6 Minimally Invasive Lateral Transpsoas Interbody Lumbar Fusion
Keywords: adjacent segment, femoral nerve, genitofemoral nerve, iliohypogastric nerve, ilioinguinal nerve, interbody fusion, lumbar plexus, scoliosis, transpsoas
The more original a discovery, the more obvious it seems afterwards.
Arthur Koestler
6.1 Introduction
No statement better describes the transpsoas interbody approach to the lumbar spine than Arthur Koestler’s astute observation on discovery. For decades of modern spine surgery, surgeons have traversed the paraspinal muscles for posterior approaches, mobilized the peritoneum and the iliac vessels for anterior surgery, and taken down the diaphragm and mobilized the psoas muscle in lateral thoracolumbar approaches. All these procedures were done in the name of accessing the thoracic, lumbar and sacral spine. Initially, none of these exposures were minimal; however, all were safe, comprehensive and reliable. In the end, these approaches were the necessary first steps in the evolution of spine surgery that would ultimately yield an elegant solution for accessing the lumbar interbody space.
The sophisticated understanding of the anatomy and the increasing experience gained from these approaches became the stepping-stone for the development of minimally invasive approaches to the spine. An entire array of minimally invasive techniques began to evolve to accomplish the same goals that were being otherwise accomplished with traditional midline incisions. Lumbar and cervical decompressions could now be reliably achieved through minimal access ports. Soon after, the access ports allowed for instrumentation of the lumbar spine. Building upon these maturing minimally invasive platforms, innovative surgeons pioneered a solution to a problem that had been staring all of us in the face for decades. Spine surgery then took a colossal step forward with the straightforward and well-conceived transpsoas approach to the lumbar interbody space.
At the end of the last century, the trend toward minimizing the extent of exposure required to accomplish the same goals of traditional midline surgeries quickly gathered momentum. Adhering to the principle introduced by Caspar regarding the ratio of the surgical target to the surgical exposure, a series of dilators passed through a paramedian incision expanded a corridor for a minimal access port and became a viable alternative to traditional larger midline exposures with self-retaining retractors. These techniques began by addressing more straightforward pathologies such as herniated discs and lumbar stenosis. The skill sets developed to deal with these pathologies were, in turn, translated to instrumentation of the lumbar spine and lumbar interbody approaches. By the mid-2000s, a firm foothold and comfort level had been established using dilators, table-mounted arms, and fixed diameter and expandable minimal access ports in the posterior lumbar and posterior cervical spine. These new techniques would lay the foundation for a more direct lateral approach to the lumbar interbody space.
Interest in a more direct approach to the spine from a lateral approach may be traced back to as early as 1985 when McAfee and colleagues 1 described decompression and stabilization of thoracolumbar burst fractures. Comfort with the thoracolumbar exposures eventually enabled these authors to explore a more focused exposure within the retroperitoneal space, which led directly to one of the first descriptions of lateral lumbar interbody fusions by Mayer2 and McAfee et al3 in the late 1990s. Running in parallel with spine surgeons’ increasing interest and comfort in the retroperitoneal space were continued improvements in minimally invasive techniques. Building upon the existing platforms of both endoscopic and minimal access port-based approaches, surgeons now began to narrow their focus on one target: the disc space. As early as 2004, Bergey and colleagues4 described an endoscopic transpsoas interbody approach to the lumbar spine. In due time, a transpsoas approach built upon the muscle dilating tubular retractor platforms would follow. In 2006, Ozgur and colleagues5 reported the current technique of the lateral transpsoas approach and thus commenced the decade of lateral transpsoas interbody lumbar fusions.
I still remember learning about the transpsoas interbody approaches to the lumbar spine as a fifth-year resident. My first reaction was, “Why did it take so long to think of that?” It seemed like such an obvious solution to the upper lumbar spine, especially in cases of adjacent segment degeneration at L2–3 or L3–4. I recalled case after case where I painstakingly exposed and then extended previous fusions, an operation that caused equal discomfort to the patient and the surgeon alike. All those cases now fell into the precinct of a transpsoas approach, which avoided the previous posterior surgery altogether, restored the disc height, corrected the coronal imbalance and indirectly decompressed the neural elements. I marveled as I considered the implications of this clear answer to a difficult circumstance that, as spine surgeons, we had been facing all this time. Again, the more original the solution, the more obvious it seems afterward. My second reaction was concern. I had completed my spine rotation, and I feared that the opportunity to learn this technique in residency might have passed. I dreaded the possibility that I might stumble through a career in spine surgery without this procedure in my arsenal.
6.2 Advantages of the Transpsoas Approach
A lateral exposure avoids all the consequences of a posterior exposure, specifically iatrogenic instability from resection of the bony elements and disruption and denervation of the paraspinal muscles. The distinct advantage of the transpsoas approach to the interbody space is that it is a minimally invasive approach in the purest sense. An access corridor in the safety of the retroperitoneal space allows for direct exposure of the disc space through the psoas muscle without the need for any significant dissection or disruption of the musculature. The very nature of a retroperitoneal corridor onto the lateral spine makes the risk of vessel injury substantially less than in anterior approaches, where the aorta and vena cava need to be directly exposed and mobilized. The decreased risk of vessel injury is especially true in the upper lumbar segments where anterior exposures require extensive mobilization of the great vessels (▶ Fig. 6.1).
Fig. 6.1 Illustration demonstrating the various corridors to the lumbar disc space. (a) Anterior view of the vascular anatomy of the lumbar spine. At L5–S1 a generous corridor becomes available with minimal mobilization of the iliac arteries and veins. However, the anterior corridor to the disc space becomes constrained in the upper segments of the lumbar spine because of the aorta and vena cava. Mobilization of these vessels is necessary to access the upper levels of the lumbar spine but with a subsequently increased risk of vascular injury. (b) Illustration showing the transforaminal corridor at L4–5. Access to the disc space is constrained by the exiting and traversing nerve roots. (c) Lateral view of the lumbar spine demonstrating the vascular anatomy. The aorta and vena cava remain anterior, and the trajectory of approach does not require visualization or mobilization of these vessels. The segmental vessels course over the lateral vertebral body and remain vascular structures at risk with a lateral approach, but these vessels do not course over the disc space. The main disadvantage is the need to traverse the psoas muscle and navigate the lumbar plexus. The combination of a thinner psoas muscle and more posteriorly located lumbar plexus makes the upper segments of the lumbar spine ideal for the transpsoas approach.
In an anterior approach, the iliac vessels, the veins in particular, have the potential to constrain access to the disc space. In a posterior transforaminal approach, access to the disc space is constrained by the exiting and traversing nerve roots (▶ Fig. 6.2). The single greatest advantage of the transpsoas approach is the unconstrained wide corridor into the disc space. A direct trajectory into this wide corridor provides the ability to span the entire breadth of the disc space and cover a substantial amount of the apophyseal ring. The correction of a coronal imbalance within a segment is unparalleled with this approach.6
Fig. 6.2 Access corridors to the lumbar disc space. (a) Intraoperative photograph of an anterior lumbar interbody fusion at L5–S1. The vascular anatomy at L5–S1 level offers a generous corridor into the disc space of up to 40 mm. The corridor becomes more constrained at the levels above L5, resulting in a greater vascular risk, especially because of the proximity of the vena cava. (b) Intraoperative photograph of a transforaminal corridor on the right at L4–5. The access to the disc space is constrained by the traversing nerve root and exiting nerve root. The resulting corridor is 10–12 mm. (c) Intraoperative photograph of a transpsoas approach at L2–3. The corridor into the disc space is not constrained by vascular or neural anatomy. After a safe corridor has been established through the psoas with electrophysiologic monitoring, and the genitofemoral nerve has been safely mobilized, there are over 20 mm of access to the disc space. (Photograph (c) is provided courtesy of Juan S. Uribe, MD.)
6.3 Disadvantages
The main limitation of the transpsoas approach is the unseen formation of the lumbar plexus and its branches within the psoas muscle. Navigating around these unseen branches of the plexus while traversing the psoas muscle to reach the disc space is the most technically demanding aspect of the operation. The lower the segment on the lumbar spine, the more anterior the lumbar plexus becomes and the greater the risk of neurologic injury. One lumbar plexus branch in particular that remains at risk with this approach is the genitofemoral nerve, which has the uncanny ability to unveil itself immediately over the disc space that requires the operation. A common course for the genitofemoral nerve is to travel within the psoas major muscle over the top of the L2–3 disc space. Identification and mobilization of the nerve are essential to mitigate the risk of neuropraxia or disruption. But it is not only the lumbar plexus that is a concern in this approach. The lower one descends in the lumbar spine, the thicker the psoas becomes, adding to the distance that one has to traverse to reach the spine. Traversing the psoas comes at the consequence of hip flexion weakness and soreness in the coming weeks after the operation. Discussions with patients regarding these risks are an important part of the patient education and informed consent for this operation.
6.4 Patient Selection and Lumbar Segment Selection
Ideal patients present in three broad categories: an isolated degeneration at one of these levels (L1–2 or L2–3), multiple levels of degeneration that include these segments resulting in deformity or an adjacent segment degeneration above a previous fusion construct. The upper lumbar segments, L1–2 and L2–3 in particular, are ideal for a transpsoas interbody approach. At both of these levels, the psoas is thin, and the lumbar plexus has yet to become fully formed, and what has formed is in the posterior aspect of the disc space. This arrangement leaves the anterior two-thirds of disc space as a corridor for entry, once one has appropriately identified and mobilized the genitofemoral nerve.
While single-level degeneration of the upper lumbar spine is not common, when these patients do present, a minimally invasive same-day, standalone surgical option is a valuable intervention to offer (▶ Fig. 6.3).7 The most common scenario is the involvement of L1–2 and L2–3 within a multilevel degenerative deformity. The transpsoas technique in that circumstance is part of a more comprehensive strategy to address the deformity. A single incision can offer access to up to three levels as the first phase of the operation. Posterior instrumentation, posterior column osteotomies, and additional lower lumbar segments are addressed during the second phase of the operation (▶ Fig. 6.4).8,9
Fig. 6.3 Isolated degeneration at the L2–3 segment. The ideal candidate for a transpsoas approach is an individual with a degenerative segment in the upper lumbar segments. In this case illustration, the patient presents with single-level disc degeneration at L2–3. The location of the lumbosacral plexus and the thin psoas muscle at this level makes a transpsoas approach ideal. (a) Anteroposterior radiograph of the lumbar spine showing no coronal deformity. Note the lateral osteophyte on the right at L2–3 (arrow). (b) Lateral radiograph of the lumbar spine showing single level degeneration at L2–3. The collapse of the disc space has resulted in a loss of the segmental lordosis. The presence of the vacuum disc phenomenon that is evident within the disc space (arrow) assures the ability to reliably restore disc height and segmental lordosis with an interbody spacer. (c) Sagittal T2-weighted magnetic resonance imaging of the lumbar spine showing the focal stenosis at the segment of L2–3 (arrow). This patient presented with elements of neurogenic claudication and L2 radiculopathy.
Fig. 6.4 Transpsoas approach as part of deformity correction. (a) Preoperative anteroposterior (AP) radiograph demonstrating severe coronal imbalance, lateral listhesis and focal deformity in the lumbar spine. The patient has a Cobb angle of 32 degrees. (b) Postoperative AP radiograph demonstrating the application of the transpsoas approach as part of a more comprehensive strategy to correct the coronal imbalance at multiple levels (L1–2, L2–3 and L3–4). The patient underwent pedicle screw fixation from L1 to L5, transforaminal lumbar interbody fusion at L4–5 and posterior column osteotomies at L1–2, L2–3, L3–4 and L4–5.
The third category of patients that is ideal for the transpsoas technique is patients with extensive constructs in the lower lumbar spine with a symptomatic adjacent segment. In these patients, the transpsoas approach is an appealing alternative to exploration, explantation and extension of the fusion construct (▶ Fig. 6.5). In the absence of a severe coronal imbalance or instability, a standalone construct for the management of the adjacent segment is an option.10,11,12 Relying primarily on the principle of indirect decompression by restoring the disc height, the adjacent segment may be adequately treated. Patients are then observed for the coming weeks and months. It becomes evident when and if a posterior operation will be necessary for additional decompression and stabilization. In this scenario, the transpsoas approach transforms a 3- to 4-hour operation with a 3- to 4-day hospital stay into a procedure performed in less than 1 hour with a 23-hour admission period.
Fig. 6.5 Transpsoas approach for the management of adjacent segment degeneration. Adjacent segment degeneration is seen at L2–3 above the level of an instrumented decompression and fusion at L3–4 and L4–5 on (a) an anteroposterior (AP) standing radiograph and (b) a lateral standing neutral radiograph. (c) Sagittal T2-weighted magnetic resonance imaging (MRI) demonstrating severe stenosis at the L2–3 segment. The redundancy of nerve roots is clearly evident. (d) Axial T2-weighted MRI revealing the ligamentum flavum and facet arthropathy contributing to the central stenosis. (e) Postoperative AP radiograph after placement of interbody placed through a transpsoas approach. (f) Lateral radiograph demonstrating correction of segmental lordosis and restoration of disc height. No additional surgery was needed for this patient.
6.5 Transpsoas at L4–5
A transpsoas approach to the L4–5 segment presents unique challenges that are not present in the upper segments of the spine (L1–2, L2–3 and L3–4). The iliac crest, the thickness of the psoas and the femoral nerve are all factors that need to be considered. Anteroposterior (AP) and lateral radiographs provide the information needed regarding the iliac crest. The lateral radiograph, in particular, shows whether the iliac crest prevents access or limits an ideal trajectory to the disc space. If the patient has a favorable iliac crest for a transpsoas approach, then the focus of the operation becomes navigating the lumbar plexus through the psoas muscle, with particular attention to the femoral nerve. The experience in the literature has demonstrated the safety and efficacy of the transpsoas approach at the L4–5 segment.13 As I consider the anatomy of the lumbar plexus at L4–5 and its branches, and the femoral nerve in particular, I admire the thickness of the psoas muscle and assess the corridor provided to me by the iliac crest, and I take pause. The L4–5 transpsoas approaches that I have performed have resulted in substantially more discomfort in the hip flexors for a longer duration than at the segments above it. I readily concede that there are techniques that mitigate the risk to the lumbar plexus and the disruption of the psoas muscle, such as shallow docking of the access port. With experience comes efficiency, and there is little doubt as to the impact of retraction time on the psoas muscle. I have several colleagues who have mastered a transpsoas interbody technique at the L4–5 segment.
However, it is difficult for me to approach the L4–5 segment with a transpsoas approach when, in my hands, the transforaminal approach is a perfectly viable option. Furthermore, the degenerative pathology that is addressed at L4–5 is typically posterior, specifically spondylolisthesis, facet arthropathy or lumbar stenosis. While distraction of the disc space allows for indirect decompression of the neural elements, I find great comfort in direct visualization and direct decompression of the symptomatic nerve roots and the thecal sac. In cases of a mobile spondylolisthesis at L4–5, it is difficult to have a standalone construct. In these cases, the transpsoas approach is typically augmented with the placement of pedicle screws and at times, additional decompression. Although a transpsoas approach is a perfectly viable surgical solution, from a philosophical standpoint, if there is one comprehensive solution to a problem with a single approach that accomplishes all of the goals of surgery in a 1.5-hour procedure from one position, that would be the approach I would intuitively favor. Finally, I have not had transient or permanent motor or sensory deficits in my L4–5 transforaminal approaches at the rate reported for the transpsoas approach.13
The L3–4 segment falls somewhere in between. For single-level L3–4 degeneration, even in the context of a coronal imbalance, I tend to favor a transforaminal approach. Where the transpsoas interbody approach has become transformational is in cases of adjacent segment degeneration, specifically in those patients who have had L4–S1 instrumented fusions (▶ Fig. 6.6).
Fig. 6.6 Transpsoas approach at L3–4. A patient who presents 12 years after an L4–5, L5–S1 instrumented interbody fusion with adjacent segment degeneration at L3–4. The patient had been fused without lordosis at L4–5 or L5–S1, likely contributing to the adjacent segment issue. (a) Sagittal T2-weighted magnetic resonance imaging (MRI) showing anterolisthesis of L3 on L4. The MRI primarily shows foraminal stenosis, and the patient presented with L3 and L4 symptoms exacerbated by any degree of ambulation. (b) Lateral radiograph again demonstrating the collapse and anterolisthesis of L3 on L4. Note the vacuum disc phenomenon, which is indicative of the ability to restore disc height and alignment. An exploration of the previous fusion, explantation of hardware and extension to L3–4 is an extensive operation for this particular patient with multiple comorbidities. By comparison, a transpsoas approach is an efficient minimally invasive option. (c) Anteroposterior radiograph demonstrating the restoration in disc height with a transpsoas approach. (d) Lateral radiograph demonstrating disc height restoration, reduction of the listhesis and the return of some segmental lordosis. In this case, the patient was discharged 23 hours after management of the adjacent segment with a transpsoas approach.
6.6 The Literature and the Lumbar Plexus
As experience among surgeons grew, and complications became increasingly recognized, the literature began to reflect a more sophisticated understanding of the anatomy of the lumbar plexus. As I read these manuscripts over the years, I breathed a sigh of relief that, except for one genitofemoral injury and an abdominal hernia, my undeveloped understanding of the lumbar plexus did not lead to an irreversible catastrophic injury to the plexus or one of its branches.
In the current transpsoas literature, surgeons have reported their extensive breadth of experience and their complications, along with comprehensive anatomical studies of the lumbar plexus.8,14,15,16,17,18,19 Reviewing these experiences and studying this anatomy enable you to possess a sophisticated understanding of the lumbar plexus in the context of a transpsoas approach that was not available when this procedure was first introduced. Take full advantage of that body of literature, which represents the growing pains of a novel technique. That knowledge is a key component of complication avoidance. A high-level understanding of the lumbar plexus positively impacts your decision-making to proceed with this technique for surgery, during surgery, and even after surgery as you guide patients through the postoperative course. Whether it is mobilizing the genitofemoral nerve or recognizing that a safe corridor into the L4–5 disc space is not feasible because of the location of the femoral nerve, it is anatomical certainty that empowers you to make a decision.
In the paragraphs below, I highlight some of the most valuable aspects of this anatomy. Still, I encourage you to read the anatomical studies in the bibliography of this chapter. Those manuscripts filled the void of my knowledge and served as a foundation for my understanding of the lumbar plexus and its branches. That knowledge has given me the confidence to manage the complications that have arisen from this technique and, at other times, to altogether avoid complications for patients who have trusted me with their care. You have the distinct advantage of reading about the learning curve of a novel surgical technique that occurred in real time. That literature accurately describes the evolving understanding of the innervation of the psoas muscle, 20 the implications of the sensory branches,13,21 which cannot be monitored and the potential motor deficits that arise based on the surgical level.13 Harness all that literature to your advantage for the benefit of your patients. One thing is certain: your mind should be able to visualize the lumbar plexus and its branches far better than the brachial plexus and its branches.
6.7 Anatomy of the Lumbar Plexus
The lumbar nerve is numbered the same as its vertebra and courses beneath the vertebral pedicle. It then exits its foramen to merge with other lumbar nerve roots at several points. The confluence of these nerves that occurs at several points both outside and within the substance of the psoas muscle forms the lumbar plexus. The following section describes the plexus and its branches, first in the abdominal wall and then in the psoas muscle (Video 6.1).
6.8 Branches of the Lumbar Plexus in the Abdominal Wall
Early in my transpsoas experience, I placed all my emphasis on the branches of the lumbar plexus within the psoas muscle itself and not enough emphasis on the nerves coursing outside of the psoas muscle. The reality is that the nerves that course outside of the plexus and instead through the abdominal wall may be at greater risk. Unlike the plexus in the psoas major muscle, no electrophysiologic monitoring identifies these nerves. The nerves of the abdominal wall include the subcostal, iliohypogastric, ilioinguinal and lateral femoral cutaneous nerves, and they are the first nerves that you have the potential to encounter as you make your way through the various muscle layers of the abdominal wall and into the retroperitoneal space.
6.9 Subcostal Nerve
The first nerve to consider is not even a part of the lumbar plexus. Instead, it is a branch of the ventral ramus of the T12 nerve root, which forms the subcostal nerve. The nerve courses anteriorly to the upper part of the quadratus lumborum and then travels in between the transversus abdominis muscle and internal oblique. It is important to recognize that the subcostal nerve has a motor and sensory component, supplying the muscles to the anterior abdominal wall, especially the external oblique. Injury to this nerve may result in weakness to the musculature of the anterior abdominal wall, anterior abdominal numbness and even painful paresthesias.
6.10 Iliohypogastric and Ilioinguinal Nerves
The iliohypogastric and ilioinguinal nerves represent the first branches of the lumbar plexus. Both of these branches originate from the ventral rami of the T12 and L1 nerve root (▶ Fig. 6.7). The iliohypogastric nerve emerges from the lateral border of the psoas major muscle, continues anteriorly to the quadratus lumborum and then travels anteriorly between the muscle layers of the transversus abdominis and internal oblique muscles, eventually completing its course in between the internal and external oblique muscles (▶ Fig. 6.8). The ilioinguinal nerve travels along a similar path caudal to the iliohypogastric nerve with the main difference between these two nerves being the site of termination. The ilioinguinal nerve travels to the inguinal canal and emerges superficially to the inguinal ring.
Fig. 6.7 The formation of the subcostal, iliohypogastric and ilioinguinal nerves (emerald green). The T12 ventral ramus branches to give rise to the subcostal nerve and a second branch that merges with the ventral ramus of the L1 nerve root and gives off the iliohypogastric and ilioinguinal nerves.
Fig. 6.8 Illustration demonstrating the course of the subcostal, iliohypogastric and ilioinguinal nerves as they leave the spine and traverse through the muscle layers of the abdominal wall. (a) An oblique illustration of the lumbar spine and the abdominal wall muscles with the subcostal, iliohypogastric, and ilioinguinal nerves leaving the spine and entering the abdominal wall. The dilators are seen traversing the abdominal wall en route to the lumbar disc spaces of L2–3 and L3–4. Blunt dissection and avoiding cautery minimizes the risk of injury to these nerves. (b) A lateral illustration of the image in a showing the abdominal wall and lumbar spine where the three muscle layers have been cut away. The black rings represent the position of the dilator over the disc spaces of L2–3 and L3–4. The iliohypogastric nerve courses just above the L3–4 dilator. The ilioinguinal courses well below the iliohypogastric. Note the genitofemoral nerve courses within the psoas just anterior to the dilator over the L2–3 disc space and then pierces through the psoas muscle and continues on the surface of the muscle.
Similar to the subcostal nerve, both of these nerves have motor and sensory components. The iliohypogastric nerve gives rise to an anterior cutaneous branch that innervates the suprapubic skin. The ilioinguinal nerve provides sensation to the medial skin of the thigh. In women, it provides sensation to the mons pubis and labia majora, while in men it provides sensation to the base of the penis and upper part of the scrotum. Both of these nerves supply the muscles of the anterior abdominal wall, and injury may result in paresis of the abdominal wall that may lead to herniation.
6.11 Lateral Femoral Cutaneous Nerve
6.12 Lumbar Plexus within the Psoas Major Muscle
6.12.1 Genitofemoral Nerve
Branches of the ventral rami of L1 and L2 combine within the substance of the psoas major muscle to form the genitofemoral nerve (▶ Fig. 6.9) as it descends through the psoas and tends to emerge on the medial border opposite L3 or L4. It proceeds beneath the peritoneum on the psoas major muscle and divides into its genital and femoral components above the inguinal ligament. However, it is not uncommon to see both branches coursing on top of the psoas major muscle. As the name implies, it provides sensation to the femoral and genital areas. The femoral component provides sensation to the upper medial thigh and skin over the femoral vessels. The genital branch enters the inguinal canal and supplies the cremaster and the skin of the scrotum in men, whereas in women, it follows the round ligament to supply sensation to the mons pubis and labia majora. Injury to this nerve results in decreased sensation or painful paresthesia, or both in these distributions.
Fig. 6.9 The genitofemoral nerve. (a) Schematic illustration demonstrating the branches of L1 and L2 combining to form the genitofemoral nerve. (b) Anatomical illustration demonstrating the course of the genitofemoral nerve (emerald green) with the psoas illustrated on the right of the spine and without the psoas illustrated on the left of the spine. The course of the genitofemoral nerve is on the medial border of the psoas major, which means that in a transpsoas approach, the genitofemoral nerve is approached blindly. Electrophysiologic monitoring does not reveal the location of this nerve. It is worthwhile to dissect through the psoas to identify this nerve to avoid injury to it. The left side of the spine shows the course of the genitofemoral nerve (emerald green) relative to the other nerves of the lumbar plexus.
When performing the dilatation through the psoas, it is important to recognize that the genitofemoral nerve courses on the medial aspect of the psoas and more often than not, you will be approaching the nerve blindly. Second, electrophysiologic monitoring does not identify the location of this cutaneous nerve. In my experience, I have found this particular nerve to be the most variable nerve of the lumbar plexus, and as such, I always take a moment or two to bluntly dissect through the psoas muscle in search of this nerve before proceeding to the disc space. If I have exposed an adequate amount of disc space for the discectomy and have not found it, I proceed with the interbody work. If during my dissection, I do identify it, I take the time to mobilize it out of the way of the working corridor to the disc space.
6.12.2 Femoral Nerve
The dorsal branches of the ventral rami of the L2, L3 and L4 combine within the substance of the psoas major muscle to form the femoral nerve, which is the largest branch of the lumbar plexus (▶ Fig. 6.10). Even after it is fully formed, the femoral nerve continues within the psoas major muscle and does not emerge within the corridor provided by transpsoas access. The nerve continues beneath the inguinal ligament and then divides into its anterior and posterior branches to innervate the quadriceps and provide sensation to the leg. Since it remains unseen, identification of the nerve relative to the access corridor into the disc space is essential and is discussed in greater detail later.
Fig. 6.10 The femoral nerve. (a) Schematic illustration demonstrating the branches of L2, L3 and L4 (emerald green) combining to form the femoral nerve. (b) Anatomical illustration demonstrating the course of the femoral nerve (emerald green) within the substance of the psoas on the right of the illustration emerging only after outside the surgical window of the transpsoas approach. The nerve emerges on the inferolateral aspect of the psoas muscle just above the inguinal ligament. The branches contributing to the femoral nerve and the femoral nerve itself must be identified with electrophysiologic monitoring to avoid injury. The left side of the illustration shows the course of the femoral nerve relative to the other nerves of the lumbar plexus.
6.12.3 Practical Application of the Lumbar Plexus Anatomy to Traverse the Psoas
Moro and colleagues22 published one of the first anatomical studies in the literature, specifically assessing the lumbar plexus relative to the psoas. Interestingly, the study was written in 2003 with endoscopic approaches to the lumbar disc space in mind and predated the landmark 2006 publication by Ozgur and colleagues5 describing the current transpsoas interbody technique. Regardless, it represents the first study to analyze the lumbar plexus and its branches relative to the psoas muscle and the disc space. Therefore, it was immediately applicable to the transpsoas approach, which had no other anatomical study that provided the necessary information. Moro and colleagues divided the vertebral bodies and the disc space into four quadrants (▶ Fig. 6.11).22 The most anterior quadrant has been designated zone I and the most posterior quadrant has been designated zone IV.15 The authors sectioned their specimens and reported the frequency of finding one of the branches of the lumbar plexus within a particular zone. There is no question as to the value of this grid system for anatomical probabilities of the location of the various branches, especially when considering that the study was not even written with the transpsoas interbody technique in mind. However, there is a limitation to its practical application in surgery.
Fig. 6.11 Illustration from Moro and colleagues22 determining the frequency of occurrence of the branches of the lumbar plexus in the four zones. The number correlates with the vertebral body. The “s” is indicative of the superior aspect of the vertebral body, and the “i” is indicative of the inferior aspect of the vertebral body. The disc spaces are labelled. It is important to note that the authors did not incorporate the genitofemoral nerve into this illustration. (Reproduced with permission from Moro T, Kikuchi S, Konno S, Yaginuma H. An anatomic study of the lumbar plexus with respect to retroperitoneal endoscopic surgery. Spine. 2003; 28(5):423428, discussion 427428.)
Application of the four-quadrant system locates the branches that make up the ilioinguinal and iliohypogastric nerves in zone IV at the L2–3 disc space. When moving from rostral to caudal, the next branch is the lateral femoral cutaneous nerve. The lateral femoral cutaneous nerve arises from the posterolateral border of the psoas at L3–4 in zone IV. As mentioned earlier, these nerves may be more at risk from the preliminary exposure through the abdominal wall at the distal end of their course than traversing the psoas muscle into the disc space and encountering them at the beginning of their course.
Branches from the L1 and L2 nerve root form the genitofemoral nerve. The L1 branch tends to cross the L1–2 disc space on a course in the anterior half of the vertebral body. The L2 contribution joins the L1 branch on its anterior course. Several anatomical studies corroborate my experience with reliably identifying this nerve over the top of the middle or anterior aspect of the L2–3 disc space (zone II). When this nerve is identified, it is essential to mobilize and protect it from the corridor that is being used to access the interbody.
Finally, the branches of L2, L3 and L4 coalesce to form the femoral nerve, which is the largest caliber branch of the lumbar plexus that resides deep within the psoas muscle. It courses along a trajectory gradually proceeding from its posterior to anterior position and can reach the midvertebral body (zone III) at the L4–5 disc space.
6.12.4 Major Sensory and Motor Nerves Relative to the Disc Space
Since the objective of the transpsoas interbody approach revolves entirely around accessing the disc space, the concept that you must master is the anatomical location of the lumbar plexus and its branches in the retroperitoneal space relative to the disc spaces that you intend to access. Uribe and colleagues15 evolved the four-quadrant system into a more practical anatomical study of the lumbar plexus along with its branches relative to the disc spaces from L1–2 to L4–5 (▶ Fig. 6.12). Identifying the safe zone is tremendously helpful as a starting point when performing the dilation phase through the psoas muscle. To this day, as I traverse the psoas to access the disc space, I keep in mind the observations made by these authors, which I have summarized below.15
Fig. 6.12 The transpsoas safe working zones to access the disc space. Illustration incorporating the safe working zones as reported by Uribe and colleagues.15 The safe working zones at the various disc spaces have been marked with a magenta fiducial with the branches of the lumbar plexus in full view (a) without the psoas (b) and with the psoas. It is important to be able to visualize the image in a in your mind’s eye during surgery when, in reality, all you see is the image in b.
L2–3: The genitofemoral nerve can take a course in the midportion of the disc space (zone II). The remaining nerve roots and divisions all course in the posterior aspect of the disc space (zone IV).
L3–4: The variable genitofemoral nerve may traverse in the mid to anterior portion of the disc space (zone II) or even the anterior aspect of the disc space. The remaining nerves traverse the disc space posterior to the mid vertebral body line (zone IV). Dissection through the psoas muscle to identify and mobilize the genitofemoral nerve should be considered.
L4–5: The femoral nerve and the obturator nerves may course right up to the mid vertebral body line (zone III). Once again, the genitofemoral nerve may be found in the anterior aspect of the disc space (zone I).
Having in your possession the knowledge of all the anatomical studies in the literature does not change the reality of surgical anatomy that lay before you or the information provided by intraoperative monitoring. Nothing replaces what you can see directly with your own eyes. The anatomical studies and electrophysiologic monitoring serve as guides through the psoas and into the disc space, but it is the anatomy at depth that dictates your surgical decision-making, especially in the case of the genitofemoral nerve, which does not reveal its position with electrophysiologic monitoring.
6.13 Vascular Anatomy
Understanding the vascular anatomy of the lumbar segment is as important as understanding the lumbar plexus, especially on the rare occasion that you encounter vigorous bleeding. I did not have that perspective when I first began performing these procedures, but bleeding from a lumbar segmental artery quickly cured my myopic focus on the lumbar plexus and expanded my vision to include the vascular structures. Review of ▶ Fig. 6.13 reveals the lumbar segmental arteries branching off the aorta and coursing with a slightly upward slope before settling into the midvertebral body. The lumbar vein runs in parallel to the segmental artery draining into the vena cava. It is important to recognize the ascending lumbar vein, which courses in the posterior aspect of the vertebral body within the substance of the psoas muscle. Examination of the vascularity leads to the intuitive conclusion that the safest corridor devoid of vascular structures is the center of the disc space. In the event of bleeding, identifying its nature (i.e., arterial or venous) leads to its control and resolution. A bleeding segmental artery cannot be controlled with a hemostatic agent and tamponade, whereas venous bleeding can. Direct visualization and cauterization are needed for the control of arterial bleeding. Hemostatic agents, pressure and patience are needed for venous bleeding. At times, visualization through the operating microscope and dissection through the psoas onto the vertebral body vastly facilitate identification and cauterization of the bleeding vessel.
Fig. 6.13 Vascular anatomy of the lumbar segment. Illustration of the arterial and venous structures at the L2–3 lumbar segment. The psoas muscle is diaphanous in this image to reveal the vascular anatomy. The lumbar segmental artery comes directly off the aorta, tends to have an upward slope and course in the midvertebral body. The segmental artery is accompanied by the vein that drains into the vena cava. The ascending lumbar vein courses in the rostrocaudal direction along the posterior aspect of the disc space within the substance of the psoas. The nerves of the lumbar plexus are also illustrated in this image. Note the genitofemoral nerve piercing the psoas muscle and then coursing on its surface.
6.14 Anatomical Basis and the Requisite Anatomical Unit
Assembling the dimensional values of the lumbar vertebrae reported by Panjabi and colleagues,23 incorporating the safe working zones reported by Uribe and colleagues,15 and finally superimposing the lumbar plexus and vascular anatomy establish the anatomical basis for the transpsoas procedure (▶ Fig. 6.14). Even more specific is establishing what anatomy must be exposed for a seamless, efficient and low-risk procedure. What I refer to as the requisite anatomical unit for the procedure centers within 20 to 22 mm on the anterior two-thirds of the disc space, which I have evaluated to be devoid of the structures of the lumbar plexus with electrophysiologic monitoring (▶ Fig. 6.14). The exposure encompasses at most 10 mm of the rostral and caudal vertebral body, keeping the working corridor a safe distance from the vascular anatomy. The next section describes the technique for placement of the access port within the ideal requisite anatomical unit.
Fig. 6.14 The requisite anatomical unit for a transpsoas approach at L2–3. Illustration of a lateral view of the L2–3 level with the vascular and neural anatomy in place. The dimensions (mm) reported by Panjabi and colleagues23 demonstrate the anatomical basis for 22 mm of exposure of the lateral aspect of the disc space, which when centered over the disc space is a safe distance from the segmental artery and vein as well as from the various branches of the lumbar plexus. A posteriorly placed access port increases the risk of an injury not only to the branches of the lumbar plexus but also to the ascending lumbar vein. Note the genitofemoral nerve coursing within the requisite anatomical unit at L2–3. Identification and mobilization may be necessary.
6.15 Room Setup
A standard operating table that can slide and break at the middle is ideal for this procedure. The image intensifier component of the fluoroscope is opposite the side of the approach (▶ Fig. 6.15). A special C-arm drape that maintains sterility while allowing the fluoroscope to alternate from AP to lateral is invaluable for this procedure (C-Armor; CFI Medical, Fenton, MI). The C-Armor drape eliminates the need for multiple sterile C-arm covers and adds efficiency to the operation.
Fig. 6.15 Operating room setup. Photograph of the operation room setup. The patient is positioned laterally on a standard operating table that can break with the left side up for management of an adjacent segment issue at L3–4, which can be seen on the screens of the fluoroscope. The surgeon stands on the posterior side of the patient, and the image intensifier is opposite the surgeon.
6.16 Patient Positioning
After induction of anesthesia, I position the patient in the lateral position. In the context of scoliosis or severe coronal imbalance, it is the convexity of the deformity curve that is accessed and therefore is positioned up to facilitate the approach. All other aspects being equal, I prefer a left-sided approach because of the vascular anatomy, specifically keeping the vena cava as far away from the working corridor as possible. The anterior superior iliac spine of the patient should rest just below where the table breaks. Early in my career, I always broke the table to an acute angle to facilitate access to the disc space. The observation made by surgeons that excessive breaking of the table could stretch the femoral nerve and make it more vulnerable to injury resonated with me as I listened to patients reporting their subjective complaints in the days and weeks after surgery.
One valuable article on positioning written by Molinares and colleagues24 examined the effect of positioning on 50 healthy individuals. The subjects were placed into one of two positions: straight lateral or lateral jackknife for 60 minutes. None of the patients in the straight lateral position experienced a neurologic deficit, but all the patients in the lateral jackknife position did. These results provide compelling evidence that it may be the position more so than the dilatation through the psoas that leads to neurologic deficits.24 For a time after reviewing this article, I refrained from breaking the table altogether to prevent tension on the psoas muscle, lumbar plexus and, in particular, the femoral nerve. However, I soon discovered that a patient in a purely lateral position does not provide for an adequate access corridor. I found that without a break in the table, the ribs are a more prominent factor limiting the access to the disc space. I have since returned to breaking the table to facilitate access, but I limit the extent to which I do it. Equally important is that I limit the time that the patient is in that position (▶ Fig. 6.16).24
Fig. 6.16 Patient positioning: breaking the table. Photograph of two different patients positioned for an L2–3 transpsoas interbody approach. (a) The first patient is positioned in a lateral jackknife position at 25 degrees. Molinares and colleagues24 identified that patients in this position are at risk for a neurologic deficit after 60 minutes. The positioning was modified to eliminate such an extreme angle in the table. (b) The second patient is positioned lateral, with only a modest break in the table to displace the ribs rostral enough to optimize a corridor to the lateral spine but decrease, if not eliminate, the tension on the lumbar plexus.
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