Transpsoas Anatomical Approach to the Spine

39 Transpsoas Anatomical Approach to the Spine


Steven L. Gogela and William D. Tobler


Abstract


This chapter discusses in detail the anatomy and clinical nuances of operative technique based on our ongoing refinements and deeper understanding of this complex anatomy. Although a lateral lumbar interbody fusion can be fraught with risks of complications and neuromuscular injury, mastery of the anatomy and technique can make this a valuable operation with minimal morbidity. The lateral transpsoas approach is an elegant, albeit unfamiliar, surgical corridor for many practicing surgeons, especially those trained before the approach became mainstream. Therefore, information is divided into five categories: abdominal wall with its neuromuscular structures, retroperitoneal cavity and contents, psoas muscle, lumbar plexus, and vertebral column. This minimally invasive procedure (using a shallow docking technique) affords excellent passage for interbody grafting, and credible clinical experiences have been well established. A relative decrease in morbidity compared with other approaches may be achieved with utilization of a natural anatomical corridor and use of the “shallow docking,” our preferred technique for dissecting the psoas under direct visualization. When indicated, properly prepared surgeons should not hesitate to approach the L4–L5 disc space laterally despite the robust psoas muscle and risk of injury to the lumbar plexus at this level. Further modification and experience with the lateral transpsoas approach will inevitably continue into the future.


Keywords: anatomy, operative technique, lateral lumbar interbody fusion, lateral transpsoas approach, abdominal wall with its muscular and neural structures, retroperitoneal cavity, lumbar plexus, vertebral column, shallow docking technique


39.1 Introduction


The minimally invasive lateral transpsoas approach to the lumbar spine has rapidly grown in popularity since its original description in 1998.1 The approach gave new options and advantages compared with standard posterior or anterior surgical corridors. One primary advantage relates to robust restoration of coronal balance at one or multiple segments. Other benefits include the preservation of stabilizing structures, such as the anterior and posterior longitudinal ligaments, facet joints, and other posterior elements, and improved graft–host interface with a larger interbody device spanning the entire ring hypophysis. Restoration of disc and foraminal height can provide indirect decompression of neural elements. With its overall ease of access, this minimally invasive approach has led to decreased blood loss and shorter hospital stays.2,3,4,5,6,7


For the inexperienced surgeon, the anatomy encountered during a lateral lumbar interbody fusion (LLIF) can be entirely new and fraught with risks of complication and neuromuscular injury. However, mastering the anatomy and clinical nuances of the approach will make this a valuable operation with minimal morbidity. In this chapter, we detail the relevant anatomy and our operative technique, which reflects our ongoing refinements as we gained greater understanding and experience with this complex anatomy.


39.2 Relevant Anatomy


In reviewing the anatomy of this approach, we divided the information into five categories:


Abdominal wall with its muscular and neural structures.


Retroperitoneal cavity and contents.


Psoas muscle.


Lumbar plexus.


Vertebral column.


39.2.1 Abdominal Wall


The wall of the abdomen begins with the skin and subcutaneous fat superficial to the lateral abdominal musculature. The external oblique, encased by a fascial layer, overlies the internal oblique and transversus abdominis superficial to the retroperitoneum. Nerves commonly encountered in this territory are as follows (image Fig. 39.1, image Fig. 39.2, image Fig. 39.3):


Subcostal nerve. Receiving innervation from T12, the subcostal nerve is responsible for sensation of the anterior gluteal skin and motor supply to the rectus abdominis and external oblique (see image Fig. 39.1). It courses inferior to the 12th rib and anterior to the quadratus before perforating the transversus to travel between it and the internal oblique. The nerve then courses inferomedially until it perforates the rectus abdominis. Injury to this subcostal nerve can occur either during the approach or more commonly during harvesting of the iliac crest near the anterior superior iliac spine (ASIS). The effect varies depending on which portion of the nerve is injured: notalgia paresthetica with injury to the dorsal cutaneous branch, rectus abdominis syndrome with the anterior cutaneous branch, or abdominal wall weakening and potential hernia formation.


Iliohypogastric nerve. With input primarily from L1 (and lesser T12), this nerve supplies sensation to the gluteal and hypogastric skin via the lateral and anterior cutaneous branches, respectively (see image Fig. 39.2). It also contributes to the innervation of the transversus and internal oblique muscles. The nerve emerges along the superolateral psoas and travels anteriorly until it perforates the transversus above the iliac crest, traveling between that muscle and the internal oblique like the subcostal nerve. Branching about 3 cm medial to the ASIS, the iliohypogastric nerve then divides into lateral and anterior cutaneous branches. Injury to the nerve can cause painful paresthesias and/or paresis of the abdominal wall, causing either abdominal wall or direct inguinal hernias depending on the location of injury.


Ilioinguinal nerve. Innervated primarily by L1 (lesser T12, L2), this nerve transmits sensation to the anterosuperomedial thigh as well as the root of the penis and scrotum in males or the labia majora in females. It also supplies motor innervation to the lower transversus and internal oblique muscles. The nerve emerges from the psoas at the level of L1 just caudal to the iliohypogastric; it pierces the transversus to run between it and the internal oblique, giving off muscle branches to both. It continues toward the pubic symphysis, with its sensory branch traveling through the inguinal canal and past the superficial inguinal ring. Injury to this nerve is most common on approach and may cause painful paresthesias and/or paresis of the abdominal wall, resulting in either direct inguinal or abdominal wall hernias.



Lateral femoral cutaneous nerve (LFCN). The LFCN receives its greatest contribution from L2 and L3 (lesser L1). This purely sensory nerve supplies sensation to the lateral thigh in a well-known distribution. After emerging from the lateral psoas around the level of L4, this nerve travels underneath iliac fascia, across the anterior iliacus, and through the retroperitoneum en route to the ASIS. It then dives under the inguinal ligament roughly 1 cm medial to the ASIS. Injury results in meralgia paresthetica.


39.2.2 Retroperitoneal Cavity


The borders of the retroperitoneum are the psoas muscle and vertebrae medially, peritoneum and internal organs anteriorly, quadratus lumborum and iliacus posteriorly, diaphragm superiorly, and pelvic structures inferiorly. Contents vary depending on the craniocaudal level, but may include the kidneys and/or adrenal glands, ureters, aorta, inferior vena cava, ascending and descending colon, rectum, duodenum, and pancreas.8


39.2.3 Psoas Muscle


The psoas muscle is the major flexor of the hip and principal anterior stabilizer of the lumbar spine (see image Fig. 39.1 and image Fig. 39.2). The deep portion originates from the transverse processes of the T12–L4 vertebral bodies, with a superficial and anterior psoas minor component originating from the lateral bodies and discs between those vertebrae. The muscle belly significantly increases in mass as it travels inferiorly from T12, becoming quite large at the L4–L5 level where it begins to move anteriorly to the spine as it courses into the pelvis. Extensive study of the psoas muscle is a must before surgery because its shape and configuration varies among patients: it may be asymmetric, making one side more approachable in many cases. As the psoas moves anteriorly, the femoral nerve courses through the muscle and exits anteriorly, where it is at risk of injury at the L4–L5 level (image Fig. 39.4). Above this point, the L3 contribution to the lumbar plexus tends to be far posterior in the psoas and at lesser risk of damage on approach.


39.2.4 Lumbar Plexus


Genitofemoral Nerv

Generally supplied by L2 (lesser L1), the genitofemoral nerve has two components: a genital branch going to the scrotum in males and the mons pubis and labia majora in females, and a femoral branch supplying skin over the femoral triangle. Notably, this nerve also supplies motor innervation to the cremaster muscle in males, which may be monitored during surgery (see image Fig. 39.1).



The nerve travels anteriorly between the two bellies of the psoas muscle at the L2–L3 level before piercing the psoas around the L3–L4 disc and traveling down the anterior or middle third of the muscle belly.9,10 Risk of injury to the nerve is greatest at L2–L3, below which it is usually not visualized because it lies more anterior. It then branches, with the genital branch traversing the inguinal ligament and the femoral branch remaining deep to the ligament. Injury results in numbness and potentially paresthesias of the supplied areas and loss of the cremaster reflex in males (see image Fig. 39.3).


Femoral Nerve

The femoral nerve, which receives input from the L2–L4 trunks, is the largest branch of the lumbar plexus and carries the most important motor function placed at risk in the transpsoas approach. Supplying the quadriceps muscles, the femoral nerve is almost entirely responsible for knee extension. It also supplies a portion of the iliacus and pectineus muscles that assist in hip flexion. Through the sartorius muscle, it is involved in flexion, adduction, and rotation of the hip. The femoral nerve supplies sensation to the anteromedial thigh via the anterior femoral cutaneous branch and the medial leg and foot via the saphenous branch. The nerve travels deep in the belly of the psoas and gradually moves anteriorly within the muscle as it progresses inferiorly. It crosses the L4–L5 disc space in the posterior middle quadrant, where the nerve is at greatest risk for injury.11 At this level, the femoral nerve is large and can be easily visualized (image Fig. 39.4b, c). Injury may result in anesthesia or paresthesias in its sensory territory, weakness of hip flexion, knee extension, and lateral thigh rotation, and loss of the patellar reflex. Injury to this nerve can result in profound and permanent disability, which is a major topic of concern cited by surgeons who are reluctant to use this approach, especially at the L4–L5 level.


Obturator Nerve

The obturator nerve, chiefly supplied from L3 (lesser L2, L4), supplies sensation to the medial thigh. Unlike the genitofemoral nerve, it has significant motor function. The nerve emerges from the medial psoas and travels inferiorly over the sacral ala en route to the obturator canal, where it exits to supply the hip joint and muscles of hip adduction.8 This nerve is at lower relative risk during the transpsoas approach because its origin from the femoral nerve is usually far anterior to the psoas muscle and not typically in the surgical corridor.



Sympathetic Trunk

The sympathetic trunk runs deep to the medial border of the psoas along the anterior one-third of the vertebral bodies throughout the lumbar spine. Injury during this approach rarely occurs but may have broad-ranging autonomic effects, typically resulting in a warm, dry leg.


39.2.5 The Spine


As at all levels in the spine, lumbosacral intervertebral discs are surrounded by an annulus fibrosus with a central nucleus pulposus. They are buttressed by the anterior and posterior longitudinal ligaments, but laterally are exposed. Although safe working zones have been described,12 others have shown that aberrant anatomy in these zones can place the lumbar plexus at risk up to 37% of the time, especially when blindly dilating the psoas.9,13 In general, considering all published cadaveric studies and experience, the center of the disc space provides a relatively safe target provided that the surgeon dissects the psoas under direct visualization and, in most cases, utilizes neuromonitoring with continuous electromyogram (EMG) and direct stimulation of the operative field.9,12,13,14,15,16,17,18


39.3 Surgical Technique


The lateral transpsoas approach to the lumbar spine is an elegant approach. Our operative technique is detailed, from positioning through stepwise dissection to the desired disc space, neuromonitoring, and appropriate imaging studies. Finally, we give some pearls of lessons learned and comment on some ongoing refinements in technique for approaching this complex anatomy.


39.3.1 Positioning


The patient is positioned in the lateral decubitus position on a table and firmly secured with silk tape; the head faces toward anesthesia. The table must be capable of flexing with the break of the bed at the hips. Flexion of the table should optimize access between the patient’s iliac crest and the lower limits of the rib cage. However, extreme flexion of the operating table is strictly avoided to prevent undue tension on the lumbar plexus and psoas muscle, which places these structures at greater risk of ischemic injury during retraction for the approach. The ipsilateral hip is flexed significantly to relax the psoas, and thus helps mitigate this potential complication. Access to the upper lumbar levels may necessitate an intercostal approach. An axillary roll is placed and all bony prominences are padded, including the contralateral side, where positioning-related injury to the peroneal and superior gluteal nerve has been reported.19


Using fluoroscopy, the surgeon obtains true orthogonal anteroposterior (AP) and lateral images with the C-arm locked at 0 and 90 degrees, respectively (image Fig. 39.5). Minor adjustments of the table can be made, but any major adjustments should be avoided because the side rails can interfere with imaging. After the disc(s) of interest are localized with orthogonal lateral X-rays, the incision is marked on the skin at approximately the middle third of the targeted disc space.


39.3.2 Approach


The patient is prepped and draped in sterile fashion. After skin incision, monopolar cautery may be used until the abdominal fat is encountered. The fat is then bluntly dissected until the external oblique muscle fascia is reached and incised sharply. Muscle layers are split longitudinally using gentle dissection with a finger or blunt instrument, taking care to mobilize any encountered neural structures en route to the retroperitoneal space. The iliohypogastric and ilioinguinal nerves flow through these muscular layers and must not be damaged.


The retroperitoneum is identified by the presence of fatty tissue and a palpable sense of entering a cavity of free space. After locating the posterior wall (quadratus lumborum and iliacus), the retroperitoneal contents are swept bluntly with a finger from a posterior to anterior and cephalad to caudad direction. The vertebral transverse processes and psoas muscle are identified by finger palpation, while taking care not to probe into the neuroforaminal areas. Patients with scarring secondary to prior diverticulitis, radiation, or abdominal or retroperitoneal surgery may not be candidates for the lateral approach


Oct 17, 2019 | Posted by in NEUROSURGERY | Comments Off on Transpsoas Anatomical Approach to the Spine
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