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Anterior and Anterolateral Lumbar Spine Decompression and Fusion: Minimally Invasive Surgery Approaches


Summary of Key Points

  • Patient positioning is paramount for a successful procedure.

  • Triggered directional electromyography findings should guide retractor placement anterior to the femoral nerve for the transpsoas approach.

  • Open (expanded) retractor time should be minimized as much as possible during transpsoas surgery to minimize ischemia to the surrounding muscles and nerves.

  • Preoperative evaluation of vascular structures on magnetic resonance imaging can potentially avoid catastrophic complications.

  • Minimally invasive procedures are dependent on the use of fluoroscopy.

  • A thorough knowledge of lateral regional anatomy is required to reduce complications.

  • A prepsoas approach provides an alternative lateral retroperitoneal surgical corridor to approach the disc space at L4‒L5, which may reduce femoral nerve retraction.

  • The anterior approach provides direct access to the anterior spinal column.

Acknowledgment

The authors thank the staff of Neuroscience Publications at Barrow Neurological Institute for assistance with manuscript preparation.

Minimally invasive surgery (MIS) on the spine is an ever-evolving continuum, with expanding applications to a spectrum of patient-specific pathologies. In the lumbar spine, minimally invasive anterior and anterolateral approaches are gaining increasing traction. Traditionally, the lumbar spine was accessed from a posterior corridor, and interbody arthrodesis was achieved via the posterior lumbar interbody fusion or the transforaminal lumbar interbody fusion. These traditional open techniques require dissection through the paraspinal musculature and can cause disruption of the posterior spinal ligamentous complex. Additionally, the transforaminal lumbar interbody fusion requires removal of the facet joint to gain access to the neural foramen and underlying disc space of Kambin’s triangle. These techniques were adapted to be performed in a minimally invasive fashion, with the goal of minimizing native tissue disruption.

Anterior and anterolateral approaches have been developed as alternatives to these traditional techniques. These techniques include the anterior lumbar interbody fusion (ALIF) and the lateral lumbar interbody fusion (LLIF), which can be performed in a prepsoas or transpsoas fashion ( Fig. 132.1 ). The lateral approach was repopularized thanks in part to Dr. Luiz Pimenta. In 2001, he first introduced the retroperitoneal transpsoas minimally invasive LLIF as a safe and effective alternative to both traditional and minimally invasive anterior or posterior approaches to the lumbar spine. , While first described in 1997 by Mayer, the prepsoas approach to the lumbar spine, known as the oblique lumbar interbody fusion, was not popularized until after 2010 as an alternative to LLIF at L4‒L5 as a means to reduce femoral nerve injury risk at that level.

Fig. 132.1
Overview of anterolateral approaches to the lumbar spine. The anterior lumbar interbody fusion ( ALIF ) allows the spine to be approached from an anterior trajectory. The lateral lumbar interbody fusion ( LLIF ) approaches the spine from a lateral retroperitoneal corridor and may use a transpsoas or antepsoas trajectory. The oblique lumbar interbody fusion technique uses a prepsoas approach to the lumbar spine as an alternative to LLIF. Oblique lumbar interbody fusion (OLIF) ,

From Barrow Neurological Institute, Phoenix, Arizona. With permission.

Anterolateral access to the spinal column has some distinct advantages over traditional posterior approaches. These include indirect neurological decompression with minimal tissue disruption, decreased blood loss, and shorter operative times. The normal, stabilizing posterior musculature, facet joints, and ligamentous complexes are not sacrificed as compared with posterior techniques. Surgical access to the disc space is larger, allowing for a more efficient and thorough disc removal and end plate preparation. Moreover, anterolateral access facilitates placement of larger and more lordotic interbody grafts that can span the apophyseal ring to provide maximum support for fusion and reduce the risk of subsidence ( Figs. 132.2 to 132.4 ).

Fig. 132.2
Patient positioning, final surgical exposure, and interbody placement for an anterior lumbar interbody fusion ( ALIF ).

From Barrow Neurological Institute, Phoenix, Arizona. With permission.

Fig. 132.3
Patient positioning, final surgical exposure, and interbody placement for an oblique lumbar interbody fusion ( OLIF ).

From Barrow Neurological Institute, Phoenix, Arizona. With permission.

Fig. 132.4
Patient positioning, final surgical exposure, and interbody placement for a lateral lumbar interbody fusion ( LLIF ).

Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

The clinical applications of the retroperitoneal transpsoas and prepsoas 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 anterolateral approaches, as well as electromyography (EMG) evaluation, surgical technique, and complication avoidance.

Regional Anatomy

Adequate knowledge of the regional anatomy is necessary to gain access to the retroperitoneal corridor while avoiding complications because of injury to nearby structures. Lateral lumbar spine access is limited by the rib cage superiorly and the iliac crest inferiorly. 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), followed by the external oblique, internal oblique, and transversus abdominis muscles. The details of blunt muscle splitting dissection (as opposed to electrocautery) are discussed later because both the L1 branches of the iliohypogastric and ilioinguinal nerves travel through the internal and external oblique muscles. The retroperitoneal space is encountered after the transversalis fascia is breached and is highlighted by the presence of yellow adipose tissue. The quadratus lumborum can be palpated along the posterior wall of the abdomen. This muscle originates from the twelfth rib and upper lumbar transverse processes and inserts on the iliac crest. The quadratus lumborum is followed forward, and the psoas muscle is encountered anterolateral to the vertebral body. The psoas muscle originates from the upper lumbar vertebra transverse processes and inserts on the lesser trochanter of the femur with the iliacus.

Lumbar Plexus

The objective of the transpsoas approach is to dock anterior to the lumbar plexus (specifically the femoral nerve) to prevent distraction of the nerves from their origin when opening the retractor ( Fig. 132.5 ). 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‒L4) and the obturator nerve (L2‒L4). The major sensory branches (not detected by EMG) include the iliohypogastric (T12‒L1), ilioinguinal (L1), genitofemoral (L1‒L2), lateral femoral cutaneous (L2‒L3), and anterior femoral cutaneous (L2‒L4) nerves.

Fig. 132.5, Schematic drawing of the lumbar plexus from a lateral view with the location of the nerves relative to the spine and disc spaces.

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 132.1 for a full description of each nerve with its corresponding innervation.

Table 132.1
Nerves and Corresponding Innervation in the Lumbar Plexus
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 . 2010;13(2):260-266.
Nerve Segment Innervated Muscles Cutaneous Branches
Iliohypogastric T12-L1

  • Transversus abdominis

  • Abdominal internal oblique

  • Anterior cutaneous

  • Lateral cutaneous

Ilioinguinal L1

  • 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

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‒L5 disc space seems to be the most vulnerable for neural injury.

Work by Moro et al. and Benglis et al. have helped establish a safe zone for lateral access to the spine through the psoas muscle at levels L4‒L5 and above. However, the genitofemoral nerve was susceptible to injury, specifically at the L3‒L4 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, with zone 1 representing the most anterior quartile and proceeding posteriorly to zone 4 ( Fig. 132.6 ). From this dissection, they developed generalized “safety zones” for each disc space from L1‒L5.

Fig. 132.6, Lateral radiograph of the lumbar spine demonstrating the four “zones” of the vertebral body, labeled I to IV from anterior to posterior. Zone III, or the relative “safe zone,” is highlighted in green. The recommended safe working zones for each level are indicated by the black circles in the disc space.

For the first three lumbar disc levels (L1‒L2, L2‒L3, L3‒L4), they found that retractor placement was safe in an area beginning at the midpoint of zone III and moving anteriorly. At L4‒L5, the safe zone starting point moved anteriorly and was defined at zone junction II/III or the mid-disc space ( Fig. 132.6 ). The genitofemoral nerve was the only nerve found to be ventral to zone III. It moves ventral at L2‒L3 and migrates caudally to L4‒L5. 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 in males and 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. Discussion of the risks of potential groin and thigh sensory loss and pain preoperatively helps with patient education and anticipation of the predominantly transient nature of these symptoms.

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, the inferior vena cava, and the common iliac arteries and veins. 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 lateral lumber fusion in their morphometric study. They concluded that the risk of injury to the great vessels was greatest at the L4‒L5 level owing to the posterior migration of those vessels. All major vessels (inferior vena cava, aorta, femoral vein/artery) should be identified before 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.

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