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