The lateral transpsoas approach to the lumbar spine is increasingly being used to treat degenerative changes requiring fusion. 1 In contrast to conventional posterior spinal fusion techniques, this minimally invasive approach spares extensive posterior tissue dissection and resection for cage implantation and decreases operative time, blood loss, postoperative pain, and tissue trauma. 1 Although minimally invasive, this procedure has the approach-related risk to cause lumbar plexus nerve injuries secondary to the insertion and dilation of dilatators or retractors (▶ Fig. 14.1, ▶ Fig. 14.2, ▶ Fig. 14.3, ▶ Fig. 14.4). Plexus injuries are reported from 6 to 33%, often presenting as neuropathic pain and motor or sensory deficits. 1, 2, 3
Fig. 14.1 Schematic drawing of the right-sided segmental nerves and their course from a lateral position. The subcostal, iliohypogastric, and ilioinguinal nerves are seen in their superficial course around the anterolateral abdominal wall. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U, Ross LM, Zeberg H, Atlas of Anatomy. New York, NY: Thieme Medical Publishers; 2015. Illustration by Karl Wesker)
Fig. 14.2 Schematic drawing of the left-sided lumbar plexus from a lateral perspective.
Fig. 14.3 Right-sided, lateral approach to the lumbar spine. The psoas major has been retracted anteriorly to illustrate the many branches of the lumbar plexus. Labeled are the ventral rami of T12–L5. Note how the branches of the lumbar plexus are draped over the lateral vertebral bodies of the lumbar spine.
Fig. 14.4 Anterior view of the right retroperitoneal space after a transpsoas, lateral approach to the L4 vertebra. The psoas major has been retracted laterally to illustrate the docking location placed under fluoroscopy. The juxtaposition of the lumbar plexus is obvious and seen posterior to the instrument. The femoral (upper arrow) and obturator (lower arrow) nerves are seen extending from L2–L4.
Several cadaveric anatomical studies on plexus nerve anatomy for lateral approaches have been published 4, 5; however, few have systematically documented the types of injury typically observed at each spinal level after such procedures. In the cadaveric study of Grunert et al, approximately 50% of all operated-upon segments had plexus nerve injuries occurring at segments L1–L4 and involving nerve roots as well as motor and sensory nerves. 6 In a similar cadaveric study, Banagan et al found direct nerve injuries from lateral approaches in 25% of operated segments. 7 The higher percentage in the former study could be explained as follows.
First, Banagan et al openly dissected the surface of the psoas major muscle. Then, under direct visualization, they inserted dilator tubes through the anterior third of the muscle in order to achieve an “ideal position.” 7 Alternatively, Grunert et al used a standard operative technique, inserting tubes without direct visualization. 6 Second, the Banagan et al study only assessed segments L3–L5, whereas Grunert et al studied L1–L5. 6, 7 In Grunert et al’s study, most of the lumbar plexus nerve injuries observed were in the upper lumbar levels (L1–L3). 6 Thus, solely investigating levels L3–L5 could significantly underestimate the injury potential of these procedures.
Other groups have conducted cadaveric anatomy studies without performing transtubular procedures in order to locate safe zones or docking points. Benglis et al described a ventral migration of the lumbosacral plexus on the lumbar disc spaces from L2 to L5, indicating that the tubular docking point should be placed more anteriorly at more caudal levels. 5 Similar results were described by Moro et al and Uribe et al, who subdivided each vertebral segment into four quarters (zone I to zone IV) from the anterior to the posterior border of the vertebral body. 4, 8 Although describing the relationship of the plexus nerves to the lateral vertebral body surface helps in determining ideal docking points, it oversimplifies the complex plexus anatomy, as the nerves run in both anteroposterior and mediolateral directions. As shown in the study by Grunert et al, injuries can occur throughout the entire trajectory of the lateral approach. Over 50% of the nerve injuries occurred either at the lateral aspect of the psoas major muscle, in the retroperitoneal space, within the outer abdominal muscles, or in the subcutaneous tissue of the abdominal wall, predominantly affecting the subcostal, ilioinguinal, iliohypogastric, and lateral femoral cutaneous nerves. 6 Dakwar et al described the anatomical course of the plexus nerves lateral to the psoas major muscle in great detail. 9 However, the surgically relevant question—for each lumbar level, at what anatomical location can these nerves be encountered and injured—has not been addressed.
To clarify where those nerves can be encountered, Grunert et al subdivided their approach into four mediolateral anatomical zones superficial to deep and medial to lateral: zone I, the psoas major muscle; zone II, the retroperitoneal fat tissue; zone III, the outer abdominal muscles; zone IV, the subcutaneous tissue of the outer abdominal wall. 6 Based on their findings, the following section outlines the course of the plexus nerves within these anatomical zones, identifies locations at which nerve injuries typically occurred, and suggests how to avoid these injuries in clinical practice.
14.2.1 Subcostal Nerve
The subcostal nerve emerges from the T12 nerve root and runs along the inferior border of the 12th rib before it pierces through the outer abdominal muscles. At L1/L2, it is only encountered within the outer abdominal muscles (zone III) and the subcutaneous tissue (zone IV). This nerve was injured in zone III and zone IV. 6
Careful blunt dissection through the subcutaneous and muscle tissues, without excessive monopolar coagulation, could avoid such injuries. The fasciae of the abdominal muscles should be opened bluntly in the direction of their muscle fibers.
14.2.2 Ilioinguinal and Iliohypogastric Nerves
Emerging from the T12/L1 nerve root, the iliohypogastric and ilioinguinal nerves run posterior to or within the psoas major muscle (zone I). Leaving the psoas major, both nerves continue their oblique descent on the anterior surface of the quadratus lumborum, along the posterior abdominal wall (zone II). As it reaches the outer abdominal muscles (zone III), small branches pierce the surrounding abdominal muscles until they reach the subcutaneous tissue (zone IV). At L1/L2 level, these nerves can potentially be injured in all anatomical zones. 6