Lateral Transpsoas Approach to Interbody Fusion

Introduction

The transpsoas lateral lumbar interbody fusion (LLIF) technique was first introduced by Pimenta and Taylor in 2006 as an alternative to traditional anterior lumbar interbody fusion. Over the past decade, LLIF has established itself as an effective means and adjunct when treating an array of spinal pathologies. The technique enables access to the spine laterally via the retroperitoneal corridor by splitting the fibers of the psoas muscle longitudinally. Lateral tissue planes and the adjacent anatomic structures are often less familiar to traditional spine surgeons; thus it remains essential to develop an understanding of the numerous complexities of the LLIF procedure prior to performing this technique. This chapter discusses the unique anatomy, surgical technique, surgical indications, and postoperative care relevant to the LLIF technique. Patient outcomes are also briefly described.

To understand the LLIF and its potential uses and limitations, the surgeon must have a thorough understanding of the relevant anatomy to this approach. The psoas muscle originates from the transverse processes and the anterolateral aspect of the lumbar vertebral bodies. It descends deep to the inguinal ligament until it meets the iliacus muscle and inserts into the lesser trochanter of the femur. The psoas muscle becomes more robust as it descends caudal from its L1 origin owing to additional contributions from each lumbar segment. It is innervated by the L2-4 nerve roots and functions as the primary hip flexor muscle.

The LLIF corridor runs in proximity to the complex anatomy of the lumbar plexus, which is enveloped by the psoas muscle adjacent to the lateral vertebral bodies and disk spaces ( Fig. 11.1 ). In general, the plexus migrates anteriorly as it descends caudally along the psoas ( Fig. 11.2 ). The iliohypogastric and ilioinguinal nerves arise at L1 and remain posterior until the L4-5 level, where they take a sharp turn anteriorly. The genitofemoral nerve arises from the L1 and L2 nerve roots and descends within the psoas muscle parallel to the spine until it emerges from the muscle and runs along the superficial surface around L3. The largest nerve within the lumbar plexus is the femoral nerve, originating from the L2, L3, and L4 nerve roots. It lies deep within the psoas and courses anteriorly as it descends along the spine, often crossing the L4-5 disk space. Here, the nerve has two branches as the descending nerve accepts its final contribution from the L4 root. Of note, the L4-5 disk space is the most challenging lumbar level to access via the LLIF approach owing to the close proximity of the femoral nerve, and the risk of nerve injury is highest at this level.

Surgical Indications

The LLIF can successfully be utilized to treat an array of spinal conditions, including (1) degenerative disk disease, (2) recurrent disk herniations (without fragment extrusion), (3) mild to moderate lumbar stenosis, (4) grades I and II spondylolisthesis, (5) adjacent level degeneration after previous arthrodesis, (6) far lateral disk herniations, (7) diskitis, (8) pseudarthrosis, and (9) degenerative scoliosis. In general, much of the same pathology traditionally treated with posterior arthrodesis and decompressive techniques can be addressed with LLIF. The primary difference from open surgical procedures is that LLIF relies on indirect decompression and ligamentotaxis to decompress neural elements as compared with direct visualization and bony decompression.

Limitations

Selection of the appropriate surgical candidate for an LLIF procedure relies heavily on understanding the structural and anatomic limitations that may compromise the success and safety of the procedure. First, despite being a very useful decompressive technique, LLIF has limitations in the extent that neural elements may be decompressed. Patients with severe central canal stenosis are not ideal LLIF candidates because indirect decompression may prove inadequate. Furthermore, if neural compression is primarily caused by posterior pathology, such as hypertrophied ligamentum flavum, LLIF may not be suitable for decompression. Other examples would include extruded disk fragments, facet cysts, osteophytic disease, or severely hypertrophied facets. Similarly, grade II or higher spondylolisthesis is better approached through a posterior corridor as the disk center may be difficult to access without injuring the adjacent neural structures, or the anterior longitudinal ligament.

Another anatomic consideration unique to the LLIF approach involves the position of the iliac crest relative to the lower levels of the spine, particularly L4-5. Plain anteroposterior (AP) radiographs are most helpful to assess the iliac crest and determine if lateral access is possible ( Fig. 11.3 ). As previously noted, as the femoral nerve travels distal, it courses anteriorly, limiting the safe working space within the psoas at L4-5 and the subsequent risk of femoral nerve traction/injury is higher. The position of the psoas muscle relative to the vertebral body should be examined; it should rest immediately lateral to the vertebral body just anterior to the transverse process. Occasionally, the psoas can be found anterolateral to its typical position, thus displacing the neural elements with it. Failure to recognize, anticipate, and adjust for this anatomic variant will increase the risk of vascular and/or neural injury. The position of the great vessels must be evaluated, particularly in the setting of deformity as their anatomic position can be severely disturbed. The ribs can limit lateral access superiorly, necessitating either traversal of the intercostal space or resection of a portion of the rib. Importantly, these anatomic structures can be visualized with magnetic resonance imaging and/or computed tomography (CT) myelography. Also notable, prior retroperitoneal surgery predisposes the patient to internal adhesions and fibrosis, further increasing the complexity and risk of the LLIF approach. Lastly, consideration of supplemental hardware (lateral plating, pedicle screw, or spinous process fixation) is influenced by the patient’s intrinsic bone quality as well as intraoperative disruption of the anterior longitudinal ligament (ALL), and endplate violation.

Surgical Technique

Step 1: Required Equipment

To perform LLIF, the required equipment consists of a series of dilators, a table-mounted lateral access retractor with light source, a neuromonitoring system, and specially designed instruments for disk resection. Neuromonitoring is critically important to ensure the safety of the LLIF procedure. An appropriate neuromonitoring platform consists of electromyography (EMG) and allows for direct stimulation of the instruments through all steps of the procedure beginning with serial dilation through the psoas to disk space until the retractor is firmly anchored in place. Once the retractor is docked and prior to incising the disk space, a stimulation probe permits direct stimulation of any tissue overlying the disk space and excludes the presence of a motor nerve. Successful use of EMG and neuromonitoring during the LLIF procedure requires the avoidance of neuromuscular blockade during anesthesia.

Other equipment needed includes an operative table with the capability to articulate between the greater trochanter and the iliac crest. Like many minimally invasive techniques, direct visualization is limited, and a significant portion of the procedure is carried out with fluoroscopic guidance. Accurate fluoroscopic imaging during LLIF is technically challenging and paramount to successfully performing the procedure. Therefore, it is recommended that the fluoroscope is managed by an experienced fluoroscopy technician.

Step 2: Positioning

The patient should be positioned in the operating room in the lateral decubitus position with the knees slightly flexed to approximately 30 degrees to relax the psoas muscle, and the articulation point of the table should be equidistant between the greater trochanter and the top of the iliac crest. The back and shoulders should be parallel just medial to the edge of the bed with an axillary roll placed under the dependent axillae. Padding of dependent and exposed bony prominences is ensured. If the patient is positioned too close to the edge, the bed frame may obstruct the fluoroscopic images. A pillow is typically placed between the knees. Finally, a pillow is placed folded in half between the arms with the dependent arm supported by an arm board ( Fig. 11.4 ). It is important to position the arms of the patient in 90 degrees of forward flexion to create a clear path for the fluoroscope. Once the patient is positioned satisfactorily, three-inch cloth tape is used to fix the patient to the table at the upper torso superiorly and around the iliac crest inferiorly. When applying tape inferiorly, it is important run the tape inferior to the articulation point of the table. Another point of fixation is accomplished by running tape from the iliac crest to the anterior inferior corner of the table, thus maximizing the costopelvic angle, and enabling a secure lateral jack-knife position ( Fig. 11.5 ). Securing the patient in this manner allows for rotation of the operative table in order to obtain the correct orientation of the spine. The table is then brought into reverse Trendelenberg position and the back of the table is brought down to create the lateral jackknife position. Care must be taken to avoid excessive breaking of the table as neural stretch can occur from positioning with resultant neurapraxia.

Step 3: Flouroscopy Setup and Incision Planning

Fig. 11.6 demonstrates the typical operating room (OR) setup for the LLIF approach ( Fig. 11.6 ). The OR tables is positioned with the head toward anesthesia. The surgeon and surgical technician are located posterior to the patient, and the C-arm is brought in from the abdominal side of the patient with the fluoroscopy monitor at the feet. The two most common incision techniques used to access the retroperitoneal space are the single and double incision method. The nuances of these two incisions are compared below. Regardless of which technique is chosen, the first step in incision planning is to obtain a true AP radiograph with a C-arm in a cross table orientation ( Fig. 11.7 ). To accomplish this, the C-arm is brought into the field such that the long axis of the machine matches the lordotic angle of the targeted disk space. The operative table is then rotated left or right with the C-arm remaining at 0 degree to obtain a true AP view by lining up the spinous processes symmetrically between the pedicles. If a multilevel surgery is planned, the C-arm and the patient may require repositioning for each level, depending on the anatomy, to ensure proper positioning and approach trajectory. This is critically important in patients with scoliosis with a significant rotatory component.

To mark the incision, the C-arm is rotated into the lateral position, and a true lateral radiograph is obtained by lining up the endplates and pedicles of the targeted level such that the disk space and neural foramen are sharply defined. To mark the disk space, a K-wire is then placed on the patient’s flank ( Fig. 11.8 ) and positioned until it is radiographically parallel with the disk space with the tip of the K-wire located at the anterior border of the disk space ( Fig. 11.9 ). After marking the skin, it is common to define and mark the posterior aspect of the disk space with the K-wire. Some surgeons find it advantageous to mark the different quartiles of the disk space as well in order to define their planned docking location. The incision line should then be drawn approximately one-third of the distance from the posterior to anterior spinal line at the same angle. For multiple levels, especially in scoliotic cases, a true lateral must be obtained and the disk space marked at each level. This is accomplished by adjusting the operative table using the Trendelenburg and reverse Trendelenburg positions and maintaining the C-arm at 90 degrees. For surgery involving multiple levels, it is often beneficial to draw the anterior and posterior spinal lines on the flank of the patient with disk spaces marked ( Fig. 11.10 ). Often, multiple levels can be accessed through a single incision by creating an incision the takes a more diagonal course from posterior/superior to anterior/inferior (simulating the course of the lumbar plexus) between the targeted disk spaces.