Spinal stability is classically defined by White and Punjabi as the ability of the spine under physiologic loads to limit patters on displacement so that neither the spinal cord nor nerve roots are damaged or irritated and to prevent incapacitating deformity or pain caused by structural change. 1, 2 Denis 3 proposed a three-column spine classification to help define the involvement of different parts of the thoracolumbar spine in common injuries. The anterior column includes the anterior longitudinal ligament and anterior half of the vertebral body, whereas the middle column includes the posterior half of the vertebral body and the posterior longitudinal ligament. Disruption of the anterior and middle spinal columns at the thoracolumbar junction may result in abnormal spinal angulation.
38.2 Patient Selection
The most common causes of spinal instability are trauma, infection, osteoporosis, and neoplasm. Spinal infiltration from tumor may induce pathological instability or necessitate surgical spinal destabilization as a consequence of tumor resection. Initial signs and symptoms of spinal instability may be as minor as a dull backache or as dramatic as complete neurologic dysfunction below the site of instability. Spinal instability is a clinical diagnosis using a combination of radiographic modalities, including magnetic resonance imaging (MRI), computed tomography (CT), and dynamic radiographs.
Instability owing to dysfunction of the anterior axial load–bearing structures is the indication for anterior spinal reconstruction and stabilization, particularly if there is not a complete loss of the posterior osseous elements and ligaments. Posterior spinal column dysfunction, typically as a result of a fracture dislocation, is often an indication for posterior spinal fixation and fusion. With complete osseous and ligamentous three-column dysfunction, a combined front-and-back approach may be required. Because instrumentation only serves as a temporary foundation through which bony fusion takes place, healthy bone at the levels to be instrumented is crucial. Poor bone quality hinders immediate solid fixation and predicts pseudarthrosis, as well as eventual instrumentation failure.
38.3 Preoperative Preparation
Several factors must be considered preoperatively before performing an anterior thoracolumbar approach. Review of the pertinent history, physical examination, and imaging will help in this regard. The location of the ventral dural compression in the presence of the spinal cord is the most crucial fact in determining which the surgical approach provides optimal access. When the site of pathology or compression is symmetric, consideration of the location of the aorta and vena cava is needed. Typically, the aorta lies along the left anterolateral border of the vertebra, with the vena cava lying along the right anterolateral border. It is not uncommon for either of these structures to lie directly lateral to the spine, potentially impeding direct access to the spine without circumferential vascular dissection and mobilization. Typically, the aorta is considered easier to mobilize and repair than the vena cava. In cases where the aforementioned factors do not determine the optimal side of approach, a left-sided approach is used to avoid obstruction by the liver.
A variety of intervertebral strut grafts are available, including structural allograft, synthetic cages, methylmethacrylate, and structural autograft. Potential durability, efficacy in promoting bony fusion, ease of use, price, and patient morbidity are factors to consider when choosing a strut graft. Autograft costs nothing (except increased operating room time) and offers the best chance for bony fusion; however, potential patient morbidity and difficulty with graft harvesting limit its application. Synthetic cages are the easiest to use, especially with the advent of expandable cages that are designed to fit snugly into a vertebral defect and stand alone without supplemental vertebral instrumentation; however, synthetic cages lack intrinsic osteoconductivity and are expensive. Metal cages also tend to subside through the vertebral end plates because of their disproportionate strength (elastic modulus) compared with cortical bone. Newer synthetic materials such as carbon fiber and polyetheretherketone match the elastic modulus of vertebral bone more closely in hopes of preventing subsidence. Structural humeral and femoral ring allografts provide a variety of different-sized implants that provide increased rates of bony fusion at a lower cost than synthetic cages. Both synthetic and allograft rings are filled with morselized allograft or autograft to promote fusion with the exposed vertebral end plates.
Methylmethacrylate implants are presently rarely used since they do not promote fusion; however, they are inexpensive and may offer immediate stability in in the setting of limited life expectancy. Specific pathologies demand additional individual considerations. For instance, vertebral strut grafting in the setting of tumor demands a material compatible with MRI and resistant to radiation effects. Infection is not a contraindication to synthetic strut implantation, but typically autograft is used. Ultimately, surgeon preference and familiarity often dictate the choice of grafting material.
Several anterolateral thoracolumbar screw–plate and screw–rod systems supplement vertebral stability, in addition to strut grafting. These systems maximize stability and deformity correction by facilitating intervertebral distraction and compression. Disadvantages include (1) increased risk of immediate vascular injury due to the need for greater rostrocaudal vertebral exposure, (2) potential risk of delayed vascular and soft tissue injury by increasing the lateral vertebral profile ( 
Fig. 38.1), and (3) greater expense.
Fig. 38.1 Complication of anterior screw–rod system placement with screw backing out into the kidney.
Intraoperative electrophysiological monitoring may be useful, especially in cases involving a high degree of epidural compression. Motor evoked potentials are the optimal method to monitor anterior spinal cord function intraoperatively. Somatosensory evoked potentials are less helpful since they monitor the spinal cord’s posterior columns.
38.4 Operative Procedure
38.4.1 Patient Positioning
Intubation is performed either with a double-lumen endotracheal tube or a tube that allows selective unibronchial ventilation if the surgeon requires. Often this is not required with a T10-distal exposure because only a minority of the lung will be exposed. The patient is placed in the lateral position on an operating table that allows intraoperative radiographs to be taken, typically by intraoperative fluoroscopy ( Fig. 38.2 a, b). The ideal operating table is a radiolucent four-poster frame such as the Jackson table (OSI, Union City, California). Regular operating tables may be swiveled in a manner to allow the bed podium to be positioned out of the way of the fluoroscopy unit. The patient is maintained in the lateral position with a combination of side braces, tape, and straps. Sometimes a vacuum-suction beanbag is used as well, but it might not be radiolucent. An axillary roll is placed to avoid compression of the brachial plexus. The arms are placed on a padded Mayo stand or in a sling out of the way of the mobile fluoroscope. A deflatable or removable kidney rest is placed under the patient’s flank to increase lateral flexion and operative exposure. Likewise, when a standard bed is used, the bed may be broken by flexing the bottom portion down to increase operative exposure. This needs to be done before deflating the beanbag position.
38.4.2 Surgical Exposure
Access to the thoracolumbar junction is typically gained by a posterolateral transverse incision centered over the 11th intercostal interspace ( Fig. 38.2 a, b). Another technique uses fluoroscopy to localize the appropriate disk space or vertebral level. The skin, subcutaneous fat, and muscle are divided down to the level of the appropriate rib which is usually one or two levels above the pathologic spinal level ( Fig. 38.3 a, b). After deflation of the ipsilateral lung, the interspace is entered. The rib may be cut and shingled over the 10th rib to increase exposure. A portion of the rib may also be removed for use as autograft in fusion. Retraction of the ribs is maintained by a Finochietto or Burford retractor. The lung is gently mobilized from the lateral aspect of the spine, and any adhesions are coagulated and sharply divided. The diaphragm is then divided. Retraction of the intrathoracic contents is maintained by a self-retaining retractor system with radiolucent blades such as the Farley-Thompson (Thompson Surgical Instruments, Inc., Traverse City, Michigan) or Omni-Tract (Omni-Tract Surgical, St. Paul, Minnesota) retractor systems ( Fig. 38.4 a). These systems are attached to the operating table in a position that least obstructs the surgeon, typically dorsal and rostral to the surgical field. One blade retracts the lung rostrally, and, in a right-sided approach, one blade retracts the liver caudally. Spinal decompression is then completed. Important landmarks during the decompression procedure are the ipsilateral vertebral pedicle and neural foramen that delineate the ventral spinal canal. The rib head that overlies and obscures the pedicle from view is removed either with an osteotome or a high-speed drill. The pedicle is palpated with a Penfield no. 4 instrument. Just caudal to the ventral-most portion of the pedicle is the neural foramen, which is also palpated. The neural foramen marks the anterior margin of the spinal canal, which is typically decompressed as a final maneuver prior to instrumentation.
Fig. 38.2 (a) Schematic diagram and (b) intraoperative photograph of incision typically used for posterolateral thoracic exposure. Although not demonstrated here, the incision for thoracolumbar exposure is centered over the 11th intercostal interspace. The photograph shows the final intraoperative patient positioning.
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