Fig. 17.1
Psoas and iliacus muscles
The iliopsoas muscle consists of three distinct muscles: the iliacus, psoas minor, and psoas major. The psoas minor, absent in 40 % of the population, attaches proximally to the bodies of T12 and L1 and travels anterior and medial to the psoas major, inserting distally in the iliopectineal eminence of the innominate bone and the iliac fascia. The iliacus attaches superiorly at the iliac crest and lumbosacral ligaments, forming a triangle-shaped muscle that attaches inferiorly to the lateral fascia of the psoas major, joining that muscle at the levels of L5 to S2 as it travels deep to the inguinal ligament and attaches to the lesser trochanter. The psoas major attaches proximally to the inferior transverse processes, vertebral bodies, and disks, as well as the intervertebral tendinous arches, from the lower body of T12 down through L5. It travels distally parallel to the lumbar spine and crosses deep to the inguinal ligament and superficial to the capsule of the hip joint, ultimately inserting distally in the lesser trochanter of the femur. The iliopsoas compartment is covered by a fascial layer contiguous with the transversalis fascia laterally, the endothoracic fascia superiorly, and fascia lata inferiorly [10, 23]. Though technically separated by these fascial layers from the retroperitoneal space, the psoas major is frequently referred to as retroperitoneal due to its local relationship with important retroperitoneal structures [10].
The psoas major is innervated by the ventral primary rami of the lumbar roots from L1 to L3. Its blood supply is variable, derived from the lumbar, iliolumbar, obturator, external iliac, and common femoral arteries. The principal supply to the psoas is frequently the lumbar branch of the iliolumbar artery, which arises from the internal iliac [7].
The psoas muscle’s primary function is hip flexion. Depending on position, it can also assist in adduction and external rotation of the hip. More controversial are its proposed functions as a “stabilizer” of the lumbar spine, but biomechanical studies have shown function as an erector of the lumbar spine and as an axial de-rotator at the inferior lumbar levels [17, 21].
The lumbar plexus lives within the psoas major. Some of the branches of the plexus perforate and lie superficial to the psoas fascia and include the genitofemoral nerve which is anterior; the lateral femoral which is cutaneous; iliohypogastric and ilioinguinal nerves, which are lateral; and the obturator nerve and the lumbar trunks, which lie medial to the psoas [5].
The precise anatomy of the psoas muscle and its associated structures varies widely from patient to patient. Morphometric studies have demonstrated patient-to-patient variability in the location of the aorta, inferior vena cava, common iliac arteries and veins, and lumbar plexus, as well as variability in the shape of the psoas major itself. This variability becomes more important as the so-called safe working zone shrinks at the lower lumbar levels, thereby allowing less room for error in the retroperitoneal approach [5, 8, 24]. This variability can be more of an issue in patients with an element of rotatory deformity. Preoperative imaging, and identification of at-risk structures, is key to safely navigating the lateral approach to the spine [13, 24] (Fig. 17.2).
Fig. 17.2
The surgeon must be mindful of the vital vascular and neurologic structures in the vicinity of the spine during the lateral transpsoas approach
17.3 Transpsoas Versus ATP
Once the psoas is encountered, the operating surgeon is faced with the choice of dissecting through the psoas major or docking anterior to the muscle, on the vertebral body, and retracting it posteriorly for access to the disk space. Each approach has its own risks and benefits (Fig. 17.3).
Fig. 17.3
This axial cross-section cartoon (L) and T2 MRI (R) show the common anterior approaches to the disc with respect to position of vital structures
The transpsoas approach has the benefit of an en-phos lateral approach. This minimizes the risk of an anterior or posterior breach of the disk space during diskectomy and cage placement and concurrently limits danger to contents of the spinal canal posteriorly and the retroperitoneal vessels anteriorly. The cost of this approach to the disk space is the risk of dissecting and retracting the psoas muscle. Manipulation of the muscle itself can frequently lead to temporary weakness. The more significant risk is that to the lumbar plexus, contained in the psoas major from the levels L1 to L5. Defining the working corridors through the psoas muscle has been the subject of much study.
The ventral migration of the lumbar plexus from L1 to L5 has been noted in multiple cadaveric and radiographic studies. This observation lent itself to the notion of “safe working zones” as defined by Uribe et al. When dissecting through the psoas at the level of the discs from L1 to L4, dissection through the middle posterior quartile of the vertebral body (“Zone 3”), and when dissecting at L4/L5 level, dissection in line with the anterior-posterior midpoint of the body [5, 24] are the safety zones.
Dissection through the middle posterior quartile (“Zone 3”) at the levels of L1 through L4 and through the midpoint of the body at L4/L5 is thought to minimize risk to the lumbar plexus posteriorly and the genitofemoral nerve anteriorly. Even when working in these “safe zones,” care must be taken during both dissection and retraction of the muscle to minimize danger to neurological structures.
Docking anterior to the psoas muscle and dissecting posteriorly have the advantage of avoiding the posterior lumbar plexus, though care must be taken to minimize manipulation or damage to the genitofemoral nerve, found in the anterior quartile of the psoas at the level of L2/L3 and overlying the anterior aspect of the muscle from L3 down. Also, anterior docking minimizes trauma to the muscle itself, which can cause weakness and confound diagnosis of injury to the superior lumbar plexus and nerve roots.
Additionally, as the approach to the psoas rotates the operative corridor anteriorly, a more anterior skin incision is indicated. As this approach demands a more anterior skin incision and muscle dissection, injury to the iliohypogastric, ilioinguinal, and subcostal nerves is less likely. This decreases the risk of abdominal asymmetry, pseudohernia, or genital numbness—known complications of the direct lateral approach [3, 6, 11].
Anterior docking carries the disadvantage of frequently requiring manipulation and concomitant risk to anterior vascular structures, as well as the sympathetic chain. The oblique approach to the disk space, when contrasted with a more directly lateral, transpsoas dissection, also increases risk to the contralateral nerve root (on the downside), which can be encountered deep in the working area during diskectomy or graft placement. Anterior docking, by rotating the working aperture for diskectomy and graft placement posteriorly, may also increase the risk to structures within the spinal canal itself [9]. These risks may be mitigated by using a banana-shaped cage [6].
17.4 Traversing the Psoas Major
As mentioned above, dissection and retraction of the psoas major in order to gain access to the disk space are fraught with risk to the lumbar plexus. Different techniques have been developed to minimize this risk.
In the original percutaneous XLIF technique, the psoas is traversed using serial dilation. Potential working zones are explored using stimulated discrete threshold electromyography (EMG)—the posterior middle quadrant at L1 through L4, the midline at L4/L5—and docked on the disk space. Once placement of the wire is confirmed with x-ray, and EMG used to detect nearby neurological structures, the psoas can be expanded using dilators of increasing caliber, until a suitable working corridor is available. Both active neuromonitoring and passive neuromonitoring are used throughout to identify potential damage from blunt dissection to or traction on the lumbar plexus [18].
While anatomic knowledge and neuromonitoring minimize risk to the lumbar plexus, they are not perfect [4]. The “shallow docking” technique allows for direct visualization of the psoas muscle dissection—thereby eliminating the need for blind passage of blunt instruments through the muscle. Direct visualization allows another opportunity to identify nerves and vessels that may otherwise be injured by retractors [1, 22] (Fig. 17.4).
