dorsolateral thoracotomy (third-rib approach with mobilization of scapula) |
| T6-12 | Dorsolateral thoracotomy |
| Thoracolumbar T12-L2 (see Figs. 74-2 to 74-7 ) | Transthoracic/retroperitoneal with 10th to 12th rib resection; division of diaphragm |
| Lumbar | Retroperitoneal/flank |
| L2-5 | Transabdominal |
| Lumbosacral L5-sacrum | Ventral retroperitoneal (“pelvic brim” approach) |
Spinal Decompression
Spinal decompression requires a sequential approach that can be divided into four stages, which are discussed in the next four sections.
Exposure
A rib that overlies the level of the pathology in the midaxillary level is resected. The appropriate rib is identified using intraoperative imaging. The pleura is sharply incised and reflected. The segmental vessels at the level of the pathology and of the vertebral bodies above and below the lesion ( Fig. 74-3 ) are ligated and divided. Division of segmental vessels over the vertebral body in the middle of the body reduces the risk of vascular compromise of the spinal cord by taking advantage of collateral vessels from adjacent levels. The periosteum is reflected medially, and the anterior longitudinal ligament is identified.
Vertebral Body Decompression
The intervertebral discs above and below the involved vertebral body are identified and resected initially by sharp dissection ( Fig. 74-4 ). Disc material is cleared with curettes and pituitary rongeurs. Removing the rib head will allow identification of the ipsilateral pedicle and its continuation into the vertebral body. The pedicle is an important marker for the orientation and position of the spinal canal. The pedicle may be removed at this point using a high-speed bur initially followed by rongeurs. This early removal identifies the dural margin for safe resection of the vertebral body in the subsequent steps. Using sharp curettes, rongeurs, and a high-speed drill, the vertebral body is resected ventrally to dorsally, except for a rim of the anterior portion of the vertebral body. This rim protects the aorta and inferior vena cava from accidental trauma. Resection of the vertebral body can progress as far as the opposite pedicle ( Figs. 74-5 and 74-6 ), and the entire posterior aspect of the vertebral body can be removed. Sufficient bone needs to be removed to clear the posterior longitudinal ligament of any compression of the dura. The dissection can also be continued posterolaterally to allow decompression of the spinal nerve roots. The tumor involvement and the quality of the residual bone for instrumentation will determine the extent of bony removal. Techniques to augment the strength of the purchase into the bone are discussed in subsequent paragraphs.
Rostrocaudal Dissection
Special care is afforded to the cartilaginous end plates and the central regions of cancellous bone of vertebral bodies adjacent to the corpectomy site. Removal is performed using a small high-speed bur or curette ( Fig. 74-7 ) or osteotomes and rongeurs, depending on the bone consistency. This allows troughs to be created in the vertebral bodies above and below the corpectomy site to allow subsequent reconstruction with a bone graft, an implant, or an acrylic graft. Preparing the end plates to accommodate the construct requires special attention. When the construct involves bone, either autograft or allograft, an eventual fusion will be desired. In this circumstance, the end plates require adequate vascular supply for achieving fusion. The structural integrity of the graft can also fail if it is suboptimal or radiated. Methylmethacrylate (MMA), on the other hand, will not fuse, and the overall construct strength can be weakened by aggressive removal of the end plates. The risk of telescoping and the graph imploding into the vertebral body can also occur when the end plates are destroyed. MMA, however, will tolerate radiation.
Intraspinal Decompression
After adequate bony resection, decompression, and removal of all devitalized bone and tumor tissue, the posterior longitudinal ligament is resected to expose the dura mater that encloses the spinal cord and segmental nerve roots. Any tumor or bone impinging on the dural sac or nerve root is carefully removed to allow decompression of these structures. The goal of surgery in some cases should be radical tumor resection and decompression. In patients who have previously received RT, the posterior longitudinal ligament is frequently adherent to the dura mater and may be difficult to separate. In these cases, it may be advisable to leave it in situ. In most virgin cases, the dissection plane between tumor and dura is easily exploited.
Avoiding Complications during Spinal Decompression
A thoracotomy carries pulmonary risks such as atelectasis and pneumonia. The retroperitoneal exposure may injure the spleen, kidney, or ureter, and a prolonged postoperative ileus may occur. Any unrepaired defect in the abdominal wall or diaphragm may be the site of visceral herniation. Using vertebral body screws with manual confirmation of bicortical penetration requires considerable dissection of the contralateral aspect of the vertebral body, placing the aorta, inferior vena cava, and iliac vessels at risk if this is not done meticulously. Injury to the superior hypogastric plexus at L5 in males may be complicated by retrograde ejaculation. Chyle leak from damage to the thoracic duct is best managed with immediate primary repair if possible, or thoracic duct ligation if needed. Chyle leaks that go undetected at the time of surgery can be managed with chest tube drainage and total parenteral nutrition, but occasionally revision thoracic surgery is necessary.
Inadequate Spinal Decompression
Inadequate decompression reduces the chance of adequate neurologic recovery. It is important that the decompression be performed to the contralateral pedicle. Complete visualization of the dura mater and confirmation with intraoperative radiograph that the appropriate level has been decompressed are required.
Neurologic Injury
A carefully staged approach to spinal decompression with adequate exposure and identification of segmental vessels, nerve roots, and the dural canal markedly reduces the risk of nerve root and spinal cord injury. Nuwer concluded that intraoperative neurophysiologic monitoring is a cost-effective way of reducing the potential for a neurologic deficit. Intraoperative information is valuable not only to the prognosis but also for altering intraoperative and postoperative management. Hardware revision and removal during the operation may be performed on the basis of intraoperative neurophysiologic changes, and neurophysiologic monitoring has identified vascular injuries from anterior approaches that were otherwise undetected.
Dural Tears
Dural tear from direct surgical trauma may occur while the tumor is being dissected or because of erosion of the dura caused by the tumor. In these cases, the precise site of dural tear should be identified and the tear repaired with a nonabsorbable suture (e.g., 4-0 Nurolon [Ethicon]). In less discrete or poorly visualized dural tears, fibrin glue can be layered over the cerebrospinal fluid leakage site and allowed to adhere to underlying dura mater. A lumbar drain should be placed for 5 days postoperatively to assist dural closure and reduce cerebrospinal fluid leaks.
Excessive Epidural Bleeding or Bleeding from Tumor
Preoperative angiography and embolization of vascular tumors reduce the risk of intraoperative bleeding. After careful identification and mobilization, segmental vessels above and below the corpectomy site should be ligated and cut to avoid bleeding from these vessels when the vertebral bodies adjacent to the corpectomy site are spread apart. Other sites of epidural bleeding should be identified, and hemostasis should be attained with bipolar coagulation. Epidural bleeding can be particularly troublesome and requires the use of various packing agents such as hemostatic gelatin (Gelfoam, Baxter Healthcare, Glendale, CA) and thrombin. Dealing with epidural vessels by coagulation before sharp dissection is the optimal strategy. Tumor bleeding can also be difficult to address. Remember to remove all visible tumor, because the most frequent site of bleeding is from the tumor bed. Preoperative planning and gross total resection of the lesion is the best strategy for tumor bleeding. Tumor bleeding can also be controlled using various packing material as described earlier. Polymethylmethacrylate (PMMA) in the resection cavity can be used. The primary function is to reconstruct, but one of the secondary benefits will be from the thermal reaction of the cement, which can cauterize the remaining tissues.
Spinal Reconstruction
Graft Material
The appropriate type of material to use for spinal column reconstruction depends on the nature of the lesion and the patient’s life expectancy. In patients with benign lesions, or patients with malignant tumors who have a relatively long life expectancy (> 2 years), reconstruction is best when using autogenous bone from the iliac crest or rib for single vertebral body defects. If two or more vertebral levels are involved with the neoplasm, an allograft (e.g., from the fibula or humerus) can be used. It is often useful to supplement the allograft strut with local autograft bone ( Fig. 74-8 ).
In patients with malignant disease and a short life expectancy, autogenous bone grafts have certain disadvantages: (1) if life expectancy is less than 18 months, solid bony fusion over the long term is unnecessary; (2) the use of adjunctive radiation and chemotherapy will slow or prevent the bony fusion needed for stability; (3) any remaining local tumors may infiltrate the bone graft and weaken the construct; and (4) autogenous donor sites may not be suitable because of tumor involvement.
For patients with an expected survival of 18 months or less, a synthetic construct using PMMA with or without titanium cages can be used. Expandable cages with only titanium construct have also been used to provide ventral support and reconstruction in difficult cases (see Figs. 74-8D and E ). Biomechanical studies looking at intervertebral fixation have shown resistance of the implants to cyclical fatigue within typical normal physiologic loading and superior reduction of intervertebral motion and increased spinal stiffness. The use of expandable vertebral prostheses or cages is gaining popularity. A series by Viswanathan and colleagues demonstrated that an expandable titanium cage used for reconstruction in patients undergoing vertebral body resection for spine tumors had a median height correction of 14%, improvement in sagittal alignment of 6 degrees, and significantly improved postoperative pain. The device used in this study was the Synex (Synthes USA, West Chester, PA); another example is the X-mesh cage (DePuy Spine, Raynham, MA).
Reconstruction Techniques
Reconstruction techniques aim to provide solid fixation of adjacent spinal segments. Failure of these constructs is usually the result of reconstruction material dislodging at proximal or distal ends where the material fits into adjacent spinal segments. Early spinal changes in the cancellous bone of adjacent vertebral segments, seen on postoperative MRI scans, can indicate the potential failure of these regions to anchor the construct. Another important situation is one in which adjacent vertebral segments are involved with disease but are not collapsed and are not causing spinal cord compression. PMMA can be used to strengthen the adjacent bone. Alternatively, supplemental dorsal instrumentation may be required. The technique of vertebroplasty has grown significantly in popularity for osteopenic fractures in the elderly. This technique can complement and add strength to a ventral or even dorsal construct.
Synthetic Constructs
The technique of Errico and Cooper, in which PMMA is pressure injected into a Silastic tube that is fitted against the vertebral bodies above and below, provides an ideally suited construct for patients with metastatic lesions ( Fig. 74-9 ). Silastic tubing of varying diameters (typically 15 to 20 mm) is cut to a measured length (from the outer edge of the upper and lower troughs of adjacent vertebral segments to the corpectomy site). One 6-mm-diameter hole is made in the center of the tubing with a rongeur, and three small holes are made laterally, two at the rostral end and one at the caudal end. Small bites are also made at the ends of the tubing to allow extrusion of cement overflow. The three smaller lateral holes allow air bubbles and excess cement to flow out easily. The side of the Silastic tubing facing the spinal cord is free of the central and lateral holes to avoid cement extrusion into the spinal canal. The Silastic tubing is passed into the space between two adjacent vertebral bodies at the corpectomy site and positioned so that there is no bending of the tubing that could obstruct cement flow. Low-viscosity, slow-curing PMMA is prepared and is kept in a large 50-mL syringe. When it has become semiliquid, the PMMA is injected through the center hole of the Silastic tubing, filling the tubing until PMMA can be seen passing out from the ends of the tube (see Fig. 74-9 ). Certain PMMA mixtures are now available, allowing for more controlled timing of the reaction and more reliable handling characteristics. The tube must be observed carefully to avoid spilling the PMMA into the spinal canal. Curved Penfield dissectors can be used to protect the dural canal. As the PMMA in the Silastic tubing becomes harder, more PMMA is prepared and placed ventral and lateral to the Silastic tube until it is continuous with the borders of the upper and lower vertebrae. During polymerization and hardening of the PMMA, copious saline irrigation is used to help dissipate the heat. Hemostasis is attained with bipolar coagulation.
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