Indications
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Indications are any situation or condition requiring interbody fusion at the level of L1-2 through L4-5 including adult spinal deformity, degenerative disk disease, adjacent segment disease, low-grade spondylolisthesis, and foraminal stenosis where direct neural decompression is not required.
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The direct lateral approach can be used to correct coronal imbalance or degenerative disease by restoring alignment and providing indirect foraminal decompression.
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The patient must have favorable anatomy in terms of the ability to access the disk space via the direct lateral transpsoas approach. There must be enough space in the working channel between the 12th rib and the iliac crest. The size of the iliac crest in men may prevent access to the L4-5 disk space.
Contraindications
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The direct lateral approach does not allow access to the L5-S1 disk space because of the significant risk of injury to the lumbar plexus at that level and the fact that the iliac crest prevents an appropriate working angle.
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The use of stand-alone direct lateral transpsoas interbody fusion is controversial; however, in situations in which the posterior tension band is intact without any evidence of instability, stand-alone direct lateral transpsoas interbody fusion may be appropriate. The direct lateral transpsoas technique is likely not indicated, however, at a level of high mechanical demand, such as adjacent to a previously fused segment.
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Grade II or higher spondylolisthesis is a contraindication.
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The procedure is also contraindicated in patients with prior retroperitoneal surgery or presence of a psoas abscess.
Planning and positioning
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Preoperative planning includes detailed study of the patient’s psoas muscle (seen best on magnetic resonance imaging [MRI]) and bony anatomy including the accessibility of the appropriate disk space with regard to the working channel between the 12th rib and the iliac crest. A high iliac crest, which is more prevalent in men, may prevent access to the L4-5 disk space. The presence of long 11th or 12th ribs does not preclude access to the upper lumbar spine but may require an intercostal approach or rib resection. In scoliosis cases, the disk space can be accessed from the concavity or the convexity of the curve. Using the concavity has the disadvantages of working through a longer tube and needing to access the disk space through the collapsed side but has the advantage of rendering multiple levels accessible through a single incision. Conversely, using the convexity of the curve brings the disk space closer to the operating surgeon and takes advantage of more open disk spaces but typically requires more incisions per disk space instrumented. Likewise, abnormal vascular or muscular anatomy may determine which side provides safer or easier access.
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The use of radiography is essential in the direct lateral approach. True lateral and anteroposterior images must be obtained. Fluoroscopy (C-arm) provides real-time data and adjustable angulation but is associated with a significant risk of radiation exposure, the need for repeatedly adjusting from anteroposterior to lateral views, and the challenge of having to work around the C-arm tube. Alternatively, intraoperative stereotactic navigation can be used to reduce radiation exposure and eliminate the obtrusive C-arm tube. The navigation images are static, however, and changes in alignment that occur after placement of even a single interbody graft can render the registration inaccurate. Likewise, the registration can be ruined by even minute alterations in the position of the reference device.
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Intraoperative neuromonitoring must be established using free running electromyography (EMG) and active or triggered EMG set at 6 to 8 mA. Free running EMG helps to identify any nerve irritation or stretch at any point during the procedure, whereas triggered EMG can identify neural structures during the dissection via direct stimulation. The genitofemoral nerve cannot be identified by neuromonitoring and must be visualized. Postoperatively, 25% of patients may experience a burning groin pain because of genitofemoral nerve irritation.
Figure 86-1:
The patient must be in a position to obtain true lateral and true anteroposterior imaging. The patient is placed in a lateral decubitus position with the hip over the break in the operating table with the top leg flexed to relax the ipsilateral psoas muscle. Bony prominences are padded thoroughly. The operating table is broken to increase the working distance between the 12th rib and iliac crest and to open up the disk space toward the surgeon. The use of a beanbag can aid in maintaining patient positioning, and thorough taping must be used to secure the patient fully in position on the operating table. Ultimately, the bed, not the C-arm, is moved during the procedure to obtain true anteroposterior and lateral views. The patient must also be positioned on the operating table such that the C-arm is able to move freely to visualize the area of interest.
Figure 86-2:
Position the C-arm before draping. The C-arm gantry should remain at 0 degrees throughout the case. Adjust the position of the bed with the patient secured to obtain true anteroposterior and lateral images. The disk space must be visualized parallel to the fluoroscopy beam so that the end plates are seen cleanly and only one pedicle is visualized in the lateral view. In the anteroposterior view, the spinous process must be in the midline with the pedicles visualized equally bilaterally. In cases involving multiple levels, the bed position must be adjusted at every level to reorient and obtain true anteroposterior and lateral views for that level. Accurate and clear fluoroscopy is key.
Figure 86-3:
After true anteroposterior and lateral images have been obtained with the C-arm gantry at 0 degrees, the patient can be prepared and draped. This positioning allows the surgeon to orient his or her trajectory in a direction perpendicular to the floor, accessing the disk space in a consistent fashion and at a comfortable working angle. Such orientation minimizes the risk of taking a trajectory that is too anterior, risking the aorta, inferior vena cava, or iliac vessels or placing the interbody graft off target into the foramen. The surgeon must reorient at each level by adjusting the table.
Procedure
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