MIS Lateral Lumbar Interbody Fusion

Summary of Key Points

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Patient positioning is paramount for a successful procedure.
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Triggered directional electromyography findings should guide retractor placement anterior to the femoral nerve.
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Open (expanded) retractor time should be minimized as much as possible.
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Preoperative evaluation of vascular structures on magnetic resonance imaging can potentially avoid catastrophic complications.
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The procedure is dependent of the use of fluoroscopy.
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A thorough knowledge of lateral regional anatomy is required.

Minimally invasive spine surgery is an ever-evolving continuum with expanding applications to an increasing spectrum of patient-specific pathologies. The lateral approach has become repopularized thanks in part to Dr. Luiz Pimenta. In 2001, he first introduced the retroperitoneal transpsoas minimally invasive lateral interbody fusion (LIF) as a safe and effective alternative to both traditional and minimally invasive anterior or posterior approaches to the lumbar spine. The lateral approach, however, is not a new concept in spine surgery. In fact, it was initially described as a treatment for Pott disease by Drs. Ménard and Capener in the early 20th century. Dr. Larson and his team at the Medical College of Wisconsin included traumatic lesions and a description of the lateral extracavitary approach in 1976. Though these techniques were largely abandoned due to patient morbidity, these pioneers laid the foundation for our current minimally invasive techniques to the lateral lumbar spine.

Lateral access to the anterior spinal column has some distinct advantages over traditional posterior approaches. These include indirect neurologic decompression with minimal tissue disruption, decreased blood loss, and shorter operative times. The normal, stabilizing posterior musculature and ligamentous complexes are not sacrificed as compared to posterior techniques. Surgical access to the disc space is larger, allowing for a more efficient and thorough disc removal and end plate preparation. Moreover, lateral access facilitates larger interbody grafts that can span the apophyseal ring to provide maximum support for fusion.

The clinical applications of the retroperitoneal transpsoas lateral lumbar interbody fusion (LLIF) are numerous and continually expanding. They include degenerative disc disease, lumbar stenosis, lumbar spondylolisthesis, adjacent segment failure, trauma, neoplasm, and adult degenerative scoliosis. This chapter outlines the specific anatomic considerations and safety zones for the lateral approach as well as electromyography (EMG) evaluation, surgical technique, and complication avoidance.

Regional Anatomy

Flank, Retroperitoneal, and Psoas Anatomy

The lateral approach exposes surgeons to unfamiliar anatomy that they may not have experienced during training. For any minimally invasive procedure, it is paramount to have a through understanding of regional anatomy, and this especially holds true for the retroperitoneal space in which there is risk of major vascular or visceral injury. Retroperitoneal structures include the psoas muscle, lumbar plexus (within the psoas), aorta/vena cava and their segmental tributaries, adrenal glands, kidneys, and the ureters. Thorough evaluation of preoperative imaging specifically identifying major vascular structures and their relationship to the disc space can help prevent catastrophic complications.

After the skin incision and dissection of the subcutaneous tissue, the lateral abdominal musculature is encountered. From superficial to deep, the first layer is the external oblique fascia (split transversely with electrocautery), external oblique, internal oblique, and transversus abdominis muscles each oriented in a different direction. The details of blunt muscle splitting dissection (as opposed to electrocautery) are discussed later as both the L1 branches of the iliohypogastric and ilioinguinal travel through the internal and external obliques. The retroperitoneal space is encountered after the transversus abdominis muscle is dissected and is highlighted by the presence of yellow adipose tissue. The quadratus lumborum can be palpated posteriorly, and the psoas is encountered directly lateral to the vertebral body.

Lumbar Plexus

The objective of the transpsoas approach is to dock anterior to the lumbar plexus (specifically the femoral nerve) so opening the retractor does not distract the nerves from their origin ( Fig. 77-1 ). The lumbar plexus is integrated within the psoas muscle and consists of the primary ventral rami from the first four lumbar nerves and the T12 subcostal nerve. The two major motor branches include the femoral nerve (L2-4) and the obturator nerve (L2-4). The major sensory branches (not detected by EMG) include the iliohypogastric (T12-L1), ilioinguinal (L1), genitofemoral (L1-2), lateral femoral cutaneous (L2-3), and anterior femoral cutaneous (L2-4) nerves.

The psoas muscle is directly innervated by the ventral rami of the first four lumbar nerves along its axis. The femoral nerve has both sensory and motor components and is subdivided into anterior and posterior divisions. Typically, the anterior division provides innervation for the anterior cutaneous nerve and the motor branches of the pectineus and sartorius muscles, whereas the posterior division covers the saphenous nerve (sensory) and the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius).

The obturator nerve is primarily a motor nerve but does supply sensory innervation to the proximal, medial thigh. Its motor components include the external obturator, adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus muscles. See Table 77-1 for a full description of each nerve with its corresponding innervation.

TABLE 77-1

Nerves and Corresponding Innervation in Lumbar Plexus

Nerve	Segment	Innervated Muscles	Cutaneous Branches
Iliohypogastric	T12-L1	• Transversus abdominis • Abdominal internal oblique	• Anterior cutaneous • Lateral cutaneous
Ilioinguinal	L-1		• Anterior scrotal nerves in males • Anterior labial nerves in females
Genitofemoral	L1-2	• Cremaster in males	• Femoral ramus • Genital ramus
Lateral femoral cutaneous	L2-3		• Lateral femoral cutaneous
Obturator	L2-4	• Obturator externus • Adductor longus • Gracilis • Pectineus • Adductor magnus	• Cutaneous ramus
Femoral	L2-4	• Iliopsoas • Pectineus • Sartorius • Quadriceps femoris	• Anterior cutaneous branches • Saphenous
Short, direct muscular branches	T12-L4	• Psoas major • Quadratus lumborum • Iliacus • Lumbar intertransverse

From Uribe JS, Arredondo N, Dakwar E, Vale FL: Defining the safe working zones using the minimally invasive lateral retroperitoneal transpsoas approach: an anatomical study. J Neurosurg Spine 13:263, 2010.

Safety Zones

Avoiding neural injury is paramount to a successful surgery. Multiple studies have attempted to map the lumbar plexus within the psoas muscle and establish “safe zones” in relation to each disc space. Not surprisingly, the results are variable owing to the lumbar plexus heterogeneity. As a general rule, the plexus migrates anteriorly as the psoas muscle descends into the pelvis and the L4/5 disc space seems to be the most vulnerable for neural injury.

Early work by Moro in 2003 helped establish a safety zone for the lateral approach, stating it was safe to traverse the psoas muscle at levels L4/5 and above. However, the genitofemoral nerve was susceptible to injury specifically at the L3/4 level. In 2010, Uribe and colleagues published a cadaveric study of sagittal spinal anatomy defining four zones of the vertebral body in the sagittal plane. Each zone represents a quartile of the vertebral body, zone 1 representing the most anterior quartile to zone 4 representing the most posterior quartile ( Fig. 77-2 ). From this dissection, they developed generalized “safety zones” for each disc space from L1-5.

For the first three lumbar disc levels (L1/2, L2/3, L3/4), they found that retractor placement was safe in an area beginning at the midpoint of zone III and moving anteriorly. At L4/5, the safe zone starting point moved anteriorly and was defined at zone junction II/III or the mid-disc space (see Fig. 77-2 ). The genitofemoral nerve was the only nerve found to be ventral to zone III. It moves ventral at L2/3 and migrates caudally to L4/5. It is primarily a sensory nerve and consists of two branches, the genital and femoral branches. The femoral branch innervates a small portion of skin over the femoral triangle and the genital branch controls the cremaster muscle and gives sensation to the scrotal skin/skin over the mons pubis and labia majora in females. Caution must be used during retractor placement/dissection, as this nerve will not be recognized by EMG (mainly sensory) and can be easily injured if the surgeon is unaware of its location.

Other studies have performed similar dissections and attempted to establish lateral transpsoas working zones with a mild variation. Regardless of the dissection findings, there will always be variability of the lumbar plexus, and the EMG findings and visual inspection of the surgical bed are the true indicators of safety. These zones should only be considered guidelines as a starting point for each level.

Vascular Anatomy

The retroperitoneal space includes major vascular structures such as the aorta, inferior vena cava, and the common iliacs. Meticulous evaluation of preoperative imaging is paramount to a safe and successful surgery. Regev and associates demonstrated the relationship of vascular structures relative to the MIS LIF in their morphometric study. They concluded that the risk of injury to the great vessels was greatest at the L4/5 level due to the posterior migration of those vessels. All major vessels (inferior vena cava, aorta, femoral vein/artery) should be identified prior to any surgical intervention. Furthermore, if available, the surgeon should review coronal reconstructions of the lumbar spine to evaluate for aberrant segmental arteries at the level of the disc space. Our institution has reported a contralateral injury from a segmental vessel crossing the disc space that occurred during release of the annulus that required operative intervention. Knowledge of the vascular anatomy is paramount to a successful lateral procedure.

Surgical Technique

The technique for the retroperitoneal transpsoas MIS LIF has been an evolving process for our team. We have made modifications to the originally described technique based on our institutional experience and published data to minimize complications. The technique described here specifically applies to the XLIF (NuVasive) procedure but the general principles can be applied to any lateral access system. A major factor that is not employed by other systems is the use of directional, triggered-EMG, which will be detailed later in the discussion.

Preoperative Planning

Preoperative evaluation is imperative for proper patient selection. Once a patient is considered for lateral access surgery, our institution uses a preoperative checklist to assure minor details are not overlooked (see Fig. 77-1 ). The specific indications for lateral access are surgeon dependent but range from simple spondylosis to major deformity correction.

Once the level of pathology is determined, the magnetic resonance imaging (MRI) must be reviewed to evaluate the vascular anatomy. The surgeon should identify both the aorta and inferior vena cava or the common iliac arteries/veins to assure they do not hinder access to the disc space. If there is any question of compromised access, then a different approach should be considered as a major vascular injury can be potentially life threatening. We do not choose approach side based purely on vascular anatomy.

Either a computed tomography (CT) or anteroposterior (AP) radiograph is reviewed to determine the optimal side for lateral access. This is especially important for the L4/5 level in which the iliac crest can impede the approach. If a spinal deformity is present, the patient is typically placed with the concave side facing up so one incision can have access to multiple disc spaces. This also provides better access to the L4/5 disc space if desired.

Positioning

The goal of positioning is to establish a true AP and lateral plane to the desired disc space. The reason for this is twofold. First, it allows the surgeon to work in a straight vertical plane perpendicular to the floor (slight movements in the anterior or posterior direction can have dire consequences); second, it allows quick change of the fluoroscopy from AP to lateral and vice versa. Our institution has excellent fluoroscopic technicians who are very familiar with the MIS LIF. For those who have never been exposed the lateral access, a significant amount of time should be spent on proper positioning, as this will save time and frustration in the long run. With any minimally invasive procedure, gross anatomic landmarks are not visualized, so precise fluoroscopic guidance is paramount to avoid major vascular or neurologic injury, which is greatly dependent on positioning.

The patient should always be placed in the lateral decubitus position with both an axillary and hip roll ( Fig. 77-3 ). Prior to lateral positioning, all appropriate EMG monitoring leads should be placed. The predetermined side of lateral access should face up. At our institution, a Cmax table (Steris, Mentor, OH) is used, but any radiolucent operating table with the capabilities of flexion, extension, Trendelenburg/reverse Trendelenburg, and lateral tilting will suffice. The patient’s back is placed near the lateral border of the bed to decrease the surgeon’s working distance, and the iliac crest is placed at the level of the table break where flexion occurs. Both the hips and the knees are maximally flexed to relax tension on both the psoas muscle and the lumbar plexus. Pillows are placed between the patient’s legs. An axillary roll is placed to prevent brachial plexus injury, and the top arm is placed on an elevated arm board out of the way of fluoroscopy. A hip roll is placed just under the iliac crest to promote flexion for better access to L4/5.