Introduction
An exponential rise in rates of spinal surgery over the past decade has been dominated by a concomitant increase in the utilization of intervertebral cages for spinal fusion. The modernized minimally invasive retroperitoneal transpsoas approach for lateral lumbar interbody fusion (LLIF) has congruently achieved growing surgeon adoption since it was initially reported by Ozgur et al. in 2006. Proponents of the lateral approach assert that the direct view of the intervertebral space that this approach enables a robust discectomy and large interbody graft placement for indirect decompression of the neural elements, and disc height restoration with a reduced risk of iatrogenic injury to the thecal sac and neural elements. The LLIF working corridor thus allow surgeons to address recurrent pathology through virgin territory, potentially avoiding an array of complications associated with operating through scar tissue in revision surgery.
Concurrent with growth in popularity of the LLIF is the expansion of its indications, including that of revision spine surgery. Spinal imbalance and instability can occur at the index and/or adjacent levels following primary surgery attributed to aggressive pars or facet resections, resulting in pseudarthrosis, adjacent segment degeneration, proximal junctional kyphosis (PJK), and deformity. A lateral technique provides distinct advantages when used as a fresh surgical corridor in patients with previous lumbar spine surgery where adherent scar tissue riddles the traditional dorsal passageway. The larger grafts and streamlined access tools and technique further enable improved lordosis restoration and spinal realignment in the revision setting. This chapter will focus on the LLIF technique and its application in revision lumbar spine surgery.
Anatomical Considerations
A precise understanding of the relevant anatomy for the transpsoas approach is first necessary. The psoas muscle typically originates at the L1 level and becomes more robust as it descends in the retroperitoneum. The lumbar plexus, including the iliohypogastric, ilioinguinal, genitofemoral, and femoral nerves, travel intimately in and around the psoas muscle. The L4–L5 level is historically the most difficult level to access owing to a combination of the larger psoas muscle and the femoral nerve coursing through the operative field.
Several radiographic features must be assessed when considering the transpsoas approach. The level of the iliac crest must be evaluated relative to the desired level of interbody fusion. This is readily apparent on plain radiographs; if the iliac crest reaches well above the desired disc space, alternative approaches to LLIF must be considered. Assessment for a rising psoas sign on axial computed tomography (CT) or magnetic resonance imaging (MRI) must also be considered ( Fig. 14.1 ). If the psoas muscle is “rising” away from the vertebral body ventrally and laterally, it congruently stretches the lumbar plexus, placing it at greater risk of injury, and is another relative contraindication to the approach. Vascular structures should be carefully assessed to ensure lack of aberrant branches and presence of a safe corridor that can be traversed without added risk, particularly at L4–L5 where the inferior vena cava (IVC) or iliac vessels often closely approximate the spinal column. Additionally, the presence of scoliosis, particularly in the axial and coronal planes, should be evaluated to determine whether a safe passageway exists without undue risk to visceral and vascular structures. In revision surgery in particular, many of these structures can be obscured or perturbed from their native locations, increasing the necessity for scrupulous preoperative radiographic appraisal.
Intraoperative neuromonitoring is crucial to successful LLIF because of the close association of the lumbar plexus with the psoas muscle. Typically, both somatosensory evoked potentials (SSEPs) and electromyography (EMG) are monitored. In certain cases, particularly in deformity correction, motor evoked potentials (MEPs) may be additionally employed. EMG is principally employed with direct stimulation throughout sequential dilation to help mitigate iatrogenic nerve injury.
Brief Surgical Technique
There are multiple indications for LLIF in the revision spine surgery patient. However, the technique remains similar to when performed in a patient undergoing surgery for the first time. The patient is positioned in the lateral decubitus position. Although either side can provide access to the spine, the authors often prefer a left-sided approach because of proximity to the aorta in contrast to the IVC. However, the side of approach is also dictated by the spinal anatomy (i.e., concave or convex side of the scoliosis, rotational deformity, etc.) and any previous peritoneal or flank surgery.
The patient is placed on an operative table with the articulation centered between the greater trochanter and the iliac crest. The knees are slightly flexed and an axillary roll placed. The patient is secured to the bed with tape rolls and flexed to create a gentle jackknife position. This should not be performed in excess, as positioning alone can cause a stretch injury to the lumbar plexus. The incision is planned using fluoroscopic true lateral and anteroposterior (AP) images. Following incision, either hemostat or blunt finger dissection is performed through the oblique and transversalis muscle layers, following which “popping” through the fascia into the retroperitoneum is achieved. Once in the retroperitoneum, blunt finger dissection is used to sweep the peritoneal contents anteriorly followed by palpation of the psoas muscle and then dorsal movement to locate the transverse process. The primary blunt dilator is placed onto the psoas muscle and neuromonitoring is used to ensure the lumbar plexus remains posterior and at a safe distance away. Lateral fluoroscopy is used to dock at approximately the midpoint of the disc space. A K-wire is inserted through the dilator and its position verified with AP and lateral fluoroscopy ( Fig. 14.2A ). Serial dilation, again using neuromonitoring, is used to enlarge the corridor to the disc space. The retractor is then advanced and secured to the table. The dilators are removed and a blunt probe EMG stimulator is used to ensure that there are no traversing nerve roots. A shim is placed into the posterior aspect of the disc space, but anterior to the posterior longitudinal ligament (PLL). A curved retractor is used to sweep any residual psoas fibers anterior to the sloping disc space and secured in place.
A bayonetted blade is used to perform the annulotomy, taking care not to violate the anterior longitudinal ligament (ALL). A Cobb elevator is passed along the end-plates and, under fluoroscopic guidance, tamped across the contralateral annulus (see Fig. 14.2B ). Rongeurs and curettes are used to perform the discectomy and remove the cartilaginous end-plate. Serial trials confirm the appropriate graft size to traverse the width of the disc space and capture the apophyseal ring (see Fig. 14.2C ). Ideal graft size and graft location vary depending on the individual surgical goals, particularly in cases with kyphoscoliosis or in primary need of indirect decompression. The graft itself is then inserted into the disc space under fluoroscopic guidance (see Fig. 14.2D ); graft material can range from an array of options including various allograft, polyetheretherketone (PEEK), and titanium sources, in addition to the use of an array of lateral plating and screw systems that fall outside the scope of this text. Final AP and lateral images are taken and hemostasis is performed, following which the retractor is carefully removed under direct visualization. The superficial fascia and subcutaneous tissues are closed with absorbable suture and the skin is reapproximated with a subcuticular monofilament.
Image Guidance
An intraoperative cone-beam CT and image-guided navigation system may be used as an alternative to standard fluoroscopic guidance. This can be particularly helpful in revision cases where anatomy can be distorted or obscured. In this variation, a Jackson flat table is used and a large stiff gel roll is placed under the flank to open the costoiliac angle. An iliac pin is placed into the anterior or posterior superior iliac spine to secure a reference frame. An intraoperative CT is obtained and navigated instruments can be used throughout the procedure as detailed above.
Surgical Indications and Lateral Lumbar Interbody Fusion Revision Surgery
Adjacent Segment Disease
Adjacent segment disease (ASD) after short-segment lumbar fusion has been reported in as many as 20% of patients within 10 years. Many of these symptomatic patients fail conservative therapy and require revision operations. Traditional approaches to treating ASD involve posterior laminectomy and extension of fusion, often necessitating dissection through scar tissue with a greater risk of dural tear and/or nerve injury. LLIF provides an alternative approach for these patients. In some cases, a standalone LLIF may be sufficient for indirect decompression and arthrodesis of the adjacent level ( Fig. 14.3 ), whereas in other cases supplemental percutaneous or open posterior extension of fixation may be required to provide a more stable biomechanical construct. In the latter case, previous instrumentation may require removal and replacement with extension of the construct.
