Course of Lumbar Nerve Root on Lateral Surface of the Lumbar Vertebral Body

Each lumbar nerve root exits through the superior portion of the corresponding intervertebral foramen. At the beginning of the extraforaminal region, the exiting root runs down the lateral surface of the caudal pedicle, where it traverses the vertical leaf of the intertransverse ligament to take a ventrolateral direction and then join the descending lumbar trunk within the fascia of the posteromedial portion of the psoas muscle. The lumbar trunk runs downward vertically, as a compact cord-like bundle along the surface of the junctional area between the body and the pedicle of the lumbar spine, outside the foramen. The compact cord-like trunk is close to the most dorsolateral surface of the vertebral body and the intervertebral disc space. The width of the nerve trunk gradually increases in the lower lumbar level. The lumbar nerve trunk is topographically arranged dorsoventrally with the L5, L4, L3, and L2 nerve roots, in that order (Fig. 41-2).

Fig. 41-2 The course of lumbar roots on the lateral surface of the vertebral body

The mean distances from the anterior border of the transverse process to the upper segment of the lumbar nerve root are estimated to be 5.1 to 6.4 mm (range 3–8 mm) at L2–5.² There is little variation between the upper and lower levels. When surgeons dissect the psoas muscle, the expected location of the nerve roots is near the foramen.

Anterior Margin Dissection Method

The spine is exposed, following the anteromedial slope of the psoas muscle. The genitofemoral nerve, which passes along the psoas muscle, must be preserved. The anterolateral attachments of the psoas muscle are sharply dissected from the vertebral body. Care is used not to injure the underlying sympathetic chain and the segmental vessels. As in the vertebral bodies of other levels, the segmental vessels are arranged on the mid-portion of the vertebral body. On the axial view, the anterior margin of the psoas muscle corresponds to the anterior one-third of the vertebral body (see Fig. 41-1). When the medial half of the psoas muscle is swept away from the vertebral body, the posterior cortex of the vertebral body is visible (Fig. 41-3). For complete decompression, the pedicle should be palpated and the foramen should be recognized. The extraforaminal nerve roots are enclosed in the fascia of the muscle, so the possibility of nerve injury is small. However, because of the bulkiness of the psoas muscle, sufficient exposure of the lateral body surface may sometimes be difficult.

Fig. 41-3 Anterior margin dissection method. A, Retraction of the medial half of the psoas muscle can expose the vertebral body. B, Axial view of anterior margin dissection.

Trans-Psoas Dissection Method

When the psoas muscle is split longitudinally on the mid-portion of the muscle belly, the line of access corresponds to the posterior two-thirds of the vertebral body (Fig. 41-4). This approach route has a greater possibility of injuring the exiting nerve roots.

Fig. 41-4 Trans-psoas dissection method. A, The psoas muscle is split longitudinally on the middle of the muscle belly. B, Axial view of the trans-psoas dissection.

If care is taken to peel off the fascia from the vertebral body, nerve injury can be avoided. For the safe approach, the access line may be shifted to the medial one-third line.

If the psoas is accessed correctly, the muscle fiber is dissected in the longitudinal plane.³ The dissection approaches to the vertebral body surface, and the muscle fascia is peeled off from the body surface. Once dissection through the psoas muscle is complete, the corpectomy is started.

Anterior Stabilization at L1–3 Levels

At L1–3 levels, anterior decompression is performed via the lateral retroperitoneal approach.

After decompression, the implant fixation is made on the lateral surface of the vertebral body. The vertebral body size in the lumbar spine is larger than in other levels. For stable construct, the posterior screw should be positioned near the posterior cortex. The transverse diameter of the vertebral body is longest along the posterior cortex. The entry point of the posterior screw is about 1 cm apart from the posterior cortex. The distance from the adjacent endplate is 0.8–10 mm. The length of the screw should be longer than the transverse diameter of the vertebral body for the bicortical purchase.

Anterior Stabilization at L4–5 Levels

In cases of L4 body tumor, vertebrectomy is performed by the paramedian or lateral route. Decompression is possible with either route. However, rod or plate implantation may be difficult because the iliac vessels are located nearby. When the lateral approach is adopted, the pelvic ring can hinder the instrumentation. In these cases, small-diameter screws are used for the enforcement of the anchorage of the graft material (Fig. 41-5). The trajectory of the screw is not parallel to the endplate. When the paramedian approach is applied, the screw is directed 45 degrees to the adjacent disc space. In cases of L5 body tumor, the lateral approach is impossible. The approach is made with the anterior paramedian or midline route. A similar method can be used to apply screws in patients with L5 lesions.

Fig. 41-5 Small anterior screws for anchorage of the cage. The screw head should be immersed into the bony cortex (anteroposterior and lateral views).

Posterior Procedure

The operation is begun with the patient in the prone position. The abdomen is freely decompressed in a dependent position to minimize bleeding. In addition, all bony prominences are padded. A standard midline incision is performed with wide exposure of the bony elements at the L4 level as well as the levels above and below. The lateral extent of the exposure must include the tip of the transverse process (L3, L4, and L5). Using a combination of rongeurs, all bony posterior elements are removed. This includes the following elements of L4: spinous process, bilateral laminae, bilateral superior and inferior articular facets, bilateral pars interarticularis, and bilateral transverse processes. Beneath the ventrolateral surface of the L4 transverse process, the L3 nerve root passes down around the L4 vertebral body. Care should be taken not to injure the L3 nerve root. In addition, the pedicles are removed until they are flush with the L4 vertebral body. This is accomplished by drilling the center of the pedicle with a high-speed drill followed by use of rongeurs to remove the cortical shell of the pedicle. After all of the posterior bony elements are removed, the annulus fibrosis and the nucleus pulposus of the intervertebral discs at the L3–4 and the L4–5 levels are removed with pituitary rongeurs. For an easier anterior procedure, it is useful to remove as much disc material as possible with the posterior approach. This disc removal must extend lateral and ventral to the exiting L3 nerve roots. Using large periosteal elevators, the psoas muscle just lateral to the L4 body is dissected away from the vertebral body as far ventrally as is safely possible (Fig. 41-6).⁴ Some surgeons suggest that the lumbar spondylectomy can be performed via the posterior approach only. This aggressive operation carries the risk of injury to the great vessels, which are not well visualized from the posterior route. Especially at the L4 level, the inferior vena cava is in direct contact with the vertebral body. Lesions at the L3 level and above may be safely treated with a purely posterior approach. An anterior staged operation should be considered for lower lumbar spondylectomy.

Fig. 41-6 The psoas muscle is dissected from the lateral surface of the lumbar vertebral body from the posterior side. Great care should be taken not to injure the segmental vessels.

After completion of posterior decompression, large cotton patties are placed in the plane between the ventral dural surface. This cotton helps the surgeon identify the interface between the tumor and ventral dura at the second anterior surgery. After posterior decompression, an instrumented fusion is performed.

Anterior Procedure

In lesions of L2 and L3, the lateral approach is favored for anterior decompression. In an L4 lesion, the anterior paramedian or lateral approach may be possible. In cases of high iliac crest, the anterior paramedian approach is chosen.

The patient is moved into the supine position. The operative field must be prepared to include the left iliac crest region for harvesting of additional autologous bone graft, if necessary. After a standard retroperitoneal approach, the iliac vessels are dissected. Before performance of the en bloc resection, the iliac vessels have loops placed proximally and distally to the operative region to ensure vascular control. In addition, the ureter is identified, mobilized, and protected.

When the L4 level is confirmed, the intervertebral discs above and below are incised and removed. This disc excision is continued dorsally until the lateral-most aspects of the disc removal meet the defect created during the posterior procedure. When this is completed, the psoas muscles are dissected laterally from the lateral aspect of the vertebral body. This is relatively easily accomplished on the left side and is continued dorsally until the previous dissection is encountered. At this point, cotton patties placed ventral to the left L4 nerve root will be visible. Because of the location of the large arterial and venous structures in this area, dissection of the right side of the vertebral body is significantly more difficult. With gentle retraction of the iliac vessels toward the right side, periosteal dissectors are used to strip the psoas muscles from the right side of the vertebral body. Once again, the dissection is continued dorsally until the cotton patties ventral to the right L4 nerve root are identified. At this point, the vertebral body/tumor is lifted out of the operative field en bloc and is passed off the operative field. After the vertebrectomy, the previously placed cotton is removed. In addition, the ventral dura and exiting nerve roots are seen (Fig. 41-7). Anterior stabilization is accomplished using either a titanium cage or a femoral ring allograft. Autologous iliac crest bone graft is used to fill either the cage or the femoral ring.

Fig. 41-7 After the vertebral body is removed, the ventral dura and bilateral L4 nerve roots are visualized.