The Minimally Invasive Retroperitoneal Transpsoas Approach

First introduced by Luiz Pimenta in 2001, the retroperitoneal transpsoas minimally invasive lateral interbody fusion (MIS LIF) is a safe and effective alternative to anterior or posterior approaches for lumbar fusion. ¹^, ² Advantages include indirect neurologic decompression with less tissue trauma, smaller incision, minimal blood loss, shorter operative times, lower rate of wound issues, placement of a larger cage, and early patient mobilization. ³^, ⁴^, ⁵^, ⁶ In addition, stabilizing ligaments are not sacrificed compared with other interbody techniques.

This mini-open technique was an adaptation of the endoscopic lateral transpsoas approach to lumbar fusion described by Bergey et al. ⁷ The authors found that the endoscopic lateral transpsoas approach to the lumbar spine was a safe method to fuse the lumbar vertebrae, which allowed for exposure of the lumbar spine without mobilization of the great vessels or sympathetic plexus. Perhaps because of the steep learning curve associated with endoscopic techniques, MIS LIF has preferentially gained popularity over the endoscopic procedure. Today, there are several mini-open systems from various manufacturers that will allow for an MIS lateral retroperitoneal transpsoas approach. The two most common are the eXtreme Lateral Interbody Fusion/XLIF (NuVasive, San Diego, California) and Direct Lateral Interbody Fusion/DLIF (Medtronic, Memphis, Tennessee).

58.2 Patient Selection

Surgical indications include degenerative disk disease, spondylosis with instability, lumbar stenosis, spondylolisthesis, adult degenerative scoliosis, adjacent segment failure, and trauma. Early outcome studies have demonstrated that MIS LIF is associated with shorter operating times, less blood loss, fewer complications, shorter hospital length of stay, and quicker recovery compared with traditional open approaches. ⁸^, ⁹ Long-term outcomes are generally favorable, with maintained improvements in patient-reported pain and function scores as well as radiographic parameters, including high rates of fusion.

58.2.1 Degenerative Spine Disease and Deformity

Both as an adjunct to open procedures and as a stand-alone entity, MIS techniques are increasingly being used to treat degenerative spine disease and deformity. In addition to the benefits already mentioned, MIS approaches to the lateral spine can be powerful tools in the reversal of coronal and sagittal imbalance. ⁸^, ¹⁰^, ¹¹^, ¹²^, ¹³^, ¹⁴

Adjacent segment failure is a commonly encountered phenomenon for spine surgeons. Operations to address this issue often involve further posterior muscle dissection and revision of existing instrumentation while negotiating previous scar tissue. This can lead to longer operative times, increased risk of infection, and higher risks of cerebrospinal fluid leak and iatrogenic neurologic injury. The MIS LIF offers an alternative that avoids re-entering a scarred surgical corridor. Virgin tissue is traversed, and lateral approaches afford placement of large interbody cages that may decrease the incidence of subsidence. Increased biomechanical strength can easily be obtained by placing lateral plates. Literature regarding the use of MIS LIF for adjacent segment failure revision surgeries is lacking, yet preliminary studies using this approach for revisions have been encouraging. ¹⁵^, ¹⁶^, ¹⁷

58.2.2 Trauma

Another area where there has been increased interest in the use of MIS LIF is trauma. Traumatic fractures commonly occur in the thoracic and lumbar spine, with many occurring at the thoracolumbar junction. The decision of when to treat operatively versus nonoperatively is beyond the scope of this discussion; however, MIS LIF may be considered an option when stabilization and arthrodesis are deemed necessary.

In a study by Smith et al with a follow-up of 2 years, traumatic fractures treated via MIS lateral corpectomy with posterior instrumentation were found to have favorable operating times, estimated blood loss, and hospital length of stay. ¹⁸ None of the patients required reoperations, and there was a significant improvement in neurologic function based on the American Spinal Injury Association categorization, with none experiencing a neurologic decline.

58.3 Anatomic Considerations

The lateral approach may be unfamiliar to spine surgeons who are accustomed to the posterior approach. Because of this, an understanding of the key structures encountered during the lateral approach is critical. In the order encountered, the abdominal wall muscles, including the external oblique, internal oblique, and the transversus abdominis muscle, are traversed. Once the retroperitoneal space is entered, the quadratus lumborum and psoas muscle are encountered. The details of blunt dissection, as opposed to electrocautery, are discussed later. Careful attention must be given to avoid injuring the nerves of the abdominal wall and lumbar plexus, which can lead to postoperative deficits. ¹⁹

58.3.1 Lumbar Plexus

The lumbar plexus is found within the substance of the psoas muscle. It is a part of the lumbosacral plexus, and it is made of the primary ventral rami of the first four lumbar nerves and a contribution of the subcostal nerve (T12). Multiple motor and sensory nerves are given off. The femoral (L2–4) and obturator (L2–4) nerves are the major motor branches. The iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1–2), lateral femoral cutaneous (L2–3), and anterior femoral cutaneous (L2–4) nerves are the major cutaneous sensory branches. Most nerves are mixed motor and sensory. The intrinsic psoas nerves are the only purely motor nerves, and the lateral femoral cutaneous nerve is the only purely sensory nerve.

58.3.2 Motor Nerves

The femoral nerve is a mixed motor and sensory nerve arising from the medial border of the psoas muscle. It has an anterior and a posterior division. The anterior division gives off the anterior cutaneous nerve and muscular branches. It gives motor innervation to the pectineus and sartorius muscles. The posterior division gives off the saphenous nerve (sensory) and muscular branches. It gives motor innervation to the quadriceps femoris, which is composed of the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The obturator nerve is also a mixed motor and sensory nerve that arises from the medial border of the psoas muscle. It innervates the adductor muscles of the lower extremity. These include the external obturator, adductor longus, adductor brevis, adductor magnus, gracilis, and the pectineus (inconstant) muscles. It does not innervate the obturator internus. It also supplies the sensory innervation of the skin of the medial aspect of the proximal thigh.

58.3.3 Sensory Nerves

The iliohypogastric nerve consists of two branches that innervate the skin of the lower abdominal wall. The lateral cutaneous branch innervates the skin of the gluteal region. Of note, this nerve can also be injured when harvesting an anterior iliac crest bone graft. The anterior cutaneous branch innervates the hypogastric, or lower abdominal region. The ilioinguinal nerve innervates the skin at the base of the penis and upper scrotum in males and the skin of the mons pubis and labia majora in females. The genitofemoral nerve consists of two branches, the genital and femoral branches. The genital branch innervates the cremaster muscle and scrotal skin in males and the skin of the mons pubis and labia majora in females. The femoral branch innervates the skin over the femoral triangle. This nerve is distinct from the other sensory nerves in that it does not follow a lateral trajectory to the site of innervation, but rather it emerges on the anterior surface of the psoas and descends on the ventral surface. The lateral femoral cutaneous nerve innervates the lateral aspect of the thigh. It consists of an anterior and a posterior branch. The anterior branch innervates the skin of the anterior and lateral surfaces of the thigh, as far as the knee. The posterior branch innervates the lateral and posterior surfaces of the thigh, from the level of the greater trochanter to the middle of the thigh. The anterior femoral cutaneous nerve innervates the anterior and medial aspect of the thigh.

58.3.4 Safe Zones

Early anatomical work related to the retroperitoneal transpsoas approach by Moro et al helped to establish a safety zone to prevent nerve injuries. ²⁰ Specifically, they found that it was safe to traverse the psoas muscle at levels L4–5 and above, with the exception of the genitofemoral nerve, which is at risk at between L3 and L4. Further studies described the course of the plexus and found that the plexus lies within the substance of the psoas muscle between the junction of the transverse process and vertebral body, while exiting along the medial edge of the psoas distally. ²¹ It is most dorsally positioned at the posterior endplate of L1–2 with a general trend of progressive ventral migration down to the level of L4–5. When a ratio of the distance from the posterior vertebral body wall to the total disk space length was calculated, it was found that there was a 0, 0.11, 0.18, and 0.28 ratio for L1–2, L2–3, L3–4, and L4–5, respectively. These findings suggest that an overly posterior placement of the dilator and/or retractor can lead to nerve injuries, especially at L4–5, where the ventral migration is nearly one third of the disk space from the posterior vertebral body wall.

A cadaveric study by Uribe et al. defined four different zones and described safe working zones for MIS LIF ⁵ ( ▶ Fig. 58.1). The four zones represent different quartiles of the vertebral body, with zone I representing the most anterior and zone IV representing the most posterior quartile. The lumbar plexus, along with nerve roots, lie within the substance of the psoas muscle and dorsal to zone IV. The genitofemoral nerve was the only structure found to be ventral to zone III, starting at L2–3 and progressing caudally to L3–4 and L4–5. It was determined that the safe anatomical zones to avoid nerve injury from L1–2 to L3–4 are the midpoint of zone III (posterior third of the disk space), and the safe zone for L4–5 is at the zone II and III junction (mid disk space). The genitofemoral nerve is at risk in zone II at L2–3 and in zone I at L3–4 and L4–5.

Fig. 58.1 Safe anatomical zones for minimally invasive lateral interbody fusion. There are four quartiles, I–IV, from anterior to posterior. The open circles indicate a safe zone for placement of the retractor and for subsequent exposure. From L1–2 to L3–4, the posterior third is generally safe. At L4–5, placement at the midpoint between zone II and III is generally safe because this will decrease the risk of injuring the femoral nerve.

The subcostal, iliohypogastric, ilioinguinal, and lateral femoral cutaneous nerves in the retroperitoneal space are also at risk for injuries because they travel obliquely, inferiorly, and anteriorly to the reach the iliac crest and the abdominal wall outside of the psoas in the retroperitoneal space. There is a chance of injury of these nerves in the early stages of the operation while obtaining access to the retroperitoneal space.

In addition to nerve injury, visceral and vascular structures should also be considered during MIS LIF. Regev et al illustrated the importance of meticulous preoperative planning in their morphometric study looking at the relationship of vascular structures relating to MIS LIF. They found that the safe corridor for performing a diskectomy and intervertebral cage placement progressively narrows from L1–2 to L4–5. ²² In the presence of scoliosis, these corridors can potentially be further narrowed. One should also keep in mind that the kidneys are in the retroperitoneal space.

The technique of the retroperitoneal transpsoas MIS LIF by our team has evolved with time and experience. Significant changes were made to our technique in 2010, and these changes have been the standard method we currently use for every patient. Specifically, the technique below refers to the use of the XLIF procedure. In general, the main principles apply to any lateral access system; however, a significant difference that will not apply to other systems is the use of a directional, triggered-electromyelography (t-EMG), which is explained further in this discussion.

58.4 Preoperative Preparation

The identification of good surgical candidates through appropriate preoperative planning is critical. A Preoperative magnetic resonance imaging (MRI) should be reviewed to ensure that the great vessels of the abdomen will allow surgical access to the desired disk space and that the position of the lumbar plexus within the psoas permits a transpsoas approach. A preoperative AP X-ray is evaluated to determine which side will provide the best access to the desired level, especially at L4–5, in relation to the iliac crest.

The patient is then placed in the lateral position with surgical site up. In cases of a scoliotic deformity, entrance through the concavity of the curve may allow surgical access to several levels with fewer incisions. The L4–5 disk space is more easily accessed through the concavity as well. In severe deformity, aberrant anatomy may preclude transpsoas retroperitoneal approaches, and each disk space should be meticulously reviewed to ensure there are no contraindications to the procedure.

At our institution, patients are placed on a Cmax table (Steris, Mentor, Ohio), but any radiolucent operating table that allows for adjustment of flexion–extension, tilting and Trendelenburg/reverse Trendelenburg will suffice. The iliac crest is placed at the level of the table break where table flexion occurs. The legs are flexed maximally at the hip and knee to reduce tension on the psoas muscle. A roll is placed beneath the axilla to prevent brachial plexus injury, and under the iliac crest to lower iliac crest to improve access to the L4–5 level.

Intraoperative fluoroscopy is then used to position the patient such that a symmetric AP image with the pedicles equidistant from the spinous processes is achieved. It is essential that these images be as accurate and symmetric as possible to prevent inadvertently dissecting too far anteriorly or posteriorly. Caution should be exercised if a prior laminectomy exists over the desired level.

Once properly positioned, the patient is taped and secured in place at the iliac crest and chest. The ipsilateral hip and leg are then taped to pull the iliac crest inferiorly and then secured to the table to prevent movement during surgery ( ▶ Fig. 58.2). Anteroposterior (AP) films are repeated to ensure that positioning was not changed during taping. The bed can be “airplaned” slightly if correction of the image is needed. ( ▶ Fig. 58.3) The relationship of the ipsilateral iliac crest to the lowest surgical level is then evaluated. The disk space should be approachable by a directly perpendicular surgical corridor; if it is not, the table can be flexed to allow access to the disk space. Table flexion should be used judiciously, as it places tension on the psoas muscle and lumbar plexus, potentially leading to neurologic injury. It is advisable to avoid table flexion if it is not necessary.

Fig. 58.2 Lateral decubitus positioning for minimally invasive lateral interbody fusion. The patient is placed on an adjustable operative table and secured with silk tape. There is a bend in the table to allow for better access during surgery.

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